Claus F. K. Diessel
Coal-Bearing
Depositional Systems
With 356 Figures and 40 Tables
Springer-Verlag
Berlin Heidelberg New York
London Paris Tokyo
Hong Kong Barcelona
Budapest
Claus F. K. Diessel
Professor of Geology
Department of Geology
The University of Newcastle
Newcastle, NSW 2308
Australia
ISBN-13:978-3-642-75670-2
e-ISBN-13:978-3-642-75668-9
DOl: 10.1007/978-3-642-75668-9
Library of Congress Cataloging-in-Publication Data
Diessel, C. F. K.
Coal-bealing depositional systems / Claus F. K. Diessel. Includes bibliographical
references and indexes.
ISBN-i3:978-3-642-75670-2 (alk. paper)
1. Coal- Geology. 2: Facies (Geology) - Analysis.
1992 553.2'4 - dc20
I. Title. TNS02.D47
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Preface
Because of the upsurge in coal exploration over the last two decades,
geological knowledge of coal has advanced at an increasing rate.
This activity has led to a considerable degree of specialisation among
the geological disciplines serving the coal industry, but there has
also been a convergence of knowledge, in the sense that today there
is a greater awareness of the close genetic links between organic and
inorganic sediments which share the same depositional environment.
At the beginning of this century, studies of the depositional
conditions of coal seams were initiated by botanists and botanically
oriented coal petrologists. Later, when the cyclicity of many coalbearing strata was recognised, geological aspects began to dominate
the discussions which currently are centred upon auto-sedimentational
models. The result is that much of the study of coal deposition is
carried out by sedimentologists who spend time and effort on an
interpretation of the interseam sediments but hardly ever look at
the coal itself. Conversely, coal petrologists usually leave the roof
and floor strata to the sedimentologists, and both leave the fossils
to the palaeontologists, with the result that all three tend to underutilise each other's rich source of geological information.
Authors of keynote addresses and editors of conference volumes
and symposia proceedings on coal have been lamenting the lack of
integration of coal geological knowledge obtained from different
branches of the discipline (e.g. Collinson and Scott 1987; McCabe
1984, 1987; Rahmani and Flores 1984; Scott 1987). The reason for
this dilemma is that coal science is a complex subject. It incorporates
both organic and inorganic aspects which are often difficult to coordinate. In this monograph, I have tried to build a bridge across
the conceptual gap between coal petrology and sedimentology.
However, any possible merit ofthis approach lies more in the inspiration it might give to others rather than in the perfection of its construction. It was tempting to extend the scope of the work into the
realms of palaeobotany and geochemistry, but, apart from a brief
review of geochemical applications, the subject matter has been
largely restricted to coal petrology and sedimentology. Only when
additional information from other areas of specialisation was needed,
reference was made to the respective fields of interest.
VI
Preface
This monograph is neither a textbook nor a research report
although it has aspects of both. It has been written in the style of
a textbook but much of its contents is the outcome of hitherto unpublished original research into the causal links between coal properties and geological setting. The analytical approach to this problem
is reflected in the relatively small number of examples which have
been selected in order to argue the case for coal facies analysis as a
useful tool in palaeo-environmental reconstruction on several levels.
Geological "proof" is often based on the principle of internal
consistency among the results obtained by different analytical
methods. The promotion of coal petrology as one of those methods
is designed to heighten the awareness of the many possibilities the
technique has to offer to students of palaeo-environmental analysis.
Since this aspect is of concern not only to the coal geologist, I hope
that also other members of the geological fraternity will find it useful.
It is only natural that the chosen examples reflect my own experience which has been mainly in bituminous coal in Australia and
Germany with only occasional glimpses of other areas. This restriction should not be seen as a disadvantage, because it covers two
geologically and economically significant coal-producing regions,
one of which (Australia) has had less exposu.r;e in the world literature
than it deserves on account of its economic significance and diversity
of coal-forming conditions.
More importantly, it is not the purpose of this monograph to
give a generalised geological history of the world's coal deposits and
their geological settings but to present coal facies analysis as a sophisticated method in palaeo-environmental reconstruction. In view of
this aim, it might seem incongruous that I have made so much use
of maceral analyses of composite coal seams rather than of microlithotype analyses obtained from pillar samples. The reason for the
emphasis on the simpler maceral analyses of composite seam sections
and subsections is the desire to make available to fundamental
research the many routine coal analyses which are carried out daily
in many commercial coal laboratories all over the world. In their
present form, most of them are unsuitable for palaeo-environmental
enquiry, but, at the expense of little effort and no extra costs, they
can become a rich and, as yet, under-utilised source of information.
Much of the material presented in the text has been synthesised
from ideas which evolved during discussions with my colleagues and
students. Space does not permit to list them all, but among the persons
who, knowingly or not, had an influence on the ideas formulated in
the text, the following deserve special thanks for discussions about
sedimentary and coal-forming environments: Boris Alpern, Ron
Boyd, Tom Callcott, Alex Cameron, Alan Cook, Hans Fiichtbauer,
Martin Gibling, Fariborz Goodarzi, Peter Hacquebard, Wolfgang
Kalkreuth, David Marchioni, Peter Martini, Ted Milligan, Noreen
Preface
VII
Morris, Dale Leckie, Ray Rahmani, Brian Rust, Klaus Strehlau, Geoff
Taylor, Marlies Teichmiiller, Peter Vail, Bob Wagner, Roger Walker,
Peter Warbrooke, Monika Wolf, Evamarie Wolff-Fischer and
Winfried Zimmerle.
John Calder, Martin Gibling, David Gibson, Michael Lawson,
David Marchioni and Ray Rahmani introduced me to Canadian,
and Carol Bacon to Tasmanian coalfields. Rupert van den Berg gave
me a glimpse of the Karroo Basin, while Phillipa Black, and Jane
and Nigel Newman paved my way to New Zealand's coalfields.
My teacher, the late Marie-Therese Mackowsky, and the
Bergbau-Forschung GmbH, as well as my parents-in-law, Gertrud
and Wilhelm Schafer, have been generous hosts on several study
tours of the Ruhr Basin. Of particular significance for the contents
of this monograph has been a four months' stay at the RuhrUniversitat Bochum which was financially supported by the German
Academic Exchange Service (DAAD). It enabled me to test some
concepts and analysis procedures, originally developed for Permian
cold-climate Gondwana coals on warm-climate Carboniferous coals,
and to prove their portability. In this context, thanks are also due
to The University of Newcastle, N.S.W., for relieving me from my lecturing duties through its Outside Studies Program. The University's
Institute of Coal Research and its Director, Konrad Moelle, are thanked
for making available a computer for the preparation of the manuscript.
The work in Germany was greatly assisted by many people. In
particular I wish to thank Albrecht Rabitz of the Geologisches
Landesamt Nordrhein-Westfalen for directing me to field outcrops
in the Ruhr Valley, and making valuable suggestions. Ronald Conze,
Eric David, Frank David and Thomas Kraft of the Ruhr-Universitat
Bochum assisted in core logging and outcrop surveys in the Ruhr
Basin. Werner Pfisterer, Herbert Schniggenfittich and Heinz-Herbert
Sawitzki of the Ruhrkohle AG-BAG Lippe made available bore
cores, logs, analysis results and many coal samples from the Ruhr
Basin. Additional support by Australian individuals and organisations
is acknowledged in the text.
Janice Crawford, Wal Crebert, Esad Krupic, Larissa Gammidge,
Geraldene McKenzie and Jocelyn Pitts of The University of Newcastle,
N.S.W., assisted in photography, drafting, laboratory and secretarial
work. Beth McHugh read most of the text while Greg Dean-Jones,
Luise Diessel, Ron Boyd and Robin OIDer read parts of it. All made
many valuable suggestions which improved both subject matter and
literary style of the manuscript. The former Head of the Geology
Department, Ian Plimer, is thanked for keeping my workplace
reasonably free from unwanted interference when the pressure was
on, while my wife, Luise, deserves much credit for providing untiring
support throughout the project.
VIII
Preface
Some of the figures and part of the text presented here have been
taken from an AMF course manual entitled Coal Geology, which I
prepared in support of a series of workshops held between 1980 and
1985 in Adelaide (SA), Newcastle (NSW), Christchurch (NZ), and
Bandung (Indonesia). I wish to express my gratitude to the Australian
Mineral Foundation Inc. and its former Director, Dean Crowe, for
the permission to use this material.
Finally, I thank the Springer-Verlag, in particular Wolfgang
Engel, for having initiated the project. Monika Huch, Gustava HeB,
and their colleagues in the Abteilung Copyediting and Abteilung
Herstellung deserve much credit for their counsel and care in the
preparation and printing of the manuscript.
Newcastle, NSW, June 1992
Claus F. K. Diessel
Contents
Preface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..
1
V
Introduction. . . . . . . . . . . . . . . . . . . . . . . . .
2
The Conditions of Peat Formation
2.1
Peatland Ecology . . . . . . . . .
2.1.1 Coastal Marshes and Swamps . .
2.1.1.1 The Marine Influence on Peat Accumulation
2.1.1.2 The Influence of Marine Transgressions
and Regressions . . . . . . . . . . . . . . .
2.1.2 Fresh-Water Peatlands . . . . . . . . . . .
2.1.2.1 Upper Delta and Alluvial Plain Swamps,
Marshes and Bogs . . . . . . . . . . . . . . .
2.1.2.2 Limnic Environments . . . . . . . . . . . . .
2.2
Peat Composition and Peat-Forming Plants
2.3
Climate and Peat Accretion . . . . . . . . . .
2.4
Evolutionary Trends in Peat and Coal Formation
3
The Coalification Process
3.1
Biochemical Coalification
3.1.1 The Vitrinitisation Path .
3.1.2 The Fusinitisation Path .
3.1.3 Plant-Specific Coal Components.
3.2
Physico-Chemical Coalification
3.2.1 The Concept of Coal Rank . . . .
3.2.1.1 Solubility in Alkali Hydroxides .
3.2.1.2 Moisture Content and Specific Energy
3.2.1.3 Volatile Matter and Fixed Carbon ..
3.2.1.4 Elemental Carbon . . . . . . . . . . . .
3.2.1.5 Vitrinite Reflectance and Other Physical Rank
Parameters ... .. . . . . . . . . . . . . . . . . . .
3.2.2 The Effects of Pressure, Temperature and Time
5
5
12
13
14
16
16
17
18
25
32
41
41
49
58
68
71
74
75
75
75
77
77
81
x
Contents
4
Coal Petrographic Entities
4.1
Macerals...........
4.1.1 The Huminite/Vitrinite Group .
4.1.1.1 The Humotelinite/Telovitrinite Subgroup
4.1.1.2 The Humodetrinite/Detrovitrinite Subgroup
4.1.1.3 The Humocollinite/Gelovitrinite Subgroup
4.1.2 The Inertinite Group .... ..
4.1.2.1 The Telo-Inertinite Subgroup
4.1.2.2 The Detro-Inertinite Subgroup
4.1.2.3 The Gelo-Inertinite Subgroup
4.1.3 The Liptinite Group
4.1.3.1 Primary Liptinites .
4.1.3.2 Secondary Liptinites
4.1.4 Maceral Analysis ..
4.2
Microlithotypes...
4.2.1 Microlithotype Analyses
4.2.2 The Relationship Between Microlithotypes
and Macerals . . .
Lithotypes.................
Lithotype Analysis . . . . . . . . . . . .
The Relationship Between Lithotypes,
Macerals and Microlithotypes
4.4
Minerals........
4.4.1 Phytogenic Minerals .
4.4.2 Adventitious Minerals
4.4.2.1 Silicate Minerals ..
4.4.2.2 Silica Minerals ...
4.4.2.3 Carbonate Minerals
4.4.2.4 Phosphate Minerals
4.4.2.5 Sulphide Minerals
4.4.3 Mineral Analysis ..
4.3
4.3.1
4.3.2
5
Coal Facies and Depositional Environment . . . . . ..
Phyterals and Macerals in Palaeo-Environmental
Analysis . . . . . . . . . . . . . . .
5.1.1 Botanical Attributes of Macerals
5.1.1.1 The Topogenous Model
of Densosporinite Formation.
5.1.1.2 The Ombrogenous Model
of Densosporinite Formation .
5.1.1.3 Densosporinite as Part of a Sedimentary Sequence
5.1.2 Scalar Properties of Macerals
5.1.2.1 The Tissue Preservation Index
5.1.2.2 The Gelification Index. . . . .
87
88
90
90
100
102
103
104
109
112
112
113
121
121
123
124
127
127
134
135
137
138
140
141
149
150
151
154
157
161
5.1
161
162
168
172
174
177
181
189
Contents
Microlithotypes as Tools in Palaeo-Environmental
Analysis . . . . . . . . . . . . . . . . . . . . .
5.2.1 Microlithotype Proportions and Bandwidth ..
5.2.2 Hacquebard's Double Triangle . . . . . . . . . .
5.3
Lithotypes as Palaeo-Environmental Indicators
5.3.1 Black Coal Lithotypes . . . . . . . . . . . . . .
5.3.2 Brown Coal Lithotypes . . . . . . . . . . . . .
5.4
Optical Properties as Palaeo-Environmental
Indicators . . . . . . . . . . . . . . . . . . . . .
5.4.1 Vitrinite Fluorescence . . . . . . . . . . . . . .
5.4.2 Vitrinite Reflectance and Other Rank Parameters
5.5
Geochemical Palaeo-Environmental Signatures ....
5.5.1 Elements of Palaeo-Environmental Significance
5.5.1.1 Sulphur ... .
5.5.1.2 Boron . . . . . . . . . . . . . . . . . .
5.5.1.3 Other Elements . . . . . . . . . . . . .
5.5.2 Organic Geochemical Characteristics
5.5.2.1 Alkanes
5.5.2.2 Fatty Acids . . . . . . . .
5.5.2.3 Isoprenoids . . . . . . . .
5.5.2.4 Heterocyclic Compounds
5.5.2.5 Aromatic Compounds . .
5.5.2.6 Amino Acids . . . . . . .
5.6
Epiclastic Minerals and Palaeo-Environments.
5.7
Summary of Palaeo-Environmental Indicators
XI
5.2
6
The Relationship Between Coal
and- Interseam Sediments . . .
Some Characteristics and Properties
of Interseam Sediments . . . . . . . .
6.1.1 Single Particles . . . . . . . . . . . . .
6.1.1.1 Origin and Composition of Single Particles
6.1.1.2 Particle Size . . . . . . . . . . .
6.1.1.3 Particle Shape and Roundness . .
6.1.2 Depositional Fabric . . . . . . . .
6.1.2.1 Classification of Fabric Elements
6.1.2.2 Types of Aggregation ..
6.1.2.3 Symmetry Relationships
6.1.3 Coal Measure Structures
6. t .3.1 Stratification
6.1.3.2 Bed Undulations . . . . .
6.1.3.3 Cross-Stratification
6.1.3.4 Surface and Sole Markings
6.1.4 Coal Measure Lithosomes
6.1.4.1 Natural Gamma-Ray Log
192
192
194
199
200
207
214
215
223
228
228
240
242
243
245
248
250
251
255
256
257
258
261
265
6.1
266
267
267
269
271
272
272
274
276
280
280
282
287
290
297
301
XII
Contents
6.1.4.2 Density (Gamma-Gamma) Log
6.1.4.3 Neutron-Neutron Log . . . . . . . .
6.1.4.4 Caliper Log . . . . . . . . . . . . . . .
6.1.4.5 Sonic Properties
. . . . . .
. .....
6.1.4.6 Resistivity Log ...
6.1.4.7 Spatial Attitude .. .
6.1.4.8 Combination Tools
6.1.4.9 Data Management .
. .....
6.2
Coal Seams and Their Floor Rocks
6.3
Coal Seams and Their Roof Rocks
6.3.1 Concordant Coal/Roof Couples .. . . . . . . . ..
6.3.1.1 Abrupt Contacts Between Coal and Roof Rocks ...
6.3.1.2 Gradational Contacts Between Coal
and Roof Rocks . . . . . . . . . . . . . . . . . . . . . .
6.3.2 Discordant Coal/Roof Couples . . . . . . . . . . . . .
6.3.2.1 Coal/Roof Discordance Due to Unequal Loading .
6.3.2.2 Coal/Roof Discordance Due to Erosion . . . . . . .
6.4
Coal Seam Splitting . . . . . . . . . . . . . . . . . . .
6.4.1 Seam Splitting Due to Differential Subsidence
6.4.2 Seam Splitting Due to Autosedimentational Causes
302
302
302
303
303
303
304
305
306
311
312
312
329
329
330
336
339
343
346
7
Coal-Producing Sedimentary Environments . . . . . . . 349
7.1
7.1.1
7.1.2
7.1.3
7.2
7.2.1
7.2.2
7.2.3
7.2.4
The Braid Plain . . . . . . . . . . . . . .
. . . . . .
The Gravelly Braid Plain . . . . . . . . . .
The Sandy Braid Plain . . . . . . . . . . .
The Coals of the Braid Plain . . . . . . . .
The Alluvial Valley and Upper Delta Plain
The Point Bar .
. . . . . . .
The Flood Plain . . . . . . . . . . . . . .
The Flood Basin . . . . . . . . . . . .
The Coals of the Alluvial Valley
and Upper Delta Plain .. .
The Lower Delta Plain .. .
The Prodelta . . . . . . . . .
The Delta Front . . . . . . .
The Distributary Channel
The Interdistributary Bay . . . . . ..
The Coals of the Lower Delta Plain
The Barrier Beach/Strand-Plain System . . . . . . . .
The Offshore Transition Zone ... .
The Shoreface . . . . . . . . . . . . . .
The Foreshore
The Backshore .
The Tidal Inlet .
The Backbarrier
7.3
7.3.1
7.3.2
7.3.3
7.3.4
7.3.5
7.4
7.4.1
7.4.2
7.4.3
7.4.4
7.4.5
7.4.6
354
360
371
374
380
385
388
393
404
409
410
411
413
416
423
427
430
432
435
437
439
442
Contents
XIII
7.4.7
7.4.8
7.4.9
7.5
7.5.1
7.5.2
Marine Transgression (Barrier Retrogradation)
Marine Regression (Barrier Progradation) . . .
The Coals of the Backbarrier Strand-Plain System
The Estuary . . . . . . . . . . . . . . .
Present and Past Estuarine Deposits
The Coals of the Estuarine System .
444
448
451
456
457
459
8
8.1
8.2
CoarFormation and Sequence Stratigraphy . . . . . .
461
463
Basic Concepts of Sequence Stratigraphy . . . . . .
Sequence-Stratigraphic Time and Space Constraints
on Coal Formation . . . . . . . . . . . . . . . . .
8.3
The Influence of Sequence-Stratigraphic Settings
on Coal Formation . . . . . . . . . . . .
8.3.1 Properties of Transgressive Coal Seams
with a Marine Roof . . . . . . . . . . . .
8.3.1.1 Chemical Signatures of Transgressive Coals
with a Marine Roof ... . . . . . . . . . . .
8.3.1.2 Mineralogical Signatures of Transgressive Coals
with a Marine Roof ... . . . . . . . . . . . . .
8.3.1.3 Petrographic Signatures of Transgressive Coals
with a Marine Roof .. . . . . . . . . . .
8.3.2 Properties of Transgressive Coal Seams
Without a Marine Roof . . . . . . .
8.3.3 Properties of Coal Seams
Formed During Marine Regression
8.4
Sequence Stratigraphic Interpretation
of Coal Seam Settings . . . . . . . . .
9
9.1
Coal-Producing Tectonic Environments . . .
Early Examples of a Tectonic Classification
of Coalfields . . . . . . . . . . . . . . . . . . .
Basin Formation as Part of Plate Tectonics
9.2
Coalfields Situated Near Convergent Plate Edges
9.3
9.3.1 Molasse Foredeeps Associated
with Subduction Zone Continental Margins
9.3.1.1 The Sunda Arc . . . . . . . . . . .
9.3.1.2 The Sydney Basin . . . . . . . . .
9.3.1.3 The Rocky Mountains Foredeep
9.3.2 Molasse Foredeeps Associated
with Continental Collision Margins
9.3.3 Coal-Bearing Intradeeps . . . . . .
9.3.4 Coal-Bearing Transtensional Lateral Basins
9.4
Coalfields Situated Near Divergent Plate Edges
9.4.1 The Kinematics of Continental Rifting . . . . . .
467
471
475
477
480
484
490
498
505
515
516
518
521
526
527
531
544
555
559
564
567
568
Contents
XIV
9.4.2
9.4.3
9.5
9.5.1
9.5.2
9.6
9.6.1
9.6.2
9.6.3
Coal Formation in Rift Valleys . . . . . . . . . . . . .
Coal Formation in Nascent
Continental Margin Settings . . . . . . . . . . . . . . .
Coalfields on Midplate Continental Margins . . . . .
Coalfields of Mobile Shelf Basins . . . . . . . . . . . .
Coalfields on Stable Shelf Margins . . . . . . . . . . .
Coalfields of the Continental Interior . . . . . . . . . ,
Epeirogenic Coal Basins . . . . . . . . . . . . . . . . .
Coalfields Associated with Wrench Faults . . . . . ..
Coal Formation in Non-Tectonic Basins. . . . . . ..
571
10
Concluding Remarks . . . . . . . . . . . . . . . . . . .. 597
581
583
583
589
591
592
592
594
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601
Author Index
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 661
Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . 679
1 Introduction
Ever since the term "facies" was introduced into the geological literature by Gressly
(1838/41), it has been subject to various interpretations. In this book the concept
of Walther (1893/94), based on Gressly and recently reiterated by Murawski (1972),
Middleton (1973) and Woodford (1973), is followed, in which facies encompasses
all the physical, chemical and biological characteristics of an areally defined
geological body in its present state. Facies is not synonymous with palaeo-environment but conclusions about the latter can usually be drawn after facies characteristics have been analysed. Facies characteristics are therefore indicators of the
palaeo-environmental conditions under which a rock body has been formed.
Coal being a biochemically formed sediment offers a wide range of organic
and inorganic palaeo-environmental indicators which cannot be utilised in
sedimentological studies concentrating only on interseam sediments. Taking into
consideration that at the time of deposition the accumulating peat was probably
eight to ten times thicker than the present coal, any restriction of facies analysis
to interseam sediments to the exclusion of coal would disregard a volumetrically
very significant portion of the stratigraphic column. Furthermore, if one accepts
that one metre of bituminous coal took probably between 5 and 10 Ka to
accumulate as peat in contrast to the mere hours or days which would have been
sufficient for some rivers to deposit a metre of sand, a consideration of coal facies
in any palaeo-environmental analysis of coal-bearing sediments becomes even
more desirable.
The accuracy of palaeo-environmental assessment depends to a large extent
on the precision with which the facies characteristics have been analysed, and on
the degree of agreement between them. If, for example, a coal seam is relatively
thin and contains a high proportion of dispersed components, algae, framboidal
pyrite, and shale bands with a high boron content, the listed facies characteristics
constitute a set in which all elements are consistent with the assumption that the
coal has been formed under marine influence. A matching set of palaeo-environmental indicators in the interseam sediments could consist of clean, even-grained
sandstone in the seam floor and bioturbated or fossiliferous shale in the roof, thus
suggesting peat accumulation in a near shore environment during a marine
transgression. By using this kind of internal consistency among facies characteristics
it is the aim of this book to identify the main coal-forming environments and to
correlate their organic and inorganic indicators.
Although the examples discussed in the text have been drawn from a variety
of sources, it has been mentioned in the preface that they are biassed towards
Introduction
2
Australian and German coals due to the author's long association with both
regions. It might therefore be of some value to the reader who is not familiar with
the consequences of spatial and temporal differences in coal formation to highlight
this aspect by comparing some broad features of the Permian Gondwana and
Carboniferous Euramerican coals.
It is now widely accepted that there existed a Late Palaeozic landmass which
included much of present-day Australia, Antarctica, southern Africa, peninsular
India, Madagascar and parts of South America (White 1986). This large southern
continent, called Gondwana, was host to a largely deciduous vegetation which
grew in a varied but mainly cool to cold climate and differed from the evolutionary
trends occurring elsewhere at the time. The largest coal deposits occur in Australia.
They are of Permian age and, together with the quantitatively less important
Carboniferous and Triassic coals (Gould and Shibaoka 1980), belong to the
Gondwana lineage which ended with the breakup of the continent beginning in
the Jurassic Period.
The Late Carboniferous (Pennsylvanian) coals of the Northern Hemisphere
were formed in an equatorial belt which comprised large portions of what is now
Europe and North America. Differences in evolutionary development and the
contrasting climatic settings of the phytogenic progenitors of the respective coals
account for much of their compositional variations. Some of these are demonstrated
in the comparison of the maceral composition between Carboniferous European
and Permian Australian coals listed in Table 1.1. The maceral classification used
here is based on Australian Standard 2856 (1986) in which each of the maceral
groups vitrinite and intertinite has been subdivided into three subgroups with the
Table 1.1. Comparison between average maceral compositions of Australian Permian and
European and American Carboniferous coals (all washed composite seam samples)
Group
MaceraljSubgr. Permian (n = 67)
Carboniferous (n = 40)
Mean %
Range
Std. Error
Mean %
Range
Std. Error
23.5
31.7
38.6
40.1
1.242
1.100
31.8
32.6
33.6
33.4
1.102
1.066
55.2
53.2
1.733
64.4
35.3
1.181
3.2
0.7
2.5
11.2
3.6
11.4
0.671
0.104
0.266
6.7
0.7
2.5
13.1
2.6
8.8
0.504
0.119
0.261
6.4
21.0
0.562
9.9
16.2
0.617
2.1
3.9
15.7
4.5
8.4
5.6
17.6
34.8
11.0
25.0
0.l36
0.392
1.026
0.332
0.590
5.1
3.4
6.1
2.9
5.0
19.1
8.1
13.8
5.8
8.0
0.621
0.319
0.555
0.228
0.310
Inertinite
34.6
55.2
1.880
22.5
33.7
1.192
Minerals
3.9
10.0
0.228
3.3
7.3
0.279
Telovitrinite
Detrovitrinite
Vitrinite
Sporinite
Cutinite
Resinite
Liptinite
Micrinite
Macrinite
Semifusinite
Fusinite
Inertodetrinite
Introduction
3
prefixes telo- (for structured), detro- (for fragmented), and gelo- (for gelified), respectively. Explanations of the terms are given in Chaps. 3 and 4.
It is noticeable in Table 1.1 that within the vitrinite group of macerals the
proportions of detrovitrinite are very similar and that the vitrinite percentages are
due to differences in telovitrinite. The magnitude of this difference is similar to the
contrast between the high semifusinite content in the Australian coals compared
with their Carboniferous counterparts. This means that the average proportion of
preserved plant tissues is similar in both sets of coals although the degree of gelification is stronger in Carboniferous coals. Another difference between the two coals
is the higher proportion of sporinite, which is related to differences in the contributing vegetable matter. Spore-producing pteridophytes dominated the Carboniferous flora whereas the Permian Glossopteris flora contained more gymnospermous
plants, resulting in a lower availability of spores.
The percentage ranges of most macerals are greater in the Australian coals
compared with their Carboniferous counterparts, which suggests that, when
comparing Carboniferous and Permian coals, environmental conditions of coal
formation were more varied in Gondwana than in Northern Hemisphere. The
result is not only a global provincialism of coal properties, i.e. systematic variations
between continents and stratigraphic periods, but also between"different coalfields"
of the same region and within a broadly similar time frame (Bennett and Taylor
1970; Cook 1975a; Taylor and Shibaoka 1976).
The regional differences between coals mentioned above are only a small
sample of variations in response to plant evolution, palaeoclimate and other
geological conditions. It seems almost futile, therefore, to apply the parameters of
coal facies analysis outside the region for which they have been developed. However,
irrespective of the considerable diversity of plants and peat-forming environments,
there is a limited number of biological constraints which govern the ecology of
wetlands and a likewise limited set of biological, chemical and physical controls
which determine the course of peat formation. Foremost among these is the position
of the groundwater table in relation to the depositional interface, followed by hydrogen ion concentration, redox potential, provision of nutrients, and some others
which are related to the geological setting of the mire. The response of plants and
their degradation products to these basic environmental conditions is sufficiently
universal to leave behind distinctive petrographic signatures by which the
depositional environment can be recognised, irrespective of age and regional origin
of the coal.
The text has been organised such that, following this introduction, Chap. 2
will discuss some aspects of peat composition in relation to modern peat-forming
environments. There has been considerable progress in recent years in the understanding of the conditions of low ash peat formation, resulting in a swing away
from topogenous peatlands as the main coal-forming environments towards an
acceptance of raised bogs as important contributors to coal. The development of
actualistic models of peat formation has led to a rejection of the delta environment
as the most likely birthplace of major coal deposits. However, the desire to measure
400 Ma of coal formation by the rules of 4 to 5 ka (at best, 10 ka) of post-glacial
peat accumulation, has produced some quite rigid interpretations of coal formation,
yet most of today's peat deposits can be compared, in time and volume, only with
4
Introduction
the (often quite dirty) bottom portion of many economic coal seams. The origin
of the up to 80-m-thick anthracite seam (Grande Couche) in the Hongai Coalfield
of Vietnam's Tongking Basin (Dannenberg 1937), or the composite thickness of
the 300 m of brown coal in a mere 800 m of coal measures in the Latrobe Valley
of Victoria, Australia (George 1982), to mention only two of many examples,
require conditions in time and space for which there are no current equivalents
on Earth. The chapter will conclude with a brief survey of climatic influences on
peat formation and a summary of evolutionary trends in former peat-producing
plants and their influence on coal types.
Chapter 3 deals with the coalification process, particularly with its biochemical
stage. The degree of humification of the phytogenic progenitors of coal will be
presented as having a decisive influence on maceral formation from cell tissues.
The latter follow either a vitrinitisation or fusinitisation pathway in response to
the position of the groundwater during peat accumulation. The second phase, or
physico-chemical stage of coalification, will be discussed only briefly because it is
of lesser relevance to the objectives of the book.
Chapter 4 consists essentially of a classification of the organic and inorganic
petrographic components of coal. i.e. macerals, microlithotypes, lithotypes and
minerals. Some emphasis will be put on illustrations and analysis procedures for
the benefit of readers not very familiar with coal petrographic nomenclature and
techniques.
Chapter 5 introduces various aspects of coal facies analysis based on different
coal components. Several ratios and petrographic indices of high diagnostic value
will be defined and some reference will be made to the geochemical aspects of
depositional environments.
Chapter 6 looks at coal as an integral part of a group of sediments sharing a
common depositional setting. Of particular interest is the coal/sediment interface
and the relationship of coal seams to their roof and floor strata. Included in the
consideration are intercalations of inorganic sediments in coal seams and the
contrasting nature of seam splits of different origin.
Chapter 7 is a classification and discussion of coal-forming sedimentary
environments including gravelly and sandy braid plains, upper delta/alluvial plains,
lower delta plains, back barrier strand plains and estuaries. Each of this sedimentary
environments is characterised by typical coal facies except for the backbarrier
coals, which show significant compositional differences, depending on whether
they were formed under a regime of marine transgression or regression.
Chapter 8 is an attempt to apply the principles of sequence stratigraphy to
coal formation, i.e. the operational independence enjoyed by the autosedimentational depositional models of the "postcyclothem era" (Rahmani and Flores
1984) will have to be somewhat restrained by reaffirming the fundamental importance of eustatic sea-level changes on depositional trends in coastal environments.
The question of marine influence on coal seams will be discussed in reference to
the contrasting composition of some back barrier coals.
Finally Chap. 9 discusses the geological setting of coal-forming environments
in the context of plate tectonics. Systematic changes in coal composition will be
found which reflect the historic development of sedimentary basins in relation to
their geotectonic affinity.
2 The Conditions of Peat Formation
Coal is an organic sediment which consists of coalifield vegetal matter. A broad
distinction is made between humic and sapropelic coals of which the first type
is far more frequent than the second type, which has been formed by subaquatic
sedimentation of floating vegetation (algae) and allochthonous (= redeposited
material, not formed in situ) organic matter. The' phytogenic precursors of humic
coals derived mainly from rooted autochthonous (= formed in situ) vegetation
which grew in mires where they accumulated as peat. The latter is the first step
in the coalification process by which the biomass is transformed into successive
coal ranks which are expressed by such terms as (in order of increasing rank),
brown coal, subbituminous, high, medium and low volatile bituminous coal,
metabituminous coal, semi-anthracite and anthracite.
During peat formation it is important that atmospheric oxygen has only limited
access to the organic matter in order to allow its maximum preservation under
reducing conditions (Overbeck 1975). This requires the maintenance of a
consistently high groundwater table and complete water saturation of the peat
(McCabe 1984; Boron et al. 1987), with the result that an undrained peat deposit
may contain over 90% water (Cameron 1973; Succow and Jeschke 1986). Even
after the peat has been consolidated into brown coal its bed moisture~content may
still be as high as 60%.
2.1 Peatland Ecology
Prolific plant growth requires considerable quantities of plant nutrients in the form
of mineral salts. In this respect peat producing wetlands or mires are commonly
divided into ombrogenous (= owing their origin to rain) peatlands, and topogenous
(= owing their origin to a place) peatlands. Many additional names and subdivisions, often with imprecise or overlapping meanings, have been introduced into
the literature (Moore 1987). The usage followed here is illustrated in Fig. 2.1.
As the name "ombrotelmite" (Grosse-Brauckmann 1980) indicates, this kind of
peat forms in a mire which receives excessive precipitation. This favours the growth
of mosses, such as Sphagnum, which is capable of absorbing large quantities of
water and keeping them above the general groundwater table by capillary action.
Continued plant growth and protection and storage of the dead vegetable matter
The Conditions of Peat Formation
6
PEATLANDS
(MIRES)
ombrogenous _ _ _ _....._ _ _ topogenous
!
!
OMBROTELMITE
TOPOTELMITE
(ombrotrophicl
(oligotrophic)
RAISED BOG
Tree
(Sphagnum Bog)
cover
increases
(Forest Bog)
L
(minerotrophic)
(rheotrophic)
(eutrophic)
1
TRANSmONALOR MIXED MIRES
(mesotrophic)
MARSH
FEN
SWAMP
---1
Fig. 2.1. Classification of mires
and their peats. (Partly after
Grosse-Brauckmann 1980, Martini and Glooschenko 1984, and
Moore 1987)
within the waterlogged body of moss causes the surface of the living moss to be
raised above ground level (therefore the terms "raised bog" or "high moor", from
the German: "Hochmoor"), which makes it difficult for the vegetation to obtain
mineral salts and other plant nutrients from the soil deeply buried below a
thickening layer of peat. Flood waters, the other common source of nutrients for
plants, are likewise ineffective because the upward convex shape of the high moor
prevents surface water from flowing into the bog. The result is an environment
which is poorly supplied with nutrients, which is the reason for calling such
peatlands oligotrophic (= poorly fed) or ombrotrophic ( = rain-fed).
A corollary of the low influx of mineral nutrients into a raised bog is the high
acidity of its peat. The most common minerals are salts of the weak silicic acid and
rather strong bases either of alkalis or alkaline earths. Orthoclase is given as an
example below:
On hydrolysis, the unstable silicic acid changes to silica and water, the amphoteric
aluminium ion forms an immobile hydroxide which dehydrates to alumina, thus
allowing the strong potassium hydroxide to neutralise, at least partially, the organic
acids formed in the peat of a topogenous swamp. However, in a raised bog, which
lacks the supply of mineral-charged surface water, no acid neutralisation takes
place, with the result that only a few hardy plant species can cope with the high
acidity and low nutrition levels. Among the plants that thrive under such
conditions, the above mentioned Sphagnum is common to most climates, while in
the tropical raised bogs of Southeast Asia arborescent vegetation might also be
supported (Polak 1950). However, some of the Indonesian high moors with their
rich and varied vegetation are not purely ombrotrophic because they recieve
episodic showers of volcanic ash, which constitute an important source of plant
nutrients.
Along their less elevated, moist, and occasionally flooded margins, raised bogs
receive a larger amount of nutrients (Anderson 1964) which also lowers acidity
Peatland Ecology
7
and increases vegetational variety and growth conditions. The most acid peats are
found in the central portions of raised bogs, which therefore sustain only the most
hardy plant species (Grosse-Braukmann 1969). Although the central portion is
commonly the most elevated part of the bog and therefore more subject to drying,
ponds and moist depressions are usually found. The origin of the water-filled
depressions on the bog surface will be discussed in Chap. 3.
A pure ombrotelmite is characterised by a low ash content and often by a high
degree of tissue preservation because high acidity suppresses microbial activity in
the peat. The lack of mineral impurities is regarded by Teichmuller (1962) as a
possible explanation for the origin of low ash coals, a notion which has been
supported by McCabe (1984,1987), Fulton (1987), Bartram (1987) and others, and
has recently been extended to encompass almost all coal. Moore (1987, p. 12) states
that "the modern successors to the 'coal swamps' are clearly not swamps at all,
but are bog forests of an ombrotrophic nature", and Clymo (1987) regards the
ombrotrophic forest bogs of Southeast Asia as present-day models of Carboniferous
coal formation whereas the boreal raised bogs of northern Europe, Canada and
Siberia are considered to be genetically analogous to the Permian Gondwana coals.
Although ombrotelmites have probably played a bigger role in coal formation
than was previously recognised, the order of superposition of coal facies and their
documented coexistence with clastic sediments in a geological setting where tectonic
subsidence was relatively accelerated and retarded by eustatic sea-level variations,
seem to require more complex models than the comparisons with today's bmbrotrophic peatlands mentioned above. There are some well-documented occurrences of
rheotrophic and mixed ombrotrophic/rheotrophic peatlands (Anderson 1964;
Spackman et al. 1966, 1976) in which the effects of marine transgressions and regressions on peat types and their order of superposition are well suited to serve as
Table 2.1. Compilation of various average properties of peat from different European mire types.
(After Hohenstatter 1973 and Schuch 1980). Percentages are by weight
Peat type
% Bed moisture
-Average
-Range
% Ash (db) at 550°C
-Average
-Range
% Org. matter (db)
-Average
-Range
pH
-Average
-Range
Spec. Energy (MJjkg)
-Average
-Range
Ombrotelmite
Transitional mires - Topotelmites
Sphagnum
Herbaceous
Woody
Woody
Herbaceous
88.7
82.2-89.2
89.7
84.2-92.7
85.8
79.1-89.4
86.3
69.9-91.9
89.5
83.8-92.0
2.3
0.6-8.2
3.3
1.2-8.1
6.5
1.7-22.5
13.3
2.7-33.7
8.5
1.5-25.0
97.3
99.4-91.8
96.7
98.8-91.9
93.5
98.3-77.5
86.7
97.3-66.3
91.5
98.0-75.0
3.4
2.5-5.5
3.9
3.2-4.9
3.9
2.6-5.5
5.0
3.2-6.4
4.8
3.6-6.0
21.3
18.8-23.0
21.8
19.1-23.4
19.9
17.9-21.7
19.4
16.9-21.3
20.9
19.5-22.7
8
The Conditions of Peat Formation
examples of coal formation in nearshore environments. They are not favoured by
some as models of coal formation because of their higher contamination with
mineral impurities when compared with raised bogs (McCabe 1984, 1987).
A comparison of average ash contents and some other properties of ombrotelmites and topotelmites is given in Table 2.1, which demonstrates quite clearly
the very low amount of inorganic impurities found in ombrotrophic raised bogs.
Even the respective figures given for topotelmites seem low but it should be realised
that Table 2.1 is based on Central European peats which, by definition, should
contain more than 75% (by weight) of organic matter (dry basis = db). As shown
by the range of ash percentages in the table, some forest swamp histosols ( = soil
with high proportion of organic matter) have been included with ash contents
exceeding 30% but the majority of the ash values is well within the defined limit
of 25%. By using Callcott's (1986) mass balance calculations of coalification yields,
it can be assumed that a peat which coalifies to a rank of medium volatile bituminous coal loses approximately 50% of its dry organic matter to devolatilisation
which, up to the stated rank, consists essentially of decarboxylation (= H 2 0 + CO 2 ).
While peats with such extremely high ash contents as listed in Table 2.1 would
not qualify to be regarded as coals, the topotelmites including transitional or mixed
peats with average or below average ash figures, would yield coalification products
which would fall well within the defined limits of coal [between 20 and 30% ash
(db) depending on national standards]. For example, the average woody topotelmite (= forest swamp peat) is listed in Table 2.1 with an average ash content
of 13.3%. On coalification to bituminous coal its residual organic matter wiil be
halved, which would increase the ash content to approximately 24% assuming that
its mass is not reduced during coalification. However, there is good reason to
believe that the reduction of the bed moisture content from around 90% in peat
to approximately 2% in medium volatile bituminous coal will also remove much
ofthe soluble mineral fraction. According to Naucke (1980), the leaching of German
bed-moist peat at a proportion of 1 part of db-equivalent peat in 50 parts of distilled water yields in average 1.45% organic and 0.2% mineral matter in ombrotelmites and 0.58% organic and 1.1 % mineral matter in topotelmites. Leaching
experiments with distilled water carried out by Kosters and Bailey (1986) on
Louisiana peats reduced their ash contents by up to 30%. The laboratory experiments are supported by field observations by Cohen et al. (1987), which also suggest
that some mineral leaching occurs during the peat stage.
Even if the likelihood of a reduction of the mineral content during the early
stages of coalification is ignored, the coal ash derived from the peat ash of Table
2.1 requires an adjustment on technical grounds because it has been determined
at a temperature of 550°C whereas coal ash is conventionally determined at 815 dc.
This difference has important consequences for the different ash yields of both
products because at the temperature at which coal ash is determined, all clay
minerals have dehyd~oxylated and most carbonates have lost their carbon dioxide.
Depending on the mineral composition of the original peat, the mass yield of coal
ash is lowered by at least 10%. The resulting coal would have raw ash content of
little more than 20%, which is better than that of a large proportion of coals
currently minded in many parts of the world. It seems therefore that the ash
Peatland Ecology
9
content of many topotelmites would not preclude them from forming economic
coal deposits. The large areal extent of many coal seems (some are continuous
over hundreds of kilometers), their internal sequence of facies, the repeated vertical
stacking of seams, their relationship to roof and floor strata, and other
characteristics suggest that the major coalfields of the world were formed and
survived because of their topogenous setting rather than the ombrogenous nature
of their peats. The geological environment is probably quite irrelevant to the
formation of the 5, 10 or, at best, 15 m of ombrotrophic peat formed since the end
of the Pleistocene glaciation. However, without a regime of subsidence and
sediment supply for their protection, not many of today's peat deposits would
survive to be transformed into thick coal seams and stacked in vertical succession in the subsurface of several-kilometers-deep basins. During the main peat
producing periods in the development of a seam the mire might very well have
been under ombrotrophic regimes but the beginning and end of the formation of
those former peats which now constitute coal deposits appear to have been dictated
by the consequences of variations in absolute and relative basin subsidence.
There are many examples of mires in either a coastal or alluvial situation whose
peat surfaces have been raised above ground surface. Although they originated as
minerotrophic or rheotrophic ( = flow fed) deposits and, in composition and distribution, still have strong topogenous affinities they are now either transitional
between the two mire types or have become ombrotrophic. Examples are found
in the Southeast Asian peatlands described by Anderson (1964, 1983) which have
been referred to as possible modern equivalents of Carboniferous coal forming
environments. As will be discussed below, the interior portions of the extensive
lowland forest bogs in Sumatra, Borneo and the Malay peninsular are, in parts,
ombrotrophic but their seaward spreading behind a prograding coastline is
primarily a topogenous event. Irrespective of their origin, the raised peat surfaces
confine river, as well as tidal floods, to their respective channels with the result
that these peats are mostly only marginally flooded and are therefore relatively
clean, having ash contents which rarely exceed 5% and usually are less than 2%.
Similarly low ash figures have been reported by Styan and Bustin (1983a, b) from
the raised peat deposits ofthe Fraser River delta in· western Canada.
Examples of predominantly ombrogenous mires occur in many parts of the
Indo-Pacific region but according to Whitmore (1984), continuous blankets of such
peats are restricted to the coastal lowlands and to high elevations, above 1000 m
a.s.l., i.e. within the cloud zone. The latter provides for a greater availability of
surface water because of higher humidity and lower evapotranspiration. The high
elevation suggests that the preservation potential of these deposits must be extremely
small.
Whereas raised bogs can occur at high physiographic elevation and even on
slopes in mountainous terrains, rheotrophic swamps are restricted to areas which
provide for a constantly high groundwater table because they do not rely on
precipitation for their moisture requirements. This gives them a greater climatic
flexibility than applies to raised bogs, as is demonstrated by the wetlands (though
not necessarily peatlands) ofthe Nile and Tigris Valley, both of which are situated
in the arid climatic zone.
10
The Conditions of Peat Formation
The vegetation of topogenous peatlands is either rooted in a soil from which
a good supply of nutrients can be obtained or, when the peat layer becomes too
thick for roots to penetrate into the substratum, nutrients are provided by the
underlying peat and by flood waters which carry nutrients in solution and may,
occasionally, spread muds and silts across the surface of the peatland. Such an
environment is therefore said to be eutrophic (= well-fed) or, more specifically,
minerotrophic (= mineral-fed).
Martini and Glooschenko (1984) distinguish between three kinds of Recent
minerotrophic peatlands:
1. A marsh is a wetland periodically inundated by either fresh or salt water. It is
usually devoid of trees. Grasses, sedges and, locally, shrubs (e.g. Salix), constitute
much of the vegetation.
2. A/en may support a rich and varied surface flora composed of grasses, sedges,
herbs, shrubs and clusters of trees, the latter taking up usually less than 25% of
the total surface, for example, in the form of the so-called tree islands in the
Everglades of Florida.
3. A swamp is a wooded wetland which in cool and temperate zones contains both
trees plus an undergrowth of shrubs, herbs and mosses. In tropical and subtropical regions a large variety of plants is common with mangrove and swamp
cypress occupying portions covered by shallow water.
According to Frenzel (1983) and Boron et al. (1987), peat formation can be initiated
either by
1. terrestrialisation which is the replacement, due to silting-up, of a body of water
(pond, lake, lagoon, interdistributary bay etc.) by a mire, and
2. paludification which is the replacement of dry land by a mire, for example due
to a rising groundwater table.
Figure 2.2 illustrates examples of both processes. Paludification, for example, as
a landward expression of a marine transgression has been an important process
in the formation of extensive and thick coal seams. Peat formation under the
conditions ofterrestrialisation is possible under both static and dynamic conditions.
In the first case peat continues to accumulate as an ombrogenous high moor once
the body of water has been silted-up (Pfaffen berg 1954; Scheffer and Schachtschabel
1966). In the second case a mechanism exists which allows the groundwater table
to rise commensurate with peat deposition. An example would be the coast of a
subsiding basin which, due to a high rate of sediment discharge at the strandline,
pro grades into the sea leaving behind lagoons and strand plains with a rising
groundwater table thus affording possible conditions for peat accumulation. Basin
subsidence combined with ombrotrophic peat accumulation such that the rise in
the peat surface continues to outstrip the rate of subsidence will lead to the
formation of thick and clean coal seams (McCabe 1984).
For a thick peat layer to form in a topogenous setting it is essential that the
rise in the groundwater table and the rate of peat accretion are balanced. In the
11
Peatland Ecology
Fresh-water peat fmrnation byterrestrialisatian
- Europe
I - ombrotrophic
I
rheotrophic - - -- -------1
I tnnsitional
I to1m.ti< I limnotelmatic I \imnic
Sphagnum
bog peot _
Mixed pe.t -.... rmtrtrlTrmtrtrlrhtrfmm
Swomp " .. t ..
Sedge peot...r+
peot-=F~EEElE~~~~=2===~~~~~~~~;S;;~
Sopropel
Reed
-,... t:
LoX. mud...J
r - - - - - - -fresh-wa.ter
saw gras, and
waterlily marsh
rheotrophic - - - -- - - - - - marine mangrove Swamp
I mixedz:one
bntc:ltish I
JP..
MO.rl
Rhl<ophon
t
Rhizophorol
~M .. riscus pea.t
~Moriscu. put
Peat
formation by
paludification (marine transgression)
- Flarida. USA
J
LNymphaea
peat
bnckish
(.. It-water
p..lms)
marine
(mangroves)
Young
forest ~$~~~~'~~~
pea.t ---+M
forest bog
pe.t ____
Rhizophon/
Nyp. pea t---->
RhizoJ)horoJ'
~.~~~~~~~~~;~~I~III
peat +Shoj
cloy
Upper
li
nee/lower
delto
Peat formation by t:errestrlalIsation (marine regression) - SE AsIa
Fig. 2.2. Three examples of peat accumulation under different circumstances. (After Overbeck
1950 - top; Spackman et al. 1966, 1969 - cenlre; Anderson 1964 and Whitmore 1984 - bOllom).
Not to scale
case of a slower rise in water level, peat accumulation could be terminated by
oxidation but in a very wet climate peat formation might continue under high
moor conditions.
Actual rates of peat accretion vary in different climates, and they vary with
with the type of vegetation. Falini (1965) has calculated that although the growth
rate of some herbaceous plants may be substantial, the considerable loss of biomass
during the conversion of vegetable matter into peat reduces the maximum annual
accumulation of peat resulting from non-woody vegetation to approximately 10 cm.
Other estimates (Stach et al. 1982) are considerably lower and vary between 0.5
and 4 mm/ a. Moore and Bellamy (1974) consider peat accumulation rates to range
12
The Conditions of Peat Formation
from a few tenths of a millimetre do not more than 2 mm/a. McCabe (1987) reaches
a similar conclusion and calculates accretion rates for Recent peat to vary from
2.3 mm/a in the tropics to 0.1 mm/a in arctic regions.
Assuming a compaction ratio of perhaps 10:1 (Ryer and Langer 1980) to
operate in the transition from peat to bituminous coal and considering that some
of these seams are several tens of metres thick, optimum peat-forming conditions
must therefore require the maintenance of a high groundwater table over very
long periods of time, i.e. between 5 ka (minimum) and 10 ka for every metre of
clean bituminous coal. As indicated above, in situations where the water level rises
faster than peat can accumulate, a facies shift towards wetter conditions would
occur, and in the case of a slower rise, a more terrestrial environment would be
established. Because of the dominance of trees, the growth rate of the latter is
considerably slower than that of herbaceous plants, which means that when the
water level rises at a rate equal to the maximum accretion rate of herbaceous
vegetation most trees would not be able to cope and would be destroyed by too
great a depth of water (Falini 1965). This means that rates of peat accretion vary
in response to the rate at which the water table rises and that the presence within
a seam of horizons with thick tree stumps indicates retardation in peat formation
for up to several hundred years.
2.1.1 Coastal Marshes and Swamps
Coastal lowlands offer a number of environments in which a variety of plants can
grow and accumulate to form fossil fuels. These environments range from the
swamps of the lower delta plain and coastal marshes to the sub-aqueous algal
concentrations in lagoons, lakes and sheltered bays.
Coastal marshes are low-lying tracts of land which are periodically inundated
by water and support grasses, reeds and rushes which tolerate brackish conditions.
Near the coast the marsh surface is situated close to mean high tide level but
landward it rises gently above the tidal influence and grades into a fresh-water
marsh. It is separated from the sea either by a barrier beach, or by a mangrove
belt, or, in high latitudes and under low energy conditions, the fresh-water marsh
grades into a salt marsh which merges with the sea. The first example applies to
exposed, wave-dominated shorelines where bars are formed by surf action and/or
long-shore drift.
Lower delta plain deposits commonly cap thick coarsening-upward prodelta
and delta front sediments. If they contain peat they are situated between active
or abandoned distributaries whereby former positions of the latter may influence
the thickness of overlying peat. Styan and Bustin (1983a, b), who studied Recent
peat formation on the delta plain of the Fraser River in British Columbia, Canada,
found lower delta plain peats to be quite irregular and discontinuous in configuration but generally extensive (= blanket peat of Coleman and Smith 1964), thin
and high in sulphur. Because of the interaction of storms, tides and river floods,
Peatland Ecology
13
ash contents were high in most samples and were emplaced mostly as wash over
deposits. However, samples taken from domed (raised) peat in transitional positions
to the upper delta plain showed both low sulphur and ash contents. Another
difference found by Styan and Bustin (1983a, b) was the lower pH and higher
degree of tissue decomposition in the marine influenced lower delta plain peat
compared with its more freshwater influenced equivalents.
2.1.1.1 The Marine Influence on Peat Accumulation
For peat accretion to occur it is essential that tidal fluctuations are either low or
are channelled through established inlets which prevent sweeping of the marsh
floar and removal of the accumulating vegetable matter. In tropical and subtropical
climates mangroves frequently line the tidal inlets and muddy coastlines behind
which grass and reed marshes expand, as is the case in saw grass marshes of
Florida (Spackman et al. 1966; Cohen and Spackman 1972). The roots of the
mangroves form an interlocking network which is an efficient trap for sediments
carried by the flood tide and, at the same time, protects the plant litter from being
flushed out by ebb-tides or floods.
Coastal peatlands can also be found Dn land only recently reclaimed from the
sea by prograding deltas. Given the right climate, their most noticeable feature is
the abundance of plant life which occupies the raised levees of distributary channels,
as well as the interdistributary troughs (Fisk 1960). This, coupled with the proximity
of the water table, affords suitable conditions for the accumulation and preservation of plant material. Organic sedimentation is interrupted periodically
by the intrDduction of fine clastics during floods, which raises the proportion of
mineral impurities Df the peat and subsequent coal. On the other hand, flDoding
also causes the dispersal of nutrients and spreads fresh water across the swamp,
both of which provide for better growth conditions than prevail in coastal swamps
away from fluvial influences. As in the coastal marsh, the tidal effects leave their
mark in the relatively high pyrite content of such deposits. Sea water contains,
among other dissolved salts, sulphates in solution. Under the reducing conditions
which are sustained in very wet portions of the swamps or at the bottom of
water-filled depressions in the marsh surface the sulphates are reduced by bacteria
to form hydrogen sulphide and/or iron sulphides.
Lagoons and interdistributary bays are prominent features of deltas and
sunken coasts in which the build-up of barrier beaches by strong surf action or the
formatiDn of spits and bars by waves and long-shore drift has partly closed lagDons,
drowned valleys and other indentations in the coastline. Continued sedimentation
within lagoons, partly by rivers debouching their load within their confines, partly
by the sea which enters through inlets or washes over the barrier, and partly by
organic sedimentation, fills the lagoons and converts them into coastal swamps.
While restricted water conditions prevail, algae are the main contributDrs to the
accumulating Drganic mud which may be covered and laterally replaced by peat
when paludal conditions succeed the lagoons.
14
The Conditions of Peat Formation
Apart from the relatively restricted oil shale-producing coastal lagoons and
bays or comparatively small lakes and ponds in which algae accumulate, there
are fossil examples of whole epicontinental seas becoming so restricted in water
circulation that oil shale deposits extending over hundreds and thousands of square
kilometres with a thickness of between a few metres to a few tens of metres can
be formed. Fossil examples have been found in the Cambrian System of Siberia,
the Silurian System in North Africa, Permian deposits of southern Brazil, Uruguay
and Argentina, as well as in Jurassic sediments of western Europe (Tissot and
Welte 1978). Of similar shallow water origin in a marine environment of restricted
circulation are the Early Cretaceous, brown to black, well-laminated and somewhat
calcareous oil shales of the Eromanga Basin in northern Queensland (Hutton et al.
1980).
2.1.1.2 The Influence of Marine Transgressions and Regressions
The world-wide post-glacial, Holocene marine transgression has, in many places,
initiated peat formation within a swamp and marsh belt which has migrated inland,
in front of the invading sea and has, in parts, been overrolled by it. Examples of
this response to a rising sea level have been reported from Florida, where some
marine influenced peats contain up to 10% sulphur (Casagrande et al. 1980) and
from the Gulf coast of the United States, where Coleman and Smith (1964) have
estimated that 8 ka ago sea level was approximately 9 m lower than it is today.
Spackman et al. (1966) describe an up to 4-m-thick peat seam from southwestern Florida (Fig. 2.2) which underlies a 10 to 20 km wide inland zone of freshwater saw grass marsh (approx. 1 m thick peat) and extends seaward underneath
a 1 to 2 km wide brackish mangrove swamp. The seam could be traced up to
2.4 km into the Gulf of Mexico underneath a thickening overburden of marine
marls. Mangrove pollen (Rhizophora mangle) are very common in the upper portion
of the peat seaward from the mangrove swamp but they are virtually absent from
the marsh peat landward of the mangrove swamps. The seam contains pollen
assemblages characteristic of fresh-water environments which also dominate the
marsh peat underneath the mangrove peat. Because water transportation is directed
from the inland marsh to the mangrove swamp its autochthonous brackish pollen
has been mixed with redistributed fresh-water pollen thus resulting in a more
diversified assemblage than in the marsh peat.
The seam has been formed over the past 4.5 ka in response to the Holocene
marine transgression over the coastal area. Evidence for this is seen in the vertical
peat profile consisting of fresh-water-derived saw grass peat at the base followed
by brackish and then marine mangrove peat at the middle and upper portions of
the seam. Although there is no unanimity about the rate of relative sea-level rise
in the area over the last 3 ka, estimates by Coleman and Smith (1964) carried out
on the coast of Louisiana suggest a rise in sea level of almost 2 mm/a. This covers at
least past of the time span during which the belt of active peat accumulation
migrated inland because the vegetation could not keep up with the rise in water
level on its seaward side. It follows that the accumulation rate of the coastal
Peatland Ecology
15
peat must have been somewhat less than 2 mm/a. The superposition of marine
Rhizophora (mangrove) peat on fresh-water Mariscus (saw grass) peat described
by Cohen and Spackman (1972) from the Joe River of southern Florida is likewise
indicative of a marine transgression.
Coasta:l peatlands formed in response to a marine regression are found in
regions of strandline progradation due to either a eustatic fall in sea level or high
sediment discharge. Haggart (1988) discusses an up to 4-m-thick peat deposit in
the Beauly Firth of northeastern Scotland which commenced accumulating on a
marine substrate abandoned during a Holocene drop in relative sea level between
9.6 and 9.2 ka BP. In the subsequent rise in sea level, which culminated between
7.1 and 5.5 ka BP, the peat-producing reed marshes and fens were covered by
estuarine and marine silts and clays, until peat accretion recommenced on top of
the latter following a renewed fall in sea level after 5.5 ka BP.
Regressive peat formation on a very large scale has been reported from
Southeast Asia where some of the coastal mangrove and salt-water palm swamps
have a regressive signature (Fig. 2.2) Pollen analyses by Anderson (1964) on cores
obtained from a 13-m-thick peat seam in the Baram Delta in Sarawak have revealed
a distinct zonation beginning with mangrove peat at the base and changing into
an upward succession of several peat types which have been formed from the same
plant associations which are presently active peat producers and follow each other
laterally with increasing distance from the sea~ The sequential trend represents a
change from large numbers of plant species and high forest canopy'near the coast,
to fewer species with more stunted and xeromorphic forms in the fens and savanna
woodlands of the inland portions of the peatlands away from the coast or the
controlling trunk streams (Whitmore 1984). The reason for the deterioration in
plant growth is the domed nature of the peat surface which, in the inner portions
of the swamp, extends up to 6 m above sea level. In a dryer climate peat accumulation would probably have stopped in the parts now raised above ground level,
but because of the high rainfall recorded in the area it continues ombrotrophically,
albeit at a reduced rate due to poor supply with nutrients.
According to Anderson (1964), the reason for the raised peat surface away from
the coast is related to the interaction between the respective rates of coastal
progradation and peat accumulation. Wilford's (1960) radiocarbon dating along
the Brunei and Sarawak coast infer a beginning of both delta expansion and peat
accumulation at approximately 4.5 Ka BP. While the delta has been advancing
since then at an average rate of 9 mfa, peat accumulation began with a high rate
of 0.475 mm/a, which more recently slowed to 0.222 mm/a (average rate = 0.29 mm/a).
This means that the inland portions of the swamp are underlain by a thick layer
of older peat which is capped by a thin layer of younger peat whereas in the more
recently reclaimed coastal land only the more slowly growing younger and therefore
thinner peat is found (Fig. 2.2). The areas of peatland involved are quite large and,
on the Maludan peninsular, extend up to 64 km inland (Anderson 1964; Whitemore
1984).
16
The Conditions of Peat Formation
2.1.2 Fresh-Water Peatlands
The above examples have shown that the landward portions of marine-influenced
coastal swamps commonly grade into fresh-water swamps. In addition there are
many inland swamps which never had any connection with the sea. The latter are
called limnic (Gr. = lake) whereas those occurrences which are hydrologically
connected to' the sea constitute paralic deposits, even though they may represent
fresh-water environments. The term paralic does not, therefore, imply a necessary
physical contact of the peat with sea water, it merely indicates an environmental
link. In this sense, the upper delta plain is as paralic as the lower delta plain is,
although the former houses fresh-water environments, just like the alluvial plain
sequence, into which it grades further upstream. Since, however, such environments
are open to the sea, they are affected by marine processes, such as marine transgressions and regressions. A feature of paralic coals is therefore their intercalation
with marine deposits even though peat formation took place under fresh-water
conditions.
2.1.2.1 Upper Delta and Alluvial Plain Swamps, Marshes and Bogs
Away from the coast, peat can accumulate in the backswamps and marshes of
flood plains and flood basins between or adjacent to rivers. They are separated
from the river channels by levee banks, the occasional breaching of which spreads
silt and mud over the peat surface, which later appear as stone bands in coal
seams. In the process of coastal progradation, where originally nearshore topogenous swamps have changed into inland ombrotrophic forest bogs, rivers such
as the Baram River in Sarawak (Anderson 1964) may become stabilised and may
change from a meandering to an anastomosing pattern.
Alluvial plain peatlands occur mainly in shallow basins which are poorly
drained because of extremely low gradients. Rivers passing through the lowlands
often overflow and branch into numerous minor channels which may feed into
fresh-water lakes serving as receptacles for both organic and inorganic detritus.
Depending on the relative proportions of the two kinds of allochthonous
deposits, sedimentation in such lakes may lead to the formation of sapropelic mud
lenses surrounded by peat. The largest present-day swamp of this kind is the
Vasyuganskoye Swamp between the rivers Ob and Irtysh in western Siberia.
According to Neustadt (1966, 1977) and Walter (1977), it occupies an area of
approximately 53700 km 2 uninterrupted peatland but it is part of the much larger
West-Siberian Basin of approximately 500000 km2.Although much of the present
swamp is wooded, its peat contains a large amount of Sphagnum, thus indicating
transitional or oligotrophic conditions.
Other poorly drained wetlands have distinctive marsh character, such as
Europe's largest peatland in the Pripyat-Polessye Basin in the upper reaches of
the Dnjepr River, southwest of Moscow. It houses vast expanses of reeds and
sedges which cover approximately 100000km 2 in area (Schneider 1980), although
Peatland Ecology
17
part of this has now been drained. Such occurrences are similar (apart from climatic
and vegetational differences) to the floating Papyrus meadows of Africa near Lake
Victoria and other banked-up lakes in Uganda (Eggeling 1935; Carter 1955; Lind
and Visser 1962) and Tanzania (Lind and Morrison 1974). One of the largest
wetlands of this kind occurs in southern Sudan, where the White Nile and its
many tributaries branch out into the Papyrus and Vossia marshes of the El Sudd
region (Hurst, 1933; Migahid 1974). According to Rzoska (1974), the Vossia cuspidata grass grows mainly into the waterways, where its runners are particularly
effective in retarding water flow.
Apart from some shifting and redeposition of vegetable matter within the
swamps (hypautochthony), the bulk of the vegetable source of peat and coal is
autochthonous, i.e. it has been produced by plants which grew in the swamp.
Allochthonous organic components, i.e. those which were brought into the peat
from outside, are of small volume only and usually confined to wind-blown material
such as spores, pollen, leaves etc. Completely allochthonous peat deposits are
exceedingly rare; they are small and display indications of transportation, such
as high detrital mineral contents, including water-worn sand grains and pebbles,
oriented tree trunks and, occasionally, cross-bedding. Some fossil examples of
allochthonous coal occur at Leigh Creek in South Australia and at Shag Point
north of Dunedin in New Zealand.
Being situated further inland than the coastal peatlands, only major sea-level
rises will push the strandline far enough landward to cover the upper delta or
alluvial plain with a marine transgression. However, the hydrological connection
to the sea will raise the ground-water table such that peatlands may be replaced
by lakes. A modern example of the termination of peat formation because of the
spreading oflacustrine conditions in consequence of the Holocene transgression is
Mud Lake near Ocala in Central Florida in which a layer of algal mud (sapropel)
caps a lO-m-thick seam of peat (Spackman et al. 1966).
2.1.2.2 Limnic Environments
Topogenous peatlands without any hydrological connection to the sea occur in
inland areas, for example, at high altitudes such as inter- and intramontane basins.
An example is Lake Titicaca in the Altiplano Basin of the South American Andes,
where some limited peat accumulation takes place as part of the terrestrialisation
process. Settings of this kind may serve as models for coal formation in the small
intramontane basins of the French Massif Central.
Algal muds and oozes currently accumulating in some rift valleys (e.g. East
and Central African graben zone) may serve as forerunners of peat and boghead
coals. Fossil examples include the Upper Rhine Graben along the Franco-German
border and the Rundle oil shale in Queensland, Australia. The latter is of Late
Eocene to Late Oligocene age and includes a number of laminated oil shale units
separated by lacustrine lutites occurring in a graben structure along the present
coastline north of the port of Gladstone (Hutton et al. 1990). Whilst lacustrine
conditions prevailed in the trough for much of the time interval during which the
18
The Conditions of Peat Formation
deposit was formed, it appears that laterally peat-producing swamps existed which
shed humic detritus into the lake. Towards the end of its development, peat
formation became more widespread, which is indicated by brown coal seams
overlying the oil shale.
Other examples of accumulation of organic matter in limnic environment
include playa lakes which can be high in salt content. The latter does not preclude
algal life, which is evidenced by the Eocene Green River oil shales and many other
similar deposits. The Green River deposit constitutes an up to 600-m-thick sequence
of alternating oil shale" seams, evaporites and clastics in Colorado, Utah and
Wyoming, where they have been formed in an intermontane basin (Tissot and
Welte 1978). Thin lamination, desiccation cracks and saline deposits indicate
sedimentation in shallow water in an arid or, at least semi-arid environment
(Bradley 1931).
2.2 Peat Composition and Peat-Forming Plants
Coal type, i.e. the petrographic composition of a coal seam, is genetically linked
to the composition of its ancestral peat deposit. The type and composition are
related to the various kinds of peat-forming plants and the biochemical conditions
under which they were converted into peat. Biochemical coalification will be
discussed in Chap. 3, while in this chapter emphasis is on the strong influence on
peat and coal exerted by peat-forming plants.
Having observed variations in rates of decay between different components of
vegetable matter, Waksman and Stevens (1929) established a "stability series" for
Fig. 2.3. Transverse section through a
6-year-old stem of Pinus sylvestris. (After
Shaw et al. 1968)
Peat Composition and Peat-Forming Plants
19
plant constituents, which begins with cell protoplasm as the least stable component
and ends with waxes and resins as the most stable ones. Because of its considerable
volume and relatively high resistance, the wood of trunks, branches and, above
all, roots has often contributed the largest proportion of vegetable matter to the
formation of coal and, among the various types, gymnosperm wood was probably
more common than any other variety. A schematic section of a conifer stem is
therefore displayed in Fig. 2.3 in order to illustrate some of the most important
wood tissues.
Basically, the tissue in' a plant stem appear like a set of cylinders which have
been telescoped into each other. They differ in their morphology and function.
The innermost cylinder is formed by the pith which in some plants is hollow but
consists of parenchyma tissue in gymnosperms. Its cells are elongated and contain
thin walls consisting mainly of cellulose. They have no specific function.
The pith is surrounded by xylem tissue which extends from the roots to the
tree tops. It forms the wood proper, and apart from giving strength to the plant
body, it contains parts of the vascular system, i.e. it serves to distribute water and
minerals, drawn up by the roots, throughout the plant and to the leaves. The
xylem consists of three types of cells (after Francis 1961):
1. Long tube-like, thick-walled tracheids, rich in lignin and forming the bulk of
the wood.
2. Roughly cylindrical, irregular thin-walled cells forming resin ducts.
3. Short, box-shaped, thin-walled cells arranged in bands at right angles to the
tracheids and radiating from the centre, thus called pith rays.
The xylem is surrounded by the phloem with the cambium in between. The latter
consists ofliving tissue which is responsible for the secondary growth or thickening
of the stem because it produce~ new plant cells on either side of itself, xylem cells
on the inside and phloem cells on the outside. In the course of this process the
older xylem cells are buried deeper in the stem whilst the primary phloem is pushed
to the outside.
The cambium moves outward too and increases in circumference because of
the accumulation of new xylem tissue inside it. In tropical climates this process is
continuous. However, in climatic zones characterised by pronounced seasonal
changes from cold to warm or dry to wet, growth is interrupted during the adverse
season. As the latter is approached the cells become smaller and thick-walled,
which shows in sections as annual growth rings (Figs. 2.3 and 2.4)
The pholem consists of mainly three types of tissues:
.
1. Rows oflarge bulging cells often containing waste products and with cell walls
consisting mainly of cellulose.
2. Small thin-walled cells grouped around the large ones.
3. Rows of thick-walled lignified cells radially arranged like the pith rays.
The main function of the phloem is the distribution of food stuffs produced in the
leaves by photosynthesis downward throughout the plant body.
20
The Conditions of Peat Formation
Fig. 2.4. Photomicrograph showing the change from spring to autumn growth (left to right) in
annual rings. Actual length of field of view = 5 mm
The phloem is surrounded by the cortex, which consists of an irregular
arrangement of parenchyma tissue. In young shoots the cortex is covered by a
cuticle which is a non-cellular protecting membrane covering the outside not only
of young parts of the plant body but also of leaves. In the course of secondary
thickening the cuticle peels off and, in order to provide continued protection to
the plant, the outer parenchyma cells change into cork cells which constitute the
bark or periderm.
Because most of the cell contents (protoplasm) decay very quickly, the walls
of plant cells contribute more than any other kind of vegetable matter to the
formation of peat and coal. Commonly such cell walls consist of up to three layers:
1. A primary wall which consists mainly of cellulose and some pectin plus lignin
in the case of wood. Cellulose (C 6 H 1005) occurs in the form of long-chain
molecules which are secreted by the protoplasm of the living cell. The cellulose
chains are bundled into the so-called microfibrils, which in turn are grouped
into macrofibrils.
2. A secondary wall is not always present and frequently it does not cover the
primary wall completely. Cellulose is its basic component with the addition of
lignin in wood cells. Similarly to cellulose, lignin (C30H34011) is not a simple
chemical compound but an association of various closely related substances.
Lignin adds firmness to the cell walls and renders them resistant.
3. Adjacent cell walls are joined by the intercellular substance, or middle lamella,
which acts as a cementing material. It consists of pectin, (C 6 H 10 0 7), which is
the methoxy ester of the pectic acid. A small hole, intercellular space, IS
commonly left at the juncture between four adjacent cells.
Peat Composition and Peat-Forming Plants
21
From the above, it follows that pectin, cellulose and lignin form the bulk of the
substances contained in the walls of plant cells and they therefore contribute greatly
to the composition of coal. In terms of quantity the substances related to both
cellulose and lignin are more important than pectin, which forms only the rather
thin middle lamellae. As mentioned before, lignin is not always present in cell
walls. It is missing altogether in a large number of plants and, where it is present,
its composition (and properties) may vary slightly between different taxa (Clymo
1987). Lignin renders the cell walls rather rigid and so it is concentrated mainly
in those parts of plants which consist of solid wood. However, non-woody
herbaceous plants may also contain partially lignified cells. In the context of peat
formation an important example is Sphagnum moss, whose surprisingly good tissue
preservation in many peats has been attributed to the high proportion oflignin-type
compounds in their cells.
The lignin content of present day pine wood, according to Schmidt and
Graumann (1921), is about 37% on a dry, resin free basis. Assuming that the wood
of other plants, including those of earlier geologic periods, had a similar lignin
content, Potonie (1924) used this figure to estimate the percentage of lignin and
related wood producing substances contained in the vegetable source of the Carboniferous coals in Europe. When viewed under the microscope, remnants of woody
tissues are usually visible in thin sections and polished coal blocks (Fig. 2.5). They
are particularly well developed in syngenetic dolomite nodules and other carbonate
concretions occasionally found in coal seams. These have been formed during the
very early stages of diagenesis and they contain therefore almost ur1compressed
portions of the former peat preserved by petrification (Fig. 2.6).
Fig. 2.5. Photomicrograph of fusinitised wood tissue (charcoal) in a high volatile bituminous
coal from the Gunnedah Basin, New South Wales. The real length of the long edge is 0.36 mm;
incident light, oil immersion
22
The Conditions of Peat Formation
Fig. 2.6. Photomicrograph of the margin of a siderite nodule in medium volatile coal of the
Bowen Basin, Queensland. Note the uncompressed cell tissue in the carbonate and the traces of
compression in the adjacent coal to the right of the concretion. Actual length offield of view = 0.23
mm; incident light, oil immersion
Based on a quantitative analysis of 94 thin sections of dolomite nodules,
Potonie (1924) concluded that 40% of the vegetable matter contained in them
consists of woody tissues. Using Schmidt and Graumann's (1921) figure of 37%
for the lignin content of wood, it would appear that 14% of the coal has been
derived from lignin and related compounds, the rest being cellulose and other
substances. This is mentioned here because earlier students of coal, particularly
Fischer and Schrader (1922), used to hold the view that lignin contributed more
to the formation of coal than any other source material. They argued that humic
coals are rich in aromatic hydrocarbons, which on distillation produce phenols.
The same is true for lignin when subjected to dry distillation. Cellulose has an
aliphatic structure and was thought therefore to be less suitable to produce the
aromatic compounds of coal. However, a large number of substances which are
known to be devoid of lignin have undergone coalification. Important examples
which contradict the lignin hypothesis are herbaceous plants and coalified plant
leaves from which much coal has been formed, although this kind of vegetable
matter contributed little lignin.
A summary of peat types is given by Francis (1961) which includes the following
characteristics:
1. Fibrous or woody peat (Fig. 2.7) is firm, moderately tough, and not plastic;
shows the original plant structures only slightly or partly altered by decay.
Such peats, when cut and dried, display only moderate shrinkage, at least in
the direction of plant growth. Large branches, trunk or roots of trees may
persist when the deposit contains much forest debris.
23
Peat Composition and Peat-Forming Plants
Fig. 2.7. Photograph of fibrous peat (top) with rootlets and enclosed piece of wood (bottom) from
Jewell's Swamp, Belmont, New South Wales
Table 2.2. Some properties of six peat types identified in the Okefenokee Swamp, Georgia,
U.S.A. (After Cohen 1973)
Depositional Leading
Colour
setting
plant species
Open water
marsh
Texture
macro-
microscopic
Plant
fragments
F/M
N/S
Leaves
seeds, roots
cell fillings
0.79
3.00
Nymphaea
odO/'ala
Reddish
brown
Light
Fine
yellow
fibrous
granular
Carex
hyalinolepis
Reddish
brown
to brown
Light
yellow
to light
brown
Light
yellow
Coarse
fibrous
Redbrown
Fibrous to
granular
0.85
Leaves
roots,
mostly
debris
Leaves
1.08
roots,
debris
Rhizomes
0.96
lea ves, roo ts
-..- --
As above
-
1.50
Shallow
glades
and
island
fringes
Panicwn
hemitomon
Tree
islands
and
swamps
Cyrilla
racemiflora
As above
Light
brown
Coarse
granular
woody
Leaves,
1.94
roots wood,
cell fillings
0.54
Taxodium
distichum
Dark redbrown
As above As above
1.00
Leaves,
roots twigs,
wood debris
0.67
Reddish
brown to
dark brown
Woodwardia As above
virginica
3.00
1.50
24
The Conditions of Peat Formation
2. Pseudo-fibrous peat, in spite of its fibrous appearance, is soft, non-coherent
and plastic. When dried, this type shows varying shrinkage depending on the
source material.
3. Amorphous peat. In this type, the original structure of the plant's cell tissue
has been destroyed by decomposition. The resultant peat is the organic
counterpart of clay, being composed of fine grains, which form a plastic mass,
similar to wet, heavy soil.
4. Intermediate forms of peat and mixed peats consist of mixtures of the more
resistant elements set "in a strongly altered matrix. The name "mixed peat" is
generally applied in modern peats to alternating layers of fibrous and amorphous
peats.
In his work on the Okefenokee Swamp, Cohen (1973) found macroscopic peat
types of the kind listed above to be genetically not particularly useful because
peats of different origins in terms of vegetational precursors and mire setting
displayed overlapping characteristics. On the basis of the dominant peat-forming
plant species, he distinguished between six peat types which were referred to three
depositional settings. A list of their main characteristics is given in Table 2.2 and
discussed below.
Parts of the Okefenokee Swamp consist of open marsh with a water depth of
up to 1 m. It is covered by floating vegetation among which the white water-lily
Nymphaea odorata is dominant. Its peat consist mainly of intertwined roots
resulting in a distinctly fibrous texture. In a hand sample, the peat is reddish brown
but its microscopic appearance in transmitted light is more yellowish and somewhat
granular. The latter results from the high degree of decomposition of the soft cell
tissues which is responsible for the comparatively low framework to matrix ratio
(F/M in Table2.1). Framework particles have been defined by Cohen (1973) as
discrete organs or relatively intact cell tissues in excess of 0.2 mm, whereas the
matrix constitutes the continuous phase of degraded interstitial matter. Framework
particles are further characterised by their ratio of non-sedimentary to sedimentary
material (N/S ratio). Non-sedimentary consists mainly of subsurface organs, such
as roots and rhizomes, while any vegetal matter that accumulates on the peat
surface, for example leaves, spores, pollen, twigs, stems etc. make up the sedimentary
portion.
An interesting aspect of the Nymphaea marsh is the occurrence of floating
patches of peat, up to several tens of metres in diameter and less than a metre
thick. According to Cohen (1973), these separate from the underlying peat when
they become buoyant due to the entrapment of marsh gas in the near-surface
layers. They are hosts to a variety of plants preferring drier habitats, which adds
more peat to the patches and causes them to become grounded. By the time shrubs
and trees are established on them they do not float anymore but are grounded.
Their former frequency and distribution is indicated by the many clumps of trees
which occur as islands throughout the marsh.
The shallow water glades and transitional zones between marsh and wooded
swamp are occupied by sedges, reed-like vegetation, and ferns dominated by Carex
hyalinolepis, Panicum hemitomon, and Woodwardia virginica, respectively. Both
Climate and Peat Accretion
25
Carex and Woodwardia peats are quite decomposed which is indicated by the low
FjM ratio but, as shown by their lower NjS ratios, compared with Nymphaea and
Panicum peat, both of which owe their high NjS ratios to the abundance of
well-preserved roots, much plant debris is still present.
Wood in the form of twigs, branches and smaller fragments, including charcoal,
as well as leaves and other surface accumulations, dominate the Cyrilla and
T axodium peats formed on the tree islands and in swamps. The results are increased
FjM and strongly reduced NjS ratios, although the FjM ratio of the Taxodium
peat is only moderate due the higher proportion of herbaceous debris.
From the above follows that, given similar source material, the F jM ratio is
a measure of surface degradation, i.e. a low value would indicate a high degree of
plant decomposition, which is true in the case of the Nymphaea and Panicum peat,
but compositional differences between peat-producing plants (e.g. woody versus
herbaceous) will certainly modify this relationship. The NjS ratio is likewise an
indicator of the degree of plant decomposition. A high ratio suggests considerable
surface degradation and advanced tissue decay whereby, once again, the different
preservation potentials of different plants growing in similar circumstances will
affect its numerical value. Both ratios are thus not only affected by the physical
and biochemical conditions of peat formation but also by the kind of flora that
is converted into peat. This is an important aspect which will be further discussed
in Chaps. 5,7 and 8, where the ratio concept will be applied to coal facies analysis.
2.3 Climate and Peat Accretion
Plant growth and thus the formation of peat and coal depends on the availability
of liquid water. The kind of flora (within limits set by evolution), its variety, and
the quantity of vegetable matter produced are regulated by temperature and
precipitation. Temperature also governs the rate of evaporation, i.e. in cool climates
with medium to low annual rainfall, plants can be provided with more surface
water than is the case in warm regions of similar precipitation rates. Warm arid
zones carry only sparse vegetation and the paucity of coal deposits between the
palaeolatitudes of 15° and 30° illustrated in Fig. 2.8 suggests that similar conditions
operated in the geological past. Also of note in the illustration is the latitudinal
shift in coal formation towards higher latitudes from the Permian Period onwards.
Indeed, during the Mesophytic Era (see Chap. 2.4), coal formation appears to have
been concentrated in temperate to cool climatic zones incorporating very high
latitudes which, by the present meteorologic configuration, would be incapable of
producing peat. Reasons for such flora extensions into polar regions include
changes in the tilt of Earth's rotational axis (Wolfe 1978), a fossil "greenhouse"
effect (Fischer 1981), different rainfall patterns due to a widening of the Intertropical
Convergence Zone (Parrish et al. 1982; Ziegler et al. 1987), and also suggestions
that the high latitudes are erroneous because palaeomagnetic determinations may
be subject to a long-term bias towards the rotational and magnetic pole locations.
26
The Conditions of Peat Formation
Relative frequency
o
30
60
Degrees latitude
_
90
Triassic
OJ] Early Tertiary
IE Penman
~ Cretaceous
. . Carboniferous
8
Jurassic
EE Devonian
Fig. 2.8. Diagram of the palaeolatitudinal distribution of coal-forming areas
throughout the ages. The relative frequency approximates present sizes of
coal fields. (After Irving 1964 and
Habicht 1979)
In the latter context Donn (1982) suggests that the angular difference between
Earth's rotational and magnetic dipole axes varies over a wider margin than has
hitherto been assumed. The resultant differences between magnetic and geographic
latitudes (depending on the longitudinal position) increase towards the poles. A
large angle might have existed during the Mesophytic Era, which would require
a lowering of the geographical latitude.
On account of their prolific plant growth, tropical and subtropical regions
might be expected to be particularly well suited for the production of large peat
deposits. However, not only plant growth but also the biochemical processes
leading to the complete removal by decay of vegetable matter are accelerated in
a hot climate, particularly when it is continuously wet. Cellulose-decomposing
bacteria thrive best at temperatures between 35° and 40 °C (Teichmiiller 1958). At
lower temperatures not only the biological action is retarded but also chemical
decompostion proceeds at a lower rate. It is therefore possible that even under
conditions of slow plant growth peat can be formed in large quantities, which is
borne out by today's concentration in high latitudes oflarge peat deposits in which
the proportion of undecomposed cell tissues is commonly higher than in tropical
peats. The long and severe winters of high latitudes do not adversely affect the
peat since during such periods plant decomposition is practically nil while during
the summer months the abundance of moisture and the short nights favour steady
plant growth.
Even if the very high latitudes indicated in Fig. 2.8 should be revised downward
in order to account for a greater difference between magnetic and rotational poles,
27
Climate and Peat Accretion
- - ' " Namurian
_ • .." KazaRian
p (Stephonion)
•
•••• ' Sokmarion
"""""" Malillium .... nt of inland ic..
MESOZ ole
UPPER
PERMIAN
4 lIarine-Glaciol deposits
,-------59
r + . - - - - 5.
57
56
'--T--+--+- 55
:";tl~.,.-+-+-
54
5'S
52
5t
50
41
4.
47
41
45
44
.., Tialsic
,..- • ..- Kazani ••
0:;:::::> Sedimentar, bOlin,
~
,0
~ II ~ Crtto"ous
• ,,'
- I ...... -* Triassic
~ Sedimentor, buin,
45
-'-"a"---- 42
....- - " - - - - - 4,
Fig. 2.9. The correlation between palaeolatitudes and the geographic and stratigraphic
distribution of Australian hard coal deposits. After Diessel (1969a), who incorporated information
from Brown et al. (1968), Engel (1962), Hawthorne (1965), Hill (1968), Hill and Denmead (1960),
Irving (1964), Irving and Gaskell (1962), Irving and Parry (1963), Cook (1975b), Knight et al. (1975),
Malone (1964), McWhae et al. (1956), Power (1967), Rattigan (1966), Standard Association of
Australia (1955), Spray and Banks (1962), Sprigg (1967).
Explanation of numbers:
Upper Carboniferous:
1 Inferior coal seams of the Italia Road Formation (Westphalian-Stephanian?) north of Raymond
Terrace, and other occurrences in the Hunter Valley. 2 Inferior coal seams in Alum Mountain
Volcanics at Bulahdelah (Stephanian-Lower Permian?).
Lower to Middle Permian:
3 Inferior coal seams in the Lower Bowen Volcanics (Sakmarian). 4 Thick coal seams in Reids
Dome Beds (Sakmarian-Artinskian) of the Denison Trough. At present not economic because
of great depth. 5 Steam and coking coals in the Collinsville Coal Measures (Artinskian). 6 Some
economic seams in upper portion of Aldebaran Sandstone and Freitag Formation (Artinskian)
of the Denison Trough. 7 Some coals in the Peawaddy Formation (Kazanian) of the Denison
Trough and on the Springsure Shelf. 8 Coal seams in the Calen Coal Measures (Artinskian) of
the Yarrol Basin near Mackay. 9 Several thick seams of steam coal in the Ashford Coal Measures
(Artinskian) of the Ashford Basin. 10 Coal seams (Artinskian) in the Stroud-Gloucester Trough.
11 Inferior coals in the Markwell Coal Measures (Artinskian?) at Bulahdelah. 12 Irregular seams
in the Clyde River Coal Measures (Sakmarian-Artinskian). 13 Inferior coals at the base of
Shoal haven Group (Artinskian?) between Kangaroo River and Tallong. 14 One coal seam at the
28
The Conditions of Peat Formation
there is sufficient independent evidence to suggest that most Gondwana coal
deposits have been formed under cool to temperate conditions. The assumed
palaeomagnetic distribution is illustrated in Fig. 2.9 for Australian coals. Palaeoclimatic indicators include the presence of strong annual growth rings in Permian
Gondwana trees (Fig. 2.10) and their absence in many Carboniferous trees from
the Northern Hemisphere (Potonie 1920; DiMichele and Phillips 1985), as well as
the occurrence of imprints of ice crystals in the Greta Coal Measures of New
South Wales (Tobin 1980) illustrated in Fig. 2.11. Further evidence for coal
formation in Gondwana under cool conditions is contained in the coal itself, in
which leaf cuticles and the enclosed mesophyll tissue often occur densely packed
as one would expect from an autumn leaffall (Fig. 2.12). There are also occurrences
in Gondwana coals where mesophyll tissue grades into semifusinite (Diessel 1983),
Fig. 2.9. (Caption continued from page 27)
base of Dalwood Group (Artinskian?) near Raymond Terrace. 15 Thick seams of steam coal in
Greta Coal Measures and regional equivalents (Upper Artinskian) in the districts of MaitlandCessnock-Greta, Muswellbrook-Singleton, and Werris Creek-Gunnedah-Curlewis. Small
Tasmanian coalfields with some economic seams in the Mersey Group (Artinskian) at 16
Preolenna 17 Mersey 18 Midlands District 19 Mt. Pelion and 20 Southeastern district. 21 Some
coal in the Irwin River Coal Measures (Artinskian) of the Perth Basin. 22 Steam coal seams in
the Collie Beds (Sakmarian-Artinskian) of the Collie and Wilga Coalfields.
Upper Permian:
23 Inferior coals in the Little River Coal Measures (Tartarian?) near Cooktown. 24 Some
economic seams in Mount Mulligan Coal Measures (Tartarian?) near Cairns. 25 Outcrops of
little-known coal beds (Tartarian?) at the Oxley River. The Bowen Basin contains many Upper
Permian (Tartarian) coal seams displaying a wide range of rank. The main districts are
26 Elphinstone 27 Blair Athol (could be Artinskian) 28 Bluff and Blackwater 29 Baralaba
30 Kianga and Moura. The Sydney Basin contains thick Upper Permian coal measures with
many coal seams. The main occurrences are 31 Ulan 32 Singleton Coal Measures (Tartarian)
contain mainly steam coal in the Hunter Valley and in the Northwestern Coalfield (Black Jack
Coal Measures); some coking coal is also present; 33 Newcastle and East Maitland districts in
the Northern Coalfield with Newcastle and Tomago Coal Measures. 34 Western Coalfield with
Illawarra Coal Measures. 35 South-western and 36 Southern Coalfields with Illawarra Coal
Measures. 37 Coals of the Coorabin Series (Tartarian) in the Riverina. Inferior coals of the
Cygnet Coal Measures (Kungurian-Tartarian) in Tasmania at 38 Mt. Pelion and 39 Cygnet
40 Steam coals of the Collie Burn Series and Cardiff Series (Kungurian-Tartarian) at Collie.
Triassic:
41 Leigh Creek Coal Measures (Middle to Upper Triassic) at Leigh Creek. 42 Triassic coals at
Fingal, Merrywood and in south-eastern Tasmania. 43 Thick seam of steam coal in Callide Coal
Measures (Lower Triassic?) near Gladstone. 44 Some economic coal in the Nymboida Coal
Measures (Lower to Middle Triassic) of the Clarence Basin. 45 Mainly steam coals in the Ipswich
Coal Measures (Lower to Middle Triassic) at Ipswich. 46 One seam at Nundah near Brisbane.
Jurassic:
47 Several seams in the Wonthaggi Coal Measures of South Gippsland. 48 Inferior coal in the
Ballimore Beds (Dogger?) near Dubbo. 49 Seams in the Walloon Coal Measures (Dogger to
Lower Maim) and the Grafton Shales (Upper Maim) of the Clarence-Moreton Basin. Coals also
in the Walloon Coal Measures at 50 Rosewood-Walloon and 51 Darling Downs. Small basins
with some Jurassic coal at 52 Tiaro 53 Mulgedie 54 Pascoe River and 55 Laura.
Cretaceous:
Lower Cretaceous (Albian) coal at 56 Burrum and 57 Styx 58 Little known coals occur in the
Winton Formation (Cenomanian) of the Great Artesian Basin. 59 Thin seams at Kuntha Hill
Climate and Peat Accretion
29
Fig. 2.10. View of transverse section of a petrified tree trunk found in the Newcastle Coal
Measures, New South Wales. Note the strongly developed annual growth rings
Fig. 2.11. Photograph of casts of ice crystals on the underside of the Ayrdale Sandstone overlying
the Balmoral Seam in the Greta Coal Measures, Upper Hunter Valley, New South Wales
30
The Conditions of Peat Formation
Fig. 2.12. Photomicrograph showing
densely packed leaf cuticles (thin dark
llnes) with enclosed mesophyll vitrinite
(dark grey). Actual length of field of
view = 0.36 mm; incident light, oil
immersion
as illustrated in Fig. 2.13. This, too, appears to be related to the deciduous nature
of the Glossopteris flora which during winter months allowed many leaves to wilt
on stems and frozen ground thus causing a freeze-drying effect before they could
be incorporated into the peat during the following spring (Taylor et al. 1989).
In Europe, Tertiary brown coal formation began under tropical conditions as
shown by remnants of palm trees and other tropical and subtropical plants in
Eocene lignites. Subsequently, pines, firs, alders, birches and other temperate zone
plants associated with Miocene brown coals indicate a marked climatic cooling
(Magdefrau 1953).
Another aspect of climatic influence of plant growth and peat accretion
concerns the availability of space for the development of peat. Apart from relatively
small occurrences in mountainous regions with a low preservation potential, the
majority of the world's 480 Mha of Holocene peatlands (Kivinen and Parkarinen
1981) is situated in low terrains not far above sea level. The current position of
the sea level divides the continental platform into a sub-aqueous portion ranging
approximately from the shelf edge at - 200 m to mean sea level and into a sub-aerial
section from sea level to a height of nearly 1000 m. This means that the present
sea-level position favours large-scale swamp formation on the wide tracts of
low-lying land. Moreover, current sea-level position favours coastal progradation
Climate and Peat Accretion
31
-_.- .
-t....
~ .~
...
!-
'.,.' c . . .
-.
.....,...
..".
,.
•
.=-.. . '.. ~.
~.
'<.
');-
•
Fig. 2.13. Composite photomicrograph showing, from top to bottom, increasing stages in the
transformation of leaf tissue (mesophyll) into semifusinite. All examples are from high volatile
bituminous coal from the Gunnedah Basin, New South Wales. The real total length of the
ilustration is 0.76 mm; incident light, oil immersion
32
The Conditions of Peat Formation
since marine regimes overlap many continental margins wide enough to form
broad continental shelves and shallow epicontinental seas (e.g. Sunda Sea in
Southeast Asia). The influx of even moderate amounts of sediments into shallow
water causes marine regression and build-up of coastal marshes and swamps. Only
30000 years ago, when during the Pleistocene Epoch much sea water was locked
in the greatly increased polar ice caps and mountain glaciers, the concomitant
lowering of the sea level by approximately 100 m shifted strandlines much closer
to the edges of the shelf break. Under such conditions rivers debouched their load
not far from the rapidly deepening continental slopes, which precluded the
construction of large coastal lowlands that appear to have been the loci of much
coal formation in the past. The influence of climate-dependent eustatic sea-level
changes on coal formation will be considered further in Chap. 8.
2.4 Evolutionary Trends in Peat and Coal Formation
A brief consideration of the evolution of the plant kingdom serves to establish the
stratigraphic range of coal formation because a certain evolutionary stage had to
be acquired before plants could spread into the environments which offered the
other parameters necessary for the development of large coalfields. The stratigraphic range of the major members of the plant kingdom is displayed in Fig. 2.14.
It shows the successive appearance of the various members of the plant kingdom
which, similarly to the animal kingdom, can be grouped into several stratigraphic
categories of increasing differentiation and complexity. The oldest group is the
Thallophyta (algae) which dominated the plant kingdom to the end of the Silurian
Period when the Pteridophyta (fern-like plants) developed.
The plant body of algae (thallus) shows little differentiation. Algae do not
possess a vascular circulatory system and are therefore dependent on osmosis and
the presence of water to sustain their life cycle. This restriction renders them
unsuitable for large-scale coal formation and thus only algal coals (boghead coals
or torbanites) and oil shales are known from pre-Silurian periods. Among these
are Precambrian torbanites of anthracite rank in North America, the thucholite
deposits of the Witwatersrand (Snyman 1965; Hoefs and Schidlowski 1967;
Schidlowski 1968; Plumstead 1969; Hallbauer and van Warmelo 1974; Hallbauer
1975), numerous occurrences of graphite schists in Australia and elsewhere and
the Kukersite deposits of Silurian age which are mined and utilised in Estonia.
The latter have been formed mainly from one alga, Gloeoeapsomorpha prise a, so
named because of its similarity with the Recent Cyanophyta genus Gloeoeapsa. It
is a colonial alga which occurs together with brachiopod shells and remnants of
bryozoa. Individual cells are 40 to 100 flm in diameter but form compound structures within a mucilagenous matrix. The associated fauna indicates a marine origin,
probably in a protected bay. Several seams occur which are up to 80 cm thick
(Miigdefrau 1953).
Evolutionary Trends in Peat and Coal Formation
.,..
,''-
33
M""Vt'1
"'"
_
ERA
ICAYTONIALES
ICONIFERALES
; ~
I """'''''' I ITA.L,I'S_
,n, I F~
" ...It",,, ''''
,",vnu""ALES
n
~co
EQUISETALES
r.AI
IARIAr.FAF
.~
..
fLLALES
~,
g
'Ar~A~1 ~
I ~I"" ARIAr.FAF
I,,,,,
I
A"'OJC, ACEAF
I LYCOPODI!,rcAc
';
I PTEROP~lnA
IDcollnD",n.
I BRYOPHYTA
IFUNGI
i~ Ii
2
I ALGAE
0
m
-t
I~'i 1'l
~en
~
:;;
Z
:;;
PALAEOZOIC
'-
c:
%I
>
en
en
(j
rri
~
~I~ :;; ~~
%I
-t
~¥
MESOZOIC
%I
-<
PERIOD
~
CENOZOIC
ERA
Fig. 2.14. The stratigraphic range of the major members of the plant kingdom. Modified after
Gothan and Weyland (1954). Note the difference in stratigraphic range between plant- and animalbased eras
Torbanites and oil shales continue to form after the advent of higher plants
and many of them are of considerable interest as a source of hydrocarbons, among
them the Green River deposits of the western U.S.A. and the Tertiary oil shales
of Rundle and Julia Creek in Queensland, Australia. Also these deposits have been
devived from Cyanophyceae (blue-green algae) which form algal mats at the
depositional interface. Torbanites contain mainly the Chlorophyceae (green algae)
genus Pila, for example in the Permian bogheadcoals of Autun in France, and
Reinschia in many Permian boghead coals of Australia, South Africa (Ermeloo)
and South America. As illustrated in Fig. 2.15 they form floating, somewhat
globular colonies containing up to several hundred individual cells. Both Pila and
Reinschia appear to be closely related to the Recent Botryococcus and (Gothan
and Weyland 1954). The latter is found in both fresh and brackish water and the
present species Botryococcus braunii is commonly regarded as the source of
coorongite, a bituminous substance found at Coorong Lagoon in South Australia.
Tasmanites is another chlorophycea found in some oil shales (tasmanite). It is
34
The Conditions of Peat Formation
Fig. 2.15. Two photomicrographs of a polished thin section of Pi/a algae in Greta coal of the
Sydney Basin, New South Wales. Left incident light in oil immersion; right transmitted light.
Note the contrasting appearance of the algae in incident (dark) and transmitted (white) light.
Actual length of each field of view = 0.2 mm
related to the present genus Pachysphaera and is mainly known from Permian
deposits in Tasmania and Jurassic to Cretaceous occurrences in Alaska (Tissot
and Welte 1978).
The possibilities for coal formation improved when the first land plants
appeared in Early Devonian time. These, the Psilophyta also called Psilopsida or
psilophytes, were descendents of algae but are regarded as a separate group by
some authors and as a class of the pteridophytes (spore plants) by others. In any
case, they occupy a special place in the phylogeny of plants because they are the
first to achieve the transition from water to land-life. This was made possible by
the acquisition of a simple vascular system and differentiation of the plant body,
parts of which became firmly anchored in the ground by means of roots, thereby
permitting the plant body (stems, branches and leaves) to be raised into the air.
The early, transitional forms still lived half-submerged in water and bore sporangia
(spore capsules) at the tips of their leaf-less stems. Later forms bear evidence of
more terrestrial habits but all psilophytes, indeed, all pteridophytes were well
equipped for a life in swampy environments. Psilophytes have thus produced the
first real peat deposits from which the banded, so-called humic coals were formed
Evolutionary Trends in Peat and Coal Formation
35
Fig. 2.16. Sketch of a
Sigillaria stem with nearly
horizontal stigmarian
roots. The stem is
approximately 1.5 m high.
(After Gumz and Regul
1954)
after coalification of the vegetable matter. Noteworthy are a number of coalfields, for instance, on Bear Island and Spitzbergen, as well as in the Kuznetsk,
Kazakhstan and other basins.
Coal formation reached an initial-peak in the Carboniferous Period of the
Northern Hemisphere when towards the end of the worldwide orogenic movements, slow but repeated subsidence within and adjacent to the mobile belts turned
large areas into peatlands. Peat accumulation was sustained on an unprecedented
scale and for many millions of years was based on the then domineering plant
group, the pteridophytes. These plants belong to the vascular cryptogams because
all members show well developed vascular strands, such as leaf veins and stems
differentiated into phloem, xylem and pith. Apart from the above-mentioned
psilophytes, which occupy a transitional position, the main subdivisions of the
Pteridophyta are the lycopsida (lycopidiales Lycophyta, lycopods or lycopsids), the
Sphenopsida (Equisetophyta or Articulatales or horsetails) and the Pteropsida
(Filicophyta Filicales pteropsids, pteropods or true ferns).
The above-mentioned pteridophytes are wetland plants which, among other
features, is shown by their widespread but shallow root system. As illustrated in
Fig. 2.16, the stigmarian root axes, to which numerous rootless were spirally
attached, radiated horizontally away from the stem. They interlocked with the
Stigmaria of adjacent trees (e.g. lycopods) thus forming a dense network of roots
(Wnuk 1985) which rendered the peat rather resistant to erosion. Conversely, the
horizontal anchoring and absorptive system of the pteridophytes made them very
sensitive to even small variations in the groundwater table, such that a minor
lowering of the water table would starve them (Collinson and Scott 1987). This
may explain the frequent occurrence in Carboniferous coal measures of rooted
horizons. in interseam positions, or the likewise frequent occurrence of welldeveloped seat earths with rootlets overlain only by a few stringers of inferior coal
(see stratigraphic column in Chap. 7). These indicate that potentially favourable
conditions for peat formation developed quite frequently but they failed to produce
economic coal seams because of the low tolerance span of the not very diverse
Carboniferous flora towards groundwater fluctuations. This was not the case, at
36
The Conditions of Peat Formation
Fig. 2.17. Photograph of Vertebraria
roots underneath the Piercefield Seam
at Saxonvale Mine, Hunter Valley, New
South Wales. Single roots can be traced
for 1m into the floor which consists of
barely disturbed and altered sandlaminated siltstone. Note the pencil as
scale at the base of the seam.
least not to the same extent, in post-Carboniferous coal measures in which the
arborescent vegetation was dominated by seed plants. Although these too, form
shallow root systems in mature wetland environments, it is not rare to find deeply
penetrating roots in the floors of some Australian Permian coal seams (Fig. 2.17),
particularly in situations where peat began to accumulate in response to
paludification. In such situations arborescent gymnosperms were among the vegetational pioneers which began to grow on relatively dry ground and therefore had
to reach deep in the search for water.
The ability of the more robust post-Carboniferous vegetation to adjust to a
larger variety of groundwater positions enabled it to utilise more fully even marginal
peat-forming conditions including those which would have been abandoned by
the the Carboniferous flora. The consequences for the subsequent course of coal
formation are immense. Not only did the Mesophytic flora occupy a wider range
oflatitudes, but the greater tolerance of the vegetation to groundwater fluctuations
also allowed for the formation of thicker coal seams. According to Stille (1926),
the 3000-m-thick coal measures of the Ruhr Basin in Germany contain an aggregate
thickness of 180 m coal distributed over a 200 separate coal seams. This results in
a mean seam thickness of 0.9 m and a proportion of coal of 6%. The Sydney Basin
of New South Wales, which has a comparable tectonic setting, contains a similar
proportion of upper Permian coal in its 1200 m of combined Newcastle and
Evolutionary Trends in Peat and Coal Formation
37
Tomago Coal Measures but its aggregate thickness of 78 m are distributed over
only 36 seams (Diessel 1980a) resulting in an average seam thickness of 2.2 m with
a range from 0.6 to 11.9 m. It may well be that Stille's figure of 200 seams represents
a maximum number and includes thin seams which would be disregarded elsewhere,
but the fact is that many of the approximately 90 coal seams which have been
named and correlated in the Ruhr Basin occur in numerous splits and, based on
a survey of 2660 m of Ruhr Coal measures (Diessel 1988), any reduction of Stille's
seam number would be minor. It would not affect the conclusion that the greater
tolerance of the Permian flora towards environmental changes enabled it and its
vegetational successors to continue accumulating peat where the Carboniferous
flora would have ceased to do so.
lf it is accepted that the post-Carboniferous flora could adjust itself to a wider
range of peat-forming conditions, including drier ones than the Carboniferous
pteridophytes could tolerate, the greater variety in petrographic composition of
Permian coals expressed in Table 1.1 does not come as a surprise. The percentage
ranges of most macerals are greater in the Australian coals compared with the
Carboniferous coals, which means that in spite of the higher mean percentage of
vitrinite and lower inertinite in the latter, some Australian coals are evidently
very bright, as has been demonstrated by Smyth (1968), Edwards (1975), Bennett
and Taylor (1970), and Taylor and Shibaoka (1976). According to Cook (1975a),
vitrinite contents of Australian coals range frbm 5 to over 80%. This means that
the Permian vegetation was just as capable of producing bright coals as the
Carboniferous flora was but that in addition to vitrinite-rich bright coals, Permian
coal measures contain many dull coals with high proportions of inertinite (group
of coal components formed under relatively dry conditions, see Chaps. 3 and 4).
It follows that the restriction to a moist habitat with permanently high groundwater levels imposed on the Carboniferous pteridophytes by their shallow root
system and other biological constraints including reproduction, implied a likewise
restriction of biochemical coalification to the relatively narrow vitrinitisation
pathway (see Chap. 3). Apart from some inertinite concentrations in the uppermost
portion of many seams, the Carboniferous coal measures of the Northern
Hemisphere contain few coal seams with consistently high inertinite contents. This
supports the notion that, as the water table dropped and the habitat became a
little drier, peat forming was abandoned altogether whereas in the Permian mires
it would have continued and resulted in an inertinite-rich seam or part of a seam.
A simi air process can also be seen operating on a large scale. As will be discussed
in Chap. 9, foreland basins (molasse foredeeps) marginal to a fold belt, like the
Ruhr Basin or the Sydney Basin, commonly begin their depositional history with
predominantly marine sediments which have gradually been replaced by terrestrial
deposits. In the Sydney Basin of New South Wales this large-scale terrestrialisation
is mirrored by a succession of Upper Permian coal seams beginning with initially
thin, discontinuous and dirty lower delta plain coals to thick, vitrinite-rich coals
formed in alluvial/upper delta plain settings and terminating with likewise thick
but very dull and inertinite-rich seams interbedded with coarse braid plain
sandstones and conglomerates. Apart from being somewhat more complicated
because of a higher frequency of marine transgressions, the development in the
Carboniferous basins of the Northern Hemisphere does not proceed beyond the
38
The Conditions of Peat Formation
high vitrinite equivalents of the alluvial/upper delta plain coals of the Sydney
Basin. Where in the latter the high-inertinite coals begin, the Ruhr Basin shows
only the advent of the respective coarse clastics but the coal content declines
rapidly. Instead, the already frequent occurrence of rooted horizons increases even
further (Strehlau 1990; David 1989), suggesting that repeatedly for very brief
episodes favourable growth conditions emerged but their duration was too short
to allow peat accumulation to be sustained.
Pteridophytes reproduced by using either the heterosporous (e.g. lycopods) or
homosporous (e.g. pteropods) strategy (Collinson and Scott 1987). This requires
a moist environment and free water in order to fertilise the spores. It also
necessitates that the timing of spore release coincides with favourable conditions
for fertilisation and sporophyte growth because the embryo is poorly protected
and has only a small food reserve (Phillips 1979). Because of these somewhat
adverse conditions, fertilisation had a low success rate, which was balanced by
producing spores in large quantities in order to assure that reproduction was kept
at a high level. The spore content of Carboniferous coals is therefore higher than
that of equivalent younger deposits which, from the Permian Period onward, were
increasingly based on seed plants. Their pollen content is considerably smaller
and conventionally counted together with the spores in coal petrographic analyses.
For example, the average spore content of the Permian coals listed in Table 1.1 is
only one half of that of the Carboniferous samples. In some lacustrine environments
spores are so concentrated that they form a special type of coal, called cannel coal.
The term has been derived from the word "candle" because such coals can be lit
like a candle on account of the high hydrocarbon content of the spore exines.
The Carboniferous flora, like the preceding ones, displays only limited
provincialism. The differentiation into geographyically different floral assemblages
(i.e. Gondwana, Angara, Euramerican flora etc.) begins with the advent of the
Spermatophyta (seed plants). Some of them coexisted with the pteridophytes since
the end of the Devonian Period but they never dominated the floral assemblages
to the extent it happened from the Permian Period onwards, although according
to Patteisky (1958) gymnosperms begin to spread from mid-Westphalian B, and
Josten (1962) notes a sharp increase in Cordatitales in the stratigraphically highest
interseam sediments (Westphalian C z ) of the Ruhr Coal Measures before the end
of coal formation. Conversely, the spore content decreases and some types (e.g.
densospores) disappear altogether from mid-Westphalian C onwards (Peppers
1984; Phillips et al. 1985; Strehlau 1988). The Permian Period marks therefore the
end of the Palaeophytic and the beginning of the Mesophytic Era in which the
gymnosperms (plants with naked seeds) constituted the leading plant group.
Similarly to the psilophytes, which were the forerunners of the bulk of the
pteridophytes, the pteridosperms (seed ferns) constitute the link between the sporebearing ferns and the seed plants. They reach their maximum development in the
Southern Continents (Gondwana) during the Late Carboniferous and Permian
Periods and are responsible for the formation of rich coal deposits in Australia,
South Africa, South America, India and Antarctica. After one of their leading
genera, the whole plant association is often referred to as the Glossopteris flora,
even though members of other plant divisions, such as the Sphenopsida, Phyllotheca,
Evolutionary Trends in Peat and Coal Formation
39
Fig. 2.18. Photomicrograph (incident light) of vitrinite from the Bulli Seam New South Wales
displaying a la~ge n~~ber of resi~ bodies in former resin ducts. The surface 'of the specimen ha~
been etched with aCidified potassIUm permanganate according to Diessel (1961). Actual length
of field of view = 0.36 mm
and Sphenophyllum are commonly associated with Glossopteris. Other gymnosperms which are frequent in the Permian Period are Cordaitales, Coniferales and
(towards the end of the period) Cycadales.
The preponderance of gymnosperms among the Mesophytic flora led to a
change in the appearance of the respective coals compared with the Palaeophytic
ones. Coals formed after the Carboniferous Period are therefore frequently low
in spore content but are rich in derivatives of plant resins and waxes. Such material
is often difficult to identify under the microscope unless fluorescence microscopy
or etching techniques are employed. An example of the latter is given in Fig. 2.18.
The differences in source material between Palaeophytic and Mesophytic coals has
many consequences for their petrographic composition, as well as for their technological properties.
Gymnosperms dominated the plant kingdom from the Permian to the
Cretaceous Period when the angiosperms, which first appeared in Triassic time,
underwent a rapid development. They contain two groups, the monocotyledons
and the dicotyledons. In both, the seeds are covered inside a carpel or seed pod with
the former comprising plants with one seed leaf (grasses, palms, lilies) whereas the
latter are plants with two seed leaves (shurbs, trees). The angiosperms which dominate today's flora developed in the Cretaceous Period, which marks therefore the
beginning of the present Cenophytic Era.
In spite of the preponderance of angiosperms, gymnosperms have remained
important contributors to peat and coal to the present day because their wood is
often resinous and resists rapid aerobic decay and they cover large portions of the
temperate and colder regions in which most of the recent peat deposits have been
formed.
3 The Coalification Process
The transformation of vegetable matter into peat and coal is commonly regarded
as proceeding in two steps, called the biochemical and physicochemical stage of
coalification (Stach et al. 1982), respectively. Other terms, such as "first and second
phase" (Mackowsky 1953), or "diagenetic and metamorphic stage" (Teichmiiller
1962) have been used to describe the coalification process. During biochemical
coalification organisms initiate and assist in the chemical decomposition of vegetal
matter and its conversion into peat and brown coal. The results of this process,
i.e. the type of peat and coal formed, depend on the phytogenic input and the
environmental conditions under which it is transformed into peat. Different
biological, chemical and physical constraints. result in different peat types which
during the subsequent physicochemical coalification are transformed into different
coal types without losing their palaeo-environmental signature. Because of the
causal links between coal types and depositional setting the following discussion
will emphasise the conditions and results of biochemical coalification, whereas
physicochemical coalification will be dealt with less rigorously.
3.1 Biochemical Coalification
The discussion of peatlands in Chap. 2 has shown that large present-day mires
house a number of sub-environments ranging from open water to relatively dry
land. Because of physiological differences only a few plant types are capable of
occupying all the possible ecological settings. Much of the swamp flora has thus
been segregated into distinct plant associations relative to the position of the water
level, some present and past examples of which are listed in Table 3.1.
The table is divided into two rheotrophic and one ombrotrophic setting but
the occurrence and type of vegetation of the latter can be assessed with reasonable
accuracy only in Recent peatlands. The most diagnostic plant of today's
ombrotrophic raised bogs in Sphagnum moss but, although it has been documented
as far back as the Permian Period (Remy and Remy 1977), little evidence of it has
been found in coal deposits, either in the form of spores or tissue fragments. This
is surprising because tissue preservation in Sphagnum peat is commonly good on
account of its high lignin content (Grosse-Brauckmann 1980) and the acid nature
of the bog environment which suppresses microbial activity. Perhaps the reason
The Coalification Process
42
Table 3.1. Selection of typical plant associations in relation to their peatland habitat and
stratigraphic order
Stratigraphic
level
Recent
Temperate
Canada coastal
(Styan and
Bustin
1983a, b)
Ombrotrophic
setting
Sphagnum,
Kalmia,
Oxycoccus,
Nuphar,
Pinus,
Ledum,
Eriophorum,
Rhynchospora
Recent
Subtrophical
SE-Florida coastal (Cohen
and Spackman
1972; Spackman
et a\. 1966, 1969)
Rheotrophic setting
Position of water table in relation to ground level:
Water table at or
below ground surface
Ground is covered by
shallow water ( < 2 m)
Populus, Picea,
Carex, Betula
Fresh-water: Carex cost.
Equisetum, Typha
latifolia, Calamogrostis,
Brackish water:
Eleoeharis, Carex lyn.,
Scirpus am.
Salt-water: Salicornia,
Elymus, Distichlis
Rhizophora mangle,
Conocarpus erecta,
Laguncularia
racemosa,
Acrostichum aureum,
M ariscus jamaicensis
Recent
Tropical
SE-Asia coastal
(Anderson
1964, 1983;
Whitmore
1984)
Litsea,
Shorea albida,
Tristania,
Palaquium,
Parastemon,
Cratoxylum,
Combretocarpus,
Calophyllum
Gonystylus,
Dactylocladus,
Shorea albida,
Stemonurus,
N eoscortechina,
Phoenix paludosa
Rhizophora apiculata
Avicennia marina,
Kandelia candle,
N ypa fruticans
Recent
Temperate
Europe inland
(Gottlich
1980)
Sphagnum,
Andromeda,
Drosera,
Polytrichum,
Calluna, Vaccinium
Erica, Ledum,
Eriophorum
Alnus glutinosa,
Pinus sylvestris,
Betula pubescens
N ymphaea alba,
Schoenoplectus lacustris,
Phragmites communis,
Typha latifolia,
Carex, Scirpus
Equisetum limosum
Cyrilla racemij1ora,
/lex, Gordonia,
Magnolia, Persea,
Lyonia, Itea
N ymphaea odorata,
U trieularia,
N ymphoides aquatica,
Xyris, Orontium, Carex,
Pontederia, Sagittaria,
Vallisneria, P anicum,
Woodwardia, Nuphar,
Taxodium distichum
Recent
Warm temperate
U.S. Atlantic
coast - inland
(Cohen 1973)
Oligo/Miocene
Warmtemperate
Gippsland
Basin,
Tristania,
Baeckea,
Epacridaceae,
Restionaceae,
Gleichenia,
N othofagus,
Elaeocarpus,
Quintinia,
Lagarostrobus,
Ackama, Pullea
Biochemical Coalification
43
Table 3.1. (Continued)
Stratigraphic
level
Ombrotrophic
setting
Rheotrophic setting
Position of water table in relation to ground level:
Water table at or
below ground surface
Ground is covered by
shallow water ( < 2 m)
Australia inland (Kershaw
and Sluiter 1982)
Proteaceae .
Dacrydium, Callitris,
Podocarpus,
Phyllocladus
Eocene
Subtropical
Europe coastal to inland
(Teichmiiller
Sphagnum?
Sequoia, Araucoria,
Pinus, Sterculia,
Cupressus, Ficus,
Woodwardia,
Phoenicites, Sabal,
Myrica, Laurophyllum,
Castanea, Taxodium
mexicanum
Phragmites, Juncus,
Carex, Cladium,
Nymphaea,
Taxodium distichum,
Glyptostrobus, Nyssa
Walkomiella,
Glossopteris,
Gangamopteris,
N oeggeratiopsis,
Sphenopteris,
Gondwanidium,
Lycopodiopsis,
Cyclodendron
Phyllotheca,
Schizoneura,
(Pi/a, Reinschia
Tasmanites)
Cordaites,
Cardiopteris,
Sphenophyllum,
Rhacopteris,
Sphenopteris,
Alethopteris,
Mariopteris,
Sigillaria,
Neuropteris
Calamites, (Pi/a,
Reinschia),
Lepidodendron,
Chaloneria,
Lepidophloios,
Paralycopodites,
Selaginella
1950, 1958, 1962;
Magdefrau 1953)
Permian
Cool temperate
Gondwanacoastal to inland
(Plumstead 1969;
Falcon 1977)
Carboniferous
Tropical
Euramerica coastal to inland
(Teichmiiller
1962; Magdefrau
1953; DiMichele
et al. 1979;
Phillips and
DiMichele 1981;
Bartram 1987)
Herbaceous
lycopods?
for the absence of Sphagnum in coal deposits is related to its preference for cool
habitats. This does not explain its absence in Gondwana coals, but the other
examples of coal-forming plant associations listed in Table 3.1 represent warm to
tropical peatlands which likewise do not support much Sphagnum except at high
altitudes (Whitmore 1984), under present-day conditions.
The difference in the distribution of Sphagnum between temperate and tropical
lowland ombrotrophic mires has considerable consequences for the respective peat
facies. As mentioned above, Sphagnum peat is commonly characterised by a high
44
The Coalification Process
proportion of preserved cell tissue giving it a high framework/matrix ratio and a
fibrous appearance (Styan and Bustin 1983a, b). The F/M ratio is further increased
by the presence of wood fragments contributed by the plants listed in Table 3.1.
In contrast to the well-textured Sphagnum peat, the ombrotelmites of tropical
Southeast Asia consist of a more decomposed, semi-liquid, organic ooze incorporating large pieces of wood. Only the peat surface is reasonably coherent because
of the interlocking roots of the vegetation cover (Whitmore 1984). The reason for
the more advanced dec.omposition of the tropical ombrotelmite is the higher and
more evenly distributed annual temperature which results in more efficient tissue
degradation by microbes, in spite of the low pH. In the Sphagnum-dominated
raised bogs of temperate climates, biomass production may not be as high as in
the tropics but, because of the lower temperatures and the frequent frost conditions
in wintertime, bacterial action is slowed down, too, resulting in a better preservation
of cell tissue.
Another important difference between tropical and temperate climates is the
higher degree of specialisation of the temperate flora between rheotrophic and
ombrotrophic types. There is considerably more mixing between the two types in
the tropics and, although Anderson (1964, 1983) distinguishes between six floral
associations in the transition from coastal rheotrophic to inland ombrotrophic
mires, a large number of plant species is common to all settings but at the expense
of a substantial size reduction towards the centres of the raised bogs. The reason
for this is the decrease in nutrient supply by flood waters (increasing oligotrophy)
as the ground surface rises towards the bog.
Among the examples of temperate climates Table 3.1 lists a number of plants
(e.g. Nuphar . Drosera anglica. Eriophorum vaginatum) in the ombrotrophic setting
which require open water or a very wet habitat. In view of the elevated surface
Fig. 3.1. View of a Sphagnum bog in northern Scotland with water-filled depressions and growth
of Eriophorum and Erica (in foreground)
Biochemical Coalification
45
of the raised bog one would expect it to be subjected more frequently to dry
conditions rather than wet ones. Indeed, many bog plants display a xeromorphic
habit in response to occasional drying, yet, water-filled depressions are a common
feature of the bog surface (Fig. 3.1). As has been discussed by Styan and Bustin
(l983a, b), the ponds and lakes on the surface of the raised bogs occurring on the
alluvial and upper delta plain of the Fraser River (British Columbia) are genetically
related to periods of dryness which effect the most elevated portions of the bog
more than other parts. These relatively dry niches are characterised by ericaceous
Sphagnum peat (Styan and Bustin 1983a).
In contrast to tropical ombrotelmites, which require almost constant
replenishment of the large amounts of water lost to evaporation because of the
prevailing high temperatures, the Sphagnum peats of temperate climates are subject
to a more uneven distribution of water requirement. During the winter months,
when there is little plant growth and evaporation is low, most of the precipitation
on the mire is either lost to runoff (see November to mid-January in Fig. 3.2) or
is left on the peat surface as snow during which period there is little runoff (see
mid-January to mid-March in Fig. 3.2). The thawing period in springtime provides
more water than can be retained in the peat which results in another runoff peak
(from mid-March in Fig. 3.2) after which only occasional heavy showers will cause
excessive runoff (see July in Fig. 3.2). Towards late summer and autumn,
precipitation may not be infrequent but is volumetrically small enough to be
absorbed by the growing vegetation, with the result that no runoff occurs at all
and the groundwater table, which has been oscillating throughout the summer
months, reaches its lowest position (see August to October in Fig. 3.2). In normal
years the lowering of the groundwater table is usually restricted to the uppermost
centimeters or decimeters of the peat (Puffe and Grosse-Brauckmann 1963) but
em
o
N
10
...E
..
....
OIl
Ground...atel'" Table
1
20
30
40
250
200
150
100
50
o
Nov Dec Jon Feb Mor Apr MOY Jun Jul Aug Sep Oct
Fig. 3.2. Comparison of variations in temperature (mean pentades), precipitation, thickness of
snow cover, position of groundwater table and runoff for the Ki:inigsmoor near Hannover,
Germany, over the 12 months between the end of 1954 and 1955. (After Eggelsmann 1980)
46
The Coalification Process
this would be sufficient to cause repeated and, in late summer, prolonged
desiccation of the peat surface.
Persistent periods of severe drought have a more severe effect on the peat than
the more normal drying of the upper few centimetres because, either by lightning
strike or spontaneous combustion, the peat may catch fire and burn right down
to the water table. If the latter has been lowered well below the surface, a return
to normal conditions will fill the burnt out hollow with water. In the developing
lake the autochthonous remnants of Nuphar and other aquatic plants (including
algae) become intermingled with allochthonous and more or less oxidised plant
litter washed into the ponds from the adjacent peat surface because in times of
high precipitation both topogenous and ombrogenous mires are subjected to
considerable water movement on the peat surface itself. As mentioned above, the
reason for the high runoff is the commonly high water saturation of the peat which
cannot retain large volumes of water during heavy falls or when the winter snow
which covers peats of cool temperate climates thaws (Bay 1969). Erosion channels
(washouts) formed by the surface runoff are therefore a common feature of blanket
mires in spite of the considerable resistance of peat to erosion, due to the
interlocking texture of its plant remnants. 'Tree-less Sphagnum bogs and other peat
surfaces, covered with low shrubs only, are affected more so than mires with
arborescent growth. The reason for this difference is not only the greater effectiveness of a dense tree cover in slowing down lateral water movement but also their
larger surface area compared with a grassy or herbaceous ground cover which
allows less water to reach the ground by adsorption and subsequent evaporation
(Eggelsmann 1980). The Nuphar peat described by Styan and Bustin (l983a, b) is
therefore rich in detrital plant material and is underlain by a fire horizon with a
high concentration of charcoal. Fires also exert a considerable influence on the
composition of topogenous peat deposits as is testified by the fire splay deposit
reported by Staub and Cohen (1979) from the Snuggedy Swamp of South Carolina.
This and other geological consequences will be further discussed in the context of
seam splitting in Chaps. 6 and 7.
The plants listed for the rheotrophic setting in Table 3.1 have been divided
into two groups depending on their assumed biotope in relation to the respective
positions of the water table and the ground surface. Allochthonous spores, pollen
and planktonic vegetable matter represent usually only a small fraction of the
biomass deposited as peat most of which has been formed from rooted vegetation.
Since the latter is usually restricted to a water depth of less than 2 m (Moore
1940), peat layers tend to thin away from the telmatic (water table close to the
ground surface) and limnotelmatic (ground surface is covered by shallow water)
zones and eventually wedge out towards deeper water. A similar thinning of the
seam occurs on the terrestrial margin of the mire, where the peat layer thins under
the xerophytic forests because a low groundwater table will cause oxidation of
the biomass either by fire or by subaerial decay. Although this scenario is common
to topogenous mires, its basic principle applies to ombrogenous settings just as
much, namely, that the preservation of vegetable matter in the form of peat is
possible only when the accumulating plant material is protected from the effects
of prolonged oxidation. Since stagnant water is an efficient air lock, peat deposits
Biochemical Coalification
47
are formed preferentially in poorly drained environments with a high groundwater
table.
The influence on coal formation of the groundwater table is twofold. By
providing the moisture necessary to maintain vigorous plant growth of those floral
assemblages which have been adapted to poorly drained habitats, the water level,
firstly, affects the biological environments in which plants grow and, secondly,
it sets the biochemical conditions under which the dead vegetable matter accumulates. The position of the groundwater table therefore influences the composition
of the coal by determining which plants and parts of plants contribute to peat
accumulation, and by regulating the conditions under which vegetable matter is
converted into peat and coal.
According to Potonic (1920), dead vegetable matter is likely to undergo one
of four processes, the result of which are very different:
1. Plant debris which is exposed to atmospheric oxygen decays and the result of
this process (Verwesung after H. Potonic) are mainly carbon dioxide, water, and
some inherent plant ash. Grosse-Brauckmann (1980) refers to this process as
"mineralisation" of the biogenic matter because the residual products are inorganic constituents, i.e. minerals in sensu lato.
2. If access of atmospheric oxygen is restricted because of high moisture content
(dampness) in the depositional environment the type of decomposition of plant
material can be described as rotting, mouldering, or humification (Vermoderung
after H. Potonic), which is a slow process of converting vegetable matter into
humic colloids by hydrolytic decomposition. Fungal and bacterial action is the
part of humification which is responsible for the dark colour and acid reaction of
some soils. Since both the microbial activity and the associated oxidation involve
loss of biomass, humification is always accompanied by some mineralisation
which increases with the duration of humification.
3. When the groundwater table remains permanently high, fallen trees and other
plant debris become relatively quickly water-logged and, because of lack of
oxygen, suffer only limited humification. Under such conditions vegetable matter
undergoes peatification (Vertorfung after H. Po tonic) in which vegetable matter
is largely preserved by impregnation with humic acids in an anaerobic, reducing
environment. Depending on the contributing plants and the severity and
duration of oxidation and microbial attack of the vegetable matter before its
final burial beneath the groundwater level, different peat types are obtained.
4. Putrefaction (Fiiulnis after H. Potonic) occurs in stagnant water in which aquatic
plants (mainly algae) and redeposited debris of land plants decompose under
strongly reducing conditions. The result is an organic mud (gyttja or sapropel)
from which the various members of the oil shale family including cannel coal
(mainly spores) and boghead coal or torbanite (mainly algae) are derived.
In view of the varying position of the groundwater level relative to the peat
surface, vegetable matter may be deposited either immediately under anaerobic
conditions (high water level) or it may first accumulate in a relatively aerobic
environment (low water level) which later changes to anaerobic when the
The Coalification Process
48
groundwater table rises again, or as a consequence of further subsidence under a
thickening overburden. In the latter case the peat is affected by a changing suite
of microbes resulting in the formation of decomposition products which are
different from those formed under anaerobic conditions only. This means that any
plant debris deposited either in open but stagnant water or in areas of high
groundwater will only briefly be affected by aerobic conditions. In contrast, vegetal
matter accumulating on drier ground remains subjected to oxidising conditions
for a longer period of time until it, too, subsides into the anaerobic zone below the
groundwater table. The composition of the peat formed in the various swamp
settings relative to the position of the ground water table is therefore determined
by the contributing plant species and their modes of decomposition.
As indicated in Fig. 3.2, the position of the groundwater table undergoes many
small-scale episodic and seasonal variations but it is also governed by much larger
changes over the long time span of active peat accumulation in response to climatic
variations, changes in the rate of plant growth, and basin subsidence. Such oscillations are reflected in the stratification of peat and coals which indicate repeated
palaeo-environmental changes ranging from open water to terrestrial conditions
with their respective modes of plant growth and conversion.
In the course of coalification peat constituents become converted into components called macerals which are the basic homogeneous, microscopic, organopetrographic entities of coal. By their kind and proportions, macerals determine
"coal type", which refers to coal composition in terms of petrographic constituents.
Depending on the position of the water table in relation to the depositional
interface, different coal macerals will be formed from identical source material.
For example, woody and herbaceous cell tissues, i.e. mainly lignin and cellulose,
form the macerals of the vitrinite group under mostly reducing conditions whereas
fusinite represents a group of macerals called inertinite which were subjected to a
period of drying and oxidation during the early peat stage (Mackowsky 1953).
Littke (l985a) referred to the strong inverse proportion in Carboniferous Ruhr
coals between the two maceral groups vitrinite and inertinite in support of the notion
of their common phytogenic precursor. Figure 3.3 shows a similar correlation
80r-~--~~~--~~--~---;
70
60
y = -0.9)(+ 83.0
r = 0.958
n = 300
50
40
.... 30
Cll
'c 20
t
~ 10
~
O~~--~~--~~--~~~~~
10 20 30 40 50 60 70 80 90 100
% Vitrinite
Fig. 3.3. Diagram showing the strong
inverse relationship between the coal
maceral groups vitrinite and inertinite
in 300 seam sections and subsections of
Australian high and medium volatile
bituminous coals from the Sydney,
Gunnedah and Bowen Basins
Biochemical Coalification
49
based mainly on Australian Permian coals which is almost identical to Littke's
(1985a) Fig. 5. The strong inverse relationship between vitrinite and inertinite is
evidence for their common source material and for the existence of two genetic
tracks which can be called the vitrinitisation and fusinitisation (fusinite and
semifusinite being the main representatives ofthe inertinite group of macerals) pathways, respectively. An important feature of Fig. 3.3 is the lower maximum percentages of inertinite compared with vitrinite. It suggests that under the conditions
of fusinitisation more cell tissue is destroyed than during vitrinitisation. This leads
to a relative enrichment in partially oxidised peat of spores, pollen, cuticles, resin
bodies, and other resistant non-tissued components, including inorganic matter
(minerals).
3.1.1 The Vitrinitisation Path
Wherever vascular plants grow, humic compounds are present in the supporting
soils, their quantity being the difference between the volume of total biomass
production and the amount lost by mineralisation. As mentioned above, the latter
term is used by Grosse-Brauckmann (1980) and others in order to indicate that
the decomposition ofthe organic compounds leaves behind a residuum of inorganic
compounds, such as carbon dioxide, water and plant ash. Depending on such
factors as climate, the quantity of available vegetable matter and the position of
the groundwater table, an equilibrium exists in most mature soils between the
rates of accumulation and destruction of vegetable matter which keeps the
proportion of humus in an aerated soil relatively constant. The dynamic nature
of this equilibrium implies that the loss of organics to mineralisation is carried
not only by the vegetable matter but also by the humic substances in the soil,
i.e. as new biomass is converted into humus an equal amount of degraded humus
is eliminated (Grosse-Brauckmann 1980).
A shift in the mass balance towards the preservation of vegetable matter in
the form of peat depends on the maintenance of a continuously high groundwater
table during the biochemical stage of coalification. Ideally, the position of the
water table coincides with the ground surface such that it neither hinders the
growth of plants by drowning them, nor allows for too much oxidation, which
would result in excessive loss of organic matter and relative concentration of the
inorganic matter (mineralisation) contained in plants. The frequent variations in
the position of the groundwater table imply that some access of oxygen to the
accumulating vegetable matter is inevitable thus permitting aerobic microorganisms to initiate humification.
Humification results in the hydrolytic decomposition of dead vegetable matter
which, in accordance with Waksman and Stevens' (1929) stability series, affects
the soft cell contents first, followed by the cellulose and hemicellulose of the cell
walls and eventually the more resistant compounds, such as lignin. In an example
given by Francis (1961), the cellulose content of a peat dropped from 12.26% near
50
The Coalification Process
the surface to 11.20% at 1 m depth while the proportion of hemicellulose decreased
from 11.29% to 8.51%. This trend is in contrast to the lignin content which in the
same peat and over the same depth range showed a relative increase from 16.7%
to 28.5%. Because of the higher carbon content of lignin and its degradation
products compared with cellulose and hemicellulose the proportion of carbon in
the peat also increased with depth.
During humification, i.e. at the beginning of biochemical coalification, before
the source material is completely water-logged, both partial oxidation and aerobic
organisms, such as fungi, ray fungi (actinomycetes), insects, and aerobic bacteria
contribute a great deal to its decomposition. Depending on the intensity and/or
duration of humification, the source material becomes degraded and, eventually,
converted into carbon dioxide and water. Intermediate stages comprise a wide
range of colloidal humic substances consisting of a complex mixture of carboxylic,
phenolic and other organic acids. Advanced humification is therefore accompanied
by an increasing loss of the original plant structures. As the more or less humified
plant litter is buried under a growing overburden and subsides below the groundwater table, it becomes increasingly difficult for aerobic organisms to function,
particularly at low pH levels. The groundwater table acts therefore as a depositional
base level above which vegetal matter is destroyed in the 5- to 50-em-thick oxidation zone or acrotelm, and below which organic matter can be preserved as
peat in the reduction zone (Jacob 1961; Ingram 1983) or catotelm (Clymo 1984,
1987).
The emphasis in the above division is on groundwater table and does not
necessarily apply to the level of water above ground, which may be quite oxygenated
near the surface. An example are the peat marshes from the Fraser River delta in
British Columbia. Mainly on the basis of the contributing plants, Styan and Bustin
(1983a,b) distinguished six peat types in the marsh habitat, two of which (sedge-clay
and gyttja peats) pioneer peat accumulation on the distal lower delta plain and
are followed in upward succession by the other types. Although virtually always
covered with water, sedge-clay and gyttja peats are highly decomposed because,
being situated on the edge of the delta plain, the water is oxygenated and marine.
The significance of the last point is the elevated pH, i.e. neutral to slightly alkaline
nature of the water, which favours the destructive activity of bacteria and other
microbes. In this situation the upper limit of the reduction zone does not coincide
with the water table but is situated at some distance below the oxygenated layer
of surface water.
Depending on the nature of the source material and the duration of residence
above groundwater, different stages of humification can be observed. Based on
Sernander's (1936) "necrotisation scale", Arnborg (1943) distinguished between the
following eight stages in the decomposition of downed fir trees in the boreal
rainforests of southern Lappland:
1. Needles and bark are in place, wood is hard.
2. Needles have fallen off, small branches are in place, wood is still hard.
3. Bark is warped and has started to peel off, wood is mostly hard but decomposition has started.
Biochemical Coalification
51
Table 3.2. Estimation of the degree of peat decomposition according to the huminosity grades
(H-grades) based on the squeeze method of von Post. (After Grosse-Brauckmann 1980)
H-grades
Plant structure
in peat
Composition of material
extruded between fingers
State of residue left
after squeezing
HI
H2
H3
H4
H5
H6
H7
H8
H9
Distinct
Distinct
Distinct
Distinct
Distinct
Indistinct
Indistinct
Very indistinct
Hardly recognisable
Not recognisable
Colourless, clear water
Yellow-brown, almost clear water
Brown, distinctly turbid water
Brown, strongly turbid water
Very turbid water + some peat
Up to 1/3 of peat substance
Approx. 1/2 of peat substance
Approx. 2/3 of peat substance
Most of the peat substance
All of the peat substance
Not mushy
Not mushy
Not mushy
Not mushy
Slightly mushy
Strongly mushy
Mushy + fibrous
Resistant fibres
Minor fibres
No residue
HIO
4. Bark has fallen off and wood is mostly soft. The liverworts Ptilidium pulcherrimum and Lophozia species m. fl. have started to cover the trunk.
5. Branches have fallen off and the trunk has started to become flattened to the
ground. The lichen Dicranumfuscescens m. m. forms a thin but closed vegetation
cover over the trunk.
6. The trunk has collapsed, its wood is soft and completely decomposed. A carpet
of moss covers the trunk on which cranberries and blueberries have started to
grow.
7. The trunk has been flattened such that it extends only vaguely above the
surrounding ground. A several-centimetre-thick humus layer covers the trunk,
which is overgrown by moss and thick stands of blueberry plants.
8. The trunk has disappeared. The vegetation growing in its place does not differ
from that growing elsewhere in the surrounding soil. The remaining wood is
easily crumbled and the overlying humus layer has the same thickness as in the
neighbourhood.
Depending on the level of necrotisation reached by the time the souree material
subsides into the catotelm, i.e. below the protecting groundwater table, the resulting
peats differ in the proportion between preserved plant structures and derived humic
substances. This relationship has given rise to a number of peat classifications of
which the "huminosity scale" of von Post (drosse-Brauckmann 1980) has been
listed in Table 3.2 as an example. A huminosity grade (H-Grade) is allocated on
the basis of three observations:
1. The degree of preservation of plant structures in the peat.
2. The kind of material that extrudes between the fingers when a bed-moist peat
sample is squeezed in one hand.
3. The state of the residue after squeezing.
52
The Coalification Process
Such megascopic methods can be supplemented by microscopic point counts in
transmitted light of structured versus unstructured organics in smear samples, as
have been published by Cohen (1973), examples of which are given in Table 2.2.
Alternatively, a variety of chemical methods have been devised which seek to assess
the degree of plant decomposition (i.e. humification) by determining the proportion
between humic substances and residual plant tissues and other vegetable matter
in the peat.
Although it has been mentioned above that vegetable matter can be preserved
once it has become water-logged, biochemical coalification does not stop altogether
below the groundwater table. Under conditions of total absence of atmospheric
oxygen, the vegetable matter is subjected to further physical and chemical changes
by the action of anaerobic bacteria which extract and utilise oxygen from the
organic molecules of the vegetable matter. This action precludes any selective
destruction of plant tissues but, as has been pointed out by van Krevelen (1952,
p. 359), "in the absence of free oxygen, organic compounds containing the most
oxygen are preferentially attacked by anaerobic bacteria." This affects the rather
resistant but oxygen-rich compounds, like lignin, as much as soft tissues, although
it may proceed at a greater rate in the latter. In this context Clymo's (1987) reference
to an 11 m-thick peat deposit on Beauchene Island in the South Atlantic is of
interest. The peat has been derived entirely from the tussock grass Poa jlabellata
(Lewis Smith and Clymo 1984) which near the surface is relatively uncompacted
and contains visible remnants of plant material. With increasing depth the texture
becomes denser such that at its base the peat appears homogenised and shows
conchoidal fracture when dry. This form of gelification results from the
continuation of biochemical processes and is referred to as biochemical gelification
by Stach et al. (1982). Although this process of obliterating cell structure has some
superficial similarity with the formation of humus colloids during humification, it
is not the same and leads to different results. Under the conditions of prolonged
humification the softening of wood tissue (beginning at necrotisation stage 4) is
merely a transitional phase towards its complete disintegration and eventual
elimination by mineralisation. Conversely, in the case of anaerobic biochemical
gelification of non- or only mildly prehumified tissue the swelling of the cell walls
close the cell lumens such that the tissue loses definition but it does not disintegrate
and remains a coherent entity in which relic cell tissue remains.
The partial extraction of oxygen from vegetable matter by anaerobic bacteria
causes a high concentration of hydrogen in the remaining humic substances part
of which is released as marsh gas in the form of methane (Clymo 1984; Svenssop
and Rosswell 1984) while another portion is absorbed by humic colloids. Their
capacity to absorb H2 is partly responsible for the relatively high H/C ratio of
unstructured gelified coal components compared with those that have retained \
cell structure.
According to Grosse-Brauckmann (1980), the most active anaerobic bacteria
are situated close to the groundwater table, i.e. not far below the aerated acrotelm
or humification zone, whereas high bacteria counts in deeper levels of a peat
deposit usually incorporate a large number of non-vital individuals. This means
that the degree of anaerobic biochemical gelification depends not only on the
53
Biochemical Coalification
__ .. ·.1 MINERAIlSAllONI
INfENSfIY OF 1HUMIFICAllON 1
.~
_ _, . . . _ . . , ._ _. . . . ,_ _ _ _, .• • • _ _ _ _ • • • • • • • • •
(mainlycellube)
I
I
I
WOODY TlSSUES
(mainlya:llube
uullignin)
I
I
I
I
'Ii
. :I
I
l. ..... . f""
' ,
.;
VV'VVVV
~
~
~
State of preservation
of vegetable matter cell tissues cell tissues llcell tissues disintegrated humus~
as deposited below
intact
mildly affected collapsed cell fragments colloids
ground water t a b l e !
! !
!
II
Macerals formed
in brown coal
after compaction
and dehydration
of peat
Macerals formed
in bituminous coal
after gelification
and polymerisation
of humic colloids
TEXTINITE
TEXTOULMINITE
u
p
PORI-. EUGELINITE
HUMOCOLLINlTE
EU·
ULMINITE
HUMOTELINITE
..
...0
G
.:
G
I
ATTRINITE DENSINITE
HUMODETRINITE I
+
TELINITE
I TELOCOLLINITE DESMOCOLLINITE g6k~O_COLLINITE
TELOVITRINlTE
. DETROVITRINITE
GELOVITRINITE
Fig. 3.4. Schematic outline of the formation of macerals of the vitrinite group from tissued
vegetable matter subjected to varying intensities of humification
source material but also on the rate at which the plant material passes through
the active upper portion of the catotelm before reaching its inactive lower realm.
This residence time is a function of the rate at which the groundwater table rises
which is often dependent on the rate of basin subsidence.
In Fig. 3.4 an outline is given of the kind of coal macerals that are derived
from tissued vegetable matter which has been subjected to varying degrees of
humification followed by submergence below the ground water level. The soft
tissues of herbaceous plants have been listed separately from the more resistant
tissue of wood (the same applies to bark) becallse they decompose faster and enter
the mineralisation stage earlier than the former. Evidence of such varying responses
of different tissue types to humification is, for example, provided by the differential
colouring found by Cohen et al. (1987) in microtome sections of peat. Although
the degree of decomposition was generally low, cellulose-rich tissues appeared
quite stained (brown) and had lost much of the characteristic cellulose birefringence
in polarised light, while lignin-rich cell walls showed hardly any staining at all.
Styan and Bustin (1983a) likewise point to the lack of lignin and high cellulose
content as a major reason for the high degree of decomposition found in the
sedge-grass peats of British Columbia.
Stability differences also exist between different kinds of wood. Most
angiosperm wood decays faster than gymnosperm wood, which explains the high
amounts ofhumodetrinite found in angiosperm-derived portions of Tertiary brown
54
The Coalification Process
Fig. 3.5. Photomicrograph showing transition from textinite to textoulminite in a Jurassic coal
from the Perth Basin. Note the plastic deformation of the cell walls and the beginning of closure
of the lumens. Incident light, oil immersion; actual length of field of view = 0.36 mm
Fig. 3.6. Photomicrograph of etched telocollinite from the Bulli Seam, Sydney Basin, New South
Wales, illustrating the dense packing of cell walls due to the collapse of the intervening lumens.
Without etching the polished surface of the telocollinite is without any distinguishing features.
Incident light, oil immersion; actual length of field of view = 0.31 mm
coals (von der Brelie and Wolf 1981a, b). It is also suggested that soft-tis sued plants
will probably not leave behind any completely intact cell material because the
largely non-humified, structured plant residues found in peat and brown coal (the
maceral textinite) consist invariably of bark or wood fragments, mostly from roots
because of the protection provided by the ground cover.
Biochemical Coalification
55
With increasing degree of humification the cell walls begin to swell, mainly
due to the hydration and incipient hydrolysis of their cellulose content. This leads
to plastic deformation, a brown coal example of which is shown in Fig. 3.5.
Megascopically, the result of this is the softening referred to under necrotisation
stage 4, and it equates with the fusion of the cell wall layers, as well as closure of
the intracellular spaces, described by Cohen and Spackman (1980). Using etched
bituminous coal as an example, completely closed cell lumens are illustrated in
F~g. 3.6. Further humification leads to disintegration of the cell tissue due to its
hydrolytic decomposition and the generation of fluid humus colloids. The process
is illustrated in Fig. 3.7, which shows the generation of droplets of fluid humic
material from the swollen cell walls of present day humified wood (necrotisation
stage 4 to 5). A coalified equivalent of this hydrolytic decomposition of cell tissue
is illustrated in Fig. 3.8, which shows residual patches of vitrinitic wood tissue
"floating" in completely decomposed, colloidal vitrinite.
The humic fluids generated during humification, have acid character (= humic
acids) and occur in peat as a fine aqueous dispersion which can flow and fill the
gaps between the solids. A coalified example of colloidal vitrinite with flow texture
is illustrated in Fig. 3.9. On compaction and dehydration of the peat due to an
increasing weight of overburden, the fluid humic substances coagulate and form
dopplerite which in brown coal is referred to as humocollinite (see Fig. 3.4) when
derived from relatively pure colloids, or as humodetrinite (see Fig. 3.4) when mixed
with disintegrated residual cell fragments. Some of the small egg-shaped vitrinite
bodies called corpocollinite in Fig. 3.4 have probably been formed in this manner.
Some examples are illustrated in Fig. 3.10.
Fig. 3.7. Photomicrograph of partially humified wood from a rainforest near Fingal in NE
Tasmania. The swollen cell walls in the centre of the illustration are dissolved by cellulose
hydrolysis to form humus colloids, droplets of which occur within the cell lumens (grey spheres
and hemispheres) and attached to some cell walls. Incident light, oil inmersion; actual length of
field of view = 0.18 mm
56
The Coalification Process
Fig. 3.8. Photomicrograph of etched vitrinite from the Katharina Seam, Ruhr Basin, Germany,
illustrating the disintegration of cell tissue during humification. Oxidative etching (with acidified
potassium permanganate) has removed the hydrogen-rich colloidal vitrinite but left residual
tissue fragments intact. Without etching the polished surface of the vitrinite would be uniformly
grey without any distinguishing features. Incident light, oil immersion; actual length of field of
view = 0.6 mm (Diessel 1961)
Fig. 3.9. Photomicrograph of etched vitrinite from the Katharina Seam, Ruhr Basin, Germany.
A fragment of cell tissue is still visible in the upper portion but the majority of cells have been
converted into humus colloids. The (formerly) fluid state is indicated by the flow texture in the
centre of the illustration. Incident light, oil immersion; actual length of field of view = 0.6 mm.
(Diessel 1961)
Biochemical Coalification
57
Fig. 3.10. Photomicrograph of corpocollinite (grey oblong bodies mainly in
upper portion of illustration) set in
detrovitrinite (grey groundmass) and
other maceral debris in high volatile
bituminous coal from the Gunnedah
Basin, New South Wales. Incident light,
oil immersion; actual length of field of
view = 0.22 mm
Humus colloids can also migrate into the cell lumens of other plant tissues
where they solidify and, after coalification to bituminous coal rank, appear as
gelocollinite (see Fig. 3.4), an example of which is shown in Fig. 3.11. After
precipitation humic colloids may initially be granular (listed as porigelinite in Fig.
3.4) but then gelify to form a clear substance (listed as eugelinite in Fig. 3.4).
Spackman and Barghoorn (1966) and Cohen et at. (1987) have described similar
material from Recent peat deposits.
The huminite macerals listed in Fig. 3.4 under the subgroups humotelinite,
humodetrinite, and humocollinite are the precursors of the members of the vitrinite
group of macerals, before polymerisation and subsequent condensation (geochemical
gelification of Stach et at. 1982) have obliterated much ofthe original cell structure.
For an outline of the correlation between huminite and vitrinite macerals see
International Committee for Coal Petrology (1971) or Stach et at. (1982). According
to Australian Standard 2856 (1986), no terminological difference is made between
pre- and post-polymerisation macerals. Following Smith (1982), the maceral group
term vitrinite is therefore applied to both brown and black coals and is further
divided into the subgroups telo-, detro-, and gelovitrinite the derivation of which
is illustrated in Fig. 3.4. This aspect will be discussed further in Chap. 4.
58
The Coalification Process
Fig.3.11. Photomicrograph of gelocollinite (grey) filling cell lumens and gaps
between cell walls of semifusinite (light
. grey) in high volatile bituminous coal
from the Sydney Basin, New South
Wales. Incident light, oil immersion;
actual length offield of view = 0.36mm
3.1.2 The Fusinitisation Path
Although the majority of peat and coal constituents have followed the vitrinitisation
path of biochemical coalification, most coal seams contain small and some even
large amounts of tissue-derived inertinite macerals whose high carbon and
relatively high oxygen contents suggest that the more or less humified plant
material suffered a period of intense desiccation and oxidation before final burial.
A rather extreme example is the widespread but commonly dispersed and rarely
concentrated maceral fusinite (Cook 1975a), which shows a high degree of
preservation of cell tissue (except for some mechanical breakage), reflectance, and
polishing relief. Most fusinite consists of fossil charcoal formed by incomplete
combustion of wood (pyrofusinite) either before its incorporation into the peat
or as a result of peat fires during dry periods. Examples of both modes of charcoal
formation have been reported by Cohen (1974) from the Okefenokee Swamp in
Georgia, U.S.A.
According to Cohen et al. (1987), woody plants are more likely to produce
charcoal than soft-tissued herbaceous plants. Furthermore, charcoal (or pyrofusinite in coal) formed from burning vegetation will be dispersed by wind and water
Biochemical Coalification
59
in the form of relatively small fragments and in such a manner that on sedimentation
it will be incorporated in a normal, i.e. unbumt, peat matrix (or detrovitrinite in
coal). An example of this is illustrated in Fig. 3.12. Conversely, the combustion of
dehydrated peat will, on resumption of peat accumulation, result in a discrete
band of charcoal which will form a fusinite-rich layer in coal. An example of this
is illustrated in Fig. 3.13.
Because the dehydrated, charred and commonly thinned cell walls of fusinite
are composed of almost" pure carbon, they are rather inert and resist any
post-depositional alteration by anaerobic bacteria (e.g. gelification) that might
affect less stable macerals. However, between the gelified and low reflecting vitrinite
and charcoal in the form of pyrofusinite exist several coal components, all
belonging to the inertinite group of macerals, which possess lower HIC and higher
OIC ratios than vitrinite macerals without reaching the extreme values of fusinite.
These are grouped under the term semifusinite because they have been derived
from cell tissue which was subjected to partial fusinitisation only. Under the
microscope their lower carbon content is shown by their lower reflectance
compared with fusinite, their lower polishing relief and less well-defined cell
structure, as illustrated in Fig. 3.13.
There are probably three main modes of semifusinite origin. The course of the
fusinitisation path and its products as outlined in Fig. 3.14 suggests that desiccation
of the hydrated cell walls and the dehydration of the humus colloids altered them
so profoundly that they could not rehydrate and continue to hydrolyse when the
Fig. 3.12. Photomicrograph of a bituminous coal from the Sydney Basin,
New South Wales, showing dispersed
fusinite fragments (white with high
polishing relief) in a matrix of detrovitrinite (desmocollinite, grey). The black
material near the fusinite splinters is
clay. Incident light, oil immersion; actual
length of field of view = 0.34 mm
60
The Coalification Process
Fig. 3.13. Photomicrograph of part of a
layer of concentrated fusinite (white, high
relief) and semifusinite (light grey) from
a high volatile bituminous coal of the
Bowen Basin, Queensland. The smooth,
medium grey material consists of vitrinite
incorporating some inertodetrinite.lncident light, oil immersion; actual length
of field of view = 0.36 mm
moist conditions of normal humification returned, or when the dehydrated humic
substances were buried below the groundwater table (Dormans et al. 1957; Zanger!
and Richardson 1963; Teichmiiller and Teichmiiller 1979b). This notion is
supported by the observation by Cohen et al. (1987) of humified cell tissues in a
Recent peat sample which were transparent in transmitted light but became opaque
after desiccation and oxidation. The resulting semifusinite (after further coalification to at least subbituminous coal rank) can be called oxi-semifusinite, although
the initial humification was affected with the aid of microbes as well.
The second mode of semifusinite formation is related to a more direct
intervention by organisms leading to selective decomposition of mainly wood
tissues. Examples are fungal attack (Fig. 3.15) and the feeding traces of insects and
termites. All of these affect soft tissues and parts thereof more than, for example,
cell walls that have been strengthened by lignin. The extraction of hydrocarbons
from the plant tissue leaves behind a cell skeleton which is relatively enriched in
carbon and has a higher O/H ratio than the original vegetable matter. The result
is a degrado-semifusinite, an example of which (with attached fungal tissue) is
shown in Fig. 3.16.
The third mode of semifusinite genesis is due to incomplete combustion as in
the formation of pyrofusinite. However, in this case, the source material was,
Biochemical Coalification
61
INTENSITY OF 1HUMIFICATIONI-I- - - - - - , - - - ...1MINERAIlSATIONI
SOFT T.I$UES
-
r---.....---r----- ....... -......
__
.~
,Q
I
••
I
(~oill~) -~==~~~~~I~:::LI=====1
Ii
~~
I
I
--1'"
g
ill
f
""
INTERVAL OF DEHYDRATION AND OX IDA TION
Macerals funned
in brown coal
after compaction
and dehydration
of peat
~
TEXTINITE ~ :EI_-------,..---------l
eli:':
~1l
INERTODETRINITE
!::~
~Z
~----_4~~I-----~-------------------------4
Macerals funned
TELINITE f-.::::J
with inc:reasingrefiectance
in bituminous coal L ____---1---.:I>.::.L________________________________...1
Fig. 3.14. Schematic outline of the formation of coal macerals from tis sued vegetable matter
subjected to varying intensities of humification and drying before burial below the groundwater
table
firstly, wood at an advanced stage of humification and, secondly, at the time of
burning the partially humified wood was still moist enough to be only mildly
affected by the combustion. In the case of partially humified wood of necrotisation
stage 4 and above, the outward sign of drying is the development of tangentional,
longitudinal and transverse desiccation cracks which give the wood a blocky
appearance, as shown in Fig. 3.17. More severe fissuring is shown by the wood
illustrated in Fig. 3.18 because it was more humified and its surface has been
signed by a forest fire. The three photomicrographs of this wood illustrated in Fig.
3.19 reveal considerable reflectance differences of the cell tissue at different depth
levels below the surface. A traverse of reflectance measurements, the results of
which are illustrated in Fig. 3.20, indicate a 3- to 4-mm-thick surface layer offusinite
which changes to semifusinite for another 5 to 6 mm, followed by partially humified
wood without any signs of heat effects from about 13 mm below the surface.
The blocky desiccation pattern illustrated in Figs. 3.17 and 3.18 is also found
in wood retrieved from peat (Fig. 2.7) and brown coal, and it is identical to the
blocky pavement structure that is commonly observed on the surfaces of fusinite
and semifusinite layers which have been split parallel to bedding. An example of
this is shown in Fig. 3.21.
The interval of dehydration and oxidation inferred in Fig.3.14 may have several
causes. In most cases it would be related to a lowering of the water table although
62
The Coalification Process
Fig.3.1S. Downed tree showing fungal
attack in swampland of coastal New
South Wales
Fig.3.16. Photomicrograph of degrado-semifusinite in the Katharina Seam of the Ruhr Basin,
with attached fungal tissue. Incident light, oil immersion; actual length of field of view = 0.6 mm.
(Diessel 196 I )
Biochemical Coalification
63
Fig. 3.17. Photograph of part of a dried tree trunk of necrotisation stage 4 showing longitudinal
and transverse desiccation fissures. Sampled in rain-forest in NE Tasmania
Fig. 3.18. Similar to Fig. 3.16 but with stronger fissuring and charring by forest fire
the influx of aerated surface water, as suggested by Gould and Shibaoka (1980)
and Beeston (1982), could bring about the same result. Warm temperatures are
likely to accelerate desiccation but freeze-drying, as suggested by Taylor et al.
(1989) for some Gondwana coals, could have a similar effect. In this case it has
been envisaged that plant detritus becomes partially or totally humified during
the summer months. Such material, if incorporated into a rapidly accumulating,
water-saturated peat without prior desiccation, would later be represented as
64
The Coalification Process
Fig. 3.19. Three photomicrographs through the outer 10 mm of one of the charred desiccation
blocks illustrated in Fig. 3.18. The upper frame shows the surface of the charcoal, the centre frame
has been taken from 5 mm below the surface and the bottom frame is from 10 mm below the
surface. Note the decrease in reflectance from top to bottom and the increase in cell wall thickness.
Incident light, oil immersion; actual length of each field of view = 0.36 mm
65
Biochemical Coalification
3_5
•
3.0
•
2.5
•
2.0
•
••
1.5
1.0
,g 0.5
l-
Fig. 3.20. Diagram showing the decrease in reflectance with depth below
surface of charred cell walls from the
specimen illustrated in Figs. 3.18 and 3.19
~
0
0
2
4
6
••
8
••
10
••
12
14
16
Depth below surface (mm)
Fig.3.21. View of a bedding plane in the Balmoral Seam of New South Wales exhibiting pavement
texture due to a concentration of blocky fusinite and semifusinite (emphasised with inked rims)
following humification of Permian wood to stage 4 to 5 of Arnborg (1943)
vitrinite in accordance with the scheme outlined in Fig. 3.4. Shrivelled humic
material which had been freeze-dried as a consequence of exposure to persistent
sub-zero temperature in the winter months would lose the capacity to hydrolyse
completely once biochemical activities resumed in the following spring. Yet, the
resulting inertinite might not be chemically altered as much as hot dried humic
material, which could account for the considerable fusibility and other forms of
chemical activity of Gondwana inertinite (DiesseI1983, 1985a; Diessel and McHugh
1986; Diessel and Wolff-Fischer 1986, 1987; Brown et al. 1985a, b).
Irrespective of the reason for the desiccation, it seems essential that the
vegetable matter be more or less humified in order to follow the fusinitisation path,
except in the case of charcoal, which can be formed directly from unaltered plant
material. The humification scheme outlined in Fig. 3.14 is therefore identical to
66
The Coalification Process
Fig. 3.22. Photomicrograph of telinite
in a high volatile bituminous coal from
New South Wales. Note the regular
pattern of resin bodies (dark grey), which
indicates that the wood tissue is still
intact in spite of the gelified appearance
of the unetched surface. Incident light,
oil immersion; actual length of field of
view = 0.56 mm
that of Fig. 3.4. Unhumified wood leads to the formation of firstly textinite (Fig.
3.5) and later telinite (Fig. 3.22) because wood can be rehydrated and gelified but
humic substances cannot once they have been dried. Wood unaffected by humification (necrotisation stages 1 and 2) forms therefore either charcoal, i.e. pyrofusinite
after coalification, when subjected to incomplete combustion during forest fires,
or it becomes telinite when unaffected by atmospheric oxygen. Telinite formation
also applies to the roots and rhizomes of the vegetation covering the peat surface.
Given the wet substratum of the mire, atmospheric oxygen has little access to
these subsurface organs, which provide the largest proportion of wood in many
present-day forest peat deposits (Cohen and Spackman 1972; Grosse-Brauckmann
1980). Because of either complete absence or low levels of humification, the cell
tissues of the roots are commonly quite intact but become gelified and thoroughly
impregnated with humic acids thus leading straight to the formation of telinite
when coal rank increases. The frequently observed transitions between telinite and
semifusinite (Fig. 3.23) indicate therefore the depth of penetration of humification
into the wood before the onset of desiccation and its final burial in the peat.
With increasing humification (necrotisation stages 4 and 5) any intense or
repeated desiccation will cause severe dehydration and shrinkage of the partially
softened wood which has been stripped of most of its cellulose and hemicellulose
by hydrolytic splitting. The dried surfaces of the remaining wood break up into
Biochemical Coalification
67
Fig. 3.23. Photomicrograph illustrating the transition between semifusinite (centre) and telinite
(top) in the Katharina Seam, Ruhr Basin. Incident light, oil immersion; actual length of field of
view = O.6mm. (Diessel 1961)
the small cubes or rectangular plates illustrated in Figs. 2.7, 3.17, 3.18 and 3.21 at
various stages of their development. After they have become separated from the
trunk they can be picked up by flood waters and deposited as allochthonous debris
in water-filled depressions.
Dehydration and oxidation of partially decomposed cell fragments and humus
colloids formed as a result of advanced humification (necrotisation stage 6) leads
to concentrations of either inertodetrinite or macrinite. The former, which is
illustrated in Fig. 3.24, consists of remnants of plant tissue, mainly in the form of
cell fragments of fusinite and semifusinite. The second maceral, macrinite, has a
complex origin which includes colloid formation by complete homogenisation of
plant tissue followed by drying and oxidation. Two varieties of macrinite can be
distinguished, one which consists of detrital angular to rounded bodies (Fig. 3.25),
many of them small enough to be grouped as inertodetrinite (Taylor et al. 1989).
This "corpomacrinite" is probably the result of desiccation of humic colloids
(angular) followed by dispersal (rounded). The other variety, illustrated in Fig.
3.26, occurs as elongated bands or laminae ("lammacrinite") probably representing
flocculated humic groundmass which would have formed desmocollinite had the
peat surface not been dried out.
The differential rate between the humification of wood and soft tissues leaves
its signature on the respective coal macerals as well. Wood-derived semifusinite
displays better-preserved plant cells than, for example, leaf-derived (mesophyll)
semifusinite. The latter is less frequent but seems to be more common in Gondwana
coals than in the Carboniferous coals of the Northern Hemisphere, which is
68
The Coalification Process
Fig. 3.24. Photomicrograph of the Greta Seam, New South Wales, illustrating inertodetrinite
(white fragments) in a matrix of detrovitrinite (grey to dark grey) together with micros pores (black.
elongated) and micrinite (very small white specks). Incident light, oil immersion; actual length of
field of view = 0.22 mm
probably due to the deciduous nature of the Permian cold climate Gondwana
flora (Diessel 1983~ Taylor et al. 1989). The soft mesophyll cells disintegrate more
easily than wood cells, which is the reason for the occurrence of transitions between
leaf-derived semifusinite and macrinite, as illustrated in Fig. 3.27.
3.1.3 Plant-Specific Coal Components
Whereas all peat and coal components mentioned above can be derived from a
wide range of plants tissues, there are other constituents of coal which are related
to specific plants or parts of plants. Most of these microcomponents constitute
the liptinite group of macerals which, according to Teichmiiller (1974), is characterised by very high hydrogen contents. Their atomic Hie ratio is therefore higher
than in all other macerals. In thin section liptinite macerals show a high degree
Biochemical Coalification
69
Fig. 3.25. Photomicrograph of the Bayswater Seam showing macrinite in the
form of oxidised patches of humic
colloids ("corpomacrinite", two examples with differing reflectance in lower
centre) together with inertodetrinite
(light grey to white angular cell wall
fragments) and clay (black). Note the
imbrication of many elongated particles
which indicate transport from right to
left. Incident light, oil immersion; actual
length of field of view = 0.22 mm
Fig. 3.26. Photomicrograph of layered macrinite ("Iammacrinite", light grey in centre) in a high
volatile bituminous coal from the Bowen Basin, Queensland, set in detrovitrinite (grey matrix),
semifusinite (light grey, near top) and inertodetrinite (white fragments at bottom). Incident light,
oil immersion; actual length of field of view = 0.22 mm
70
The Coalification Process
Fig. 3.27. Photomicrograph of a high
volatile bituminous coal from the Bowen
Basin, Queensland, showing gradation
between semifusinite and macrinite in
the two light grey layers in centre. Other
macerals include the matrix of detrovitrinite (grey), some inertodetrinite (white
fragments) and dark, elongated microspores. Incident light, oil immersion;
actual length of field of view = 0.28 mm
of translucency in low rank with a concomitant low reflectance when viewed in
incident light. The algal matter illustrated in Fig. 2.15 is a good example of the
contrasting appearance oflow carbon constituents in transmitted and incident light,
respectively. Another common optical characteristic is the strong microfluorescence
which is emitted after excitation with short wave radiation (ultra-violet to blue
light).
Although the proportion of liptinite is small in most coals and rarely exceeds
20%, its high hydrogen content influences the technological properties of coal
while some of its members have considerable palaeo-environmental significance,
as will be discussed in subsequent chapters.
A large proportion of the liptinite group of macerals consists of remnants of
the waxy protective cover (cuticle) of leaves and young shoots, and of the resistant skins (exines) of spores and pollen. As has been discussed in Chap. 2, Carboniferous coals are often spore-rich because at the time of their formation, sporebearing plants (pteridophytes) were dominant. According to Alpern (1960) 1 g of
Carboniferous coal contains in average five million spores. The reason for the
large number is that reproduction by means of spores is not particularly effective
thus necessitating the production of large quantities of spores in order to assure
the survival of the species. As pteridophytes were largely replaced by seed-bearing
Physico-Chemical Coalification
71
gymnosperms, spore production declined in post-Carboniferous peatlands without
being replaced by a matching quantity of pollen.
Other members of the liptinite group of macerals consist of derivatives of plant
resins and waxes. These and further constituents will be discussed in detail in
Chap. 4, where a comprehensive classification of all the petrographic components
of coal is given.
3.2 Physico-Chemical Coalification
Biochemical coalification begins with the accumulation of dead vegetable matter
and ends with the polymerisation of humic colloids at the rank of sub-bituminous
coal. The interaction of its many different physical, chemical and biological
processes leads to the development of a diverse range of degradation products of
vegetable matter which appear as macerals in coal. The biochemical stage of
coalification is therefore characterised by a divergence of maceral properties which,
from the same source material, can produce a fusinite consisting of almost pure
carbon and a vitrinite containing only 79% carbon (Mackowsky 1953).
The subsequent physico-chemical stage of coalification is initiated and maintained by deep post-depositional subsidence ofthe seam, which causes both temperature and pressure to rise in response to the local heat flow and a growing thickness
of overburden. Microbial activity ceases when the temperature approaches the
boiling point for water, and since all components contained in a seam are affected
by the same PT conditions, maceral properties begin to converge, in the course
of which the initial chemical and physical differences between the various coal
2·0
©
Fig. 3.28. The atomic HIC versus
OIC diagram after van Krevelen Hi
(1952 and 1961) illustrating the ~
convergence of maceral composition during the physicochemical
stage of coalification. A algal matter; C cellulose; E spore exines
and other Iiptinite; F inertinite; L
lignin; V vitrinite; W wood
0·0 +----r--.-----r-.,--r--.-r---r-,--,
0·0 0'1 0·2 0·3 0'4 0·5 0·6 0·7 0·8 0·9 1'0
The Coalification Process
72
macerals are cancelled. Results and some consequences of this trend are illustrated
in Fig. 3.28 by van Krevelen's (1952, 1961) H/C versus O/C diagram, or by the
successive alignment of the reflectance of the various coal macerals with that of
fusinite as illustrated in Fig. 3.29.
The H/C versus O/C diagram suggests that the latent chemical difference
between vitrinite and inertinite manifests itself only during physicochemical coalification. This does not hold for pyrofusinite, which is fully developed already in
peat but the notion is correct for a large portion of low and medium reflecting
inertinite, in particular semifusinite and macrinite. Before polymerisation of the
humic acids, these are often indistinguishable from huminite ( = vitrinite in black
coal) but Smith and Cook (1980) have shown that during the transition from
brown to sub-bituminous coal the reflectance of many inertinite macerals increases
more rapidly than that of either vitrinite or liptinite. The result is that only in
sub-bituminous and higher rank coals low to medium reflecting inertinite is readily
recognisable and follows a separate pathway (Fig. 3.29) until all coal properties
converge towards anthracite. Teichmiiller (1987) refers therefore to "inertinitisation" during coalification, but it should be realised that it was the dehydration
of partially humified plant tissue in the mire that initiated this process.
Also at the high rank end of coalification the convergence of maceral reflectance
can be more complex in detail than depicted in the general trend illustrated in
Fig. 3.29. Vitrinite and liptinite, for exam pie, acquire a stronger bireflectance than
inertinite which pushes their maximum reflectance a little higher. These physical
changes are largely the result of the relative increase in carbon which is affected
by a release of mainly oxygen and hydrogen due to thermal cracking. This trend
is illustrated in Fig. 3.30 which is Callcott's (1986) improved and extended version
of Seyler's Chart (1931, 1938). The area enclosed by the thin line connecting the
point of origin (0% Hand 0% C) with the coordinates for methane (CH 4 = 74.87%
I
I-n,mt-€
HERBACEOUS
PLANTS
;r
W'ORES,roll.EN,
CUTICLES, RESINS,
ALGAE, and WAXI.S
FUSINITE
.... 1-'"' i--"~
SEMIFUSINITE
MACRINITE
LlPTINITE
Cl>
u
c:
.... ..-:::~
VITRINITE
"'""'"
... --
...,'"
1.6
Cl>
;;:
0.8
ct
0.4
'"
0.2
u
~ ~"
Cl>
X
1:
~
Biochemical
Coalification
Peatl Brown Coal
80
85
6.4
~ 3.2
0.1
90 % Corban
(whole co,l)
Physi cochemi ca I Coalification
Sub-Bitum. Coal HV-MV-L V- Bitum. Coal
Anthrac.
Fig. 3.29. Schematic diagram illustrating the convergence of maceral reflectance during physicochemical coalification
73
Physico-Chemical Coalification
26 r--r-.,.--.,--r--,..--r--r---,r---r---,
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
C 9
E 8
Fig. 3.30. Seyler's Chart,
modified and extended by
Ca\1cott (1986), illustrating
coalification trends for
vitrinite, inertinite and
liptinite. The squares to the
upper left of the vitrinite
line are plots of peat
samples. See text for
explanations
.§ 7
'-' 6
a5 5
8' 4
is 3
>- 2
:c
1
......- ........K.'--......-
~OL..
o
10 20 30 40
:g CARBON (dmmf)
.....- ' - -......_
....._'--....JII
50
80
60
70
90 100
C and 25.13% H) and then down to 100% C and 0% H, includes the elemental
compositions of all organic C-H-O compounds, ignoring minor elements, such as
nitrogen and sulphur. Oxygen is not plotted in the diagram but is obtained by
difference, as
o = 100 - (C + H).
(3.1)
The fluid coalification products consisting of the end-members carbon dioxide,
water and methane, form the apices of a triangle, Callcott's (1986) "Effiuent
Triangle", which contains the elemental composition of the mixtures of all liquids
and gases generated during coalification. The centre line of the "bright coal band"
in Seyler' Chart is represented in Fig. 3.30 by the vitrinite line for coals ranging
in carbon content from 64% (brown coal) to 95% (anthracite) while development
lines for inertinite and liptinite are indicated separately. Peat composition (open
squares) is also plotted separately because of the difficulty of identifying and
analysing pure maceral precursors. Their position above the low rank extension
ofthe vitrinite line is due to their high liptinite contents and still immature inertinite.
In the early stages of physicochemical coalification, the release of effiuent fluids
is mainly by decarboxylation, carbon dioxide and water being the end-products,
while the generation of hydrogen-bearing fluids (e.g. by demethanation) is limited
in such a manner that hydrogen in the residual coal remains constant up to
The Coalification Process
74
the rank of medium volatile bituminous coal of approximately 80% C. The vitrinite
line remains therefore fixed at 5.35% H, which is also the hydrogen content of the
mixture of vitrinite-generated effiuent gases and liquids whose bulk composition
ranges between A and B in the Effiuent Triangle. As coalification proceeds into
high rank coals, i.e. into the curved portion of the vitrinite line, effiuent composition
can be assessed for each rank increment by extending a tangent to the vitrinite
line into the Effiuent Triangle. Examples given in Fig. 3.30 are the F-F' track for
vitrinite with 88% C (low volatile bituminous coal) and the C-D track for vitrinite
of 92% C (semi-anthracite). It follows that advanced coalification is characterised
by an increase of the CH 4 /CO Z ratio in the effiuent, i.e. by increasing demethanation.
3.2.1 The Concept of Coal Rank
Whereas the development of the organo-petrographic components of coal refers
to the concept of coal type, the various physical and chemical changes they undergo
during subsequent coalification constitute the basis for the concept of coal rank.
The latter indicates how far a coal has progressed along the coalification path,
whereby names such as those listed in Fig. 3.29 (and others) are used in order to
indicate different rank stages. Being a concept rather than a property, rank cannot
be measured, but it can be assessed by means of those physical and chemical coal
properties which change most during coalification. The following discussion gives
a brief overview of the most important rank parameters (see also Fig. 3.31).
RANK
STAGES
" In Situ
Mojsture
~ Vitrinite
ReflectHnce
rendom
mox
~ >65 ~
WOOD
PEAT
60 \
BROWN
COAL
SUBBITUMINOUS
COAL
HIGH
% Volatile
Matter (dall
71 \
80 \
<fl
86
~ e:~-J
MEDIUM -<:c <:3 90
~a::Ju
lOW
>~
91
UJ:l
SEMIANTHRACITE
ANTHRACITE
>60 '14.7 \
75 0.20 0.20
52 23.0\
30 0.40 0.42
40 33.5\
5 0.60 0.63
\ 31 35.6 \ 3 0.97 1.03
36.0 \ <1 1.49 1.58
\14 36.4 \ 1 1.85 1.97
\22
92
\ 8 36.0
95
-~ 35.2 ~
" Carbon
(dall
\ 1 2.65 2.83
Speciflc
Energy (gross
in MJ/Kg)
6.55 7.00
Fig. 3.31. Some rank parameters showing the changing pattern of coal composition with increasing coalification
Physico-Chemical Coalification
75
3.2.1.1 Solubility in Alkali Hydroxides
A large portion of the humic acids contained in soils, peat and brown coal can
be readily dissolved in diluted alkalis, from which a pitch-like precipitate can be
obtained on acidification. Polymerisation of humic substances occurs at the beginning of the physico-chemical stage of coalification, which leads to the formation
of macromolecules of essentially aromatic and aliphatic habit. The disappearance
of humic acid in sub-bituminous coal is indicated by the loss of its solubility in
alkalis, for example in diluted KOH, which marks the beginning of physicochemical coalification and is taken in some coal classification systems as the natural
boundary between brown and black (hard) coal.
3.2.1.2 Moisture Content and Specific Energy
Both moisture content and specific energy (calorific value) are good indicators of
the degree of coalification in brown and sub-bituminous coals because they lose
bed moisture quite rapidly during the early part of coalification due to increasing
compaction with increasing overburden pressure in relation to depth of burial. As
water is lost from the coal, the specific energy must rise because less heat is required
to drive out the remaining moisture during combustion. When in the course of
coalification the moisture content stabilises at around 1%, it loses its significance
as a rank parameter as does specific energy, as shown in Fig. 3.31.
3.2.1.3 Volatile Matter and Fixed Carbon
Once humil. substances have polymerised at the beginning of the physicochemical
stage, any subsequent coalification is characterised by so-called condensation
reactions (Karweil 1966). Like polymerisation, they also create macromolecules
and lead towards the orderly stacking of aromatic ring systems (micelles) which
are both separated and connected by aliphatic and other non-aromatic compounds
(side chains). As illustrated in Fig. 3.32, the aromatic clusters grow and coalesce
with increasing coal rank at the expense of the surrounding non-aromatic
compounds. The result is effiuent generation which, as has been discussed above,
is sustained initially by oxygen fugacity but, with rising temperature, the aliphatic
side-chains are subjected to increasing thermal cracking which, at a more advanced
stage of physico-chemical coalification, shifts the degassing pattern more towards
demethanation.
A simple test of the degree of devolatilisation that has taken place during
coalification is carried out by proximate analysis. It involves heating a coal sample
in an inert atmosphere to about 900°C (the exact temperatures are laid down in
national and international standards), at which temperature practically all of the
remaining oxygen, hydrogen, sulphur and nitrogen have been driven out as volatile
matter (VM). The residual char is referred to as fixed carbon (FC) the amount of
which is inversely proportional to volatile matter. The analysis results can be
76
The Coalification Process
/A A
"-
~
X
:"
'X"--..
""
///>
~
~"
""
~/
A
':X)<:<&"
/
Fig. 3.32. Cartoon illustrating the relationship between aromatic clusters and intermicellar side chains for a high (top) and low
(bottom) volatile bituminous coal. The
hexagons are the aromatic clusters, the
connecting lines are the aliphatic and other
non-aromatic bridges. (After Teichmiiller
1962)
quoted as received (as), on air-dried basis (ad), or on a dry basis (db), i.e. after the
moisture content has been determined. In order to facilitate comparison with other
coals both volatile matter and fixed carbon are conventionally quoted on a dry
ash-free (daf) basis which requires determination of the moisture and ash contents
by heating a separate coal sample in air to 11 0 °C and approximately 900°C, respectively. A slightly more accurate basis is dry mineral matter free (dmmf), whereby
the mineral matter content is obtained either directly by plasma or low temperature
ashing or by recalculating the ash content, taking into account pyritic sulphur
and (if necessary) carbonate-based carbon dioxide. A frequently used conversion
is by Parr's equation:
%Mineral Matter = 1.08 ash (db) + 0.55 sulphur (db).
(3.2)
If both carbonate and pyrite occur in only insignificant amounts in the coal, the
mineral matter content can be approximated by multiplication of ash by 1.11, but
any of these corrections is reasonably accurate only when the ash content is
relatively small, say less than 10% (Cook 1982).
Other modes of assessing coal rank by volatile matter and fixed carbon are
the Fuel Ratio and Carbon Ratio of D. White (1915):
Fuel Ratio = %Fixed Carbon/%Volatile Matter
(3.3)
Carbon Ratio = %Fixed Carbon/%Total Carbon.
(3.4)
In both ratios the numerical value increases with coal rank however, while Eq. (3.3)
involves proximate analysis only, Eq. (3.4) requires ultimate analysis, as well.
Physico-Chemical Coalification
77
3.2.1.4 Elemental Carbon
The most common elements in coal are C, H, 0, N, and S the proportions of
which are determined by ultimate analysis. The strong degassing that accompanies
the condensation of the aromatics and the splitting of the side chains during
physico-chemical coalification causes the carbon content to concentrate from
around 78% in a sub-bituminous coal to approximately 95% in a high rank
anthracite. The percentage of elemental carbon is therefore a suitable rank
parameter throughout much of the black coal range, although at both ends its
usefulness is impaired by lack of sufficient differentiation. However, in both cases
other parameters can be used, such as moisture content and specific energy in low
rank coals, and elemental hydrogen at the high rank end of the scale.
3.2.1.5 Vitrinite Reflectance and Other Physical Rank Parameters
Coals of different rank reveal major differences in their degrees of compaction and
homogenisation in both hand specimen and, more strongly, under the microscope. This trend is part of the convergence of maceral properties during the
physico-chemical stage of coalification whiCh results from the condensation
reactions mentioned above. One ofthe most striking physical changes is the increase
in the ability of macerals to reflect light from a polished coal surface (Figs. 3.29,
3.31). The reason for this is the before-mentioned systematic change taking place
in the molecular structure of coal during physico-chemical coalification. Initially,
the aromatic clusters (micelles) are small and separated from each other by a large
volume of intermicellar aliphatic and other non-aromatic material in which most
of the hydrogen and oxygen is concentrated (Fig. 3.32). As coalification proceeds,
fluids and gases are released by diffusion from the non-aromatic compounds whose
proportion decreases resulting in an increase in the aromaticity (fa = aromatic
Cltotal C) of coal from approximately 0.56 in brown coal to 0.78 in medium
volatile bituminous coal and 0.95 in anthracite (Iyengar and Lahiri 1959). The
aromaticity of coal is the weighted average of the aromaticities of its contained
macerals which show marked differences at the beginning of coalification. Fusinite
enters the physico-chemical stage with such high aromaticity that it changes very
little, whereas liptinite, being less aromatic to start with, undergoes very marked
changes. The relationship between whole coal aromaticity (fJ and mean maximum
vitrinite reflectance in oil immersion (Romv) can be approximated as (calculated
from Davis 1978a, Fig. 10):
fa = 0.34 + 0.78x - 0.33x 2 + 0.05x 3 .
(3.5)
Besides clarifying many other properties of coal macerals, the concept of increasing
aromaticity also explains the variation in reflectance of different macerals, as well
as the trend towards higher reflectance with increasing rank. The weak reflectance
The Coalification Process
78
of low rank coal or of macerals with relatively low aromaticity is due to the large
proportion of weakly reflecting, carbon-poor intermicellar aliphatic and other
non-aromatic compounds. As this part of the coal decreases during further coalification, the aromatic clusters, which can be regarded as pre-graphitic crystallites
in which the carbon atoms are densely packed and thus highly reflecting, determine
more and more the total reflectance of high rank coal. This has an adverse effect on
transmitted light microscopy in the form of rising refractive and light absorption
indices.
Microphotometry of coal in incident light has therefore become the most
important tool in the arsenal of physical methods of rank assessment. The results
of reflectance measurements correlate well with chemical methods (Hoffmann and
Jenkner 1932; McCartney 1952; Huntjens and van Krevelen 1954; Broadbent and
Shaw 1955; McCartney and Hofer 1955; Murchison 1958; Kotter 1960; McCartney
and Teichmiiller 1972; and others), which for Australian vitrinite is given as (after
Callcott 1986):
%Ro max = (0.15C o.97 - 0.59)/(98 - C).
(3.6)
Reflectance measurements have the advantage that the small measuring area
(commonly 2 to 5Jlm diameter) exposed to the photomultiplier permit determinations to be carried out on individual macerals. The chemical rank parameters
discussed above require for the analysis several grams of material which rarely
consists of a pure maceral concentrate but conventionally is obtained from representative bulk samples of coal. Differences in coal type between samples contribute
therefore a discern able modification to the results of most chemical methods of
rank assessment (Diessel and Wolff-Fischer 1987). However, photometric rank
determinations are also not completely free from ambiguity because it has been
found that reflectance values are consistently suppressed in coals that contain
alginite, i.e. the remnants of algae (H utton and Cook 1980; Kalkreuth and Macauley
1984, 1987) and large amounts of other liptinite macerals (Kalkreuth 1982) due
to absorption of lipids by vitrinite. The proportion of coals affected in this way is
probably small, and a correct rank allocation can be made on the basis of
fluorometric characteristics (Wolff-Fischer 1984).
Instrumentation and analysis procedure of reflectance measurements have all
been standardised and are described in various national and international
standards, International Committee for Coal Petrology (1963, 1971, 1975), Piller
(1977), Davis (1978), Statch et al. (1982), Bustin et al. (1985) and Robert
(1988).Conventionally, vitrinite (more specifically, telocollinite) is used for the
determination of coal rank by micro reflectance for reasons of its relative ubiquity,
homogeneity, and ease with which a well-polished surface can be obtained. Since
the condensation of the aromatic complexes is associated with an increasing parallel
orientation of their layer planes, the rise in reflectance with coal rank is accompanied by the development of optical anisotropy. This offers the choice of conducting the reflectance measurements in either polarised or non-polarised light.In
the latter case, random reflectance (Ror or Ro rand ) is determined, which is the most
time-efficient procedure. However, when measuring high rank coals it may be
Physico-Chemical Coalification
79
advantageous to determine maximum reflectance (Rom or Ro max) because its
increments against other rank parameters are larger than those of random reflectance. The "0" indicates that the reflectance measurements have been carried out
in oil immersion (ne = 1.518). Deletion of the "0" infers likewise application of oil
immersion because it is so commonly used in coal microscopy in order to enhance
contrast of the microscopic image that only alternative methods need be specified.
In the older literature the expression "mean reflectance" (from German "mittlere
Reflexion" = Rm) can be found, but the term is ambiguous and should not be used.
According to Neavel et al. (1981), the relationship between maximum and random
reflectance can be expressed as:
%Rmax = 1.106%R rand - 0.024.
(3.7)
A similar relationship has been found by Diessel and McHugh (1986) for Australian
coals:
%Rmax = 1.07%R rand - 0.01.
(3.8)
Maximum reflectance is determined in plane-polarised light (one polariser) whereby
two reflectance maxima are obtained when the microscope stage is rotated 360 0
during the measurement. The reason for this is the development of uniaxial negative
crystal properties in most high rank coals and graphites (Cornford and Marsh
1976), although some high rank vitrinites with weak biaxial behaviour have also
been reported by Cook et al. (1972), Stone and Cook (1979) and Teichmiiller and
Teichmiiller (1979a), mainly from tectonically deformed regions. Both maxima
should yield similar values, the mean of which is recorded. When reflectance
measurements are combined with fluorescence intensity determinations, it is
advantageous to replace oil by water immersion. The obtained reflectance values
can be recalculated to the more familiar oil equivalents as:
Rw = 1.09 + 1.43 Ro.
(3.9)
Correlation between reflectance measurements in oil immersion and air are given
in Prado (1975).
Other physical properties which undergo changes with increasing coal rank
are density, hardness and the microfluorescence of coal. Although application of
the latter was originally restricted to the strongly fluorescing liptinite (Jacob 1952b,
1964), improvements in instrumentation made it possible to process the relatively
weak signals obtained from vitrinite (Ottenjann 1980, 1982; Ottenjann et al. 1982;
Teichmiiller 1982) and inertinite (Diessel1985a; Diessel and McHugh 1986; Diessel
and Wolff-Fischer 1986, 1987; Brown et al. 1985a, b).
A detailed discussion of the principle of microfluorometry and its application
is given by Lin and Davis (1988a, b). A substance which is irradiated with a
high-energy beam absorbs part of the energy and converts it into heat. Part of
the energy which is not absorbed by the substance is emitted again with a longer
The Coalification Process
80
wavelength than that of the exciting radiation (Holz 1975). In the case of coal,
the wavelength of the emitted fluorescence is also rank-dependent and shifts from
low to high values with increasing degree of coalification. This offers the possibility
of measuring either the intensity of the emitted light at a fixed wavelength (monochromatic fluorometry) or to determine the spectral range of the emitted light
(spectral fluorometry). In the latter case the results can be expressed numerically
by several parameters (Ottenjann et al. 1974), such as the wavelength with the
highest relative intensity (lambda max), or the red/green ratio Q:
Q = 1650/1500.
(3.10)
1650 and 1500 are the relative fluorescence intensities determined at 650 nm and
500 nm, respectively. Both lambda max and Q shift to longer wavelength with
increasing coal rank. When measuring the relative fluorescence intensity at a fixed
wavelength (for example at 650 nm) the instrument is calibrated in reference to
a standard of known fluorescence (e.g. uranyl glass), similarly to the usage ofreflectance standards in microphotometry.
As a rank parameter the red/green quotient of either liptinite or vitrinite is
particularly useful in subbituminous and high volatile bituminous coals in which
the moisture content has decreased such that it has lost its usefulness whereas the
main rank parameters of bituminous coal (volatile matter, carbon, vitrinite reflectance and others) have not yet developed optimum resolution (Ottenjann 1985;
Strehlau 1988).
In very high rank coals X-ray diffraction has some application in distinguishing
between different anthracites because of the rapid graphitic lattice growth that
takes place towards the high end of the coalification range (Franklin 1951; French
1964; N oda 1968; Hamilton et al. 1970; Landis 1971; Grew 1974; Diessel and Offler
1975; and others). Subsequent graphitisation involves
1. the formation of planar networks of six-sided carbon rings,
2. the stacking of the planar carbon networks to form a layered structure with
two-dimensional symmetry,
3. followed by the ordering of layers so that the carbon atoms in anyone layer
lie over the midpoints of the six-member carbon rings of adjacent layers in a
regular, three-dimensional stacking sequence.
The extent of layer ordering is inversely related to the apparent dooz interplanar
spacing (Ubbelohde and Lewis 1960; Diessel et al. 1978) which is 3.354 A for
three-dimensionally ordered graphite (Oberlin et al. 1980). The end of coalification
and the beginning of graphitisation is also indicated by a reversal of the trend for
minimum reflectance. As illustrated in Fig. 3.33, during coalification both maximum and minimum vitrinite reflectance increase up to a maximum vitrinite reflectance of 3%, at which stage minimum reflectance has reached 2.5%. While this
coalification trend continues for some samples up to a maximum vitrinite reflectance
of 6% (Ro min = 4%) others enter the graphitisation path by reversing minimum
reflectance. Beyond a maximum reflectance of 6% all minimum reflectances are
Physico-Chemical Coalification
81
15
10
"
5
..
.,x
Fig. 3.33. Diagram illustrating the changing relationship between maximum and minimum vitrinite
reflectance during coalification and graphitisation.
Each data point represents between 30 and 100
measurements carried out on coal and metamorphic rock samples cut normal to bedding and/or
schistosity. (After Diessel 1983)
E
o
10:::
b'!-
o
o
1
2
3
4
%Romin
reversing and hence the bireflectance increases to a combination of both decrease
in minimum and increase in maximum reflectance values until the extreme position
of graphite is reached in which maximum reflectance and bireflectance are
numerically similar because minimum reflectance approaches zero (McCartney
and Ergun 1967; Diessel and Offler 1975; Kwiecinska et al. 1977; Diessel 1983).
The samples which in Fig. 3.33 display signs of early graphitisation in comparison to their still moderate coal rank come from high pressure metamorphic
terrains, for which the observations on naturally occurring high rank coal and
graphite by Bostick (1974), Ragot (1976), Diessel et al. (1978), Bonijoly et al. (1982),
Okuyama-Kusunose and Itaya (1987) and the experimental work of Noda and
Kato (1965); Noda (1968); and Inagaki et al. (1977) suggest that high pressure
retards coalification but accelerates graphitisation even of relatively "nongraphitisable" carbon.
3.2.2 The Effects of Pressure, Temperature and Time
Pressure, either due to an increasing weight of overburden or tectonism, has quite
contrasting effects on coalification. Towards the end of the biochemical and at the
beginning of the physico-chemical stages it causes compaction and dehydration
from peat to brown and sub-bituminous coal which is accompained by a concomitant rise in specific energy (Fig. 3.31). Dehydration is a physical process which
responds to compaction and consolidation without any rise in temperature.
82
The Coalification Process
Teichmiiller and Teichmiiller (1966a) have shown this experimentally by compressing brown coal at room temperature. Wood (textinite) from a soft brown coal,
after having been sUbjected to a pressure of 270 Kg/cm 2 , which equates to an overburden of approximately 1200m, lost all cell lumens (cavities) and appeared as
vitrinite, not unlike telinite under the microscope. Similarly, open-textured brown
coal attritus was converted into a densely packed mixture of spores and vitrinite
(desmocollinite). This trend was macroscopically accompanied by darkening and
increase in lustre but, as Mackowsky (1953) had pointed out previously, both
carbon and volatile matter contents on a dry-ash-free basis (daf) are little influenced
by such changes.
Once subsidence and depth of cover of the coal have caused the temperature
to rise sufficiently to start first polymerisation and then thermal cracking of the
intermicellar side chains, excessive pressure can exert a retarding influence on
coalification by supprissing devolatilisation. According to Karweil (1966), coalification is a first-order reaction, i.e. its effectiveness is governed by the concentration
of the reacting partners. If the developing effiuent gases and liquids are prevented
from leaving the coal by a high confining pressure, they will dilute the reacting
groups in the microfabric of the coal and thus slow down the condensation
reactions. This assumption has been partly based on artificial coalification
experiments by Huck and Patteisky (1964), who found that under vaccum
conditions, a coal heated to 350°C devolatilised from initially 33% VM to 18%
VM, whereby liptinite disappeared. Both liptinite and volatile matter remained
unchanged when the same coal, at the same temperature was subjected to a pressure
of 800 Kg/cm2. More recent experiments by Horvath (1983) and Goodarzi (1985a)
have confirmed these results.
In addition, geological evidence weights heavily in favour of temperature as a
major cause of physicochemical coalification. Comparisons between coal rank and
rock temperatures measured in deep drill holes, such as Miinsterland No.1 in northwest Germany, show a direct relationship between isovols and isotherms (Lensch
1963; Hedemann 1963). Of importance in this context is the fact that coal rank does
not only increase with depth (Hilt's Rule) but also under conditions of thermal
metamorphism, i.e. by temperature alone. The best results are obtained from a
large acid or intermediate batholith with widely spaced isotherms which intrudes
slowly and remains at some distance measured in hundreds or even several
thousand metres below the coal. This will cause steepening of the geothermal
gradient and increased heat flow over a large area but will not carbonise the coal
as is frequently the case in contact with the very much hotter dolerite dykes or
sills. A fine example of how an intrusive body at depth has influenced the rank
of coals from three geological periods (Carboniferous, Jurassic, and Cretaceous)
is the Bramsche Massif in northwestern Germany. On approaching the area affected
by the intrusion, a centripetal rank increase can be observed up to anthracite rank.
According to Teichmiiller and Teichmiiller (1966a), neither overburden pressure
nor tectonic stress in the region have been particularly high. Moreover, Patteisky
et al. (1962) have presented a coalification chart of the Sonnenschein Seam in the
Ruhr Basin which shows that where folding was most intense close to the Rhenish
Fold Belt coal rank is actually rather low. As a reason for this it has been assumed
Physico-Chemical Coalification
83
that early folding and uplift interrupted coalification. A similar situation has been
reported by Kalkreuth and McMechan (1984) from the Grande Cache region in
western Canada, where coal rank at the Lower Cretaceous level decreases from
semi-anthracite in the Alberta Syncline to sub-bituminous coal towards the Rocky
Mountains. The authors suggest that the westward decrease in coal rank is due
to the eastward migration of the Laramide deformation across the area. Its early
onset in the west interrupted coalification whereas, further east, Lower Cretaceous
strata remained buried underneath a thick pile of Upper Cretaceous and Tertiary
molasse sediments, thus allowing coalification to continue much longer.
Conversely, there are also regions where the rank of near-surface coals increases
towards an area of high deformation, for instance, in the southern Canadian Rocky
Mountain Foothills and Frontal Ranges (Norris 1971); Hacquebard and
Donaldson 1974) and close to the orogenic margins of some other foreland basins.
However, these tectonic settings were also places of particularly deep subsidence
which were subsequently incorporated in the deformation and uplift of the adjacent
orogen. As a consequence of this either stratigraphically deeper levels have been
exposed (e.g. Ruhr Basin) or coalification was largely complete before the onset
of uplift.
In view of the perceived temperature dependence of physicochemical coalification, its velocity (V) has been expressed by Karweil (1966) as an Arrhenius-type
equation:
(3.11)
whereby n = number of groups capable of reacting; A = number of reactions
actually taking place; T = absolute temperature; R = universal gas constant;
E = activation energy; RT = available thermal energy. With the exception ofn and
T the equation contains only constants, E having been calculated at 8.4 Kcaljmol
and A at 2.27 times 1O-4ja. The velocity of coalification depends therefore on
the temperature the material was subjected to and on the number of groups capable
of reacting. They are plentiful in low rank coals but as even high rank coals still
do contain some of these groups, coalification would theoretically not terminate
before the last couple has reacted with each other, i.e. at the "rank" of graphite.
Assuming therefore that a sufficient number of reacting partners is always present
in coal, temperature takes the leading role. From this link between temperature
and velocity of coalification it follows that the rank of coal depends on the residence
time the source material was subjected to the prevailing end temperature, whereby
the reaction rate decreases exponentionally with time (Robert 1988).
Karweil (1956) has summari~ed the correlation between time, temperature and
coal rank in diagram form a modified version of which is illustrated in Fig. 3.34.
In the diagram, time is represented by a family of curves which relate to temperature
on the ordinate and coal rank on the upper abscissa, either as volatile matter or
vitrinite reflectance. In order to obtain coal rank, the burial history of the deposit
is divided into discrete time segments with similar burial temperature. For each
segment the respective Z-value is obtained from the Z-scale on the lower abscissa.
The sum of the individual values provides a conversion factor from which coal
The Coalification Process
84
% Rmax (leJocollinite) 0.5
0.8
0.6
% VM (dar)
40
1
20
Residence lime in million lears
~n,
I'.
f"
o
c
':;,
100
/,1
V
-
~~
,.
:.-:~
c.
...... ~ ~I-'
E
0
0.001
Z - Scale
0.005 0.01
0.02
6
7
100
rh ~W1
~ ~ ~ ~V200
0 ~ W .xl
~::r
V
VI..-'
veX
V " 3'Ol
V
v~
1.--':":::
'Q)
....
"
vf' ...-
.....
10
Q)
h
4 5
54
10
10 12 20 30 40 50
I-'.: v:
V ...-V:: ~ra
u
Q)
3
1.5 2
35 30
~f'
III
0.05
0.1
0.2
0.5
2
5
10
Fig. 3.34. Karweil's diagram relating coalification temperature and residence time to coal rank.
(After Karweil 1956 and Bostick 1971)
rank can be obtained when its numerical value is projected from the Z-scale to
the upper abscissa.
The version of the Karweil Diagram illustrated in Fig. 3.34 was extended and
slightly modified by Bostick (1971, 1973) mainly by adjusting the predicted rank
values in comparison with measured results and by adding a vitrinite reflectance
scale. In addition to the thermo-kinetic approach by Karweil, the relationship
between time and coalification has also been studied on the basis of actualistic
models or by combining both methods. Examples have been presented by Lopatin
(1971, 1976a, b); Hood and Castano (1974), Hood et al. (1975); Cornelius (1975);
Tissot and Espitalie (1975); Shibaoka and Bennett (1977). Their results may differ
in detail, but they all come to the same conclusion, that given sufficient activation
energy, a long residence time at low temperature may yield the same coal rank as
a relatively short residence time at high temperatures. However, since coalification
is exponentially dependent on temperature [see Eq. (3.11); Lopatin 1971; and Waples
1980] a relatively short residence time at comparatively high temperatures will
affect coal rank more than long residence time at low temperature (Kalkreuth and
McMechan 1984). Of course, residence time must be long enough for an equilibrium
to be established between temperature and coal rank. In rapidly sinking rift basins
equilibrium temperature may be established more slowly than the rate of
subsidence, and likewise, it will take time for coalification to reach a level that is
commensurate with the given temperature (Teichmiiller 1987). In order to maintain
rank equilibrium the rate of basin subsidence should not exceed 10m/Ma (Lopatin
1976a, b). In a more rapidly subsiding basin, the result will be (for some time after
subsidence has ceased) a lower coal rank than would be expected on the basis of
temperatures measured at depth. Examples of this kind are the Gippsland Basin
85
Physico-Chemical Coalification
350
:.-
300
•
•
• Q.-6
Fig. 3.35. Diagram showing two sets of reflectance/temperature correlations based on fluid
inclusion data (small dots) by Barker and
Goldstein (1990a,b) and an oxygen isotopes
(large circles) by Diessel et al. (1978)
150
"a, 100
a.
E
~
• ,. Q
q;0
V'
j. rO:
'0 ••
200
~
··0
. ;:• ,.'.::n
'0 0
1()
250
.3
.• :
:9: ..
50
Rorv 1.0
2.0
3.0
4.0
5.0
in Australia (Kanstler et al. 1978; Shibaoka et al. 1978) and Upper Rhine Graben
(Espitalie 1979; Teichmiiller 1979).
More recently, Barker and Goldstein (1990a) have cast some doubt on the
importance of residence time in coalification. Based on a comprehensive laboratory
and field study of present-day peak temperatures (T peak)' homogenization temperatures (Th) of fluid inclusions in calcite, and mean random vitrinite reflectance
(Rorv) determined in associated coals or dispersed organic matter (DOM), they
found that T h correlates well with the maximum temperature to which the rock
was subjected in its geological history. A similarly close agreement was established
between T hand Rorv independent of the duration of heating, which was estimated
to have varied between thousands and millions of years. From the results presented
by Barker and Goldstein (1990a,b) maximum coalification temperature (T) may
be estimated without any reference to residence time as:
T = [(in Rorv) + 1.26]/0.0081.
(3.12)
For comparison, Barker and Goldstein's (1990b) Rorv/fluid inclusion temperatures
have been plotted against the Rorv/oxygen isotope temperatures obtained by Diessel
et al. (1978) from a high-pressure metamorphic belt in New Caledonia. There is
good agreement up to 2% Rorv after which the two data sets diverge. In view of
the many possible sources of error in both methods neither the agreement nor the
disagreement need necessarily be proof or otherwise of the accuracy of the results.
However, it is an interesting contribution to the discussion of the role of time in
coalification that many of the fluid inclusion temperatures, which are based on
samples with very short periods of heating, are slightly higher than the oxygen
isotope temperatures which were established over a metamorphic interval in excess
of 12Ma.
4 Coal Petrographic Entities
The preceding discussion has shown that similar plant tissues can form a wide
range of degradation products when subjected to varying degrees of humification
and dehydration before being incorporated in the accumulating peat. On conversion into coal some of the differences between the peat components are lost
whereas others are retained and may even become accentuated, such as differences
between macerals in reflectance which develop at the beginning of physico-chemical
coalification, before undergoing the kind of convergence illustrated in Fig. 3.29.
The differentiation into the various organo-petrographic constituents of coal
has been inherited from the biochemical stage of coalification, mainly in response
to fluctuations in the groundwater table which led to frequent changes in both
floral composition and the conditions under which the plants were converted into
peat. When viewed in outcrop or in bore core, humic coal appears therefore
stratified into lithotypes, i.e. macroscopic units which differ from each other in
such physical properties as colour, texture and desiccation patterns in brown coal,
or in texture and lustre in black coal. In contrast, sapropelic coals, such as cannel
and bog head coals (torbanite), display little differentiation because they formed
as organic oozes from spores and/or algae on the bottoms of lakes and ponds
outside the range of minor fluctuations in the position of the water table (Dulhunty
1944). Therefore they appear macroscopically unstratified and often display distinct
conchoidal fracture (Fig. 4.1).
The physical differences between the various lithotypes are an expression of
their maceral composition. These are the basic and relatively homogeneous organopetrographic entities of coal which by their chemical composition and physical
characteristics determine its properties and utilisation. Natural associations of
macerals are referred to as microlithotypes. Like macerals, they are grouped into
broad categories, i.e. microlithotype groups, which are identified by the number
and kinds of macerals contributing to the microlithotype. This hierarchical
arrangement of the organo-petrographic components of coal results in the
distinction of three assemblages of components, i.e. macerals, microlithotypes and
lithotypes, which form the basis for the International or Stopes-Heerlen classification system of the petrographic constituents of coal (International Committee
for Coal Petrology 1963, 1971).
88
Coal Petrographic Entities
Fig.4.1. Photograph of a Permian boghead coal from the Sydney Basin of New South Wales
showing conchoidal fracture
4.1 Macerals
The term maceral was introduced by Stopes (1935) in analogy to the minerals of
inorganic rocks for the purpose of classifying the elemental microscopic constituents of coal. All maceral names have the suffix "-inite". Coal macerals are conventionally combined into three groups which in brown coals are referred to as
huminite, inertinite, and liptinite, while in black coals the term huminite is replaced
by vitrinite, the other two maceral groups being the same.
The huminite/vitrinite group of macerals has been derived from plant tissue
which has been humified to varying degrees prior to burial and anaerobic gelification. Much of the inertinite group has also been derived from more or less
humified tissued source material, but has been subject to dehydration and oxidation
during biochemical coalification and before final burial in the catotelm. Liptinite
macerals have been derived from cuticular and other resistant vegetal matter high
in resinous and waxy material. In addition to the mace.rals and maceral groups,
the International Committee for Coal Petrology (1963,1971) distinguishes between
sub macerals and maceral varieties in reference to either their particular vegetal
origin or their respective states of preservation, although some of the genetic
relationships are not clearly defined. For example, irr the conventional StopesHeerlen system, the two main macerals of the vitrinite group are telinite and
collinite. Each ofthese is divided into submacerals and, in the case oftelinite, further
into maceral varieties. The submacerals of collinite all share a seemingly colloidal,
i.e. structure-less, habit when viewed in reflected light, although they show different
Macerals
89
MacO?ral
Group
Maceral
Subgroup
Humo-
--:z
W
H
Fig. 4.2. The classification
of the brown coal (lignite)
macerals of the huminite
group. (After
International Committee
for Coal Petrology 1963,
1971)
~
::J
:I:
MacO?ral
Texto- UJ mi nite
Maceral
VariO?ty
Ii (dark)
B (bright)
Ii (dark)
B (bright)
Eu- Ul mi nite
Ii (dark)
B (bright)
SubmacO?ral
Textinite
Telinite Ul mi nite
Humo- Attri nite
Detr1nite Den3inite
Levi- Detrogeli nite
Humo- Geli nite Geli- TeJogeli nite
nite Eugeli nite
Collinite
Porigeli nite
Phlobephi nHe
CorpoHuminite PsO?udo-Phlobaphinito?
degrees of cell tissue preservation after they have been etched with an oxidising
agent, such as acidified potassium permanganate (Teichmiiller 1941; Diessel1961;
.
Mackowsky 1974; Ng et al. 1987).
Etching techniques reveal that telocollinite possesses a completely intact, albeit
somewhat collapsed and slightly gelified, cell structure and is therefore genetically
more closely related to the structured telinite than to the other collinite submacerals. In the conventional brown coal classification this problem is accounted
for by dividing the huminite group (the brown coal equivalent of the vitrinite
group) into three subgroups as illustrated in Fig. 4.2, which comprises structured,
detrital and colloidal macerals, respectively, and represent increasing degrees in
humification in accordance with Fig. 3.14. The recognition of three subgroups of
tissue-derived macerals according to their state of humification prior to physicochemical gelification has been adopted by Australian Standard 2856 (1986) for a
Table 4.1. The classification of bituminous coal macerals and maceral groups. (After International
Committee for Coal Petrology 1963, 1971, 1975). Maceral subgroups are according to Australian
Standard 2856 (1986)
Maceral groups
Maceral subgroups
Macerals
Vitrinite
Telovitrinite
Detrovitrinite
Gelovitrinite
Telinite, telocollinite
Desmocollinite, vitrodetrinite
Gelocollinite, corpocollinite
Inertinite
Telo-inertinite
Detro-inertinite
Gelo-inertinite
Fusinite, semifusinite and sclerotinite
Inertodetrinite, micrinite
Macrinite
Liptinite
Primary liptinites
Resinite, cutinite, sporinite, alginite,
suberinite and liptodetrinite
Exsudatinite. flll""-"
Secondary liptinites
90
Coal Petrographic Entities
unified brown/black coal maceral classification. The same threefold subdivision of
tissue-derived coal components is also applied to the black coal maceral
classification listed in Table 4.1 which is used in this text.
4.1.1 The HuminitejVitrinite Group
Huminite/vitrinite macerals are ubiquitous in humic coals where they frequently
constitute the largest maceral group. The terms huminite and vitrinite are applied
to macerals which have followed the vitrinitisation path of biochemical coalification
as outlined in the previous chapter. These macerals have been formed from wood,
bark, root, leaf, and other cell tissues which have been sUbjected to humification
of varying intensity. The effects of this kind of biochemical coalification are easily
recognised in the huminite macerals of peat and brown coal but they are not quite
as obvious in bituminous coal because of the convergence of the coalification
tracks of the various macerals during physico-chemical coalification (Fig. 3.29). The
associated physical and chemical condensation and loss of textural definition have
their analogy in inorganic sediments, for example, in the gradual post-depositional
obliteration, due to recrystallisation, of detrital minerals and their fabric during
the transition from a loose sand ( = peat) to a sandstone ( = brown coal), and finally
into a quartzite (= bituminous coal to anthracite).
Having been derived from humified cell tissue huminite transforms into vitrinite
at the beginning of the physico-chemical stage of coalification. This is accomplished
by the polymerisation of biodegraded humic substances to form condensed
aromatic and hydroaromatic ring structures, which are connected by covalent
cross-links. With increasing coalification and at the expense of the non-aromatic
matter, the cross-links become degraded and functional groups are lost, which
leads to a tighter packing of the aromatic clusters (Sakurovs et al. 1989). Their
increasing size and degree of crystallinity results in the observed increase in both
reflectance and bireflectance in high rank vitrinites (Mackowsky 1951).
4.1.1.1 The Humotelinite/Telovitrinite Subgroup
Although the outward appearance of the members of this subgroup may vary,
their common characteristic is the retention of cell tissue in various stages of
preservation. Because herbaceous plants degrade rather quickly, it is assumed that
woody and cortical cell tissues constitute the main progenitors of this subgroup.
Except under severe and prolonged conditions of degradation, lignin-rich wood
will often tend to stop short of total breakdown of cell texture. This is related to
compositional differences between the different layers that constitute such cell
walls. For example, transmission electron microscopy (TEM) of partially humified
xylite samples from Latrobe Valley brown coals (Gippsland Basin, Victoria,
Australia) has shown that only their secondary and tertiary cell wall layers had
Macerals
91
Fig. 4.3. Photomicrograph showing the effects of termite attack on Pinus radiata. Upper [eji part of
the wood. Upper right termite excreta. Centre termite excreta in polished thin section with remnants
of cellulose (anisotropic, white, fibrous); transmitted light, crossed po lars; actual length of field of
view = 0.22 mm. Bottom. As above, fluorescent mode showing the high concentration of lignin
(light grey to white); incident light, actual length of field of view = 0.22 mm
92
Coal Petrographic Entities
been converted into colloids (Liu et al. 1982). Therefore humotelinite and
telovitrinite macerals show various stages of tissue preservation in response to
their botanical origin and the course of humification.
As summarised in Chap. 3, humification is a process of depolymerisation
of complex vegetal molecules into smaller products. It begins with the least resistant
cell contents and protoplasmic bodies and ends with the decomposition of cell
walls by the hydrolysis of cellulose to monosaccharides and of lignin to phenolic
compounds. According to the work of Hatcher et al. (1981, 1982), Russell and
Barron (1984), Wilson and Pugmire (1984), Wilson et al. (1984, 1986) and Hedges
et al. (1985), the loss of cellulose is accompanied by a loss of methoxy carbon and
oxidation to form carboxylic and OH functionality, followed by dehydroxylation,
decarboxylation, replacement of alkyl groups by hydrogen and subsequent crosslinking. These processes are aided by organisms in various ways.
Most insects, mites and other organisms feeding on vegetable matter will make
use of only a small portion of the organic intake, the remainder being excreted
either unaltered or only slightly modified in composition. An example of wood
attacked by termites is illustrated in Fig. 4.3, which shows that some portions of
the wood structure have been retained. These portions are impregnated by tannins
and resins. Lignin-rich cell walls are also frequently bypassed, or if chewed and
ingested it may concentrate in excreta, also shown in Fig. 4.3. The mechanical
destruction of such plant material enlarges the surface area of the residue which
prepares the lignin and other resistant components for more efficient microbial
and chemical digestion (Wolf 1988). This is effected by bacterial enzymes, such as
hydro lases which catalytically accelerate the hydrolysis of organic molecules. Some
kinds of wood are more susceptible than others to this kind of degradation. Among
the more resistant ones are wood tissues produced by gymnosperms on account
of their relatively high content of resins and tannins. Most of the large and
comparatively unaltered wood fragments found in peat and brown coal have been
derived from gymnosperms, whereas angiosperm wood is frequently badly
degraded.
In the soft brown coal maceral textinite (Fig. 4.4, upper left) the cell tissue
differs little from that of the original wood and, in some cases (e.g. Gippsland
Basin, Australia), has retained some unaltered anisotropic cellulose and lignin with
primary fluorescence (Russell 1984). Plant genera and species are readily identified
from textinite tissue, which is possibe only with some difficulty in the equivalent
telinite of bituminous coals. Identifications of the phytogenic precursors of some
telinites in British Carboniferous coals have been made by Hickling and Marshall
(1932, 1933).
By the time humification has reached the ulminite stage all cellulose and much
of the lignin will have been hydrolysed which often is accompanied by swelling
and deformation of the cell walls. Two submacerals have been distinguished,
texto-ulminite (Fig. 4.4, upper right and lower left) in which cell lumens are still
open and eu-ulminite (Fig. 4.4, lower right) in which the lumens are closed and
cell walls are in contact with each other and are barely distinguishable under the
conditions of light microscopy. As has been discussed in Chap. 3, this process is
part of the breakdown of cell tissues under the conditions of humification in the
Macerals
93
Fig. 4.4. Photomicrographs of macerals of the humotelinite group. Upper left textinite. Upper right
texto-ulminite. Lower left textinite grading into texto-ulminite. Lower right texto-ulminite grading
into eu-ulminite. Incident light, oil immersion; actual length of each field of view = 0.22 mm
94
Coal Petrographic Entities
Fig.4.5. Photomicrograph of wood tissue undergoing plastic deformation and fusion in a xylite
(wood) from the Latrobe Valley, Victoria, Australia. Transmitted light; actual length offield of view
=O.9mm
acrotelm. It is accompanied by softening, swelling, deformation and eventual fusion
of the cell walls, an example of which is illustrated in Fig. 4.5.
Electron microscopic observations by Liu et al. (1982) suggest that the
degradation of cell walls begins with their separation into fine lamellae followed
by their transformation into spheroidal colloidal droplets which appear to
constitute the maceral porigelinite that is frequently observed to fill cell lumens.
In support of this notion Fig. 4.6, shows porigelinite in the lumens of somewhat
degraded textinite. Its cell walls are particularly thin or have disappeared altogether
in the vicinity of porigelinite concentrations. They appear to represent a more
advanced stage of the formation of colloidal droplets illustrated in Fig. 3.7 from
a Recent partially humified wood sample.
With increasing condensation under post-depositional, anaerobic conditions,
both textinite and texto-ulminite also transform into eu-ulminite due to impregnation of cell tissues by humic acids resulting in closure of the cell lumens
and the acquisition of a homogeneous surface when polished (Fig. 4.7). This
gelification of largely unhumified wood is revealed in ultra-thin thin sections by
the widespread occurrence of traces of completely intact and un deformed wood
tissue within the otherwise homogeneous humic background (Fig. 4.7, lower right).
The remnants of cell walls which mostly appear only as faint ghost structures
behind the homogeneous background are reminiscent of the petrified cell tissues
found in carbonate concretions, an example of which has been illustrated in Fig. 2.6.
The mechanism is probably similar, except that in this case the impregnating fluid
consists of fluid humus colloids. This suggests that there are two pathways leading
to the formation of eu-ulminite (and ultimately telovitrinite):
Macerals
95
Fig. 4.6. Photomicrograph of porigelinite (grey, granular) filling the cell lumens
of textinite whose cell walls have been
degraded and thinned in the vicinity of
porigelinite concentrations. Incident
Jigh t, oil immersion; actual length of each
field of view = 0.22 mm
1. Syngenetic eu-ulminite is formed in the acrotelm by the plastic deformation,
collapse and fusion of cell tissue during humification as illustrated in Fig. 4.5.
Before their closure, cell lumens may be filled by porigelinite, i.e. by colloids
presumably generated from within the wood tissue. This eu-ulminite represents
an intermediate phase corresponding to necrotisation stages 4 to 5, in the
biologically aided hydrolysis of cell walls of wood. After polymerisation much
of this material will form telocollinite or alternatively, semifusinitejmacrinite if
the hydrolysate will be subjected to dehydration and oxidation before final
burial in the catotelm.
2. Epigenetic eu-ulminite is formed during diagenesis by the successive impregnation of textinite and texto-ulminite and the filling of their cell lumens with
fluid humic colloids, as illustrated in Fig. 4.7. In the process the components
of the host tissue are metasomatically replaced by humic substances as well,
which is shown by the loss of anisotropy and fluorescence from cellulose and
lignin, respectively. The impregnating fluids may be generated from within the
replaced tissue but they could also represent migrating humic hydro sols formed
elsewhere in the deposit. This post-depositional, epigenetic gelification of the
cell tissue begins in the catotelm during the peat stage and culminates after the
polymerisation of the humus colloids with the formation of telovitrinite, mainly
in the form of telinite.
96
Coal Petrographic Entities
Fig. 4.7. Photomicrograph of eu-ulminite/telinite in a brown coal from the Gippsland Basin,
Victoria, Australia. Upper left Morwell Seam with homogenised cell tissue, still showing resin ducts.
Upper right Latrobe Seam, of slightly lower rank than Morwell Seam, is less advanced in its
epigenetic gelification and still displays contrast between autochthonous cell tissue and infilling.
Lower left same as upper right, higher magnification. Lower right same as before, showing
undeformed cell structure partially impregnated by colloidal humic fluids. Transmitted light;
actual length of field of view = 0.9 mm in upper left and 0.36 mm in others
97
Macerals
HUMIFICATION
cell tissues
cell tissues
mildly affected
intact
1
I
cell tissues di'inte"ated, h umw ; I
collapsed cell hagment' ,.II. ids
!
!
TEXTI NITE
!
after
GELIFICATION
TEXTO- ULMINITE
1
1
after
POlYMERI SATlON
1
EU-U LM INI T E
TEUNITE
1
1
I TELOCOLUNITE
TEL OV IT RINITE
Fig.4.8. Outline of the syngenetic humification and epigenetic gelification tracks leading to the
formation of eu-ulminite as the common predecessor of telovitrinite
Fig.4.9. Photomicrographs of telinite (large homogeneous areas without inclusions) in high
volatile bituminous coal from the Sydney Basin, New South Wales. Note the weak cell structure in
the telinite. Other macerals illustrated are sporinite (flat translucent lenses) and inertinite (either
dark grey or opaque). Transmitted light, actual length of field of view = 0.9 mm (left) and 0.22 mm
(right)
Coal Petrographic Entities
98
Fig. 4.10. Photomicrograph oftelocollinite (Upper half) demonstrating the lack of other maceral
inclusions due to the preservation of cell tissue now "hidden" behind the homogeneous surface
polish. Also note the contrast in reflectance between the telocollinite and the darker grey matter in
lower half which consists of desmocollinite (subgroup detrovitrinite). Incident light, oil immersion;
actual length of field of view = 0.11 mm
'..."
1.8
Ror\ = 0 .06 + 0 .95 Rorv
0
'" 1 .6
"C
~
t:0
1.4
~
Q>
1.2
....u
1.0
u
c:
<Il
Q>
Q>
0 .8
~
0 .6
a:
Rord = 0.06 + 1.0 3 Rorv
0.6
% Rorv
0.8
1.0
1 .2
1.4
1 .6
1.8
Fig. 4.11. Diagram
showing the relationship
between mean random
reflectance of telovitrinite
(Rort closed squares) and
detrovitrinite (Rord open
sq uares) in reference to
the weighted mean of
random reflectance of
total vitrinite for
Australian and German
bituminous coals. Each
point represents at least
50 readings
99
Macerals
70
60
,...
50
. ",
.
.... ':.".-- ... ....
40
.. .....:., i ,..
......CIJ 30
'c
·C
......; 20
Fig.4.12. The proportion
between total vitrinite
content and telovitrinite
in Carbonife'rous and
Permian bituminous coals
"
•• I -
'
0
Qi 10
.. .'. . ..
.
.
.
10~
0
20
30
40
50
60
70
80
90
100
%Vitrinite
The two pathways discussed above have been outlined in Fig. 3.4 and have been
further emphasised for the huminite/vitrinite group in Fig. 4.8. In low rank
bituminous coal it is still relatively easy to identify telinite in transmitted light
(Fig. 4.9) but in incident light and with ihcreasing coalification it becomes
indistinguishable in unetched coals from telocollinite due to the obliteration of
cell structure. The term telovitrinite is therefore used to cater for both macerals. It
constitutes a maceral subgroup the diagnostic feature of which is that it is free
from contamination by other macerals because, in spite of its gelified appearance,
the cell tissue remained sufficiently coherent to resist disintegration and intermingling with other macerals (Fig. 4.10). For the same reason all telovitrinites
display a 5 to 10% higher reflectance (Robert 1979) compared with detrovitrinite
which has been subjected to a higher degree of cell destruction before final burial.
Figure 4.11 shows the relationship between mean random reflectance for telovitrinite and detrovitrinite in reference to random reflectance of total vitrinite
(weighted mean) for coal ranks ranging from high to low volatile bituminous coal,
where the two regression lines merge. A similar convergence would also be
encountered if the relationship between the two vitrinite subgroups were extended
into brown coal and peat thus leaving high volatile bituminous coal as the rank
in which the highest degree of divergence occurs. Consideration of such aspects
is important when using vitrinite reflectance as a rank indicator which caused
Brown et al. (1964) to make a distinction between vitrinite A (= telovitrinite) and
vitrinite B (= detrovitrinite). In addition there seems to be a small proportion of
telovitrinite macerals which have been found to show elevated reflectance and
anomalous fusibility during carbonisation and have therefore been called
pseudovitrinite (Benedict et al. 1968; Thompson and Benedict 1974; Gray 1982).
The proportion of humotelinite/telovitrinite varies in different coals but is
generally low in coals with low total vitrinite content. In these coals the majority
of vitrinite consists of detrovitrinite. As shown in Fig. 4.12, with increasing vitrinite
content the proportion of telovitrinite increases likewise, such that in a vitriniterich coal telovitrinite is the dominant maceral subgroup.
Coal Petrographic Entities
100
4.1.1.2 The HumodetrinitejDetrovitrinite Subgroup
When the coherence of the cell tissue becomes lost in the course of humification
the result is a mixture of fluid humus colloids which constitute the continuous
phase, and particulate plant debris in all stages of degradation which forms the
discontinuous phase. On compaction ofthe peat and its transformation into brown
coal, the mixture of colloids and solids coagulates to form the maceral subgroup
humodetrinite (see brown coal classification in Fig. 4.2), which becomes detrovitrinite during physico-chemical coalification (Table 4.1). Because the soft tissues
of herbaceous plants disintegrate more readily during humification than lignified
cell walls, much of the material. constituting humodetrinite and detrovitrinite has
probably been formed from non-woody cell tissues, although prolonged humification would affect trees and other wood producing plants in a similar manner,
as has been shown by the necrotisation scheme in Chap. 3. The genetic relationship
between the main macerals has been schematically indicated in Fig. 3.14 and is
further amplified in Fig. 4.13.
Depending on the relative proportion between continuous and discontinuous
phases in humodetrinite and their state of gelification, a distinction is made in
brown coal between the macerals attrinite and densinite (Fig. 4.2). As illustrated
in Fig. 4.14 (left), attrinite consists ofloosely packed cell fragments and other plant
debris including liptinite and inertinite. On further humification and compaction,
attrinite is converted into densinite in which the partially degraded cell fragments
begin to lose their identity and merge with the surrounding colloidal matrix (Fig.
4.14, centre). Similarly to the syn- and epigenetic formation of eu-ulminite discussed
above, densinite, too, can be the product either of humification, in which case the
dense appearance is due to the high proportion of colloids, or it can be formed
by epigenetic gelification of attrinite, as indicated in Fig. 4.13. The epigenetic
pathway is obviously quite common, as suggested by the observation that contrary
to its frequent occurrence in soft brown coal, practically all attrinite has been
replaced by densinite in the higher rank hard brown coal.
I
HUMIFICATIONlcellti"""'I,
.,ellti,,"", ,ellti,,"", disintegrated, humus colloids
lnta.d
mlldlyaffected (ol1a.p~ed cell mgments
1
after
1
DETRO- EU- PORI-i
PSEUDOGEL I NIT E
PHLOBAPHINITE
AITRINITE
GELIFICATION
I
!
I
!
DENSINITE
1
1
after
DESMOCOLLINITE
GELOCOLLINITElcORPOCOLLINITE
POLYMERlSATION
DETROVITRINITE
GELOVITRINTE
Fig.4.13. Outline of the genetic relationship between the macerals of the humodetrinite/
detrovitrinite and humocollinite/gelovitrinite groups
Macerals
101
Fig.4.14. Photomicrograph illustrating the difference between attrinite (left) in Tertiary brown
coal from the Gippsland Basin, Victoria, densinite (centre) from the same locality and detrovitrinite
(right) with pseudocorposcJerotinite (White body with high relief on right margin) containing
gelocollinite in cavities. Note the fungal spores in densinite consisting of a teleutospore slightly
above centre and Sclerotites brandonianus below centre. Incident light, oil immersion; actual length
of each field of view = 0.22 mm
The black coal equivalent of densinite is desmocollinite (= heterocollinite after
Alpern 1966) which belongs to the detrovitrinite subgroup (Fig. 4.14, right)
according to Table 4.1. It constitutes the vitrinitic groundmass and cement (Gr.:
kolla = glue) of sub-bituminous and bituminous coal. In transmitted light microscopy of high volatile bituminous coal, the heterogeneous nature of desmocollinite
is quite recognisable but without etching no differentiation can be made between
the colloidal continuous and the attrital discontinuous phase in higher rank coals.
In spite of this disadvantage, the occurrence of liptinite and inertinite inclusions
is proof of the total loss of a coherent cell structure. A large portion of the enclosed
liptinite debris is too small to be resolved by the light microscope but consists of
absorbed oils, resins and waxes. As illustrated in Fig. 4.10, this has the effect of
lowering the reflectance of the desmocollinite.
102
Coal Petrographic Entities
4.1.1.3 The Humocollinite/Gelovitrinite Subgroup
Coalified humus colloids without any inclusions of remnant cell tissue are not
frequent but occur sporadically in the form of gelinite and corpohuminite in brown
coal. In most cases they have been precipitated in cavities, cleats and fissures of peat
and brown coal from a humic hydrosol (humic acid) which separated during
humification from the decomposing vegetal matter.
Gelinite is divided into two submacerals, namely, porigelinite (Fig. 4.6) which
has a granular texture, presumably consisting of small droplets of humic colloids
(Liu et al. 1982), and levigelinite with a smooth and sometimes cloudy polished
surface. As shown in Fig. 4.2, levigelinite has been divided into further types, one of
which, telogelinite, is genetically misplaced in the inteniational classification and
should be part of eu- ulminite. The other two, detrogelinite and eugelinite, follow on
from attrinite and densinite, and are distinguished on the basis of increasing colloid
Fig.4.15. Photomicrograph of cortical tissue with phlobaphinite (tabular, smooth bodies in upper
portion on left) in brown coal from the Gippsland Basin, Victoria; transitions from phlobaphinite to
corpocollinite in Permian subbituminous coal from Collie, Western Australia (centre); and
corpocollinite in Permian high volatile bituminous coal from the Sydney Basin, New South Wales.
Note the dark suberinite partitions between the phlobaphinitejcorpocollinite bodies. Incident
light, oil immersion; actual length of field of view on left = 0.22 mm, in centre and on right = 0.22
Macerals
103
ratios. Indeed, detrogelinite is still so closely related to densinite that it, too, converts
into detrovitrinite during physico-chemical coalification, as outlined in Fig. 4.13. The
black coal equivalent of the remaining gelinite macerals is gelocollinite, an example
:>fwhich has been illustrated in Fig. 3.11 as infilling of cell lumens and gaps between
~ell walls of semifusinite. Colloidal humic matter ofthis purity is comparatively rare,
as it commonly contains inclusions of cell fragments and other organic debris.
Cell lumens in coal sqmetimes contain spheroidal and elliptical bodies of
flocculated humic colloids which are called pseudo-phlobaphinite in brown coal.
The prefix "pseudo-" is used in order to separate such secondary infillings from
"real" phlobaphinite which consists of tannin and similar non-humic substances
excreted, for instance, in the cork cells of bark tissue (Fig. 4.15). On disintegration of
the cell tissue these corpohuminite bodies become isolated. In black coal they are
called corpocollinite, examples of which have been illustrated in Fig. 3.10.
4.1.2 The Inertinite Group
This maceral group comprises members which have been thought to possess similar
technological properties, in particular, lack of fusibility during coke making, and
generally low reactivity due to a greater covalent cross-link density than commonly
found in the vitrinite group (Barton and Lynch 1986). However, as a corollary to
their wide range in reflectance and fluoresence intensities (Diessel and McHugh
1986) inertinite macerals display a likewise wide range in other properties, such that
lack of fusibility during carbonisation may be true only for fusinite and other
inertinites with a reflectance in excess of 1.8% in oil (Diessel 1983), although
inertinite fusibilities up to 2.8% reflectance have been found in laser induced
carbonisation experiments (Hall and Coin 1989). The actual degree of inertinite
fusibility is rank-dependent and is highest in medium volatile bituminous coals
(Diessel and W olff-Fischer 1987; Diessel and Bailey 1989).
As outlined in Fig. 3.4, inertinite macerals have the same precursors as vitrinite
macerals and many of them pass through the same stages of humification except
that, before reaching depositional base level below the groundwater table, they are
subjected to a period of intensive desiccation and varying degrees of oxidation
including partial burning of the accumulated vegetal matter (Gould and Shibaoka
1980). The results are coal constituents which possess relatively high OIC ratios and
high reflectance in incident light microscopy because they are rich in aromatic
carbon. Most inertinite macerals are relatively brittle and hard, which in incident
light microscopy is shown by their tendency to develop polishing relief. Similarly to
the huminite and vitrinite groups, inertinite has been divided into three subgroups in
reference to the degrees of cell tissue preservation.
The proportion of inertinite in coals varies over a wide range but is frequently
between 20 and 30%. Figures which deviate substantially either way from this
average occur in coals which have been formed under particular sets of environmental conditions, as will be discussed later. Since no terminological distinction is
104
Coal Petrographic Entities
made between inertinites contained in brown and black coal, the classification of
inertinite macerals listed in Table 4.1 applies to all coal ranks.
4.1.2.1 The Telo-Inertinite Subgroup
The term telo-inertinite implies that its members consist of structured macerals in
which gelification is either absent or subordinate. The widespread but infrequent
pyrofusinite shows the highest degree of preservation of cell tissue (Fig. 3.19, top),
reflectance and polishing relief. In ordinary incident light it appears white with a
distinct yellow tinge, whereas it is opaque in transmitted light. Unlike semifusinite,
whose cell walls are often swollen as a result of partial humification, the cell walls of
pyrofusinite are rather thin because only their resistant lignified portions survived
the effects of incomplete combustion (Barghoorn 1949). Frequently, the brittle cell
walls are broken by either overburden or tectonic pressure into curved fragments
which is referred to as bogen (bow) structure (Fig. 4.16). Next to wood, cortical
tissues with cork cells can be frequently observed in fusinitised form, an example of
which is shown in Fig. 4.17. Because of its high concentration of aromatic carbon,
pyrofusinite does not undergo further change during physico-chemical coalification
which is in contrast to semifusinite, whose initially lower reflectance, according to
Fig. 3.29, increases until anthracite rank is reached (Alpern and Lemos de Sousa
1970; Smith and Cook 1980). For the same reason fusinite does not fluoresce and
appears completely opaque in transmitted light, whereas semifusinite displays both
translucency and fluorescence, the intensity of which is inversely proportional to its
reflectance. Examples are illustrated in Fig. 4.18.
Fig. 4.16. Photomicrograph of crushed fusinite with bogen structure (curved cell wall fragments).
Incident light, oil immersion; actual length of field of view = 0.22 mm
vl.acerals
105
Fig.4.17. Photomicrograph of fusilitised bark (periderm) tissue with
:ork infillings (phlobaphinite). High
wlatile bituminous coal, Sydney
Basin, New South Wales. Incident
ight, oil immersion; actual length of
"ield of view = 0.36 mm
Reference has been made in Chap. 3 to the main modes of fusinite and
:emifusinite formation. Most fusinite is fossil charcoal, i.e. its most common mode of
·ormation is by incomplete combustion (pyrofusinite = Brandfusinit of Teichmuller
1950), whereas the lower reflecting semifusinite is a product of either aerobic
'iodegradation during humification (degradofusinite = Zersetzungsfusinit of Teichnuller 1950; also Murchison et al. 1985), or oxidation and incomplete combustion of
Jartially humified cell tissue. An accurate identification of the various modes of
Jrigin is difficult in most cases, because they commonly overlap and rarely proceed
n isolation. Important indicators are the degree of cell preservation which is a
~e1ative measure of the effect of humification and associated biodegradation, and
~eflectance, which increases with the amount of oxidation and/or the extent of
~ombustion the maceral has been subjected to. Since the brightness of coal in
ncident light microscopy is proportional to its carbon content, the reflectance of
;emifusinite, its hardness, degree of cell preservation and other properties range
Jetween those of vitrinite and fusinite. In incident light its colour ranges from light
~rey to white whereby the darker varieties show brown translucence in transmitted
light and some low reflecting varieties display long-wave microfluorescence.
As mentioned before, wood-derived inertinite displays better preserved plant
~ells than, for example, leaf-derived semifusinite, particularly when low reflecting
106
Coal Petrographic Entities
Fig.4.18. Photomicrographs in white light (left) and fluorescence mode (right) of semifusinite in
Greta Seam, Sydney Basin, New South Wales. Note the decrease in semifusinite fluorescence with
increasing reflectance. Incident light, oil immersion in white light, dry in fluorescence mode; actual
length of field of view = 0.22 mm
Macerals
107
and still closely related to vitrinite. An example ofthis is illustrated in Fig. 4.19 which
also shows that cell definition improves with increasing reflectance. Depending on
the degree of humification prior to oxidation, wood-derived semifusinite may also
show poor cell preservation and occupy a transitional position to macrinite. An
assemblage of such products which correspond to Beeston's (1987) degradofusinite,
is illustrated in Fig. 4.20. Fusinite and semifusinite occur in the form of layers and
lenses in which the cell cavities (lumens) are either empty or may be filled with a wide
range of substances including gelovitrinite, gelo-inertinite, resinous material and
various minerals.
While fusinite and semifusinite can be formed from a large variety of phytogenic
precursors, the only plant-specific inertinite is sclerotinite, which consists of
structured fungal remains, mainly in the form of spores (corposclerotinite) and, to a
lesser extent, mycelium and hyphae (e.g. plectenchyminite). As is implied by the
name (skleros = Gr. for hard) sclerotinite is the hardest of the macerals and
commonly shows high polishing relief. Its reflectance is usually also very high in
bituminous coals but may be quite low in some brown coals.
Because of their distinct morphology the recognition and identification of
fungal sclerotinite in Late Mesozoic and Tertiary coals is usually not difficult (Fig.
4.14, centre), but problems exist in older coals. Positive identifications of fungal
remains have been made by Thiessen (1920a) and Stach and Pickhardt (1964) in
108
Coal Petrographic Entities
Fig. 4.20. Photomicrograph of an assemblage of semifusinite (degradofusinite
after Beeston 1987) showing various
stages of cell preservation and reflectance in a high volatile bituminous coal
from the Bowen Basin, Queensland. Incident light, oil immersion; actual length
of field of view = 0.36 mm
Cook (1962), Lyons et al. (1982) and others, some so-called scierotinite appears to
have been formed from oxidised resinous and tissued material and is therefore
referred to as pseudocorposcierotinite (Fig. 4.14, right) or secretion scierotinite in
contrast to the corposcierotinite representing fungal spores. Based on his studies of
Bowen Basin coals in Queensland, Beeston (1987) considers most pseudoscierotinite
to have been derived from a variety of plant tissues, including wood by a two-phase
process involving geiification followed by oxidation. He distinguishes between two
varieties which he calls degradosclerotinite and sclerotodetrinite. Both consist of
commonly rounded but also irregular, elongated, even squared blisters of high relief
and high reflectance material which is sculptured either by deep notches or irregular
vesicies which, in the case of degradoscierotinite are set within degradosemifusinitic
cell tissue. In contrast, sclerotodetrinite occurs as isolated bodies and thus corresponds to pseudocorposclerotinite. Examples are illustrated in Fig. 4.21.
Semifusinite and its various varieties constitute the most common maceral type,
not only of the telo-inertinite subgroup but of the inertinite group in general. Its
proportion varies but it often accounts for more than 50% of all inertinite macerals
in a coal seam. Fusinite and sclerotinite are considerably less common and rarely
make up more than a few percent, although there are notable exceptions, such as the
"Zwickauer Rul3kohle" (= soot coal of Zwick au, a city in Saxony), which is a
Macerals
109
Fig. 4.21. Photomicrograph of pseudocorposclerotinite (sclerotodetrinite after
Beeston 1987) in the Bayswater Seam of
the Sydney ~asin, New South Wales.
Incident light, oil immersion; actual
length of field of view = 0.36 mm
Carboniferous coal seam consisting almost exclusively of fusinite and semifusinite
(H. Potonie 1920; R. Potonie 1924; Stutzer 1929).
4.1.2.2 The Detro-Inertinite Subgroup
This subgroup consists offusinitised detrital plant fragments in which two macerals
are distinguished on the basis of size. Inertodetrinite (Figs. 3.24 and 4.22) consists of
fragmented inertinite ranging in the longest diameter between 30 and 211m, whereas
micrinite (Figs. 3.24 and 4.23) is composed of smaller inertinite grains. The latter was
referred to as granular opaque matter by Thiessen (1920a) and Thiessen and Sprunk
(1936). According to Alpern and Pregermain (1965) and Teichmi.iller (1974) much of
the micrinite forms at the rank level of subbituminous coal from lipid-rich material
by a disproportionation process which results in the formation ofliquid and gaseous
hydrocarbons, such as exsudatinite and bitumen leaving behind micrinite as a
residue. The latter is called submicrinite by Taylor and Liu (1989) and associated
more closely with the liptinite group of macerals than with inertinite. Because of the
difficulty in distinguishing very fine-grained inertodetrinite (= detromicrinite of
110
Coal Petrographic Entities
Fig. 4.22. Photomicrograph of densely packed inertodetrinite on either side of the telovitrinite
band (grey, in upper half) in the Bayswater Seam of the Sydney Basin, New South Wales. Incident
light, oil immersion; actual length of field of view = 0.22 mm
Fig. 4.23. Photomicrograph of micrinite (small white specks), macrinite (white lenticular bodies in
upper left) and spores (darkjlat lenses) set in detrovitrinite (grey groundmass) of the Katharina Seam,
Ruhr Basin, Germany. Incident light, oil immersion; actual length of field of view = 0.17 mm.
(Diessel 1961)
Macerals
111
Fig.4.24. Photomicrographs of macrinite (smooth, light coloured lenses and layers on left top and
bottom) with varying reflectance levels and reciprocal intensities of translucency (top right) and
fluorescence (bottom right). Top left and right has been taken from a polished thin section of the
Duncan Seam, Tasmania; bottom left and right is from a Queensland high volatile bituminous coal.
Left top and bottom incident white light in oil immersion; top right transmitted light; bottom right
fluorescent mode, dry objective; actual length of each field of view = 0.22 mm
112
Coal Petrographic Entities
Mackowsky 1976) from micrinite (in sensu stricto), Australian Standard 2856 (1986)
regards all inertinite smaller than 2 jlm as micrinite (in sensu lato).
While the proportion of inertodetrinite in coal can vary quite considerably the
percentages of micrinite are usually small. In Carboniferous coals, which contain
generally more micriIiite, than younger coals, micrinite percentages average 3-6%
and may be as high as 19%, whereas Permian and other post-Carboniferous coals
rarely exceed 3%.
4.1.2.3 The Gelo-Inertinite Subgroup
As the name implies, this subgroup has been derived from plant material which was
first biodegraded into humus colloids and subsequently dehydrated and oxidised
(Stach and Alpern 1966). Gelo-inertinite contains macrinite as the only defined
representative but, as discussed in Chap. 3, two varieties of macrinite can be
distinguished, one which consists of detrital angular to rounded bodies, commonly
associated with inertodetrinite. This "corpomacrinite" which has been illustrated in
Fig. 3.25) is probably the result of desiccation of humic colloids (angular) followed by
dispersal and redeposition (rounded). The other variety (Fig. 3.26), occurs as
elongated bands or laminae ("lammacrinite"), probably representing dried humic
groundmass which, as mentioned in Chap. 3, would have formed desmocollinite had
the peat not been exposed to oxidising conditions due to a fall in the groundwater
table. As shown in Fig. 4.24, in both transmitted light and fluorescence mode,
macrinite covers not only a wide range of reflectance in relation to the associated
vitrinite but, like semifusinite, there is an inverse relationship between reflectance
and intensities of translucency and fluorescence.
4.1.3 The Liptinite Group
The liptinite group of macerals has been derived from specific plants or parts thereof
which are charcterised by high aliphatic (mainly long-chain alkanes) contents, as
well as higher atomic HIC ratios compared with other macerals. Reference was
made in Chap. 3 to the high degree of translucency of low rank liptinite in
transmitted light and its low reflectance when viewed in incident light. Another
optical characteristic is the strong fluoresence after excitation with short wave
radiation. As indicated in Fig. 3.29, liptinite macerals increase in carbon content
with coalification which causes them to lose their specific properties and to fade into
the background of vitrinite from which they become indistinguishable in low volatile
bituminous and higher rank coals.
Although no formal subgroups have been named, a distinction can be made
between primary and secondary liptinites. Primary liptinites consist of coalified
plants or parts of plants whereas secondary liptinites form a group of products
derived from thermal condensation and dissociation reactions. Both categories
Macerals
113
occur in only relatively small proportions and rarely exceed 20% in most humic
coals, but they can be very concentrated in sapropelic coals. The nomenclature does
not make any differences between black and brown coal liptinite macerals.
4.1.3.1 Primary Liptinites
Being relatively resistant to decay, the protective skins (exines) of spores and pollen
grains constitute the maceral sporinite. It consists of sporopollenin (Zetzschke et al.
1930, 1932) which, according to Shaw (1970) consists of oxidative polymers of
carotenoid esters. Sporopollenin has a higher degree of cross-linking than the
components of other liptinite macerals, which renders sporinite particularly
resistant to biodegradation (Taylor and Liu 1989). Although it occurs only in cannel
coal in large quantities, in humic coals it is the most common representative of the
primary liptinites, particularly in some Carboniferous coals (see Chap. 2). Since the
spore and pollen exines are usually compacted in coal they appear as small, flattened
lenses in sections normal to bedding. Figure 4.25 gives several examples of the
different forms of sporinite in bituminous coal.
In spite of the extensive spore classification in the palynological literature only
few sporinite types are distinguished in non- macerated coal, mainly by size and
shape. Large spores, ranging in (flattened) diameter between several hundred
micrometres to several millimetres referred to as macro- or megaspores, or as
macro-/megasporinite if regarded as a submaceral of sporinite. The exines are
ornamented by various protrusions, and small, semi-detached, spheroidal appendages which represent abortive spores (Strehlau 1988) are not uncommon
(Fig. 4.25, bottom). In low rank coals the dark polished surface is marked by reddish
internal reflexions, whereas in fluorescent mode macrosporinite often has a distinct
granular appearance.
In view of their size and relatively low proportion compared with microspores,
macrospores are thought to represent mostly female spores of heterosporous plants
such as lycopods (Kosanke 1969). According to Strehlau's (1988) studies of
Carboniferous coals in the Ruhr Valley the most common percentages of
macrosporinite range from 1-2% with extreme values varying between 0 and 5%.
With the reduced contribution of pteridophytes to post-Carboniferous coals not
only sporinite in general but macrosporinite percentages in particular are even
further reduced.
Microsporinite which has been derived from homosporous plants and the male
microspores of heterosporous plants constitutes the largest group ofliptinite macerals
totalling up to 15% in some Carboniferous humic coals (Strehlau 1988) and up to
80% in some cannel coals. Because of the lower contribution of spores to younger
coals, post-Carboniferous cannel coals are quite rare, while, for example, the
sporinite content ofthe Permian coals of Australia averages only 3% (see Chap. 2.4).
The diameters of microspores measure not more than a few tens of micro metres
although considerable size differences can seen under the microscope. While the
sporinite of Carboniferous and older coals consists largely of true spores, the
subsequent advent of seed plants (gymnosperms and angiosperms) has added an
114
Coal Petrographic Entities
Fig.4.25. Photomicrographs of different sporinites in high volatile bituminous coal. Upper left
Thin section of Sydney Basin coal with numerous microspores (small translucent, flat lenses)
together with vitrinite (bands = telovitrinite, matrix = detrovitrinite); transmitted light, actual
length of field of view = 0.36 mm. Upper right Carboniferous coal from the Ruhr Basin with
sporangium (spore capsule) filled with fluorescing immature microspores in upper half and part of a
macrospore in lower half. Other fluorescing matter is liptodetrinite (diffuse schlieren) and cutinite
forming elongated strands near lower edge of photomicrograph; incident light, dry fluorescent
mode; actual length of field of yiew = 0.36 mm. Bottom left and right Macrospore with abortive
microspores in high volatile bituminous coal from the Ruhr Basin. Incident white light in oil
immersion (left) and ih dry fluorescent mode (right); actual length of each field of view = 0.22 mm
Macerals
115
Fig. 4.26. Photomicrographs of Ruhr Basin coals showing the morphological contrast between
tenuisporinite (top) with thin exines and thick-walled crassisporinite (bottom), the latter consisting
of densospores. Incident white light in oil immersion (left) and in dry fluorescent mode (right); actual
length of each field of view = 0.22 mm
116
..
-
Coal Petrographic Entities
J
Fig.4.27. Photomicrograph of a cross-section through the edge of a leaf cuticle in a high volatile
bituminous coal from the Sydney Basin, New South Wales. Bottom left The most common
appearance of cutinite in bituminous coal. Bottom right Densely packed cuticles in Triassic paper
coal from Fingal, Tasmania. Incident white light, oil immersion; actual length of field of view
= 0.36 mm in upper frame; dimensions are reduced in lower frames by amount of cutoff
increasing proportion of pollen grains to the younger coals. Because of the difficulty
in distinguishing between microspores and pollen, Guennel (1952) introduced the
term miospores for both groups.
Based on the thickness of their exines Stach (1952, 1954, 1964) distinguished
between tenuispores ( < 2 !lm), examples of which are shown in Fig. 4.26 (top), and
crassispores (> 2 !lm). The latter have been further divided into torispores (Balme
1952, 1959) and densospores (Stach 1952, 1954, 1964). Torispores have very thick
protective exines and form the outer wall of a sporangium (spore capsule or
Bicoloria after Horst 1957). Densospores were first described by Thiessen et al.
Macerals
117
(1931) as "dumbbell spores" because of their characteristic shape, examples of which
have been illustrated in Fig. 4.26 (bottom). According to Butterworth (1966) their
stratigraphic :ange encompasses the Late Devonian to Permian coals of the
Northern Hemisphere.
The maceral cutinite (Fig. 4.27) is formed from cuticles, the waxy cover on the
epidermis of leaves and young shoots, although some chitinous cuticles may have
been derived from the epidermis of arthropods (Goodarzi 1984; Bartram et al. 1987).
Cuticles consist of cutin, a glycerine ester offatty acid (Stach et al. 1982), from which
hydroxy and epoxy fatty acids can be derived by depolymerisation (Kolattukudy
1976). Although not as resistant to biological and chemical attack as sporinite
(Taylor 1989), cutinite survives biodegradation better than its associated soft
mesophyll tissue. The latter may decompose quite readily causing layers of cuticles
to become densely packed, as in the paper coal from the the Fingal Valley of
Tasmania illustrated in Fig. 4.27 (lower right). It is assumed that cuticles concentrate
mainly in shallow ponds soon after detachment from the parent plant and without
much transporatation (Teichmiiller 1950, 1962; Succow and Jeschke 1986) However,
the transitions from cutinite into semifusinite observed in Australian coals (Fig. 4.19)
suggest that the leaves of the deciduous Gondwana flora often wilted before
subaquatic burial (Diessel 1983; Taylor et al. 1989).
Under the microscope, cuticles appear as straight or wavy lines of translucent
(transmitted light) or rather dark (incident light) material, usually with palisade
ridges on one side. Littke (1985a) distinguishes between the following three
morphological types of cutinite in sections cut normal to bedding:
I. Very long and thick cuticles with a width exceeding 10 11m.
2. Thin cuticles with a width of often less than 1 11m and a length of 10011m.
3. A couple of thin cuticles enclosing a liptinitic "middle lamella", possibly a
vascular strand.
The proportion of cutinite is small in most coals and rarely exceeds 2 or 3%.
Resinite incorporates a number of different source materials, although its main
precursors are resins and waxes from vascular plants. Depending on its varied origin
and postdepositional history resinite displays a wider range of optical properties
than the other liptinite macerals. According to Selvig (1945), resinite consists of highmolecular weight (mainly) aliphatic compounds including resin acid, resin esters and
terpenes. By using 13C nuclear magnetic resonance (NMR) Wilson et al. (1986)
compared fossil Agathis resin nodules obtained from Miocene brown coal of the
Gippsland Basin (Latrobe Valley) in Victoria, Australia with present-day resin of the
same genus. They found that the fossil resin is a polymer of terpenoids, such as
agathic acid, which can be isolated in large quantity from the resin of modern
Agathis trees. In reference to studies by Cunningham et al. (1983) they concluded
that the fossil resinite had been formed by photolytic polymerisation, i.e. by
exposure after its exudation from the host tree.
Resinite occurs in coal either in situ in resin ducts and cells of xylem, cortex,
mesophyll and seeds (White 1914; Selvig 1945), or in dispersed form as lumps and
nodules, some of them quite large, and rod lets (Kosanke and Harrison 1957;
118
Coal Petrographic Entities
Fig.4.28. Various modes of occurrence of resinite in high volatile bituminous coals from the
Gunnedah Basin, New South Wales. Upper left Resinite (small dark lenses) preserved in resin ducts
of telovitrinite. Upper right Resinous inclusions (irregular dark grey bodies) between leaf cuticles
(thin black lines). Bottom Diffuse schlieren (dark strands) and impregnations (irregular patches of
various shades of grey) of former humus colloids in detrovitrinite. Incident white light, oil
immersion; actual length of field of view in upper frames = 0.36 mm in upper frame; same
dimensions applies to vertical edge in lower frame
Macerals
119
Lyons et al. 1982, 1984). Much of the dispersed resllllte represents surface
exudations and coverings of wounds (White 1914) but some may have been
concentrated as a residuum after its host tissue had decayed. It seems that a
substantial portion of the resinous matter contained in vascular plants becomes
absorbed by humus colloids during advanced humification. This process may be,
in parts, responsible for the lower reflectance of detrovitrinite.
In situ resinite occurs in resin ducts preserved in some telovitrinites (Figs. 2.18,
3.22 and 4.28, upper left) within leaves (Fig. 4.28, upper right), and in the form of
diffuse schlieren and impregnations of humus colloids in detrovitrinite (Fig. 4.28,
bottom). Various specific terms have been defined in order to describe the many
morphological variations of resinite. For details see International Committee for
Coal Petrology (1963, 1971, 1875) or Stach et al. (1982).
Alginite is a maceral term which, according to Robert (1988), is ill-defined
because it encompasses remnants of very different marine and lacustrine algae. Coal
contains mainly alginite derived from lacustrine or lagoonal strands although
sapropelic coals and other transitions to oil shale may contain marine algae, as well.
An example is the Ordovician kukersite from Estonia, which has been formed from
the remnants of Gloeocapsomorpha prisca, a marine planktonic algal species
(Zalesski 1917; Gothan and Weyland 1954; Downie 1967).
Fig. 4.29. Photomicrograph of Tasmanites, in tasmanite, oil shale from Tasmania. Incident white
light in oil immersion (left) and in dry fluorescent mode (right); actual length of each field of view
=O.22mm
120
Coal Petrographic Entities
Much of the alginite found in coal belongs to the group of lacustrine colonial
algae of which Botryococcus braunii is the most widespread present-day representative (Robert 1988). It is often found floating as jelly-like masses on the surface of
stagnant water and as coorongite at Coorong Lagoon in South Australia. The
respective fossil predecessors have been called Pita and Reinschia. As illustrated in
Fig. 2.15, each colony represents a small (up to 0.5 mm in diameter), more or less
spherical aggregate of tubular algal cells, approximately 6 to 10 f.lm in diameter
(Stach et aL 1982). This "structured" alginite is referred to as telalginite in contrast to
lamalginite (Hutton et aL 1980; Cook et aL 1981) which occurs as thin, anastomosing
lamellae formed by algal mats interlayered with mineral grains. The telalginite
bodies commonly represent algal colonies which Kalkreuth (1982) found in some
Cretaceous subbituminous coals from British Columbia to average 1 to 1.5 mm in
maximum diameter, although some bodies were up to 7 mm long. The most
common shape of these algal bodies is spheroidal or flat lenticular but the "needle
coals" from the Elk Formation in the Crowsnest Coalfield of British Columbia
(Gibson 1977), now also being regarded as of algal origin (Kalkreuth 1982) are
distinctly acicular.
Apart from the colonial Botriococcus-type algae there exist unicellular algae
which are almost exclusively marine. Probably the best known among these is the
genus Tasmanites, which is common in tasmanite, a Permian oil shale from
Tasmania. This alga consists of a spherical body, not unlike that of a thick-walled
small macrospore, approximately 0.1 mm in diameter. However, as shown in Fig.
4.29 the spheres are commonly flattened due to overburden pressure.
Algae can exist only in the presence of water, which is the main reason for their
concentration in sapropelic, particularly, bog head coals and other subaqueous
deposits. However, the large amount oflichen-based algal matter produced in moist
rain forests suggests that some alginite might have accumulated in supra-aqueous
environments as well. The amounts of alginite analysed in humic coals are
commonly quite small but its presence is probably underestimated as not allliptinite
of algal origin may be recognised as such (Liu and Taylor 1987). This appears to be
particularly true for liptodetrinite, which consists of degraded liptinite fragments
and may contain a considerable amount of remnants of algae. According to Taylor
and Liu (1989), they have not only a low resistance to biodegradation but their
aliphatic walls may convert into material with the superficial appearance of humic
matter with lower fluorescence (orange to red) compared with the brilliant yellow
fluorescence of alginite in high volatile bituminous coals.
Suberinite is a liptinitic maceral which is similar to cutinite in composition. Its
main constituent is suberin which, according to Kolattukudy (1980), can be likened
to a phenolic matrix with embedded cutin-like polyester domains with long-chain
monomers of fatty acids, alkohols and dicarboxylic acids (Taylor and Liu 1989).
Suberinite originates from suberin-impregnated cell walls of cork tissue (Fig. 4.15).
Usually it traces the outline of the cells of such tissue but in biodegr.aded varieties it
becomes aligned with bedding due to the disappearance of the corpocollinitic cell
fillings. It then appears very similar to the resinite schlieren illustrated in Fig. 4.28
(bottom).
Macerals
121
In spite of their considerable resistance to decomposition, liptinites become
chemically corroded in alkaline environments and may be abraded when subjected
to transportation and redeposition. The residual liptinite debris which is too
rragmented to be identified with any of the above-mentioned liptinite species is
grouped under the maceral term liptodetrinite, some examples of which are shown in
Fig. 4.25 (upper right).
4.1.3.2 Secondary Liptinites
Whereas many of the primary liptinite macerals can be related to distinct plants or
parts of plants, there are several secondary liptinites which are formed during
coalification. Since they are products of chemical dissociation processes and pass
through a fluid stage they do not possess any morphological identity (Teichmiiller
and Ottenjann 1977). They occupy whatever empty spaces were available at the time
of their formation, mostly cell lumens and small fractures and some of them display
intensive fluorescence. Although their proportion is small in most coals they are
widespread in occurrence and have been described from many different coalfields,
sometimes as remobilised or "secondary" resinite (M urchison and Jones 1964; Stach
1966). Teichmiiller (1974) recognised the complex nature of these substances and
distinguished between fluorinite, a dark, i.e. low reflecting decomposition product of
lipid secretions, mainly from leaves (Robert 1979), bituminite, a bacterial decomposition products offats and proteins, and exsudatinite, a liquid derivative ofliptinite
which commonly migrates into cleats and fissures. Further subdivisions have been
suggested by Jacob (1975, 1981) and are under review by the International
Committee for Coal Petrology (ICCP).
4.1.4 Maceral Analysis
Detailed recommendations concerning sample preparation and analytical techniques have been made by the International Committee for Coal Petrology (1963), the
International Standards Association (ISO) and various national bodies with similar
aims; it is consequently not necessary to extend the discussion beyond some general
remarks.
Maceral analyses of bituminous coals are carried out by reflected light microscopy on polished blocks of either solid lumps of coal or of grain mounts. The latter
have been prepared from coal samples crushed to minus 1 mm and set in an epoxitype resin to give a polished surface area of approximately 4 cm 2 • Solid lumps of coal
are usually analysed as part of a petrographic study of a continuous seam profile. In
such case polished blocks are prepared from a piller sample representing either the
full seam thickness or the part to be analysed. Maceral analyses are then carried out
122
Coal Petrographic Entities
14
12
...
10
8
Ul
E
6
ell
C
.
f
4
c5
2
.....
>
%Mlnerals (vo!.) = O. 54 %Ash (mass)
• •
~
~
.............
-.
.....,.
O~~--~~~--~~--~~~~.
o
2.5
5
7.5
10 12.5 15 17.5
" (mass) Ash (db)
20 22.5 25
Fig. 4.30. The correlation between ash
content in % by weight and the optically
assessed mineral content in %by volume for
Carboniferous and Permian coals
on the blocks either by dividing them into fixed intervals of, say one, or two, or
several centimetres stratigraphic length, or by making use ofthe natural divisions of
the seam profile into lithotypes.
Semi-automatic point count methods are applied by advancing the sample by
equal steps on the microscope stage and recording the material at a suitable
reference point in the graticule fitted to the ocular. Since some macerals are defined
by size constraints (e.g. inertodetrinite from 2 to 30 !lm, according to Australian
Standard 2856, 1986) the graticule should be calibrated in reference to the total
magnification used. The latter may range from 250x to 600x. In order to enhance
contrast, oil immersion objectives are generally employed. Because the precision of
the result is determined by the size of the sample population, a minimum of 500
counts is usually carried out for most routine analyses.
Maceral analyses of brown coals are conducted in a similar manner but because
of the high moisture content sample preparation is more time-consuming. If the
polished surfaces are required to last more than a few days, careful drying and
frequent re-impregnation of the coal is necessary. In addition to the 500 points
counted in white light, it is recommended (George 1982) to repeat the maceral count
in fluorescent mode. The reason for this procedure is the difficulty of distinguishing
in white light the low reflecting brown coalliptinites from mineral inclusions. In the
first count (white light) a separation is made between the various huminite and
inertinite macerals while liptinite and minerals remain undifferentiated. They are
identified in the second count by making use of the strong fluorescence of liptinite
macerals, while huminite and inertinite are assigned to the remainder. The two
modes are then combined to give a complete brown coal maceral analysis based on a
total of 1000 points.
The identification and assessment of minerals as part of a maceral count is
limited. Quartz and many silicates are barely recognisable in reflected light because
of close similarities between their refractive indices and that of the immersion oil
used (ne = 1.518/23 QC). Minerals which occur in submicroscopic dispersion are
likewise not recognised, which leaves the opaques and minerals with either very high
or low refractive indices as the main inorganic fraction counted as part of a maceral
Microlithotypes
123
analysis. The correlation between ash contents (in weight%) obtained from
proximate analyses of Carboniferous and Permian bituminous coals and their
respective mineral contents (in volume%), as counted as part of maceral analyses, is
presented in Fig. 4.30.
4.2 Microlithotypes
Macerals are not scattered randomly throughout a coal but tend to be concentrated
in layers in which the one or another maceral group predominates. It was therefore
suggested by Seyler (1954) to define a number of typical maceral assemblages in the
form of microlithotypes. The following description of microlithotypes has been
based on the International Committee for Coal Petrology (1963,1971). A list of the
microlithotype groups is given in Table 4.2.
Three types of microlithotypes, mono-, bi- and trimaceral, are identified on the
basis of their composition and depending on whether they contain macerals of one,
two or three maceral groups. With the various possibilities for composition and
band width of these associations the following definitions apply:
1. The minimum band width of a microlithotype band must exceed 501lm.
2. The monomaceral and bimaceral microlithotypes must not contain more than
5% of macerals from maceral groups which are not characteristic of them by
Table 4.2. The Composition of the common microlithotype groups. (After International
Committee for Coal Petrology 1963, 1971)
Microlithotype group
Maceral group composition
Monomaceral:
Vitrite
Liptite
Inertite
Vitrinite
Liptinite
Inertinite
>95%
>95%
>95%
Bimaceral:
Clarite
Durite
Vitrinertite
Vitrinite + Iiptinite
Inertinite + liptinite
Vitrinite + inertinite
>95%
>95%
>95%
Trimaceral:
Trimacerite
Vitrinite + inertinite + Iiptinite
Coal/mineral associations:
Carbargilite
Carbopyrite
Carbankerite
Carbosilicite
Carbopolyminerite
Minerite
Coal + 20-60vol% clay minerals
Coal + 5-20vol% sulphides
Coal + 20-60 vol % carbonates
Coal + 20-60vol% quartz
Coal + 20-60 vol % minerals
Minerals + 0-40vol% coal
Coal Petrographic Entities
124
definition; the trimaceral microlithotypes must contain more than 5~~ of all
maceral groups.
All microlithotypes are contaminated to some degree by minerals. If the amount is
sufficiently low that the density of 1.5 g/cm 3 in the range of bituminous coals and
anthracites is not exceeded, the microlithotype term is governed solely by the
maceral constitution, as defined in Table 4.2. More strongly mineral-contaminated
microlithotypes with a density between 1.5 and 2.0 g/cm 3 (in the range of bituminous
coals to anthracite), are grouped together under the term carbopolyminerite (Table
4.2), when several minerals are involved or carbominerite, when mineral types are
not specified. In cases of monomineralic coal/mineral associations, the term is
governed by the mineral species, as listed in Table 4.2.
4.2.1 Microlithotype Analyses
The recording of microlithotype groups is sufficient for most microlithotype
analyses but for genetic and other specialised studies it is advisable to analyse for
individual microlithotypes, or subdivide them further, as shown in Chap. 5.2. The
two methods of microlithotype analysis commonly used are as follows:
I. The 20 point ocular designed by Kotter(1959) is an eyepiece graticule which, given
correct microscope magnification, covers an area of 50 by 50 ~m, the minimum
band width according to the microlithotype definitions. Each maceral covered by
the 20 cross-line intersections in the device is taken to represent 5% of the total 50
by 50 ~m area which assists in microlithotype identification according to the 5%
clause. The 20 point ocular is used in conjunction with an electric point counter.
Examples of microlithotype identification by this method are given in Figs. 4.31
and 4.32.
2. The selon la ligne method makes use of a conventional cross-line eyepiece in
which one axis is graticuled and calibrated such that the 50 ~m minimum band
width can be identified. As the specimen is advanced relative to the cross-wire the
thickness of the natural band underneath its centre point is measured and, iflarge
enough to constitute a microlithotype, its maceral proportions are estimated and
the microlithotype identified, whereby use can be made of the graticule intervals.
The main difference between the two methods is that the 20 point method works
within a rigid frame which, on occasions, will integrate maceral proportions across
natural microlithotype boundaries, whereas the selon la ligne method is more
flexible and will take account of the natural stratification of coal. For this reason the
20 point method will somewhat underscore the amount of monomaceralic microlithotypes in a coal sample but this slight inaccuracy is systematic and therefore
predictable, and it is balanced by a greater precision compared with the selon la ligne
method.
125
Microlithotypes
Fig.4.31. Examples oimicrolithotypes as identified by using the lO-point ocular. Upper left Vi trite,
because all graticule intersections (= 100%) are in vitrinite. Upper right Clarite, because five
intersections ( = 25%) are in liptinite, the remainder is in vitrinite. This c1arite consists of resinoc1arite since the liptinite is represented by resinite. Lower left Cia rite, because two graticule
intersections ( = 10%) are in liptinite, the remainder is in vitrinite. This c1arite consists of sporoc1arite since the liptinite is represented by sporinite. Lower right Vitrinertite, because four
intersections ( = 20%) are in inertinite consisting of concentrations of micrinite, the remainder is in
__ !.t._! __ : .. _
r'\~1
: ______ . __
~
_
•
•
. "
I
126
Coal Petrographic Entities
Fig.4.32. Examples of microlithotypes as identified by using the 20 point ocular. Upper left
Trimacerite, because one graticule intersection (= 5%) is in inertinite (small inertodetrinite
fragment in upper centre), two intersections (= 10%) are in sporinite and the remainder is in
vitrinite. For this reason the specific term duroclarite applies. Upper right Durite, because ten
intersections ( = 50%) are in liptinite (mainly sporinite) and the remainder is in inertinite (macrinite
and inertodetrinite). Lower left Inertite, because all intersections ( = 100%) are in inertinite. Because
the latter consists offusinite the specific term fusite applies. Lower right Carbargilite, because eleven
intersections (= ~5°J.:) ::Ire in r.()~1 the: TP:m::.innpr 110: in ,..l !:Hl nt"f"'lInvinlT ,...",,11 In ....... '''' ..... ,..,f fn ... ; ..... ;t ...
127
Lithotypes
Both kinds of microlithotype analyses can be combined with maceral counts, in
the 20 point method by assigning one of the 20 graticule point to double as the
maceral recorder and, in the selon la ligne method by using the central cross-line
point for the same purpose. Further details can be obtained from the International
Committee for Coal Petrology (1963).
4.2.2 The Relationship Between Microlithotypes and Macerals
Although the maceral content of microlithotypes is fixed within defined limits, it
may vary in bi- and trimaceralic microlithotypes over a considerable margin. However, it has been found by Diessel and Callcoti (1965), Smyth (1970), and Bennett and
Taylor (1970) that within a coal seam or in a group of genetically related seams,
microlithotypes groups are relatively consistent in their maceral group proportions.
Some ofthe relationships found in New South Wales coals are listed below (modified
after Diessel and Callcott 1965):
Vitrite
= (vitrinitef/100.
Vitrite + clarite = (vitrinite + liptinite)2/100.
Inertite
= 0.55 inertinite.
Trimacerites
= 0.6 (inertinite + liptinite) + 0.05.
(4.1)
(4.2)
(4.3)
(4.4)
The conditions which control these relationships result from the phytogenic input
and the interaction between biochemical and physico-chemical coalification.
4.3 Lithotypes
The lithotypes of humic bituminous coals consists of macro-petrographic units
which are distinguished and logged on the basis of lustre (bright versus dull) and
fracture pattern (irregular versus smooth), colour and streak of coal, as well as
texture and kind of stratification. While some of these properties also apply to subbituminous and some brown coals, the majority ofthe latter are distinguished on the
basis of colour, structure and desiccation pattern. Shrinkage on exposure is all
important feature of many brown coals because of their high bed moisture content,
which in isometamorphic coals varies with the degree of humification and
gelification displayed by individual lithotypes.
In brown or lignitous coals, several lithotype classifications exist depending on
the preference that is given to the main characteristics, colour or texture. Based on
the latter parameter, Francis (1961) gives the following description of brown coal
types. (In accordance with many authors, Francis uses the term lignite as a synonym
for brown coal. Others restrict the term lignite to "high rank" brown coals. It is used
here in sensu Francis):
128
Coal Petrographic Entities
1. Earthy-brown or fibrous lignites resemble peat in appearance and properties.
They are light brown in colour and of fibrous and earthy texture. The fibrous
habit commonly results from root penetration, and small pieces of wood are
frequent inclusions in the finely fragmented groundmass which is composed
mainly of attrinite. An example is given in Fig. 4.33 (upper right).
2. Woody or xylitic lignites are oflight brown colour and consist for the most part of
coalified wood (xylite) that has largely retained its woody structure (Fig. 4.33,
upper left). In many brown coals, xylitic bands are observed, whilst occasionally
Fig. 4.33. Photographs ofxylite (wood) on left upper side of photograph and earthy brown coal on
right upper side as two examples of textural differences between brown coal lithotypes. Below are
some examples of hard brown coal
Lithotypes
129
whole seams appear to be composed of woody lignite which microscopically
consists of textinite, occasionally leading towards texto-ulminite.
3. Amorphous brown coals are of light or dark brown colour, and of an amorphous
or uniform texture, free from obvious fibrous and woody structure, sometimes
they are soft and easily powdered, i.e. "unconsolidated," while sometimes they are
hard or "consolidated." The bulk of this material consists of highly degraded
attritus in the form of attrinite grading into densinite.
4. Black lignites (Hartbraunkohle in German) are of slightly higher rank than the
above examples of soft brown coal (Weichbraunkohle in German). They are
denser (Fig. 4.33, bottom) and of dark brown or black colour and, depending on
further rank differentiation have either a dull, matt appearance (Mattbraunkohle
in German); sometimes with a silky lustre, or display dull and bright bands
(Glanzbraunkohle in German), as in bituminous coals. In fact, the latter overlaps
with sub-bituminous coal according to the ASTM classification. Microscopically
their woody inclusions show evidence of the rank-related gelificatioin discussed
above. They consist of texto-ulminite grading into eu-ulminite, whereas the
attrital portion has been transformed into densinite.
Other classification systems stress the colour appearance first and use other physical
parameters as additional information. A example is the lithotype classification
applied to the brown coals of Australia as given in Table 4.3.
The macroscopic properties of brown coal lithotypes, particularly the assumptions relating to the degree of gelification are supported by their microscopic
composition listed in Table 4.4. The trend from dark to light coloured lithotypes is
accompanied by a decrease in tissue-derived macerals, although the least humified
Table 4.3. The lithotype classification of air dried soft brown coal. (After George 1975, 1982)
Lithotype
Colour
Texture
Gelification
Weathering
Strength
Dk: Dark
Black
to dark
brown
High wood
content in
small pieces
Extensive
Regular pattern of deep,
wide cracks
Strong,
hard,
dense
M-d:
Medium dark
Dark brown
to medium
brown
High to med. Common
wood content but not
in large pieces extensive
Regular pattern of wide
cracks
Fairly
strong,
hard,
dense
M-l:
Medium light
Medium
to light
brown
High to low
Uncommon
wood content,
well preserv.
wood + stumps
Irregular
pattern of
shallow
cracks
Medium
hardness
and
density
Lt: Light
Light
brown
Medium to
low wood
content
Rare
Random
pattern of
fine cracks
Soft,
low
density
Pa: Pale
Pale brown
to yellow
brown
Low wood
content
Very rare
Fewextensive cracks
Soft,
crumbly,
light
Coal Petrographic Entities
130
Table 4.4. The maceral composition of brown coal lithotypes (mean of five samples) in the
Yallourn Seam, Yallourn Open Cut, Latrobe Valley, Victoria. (After George 1982)
Maceral (Sub-)Group
Maceral
Lithotypes
Dk
M-d
M-l
Lt
Pa
Textinite
Texto-ulminite
Eu-ulrninite
1.2
14.6
16.2
3.2
16.4
4.0
2.1
12.9
1.8
0.7
7.8
1.8
1.5
5.3
0.5
32.0
23.6
16.8
10.3
7.3
32.2
21.3
51.1
8.6
64.5
1.2
70.5
0.4
63.5
0
53.5
59.7
65.7
70.9
63.5
5.2
0
4.3
1.4
1.0
9.5
1.6
0.8
5.7
0.2
1.2
5.3
0
0.7
1.2
9.5
11.9
8.1
6.7
1.9
0.6
1.6
0.4·
0
0
0.4
0.5
0
1.7
1.1
2.0
0.4
1.6
1.8
2.0
3.3
1.0
0.2
3.2
2.8
7.8
0.3
2.1
1.4
12.1
2.6
3.7
7.8
10.5
23.7
0.4
0.7
1.3
0.2
0
0.7
0
0
0.6
0
0
1.0
0
0
2.4
2.4
0.9
0.6
1.0
2.4
0
0.2
0.6
0.6
0.5
0
0
0.4
0
0.7
Humotelinite (Telovitrinite)
Attrinite
Densinite
Humodetrinite (Detrovitrinite)
Eugelinite
Porigelinite
Corpogelinite
Humocollinite (Gelovitrinite)
Sporinite
Cutinite
Resinite
Suberinite
Liptodetrinite
Liptinite
Semifusinite
Fusinite
ScIerotinite
Telo-inertinite
Detro-inertinite
Minerals
Inertodetrinite
cell tissue is recorded in the medium dark lithotypes (M-d), where the combined
textinite and texto-ulminite proportion reaches the highest value of tissue retention
of 19.6%. Gelified cell tissue (eu-ulminite) shows a steady decline from dark to pale
lithotypes which is also shared by humocollinite. In contrast, the liptinite content,
mainly in the form of liptodetrinite, increases significantly in the same direction.
The trends in maceral composition are mirrored by the chemical composition of
the brown coal lithotypes listed in Table 4.5. To some extent it confirms the
microscopic assessment ofthe lithotypes. For example, the increase towards the pale
end-member of both the hydrogen content and the Hie ratio is to be expected on
account of the sharp rise in liptinite. It is interesting to note that the carbon content
increases too, although the contribution of wood to the coal decreases in the same
direction. The reason for this is the higher proportion ofthe relatively heavy oxygen
(compared to hydrogen) in the darker lithotypes. It should be remembered from
Lithotypes
131
Table 4.5. The chemical composition of brown coal lithotypes in Yallourn and Morwell Open
Cuts, Latrobe Valley, Victoria. (After George 1982)
Lithotypes
Dk
M-d
M-I
Lt
Pa
Yallourn Open Cut
(Mean of 26 samples)
Ash (db)
Volatile matter (daf)
Carbon (daf)
Hydrogen (daf)
Oxygen (daf)
Specific energy (gross, db)
Atomic HIC
Atomic OIC
0.9
50.6
68.0
4.7
26.4
26.36
0.83
0.29
0.9
50.4
68.3
4.7
26.1
26.48
0.83
0.27
0.8
51.3
68.0
4.8
26.3
26.27
0.85
0.29
1.2
56.6
69.3
5.5
24.2
27.78
0.95
0.26
1.1
63.4
70.1
6.5
21.9
29.26
1.11
0.23
Morwell Open Cut
(Mean of 35 samples)
Ash (db)
Volatile matter (daf)
Carbon (daf)
Hydrogen (daf)
Oxygen (daf)
Specific energy (gross, db)
Atomic HIC
Atomic OIC
3.1
48.1
69.3
4.8
25.0
26.89
0.83
0.27
3.2
48.6
68.6
4.7
25.8
26.50
0.82
0.28
3.7
51.0
69.9
5.1
24.2
27.45
0.87
0.26
3.8
54.4
70.5
5.4
23.2
28.08
0.92
0.25
4.4
57.4
70.9
6.0
22.2
29.03
1.01
0.23
Fig. 3.30, that methane having the highest atomic HIC ratio of all hydrocarbons
with 4 hydrogen atoms for each carbon atom, contains 75% (by mass) carbon,
whereas in carbon dioxide, which contains only two oxygen atoms per carbon atom
the mass of carbon is down to 27.3%. Another interesting aspect is the increase in ash
from dark to pale lithotypes. It would be tempting to explain these simple trends as
expressions of a continuous change in either the composition of the contributing
flora, in the conditions of its transformation into peat and brown coal, or both.
However, the interpretation of the compositional trends is far from simple and has
been the subject of an ongoing debate in the literature, whereby the origin of the pale
bands attracted most attention. This problem will be discussed in Chap. 5.3.2.
Upon their transformation into sub-bituminous coal, the various types of peat
and brown coal become highly compacted and consolidated. The increase in density
is accompanied by a deepening in colour and the acquisition oflustre in lithotypes of
homogeneous composition, particularly those consisting of vitrite only. This results
in the development of even fracture (occasionally also conchoidal) and leads to a
likewise even light reflexion, which gives this coal type a bright appearance. In
contrast, lithotypes of heterogeneous composition, for example, those containing
mixtures of macerals and microlithotypes, display an irregular fracture pattern with
the concomitant diffuse reflection of light. The result is a dull appearance.
The banded character of humic coals was observed from the earliest beginning
of coal mining and it has also been long established knowledge that the suitability of
coal for various technical processes varied with the prevalence or absence of certain
Coal Petrographic Entities
132
banded constituents. In 1887 Fayol (in Freund 1952) published the results of
ultimate analyses and coking tests which were carried out on the four different
macroscopic coal types he distinguished in French coals. His four types are the same
four ingredients which Stopes (1919) in the United Kingdom later called vitrain,
clarain, durain and fusain.
The lithotype classification listed in Table 4.6 has been based partly on German
usage which is purely descriptive, provides more subdivisions than Stopes' (1919)
"four ingredients" and can be applied to a wide range of coals. It also correlates well
with radiographs, as obtained from X-rays of bore-cores (Jones 1970). As used
by Diessel (1965a), the minimum band width for an individual lithotype is 5 mm. If,
for example, a number of thin bands of bright and dull coal occur in succession and
each of the individual bands is less than 5 mm thick, the whole unit is taken as
banded bright coal if bright coal exceeds dull coal in quantity, as banded coal if the
proportion of both is equal, and as banded dull coal ifthere are more dull than bright
bands. As soon as one of the individual bands becomes 5 mm thick or more, it forms
a lithotype of its own, such as bright coal or dull coal. An example of the application
of the lithotype classification is given in Fig. 4.34, which demonstrates some of the
detailed information that can be gained by a carefully conducted lithotype analysis.
Table 4.6. The classification of black coal lithotypes. (After Diessel 1965a)
Lithotype
Description
Bright coal
(vitrain)
B
Banded bright coal
(clarain)
Bb
Banded coal
(duroclarain)
Banded dull coal
(clarodurain)
BD
Dull coal
(durain)
Fibrous coal
(Fusain)
Shaly coal
D
Db
F
Cs
Coaly shale,
Sc
mudstone,
sandstone etc.
Carbonaceous shale,
mudstone, siltstone etc.
Shale, mudstone,
siltstone,
sandstone etc.
Vitreous to subvitreous lustre; even to conchoidal fracture;
brittle; may contain up to 10% dull coal in bands less than
5mm thick
Mainly bright coal containing thin (less than 5 mm) dull
coal bands ranging in proportion between 10 and 40%;
even fracture
Contains bright and dull coal bands (all less than 5 mm)
ranging proportion between 40 and 60% each
Mainly dull coal containing thin (less than 5 mm) bright
bands ranging in proportion between 10 and 40%; uneven
fracture
Matt lustre and uneven fracture; may contain 10% of
bright coal bands less than 5 mm thick
Dull with satin sheen; friable; may contain up to 10% of
other coal lithotypes less than 5 mm thick
Contains between 30 and 60% of clay and silt either in
intimate mixture with coal or in separate bands each less
than 5 mm thick
Consists of alternating laminae (each less than
5 mm thick) of non-coal and coal, the latter not exceeding
40% of total
Any sediment containing 60 to 90% finely disseminated
carbonaceous matter
Any sediment containing less than 10% carbonaceous
matter
Lithotypes
133
Scale in em
0
4" Bond
10
20
30
40
50
H i ddll!- Band
60
70
80
90
100
BoUom Band
Miner's Floor
Bright Coal (Vitroin)
Banded Bright Coal (Clorain)
Bonded Coal (Duroclarain)
Banded Dull Coal (Clarodurain)
Steel Band
Dull Coal (Duroin)
Fibrous Coel (Fusain)
Shaly Coal
Coaly/Carbonaceous Shnle
Shale
Claystone IT onstein
Sendstone
Fig. 4.34. Lithotype section of the Borehole Seam from the Sydney Basin (Newcastle Coal
Measures), New South Wales. Four persistent dirt bands, three of which represent kaolinitic
claystones (tonsteins) have been named. Note the occurrence of gross compositional cycles in some
intervals, for example, between the Middle and Four Inch Bands
Coal Petrographic Entities
134
100
90
80
70
60
50
40
30
20
10
0
Sc
Cs
D
Db
F
Fig. 4.35. Aggregate lithotype compositions of some New South Wales coals. The
seams represented are as follows: a Wallarah
S; b Bulli S; c Borehole S; d Yard S; e Dudley
S; jVictoria Tunnel S The lithotype symbols
are as in Table 4.6
Moreover, it has been found (Diessel, 1965a) that many coal seams are characterised
by relatively constant lithotype proportions which are maintained within narrow
limits over many kilometres and do not change abruptly. When plotted as
cumulative curves, as illustrated in Fig. 4.35, generally brighter or duller coal seams
can be distinguished.
4.3.1 Lithotype Analysis
Although the method of identification of various lithotypes is somewhat subjective,
valuable information in terms of coal composition, genesis and technical properties
can be gained from lithotype analysis. When applied to bituminous coal the latter is
also referred to as "brightness log" and is frequently the first step in the assessment of
a bore core or a newly exposed coal face. Even when a full lithotype analysis is not
carried out, lithotype groupings are commonly used to divide the seam into
subsections which are sampled for further analysis.
Analysis procedure is simple and consists of recording the width of each
lithotype either in bore core or in outcrop with the aid of a measuring tape and
noting the readings either in writing or in coded form. When using bore cores, it is
advantageous to split them longitudinally with the aid of a chisel because the
fractured surface allows for a better distinction between different lithotypes than the
drilled core surface. Brown coals are best left to dry and develop their characteristic
desiccation pattern before logging them. The visual identification oflithotype colour
can be supported by the colorimetric determination of the colour index on a ground
sample of air-dried coal (Attwood et al. 1984). According to Higgins et al. (1980) and
Mackay et al. (1985), it is a measure of the "degree of brownness" which correlates
well with conventional lithotype logs.
In recent years visual lithotype identification has been supplemented by
radiographic methods whIch have been specifically developed for non-destructive
bore core analysis (Jones 1970).
Lithotypes
135
4.3.2 The Relationship Between Lithotypes, Macerals
and Microlithotypes
From Table 4.6 follows that any coal lithotype may contain up to 30% inorganic
constituents because only when this figure is exceeded, the term shaly coal is used.
The actual proportions oft'nacerals and minerals contained in the various lithotypes
follow set patterns which are specific for individual coal seams, provided they are not
affected by marked lateral facies and/or rank changes which would alter lithotype
properties. An example of the compositional relationship between lithotypes and
maceral plus minerals is given in Table 4.7, while in Fig. 4.36, trends in lithotype
composition are given for four different sets of Australian bituminous coals. Some of
these consist of several coal seams of similar composition and origin. An example
are the four coal seams of the Lambton Subgroup of the Newcastle Coal Measures
which in the lithotype diagram of Fig. 4.35, have plotted the virtually identical
cumulative curves c, d, e and f.
The diagrams of Fig. 4.36, show gross similarities in maceral composition of the
various lithotypes, although some systematic variations occur. These affect mainly
the heterogeneous lithotypes of coals which have been formed under contrasting
circumstances. For example, the rather dull coals of the Rangal Coal Measures and
the Bulli Seam contain less vitrinite and more inertinite in almost all lithotypes
compared with the generally bright coals of the Lambton Subgroup and the
Wongawilli Seam. Even stronger differences are found in the distribution ofliptinite,
for which there are several rea~ons. The similarity in the liptinite contents of the Bulli
and W ongawilli lithotypes (full circles in Fig. 4.36) is due to their comparatively
Table 4.7. The maceral composition of black coal lithotype of a bituminous coal from the Rangal
Coal Measures, Blackwater District, Queensland, Australia
Macerals and groups
Lithotype
B
Bd
BD
Db
D
F
Telovitrinite
Detrovitrinite
84.4
6.8
67.0
14.8
46.4
19.6
12.2
11.0
0.2
2.0
0
1.2
Vitrinite
91.2
81.8
66.0
23.2
2.2
1.2
Sporinite
Cutinite
Resinite
0.4
0
3.0
0.8
0
2.4
2.0
0.6
1.0
1.8
0.2
0.4
4.0
0
0
0.2
0
0
Liptinite
3.4
3.2
3.6
2.4
4.0
0.2
Telo-inertinite
Detro-inertinite
Gelo-inertinite
2.6
1.2
0.4
7.2
4.4
0.4
20.0
7.2
2.4
45.0
17.8
6.4
26.6
50.2
9.8
94.1
3.6
0
Inertinite
4.2
12.0
29.6
69.2
86.6
97.7
Minerals
1.2
3.0
0.8
5.2
7.2
1.0
136
Coal Petrographic Entities
100
80
60
40
20
<P-
O
100
Minerals
80
60
40
5
20
<P-
O
B
Bd
BD
Db
D
B
Bd
BD
Db
D
F
Fig.4.36. Four diagrams illustrating mean maceral group composition of lithotypes in four
Australian sets of bituminous coals. Open squares Rangal Coal Measures, Queensland; Open
triangles coals of the Lambton Subgroup, Newcastle Coal Measures, N.S.W.; full squares Bulli
Seam, IIIawarra Coal Measures, N.S. W.; filll triangles Wongawilli Seam, IIIawarra Coal Measures,
N.S.W. The full circles used in the Iiptinite diagram (upper right) represent the combined values for
Bulli and WongawiIli lithotypes, which are too closely spaced to be shown separately. The lithotype
symbols are as in Table 4.6
high ranks and the difficulty of identifying liptinite after it has acquired vitrinite
reflectance. The differences in liptinite distribution between the coals of the
Lambton Subgroup (open triangles in Fig. 4.36) and the Wongawilli Seam (full
circles in liptinite diagram of Fig. 4.36) are therefore related to different physicochemical coalification paths and have nothing to do with their respective vegetal
sources or the conditions of peat accumulation. This is different when the Lambton
Subgroup is compared with the Rangal Coal Measures (open squares in Fig. 4.36).
Their inertinite content increases from banded coal (or duroclarain, BD in Fig. 4.36)
to banded dull (or clarodurain, Db in Fig. 4.36) and dull coal (or durain, D in
Fig. 4.36) by very large amounts compared with the moderate increases in the
Lambton Subgroup coals (and the Wongawilli Seam). The reverse is the case in
the distribution of liptinite between the two coals over the same lithotype range.
The two sets of coals differ likewise in the distribution of minerals which display
a strong increase in the dull lithotypes of the Lambton Subgroup and the Wongawilli
Seam but only moderate gains in the Rangal Coal Measures and the Bulli Seam.
From the above discussion follows that a distinction can be made between two
kinds of dull coals or durains, one type in which the "dullness" is due to a
concentration on inertinite plus moderate amounts of liptinite and minerals, and
another type which is dull because of high liptinite and mineral contents and a
Minerals
137
Table 4.8. The microlithotype composition of black coal lithotypes of a bituminous coal from
the Rangal Coal Measures, Blackwater District, Queensland. (Analysed by J.G. Bailey, The
University of Newcastle, N.S.W.)
Microlithotype groups
Lithotypes
B
Bd
BD
Db
D
F
Monomaceral:
Vitrite
Inertite
69.3
0.7
60.7
7.0
27.0
15.8
17.6
26.2
1.3
89.3
n.d.
n.d.
Bimaceral:
Clarite
Durite
Vitrinertite
16.0
0
9.1
7.4
0
19.9
1.2
0.1
50.5
0.7
2.5
45.7
0
5.8
2.7
n.d.
n.d.
n.d.
Trimaceral:
Trimacerite
2.1
4.3
5.1
7.2
0.1
n.d.
Coal/mineral associations:
Carbominerite + minerite
2.8
0.7
0.3
0.2
0.8
n.d.
moderate admixture of inertinite. Their microlithotype composition follows the
same pattern, an example of which is given in Table 4.8 for the Rangal Coal
Measures in which dull coal (D) shows a significant concentration of inertite
followed by inertinite-rich durite. Much of this inertinite consists of oxy- and
degrado-semifusinite in contrast to the inertodetrinite and macrinite which
constitutes the inertinite of the spore-rich durite in the dull coals of the Lambton
Subgroup. Mary C. Stopes' observation that "one may say that, on the whole, durain
is essentially composed of a high proportion of opaque, fine granules, with many
macro- and microspore exines scattered through it like currants in a pre-war
pudding" (1919, p. 480) is an apt description ofthe spore-rich durites which with few
exceptions (e.g. Smith 1964) have been variously described as sapropel-durite
(Potoni6 1924), subaquatic oozes (Teichmiiller 1950; Hacquebard et al. 1964;
Reidenouer et al. 1967), or wet durite (Stach et al. 1987), in order to make a genetic
distinction from the fusinite and semifusinite rich dry durite or humus durite of
Potoni6 (1924) which represents a partially oxidised peat (Stach et al. 1987).
4.4 Minerals
Most inorganic matter contained in coal occurs in the form of mineral inclusions.
They constitute the bulk of non-combustible portion of coal which, on burning,
are left behind as ash. Its mass is smaller than that of the mineral content
due to the dehydroxylation of clay, the loss of CO 2 from carbonates and
other alterations which in average amount to a loss in mass by a factor of 1.1.
Because of their effects on coal utilisation, ten elements contained in coal ash are
routinely determined in most commercial coal analyses and listed as oxides. They
Coal Petrographic Entities
138
are:
The proportions in which these and other elements occur in the ash is governed by
their stoichiometric and crystallographic relationships in the various mineral
species and their proportions in the host coal. Because several elements are often
combined in one mineral, a large amount of one element will therefore require a
proportional presence of another element, sometimes at the expense of some
other elements. An example is silica, a large proportion of which is combined with
alumina in coal in the form of clay. In the Carboniferous coals of the Ruhr Basin this
relationship can be expressed as (Anon 1984):
Al 2 0 3 = - 16.2 + 1.91 Si0 2 - 0.02 SiO~.
(4.5)
Conversely, silica has an inverse relationship with iron which in coal is bonded to
either carbonate or sulphur.
Depending on their origin and association with the coal, three groups of
inorganic constituents can be distinguished. The first group comprises phytogenic
minerals derived from the inorganic matter contained in the coal-forming plants, the
second group includes all minerals which were either washed into the coal swamp as
detrital fragments or which were precipitated from migrating solution, whereas the
third group consists of dissolved inorganic matter in the coal's pore or surface water
(Ward 1986a). In contrast to the so-called inherent ash formed from the first group,
the minerals resulting from the second and third group are referred to as
adventitious (Francis 1961).
A clear distinction between the inherent and adventitious groups of minerals is
not always possible, especiallly when they were precipitated from solutions which
may have been derived from either plant ash or from ions transported into the area
of deposition. Commonly both possibilities together will be the source of mineral
matter. Many minerals occur in close association with the coal matrix either as
infillings of cleats, cell lumens or as partial metasomatic replacement of organic
matter (Balme and Brooks 1953; Cook 1962; Beeston 1981) but others are
concentrated in concretions, lenses or dirt bands. Although the list of minerals found
in coal is long (Kemezys and Taylor 1964; Mackowsky 1968; Davis 1982; Warne
1982), only the major groups and those with significance for palaeo-environmental
analysis will be treated here. The latter aspect will be further discussed in subsequent
chapters.
4.4.1 Phytogenic Minerals
Plants require inorganic matter for a variety of purposes, including growth,
protection and others. It is absorbed from the environment in which the plants live,
in the case of aquatic vegetation from the surrounding water whereas land plants
extract the reauired constituents from the soil
Minerals
139
Different groups of plants need different types of inorganic matter, some are
sensitive to small amounts of calcium, others can only thrive in lime-rich soils.
Grasses usually absorb large amounts of silica, which is concentrated in their blades
and renders them quite sharp. This material provides an important source of
dispersed opal found in some soils (opaline phytoliths) and may even be adventitious
to peat, such as the large proportion of dispersed opal which according to Finney
and Farnham (1968) is blown into the Minnesota bogs from the surrounding
grasslands. Also much of the fine crystalline quartz found dispersed in coal, has been
shown to be of biogenic origin (Raymond and Andrejko t 983; David et al. t 984).
Some phytogenic precursors of coal, for example, lycopods, contain substantial
amounts of alumina in their cell walls.
According to Francis (t 96 t), the more commonly occurring inorganic constituents of plants are compounds of:
Ca, Mg, Fe, AI, Na, K, Mn, Ti, S, Si, CI and P.
Most of these inorganic constituents remain in the peat, though often in changed
form. While incorporated in vegetable matter, the above elements would be part of
complex organic compounds and some of them would remain in organic affiliation,
for example by sorption to humus colloids and the formation of Me-humates after
decomposition of the host plants. Other elements would probably form inorganic
constituents, particularly, during advanced humification ( = beginning ofmineralisation). Some of the above elements may be removed altogether during the
biochemical stage of coalification. These are the chlorides of sodium, potassium and
magnesium, which are dissolved in water and leave the system during compaction
and dehydration. Also, some of the nitrogen and sulphur may be released in this
manner (Baragwanath 1962).
The frequently difficult identification of the organic or inorganic affiliation of an
element was solved by Warbrooke and Doolan (t 986) by splitting the ground coal
sample into five density fractions, namely Fl. (float) 1.30, 1.40, 1.50, 1.60 and S. (sink)
1.60 and by determining the elemental distribution separately for each of the five
subsamples. The authors established that elements in organic affiliation concentrate
in the low density fractions, whereas elements occurring in inorganic affiliation are
found in the high density fractions. Combinations of the two occurrences are
reflected in the proportional distribution of the various elements between the
different density fractions.
The contributions made by inherent inorganic constituents to the total mineral
matter content of coal is relatively small because the total proportion of inorganic
matter contained in plants is usually less than 2% and probably less than 5% in coal
(Finkelman 1982). However, variations will occur in response to the degree of
humification and mineralisation of the biomass before and during peat formation.
For example, Hamilton and Salehi (1986) and Salehi and Hamilton (1986), by
subjecting coal macerals to energy dispersive X-ray (EDX) analysis in SEM
backscattered electron mode (BSE), find both similarities (e.g. sulphur) and
significant variations in the distribution of inherent ash-producing elements
between different maceral groups, as well as between members of the inertinite
140
Coal Petrographic Entities
of inherent ash) have been found in inertodetrinite which is not surprising since in
coals formed from rather oxidised terrestrial peat it often represents the last organic
residuum before complete mineralisation of the vegetable matter. Moore (1964) goes
as far as suggesting that kaolinitic claystones (tonsteins) may consist of the residual
minerals left after oxidative removal of organic matter, and also Renton and Cecil
(1979) thought this to account for the occurrence of some clay partings in coal seams.
Although in many cases lj.lternative modes of origin might apply to the occurrence of
both tonsteins and other clay partings (see Chap. 4.4.2.1 below), the observations of
Davis et al. (1984) in the Okefenokee Swamp have demonstrated that phytogenic
minerals are likely to be concentrated at the expense of organic matter in heavily
degraded peats. A similar relationship must be expected in coals rich in autochthonous inertinite. This aspect will be explored further in Chaps. 5 and 6.
4.4.2 Adventitious Minerals
This group of minerals is added to the peat or coal from sources which, at the time of
emplacement, existed outside the organic deposit. According to Warne (1982) this
may have been effected in any of the following ways:
1. fallen or blown in by gravity or air movement,
2. washed in, for example, during periods of flood,
3. transported in solution laterally and/or vertically by surface or ground water
currents including compaction fluids,
4. related to changed chemical environments in the peat due to marine or fresh
water influences,
5. due to secondary alterations of minerals, for example, during diagenesis.
Depending on the timing of mineralisation in relation to peat formation and
coalification, a distinction can be made between syngenetic and epigenetic minerals
in coal.
Syngenetic minerals are deposited (if detrital) or precipitated (if authigenic)
concurrently with peat accumulation. Such occurrences are, for example, recognisable by the bulging of coal laminae around mineral grains or concretions which
indicates that the resistant component was already in its present position before
diagenetic compaction had affected much of the peat. Well publicised examples are
the dolomite nodules (Stocks 1902; Kukuk 1906; Stopes and Watson 1908;
Teichmiiller et al. 1953/54 and subsequent authors), whose well-preserved inclusions of uncompacted plant and peat fragments have yielded much information
about the phytogenic progenitors of coal. Other common syngenetic concretions
and petrifications of vegetal matter consist of siderite, silica and pyrite.
Epigenetic minerals are practically always the result of chemical precipitation
(authigenesis) at a time when much of diagenesis was completed. Compactional
viinerals
141
listortion of coal bands around such minerals is therefore missing and frequently
hey occur on cleats and fissures in the coal or have metasomatically replaced
)revious minerals or peat.
1.4.2.1 Silicate Minerals
)f all the silicate minerals found in coal none are so widespread and universally
Ibiquitous as the clay minerals which will dominate the discussion in this chapter.
\11 other silicate minerals pale in quantitative significance- compared with clay
ninerals which, when concentrated in the form of claystone layers constitute
mportant marker horizons in coal seams. Although they are highly variable in their
)hysical behaviour and chemical composition, clay minerals have some common
eatures, such as sheet structures with alternating linked Si0 4 -tetrahedrons and
\1203-octahedrons, a high water content and the capacity of ion exchange.
One of the most common clay minerals in coal is kaolinite. It occurs in a variety
)f forms ranging from infillings in plant cell cavities (Balme and Brooks 1953) to
inely dispersed inclusions in vitrinite, from discrete pellets and vermicular
199regates (Kemezys and Taylor 1964) to continuous claystone bands (Loughnan
1966), as illustrated in Fig. 4.37. The latter are commonly only a few centimetres
·jg.4.37. Photograph of a tontein band (Middle Band) in the
lorehole Seam of the Newcastle
:oal Measures, New South
¥ales
142
Coal Petrographic Entities
thick, but may be laterally quite persistent. Following German usage, the later are
commonly referred to as tonsteins, of which there are. several varieties.
According to Schuller et al. (1956), the following types of tonsteins may be
distinguished:
1. Crystal tonstein: contains vermicular, prismatic or tabular kaolinite crystals
which may range in colour from beige to dark brown according to the proportion
of contained carbonaceous matter. The crystals are embedded in a matrix which
consists of finely-crystalline to cryptocrystalline kaolinite.
2. Pseudomorphous tonstein: (Figs. 4.38 and 4.39) characterised by numerous pseudomorphs of kaolinite after feldspar, mica or volcanic glass shards set in a dense
kaolinitic matrix.
3. Pellet (Graupen) tonstein: (Fig. 4.40) consists predominantly of subspherical
kaolinite grains oflighter or darker shades, often surrounded by collinite. These
grains show a cryptocrystalline to finely crystalline structure; the cryptocrystalline material is isotropic.
4. Dense tonstein: consists almost entirely of a fine-grained kaolinite groundmass,
showing weak aggregate-polarisation, containing isolated corroded crystals of
kaolinite. Such bands are commonly more than 10 mm thick and light in colour.
According to Stach (1968), the kaolinitic tonsteins are not restricted to particular
positions within coal seams, nor are they confined to coal seams but have been
observed to extend also into interseam sediments (Hartung 1942; Burger 1958, 1960,
1962; Burger et al. 1962; Hartlieb 1962). When these rocks were first described in
1894 in the Saar Basin by Schmitz-Dumont (cited by Hartlieb 1960) volcanic ash
falls were considered to be the most satisfactory explanation for their surprising
lateral persistence over tens of kilometres compared with their thickness of only a
few centimetres. Moreover, volcanism was known to occur in the stratigraphic
vicinity of the coal deposits of Saxony, Silesia and the Saar where the first
observations had been made. Additional evidence for the pyroclastic nature of
tonsteins was seen in the presence of inclusions of sanidine, angular quartz and
volcanic glass shards.
Stutzer (1931) and Bode (1937) were early supporters of the volcanic ash fall
origin of tonsteins, while Hartung (1942) distinguished between fall-out tonsteins
and a second variety derived from reworked ash falls. Petrascheck's (1942) proposal
of a ton stein origin from reworked tuffs was widely supported at the time, although
some authors, among them Stach (1950) continued to adhere to the fall-out
hypothesis. Other authors rejected any connection between tonsteins and volcanism
altogether. Among them was Moore (1964) who, as mentioned above, suggested that
tonsteins may consist of the residual minerals left after oxidative removal of organic
matter. A second "non-volcanic" explanation of the origin of tonsteins is by
derivation from ordinary clay-rich sediments. This idea was first expressed by
Termier (1923) and followed by Schuller et al. (1956). Hoehne (1948, 1951, 1954,
1957) has been the most consistent supporter of another genetic concept. He
considers tonsteins to be chemical precipitates which derived from aqueous
solutions via silica-alumina gels which has also been supported by Teichmuller et al.
Minerals
143
Fig. 4.38. Photomicrograph of a tonstein (intra-seam tulTs) from the Newcastle Coal Measures,
New South Wales, with numerous biotite inclusions in kaolinite/montmorillonite matrix in various
stages of biotite replacement by kaolinite. All in transmitted light, one polar; actual length of each
field of view = 2.6 mm for upper left, = 0.9 mm for upper right 0.36 mm for centre and bottom
(1952), while Bolewski (1937) described a tonstein from Upper Silesia as a "clayey
laterite" formed by coagulation of silica and alumina colloids. This line was later
followed by Loughnan (1962), who does not entirely rl!ject the pyroclastic influence
on tonstein formation but suggests that volcanic parent material might be converted
into laterite under tropical or subtropical conditions. the the weathered material
144
Coal Petrographic Entities
Fig.4.39. Photomicrographs of tonsteins (intra-seam tuffs) from the Newcastle Coal Measures,
New South Wales. Upper left Devitrified glass shards set in a kaolinitic matrix; one polar. Upper
right As before with crossed polars. Centre left As in upper left but with more feldspar and quartz.
Centre right Pumice fragment with degassing vesicles. Bottom left Two partially kaolinitised
volcanic rock fragments, one aphanitic (on left), the other (on right) with trachytic texture. Bottom
right As before with crossed polars. All in transmitted light, one polar; actual length of each field of
view = 2.6 mm for top and centre left = 0.9 mm for centre right and bottom
then being transported into coal basins where the bauxite minerals are transformed
into kaolinite by resilicification.
From 1960 onward evidence in favour of the pyroclastic origin of tonsteins
begins to mount. Kirsch and Hallbauer (1960) describe high temperature feldspar
(sanidine) in a tonstein from the Ruhr Basin, which is followed by the discovery
(Hall bauer et al. 1962) that kaolinite is not the only clay mineral in these rocks but
that in some Ruhr and Saar tonsteins it is associated with montmorillonite-illite
Minerals
145
Fig.4.40. Photomicrographs of pellet (graupen-) tonsteins (intra-seam tuffs) from the Newcastle
Coal Measures, New South Wales. Upper left and right General view. Centre Enlargement of
central portion of upper right with biotite in kaolinitised volcanic rock fragment, left with one polar,
right with crossed polars. Lower left Pellet tonstein with partially kaolinitised biotite. Lower right
Enlargement of frontal portion of partially kaolinitised biotite. All in transmitted light, one polar;
actual length of each field of view = 2.6 mm for upper left and right and lower left, = 0.9 mm for
centre left and right, = 0.36 for lower right
146
Coal Petrographic Entities
mixed layer clays. Francis (1961) and Francis and Ewing (1961) describe kaolinitised
tuffs from Scotland, while Bouroz (1962) finds tuffaceous relicts in Japanese
tonsteins and proposes the generic term "cinerite" for all tephra-derived tonsteins.
An important contribution is made by Stoffier (1963), who concludes that the
particle size distribution of quartz and zircon in Saar tonsteins, the habit of their
quartz inclusions, the number and distribution of their accessory heavy minerals,
high beryllium and low chromium contents point towards rhyolitic tuffs as the most
likely source material. This is followed by Schellendorf's· (1964) acceptance of a
pyroclastic origin of tonsteins for reasons of composition and mode of transportation. O'Brien and McKenzie (1963) describe the widespread occurrence of
tuffaceous claystones in the coal seams of the Sydney Basin, New South Wales, to
which Diessel (1965b) applies the tonstein classification of Schuller et al. (1956) and
presents photomicrographs of well-preserved shards of volcanic glass. The application of advanced analytical techniques by Price and Duff (1969), Bohor and
Pillmore (1976), Spears (1970, 1987), Spears and Kanaris-Sotiriou (1979), Fuchtbauer and Riedel (1979), Bohor and Triplehorn (1981), Triplehorn and Bohor(1981),
Lippolt and Hess (1985), as well as more extensive and detailed field observations in
various parts of the world by Duff(1972), Zaritsky (1977, 1985), Burger (1979, 1985),
Ryer et al. (1980), Pevear et al. (1980), Zhou et al. (1982), Bouroz et al. (1983),
Addison et al. (1983), Bouroz and Spears (1985), Diessel (1985c) including the study
of modern volcanic analogues by Francis (1985) and others, have led to a virtual
consensus about the volcanic origin of tonsteins.
Some of the most convincing evidence for the pyroclastic nature of these rocks
can be found in the Permian Newcastle Coal Measures of the Sydney Basin, New
South Wales, in the form of numerous transitions from barely altered tuffs to both
genuine kaolinitic tonsteins and montmorillonitic bentonites. As discussed by
Diessel (1985c), they have been derived from explosive volcanism which was active
during coal measure sedimentation, probably several tens of kilometers outside the
present boundary of the Newcastle Coalfield. A distinction is made between thick
(up to 15 m) interseam tuffs and thin intra-seam claystones or tonsteins which occur
in large numbers in some coal seams and represent true cinerites. As an example the
coal seams and their tonstein and other dirt bands of the Lambton Subgroup in the
lower portion of the Newcastle Coal Measures are shown in Fig. 4.41.
Many of the Australian tonsteins consist of fine ash particles which have been
converted into a dense matrix of either montmorillonite or kaolinite and are too
small to be resolved by light microscopy. In colour they vary from light grey and
cream in the bentonitic varieties to light and dark brown when dominated by
kaolinite. The darker colour is usually the result of intimate mixing with
carbonaceous matter, although partially oxidised siderite which results from the
devitrification of glass also produces brown tinges. It is a common observation that
the kaolinitisation of the tonsteins has progressed furthest where their contact with
organic matter was closest. The thick interseam tuff bands are therefore hardly ever
kaolinitised.
Coarse intra-seam tonsteins varieties are rich in devitrified shards of volcanic
glass examples of which are shown in Fig. 4.39 (top and centre left). Other
components include pumice fragments illustrated in Fig. 4.39 (centre ridlt) and
Minerals
147
BOREHOlE SEAN
V IC TOR.I A TUNNEL SI: AM
DUOLEY SE AM
YARD SEAN
............
.........
............
.A ........
LEGEND
_
Coal
-=:II
Co.eJl)",ne, I~
~
TOMtelft
mmmmmmn Shaly COllI
Fig. 4.41. The distribution of tonsteins and other dirt bands in the coal seams of the Lambton
Subgroup of the Newcastle Coal Measures, Sydney Basin, New South Wales
volcanic rock fragments in various stages of kaolinitisation (Fig. 4.39, bottom).
Residual minerals consist mainly of quartz, K-feldspar, plagioclase and, most
commonly, biotite. As illustrated in Figs. 4.38 and 4.40, biotite displays numerous
examples of kaolinitisation, whereby the micaceous matter is usually replaced by
kaolinite along basal cleavage planes. The associated increase in volume causes the
resulting aggregates of kaolinite to swell into oblong, subspherical shapes
characteristic of pellet (graupen-}tonstein (Fig. 4.40, top and bottom left). Other
components which have been observed to transform into kaolinite pellets are Kfeldspar and volcanic rock fragments, as illustrated in Figs. 4.39 (bottom) and 4.40
(top right and centre). Such transitions, particularly from biotite to kaolinite pellets,
have been observed so frequently that it must be assumed to be a common
mechanism leading to the formation of pellet (graupen-) tonsteins. The process begins
shortly after the volcanic eruption in the dust cloud where biotite (and possibly other
mica), due to its flakiness travels farther than the less buyoant equant or spherical
minerals (e.g. quartz and and feldspars) and settles together with fine ash particles.
The result is a concentration of biotite set in a dense matrix, which, when
accumulated in a low energy peat environment, is subjected to the illustrated postdepositional changes. It can therefore be assumed that the various tonstein types
listed above represent a sorting sequence which can be arranged in order of
increasing distance from the vent, as follows:
Crystal tonstein > pseudomorphous tonstein > pellet tonstein > dense tonstein.
The "distance from the vent" is a relative figure which varies from ash-fall to
ash-fall because of its dependence on wind directions and wind strength prevailing
148
Coal Petrographic Entities
at the time of the eruption. It is possible therefore that different tonstein types can
occur in one locality in vertical succession although they may have been derived
from the same volcanic source.
Because of their wide lateral persistence, tonsteins are used in seam correlation
and regional stratigraphy. Being volcanic in origin, they represent brief episodes of
pyroclastic deposition, probably measured in hours or days. They constitute
therefore excellent time markers which allow not only absolute age dating to be
carried out but by their lateral extent and position within coal seams they are
invaluable aids in palaeogeographic reconstruction. A coal seam may sometimes be
traced over an entire coalfield by means of a tonstein band only a few centimetres
thick.
Volcanic ash constitutes an important source of plant nutrients from which
raised bogs benefit as much as topogenous peatlands do. In the former, oligotrophic
conditions are, in times of volcanic activity, replaced by a relative abundance of
nutrient supply (eutrophy), not only in the form of distinct cinerite bands but also
from the weaker but much more frequent ash-falls which leave only some
disseminated material behind. As has been mentioned in Chap. 2, present-day
examples are the Indonesian raised bogs of Sumatra and Java whose luxuriant
arborescent vegetation is partly due to their tropical setting and partly to the high
level of volcanic activity in the region. Since the hydrolysis of volcanic glass and
associated silicates by the peat water will raise its pH, an opportunity arises for
increased bacterial activity. Mohr and van Baren (1954) found this to be the case in
the vicinity of Recent volcanic ash layers imbedded in tropical peats, and Littke
(1985a) quotes this information in support of this observation of higher than normal
proportions of bituminite (due to bacterial plant decomposition) above and below
the tonstein in the Hagen Seam of the Ruhr Basin. It is likely that a similar
mechanism influenced the lithotype distribution in the Borehole Seam illustrated in
Fig. 4.34. Particularly, above and below the 4" Band and Middle Band heterogeneous lithotypes, i.e. concentrations of macerals formed from strongly humified or
decomposed peat (liptinite, detrovitrinite, detro-inertinite), occur, which away from
the influence of these tonsteins grade into the bright lithotypes which are more
characteristic of this part of the Newcastle Coal Measure (Lambton Subgroup).
In addition to the pyroclastic tonsteins, coal seams often contain epiclastic dirt
bands in the form of various kinds of mudrocks and fine sandstone (Britten 1979).
They contain a large variety of silicates, among which illite is common in
Carboniferous coals but it is rarely seen as mineral matter within the Permian coals
of Australia (Ward 1978). In contrast, the latter frequently contain montmorillonite
and mixed layer clays which are seldom found as coal impurities in the Northern
Hemisphere.
Kirsch and Stratmann (1959) consider most illites and mixed layer clays to be
weathered derivative minerals which have been derived by potassium leaching,
mainly from the alteration of initially sericitised feldspars. The distribution of illite
and other clay micas is of interest because they may contain increased boron contents
which is an indication of marine conditions of deposition (Degenes 1958; Adams
et al. 1965; Bohor and Gluskoter 1973; Swaine 1975). However, it should be noted
that the illite associated with high rank coals is probably not of sedimentary but
of diagenetic origin and mav have reolaced montmorillonite and kaolinite.
Minerals
149
Fig.4.42. Photomicrograph showing allophane in the lumens of vitrinitised wood cells of a high
volatile bituminous coal from the Sydney Basin, New South Wales. Incident white light, oil
immersion; actual length of field of view = 0.22 mm
As indicated above, montmorillonite and mixed layer clay minerals are
common constituents of Australian coals, where they are concentrated mainly in
tuff-derived clay bands. They are even more commonly found in interseam tuffs, for
example, in the Newcastle Coal Measures New South Wales, where several metres
thick deposits of relatively pure bentonite occur.
Chlorite is rare in coals themselves, but can often be seen as inclusions in shale
bands and interseam sediments. It seems that most chlorites in these sediments are
detrital in origin which have been deposited mainly as Mg-prochlorite with low iron
content (Diessel 1961), although Ward (1978) describes the occurrence of Fe-rich
chlorite among the minerals contained in semi-anthracite from Baralaba, Queensland. In shale bands it frequently occurs in alternating strands with sericite and clay
mica, from which it may have been formed by the absorption of magnesium and
some iron (Kisch 1968).
Since kaolinite has a preference for neutral and slightly acid environments, it is
not surprising that this mineral is more frequently found within coal seams and
intra-seam dirt bands rather than in the interseam sediments (Ward 1977). In
addition to well-crystallised kaolinite, which probably formed by authigenic
precipitation and/or as metasomatic replacement of other silicates (Keller 1956), the
amorphous variety allophane is likewise commonly found. It often occurs as an
early precipitate in the still uncompacted cell lumens of telovitrinite or teloinertinite, an example of which is shown in Fig. 4.42.
4.4.2.2 Silica Minerals
Silica occurs in coal in various modifications among with quartz is the most
common form in bituminous coal. Most of it may be detrital in origin but some
150
Coal Petrographic Entities
quartz and chalcedony has been formed from the diagenetic transformation of opal
which is not uncommon in brown coal. According to Ward (1978), quartz may
constitute half of the mineral matter in some Australian coals but usually does not
exceed 10%. Most of the macroscopically and microscopically visible quartz is
detrital in origin, which means that it was transported as mineral grains to the
depositional site. For this reason quartz is usually found together with other detrital
material; in many cases, together with clay in altered tuffs or epiclastic dirt bands left
behind by flood waters or in the form of splay deposits.
Authigenic quartz originates from aqueous solutions during the early stages of
diagenesis. Some quartz of this type seems to have passed through the stages opalchalcedony-quartz. Quartz which has derived from the alteration of opal and
chalcedony can be detected only with difficulty. Often, it consists of minute needles
which are under 1 Jlm in length, such that microscopic methods cannot be used for
its analysis. Together with this kind of quartz, true chalcedony of acicular and
spherulitic habit occurs. Acicular chalcedony is distinguished from quartz by its
lower refractive index and optically negative elongation. It has been found as
infilling in the cell lumens of some coal macerals and on cross-cutting cleats (Ward
1986).
4.4.2.3 Carbonate Minerals
The cations Ca, Mg and Fe form four carbonate minerals which are distinguished
by their relative proportions. The end-members are calcite (CaCOh), magnesite (MgC0 3 ) and siderite (FeC0 3 ). Of these, magnesite does not occur in coal.
Instead there are two mixed carbonates, dolomite (Ca, Mg)C0 3 and ankerite
(Ca, Mg, Fe)C0 3 . They can be found as both syngenetic and epigenetic minerals.
The most common occurrence of syngenetic carbonates is in the form of
spheroidal concretions (Fig. 4.43). Depending on their size and internal organisation
Littke (1985a) distinguishes between three varieties of concretions:
1. Large nodules up to several decimetres in diameter. These consist mainly of
siderite and iron-bearing dolomite which form a mass of small spherulitic bodies
near the centre ofthe large concretions while towards their margins the carbonate
becomes more crystalline although less pure because of increased clay content
plus some pyrite. In some marine influenced coals very large (> 1 m in diameter)
dolomite nodules have been found near the seam roof. They were formed in a very
early stage of diagenesis, which is indicated by inclusions of the original peat.
2. Small spheroidal concretions of siderite occur in some bright coal (vi train) bands.
Marginally the siderite grades into dolomite/ankerite.
3. Small concretions (up to 0.5 mm in diameter) consisting mainly of blocky calcite
crystals internally and of radiating fibrous Fe-calcite or Fe-dolomite near the
margins are also found in some vitrain bands. Occasionally the concretions are so
concentrated that they form continuous bands.
Carbonates are also common as in fillings in the cell lumens of fusinite and
semifusinite an example of which is illustrated in Fig. 4.44 (upper right). It is not
Minerals
151
Fig.4.43. Photomicrograph of syngenetic spheroidal carbonate concretion composed of blocky
calcite crystals in bituminous coal from the Bowen Basin, Queensland. Incident white light, oil
immersion; actual length of field of view = 0.36 mm
always clear in such cases, whether the impregnation is of syngenetic or epigenetic
origin.
Probably the most widespread occurrence of carbonate is as epigenetic precipitates on cleats and in fissures. These are mostly less than 1 mm wide and are
oriented more or less normal to bedding. Filling is mainly by dolomite/ankerite and,
to a lesser degree, by calcite, often in the form of well-developed crystals. Examples
are given in Fig. 4.44 (upper left and bottom). Syngenetic dolomite is of interest from a
palaeo-environmental viewpoint, since the work of Alderman and Skinner (1957) has
shown that sedimentary dolomite requires high pH conditions for its formation.
Cone-in-cone calcite, commonly found in marls and impure limestones as
telescoped cones consisting of fibres or (more rarely) undulous crystals whose c-axis
coincides with the cone axes (Fiichtbauer and Richter 1988), have also been reported
from concretions in coal seams, although, more commonly, from interseam sediments (Schone-Warnefeld and Dahm 1962; Franks 1969; Pearson 1979). They form
during early diagenesis by the growth of carbonate more or less normal to bedding
which, in the compacting sediment is the easiest growth direction.
4.4.2.4 Phosphate Minerals
These minerals, which in coal consist mainly of apatite and phosphorite, are not
common but sometimes play an important role (Diessel 1961; Cook 1962; Ward
1978; Corcoran 1979). The term phosphorite is a collective name for those
phosphates which were precipitated in colloidal form and later underwent partial
crystallisation. Most of the phosphorites in coal are arranged in small spherulitic
nodules of only a few micrometres in diameter, and it is rare to find lenses of
152
Coal Petrographic Entities
Fig.4.44. Photomicrographs of examples of epigenetic carbonates. Upper left Euhedral dolomite
crystal (centre) in a cleat of the Katharina Seam, Ruhr Basin. Upper right Siderite filling and
replacing semifusinite cells (could be syngenetic). Bottom Fibrous calcite filling fissures in
telovitrinite. All in reflected white light, oil immersion; actual length of field of view = 0.7 mm for
upper left, = 0.36 mm for upper right. and = 0.33 for bottom
Minerals
153
phosphorite which extend as much as 20 mm or more in horizontal diameter. Diessel
(1961) describes phosphorite nodules from a cannel coal below the roof of the
Katharina Seam in the Ruhr Basin consisting of collophane, the amorphous to
cryptocrystalline variety of Ca s(P04hOH and partly of small crystalline zones
consisting of apatite. As illustrated in Fig. 4.45 (top), the phosphorite appears yellow
in transmitted light whereby small colour variations are due to organic inclusions.
Larger crystals of apatite (Fig. 4.45, bottom right) are rare, sometimes they occur as
secondary formation on the surfaces of earlier formed phosphorites or on fragments
Fig. 4.45. Photomicrographs of various forms of phosphates in the upper cannel coal portion of
the Katharina Seam, Ruhr Basin. Top Nodular phosphorite. Bottom left Fish tooth or part of a
conodont. Bottom right Small prismatic apatite crystal from a phosphorite nodule. All in
transmitted light, one polar; actual length of field of view = 2.6 mm in upper frame, = 0.16 mm for
vertical edge at bottom left and right
Coal Petrographic Entities
154
of phosphorus-bearing fossils, such as bones, teeth (Fig. 4.45, bottom left) or
conodonts (Diessel 1961).
Additional, more complex alumino-phosphate minerals of the goyazite group
have been found by Ward (1974, 1978, 1986a) and Doolan et al. (1979) in eastern
Australian coals. They form a particularly resistant group of minerals which is
insoluble in most acids.
4.4.2.5 Sulphide Minerals
Sulphur is a common constituent in coal, where it occurs in varying quantities and
forms. Many coal seams contain less than 0.5% sulphur, others carry more than 10%.
Likewise, the proportion of sulphide minerals as a percentage of the total mineral
content varies from less than 1% (by mass) in some Australian coals to more than
30% in some coals from the Illinois Basin of the U.S.A., (Rao and Gluskoter 1973).
The sulphur content of a coal seam may show regional differences. An example is the
Herrin Seam in Illinois, U.S.A., in which total sulphur content varies laterally from 1
to over 8% (Gluskoter and Simon 1968). A distinction is made between organic and
inorganic forms of SUlphur. The former includes such functional groups as thiols,
thiophenes and thiopyrones (Given and Wyss 1961) with a preference for host
macerals in the following desending order (after Raymond 1985):
sporinite > vitrinite> resinite > fusinite.
As illustrated in Fig. 4.46 (top), the correlation between organic sulphur and
sporinite is weak in Permian Gondwana coals, and is similarly poor in other
macerals.
Inorganic forms of sulphur occur mostly as various kinds of iron sulphides,
including melnikovite, marcasite and, most frequently, pyrite, which is also the most
common iron bearing mineral in coal. Altogether, Mackowsky (1968) and Davis
(1982) list approximately 20 metal sulphides which have been found in European
and U.S. coal seams. In a study of sulphur formation and occurrence in peat and coal
Casagrande (1987) makes the following observations on U.S. coals:
1. In general, low sulphur coals contain more organic than pyritic sulphur and vice
versa but some notable exceptions occur in which coals with greater than 5%
organic sulphur contain less than 0.5% pyritic sulphur. The relationship between
organic and pyritic sulphur is similar for Australian coals, illustrated in Fig. 4.46
(bottom), which shows a reversal in most samples in the proportion between
organic and pyritic sulphur when the total sulphur exceeds 1%.
2. High sulphur coals are associated with marine roof rocks.
3. The sulphur content of a coal seem is often highest in the upper portion of the
seam and near the floor (see also Gluskoter 1977)
Pyrite and marcasite occur as syngenetic and epigenetic minerals in a variety of
forms which have been discussed in some detail by Balme (1956), Neavel (1966),
155
Minerals
.9
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Fig. 4.46. Two diagrams
illustrating the quantitative relationship in
Australian Permian coals
between organic (open
circles) and pyritic
sulphur (closed circles)
and sporinite (top), and
total sulphur (bottom)
8
6
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Gluskoter and Simon (t 968), Love et al. (t 983) and Littke (t 985a). The following list
of common Fe-sulphide occurrences is partly based on their findings:
1. Small pyrite crystals occurring as isolated inclusions in vitrinite and semifusinite
(Fig. 4.47, upper left) are frequently found in association with framboidal pyrite.
2. Small nodules of pyrite and/or marcasite ranging up to several centimetres in size
are commonly composed of elongated, radiating crystals, up to several millimetre
long (Fig. 4.47, upper right). Similar occurrences have been described by Parrat
and Kullerud (1979) and Price and Shieh (1979).
3. One of the most common forms of syngenetic Fe-sulphides are small pyrite
crystals, usually less than 211m in diameter, which form spheroidal clusters rarely
exceeding 30 11m in diameter. These framboids occur mainly in vitrinite or in dirt
bands (Fig. 4.47, lower left).
4. As with carbonate, iron sulphide may be precipitated in the cell lumens of wood
tissue, commonly in the form of semifusinite or fusinite. This is partly the case in
Fig. 4.47 (upper left), although most of the pyrite in that case has actually replaced
cell tissue.
156
Coal Petrographic Entities
Fig.4.47. Photomicrographs displaying various types of iron sulphides in Australian bituminous
coals. Upper left Single crystals of pyrite in semifusinite. Upper right. Aggregates of radiating
marcasite crystals in telovitrinite. Lower left. Clusters offramboidal pyrite in detrovitrinite. Lower
right Pyrite band formed by the fusion of aggregates of framboidal pyrite. All in reflected white
light, oil immersion; actual length offield of view = 0.36 mm for upper and lower left, = 0.22 mm for
upper and lower right
5. Concretions of type (3) may fuse together to form either flat lenses or continuous
bands in which areas of good crystallinity alternate with dense, almost cryptocrystalline pyrite. Love et al. (1983) describe such pyrite bands up to 5 cm
thick from the Halifax Hard Bed Coal northwest of Sheffield, u.K., which exhibit
both plastic deformation and fractures healed by a second generation of pyrite.
The latter is continuous with pyrite filling cleats in the adjacent coal, which
suggests that precipitation of the second generation pyrite took place during late
diagenesis, i.e. at the time of cleat formation during compaction and dehydration.
It may be regarded as being transitional to epigenetic pyrite.
6. Epigenetic pyrite and marcasite are frequently found on cleats and in fissures
(Taylor and Warne 1960). In the Katharina Seam of the Ruhr Basin, Diessel
(1961) found that cleats and joints carried more pyrite in the upper portion than
elsewhere in the seam which is probably related to the occurrence of marine
sediments in the seam roof, suggesting relatively early emplacement. Other
occurrences indicate late mineralisation, for example during tectonic deformation (Fig. 4.48).
7. Cone-in-cone pyrite is another form of epigenetic pyrite. According to Woodland
(1975) and Love et al. (1983), the pyrite is formed as metasomatic replacement of
cone-in-cone carbonate.
Minerals
157
Fig. 4.48. Photomicrograph of epigenetic pyrite (white, high relief) filling cleats in sporoclarite of
the Katharina Seam, Grillo 1/2 Colliery, Ruhr Basin. Note the displacement of the grey vitrinite
layer in the centre of the frame. Incident white light, oil immersion; actual length of field of
view = O.56mm. (DiesseI1961)
Apart from melnikovite, an amorphous form of Fe-sulphide, pyrite and marcasite
are both widespread. In a study of the Alton Marine Band and the Halifax Hard Bed
Coal, Love et al. (1983) found framboidal pyrite and associated microcrystalline
pyrite to be the first "preserved" sulphide phase but not necessarily the first
precipitate. Studies of Recent sediments have shown that the first Fe-sulphides to be
precipitated are metastable modifications, such as mackinawite (FeS O. 94) and
greigite (Fe 3 S4 ), which are short-lived and convert into pyrite (Goldhaber and
Kaplan 1974).
4.4.3 Mineral Analysis
The correlation illustrated in Fig. 4.30 between the mass percentages of coal ash and
the volume percentages of minerals determined by incident light methods in the
course of conventional maceral analyses demonstrates quite clearly that a
considerable amount of inorganic matter in coal is not recognised by the latter
technique. The distinction between different non-opaque minerals, particularly the
silicates and carbonates, is even more difficult, although the appplication of different
immersion methods, fluorescence microscopy (Teichmiiller and Wolf 1977), staining
158
Coal Petrographic Entities
(Warne 1962) and other techniques may enhance mineral recognition. Transmitted
light microscopy will overcome many of the difficulties, but at the expense of not
being able to differentiate between the opaque mineral fraction unless polished thin
sections are available. These are probably the best source of information concerning
compositional and genetical questions, particularly when combined with SEM and
electron microprobe analysis. However, the area covered is too small to give
quantitative representation of a seam, and the difficulty and costs involved in
manufacturing satisfactory polished thin sections of mineral impurities in coal
without losing the latter assure that this technique remains reserved for the solution
of special problems of a qualitative nature.
By far the best approach to quantitative mineral analysis is by a combination of
techniques which require that the minerals to be studied must first be liberated from
the coal matrix without causing too much damage to them. No method is available
at present which does not affect the minerals in some small way but minimum
damage can be achieved by removal of the enclosing coal by careful oxidation, for
which the following three methods have proved to be successful:
1. Hydrogen peroxide oxidation has traditionally been used by pedologists for the
removal of organic matter from soils (e.g. Carver 1971) and has been successfully
applied to the separation of minerals from coal by Nalwalk and Friedel (1972)
and Ward (1974). As described by Ward (1986a), the coal is reacted with 35%
H 2 0 2 at temperatures up to 100°C for several days during which the organic
matter is destroyed with a slow, effervescent reaction. The precise time required is
rank-dependent and ranges from a few days for bituminous coal to weeks for
anthracite. Damage to the minerals reported by the above authors include partial
or complete solution of water-soluble compounds, reaction of carbonates with
organic acids formed during the treatment and some oxidation of sulphide
minerals.
2. Low temperature ashing is a well-tried method for both qualitative and
quantitative mineral analysis. As used by Diessel (1961), the coal sample is
carefully crushed to minus 0.5 mm and dried at 110°C until constant weight is
established. Subsequently, the sample is thinly spread on a suitable base and kept
in a mume-furnace at a temperature 380°C ± 5 ° until the organic fraction has
disappeared whence the sample is re-weighed in order to establish loss in mass.
Depending on coal rank, the process takes several days, its completion is
recognised by the light colour of the residue and, in pyritic coals, by a reddish hue
resulting from the oxidation of pyrite to hematite. The temperature has been
chosen because it remains below the dehydroxylation temperature of even badly
crystallised clay minerals, which begin to lose OH-groups from 400°C onwards.
Likewise, no loss of CO 2 has been recorded from carbonate. The only obvious
alteration is the above-mentioned oxidation of pyrite and any other sulphides.
3. Oxygen plasma ashing (also called "radio frequency oxidation" = RFO) is today
the most widely used method of separating minerals from coal (Gleit 1963;
Gluskoter 1965, 1967; O'Gorman and Walker 1971: Frazer and Belcher 1973;
Miller et al. 1979). According to Ward (1986a), in this method the crushed coal is
exposed in an evacuated chamber to a stream of oxygen which has passed
Minerals
159
through a high energy electromagnetic field produced by a radiofrequency
oscillator. The ionised oxygen reacts with the organic matter at temperatures
between 150 and 200°C, which makes it the least damaging technique currently
available.
Once the mineral residue has been separated, the complete range of chemical and
petrographic techniques of sample preparation and analysis is available for the
subsequent investigations. Commonly they begin with further separation of the
residue into related mineral groups and fractions, including the recovery of heavy
minerals by float/sink methods, the removal of magnetic minerals, the sizing of the
residue by sieving and elutriation or sedimentation in order to concentrate the clays
for further analysis, etc. Among the analytical work a combination of microscopy of
particulate mineral concentrates and X-ray diffraction (XRD) will represent the
main techniques, supplemented by other methods directed to the solution of more
specialised tasks of a mineralogical or geochemical nature. These include differential
thermal analysis (DT A, DTG), Fourier transform infra-red spectometry (FTIR),
electron and iron probe work, scanning electron microscopy (SEM) and others for
which the reader is referred to the respective literature (see also Chap. 5 for further
discussion).
5 Coal Facies and Depositional Environment
In the preceding chapters coal components have been classified on the basis of
physical, chemical and genetic relationships. In the following discussion these will
be employed in a filtering process that is designed to detect the signatures left
behind by depositional environments in the form of a distinctive coal facies. The
concept of depositional environment will be targeted at two levels: one is specific
and refers to the type of mire and the conditions of peat accumulation within it,
whereas the other takes a broader view and seeks to establish the relationship
between coal facies and the sedimentary setting of the mire. The questions to be
asked in the first case, which is the main subject of this chapter, will concern the
hallmarks of rheotrophy and ombrotrophy, as well as the influence on coal type
of the vegetal progenitors after the local variabilities have been filtered out. The
aim is akin to what Walker (1980, Fig. 4) calls the "pure essence of environmental
summary", which constitutes a facies model obtained from the common
denominator of a variety of local examples. The results of this enquiry are then
linked to their sedimentary settings, such as delta plains, alluvial valleys and the
like, which will be discussed in detail in Chap. 7.
The techniques employed in the palaeo-environmental enquiry are based mainly
on the standard methods of coal quality testing. Analytical emphasis is on coal
microscopy, but the range of maceral and microlithotype analyses has been widened
beyond the requirements of conventional application. Related disciplines, such as
palynology and both organic and inorganic geochemistry, have been included in
the search for palaeo-environmental signatures, but not palaeobotany as such.
Since coal has been formed from vegetal progenitors, plant ecological considerations would contribute much to the enquiry, but palaeobotany is such a large
discipline in its own right, that any consideration of specifically botanic aspects
of coal formation would be beyond the scope of this text.
5.1 Phyterals and Macerals in Palaeo-Environmental Analysis
In all previous references to the smallest and relatively homogeneous pertographic
entities of coal, the term "maceral" has been used. As originally introduced by
Stopes (1935), the term emphasises material differences, rather than botanical
affinities or a specific origin. It is true that the macerals comprising the liptinite
162
Coal Facies and Depositional Environment
group have been derived from specific organs or products of the original plant
material, but the stress remains on the physical and chemical characteristics of
the material they consist of. In order to emphasise the botanical derivation of coal
components, Cady (1944) introduced the term "phyteral". The term has not been
widely accepted, although there has been an increasing tendency to include it into
the general maceral concept. Indeed, as pointed out by Stach (1971), because the
term phyteral relates to the phytogenic fossil and its morphology only, it is virtually
included in the wider context of maceral terminology. The subsequent discussion
will follow the same practice, but it may be useful to keep in mind that a term
such as "densosporinite", which is a sporinite composed of Densosporites, refers
as much to a phyteral as it does to a maceral. The same applies to other botanical
attributes of macerals.
Coal macerals contribute to palaeo-environmental analysis at two levels of
inquiry which can be described as attribute and scalar properties. According to
Potter and Pettijohn (1963, p. 3), attributes are "those properties that are specified
by their presence or abesence. Neither magnitude nor abundance need by specified".
The authors apply this concept to the recognition of a diagnostic rock type or key
mineral which may identify the depositional environment of a particular sediment,
not unlike the role of an index fossil in determining the age of a stratigraphic
horizon. This does not mean that attribute properties are totally independent from
quantitative considerations, but frequency or magnitude alone is not the deciding
factor as is the case with scalar properties. In the context of palaeo-environmental
coal analysis an example would be the presence of telalginite, which has been
derived from planktonic algal progenitors and therefore indicates aqueous
conditions for the formation of the seam section in which it occurs, no matter
whether its proportion in the coal is large or small. Conversely, small amounts of
almost all other macerals would not indicate any specific mire setting, but when
present in large quantities, their environmental significance increases considerably.
5.1.1 Botanical Attributes of Macerals
For many macerals the place of burial does not necessarily coincide with the place
of origin. Only few components would therefore, by their presence or absence,
define the type of peatland the coals originated in. For example, fusinite requires
desiccation, oxidation and, in most cases, partial pyrolysis for its formation, but
the occurrence of a few fusinite lenses in a coal sample does not infer a dry peatforming environment. In fact it may do the opposite, because a few isolated fusinite
grains set in a detrovitrinite matrix are likely to have been blown into the mire
from the site of a bush or forest fire and therefore have no palaeo-environmental
significance as far as the depositional site is concerned. Moreover, dispersed
fragments of semifusinite and fusinite in trimacerite and vitrinertite could have
been formed as a result of crown fires and fallen into waterlogged peat below
Phyterals and Macerals in Palaeo-Environmental Analysis
163
(Scott 1989), because fires spreading through foliage and branches of trees growing
in limnotelmatic environments have been reported by Komarek (1972), Cohen and
Spackman (1980) and Cohen et al. (1987). In Australia, where such fires are a
common feature and an essential part of the ecological fabric, fine char particles
have been observed to spread over tens, and in large bush fires, hundreds of square
kilometres. However, if instead of a small amount of dispersed inertinite inclusions
the coal contains a persistently large concentration offusinite plus semifusinite and
macrinite, such a paragenetic maceral assemblage acquires palaeo-environmental
significance, not merely 'by its attributes, i.e. the presence or absence of consanguineous macerals, but by their frequency, which is a scalar property.
As mentioned above, probably the only example of a maceral with palaeoenvironmental significance in its own right is alginite, in particular telalginite. It
invariably signifies lacustrine conditions and it does so irrespective of whether the
sample consists predominantly of alginite, as in a boghead coal, or whether the
coal contains only a few isolated algal bodies. Indeed, the coexistance of a high
proportion of inertinite and a small amount of alginite would be indicative of the
allochthonous (i.e. redistributed) nature of the inertinite.
The process-response relationships between the phytogenic precursors of
macerals and their biotopes can produce noteworthy attributes in macerals. Such
features are often disregarded in routine analyses, but considerable insight can be
gained from simple observations. An example is the thickness of cuticles in cutinite
which is related to the avilability of water to the host plant because the cutinite
protect the underlying tissue from drying. Plants of wet environments are therefore
characterised by thin cuticles whereas those growing in comparatively dry positions
(= xerophytic plants) possess relatively thick cuticles (Strasburger 1983). Likewise,
the thick walls in crass is pores have been interpreted as a protective cover against
dehydration. Naumova (1953) regards parctically all small spores with thin exines
and simple sculptures as having been derived from plants that either grew in or
around water, in contrast to more sculptured spores with thick exines and a
perisporium which are typical of terrestrial plants.
A considerable number of palaeo-environmental studies of coal has been based
on the botanical affinities of the maceral group liptinite. In this context a distinction
has been made in Chap. 4 between spores on the basis of size (e.g. macro- and
microspores). Because microspores are difficult to separate from pollen grains, they
are grouped under the collective term miospores, which have received more
attention than macrospores, although Bartram's (1987) work on the Low Barnsley
Seam in Yorkshire, U.K., is a notable exception.
It is known from the study of Holocene peatlands in southern Florida, SE-Asia
and elsewhere (Spackman et al. 1966, 1969; Anderson and Muller 1975) that
different swamp ecologies leave behind discrete miospore assemblages in the peats
formed in them. Although no detailed taxonomic identification is possible without
the application of specialised palynological techniques, coal petrologists have for a
long time used simple morphological features, such as the above-mentioned
thickness and shape of the exines of miospores to group them into crassi-, tenui-,
tori-, and densosporinite, the latter being a type of crassisporinite. These different
kinds of spores have been derived from different parent plants which have palaeo-
164
Coal Facies and Depositional Environment
environmental significance. For example, the thin-walled tenuispores, particularly
in the form of lycospores, are characteristic of wet, arborescent lycopod swamps
(Felix 1954; Smith 1957, 1961, 1962, 1964, 1968; Smith and Butterworth 1967;
Grebe 1966, 1972; Grebe and Josten 1964; Hagemann 1966; Mittapalli 1966; Scott
and King 1981; Peppers 1984; Fulton 1987; and others), whereas densospores have
been derived from herbaceous lycopods (Butterworth 1966; Phillips and Peppers
1984; Phillips et al. 1985). Wagner (1985) found densospores associated with
Sporangiostrobus, but the precise botanic affinity of these sporomorphs remains
largely unknown.
Due to their origin from large arborescent lycopods (e.g. Lepidodendron and
Sigillaria) lycospores are commonly associated with bright coals (vitrain) rich in
vitrite and clarite (Fig. 5.1 top). In the Illinois Basin the greatest abundance of
lycospores (concurrent with strong dominance of Lepidophloios) occurs in the
thickest coals with high vitrinite/inertinite ratios (Harvey and Dillon 1985) in close
proximity to palaeochannels which, according to Phillips et al. (1985), is consistent
with maximum preservation of biomass in continuously wet terrains with possibly
long periods of standing water on the ground. The frequent coexistence of
densosporinite with inertinite (Fig. 5.1 bottom) in durite and dull coal (durain) has
led a number of authors to conclude tqat the presence of densospores signifies
relatively dry peat forming conditions including ombrotrophic raised bog
environments (Smith 1962, 1964; Littke 1985a, b, 1987; Fulton 1987).
In addition to the frequent coexistence of densospores with inertinite, it is also
their anatomy which suggests their association with a relatively low groundwater
table. The dumbbell shape of the spore results from a wedge-shaped thickening of its
equatorial zone (cingulum), which has been interpreted by Tschudy (1969) and
others as a protection against desiccation. However, the photomicrographs of
densospores in Fig 4.26 demonstrate that outside the cingulum the spore wall is
quite thin and conforms to Kosanke's (1969) observation that the body of Densosporites is thin and transparent. Likewise, the occurrence of densosporinite in cannel
coal (Littke 1985a; Strehlau 1988) and in durite, which laterally grades into sapropelic coal (Moore 1968; Strehlau 1988) suggests hypautochthonous to allochthonous
subaqueous deposition and possibly an, at least periodically, wet environment
for the parent plants. van Wijhe and Bless (1974) consider the densosporinite
facies to be indicative of a strand-plain setting, while the work of Habib (1966),
Habib and Groth (1967), Ting and Spackman (1975), Rimmer and Davis (1988) and
Ting (1989) on the Lower Kittanning coal of western Pennsylvania suggests that the
parent plants of Densosporites oblatus. the main densospore occurring in the Lower
Kittanning Seam, were possibly halophytic and grew most likely on the shores of
the Carboniferous sea in a manner not unlike that of present-day mangroves. Indeed,
Habib et al (1966) make a direct comparison with the Recent peat deposits of
southwestern Florida discussed in Chap. 2, in which environmental relationships
and the superposition of peat types indicate peat formation during a marine
transgression, a condition which is mirrored in the Carboniferous Lower Kitanning
seam. It is overlain by dark shale, in which marine, brackish and fresh-water environments have been recognised (Ting 1989) on the basis of boron content, clay
mineral distribution and fossils (Degens et a11957; Williams 1959, 1960). The marine
Phyterals and Macerals in Palaeo-Environmental-Analysis
165
Fig. 5.1. Comparison of lycospores set in c1arite (surrounded by vitrite in top) and densospores
set in durite (bottom) from a high volatile bituminous coal of the Ruhr Basin. Left top and bottom.
Incident white light in oil immersion; right top and bottom fluorescent mode, dry objective; actual
length of each field of view = 0.22 mm
166
Coal Facies and Depositional Environment
portion of the roof shale contains Lingula along the coal contact, followed by other
brachiopods such as M esolobus higher up. Densospores are restricted to the
uppermost coal although the other spore assemblages which follow downward
through the seam are also represented. Tenuispores of the Lycospora (i.e. lycopod)
assemblage occur mainly in the lower vitrain-rich portion of the seam. The densospores near the seam roof are contained in durite-rich dull coal, and it is suggested
that the high inertinite content is largely redistributed forming Ting's (1989) genetic
lithotypes allosite and kittosite (see Chap. 5.3).
Rimmer and Davis (1988), while accepting that the original plant assemblages
had some bearing on sporinite and maceral distribution in the Lower Kittanning
Seam, have provided additional evidence for the influence of changing pH/Eh
conditions on the humification of the coal-producing plants. This has affected
mainly the axial portion of the Appalachian Basin, where due to a high rate of
subsidence during seam formation a large proportion of wood tissue was preserved
as telovitrinite, except in the late stage of peat accumulation, when the influx of
brackish and marine waters raised the pH such that woody tissues could be more
easily biodegraded. This led to a relative rise of the detrovitrinite (de5mocollinite)
content in the upper portion of the seam. A similar increase in inertinite is regarded
as being the result either of reworking and redistribution of inertinite formed
elsewhere, or by in situ oxidation due to alternating periods of drying and wetting by
oxygenated water under tidal influence. Based on the combined evidence provided
by the distribution of macerals and their botanical attributes, Rimmer and Davis
(1988) also interpret the densosporinite-rich upper portion of the Lower Kittanning
Seam as the product of increasingly wet peat-forming conditions, which eventually
led to the drowning of the seam.
The possibility of Densosporites oblatus being an indicator of marine influence
on coal formation does not mean that other species of the same genus follow the
same pattern. Indeed, many concentrations of densosporinite assemblages have
been reported from the upper portions of coal seams without marine roof sediments.
However, even then the densosporinite remained linked with high inertinite contents in contrast to lycospore-based tenuisporinite which has a general preference for vitrinite-rich coal plies. This coexistence has led to virtual unanimous
acceptance of the notion that coal seams or parts thereof with high vitrite and
vitrinite-rich clarite contents are typical products of wooded peatlands (von
Karmasin 1952; Kremp 1952; Stach 1954,1955; Smith 1957, 1961, 1962, 1964, 1968;
Navale 1962; Teichmiiller 1962; Alpern et al 1964; Hacquebard et al 1964;
Teichmiiller et al 1965; Hagemann 1966; Mittapalli 1966; Moller 1966; Kutzner
1967, 1987; Hacquebard and Donaldson (1969), Hacquebard and Barss (1970);
Schneider 1971; Phillips and Peppers 1984; Littke 1985a, b, 1987; Esterle and Ferm
1986; Strehlau 1988; Rimmer and Davis 1988; Ting 1989; and others).
Even when marine fossils are absent from the roof sediments of densosporinite-rich coal seams strong indications can frequently be found of termination
of peat accumulation by drowning of the seam. In many of such cases the coal is
part of a distal transgressive sedimentary sequence (discussed in detail in Chap. 8)
in which the roof consists of shale whose non-marine, subaqueous origin is indicated
by its low boron content (Ernst et al. 1960) and/or the presence of fresh-water
Phyterals and Macerals in Palaeo-Environmental Analysis
167
Fig. 5.2. Photomicrograph of densospores (encircled) set in trimacerite together with syngenetic
pyrite (arrows) in high volatile bituminous coal from the Ruhr Basin. Left Incident white light in oil
immersion; right fluorescent mode, dry objective; actual length of each field of view = 0.22 mm
ElITROPHY
Fig. 5.3 A, B. Two cartoons
illustrating possible modes
of origin of the densosporinite facies. See text for
explanations
F"loor Sediments
OUGOTROPHY
-
Shsly Fecles
Vttrlnlte-Fuslnlte F. Densosporln1te F.
SepropelHe F.
168
Coal Facies and Depositional Environment
fossils above a dull coal portion with densospore assemblages (Stach 1954; Fiebig
1960; Littke 1985a; Fulton 1987; Strehlau 1988). It can be assumed that the peat
of these seams gave way to spreading fresh-water lakes, or was covered by the
landward extensions oflow salinity lagoons. The pyrite content of such coal seams
may be low but the mineral is rarely missing altogether. An example is shown in
Fig. 5.2.
From the above follows that there are two schools of thought concerning the
palaeo-environmental significance of densosporinite, namely, one which favours a
wet origin from hydrophytic plants which grew in a topogenous setting along lake
and/or sea shores, and another which regards densospores to have been derived
from a vegetation, which represents the climax in ombrotrophic mire development.
However, in the same manner as oligotrophic conditions can ensue in a topogenous
setting, lacustrine environments can be accommodated in a raised ombrotrophic
bog. It is possible therefore to establish the two models of densosporinite formation
illustrated in Fig. 5.3, in which their essential features have been combined to offer
the two alternative possibilities discussed below.
5.1.1.1 The Topogenous Model of Densosporinite Formation
In model A of Fig. 5.3 seam formation is thought to take place on a delta plain
to fluvial environment. The envisaged delta may conform to the classical Mississippi
model or any other delta prograding into an ocean basin, it may be prograding
into an inland sea or into a large fresh-water lake. In either of these cases the mire
setting is truly rheotrophic because the water that sustains peat accumulation
moves slowly through the wetlands, not only within established channels, but also
outside, where it appears to be fairly stagnant. Another rheotrophic peatland
setting to which Model A could apply occurs on low-lying ground that is traversed
by rivers such that their channels split and open into lakes, marshes or swamps.
Examples are the wetlands of EI Sudd on the White Nile in Sudan, the Pripyat
marshes in Russia and many others.
Finally, some of the general features of Model A can also be found in peatlands
that are quite different from the above-mentioned examples, because they do not
occupy a transitional setting through which water is conveyed on its way from
the hinterland to the receiving sea or lake, but in this case the mire itself is situated
at the receiving end of the drainage system. The result is a delta that does not
prograde into a body of deep water but into a swampy topographic depression,
where the inflow/evaporation ratio is so low, that little permanent water is found
outside the delta itself. Several current examples of this type accumulate very
effectively large masses of peat, although the latter is mostly based on herbaceous
vegetation, which does not conform entirely with Model A. The dimensions of
the peat seams formed in this manner are very large, as can be gauged by the
Okavango Delta, or more commonly referred to as the Okavango Swamp, in
Botswana, which shows some interesting aspects of peat formation (McCarthy
et al. 1989). While much of the so-called peat in the proximal portions of the delta
contains around 60% ash (db), this proportion declines distally to an average of
Phyterals and Macerals in Palaeo-Environmental Analysis
169
14~/~ (db) with individual values as low as 6.3~/~ (db), due to the combined filtering
effect by vegetation and rainwater leaching. In evaluating these results it should
be realised that, firstly, the area producing low-ash topogenous peat takes up a large
portion of the 6000 km 2 of permanently flooded papyrus marsh, and secondly, for
reasons of access, McCarthy and his co-workers could sample only in the vicinity
of channels, where the peat is naturally more contaminated than further away
from the channels, where even lower ash values could be expected.
The following discussion of densosporinite formation under the conditions
of Model A is based on a comprehensive study of the coal facies distribution in
the Upper Carboniferous coals of the Ruhr Basin by Strehlau (1988), but it also
takes account of the implications and constraints imposed by current examples of
peat accumulation under a topogenous regime.
1. The shaly coal facies (tonige Fazies) consists of alternating thin bands of clean
coal, shaly coal, coaly shale and shale. Coal composition varies but is dominated
by detrovitrinite-rich trimacerites and only low concentrations of durite. The
conditions of peat formation are similar to the ones in the vicinity of autosedimentational seam splits (Chap. 6.3.2), where the ash content of the coal is elevated
due to its disseminated clay and silt and the occurrence of stone bands. A depositional site in the vicinity of a river is envisaged for this example because the filter
effect of the swamp vegetation (Frazier and Osanik 1969; McCarthy et al. 1989)
and flocculation due to the' acid nature of the swamp waters (Staub and Cohen
1979) would prevent the suspended load from reaching the more distal
portions of the mire.
The shale bands have probably been deposited from flood waters which either
overtopped or breached the levee banks but they are not part of the levee banks
themselves. The fact that in many coal seams containing thick sections of the
shaly coal facies, a centimetre-thick shale (or silt) band is often separated by at
least as much coal, is suggestive of long periods of peat formation between brief
stages of severe flooding, because minor episodic or periodic, e.g. seasonal, flooding
would probably not result in the deposition of discrete stone bands. For example,
Anderson (1976) observed in southern Kalimantan that the Sebangau River (Sungei
Sebangau) inundated/quite regularly the adjacent swamp forest with floodwaters
rich in organic detritus but practically free from inorganic sediment. In settings
of coastal flood plains or strand plains, major inundations could also have been
linked to influxes of brackish water, banked-up by king tides and storm waves.
The concomitant increase in pH in the tide-affected marginal portions of the
swamp may be responsible for the syngenetic pyrite occasionally found in this
facies.
Seams which consist of shaly coal facies. only have been found in many upper
delta/alluvial plain environments although more frequently this facies is combined
with the vitrinite-fusinite facies in which it often constitutes the basal portion. The
frequent intercalations in this facies of bands with high fusinite/semifusinite
contents (Hoffmann 1933) indicate a proximal setting high on the alluvial ridge
of trunk streams, where the ground surface dried out and burned down to water
level during low water stages.
170
Coal Facies and Depositional Environment
2. The vitrinite-fusinitefacies is dominated by wood-derived vitrinite, which in Car-
boniferous coals is commonly linked to the occurence of lycospores. The minerotrophic, telmatic nature of this facies is indicated by the large production and preservation in mainly gelified form of structured biomass, as well as by the occasional
occurrence of carbargilite and shale bands brought into the swamp by flood waters.
Following Remy and Remy (1977), Strehlau (1988) also stresses the importance of
the tree-supported canopy of foliage in keeping the ground moist and protecting
the accumulating vegetable matter from desiccation by wind and sun.
The setting of the vitrinite-fusinite facies is the broad transitional belt ranging
from distal flood plain to proximal flood basin, i.e. from the foot of the often broad
and flat alluvial ridge (see Model A in Fig. 5.3 and Chap. 7), where gradients flatten
and the water level begins to rise above the ground surface to the limnotelmatic
forest swamps surrounding lakes and ponds. Depending on the extent of upstream
transitions into the drier braided stream environments or onto the higher ground
of the alluvial ridge, fusain layers and lenses consisting of fire-generated semifusinite
and fusinite may be found in some seams, but vitrinite mainly in vi trite, clarite
and duroclarite usually remains dominant, thus resulting in bright lithotypes with
thick vitrain bands. Persistent horizons of fusain are probably drought-related
because they represent ground fires senso Davis (1959) and Scott (1989) and affect
the upper portions of the peat as well.
Further into the flood basin, detrovitrinite and detro-inertinite, together with
dispersed liptinite, collect in rivulets and ponds as hypautochthonous trimacerite
and durite. The distal loss of the protective canopy because of an increasing
replacement of trees by smaller vegetation suggested by Teichmiiller (1950, 1962)
and van Wijhe and Bless (1974) could cause occasional drying of the peat surface,
resulting in the formation of some autochthonous inertinite by oxidative degredation.
In terms of nutrition the vitrinite-fusinite facies appears to occupy a
meso trophic setting between the eutrophic shaly coal and the oligotrophic densosporinite facies. Its low ash content indicates that it formed beyond the reach
of water-borne particulate inorganic matter, either because its peat surface was
raised above flood level, or because any mud sespended in flood waters was trapped
on its passage through the proximal belt represented by the shaly coal facies. The
vitrinite-fusinite facies is therefore characterised by a pronounced edaphic gradient
ranging from nearly eutrophic on the boundary with the shaly coal facies to
oligotrophic towards the distal densosporinite facies. The result is a decline in
arborescent vegetation in the same direction and its replacement by herbaceous
plants.
3. The densosporinite facies is characterised by dull (durain) and banded dull
(clarodurain) coal, while under the microscope high concentrations of durites and
clarodurites rich in detro-inertinite can be seen. Densospores dominate the liptinite
content while detrital minerals are rare, and also syngenetic pyrite is not
widespread, which accounts for the generally low sulphur contents (Stach 1954).
According to Butterworth (1964), the low mineral content of this facies requires
a setting beyond the reach of adventitious minerals. The concomitant low influx
Phyterals and Macerals in Palaeo-Environmental Analysis
171
of nutrients can be assumed to cause the shift away from wood-bearing arborescent
to herbaceous and possibly even a marsh vegetation analogous to the N ymphaea
marshes in the central portions of the Okefenoke Swamp, or the papyrus marshes
of El Sudd and the Okavango Swamp. Except for their roots, such soft-tissued
plants are often more readily degraded, in spite of a likely drop in pH and lowered
bacterial activity because of the reduction in the supply of allochthonous mineral
bases. The result is a decline in the precipitation of iron sulphides and a reduction
in total vitrinite content. The latter is accompanied by a shift in composition from
predominantly telovitrinite to a greater proportion of detrovitrinite (desmocollinite). This shift is not merely due to a more thorough humification of wood
because the increased loss in biomass would result in a concomitant increase in
inherent ash for which there is no evidence. Indeed, the ash content of the
densosporinite facies is commonly very low, which is also a consequence of the
reduced bacterial activity. In the Okavango Delta McCarthy et al. (J 989) found
both a downstream decline in the contribution by allochthonous inorganics to the
composition of peat ash and a change in its composition. Based on mass balance
considerations of the distribution of elements in the ash of peat and its source
plants (see Chap. 5.5.1), they regard average Okavango peat ash to contain 90%
of adventitious (allochthonous) detrital matter plus inherent plant ash. The
remaining 10% (predominantly Fe, Mn, Ca, Na, K, and P) are thought to have
been added in complexed form by bacteria. This notion is based on the high
proportion of nitrogen in the peat, which is widely regarded (e.g. Bajor 1960;
Casagrande and Given 1980; Chaffee et al. 1986) as an indication of extensive
microbial reworking of the peat forming vegetation. It can be concluded that low
ash peats will result when the distal decline in adventitious detrital minerals is
accompanied by a reduction in microbial activity.
As mentioned above, more open mire conditions probably caused some
occasional desiccation and fire in times of low rainfall or water influx which might
have resulted in the formation of some autochthonous durite, particularly along
the shores of shallow ponds which dry out much faster than the peat, in which
water is better retained by capillary action (Thomson 1951, 1956; Jacob 1961;
Phillips et al. 1985; Strehlau 1988). It is also possible that in the densosporinitebearing Carboniferous peatlands the water budget was seasonally regulated,
although the lack of well-developed annual growth rings in Carboniferous wood
does not suggest pronounced wet and dry seasons. Indeed, even small variations
in moisture supply would have affected the distal portions of the mires more than
the areas situated close to the channels supplying the moisture. The result would
have been uninterrupted growth of arborescent vegetation near the channels but
distally more intermittent growth and periodic or episodic drying would have lead
to desiccation and burning of the exposed vegetation. This is currently a common
process in the Okavango Swamp in contrast to the more severe destruction by
fire of dried peat, which happens only when part of the water supply system has
become defunct due to channel abandonment (McCarthy et al. 1989) or during
periods of severe drought leading to the so-called fire splays reported by Staub
and Cohen (1979) from the Snuggedy Swamp in South Carolina (see Chap. 6).
With renewed water supply the charred plant and peat fragments collect in
172
Coal Facies and Depositional Environment
topographic depressions, which is evidenced by the previously mentioned transitions with cannel coal and by a high content of redistributed inertodetrinite,
sporinite and liptodetrinite. Many of these macerals have been washed in from
the adjacent rainforest in accordance with the depositional gradient towards the
poorly drained portions of the mire.
4. The sapropelic coalfacies covers both cannel and boghead coals, although pure
cannel coal seams are quite rare and seem to be restricted to the Carboniferous
Period, the flora of which provided the large amounts of spores necessary for their
formation. Deposits consisting exclusively of low ash boghead coal are likewise
not frequent (Stach and Hoffmann 1931) but occur in the form of occasional
intercalations with humic coal seams. Transitions from the densosporinite facies
into sapropelic coals observed by Moore (1968) and Strehlau (1988) represent
gradations from hypautochthonous limnotelmatic environments into deeper water
beyond the reach of rooted vegetation, where allochthonous sedimentation took
place. Vertical successions in which vitrain-rich coals grade upward into durain
of sapropelite have been known for a long time (Raistrick and Marshall 1939).
5.1.1.2 The Ombrogenous Model of Densosporinite Formation
In Model B of Fig. 5.3 the eutrophic portion on the left hand side of the diagram
is identical to Model A, and Strehlau's (1988) coal facies and their interpretation
can be applied just the same. The shaly coal facies (1) and parts ofvitrinite-fusinite
facies (2) would be equivalent to the eutrophic margins of raised bogs (rand) and
could probably be compared with the Rhizophora mangrove and the fresh-water
swamp forest peat Cohen et al. (1989) found at the base and margin of the
Changuinola peat deposit in northwestern Panama. This mire occurs as a raised
bog behind a barrier beach, but while the mangrove peat separates the domed
portion of the mire from the brackish water of the Almirante Bay to the south,
swamp peat forms the northern margin, where it interfingers with flood plain
sediments of the Rio Changuinola. Accordingly, these two marginal peat types show
the highest ash contents with 10 to 12% for mangrove and 1 to 25% for forest
swamp peat, compared to 1 to 13% sedge-grass-fern, 1 to 17% for Sagittariadominated and 2 to 12% for Nymphaea-dominated peats.
The distal (oligotrophic) portion of the vitrinite-fusinite facies with its
increased detrovitrinite and autochthonous durite may be considered to be the
beginning (rand) of the raised bog, the central portion of which consists of the
densosporinite facies. Indeed, the possibility of a raised bog setting for the distal
portion of the vitrinite-fusinite facies is also included in Model A, where this part
ofthe facies has been depicted as occupying the highest elevation of the peatland.
The main contrast to Model A is the raised bog setting for the densosporinite
facies which has been the accepted view of many authors ever since Smith (1957,
1961, 1962, 1964, 1968) published his work relating coal type to miospore
assemblages. Initial reasons for the development of the raised bog model were:
Phyterals and Macerals in Palaeo-Environmental Analysis
173
1. Densospores are a distinct group of miospores which presumably were derived
from herbaceous or shrub-like lycopods. As stunted growth is commonly an
indication of oligo trophy, the raised bog hypothesis is well served by this
observation.
2. The parent plants of densospores did not form an understory underneath a
canopy of arborescent lycopods but grew in a separate habitat, as do the
somewhat xeromorphic, hardy plants of the inner portions of raised bogs, which
do not support luxurious vegetation.
3. Densospores are mainly associated with inertinite, usually in durites or
inertinite-rich trimacerites (clarodurites), which was interpreted to be
autochthonous thus suggesting periods of dryness in the elevated portions of
raised bogs.
4. Densosporinite is associated with relatively low ash contents in general and
low pyrite in particular. The low pyrite content was thought to be related to
the absence of a strong sulphur source (e.g. sulphate from sea water) and the
acid nature of the raised bog which suppresses bacterial activity.
An important corollary to the ombrogenous interpretation of the densosporinite
facies is the necessity to accommodate the sapropelite facies within this environment
because the association with subaqueous deposits is common to the densosporinite
facies (Strehlau 1988). As discussed in Chap. 3, bog pools are a frequent feature
in many present day raised mires where they are separated by low ridges resulting
in a hummock and hollow pattern. They are part of Osvald's (1923) regeneration
complex, a hypothesis according to which hummocks result from rapid plant
growth and peat accumulation in the semi-quatic hollows. As the hummocks grow
above water level they dry out, resulting in slower peat accumulation compared
with the higher accumulation rate in the surrounding hollows, which then become
hummocks, while the previous hummocks revert to form hollows. Although the
general applicability of this hypothesis has been disputed (Gore 1983), it could
explain the coexistence of inertinite (derived from the dry peat hummocks) and
alginite (formed in the hollows). However, it fails to explain the frequently observed
systematic change from lycospore-rich bright coals in the lower portions of
Carboniferous coal seams to densospore-rich dull coals in their upper sections.
Apart from Osvald's (1923) regeneration hypothesis for the formation
of hummocks and hollows, other reasons for pool generation have been discussed
in the literature. One example is genetically related to periods of dryness which
affect the most elevated portions of the bog more than other parts. Protracted
periods of drought have a more severe effect on the peat than the normal seasonal
drying of the upper few centimetres because, either by lightning strike or
spontaneous combustion, the peat may catch fire and burn down to the water
table. A return to normal conditions will fill the burnt-out hollow with water in
which aquatic plants (including algae) become intermingled with allochthonous
and more or less oxidised, including charred plant litter washed into the ponds
from the adjacent peat surface because in times of high precipitation both
topogenous and ombrogenous mires are subjected to considerable water movement
on the peat surface itself. The Nuphar peat described by Styan and Bustin (1983a, b)
174
Coal Facies and Depositional Environment
from the Fraser River delta in British Columbia is rich in detrital plant material
and is underlain by a fire horizon with a high concentration of charcoal. In this
context reference has also been made to the fire splay deposit reported by Staub
and Cohen (1979) from the Snuggedy Swamp of South Carolina, which likewise
fill with water and allochthonous, including inorganic, matter. Fossil examples of
this kind in the form of shale-covered fusain bands have been described from
British Carboniferous coals by Scott and Collinson (1978) and from the Herrin
No.6 coal in the Illinois Basin by Phillips and DiMichele (1981). Recent peat
fires and their associated fusinite formation have been described by Johnson (1984)
from Borneo, where peat was burned to a depth of 2 m, by Teichmiiller (1989)
from Holland, and were also observed by the author during the 1980/81 drought
along the eastern seaboard of New South Wales.
Further examples of the formation of open-water bodies on the surface of
raised bogs have been reported by Cameron et al. (1989) from the United States.
One is from the Great Heath in Maine, which contains shallow, waterlogged
depressions on a mature, domed bog surface that has reached maximum elevation
and now decays more rapidly than it accumulates new peat. A different origin has
been proposed for the existance of Myrtle Lake, a water-filled depression in the
surface of a raised bog in the Lake Agassiz peatland in Minnesota. The beginnings
of Myrtle Lake pre-date paludification and it persisted as a lake beca,use, in the
absence of inorganic sedim~nt supply, it was too deep to be completely infilled
by organic matter. As peat accumulated around the margin of the lake, algal
matter plus the mire's allochthonous outwash was deposited in it, which caused
the lake floor and surface to rise at a rate similar to the surrounding peat surface.
The low degree of tissue preservation found in the densosporinite facies
corresponds well with its assumed raised bog setting and a warm Carboniferous
climate. Also in this context reference has been made in Chap. 3 to the contrast
between the well-textured Sphagnum peat of present-day temperate climates and
the ombrotelmites of tropical SE-Asia, which consist of a more decomposed,
semi-liquid, organic ooze incorporating large pieces of wood. The reason for the
more advanced decomposition of the tropical ombrotelmite is the higher and more
evenly distributed annual temperature which results in more efficient tissue
degradation by microbes, in spite of the low pH. However, the climate argument
does not particularly favour Model B, because the efficiency of tissue destruction
in topotelmites would benefit from high annual temperatures just the same.
5.1.1.3 Densosporinite as Part of a Sedimentary Sequence
In either of the two models the densosporinite facies occurs in the upper portions
of the peat deposits. As has been mentioned before, coal seams of this composition
are often overlain by subaqueous deposits ranging from lacustrine to marine in
origin. In deciding which of the two models is the more likely one, the genetic
relationship between the densosporinite facies and the overlying subaqueous
clastics is of critical importance. Either the coal seam and its clastic roof are part
of the same transgressive tract, in which the densosporinite facies marks the shift
Phyterals and Macerals in Palaeo-Environmental Analysis
175
from predominantly autochthonous organic to predominantly allochthonous
inorganic sedimentation, or the densosporinite facies represents the conclusion of
a regressive phase (from wet to dry conditions) of coal formation, with the clastic
facies marking the beginning of a new transgression (proximal, if represented by
marine shales; distal, if lacustrine). In the first case Model A applies and in the
second case Model B is the logical choice.
Additional insight into the above problem can be gained by considering
composite coal seams in which the vertical sequence depicted in the cartoons of
Fig. 5.3 are repeated several times. An example of this is the Thick Coal described
by Fulton (1987) from the Warwickshire Coalfield in the southern Pennine Basin,
U.K. As illustrated in Fig. 5.4, it consists of four subsections, each terminating
upward with a clastic band, two of which (one above Leaf 1 and the other forming
the roof above Leaf 4) contain fresh-water bivalves. Once again the previous
question arises, i.e. whether the clastic intercalations mark the beginning of
renewed subsidence following regressive phases, which began with shoreline
progradation and peat formation on the reclaimed land and climaxed with raised
densosporinite bogs, or whether the clastic bands are the deeper water terminations
of repeated transgressive trends each beginning with the leucospore-rich
vitrinite/fusinite facies. The trend lines connecting the concentrations of Fulton's
..
b
~
Dominant fJenso- LaeviFaoaL!lcOmiospore sporites !1atospo' sporites spora
species
spp
rItes
exi/Is pusillo
.
minor
Leaf 4
4
Leaf 3
3
Leaf 2
2
Fig. 5.4. Subdivisions of the Warwickshire
Thick Coal with positions of the concentrations of dominant miospore species. Their
relative frequency is indicated by the width
of the bars. (After Fulton 1987)
Leaf 1
o ::::::::::::::::::
-_-----------------...... __ ... -_ .. .
--------------------------------_............... ----------_._._---_.-
176
Coal Facies and Depositional Environment
(1987) significant and dominant miospore species in Fig. 5.4 seem, in this author's
opinion, to favour Model A and so does the observation that densospores extend
from the densosporinite facies into the overlying clastics, where they coexist,
presumably as allochthonous constituents, with freshwater bivalves. It is also
noteworthy that densospores appear to be the only dominant miospore species
present in the seat earth, and that they have been observed in other clastic sediments
by Peppers (1984) and Kosanke (1969).
The coexistence of densospores, inorganic detritus and subaquatic fossils would
be difficult to explain under the conditions of Model B, because the change from
a bog surface raised metres above the general groundwater level involves rapid
subsidence and complete submergence of the raised bog which would leave few
sources of densospores to be deposited together with the incoming clastics in the
spreading lakes. Further support for this notion can be obtained from the
sedimentation pattern in the vicinity of the area covered by the densosporinite
facies. The Warwickshire Thick Coal is restricted to an area of approximately
100 km 2 (Fulton 1987), which is likewise the extent of the maximum development
of the densosporinite facies. Outside this area the seam splits into subsections
which become' increasingly interbedded with clastic sediments. This suggests that
the combined seam occupies a portion of the basin which was less affected by
subsidence than the surrounding areas, in the direction of which the seam tends
to split. Similar situations have been reported by Strehlau (1988) from the Hagen 4
Seam in the Ruhr Basin, and also Butterworth (1964) notes that the greatest
concentration of coals with densosporinite appears to occur in areas of reduced
coal measure thickness.
In the context of the ombrogenous model (B) the portions of the Warwickshire
Thick Coal with high densosporinite concentration represent islands of raised bogs
formed in response to a low rate of subsidence which was commensurate with the
growth rate of low ash ombrotrophic peat. The islands were surrounded by open
water in areas of increased subsidence, in which peat sometimes developed, but
which were mostly sites of subaqueous inorganic sedimentation. The repeated
capping of the densosporinite facies by fossiliferous sediments indicates periodic
increases in subsidence rates resulting in inundation of the raised bog islands
followed by renewed terrestrialisation. Since in this model the parent plants of
densospores are assumed to have occupied the highest and driest portions of the
raised bogs, the change to subaqueous conditions in the peat islands makes it very
difficult to find a source for the densospore content of the clastic bands in such
seams as the Warwickshire Thick Coal.
In the topogenous model (A) the densosporinite facies within the Warwickshire
Thick Coal would be interpreted as the last vestiges of peat accumulation, which
persisted in areas of reduced subsidence, while the surrounding terrain was already
flooded due to higher rates of subsidence. The trapping of sediments in the latter
would leave the low lying peat islands free from inorganic contamination, but as
the islands were reduced in size by the rising water table a blanket of mud and
sand spread over the peat. The intra-seam stone bands, for example between the
four subsections (leaves) in the Warwickshire Thick Coal, represent stages of
maximum flooding followed by land reclamation and renewed peat accumulation.
Phyterals and Macerals in Palaeo-Environmental Analysis
177
As illustrated in Fig. 5.4, the dominance of lycospores in the lower portions of the
subsections extends into the underlying stone bands, which suggests that peat
formation had already recommenced in some parts of the mire, while in others
subaqueous conditions still persisted.
5.1.2 Scalar Properties of Macerals
The scope of the following discussion is restricted neither to the mere presence or
absence of a particular maceral, nor (if present) to its morphological features and
botanical affinities, but the emphasis of the inquiry will be centred on the
quantitative relationship between macerals at the place of burial. There are many
examples in the literature in which coal facies trends have been portrayed, ranging
from the most basic level showing vertical and regional distributions of macerals
or maceral group percentages (Abramski et al. 1951; Diessel 1961; Backer 1965;
Kutzner 1965; Marchioni 1976, 1980; Warbrooke 1981; Haverkamp 1984; Littke
1985a; Strehlau 1988; and many others), either as contoured raw data (e.g. Trinkle
and Hower 1984; Tadros 1988a; Littke and Ten Haven 1989) or as trend surfaces
(e.g. Rimmer and Davis 1988), to the portrayal of various combinations in the
form of maceral ratio maps (e.g. Diessel 1982). Also ternary diagrams of macerals
and maceral groups have been widely used (e.g. Diessel and Callcott 1965;
Mackowsky 1975; Navale 1985; Navale and Saxena 1989; Bertrand 1989; Smyth
1989; Hunt 1989; Correa da Silva 1989; Scheidt and Littke 1989; Littke et al. 1989;
and others) for the purpose of highlighting compositional similarties between
genetically related coals and of contrasting them with coals perceived to have been
formed under different circumstances.
Practically all of the above analyses have been applied to bituminous coals,
but palaeo-environmental relationships have also been illustrated by using ternary
diagrams of brown coal macerals, e.g. in Greek brown coals by Cameron et al.
(1984). By using the giant (350 to 550m thick!) brown to sub-bituminous coal
deposit of Hat Creek as an example, Goodarzi (1985b) summarised vertical seam
development in a set of facies triangles, in which the apices consisted of minerals
(top), humotelinite plus humocollinite (right), and humodetrinite plus inertinite and
liptinite (left). A gradation from "very wet" to "dry" conditions was inferred along
the base line to 20% minerals (= coal) from left to right, while the upward
transitions from coal to carbominerite and to argillaceous shale were taken to
indicate greater water depth, more turbulence and a higher rate of subsidence.
In most presentations of the results of macerals analyses, the authors have
included the full spectrum of components, either by referring to maceral groups or
by combining macerals or their subgroups in a manner deemed suitable to highlight
the desired effect. However, unless the coals in question differ very substanitially
in composition, the analysis results often overlap such that the purpose is not
achieved. A higher degree of resolution is obtained by contrasting carefully selected
key macerals in the form of facies ratios. As discussed in Chap. 2.2, Cohen (1973)
178
Coal Facies and Depositional Environment
Table 5.1. Percentages of the main constituents of the six peat types identified in the Okefenokee
Swamp of Georgia, U.S.A. (After Cohen 1973)
Plant
type
Depositional
setting
Peat type
Pre-telo- Previtrinitic fusinitic
cell walls charcoal
Presclerotinite
Pre-detro- Pre-resivitrinitic nite
debris
Marsh
Nymphaea
49
1
0.3
49
1
Herbaceous
Shallow
glades and
fringes
Carex
65
Panicum
62
. W oodwardia 46
2
2
3
0.3
0.3
0.3
32
35
49
I
I
2
Arboreous
Tree islands
and swamps
Cyrilla
Taxodium
56
60
4
5
25
24
15
10
----------
applied this principle to the peat types of the Okefenokee Swamp by defining a
ratio of framework particles to matrix (F1M) and a ratio of non-sedimentary to
sedimentary material (N/S). Because of the obliteration of much of the diagnostic
cell tissues during coalification, these ratios cannot be readily applied to coal,
particularly not the NIS ratio because it relies on the identification of roots and
rhizomes, which is rarely possible in coals. Although it is possible to distinguish
between cell tissues of different origin in brown and black coal, respectively, such
differentiation is difficult in more humified cell tissues and cannot be applied
quantitatively. Nevertheless, Cohen (1973) did consider the post-coalification
nature of his peat constituents whose quantitative relationships are listed in
Table 5.1.
The percentages of the components given for each of the six peat types in
Table 5.1 are based on point counts of vertically oriented microtome sections. The
coal maceral interpretation of the peat constituents given in the column headings
follows closely that of Cohen but two modifications have been made. One of them
is merely for the sake of brevity and refers to the pre-resinite in the last column
of the table which in Cohen's interpretation includes all cell infillings, such as
pre-phlobaphinite. The plant debris listed in the fourth column has been called
pre-detrovitrinitic, which deviates somewhat from Cohen's (1973, Table 2) interpretation, who refers to it as "predominantly pre-micrinites with some pre-vitrinite".
In view ofthe predominantly limnotelmatic environment in the Okefenokee Swamp
a reversal of the emphasis might be more appropriate, apart from the possible
confusion about the term micrinite.
Excluding the cell secretions listed in the last column of Table 5.1, all other
constituents have been derived from cell tissues. The first three columns contain
plant remnants which still display tissue fragments. This might not be true for all
of the fungal matter listed in the third column, but its quantity is too insignificant
to be of much concern. By establishing a ratio of columns 1 + 2 + 3 versus column 4,
a measure of the retained cell tissue is obtained not inlike the tissue preservation
index discussed below. When applied to the six Okefenokee peat types the ratios
give the following numerical values:
Phyterals and Macerals in Palaeo-Environmental Analysis
179
1. N yrnphaea peat = 1.0
2. Carex peat
= 2.1
3. Panic urn peat
= 1.8
4. W oodwardia peat = 1.0
5. Cyrilla peat
= 2.4
6. Taxodiurn peat = 2.7
The figures show that the two peats with the largest proportion of woody
progenitors (5 and 6) have the highest tissue ratios, although in the case of the
Taxodiurn peat, tissue preservation appears to vary widely depending on the
position of the groundwater table and the duration of dry periods (1. Dehmer,
pers. comm.). The lower tissue counts reported by Yeakel (1981) and Dehmer
(1988) for this peat type, apart from having been carried out differently to Cohen
(1973), indicate more advanced humification and oxidation. This is, for example,
shown in Yeakel's analyses by the high proportion of charcoal (6.6%), which
probably resulted from crown fires, and a likewise high humic acid content of
38.1% compared with 21.7% in the Nyrnphaea peat. In the interpretation of these
results it is also important to realise that they have been derived from near the
depositional interface where active peatification is still in progress. It is likely that
with increasing maturity the tissue ratios of the peats with herbaceous precursors
(1 to 4) will be lowered more than for peats formed primarily from woody plants.
The reason for this is the very low shoot/root ratio found in the peats of
non-lignified herbaceous plants, in which the surviving tissue is frequently restricted
to root material alone (Grosse-Brauckmann 1980). In arborescent plants too, much
of the aerial vegetal matter is lost during humification, but relatively more aerial
plant tissue may be preserved. Phillips et al. (1985) estimate a shoot/root ratio for
complete Pennsylvanian (Upper Carboniferous) lycopod trees of approximately 4
compared with an average of 1 (range 0.69 to 2.69) found in lycopod-rich (80 to
95% lycopods) fossil peats preserved in European coal balls. In the United States,
where the lycopod content of the coal balls is often smaller and the proportion
of cordaites is higher, the reported shoot/root ratios range from 0.47 for 54.5%
lycopods and 36.7% cordaites in the peat to 1.3 for 91.1 % lycopods and 1.7%
cordaites (Phillips et al. 1985, Table 3). The proportion between lycopods and
cordaites in the Carboniferous mires appears to exert some influence on the origin
of the preserved cell tissues. In lycopod-rich coals aerial bark tissue is more common
than wood tissue. An example is the Lancashire Coalified in the U.K., where Phillips
et al. (1985) found a periderm/wood ratio of up to 8.5, in contrast to the
cordaitean-rich U.S. coals, in which often more wood tissue from roots has been
preserved than bark tissue.
There have been several suggestions in the past to characterise coal and coalforming environments on the basis of the quantitative relationship between tissue
derived components. For example, von der Brelie and Wolf (1981a) correlate
different brown coal lithotypes of the Lower Rhine Valley with their gelification
quotient (Vergelungsquotient) which is defined as:
.
Ulminite + densinite + gelinite
G el1'fi'
lcatlOn quotIent = ~-"--"-"-"
--Textinite + attrinite
(5.1)
180
Coal Facies and Depositional Environment
The average gelification quotient for the Lower Rhine brown coal is 0.38 ± 0.34,
while a range from 0.08 to 1.28 is quoted for the Miocene brown coals of the
Oberpfalz in southern Germany by Dehmer (1989a). According to her, the gelification
quotient has a threefold application:
1. The gelification quotient is a measure of wet/dry conditions, because gelification
requires the continuous presence of moisture.
2. The gelification quotient is a relative indicator of pH, because effective microbial
activity requires low acidity resulting in tissue degradation followed by flocculation of the generated humic acids and other decomposed organic matter as
humates.
3. The gelification quotient can also be a measure of diagenesis during which biochemical gelification, as part of syngenetic humification, is increasingly replaced
by epigenetic gelification.
George (1982) uses a gelification index for the brown coals of the Gippsland Basin
in Victoria, Australia, which he defines as:
Gelification index = densinite + eu-ulminite + levige1inite
+ 1/2 phlobaphinite (pseudophlobaphinite).
(5.2)
This index correlates well with brown coal lithotypes showing a systematic
progression in ge1ification from pale (gelification index = 1.1) to dark (gelification
index = 44.7) lithotypes.
The application of either Eqs. (5.1) or (5.2) to bituminous coal is not possible
because of the difficulty in some macerals to make a clear distinction between the
effects of syngenetic humification and epigenetic gelification. For example, densinite
and attrinite occur in Eq. (5.1) in the numerator and denominator, respectively, but
in bituminous coal the distinction between the two macerals has been obliterated
by epigenetic gelification and subsequent polymerisation, as both macerals are
transformed into detrovitrinite (or desmocollinite). However, this disadvantage is
balanced by the better distinction in higher rank coals between the members of
the inertinite and vitrinite groups of macerals. With the exception of pyrofusinite
and pyrosemifusinite which, according to the partially charred wood illustrated
in Figs. 3.18 to 3.20, have responded instantly with elevated reflectance to
incomplete combustion, the milder effects of desiccation and oxidation experienced
by other precursors of inertinite are barely recognisable in peat and brown coal.
They are thought to develop during or immediately after polymerisation when
coal properties attain their maximum divergence at the beginning of high volatile
bituminous coal rank. Irrespective of the mode of inertinite formation, when present
in high concentration and as an autochthonous component of a bituminous coal
seam, it indicates that the higher reflecting macerals have been oxidised and/or
partially pyrolised. Unless the proportion of macrinite is particularly high, such
coal will show low levels of gelification.
Gelification is characteristic of vitrinite, the precursors of which may have
been in contact with some oxygen during humification but without ever undergoing
dehydration and desiccation, except, perhaps, for very short periods which may
Phyterals and Macerals in Palaeo-En'l'ironmental Analysis
181
have led to the formation of so-called pseudovitrinite (Benedict et al. 1968;
Thompson and Benedict 1974; Wolf and Wolff-Fischer 1984). An intermediate
position is taken by macrinite which has been first gelified and subsequently
oxidised or partially pyrolised.
Based on the above considerations, Diessel (1982, 1985b, 1986a, b) developed
two coal facies indices, a gelification index (GI) contrasting gelified (e.g. vitrinite) with
fusinitised (e.g. inertinite) macerals, and a tissue preservation index (TPI) which
emphasises degrees of tissue preservation versus destruction. These indices are
defined as follows:
Telovitrinite + telo-inertinite
TPI=------------------------------Detro- + gelovitrinite + detro- + gelo-inertinite
(5.3)
G I = __V_it_fl_·_n_it_e_+_g_e_Io_-_in_e_r_ti_n_it_e_
Telo-inertinite + detro-inertinite
(5.4)
Given similar source material, the tissue preservation index is a measure of the
degree of humification because under conditions of mild or brief humification
much of the cell tissue produced in a peatland would be retained either as
telo-inertinite (= fusinite + semifusinite) in a relatively dry peat, or as telovitrinite
(telinite + telocollinite) under more moist conditions. Advanced humification leads
to tissue destruction, which is expressed in the tissue preservation index by a low
numerical value. The complementary gelification index relates to the continuity
in moisture availability. Slow subsidence resulting in marginal peat-forming
conditions, or intermittent, e.g. seasonal drying (hot or cold) of the peat will bring
about oxidation and other irreversible changes in the more or less humified plant
tissues, which can be expressed by the gelification index. A more detailed discussion
of the two indices follows below.
5.1.2.1 The Tissue Preservation Index
The ratio of tissue-derived structured macerals versus tissue-derived unstructured
macerals is a measure not only ofthe degree ofhumification suffered by the maceral
precursors but, to some extent, it gives also an indication of the proportion of
wood which has contributed to the peat and was preserved in it. Cellulose is known
to degrade quite readily, even under anaerobic conditions (Given et al. 1973;
Chaffee et al. 1986), except when Eh and pH are very low (Francis 1961; Russell
and Barron 1984). It follows that cell tissues of herbaceous plants, which often
lack the structural support provided by lignin in their cell walls, tend to decompose
more quickly than lignin-supported wood cells. However, actual decomposition
rates of cellulose seem to fluctuate widely, as shown by the experiments Given
et al. (1983) conducted in Florida peat swamps. The preservation potential of wood
also varies and prolonged humification, for example due to an extended residence
time in the acrotelm because of a slow rate of subsidence, will also tend to lower
TPI. Conversely, an increase in the proportion of structured maceral derived from
non-woody cell tissues can be brought about by special conditions, such as a cold
182
Coal Facies and Depositional Environment
climate. An example are the Permian coals of Australia, where a notable contribution to the amount of telovitrinite and semifusinite has been made by the
seasonally abundant leaflitter produced by the deciduous Gondwana flora (Beeston
1982; Diessel 1983; Taylor et al. 1989).
Maximum tissue preservation indicates a balanced ratio of plant growth and
peat accumulation versus rise in groundwater table, for example due to basin
subsidence. This notion applies primarily to a topogenous mire setting, but it
probably holds for raised bogs just the same, since no ombrotelmite can be
preserved as coal without basin subsidence. Although cell tissues can be preserved
in both vitrinite and inertinite, the highest TPI values of approximately 2.5 recorded
by Diessel (1986a) for Permian and by Strehlau (1988) for Carboniferous coals are
based on telovitrinite rather than telo-inertinite. The conclusion that ideal
peat-forming conditions with the least loss of biomass are indicated in the coal
by telovitrinite-based high TPI values is supported by the compositional changes
observed in coal seams which have been formed under the influence of a lateral
increase in subsidence. An example of this is the Maules Creek area in the Gunnedah
Basin of New South Wales.
As illustrated in Figs. 5.5 and 5.6, the Maules Creek area is located near the
northeastern margin of the Gunnedah Basin, which is part of the 2000 km-Iong
foreland basin to the New England Fold Belt in eastern Australia. The coal
measures (Maules Creek Formation) are of Middle Permian age, they rest on
-l----"""""[li"-"',;:;:----r--------I-20'
-----<-2.'
--:,----1_2.'
-+----i-32'
-+""-='----=''''T---t/'----------i- 36'
,
30.0
Fig. 5.5. Location of the Boggabri Ridge
within the Gunnedah Basin. The ruled
area encompasses the molasse foredeep to
the New England Fold Belt in eastern
Australia. (After Diessel 1985b)
183
Phyterals and Macerals in Palaeo-Environmental Analysis
NE
SW
70
42
47
51
28
19
M
Fig. 5.6. Traverse of borelogs (numbered above the columns) showing coal seams (black) and
prominent conglomerates (circles) along the section line displayed in the inset between Boggabri
and Narrabri. (After Butel et al. 1983 and Diessel 1985b)
Permo-Carboniferous volcanics, which are exposed in the Boggabri Ridge to the
west of the traverse of boreholes displayed in Fig. 5.6. The setting of the coalfield
on the eastern flank ofthe Boggabri Ridge has resulted in an asymmetric transverse
cross-section of the Maules Creek Formation, which thins towards the Boggabri
Ridge but thickens to almost 800 m in a northeasterly direction towards the
border thrust with the New England Fold Belt. The NE-SW traverse of boreholes
illustrated in Fig. 5.6 gives the positions of coal seams, which in the deep holes to
the northeast increase in rank from 0.75% mean maximum telovitrinite reflectance
near the surface to over 0.8% at depth. The contrast between the thin coal measure·
section near the Boggabri Ridge and the increasing thickness away from it towards
the northeast suggests that the rate subsidence increased in the same direction,
thus providing an opportunity to relate coal composition to the changing
depositional regime. Petrographic analyses carried out on representative whole
seam samples of washed coal (floats at 1.6 gjcm 2 ) show an increase in vitrinite
towards the Maules Creek Subbasin.
More than one half of this increase in total vitrinite content is based on
telovitrinite. It consists of both telinite and telocollinite and appears to have been
of
Coal Facies and Depositional Environment
184
derived mainly from xylem and cortex tissues, thus indicating an origin from
wood-producing plants under rather moist conditions. It is therefore not surprising
that both the tissue preservation and gelification indices, whose isopleths are
illustrated in Figs. 5.7 and 5.8, also increase into the basin.
A low TPI suggests either predominance of herbaceous plants in the mire or
large scale destruction of wood because of extensive humification and mineralisation. In the latter case, the remaining coal should be relatively enriched in
comparatively stable liptinite macerals, including sporinite and resinite, as well
as dispersed residues offusinite and semifusinite in the form of inertodetrinite.
The presence in coal of detro-inertinite, i.e. mainly inertodetrinite, has two
aspects depending on whether it occurs in situ (autochthonous) or has been
redeposited (hypautochthonous or allochthonous). Consisting essentially of
fragmented cell walls of fusinite and semifusinite plus small corpomacrinite grains,
the phytogenic precursors of autochthonous inertodetrinite have been subjected
to severe humification followed by desiccation and, in many instances, by
incomplete combustion. In this case the inertodetrinite is associated with a maceral
debris consisting of partially oxidised resinite, corroded spores, other liptodetrinite
and inherent ash (phytoliths) in the form of disseminated clay and silica, all of
70
47
42
51
28
19
0
100
200
CONCENTRATION
D
<0.6
fZ2Zl
0.6-0.8
~
0.8-1.0
§
300
400
>1.0
500
BOGGABRI RIDGE
600
700
800
Fig. 5.7. lsopleths showing the increase in tissue preservation index (TPI) from the flank of the
Boggabri Ridge into the Maules Creek Subbasin along the section line of Fig. 5.6. (After Diessel
1985b)
Phyterals and Macerals in Palaeo-Environmental Analysis
42
47
185
51
28
19
0
100
200
CONCENTRA TlON
Dc,
~1-2
300
400
~ 2-4
§
Fig. 5.8. Isopleths showing the increase in gelification index (GI) from
the flank of the Boggabri into the
Maules Creek Subbasin along the
section line of Fig. 5.6. (After Diessel
1985b)
>4
Horizontal Scale
500
600
700
BOGGABRI RIDGE
800
which comprise the remnants of an oxidised peat formed during a period of low
groundwater conditions.
Increased inertodetrinite contents due to relatively dry peat-forming conditions
have also been found on the Boggabri Ridge referred to above. The inertodetrinite
percentages which are high above and on the flank of the basement ridge decrease
towards the adjacent Maules Creek Subbasin, i.e. in the direction of accelerated
basin subsidence resulting in a consistently higher groundwater table and more
optimum peat accumulation, which particularly affects the lower stratigraphic
sequence.
While for autochthonous inertodetrinite the places of origin and burial are
the same and may very well included raised bog environments, hypautochthonous
(redistributed locally) and allochthonous (transported to the place of burial from
some distance including from outside the swamp) inertodetrinite have separate
sites of origin and burial. The redeposited inertodetrinite is not necessarily the
product of the same extreme humification suffered by the autochthonous inertodetrinite referred to above but it has been formed during transportation by
mechanical abrasion from larger pieces of fusinite, the precursors of semifusinite,
and flocculated and oxidised humic acid. In most cases, the depositional site is a
subaqueous environment, such as a water course or a lake into which the inertodetrinite has been transported, for example, by flood waters, as envisaged by
Beeston (1982) for his detrital coal facies. Evidence for this is the coexistence of
inertodetrinite with pyrite, alginite (Fig. 5.9), the occurrence of imbrication
(Fig. 3.25) graded bedding (Fig. 5.10) and microcross-lamination (Fig. 5.11) in
inertodetrinite and associated liptodetrinite concentrations. It is also noteworthy
186
Coal Facies and Depositional Environment
Fig. 5.9. Two photomicrographs showing inertodetrinite and tel alginite in a high volatile
bituminous coal from the Maules Creek Formation, Gunnedah Basin, New South Wales. Incident
white light in oil immersion (left) and in dry fluorescent mode (right); actual length of each field
of view = 0.22 mm
that Falcon (1989) finds a posItive correlation between concentrations of
inertodetrinite and non-spinose acritarchs.
The very low TPI values commonly found in sapropelic coals and other
subaqueous maceral deposits are linked to the low total vitrinite content which,
according to Fig. 4.2 in the previous chapter entails reduced tissue preservation,
as well as to the relatively high degree of fragmentation in the form of inertodetrinite
of the dispersed maceral precursors in relation to distance of transportation from
the place of origin. An example is given in Fig. 5.12, in which the mean TPI and
GI values represented by open circles, have been calculated from Strehlau's (1988,
Figs. 10 and 11) analysis results. The marked decline in tissue preservation indices
from lycosporinite-bearing durite to the sapropelic coal appears to indicate a
change from proximal hypautochthonous to distal allochthonous depositional
conditions.
Also included in Fig. 5.12 are TPI and GI values calculated from Rimmer and
Davis (1988, Fig. 11), for two sections (full circles) of the above-mentioned Lower
Kittanning Seam, subdivided into bottom, middle and top portions. The two
sections differ in their gelification indices (they are higher in the sample from the
central portion of the Appalachian Basin in accordance with the larger proportion
Phyterals and Macerals in Palaeo-Environmental Analysis
187
Fig. 5.10. Photomicrographs showing two layers of inertodetrinite set in vitrinite. Note the
pronounced graded bedding of the inertodetrinite. Incident white light in oil immersion; actual
vertical length of the field of view = 0.36 mm
of vitrinite due to the higher rate of subsidence compared with the basin margin)
but both show the same decline in TPI and GI values towards the seam roof.
Their numerical values are not as low as the arithmetic mean for Strehlau's (1988)
densosporinite facies but the trend in the direction of the allochthonous
sapropelite facies is obvious and supports the notion . that the densosporinite
assemblage is likely to indicate a wet peat-forming environment. However, it
should be stressed that this assumption is based as much on attribute properties,
such as the observed gradation of the facies into sapropelic coal, as it is on the
TPI and GI values. After all, numerically low indices can also be obtained from
a dry durain formed in a raised bog, in which the structured inertinites
(telo-inertinite) have been degraded into inertodetrinite due to excessive oxidation.
An indication of the amounts of inertodetrinite found in coals which formed
either under exceptionally dry conditions or in which the peat was reworked and
oxidised during redistribution is illustrated in Fig. 5.13, in which the percentages
of telo-inertinite (fusinite + semifusinite) of hundreds of Australian Permian coal
samples are correlated with their associated inertodetrinite contents. The point
cloud is divided into two areas, the more concentrated portion of which (sector A)
shows a high telo-inertinite/inertodetrinite ratio of 4 to 8. This appears to be a
Coal Facies and Depositional Environment
188
Fig. 5.11. Photomicrographs showing
microcross-Iamination by the alignment
of inertodetrinite and sporinite on a
telovitrinite surface in a high volatile
bituminous coal from the Sydney Basin,
New South Wales. Incident white light
in oil immersion; actual vertical length
of the field of view = 0.36 mm
14
M
12
10
8
T
8
T~
6
4
DFO
2
SF
(j 0
DSD DST
DSl
0----00-----0
0.0 TPI
1.0
2 .0
3.0
Fig. 5.12. Bivariant plot of the tissue preservation and gelification indices of various
Carboniferous coals facies from the Ruhr and Appalachian Basins based on analyses by Strehlau
(1988) = open circles and Rimmer and Davis (1988) = closed circles. SF Sapropelic facies, mean
of three samples; DF densosporinite facies, mean of 13 samples; DSD duritic subfacies with
densosporinite, mean of ten samples; DST duritic subfacies with torisporinite, mean of three
samples; DSL duritic subfacies with Iyco-(tenui-) sporinite, mean of 17 samples. B bottom; M
middle; Ttop of the Lower Kittanning Seam
189
Phyterals and Macerals in Palaeo-Environmental Analysis
50
A
40
30
B
20
!
:~
i10
c
T
o
Qi
I-
~'O~--~~r------.------r-~--~----~
o
10
20
30
40
50
'l(, Inertodetrinite
Fig. 5.13. Diagram showing the correlation between inertodetrinite and telo-inertinite
(semifusinite + fusinite + scierotinite) in Australian bituminous coals. Sector A "Normal"
relationship; Sector B excessive inertodetrinite. (After Diessel 1982)
normal relationship because a coal with a given telo-inertinite content will always
be associated with a small proportion of likewise in situ inertodetrinite as a result
of some mechanical disintegration. The considerably smaller ratios in sector Bare
based on inertodetrinite concentrations of up to 45% on a whole coal basis. They
are the result of unusual circumstances and, as discussed above, have been brought
about either by fragmentation of inertinite during transportation or by excessive
humification, oxidation and possibly burning.
5.1.2.2 The Gelification Index
While the tissue preservation index (TPI) is affected by the duration and severity
of humification of the maceral precursors, the gelification index gives an indication
of the relative dryness or wetness of autochthonous peat-forming conditions. Hypautochthonous and allochthonous coal components, such as the "wet" i.e. subaqueous
durites plotted in Fig. 5.12 and sapropelic coals do not follow this rule because
they represent only the redistributed detrital fraction of the peat but not its entirety.
Since in many cases even macrinite has been fragmented during transportation
and is therefore counted as inertodetrinite, the GI values of such coals are
commonly quite low.
190
Coal Facies and Depositional Environment
Like the tissue preservation index, the gelification index too is related to rates
of peat accumulation and basin subsidence (or rise in groundwater table), as has
been demonstrated by the traverse across the flank of the Boggabri Ridge discussed
above. The GI values plotted in Fig. 5.8 are low above and on the flank of the
basement high, but they increase in the direction of accelerated basin subsidence
into the Maules Creek Subbasin, where a consistently higher groundwater table
inhibited desiccation and caused the vegetable matter to follow the vitrinitisation
path.
The presence or absence of remnant cell tissue is not considered in the
gelification index but emphasis is put on the degree of homogenisation of the
former vegetable matter. This is achieved by contrasting the gelified structured
and unstructured vitrinite macerals plus macrinite (that the latter has been affected
by some drying is of no consequence since it was substantially homogenised first)
with telo- and detro-inertinite macerals, all of which show no or only little
gelification. The gelification index is therefore similar to the vitrinite/inertinite
ratio of Harvey and Dillon (1985), although micrinite was excluded from the latter.
Micrinite presents a problem, because not all of it is merely very finely
fragmented inertinite (Hartlieb 1962b; Mackowsky 1976), but is a byproduct of
petroleum generation from liptinitic materials (Teichmiiller 1974). As mentioned
in Chap. 4, this secondary micrinite is called submicrinite by Taylor and Liu
(1989) and associated more closely with the liptinite group of macerals than with
inertinite. On account of their small sizes it is not possible to distinguish the two
varieties under the light microscope with sufficient accuracy, which raises the
question of whether to in- or exclude this maceral when calculating coal facies
indices. Equations (5.3) and (5.4) include micrinite as part of the subgroup
detro-inertinite, because in many Gondwana coals, the total amount of micrinite
is not only very small (see Table 1.1) but consists mainly of finely fragmented
inertinite. However, where a high proportion of secondary micrinite would
introduce a significant bias, as may be the case with some Carboniferous coals, it
might be better to exclude micrinite altogether.
Both GI and TPI define four coal facies whose palaeo-environmental
characteristics are summarised in Table 5.2. The table is based on palaeoenvironmental studies carried out in the Sydney Basin of New South Wales on
coal seams whose depositional setting had been established over many years of
field and laboratory investigations. The result of the investigations are listed in
Table 5.3, in which a type seam has been named for each depositional environment.
The tabulated indices represent composite values, which have been obtained from
whole coal samples. It is expected that a pillar sample or a set of subsamples from
a channel sample would yield a similar average signature, although the stratigraphic
progression through each seam will show considerable and systematic variations
from floor to roof (Tasch 1960; Smith 1961, 1962; Diessel1961, 1965a; Smyth 1967,
1970, 1972; Britten and Smyth 1973; Shibaoka and Smyth 1975; Shibaoka and
Bennett 1975, 1976; Marchioni 1976, 1980; Littke 1985a, b, 1987; Strehlau 1988,
1990; Ting and Spackman 1975; Ting 1989; and others). Except for the coals of
the upper delta plain and alluvial valley, which cannot be separated, and the distal
back barrier coals, the various environmental settings have exerted a distinct
Phyterals and Macerals in Palaeo-Environmental Analysis
191
Table 5.2. Summary of the relationship between coal facies indices and conditions of coal formation
High TPI
Low TPI
High GI
Coal type: Bright (vitrain) to banded
bright (ciarain); wood- and barkderived telovitrinite
Origin: In forested peatlands (telmatic
swamps), when relatively high in
coal ash and/or interbedded with
epiciastic stone' bands. In forested,
continuously wet raised bogs, when
low in ash. Mild humification and
strong gelification of plant tissues
due to high rate of subsidence
Coal type: banded bright (cia rain);
tissue-derived detrovitrinite plus
some gelovitrinite
Origin: (I) In forested peatlands from
strongly decomposed wood under
conditions of slow subsidence in
telmatic or limnotelmatic setting (high
ash and epiciastic bands). (2) From
herbaceous plants in tree-less marshes
(high ash and epiciastic bands). (3)
From herbaceous plants in continuously wet raised bogs (low ash, no
bands). Telmatic or limnotelmatic.
Advanced humification and strong
gelification of plant tissues
LowGI
Coal type: Banded dull (ciarodurain);
wood-derived telo-inertinite
Coal type: Dull (durain) to banded
dull (ciarodurain); tissue-derived inertodetrinite
Origin: (I) In slowly subsiding, intermittently dry swamps from aerobically
decomposed autochthonous plants.
(2) Redistributed as subaqueous sediment. (3) In slowly subsiding relatively
dry raised bogs
Origin: In intermittently dry forested
swamps when high in ash, or in
forested raised bogs, when coal ash is
low or moderate. Mild humification
and gelification of plant tissues
Table 5.3. Coal facies indices for different sedimentary settings in the Sydney Basin of New South
Wales. (After Diessel 1986a)
Mean GI
Std. Error
n
0.261
3.58
2.262
IO
0.85
0.059
10.76
2.139
32
0.90
0.103
2.60
0.497
5
Borehole
Bayswater
1.52
0.67
0.376
0.289
4.86
0.64
0.470
0.228
7
18
Upper delta/Alluvial
valley
Dudley
1.50
0.053
3.68
0.264
41
Braid plain (sandy)
(gravelly)
Bulli
Wallarah
1.23
1.49
0.059
0.096
1.04
0.78
0.051
0.089
13
18
Environment
Type seam
Mean TPI Std. Error
Abandoned delta
(transgressive)
Greta
0.40
Lower delta plain
Rathluba
Backbarrier
(transgressive,
proximal)
(transgr., distal)
(regressive)
Wynn
192
Coal Facies and Depositional Environment
Table. 5.4. Approximate IV and SAL ranges in relation to depositional environments for 20
Hunter Vaney coals based on 155 samples (After Warbrooke 1987)
Depositional environment
Type seam
IV Factor
SAL Factor
Reed marsh
Fen
Wet forest swamp
Wet/dry forest swamp
Relatively dry forest swamp
Greta
Borehole
Victoria Tunnel
Fassifern
Wanarah
15-35
5-35
5-35
35-65
45-85
45-60
60-80
80-90
65-85
55-75
influence on coal type which will be the subject of a detailed discussion in Chaps. 7
and 8.
An interesting approach has been taken by Warbrooke (1987) who combines
maceral analyses with coal ash-based geochemical information and obtains two
ratios which he defines as:
IV Factor
= Inertinite x 100/(inertinite + Vitrinite).
SAL Factor = Si0 2 x 100/(Si0 2 + AI 2 0 3 ).
(5.5)
(5.6)
By applying the two parameters to the Hunter Valley coals of the Sydney Basin
of New South Wales, the relationships given in Table 5.4 have been established.
The palaeo-environmental significance of minerals and ash composition is further
discussed below.
5.2 Microlithotypes as Tools in Palaeo-Environmental Analysis
Coal facies studies have been traditionally the principal field of application of
microlithotype analyses. Because microlithotypes consist of associations of
micropetrographic coal components, they reflect to a greater extent the coal
forming environment than individual macerals. For this reason the above coal
facies indices TPI and GI were estabilished, so that several macerals can be
contrasted with each other. These coal facies indices differ from microlithotypes
by their concentration on key macerals, whereas microlithotypes are more broadly
based and take account of mrtural maceral association.
5.2.1 Microlithotype Proportions and Bandwidth
Microlithotypes can be applied in various ways to palaeo-environmental analysis.
Vertical profiles and ternary diagrams have been used by Abramski et al. (1951),
Tasch (1960), Diessel (1961), Diessel and Callcott (1965), Hacquebard and Donaldson
Microlithotypes as Tools in Palaeo-Environmental Analysis
193
(1969), Hacquebard and Barss (1970), Smyth (1968, 1979), Marchioni (1976, 1980),
Warbrooke (1981), Hunt (1982, 1989), Miao et al. (1989) and many others. Smyth
(1968, 1979, 1984, 1989) used density contoured triangular diagrams of microlithotype groups in order to distinguish between coals from different depositional
environments. Falcon (1989) combined microlithotype with palynological analyses
and found systematic correlations of saccate pollen with high-vitrinite microlithotypes, trilete spores with inertodetrite (as mentioned above, this microlithotype also correlates positively with non-spinose acritarchs), and carbominerite-rich
microlithotypes, and finally, plicates with inertinite-rich microlithotypes.
Another approach has been taken by Hunt (1982) and Hunt and Hobday
(1984) with the addition of a fourth parameter called bandwidth. The facies triangle
illustrated in Fig. 5.14 displays the microlithotype groups inertite plus durite on
the left apex and vitrite plus clarite on the right apex. This leaves vitrinertite plus
trimacerite for the upper apex, i.e. the microlithotype groups with the highest
degree of mixing between vitrinite and inertinite. A coal in which the macerals are
arranged in clearly defined mono- or bimaceral microlithotypes plots near the
baseline. A coal which is coarsely banded and shows little mixing of its macerals
will have a low proportion of trimacerite and vitrinertite. It will therefore plot
near the baseline of the triangle, whereas a coal with a high proportion of these
components shows a considerable degree of mixing. This is possible only when
the bandwidth is so very fine that only a small amount of mono- and bimaceral
(other than vitrinertite) microlithotypes is present. Such a finely banded coal will
plot closer to the upper apex. For this reason Hunt (1982) has subdivided the
triangle by a set of curves, which separate areas of coarse, medium, fine and very
VITRINERTITE
+
DUROCLARITE
+
CLARODURITE
100
Fig. 5.14. The microlithotype facies triangle subdivided on the basis of
curves of relative bandwidth. (After Hunt 1982)
1 00 ""'---"'---"'---'--"--"'---:!,::---"'---"--'--"--"'--'S1 00
INERTITE
VITRITE
DURITE
CLARITE
+
+
194
Coal Facies and Depositional Environment
fine bandwidth, respectively. In reference to depositional environments Hunt and
Hobday (1984) found the following relationship:
1. braided fluvial (alluvialfans) = medium banded
2. meandering fluvial
= very finely banded
3. upper delta plain
= finely banded
4. lower delta plain
= medium banded
Other authors have overcome the restrictions imposed by triangular plots by
spreading the analysis results over two interconnected triangles. In fact the oldest
and most widespread application of microlithotypes in coal facies analysis makes
use of this technique.
5.2.2 Hacquebard's Double Triangle
When applying conventional microlithotypes of the kind listed in Table 4.2 to
palaeo-environmental studies, it was found that some adjustments had to be made.
Based on the pioneering work by Teichmiiller (1950, 1958), Teichmiiller and
Thomson (1958), Smith (1962) and Spackman et al. (1966), a system of micro lithotype analysis for the purpose of coal facies studies was developed by Hacquebard
et al. (1964, 1967). They constructed a four-component facies diagram based on a
double triangle in which the apices represent Teichmiiller's (1950) four peat facies
and Osvald's (1937) mire zones. Recent examples of the application of the technique
are given by Chandra and Chakrabarti (1989).
The analysis can be carried out by any of the methods described in Chap. 4
and the results are plotted on the double triangle shown in Fig. 5.15. If fewer than
20% D-components (limnic zone) have been recorded, the upper triangle is used
and A- and D-components are combined. Alternatively, B- and C-components are
combined and the lower triangle is used when D-components amount to more
than 20%. Their meaning and palaeo-environmental interpretation is given below
together with extensions to the original concept by Marchioni (1976, 1980),
Warbrooke (1981) and others. Such extensions consist of additional microlithotypes, which have been estabilished in order to suit local conditions and requirements.
The dispersed nature of some organopetrographic components, in particular
among the liptinite and inertinite macerals, opens possibilities for further refinement,
but it should be noted that, in its present form, the double triangle has no real
provision to accommodate ombrotrophic environments.
1. The A-components sporoclarite and duroclarite have traditionally been
associated with reed-like herbaceous vegetation, as found in topogenous
marshlands or in the fringe zones of treed swamps. When plotted on the upper
triangle, these components refer to the telmatic zone which is situated between the
high and low water mark in intertidal marshes or in other environments periodically
Microlithotypes as Tools in Palaeo-Environmental Analysis
195
TERRESTRIAL FOREST
ZONE
B
LIMNOTELMATIC
FOREST ZONE
Fig. 5.15. The double triangle according to Hacquebard et al. (1964,
1967) facilitating coal facies analysis
on the basis of microlithotypes
't-*-*---*---*---7(----'I LIMNOTELMATIC
REED ZONE
D
LIMNIC ZONE
inundated by water. As mentioned above, coal formation under ombrotrophic
conditions was not considered in the formulation of this facies concept, but a
concentration of A-components in a low ash coal could also have originated in
the tree-less central portions of continuously wet raised bogs.
In the lower triangle the depositional setting is assumed to be limnotelmatic,
i.e. transitional between the telmatic and limnic zones. The area is permanently
covered with shallow water down to a water depth of2 m, which is the approximate
limit for rooted vegetation.
The paucity of woody vegetation and the preponderance of soft-tissued
spore and pollen-bearing plants produces a coal type in which sporinite and
desmocollinite are concentrated in varying proportions. They are covered by the
microlithotypes sporite and sporoclarite, respectively. In many cases the claritic
matrix contains some inertinite which has been washed into the mire from drier
parts of the swamp and occurs in the form of inertodetrinite. Hacquebard and
Donaldson (1969) therefore recognised duroclarite as part of their reed moor
environment. The specific origin of the liptinite could be further emphasised by
referring to this microlithotype as duro-sporoclarite which is a trimacerite in which
vitrinite exceeds the inertinite content whilst sporinite dominates the liptinite. The
microlithotype vitrinertoliptite added by Marchioni (1976) is a trimacerite in which
the liptinite content exceeds that of inertinite. By referring to it as vitrinertosporite,
once again, the spore origin of the liptinite is emphasised.
2. The B-components fusitoclarite (= fusite in a clarite matrix) represent the
terrestrial zone, which is affected by periods of dryness. Also intermittently dry
Coal Facies and Depositional Environment
196
raised bogs could possibly be incorporated with this environment, although, once
again, this possibility does not appear to have been envisaged at the time the
system was established. This facies is characterised by both arborescent vegetation
and a relatively low groundwater level. Wood-derived macerals formed under partial
oxidation dominate the resulting coal types, which occupy the upper apex in Fig.
5.16. The mono-maceral microlithotypes are represented by fusite, semifusite,
sclerotite, and macroite, respectively. Of these the first two are the product of
either incomplete combustion (e.g. pyrofusinite) or biochemical degradation under
partial access of oxygen. Sclerotite incorporates both coalified remnants of fungal
tissue, as well as oxidised resinous and humic matter. Finally, macroite is the
microlithotype equivalent of macrinite and thus differs from the before-mentioned
microlithotypes by its lack of any residual cell structure. Two bimaceral microlithotypes could be added to this facies. One is durite, in which the inertinite
portion consists of either fusinite, semifusinite, sclerotinite or macrinite while the
liptinite need not be specified. The other is vitrinertite, which consists of vitrinite
TERRESTRIAL
B
TELMATIC
TELMATIC
C
•
~
':,...
6.
.6
6
A+D
~~~--~~----------~A
B+C
LlMNO- TELMATIC
o
Bright Coal
•
Banded Bright Coal
•
Banded Coal
6
Banded Dull Coal
..
Dull Coal
A Sporoclarite'Duroclarite'
Vitrinertoliptite
B Fusito-clarite'Vitrinertite-1
o
LlMNIC
C Clarite-V'Vitrite'Cuticloclarite
Vitrinertite-V
D Clarodurite+Durite·Macroite·
Carbominerite
Fig.5.16. The microlithotype composition of the Liddell Seam (Sydney Basin, New South Wales).
(After Marchioni 1980)
Microlithotypes as Tools in Palaeo-Environmental Analysis
197
and a total of more than 50% of any or all of the above mentioned macerals of
the inertinite group. The same inertinite components dominate the trimaceral
microlithotype clarodurite which may be regarded as transitional to the wet forest
zone.
3. The C-components comprise vitrinite-rich clarite and cuticloclarite ( = cutinitebased clarite). They are considered to have accumulated in a continuously wet
forest swamp with consistently high groundwater table. Similarly to the A-components, a telmatic setting is assumed in the upper triangle, leading to more limnotelmatic conditions in the lower triangle. Like the terrestrial zone this facies is
characterised by arborescent vegetation with or without herbaceous undergrowth.
Wood tissues become more or less gelified leading to a concentration of vitrite in
which telovitrinite predominates. Where humification has been more intense and
occasional drying has occurred, inertinite fragments are incorporated in a collinitic
groundmass. The resulting bimaceral microlithotype could be called inertovitrite,
which is a reversal of the term vitrinertite and indicates that the vitrinite content
exceeds that of inertinite.
Because of the high arborescent proportion and particularly in coals formed
in temperate climates with deciduous vegetation, remnants of leaf cuticles (cutinite)
are characteristic of this facies in the form of cutite, vitrinerto-cutite which is a
cutinite-based vitrinertoliptite, cuticloclarite, and duro-cuticloclarite which are
cutinite-based clarites and duroclarites, respectively. Other facies indicators are
remnants of plant resins and waxes grouped under the maceral term resinite, as
well as cork tissues in the form of suberinite. Analogous to the preceding liptinite
macerals sporinite and cutinite, the respective microlithotypes can be extended
to include resite, resinoclarite, vitrinertoresite, and duro-resinoclarite for resinite
and suberite, and suberinoclarite, vitrinertosuberite and duro-suberinoclarite for
suberinite.
4. The D-components consists of clarodurite, durite, carbargilite and other dispersed
matter. This facies occurs beyond the fringe of rooted vegetation and consists of
lakes and ponds which collect detritus blown or washed in from all other parts
of the swamp. Indigenous vegetation is represented mainly by algae which lead
to the formation of such microlithotypes as algite, algoclarite, vitrinertoalgite and
duro-algoclarite. Among the allochthonous macerals inertodetrinite may be
common. It forms the bulk of the inertinite fraction of inertodetrite, vitrinertodetrite
(= inertodetrite with less than 50% vitrinite), and inertodetrinite-based clarodurite
and durite. Although the designated association of this facies with lacustrine
conditions refers to a topogenous environment, a concentration of inertodetrite
in a low ash coal could be derived from an ombrotelmite formed in a raised bog
under a seasonally dry climate. Conversely, under rheotrophic conditions, both
allogenic and authigenic minerals collect in mire lakes together with dispersed
organopetrographic constituents. Such combinations are covered by the
microlithotype groups carbominerite and mine rite which can be further subdivided
according to the associated mineral species, as listed in Table 4.2.
198
Coal Facies and Depositional Environment
The microlithotype-based facies analysis can be applied to full seam sections as
well as to subsections or, as illustrated in Fig. 5.16, to individual lithotypes among
which the status of dull coal is of particular interest. In Fig. 5.16 all dull coals plot in
the lower triangle, which according to the above discussion could indicate an
intermittently dry ombrogenous environment or a topogenous setting under wet
conditions if the mineral content is high. According to Marchioni (1980, Table VII)
the values for carbominerite range up to 18% (mean = 4.3) for banded dull coal and
11 % (mean = 5.9) for dull coal, which appears too high for dry durain (because of the
grey colour due to a high inertinite content they are also called "grey durains")
formed under either rheotrophic or ombrotrophic conditions. The dull coals in
Fig. 5.16 appear therefore to represent wet durains (also called "black durains",
because of the dark appearance of liptinite concentrations).
A detailed facies analysis of a coal seam is obtained by subdividing a bore core
or a piller sample into individual lithotypes and subjecting them to the coal facies
analysis described above. However, this is a time-consuming procedure and, more
frequently, a seam section is divided into plies or subsections containing groups of
similar lithotypes. The subsections are then crushed and representative samples are
obtained by coning and quartering or with the aid of mechanical sample splitters.
Grain-mounts or particulate blocks are then prepared and, after polishing, the
analyses are carried out by ordinary point count method whereby the use of an
additional counter unit may be necessary if a large number of microlithotypes is
distinguished. It is obvious that the contrast between subsections of a coal seam
diminishes with the number of different lithotypes incorporated in them, since
opposing trends will cancel each other. Nevertheless, even when whole seam sections
are analysed in this way it is usually possible to determine lateral and vertical facies
patterns between adjacent seam samples.
Within the above-mentioned limits, microlithotype analyses, particularly when
modified and extended in accordance with local requirements, have proved to be a
reasonably successful petrographic approach to palaeo-environmental reconstruction. Their usefulness is enhanced when supported by additional techniques such as
geochemistry or palynology. An example of the latter is Bartram's (1987) work on
the Lower Barnsley Seam in the U.K., in which the microlithotype analysis of a full
seam section was supplemented by a detailed investigation of megaspores. One of
the many interesting results of this study was the realisation that petrographic and
palynological boundaries do not necessarily coincide, a notion which, in relation to
the colour banding of brown coal, has also been voiced by Hagemann and
Hollerbach (1979), von der Brelie and Wolf (1981), Dehmer (1988, 1989a) and others,
although Blackburn (1981) and Kershaw and Sluiter (1982) found a good
relationship between colour-based lithotypes and their floristic content in the brown
coals of the Gippsland Basin in Victoria, Australia.
A disadvantage of the microlithotype-based facies analysis is that in spite of
being considerably more time consuming, its discriminative power is not better than
that offered by the maceral-based tissue preservation and gelification indices
discussed above.
Lithotypes as Palaeo-Environmental Indicators
199
5.3 Lithotypes as Palaeo-Environmental Indicators
The origin oflithotypes in both black and brown coals has been the subject of much
discussion in the international literature. Depending on the modern analogue
chosen for comparison and the technique employed in the investigation opposing
conclusions were reached about identical lithotypes. The densosporinite-bearing
dull coals discussed above. are a typical example.
Tasch (1960) used lithotype diagrams for both seam correlation and genetic
interpretation over relatively short distances within the Ruhr Basin, but this
procedure is valid only if applied to a limited area in which the composition of the
coal is well known. For example, Tasch's conclusion that in the region where he used
this technique, dull coal represented a subaqueous environment of deposition,
cannot be applied to other areas without verification by microscopic and possibly
other analyses.
Lithotypes are used here as defined in Chap. 4.3, i.e. they are regarded as
macroscopic units of coal, which are distinguished from each other by their physical
properties. Although the origin and mode of formation of the various lithotypes is
naturally of interest, the classification used is purely descriptive and has no genetic
base. As applied here, the lithotype concept differs from that of Ting and Spackman
(1975) and Ting (1989), who distinguish between five genetic lithotypes on the basis
of the following parameters:
1. Shape, size, condition of preservation and association of vitrinite;
2. cross-sectional shapes of miospores and their association with micrinite;
3. shape and form of inertinite, i.e., whether dispersed or clustered;
4. presence or absence of flow or other synsedimentary structures.
By applying the above criteria to both thin sections and polished blocks made from
piller or column samples (continuous lump samples from top to bottom of coal
seam) Ting (1989) obtains the following genetic lithotypes, which are identified as
such by the suffix-osite:
1. Allosite (from allochthonous) was originally called detrosite by Ting and
Spackman (1975) and is composed of reworked peat. This is shown by the high
proportion of well sorted, equiangular maceral fragments resulting from
extensive redistribution of organic matter under current and possibly wave
influence.
2. Kittosite (after the Lower Kittanning Seam) is characterised by the dominance of
densospores with the addition of clustered inertodetrinite and anastomosing
vitrinite. Also common is semifusinite with swollen and poorly defined cell walls
(degrado-semifusinite) referred to as quasigelifusinite-like after Timofeev et al.
(1962). An aquatic environment is envisaged, possibly with tidal influence.
3. Klastosite (from Greek for broken) consists of large fragments of vitrinite,
inertinite and megaspores together with alginite and detrital minerals. The
inferred environment is lacustrine, possibly with some wave action.
200
Coal Facies and Depositional Environment
4. Herbosite (from Latin for herb) was formerly called pophyrosporosite by Ting
and Spackman (1975) and consists of various miospores set in a vitrinitic
groundmass as in sporoclarite. With increasing admixture of dispersed detroinertinite it may grade into duroclarite. It appears to have been formed in a
limnotelmatic reed marsh.
5. Arborosite (from Greek for tree) was previously called lignosite by Ting and
Spackman (1975). It is dominated by well developed vitrinite bands (telocollinite)
and contains fewer spores than the other units. Also present are thin layers of
inertinite, mainly as fusinite and semifusinite as the result of wildfires. A freshwater forest swamp environment in inferred.
These genetic lithotypes occur in the previously discussed Lower Kittanning Seam
from top to bottom as listed above, but similar genetic sequences have been
recognised in many other coal seams. In their composition and genetic interpretation these units are more comparable (in reverse order) with Strehlau's (1988) facies
zones 1 to 4 discussed in Chap. 5.1.1.1, rather than with the more conventional
lithotypes discussed below.
5.3.1 Black Coal Lithotypes
In view of the foregoing discussion about the origin of the densosporinite facies and
the microlithotype composition of bituminous coal lithotypes, the question of a wet
or dry origin has involved mainly the dull lithotypes. Most coalfields contain
examples of both varieties and frequently they occur in the same coal seam. Based on
Australian examples, a relatively dry formation of dull coal (dry durain) can be
assumed in the following circumstances:
1. Coal seams containing dry durains are associated with coarse, often conglomeratic interseam sediments having high sand/shale ratios (e.g. Newcastle Coal
Measures: Warbrooke 1981).
2. Increases in dull coal content towards the basin margin or basement highs
suggest presence of autochthonous inertinite. Examples are the Bulli Seam in the
Southern Coalfield of the Sydney Basin (Cook and Wilson 1969), the Dudley
Seam in the Newcastle Coalfield (Warbrooke and Roach 1986) and the high
concentration of dull coals in the Maules Creek Formation on the flank of the
Boggabri Ridge (Diessel 1985a).
3. Sedimentary structures indicative of desiccation, such as mud cracks, have been
found to penetrate into the dull top section of the Bulli Seam in W ongawilli
Colliery near the margin of the Southern Coalfield of New South Wales (Diessel
et al. 1967). An illustration is given in Fig. 5.17.
4. The oxidative removal of biomass during humification results in a concentration
of inherent ash in the residual peat (Johnson and Cook 1973; Cook and Johnson
1975; Warbrooke and Roach 1986).
Lithotypes as Palaeo-Environmental Indicators
201
Fig. 5.17. Photograph of mud cracks in the immediate roof of the BulIi Seam, Wongawilli Colliery,
Southern Sydney Basin, New South Wales. During mining the mud cracks were seen to penetrate
between 5 and 10 cm into the coal. The base plate of the roof b61t in the centre is approximately
20 cm across. (Diessel et al. 1967)
5. High proportion of telo-inertinite (structured inertinite, such as fusinite, semifusinite and sclerotinite) in dull coal is indicative of autochthonous formation,
but, as has been discussed above, excessive oxidation may result in an autochthonous concentration of inertodetrinite.
6. High proportion of layered macrinite (lammacrinite) suggests lowering of
groundwater table under a regime of autochthonous peat formation.
7. Irregular vitrinite layers cross-cutting microstratification suggest origin from
roots (Littke and Ten Haven 1989) thus indicating (but not proving) the
possibility of autochthonous accumulation of inertinite.
Much of the evidence for wet (black) durain formation has been mentioned before.
Since the place of subaquatic deposition of inertinite is separate from the place of its
generation, substantial redistribution takes place leading to fragmentation and
concentration of inertodetrinite. The above-mentioned coexistence of the latter with
pyrite and alginite, the occurrence of graded bedding, imbrication and microcrosslamination in inertodetrinite and associated liptodetrinite concentrations all
support the notion of dull coal formation under limnotelmatic to limnic (lacustrine)
conditions. Similar subaquatic environments must be assumed for the combination
of detrital inertinite and liptinite Scheidt and Littke (1989) found in mudstones and
siltstones of the German Ruhr Basin.
In order to form inertinite, the accumulating detritus need not come from a
particularly dry portion of the mire since oxidation of more or less humified plant
202
Coal Facies and Depositional Environment
debris can also take place during transportation in aerated water (Beeston 1982).
While dull coals formed in bog lakes or in the distal portions of a flood basin might
be low in mineral impurities, high proportions of adventitious mineral detritus give
evidence of the dispersed nature of these occurrences. Marchioni (1980) describes a
characteristic association of clastic dirt bands in the Liddell Seam of the Hunter
Valley with over- and underlying dull lithotypes rich in durite and clarodurite due to
"the increased level and circulation of ground-water before and after a period of
inundation during which clastic material was deposited". It is quite possible that the
concentration of dull coal around the dirt bands was assisted by a rise in pH due to
the influx of sediment such that increased bacterial activity assisted in the
biodegradation of plant tissues.
In the discussion of the topogenous model in Fig. 5.3 (Model A) it was assumed
that the bright coals ofthe vitrinite-fusinite facies would change upward (i.e. distally)
into fusinite- and semifusinite-dominated autochthonous dull lithotypes before
grading into the increasingly allochthonous densosporinite-bearing dull coal near
the top of the seam. This was considered to be possible assuming a distal, meso- to
oligotrophic flood basin setting behind a broad and possible raised belt of
arborescent vegetation which confined rivers to their channels and effectively
filtered out any suspension load that might have been carried by high flood waters. It
is almost mandatory in this scenario that the change from low ash coal to epiclastic
roof sediments is quite abrupt following the rapid filling of the flood basin, once its
densely vegetated perimeter has been breached. Without keeping adventitious
minerals out of the distal flood basin and preserving oligotrophic conditions until
the end of seam formation any gradual rise in water level combined with influx of
even small quantities of suspension load would have increased eutrophy and caused
a reversal to arborescent vegetation and the formation of vi trite-dominated bright
lithotypes. Coal seams of this kind, which grade from bright coal to dull coal and are
capped by another bright coal section have been described from many parts of the
world, for example by Smyth (1957, 1961, 1964, 1968), Smyth (1967, 1968, 1970),
Smyth and Cook (1976), Strehlau (1988), Tadros (1988a) and others.
The dualistic nature of durain (wet/dry) emphasises the problem of using
lithotypes as palaeo-environmental indicators without any microscopic or chemical
backup. In order to highlight the situation further, the carbon and hydrogen
contents of two sets of Tertiary brown and almost isometamorphic Permian black
coal lithotypes have been plotted in Fig. 5.18, which is based on Callcott's (1986)
modified and extended version of Seyler's Chart. The black coal samples come from
five bituminous coal seams, two from the Bowen Basin in Queensland (one which
has been used in Tables 4.7 and 4.8) and three from the Gloucester Basin in New
South Wales. The latter have been combined into one set of mean values.
It is significant that banded dull coal (Db) and banded coal (BD) occur in two
positions with reference to the bright coal band. The banded coal (BD) from the
Gloucester Basin plots close to the upper boundary of the bright coal band, while the
respective banded dull coal (Db) is situated even within the perhydrous field. In
contrast, the same lithotypes from the Bowen Basin occupy positions close to and
within the subhydrous field. It seems that in the first case the comparatively high
hydrogen content is related to a high proportion ofliptinite in Db and BD, although
Lithotypes as Palaeo-Environmental Indicators
203
7r-----~-r--~----~----~----~------T
perhydrous
6.5
6
70
BO
90
100
Fig. 5.1S. Various lithotypes plotted in relation to Seyler's bright coal band, modified after
Callcott (1986). The designation of brown coal lithotypes from the Yallourn and Morwell Seams
in the Gippsland Basin, Victoria, Australia (recalculated from George 1982), follows Table 4.3,
while bituminous coal lithotypes from the Sydney and Bowen Basins in eastern Australia (based
on unpublished data by J.G. Bailey, T.G. Callcott and C.F.K. Diessel), are according to Table
4.6. Also included are Russell's (1984) analysis results for Recent wood and variously gelified
xylites from the Yallourn and Morwell Seams, respectively. The thick lines indicate biochemical
and the dashed lines physico-chemical coalification tracks, respectively. The dash/dot line marks the
charcoal generation track, while the two drawn-out and curved lines trace the boundaries of the
coalification band
maceral counts in white light yielded only 3.5% more liptinite in the Gloucester Db
samples than in the Bowen Basin samples. However, the aliphatic/aromatic ratios,
analysed by Fourier transform infra-red spectroscopy (FTIRS) in the Gloucester Db
and BD lithotypes are also high, which suggests that the coal probably contains
more finely dispersed liptinite (liptodetrinite) than is recognised by maceral analysis.
The aliphatic/aromatic ratio is an inverse measure of aromaticity, which up to the
rank of medium volatile bituminous coal is lower in liptinite macerals, due to a
relatively high proportion of aliphatic compounds, than in vitrinite and inertinite. A
comparison of the FTIRS results of the two sets of coals, illustrated in Fig. 5.19,
reveals therefore contrasting trends. While in the Gloucester Basin samples the
mean aliphatic/aromatic ratios peak in banded dull coal (Db), the most perhydrous
of the bituminous coal lithotypes, the Bowen Basin samples show a steady decline in
aliphatic/aromatic ratios from bright to dull coal, which is in keeping with their
lower liptinite and very high inertinite contents.
An interesting reversal (verified by repeat analysis) of the decline in
aliphatic/aromatic ratios with increasing inertinite content is displayed by fusain
(F), the most extreme case of inertinite concentration. This suggests that the
inertinite contained the dull coal (D) has evolved differently from the pyrofusinite
and pyro-semifusinite contained in fusain. As shown by thick lines in Fig. 5.18, the
Coal Facies and Depositional Environment
204
n8
u
:;:;
n8
4
to
E
0
3
'-
~
2
u
:;:;
to
.r:
0.
:;(
0
B
Bd
BD
Db
D
F
Fig. 5.19. Comparison of mean aliphatic/
aromatic ratios obtained from FTIRS
analyses of high volatile bituminous coal
lithotypes from the Gloucester (open circles,
na no analyses available) and Bowen (dots)
Basins in eastern Australia. (Based on
unpublished data by J.G. Bailey with kind
permission)
biochemical coalification (humification) tracks take the woody precursors of brown
coal xylite (gymnospermous) into the subhydrous field, where also the variously
gelified medium dark (M-d) and dark (Dk) lithotypes plot. Infra-red and solid-state
NMR spectroscopic analyses by Russell and Barron (1984) of the wood and xylite
samples plotted in Fig. 5.18 indicate that gelification is accompanied by the
progressive elimination of cellulose and a modification of lignin, including the
reduction in the proportion of methoxyl groups, an increase in the ratio of nonoxygenated to oxygenated aromatic carbon, and an increase in the relative
proportion of carbonyl groups. Studies carried out by Stout et al. (1989) and
Hatcher et al. (1989a, b) have produced similar results. Hatcher et al. (J989a) lists the
following steps in the biochemical coalification of gymnosperm wood:
1. Selective removel of cellulosic compounds and transformation of lignin into
macromolecular components;
2. demethylation of lignin to form catechol-like (phenol-related with excess OH-)
structures, accompanied by condensation reactions and cross-linking;
3. de hydroxylation during coalification from brown to subbituminous coal rank
resulting in a decrease of catechol-like structures and increase in phenol content
of xylem.
The biochemical coalification of angiosperms follows a similar pattern of destruction of biopolymers and their replacement by geopolymers, but because of the
greater variability in composition of angiosperm wood, a more complex humification path is followed.
The overall biochemical trends involved in the humification of ungelified to
gelified xylite are elimination of cellulose, minor oxidation oflignin and increase in
aromaticity from just under 0.2 in the two wood samples of Fig. 5.18 to around 0.4 in
ungelified and 0.6 in gelified xylite (Russell and Barron 1984). A similar biochemical
increase in aromaticity has been reported by Verhey en et al. (1981) for the following
brown coal lithotypes: Pale (Pa) to medium light (M-l) = 0.54 to 0.66 and medium
dark (M-d) to dark (Dk) = 0.58 to 0.65. The accompanying loss in hydrogen
manifests itself by demethanation, i.e. by the generation of marsh-gas or the "early
biogenic gas" of the petroleum geologists. In Fig. 5.18 the hydrogen loss is shown
Lithotypes as Palaeo-Environmental Indicators
205
by the downward displacement of the xylite plots, which was also recorded by
Allerdice et al. (1977), Given et al. (1984) and Chaffee et al. (1984). The gelification
tracks of the lithotype samples in Fig. 5.18 follow a similar trend starting with the
liptinite-rich pale (Pa) lithotype above the extension of the bright coal band and
finishing with the dark (Dk) lithotype below.
The degree of oxidation, which is invariably part of the humification process,
depends on several influencing factors, among them moisture content and oxygen
availability, temperature, redox potential (Eh), which is influenced by the hydrogen
ion concentration (pH), and the diffusion rate of oxygen to the organic molecules,
which is related to the degree of fragmentation of the biomass due to mechanical
breaking and organic interference (e.g. chewing and excretion). While the
biochemical gelification track leading to the dark lithotypes and the gelified xylite
below the bright coal band in Fig. 5.18 was affected by the mild oxidation, an
increase in its severity would have no fundamentally different result. The
combination of microbial attack and increased oxidation would still lead to the
stripping of aliphatic chains from the cell walls, but cross-linking of the aromatic
residue and the occupation of its remaining reactives sites by quinone, carboxyl
and hydroxyl groups (Saxby and Shibaoka 1986) might be more complete than
in the above case of milder oxidation. It has been mentioned before that at this
low rank the physical differences between the products of mild and more severe
oxidation are not obvious, but they develop with increasing coalification by raising
the reflectance of the more oxidised ("pre-semi-") inertinite above the level of
vitrinite.
When coal rank begins to increase, the ungelified and partly gelified xylites
are likely to follow a straight coalification track along the centre of the coalification
band in Fig. 5.18, as had been envisaged by van Krevelen (1952). Its result is the
formation of telovitrinite, mainly in the form of telinite. The fate of the gelified
xylite (previously called doppleritic xylite by Jacob, 1958a), together with the more
or less gelified humic degradation products contained in the dark and medium
dark lithotypes depends on the amount of desiccation and oxidation suffered
during humification. If it was small, they will follow a coalification pathway,
characterised by the loss of oxygen-bearing compounds [mainly OH-, according
to (3) above, plus CO, CO 2 and COOH -], that takes them into the bright coal
band, where the gelified xylite will appear as telocollinite and the other humic
degradation products as detrovitrinite. This trend is accompanied by an increase
in HIC and the decrease in OIC ratios, as recorded by Hatcher et al. (1982) and
also observed by Black (1989) in a comparative study oflow and high rank lignites
from Eastern Southland, New Zealand. Conversely, ifmore severe oxidation during
humification led to substantial cross-linking, condensation and a preferential
replacement of aromatic hydrogen by oxygen (quinone formation) rather than
hydroxyl groups (catechol and phenol formation), the subsequent coalification
track will remain in the subhydrous field below the bright coal band. The resulting
bituminous coal macerals include (degrado-) semifusinite and various transitions to
macrinite.
The above-mentioned trend leading to the formation oflow reflecting inertinite
by humification is quite different from the formation of the telo-inertinite that is
Coal Facies and Depositional Environment
206
1000
M-]
100
Lt
Pa
10
/0
M-d
+/+-+,
I
+ Dk
+
•
B
Bd ' /0 B
BD e-e /
/ ° Bd
Db. IBD
D
JI
/~.F
D 0 __________
.1
°F
a
.01 +-~~~~~~~-~~~~-~
"01
10
.1
TPI
50
40
NymPh.e. pe.!
30
eo
Fi brous cool
70
(Fus.'n)
Fig. 5.20. Diagram showing the gelification
and tissue preservation indices of black and
brown coal lithotypes. Dots Example of a
predominantly bright coal, calculated from
Marchioni's (1980) analyses; open circles
example of the predominantly dull Bowen
Basin coal, calculated from Table 4.7; crosses
brown coal, calculated from George's (1982)
analyses in Table 4.4. Lithotype identifications are as in Tables 4.3 and 4.6
60
20
Or-~-U~~
____
60
so
__
40
Ccrex peat
30
~
30
10
o~---~~~----~
10
30
O~-~~---~~-
40
30
10
30
20
20
Panicum peot
SO
10
10
O ~~~~~~~~U
30
30
30
20
20
20
10
10
o~~~~-W~~~~
Woodwerd l O peat
40
30
O~~~~---~~
o~~~~-U~~~~
40
30
)0
20
20
10
O~~~~~~~~~~
10
60
so
50
40
40
40
)0
)0
30
20
20
20
10
SO
Cyr1J1~ put
10
o~~~~~~~~~.
10
o~~~~---~L-
70
Bright co.1
60
(Vllr .. n)
68~~~~~~~~~·
eright co. I
70
50
50
60
40
30
40
30
50
20
20
20
10
10
10
o~~~~----~L-
O~~~~--~~~~-
2 2 2 2 2 2 "iQ'"
L"c "c "c "c "c
:;:;
2 2 2 2 2 2 '"
"c "c "c "c "c :£ "iQL:;:;
E:;: :5
:r t ::::;~ f"
", ", ,
L-
0
":;: "
c
c c" "
0
I-
0'
q;
Q;
+
0
L-
-.;
0
0
Q;
I-
0
b
"
0
0
Q;
l!l
CI.
C
(ClorodurOln)
10
40
10
20
B.nded dull co.'
40
40
or-~~~~--------
Oul1 co. 1
(Ou'OIn)
40
20
60
(Fus.'n)
20
20
40
F"lbrous coel
40
SO
10
o
BO
so
(Vltroln)
)0
t a. "C
"'0> "'0> "
c c" "
c ::::; f
:5 :5 t
L-
Q;
0
I-
Q;
0'
+
0
L-
-.;
0
", , ",
0
Q;
I-
b
"
0
0
Q;
l!l
O~~~~----~~-
2 2 2 2 2 2
"c c § "c "c :5
:;:;
"iQ
"'0> "'0>
f
:5 :5
Q;
I-
Q;
0'
+
0
L-
-.;
0
'"
.,
L-
L-
L-
c
c
t a. c
::::;
0
0
0
"," ," .."
q; b
I-
c
Q;
'" l!l
0
Lithotypes as Palaeo-Environmental Indicators
207
concentrated in the fusain sample of Fig. 5.18. Its largest proportion consists of
pyro-semifusinite followed by pyrofusinite similarly to the gradation between the
two macerals illustrated in the charred wood in Fig. 3.19. Fusain has been derived
by incomplete combustion mainly directly from wood along the charcoal
generation track shown by the dash/dot line. Although charred peat might have
contributed to the formation of fusain, its microscopic appearance reveals little
evidence of biochemical gelification, which may account for its slightly elevated
aliphatic/aromatic ratio. It suggests that the large semifusinite fraction contained
in fusain was able to retain some aliphatics, because it missed out on the biochemical
increase in aromaticity Russell and Barron (1984) found to be a part of gelification.
Figure 5.20 displays a comparison of GI/TPI plots for two high volatile
bituminous coals, one consisting of predominantly bright lithotypes,and the other
being mainly dull and representative of one of the Bowen Basin coals listed in
Tables 4.7 and 4.8, and included in Fig. 5.18. Although their respective coal facies
indices differ in actual values, they follow the same trend in both examples with
bright coal (B) and fusain (F) displaying the highest tissue preservation, followed
by banded bright coal (Bd). High TPI and GI values have been interpreted as the
result of a high input by wood tissues and optimum biomass retention under
predominantly wet conditions. The position of the xylite (brown coal wood) samples
at the beginning of the bright coal band in Fig. 5.18 supports this notion, as well
as the comparison of the composition of Cohen's (1973) peat types, listed in Table
5.1, with the black coal lithotypes illustrated in Fig. 5.21. There is a striking similarity
between the Taxodium and Cyrilla peats and the vitrinite-rich lithotypes banded
bright and, to a lesser extent, banded coal. Bright coal (vitrain) could be included
too, although its progenitors would be pieces of wood or bark rather than a mixed
peat. Nymphaea and Woodwardia are the other peat types with some similarity
with coal lithotypes, in this case, banded dull coal. The Okefenokee peats, as well
as the bituminous coal samples, come from topogenous mires, which may account
for some of their similarities.
5.3.2 Brown Coal Lithotypes
As has been discussed in Chap. 4.3, brown coal lithotypes can be classified on the
basis of several physical properties, including texture and colour. See also
Hagemann (1978), who distinguishes between different lithotypes on the basis of
their wood and mineral content, matrix/tissue ratio, stratification, colour,
gelification, accessory components, and fragmentation pattern. In spite of some
~---------------------------------------------------------------
Fig. 5.21. Comparison of the components of different peat types from the Okefenokee Swamp
(left column, after Cohen 1973) with the macerals composition of lithotypes from a predominantly
bright HV bituminous coal from the Sydney Basin, NSW (centre, after Marchioni 1980) and the
predominantly dull HV bituminous coal from the Bowen Basin used in Tables 4.7 and 4.8, and
Fig. 5.18 (right column). All components have been expressed in equivalent maceral terms
208
Coal Facies and Depositional Environment
reservations about its petrographic significance (von der Brelie and Wolf 1981a)
the colour coding of lithotypes is quite common. Australian examples are listed
in Table 4.3, together with the respective textural features also used to distinguish
between different brown coal lithotypes. Beside the visual determination of
lithotype colour, there are optical methods available, by which a colour index or
the brightness of the coal can be determined. An example is the Hunterlab
Colorimeter used in Australia (Hunter 1979), which, as mentioned in Chap. 4.3.1,
gives a numerical value for the degree of "brownness" of ground and air-dried
26
66
28
68
30
70
32
72
34
74
36
76
38
78
40
80
42
82
44
84
46
86
46
88
50
90
92
52
54
94
56
96
58
98
60
100
62
102
64
104
Depthin
Metres
66
60
80 100 120 140 160 180
Colour Index
60
80
100 120 140 160 180
Colour Index
Fig. 5.22. Comparison of a lithotype and colour index log of an Australian brown coal seam
from the Gippsland Basin, Victoria. (After Mackay et al. 1985)
Lithotypes as Palaeo-Environmental Indicators
209
coal (Higgins et al. 1980; Attwood et al. 1984; Mackay et al. 1985). The colorimetric
determination is expressed by the colour index consisting of three parameters:
L = opacity, ranging from (black) to 100 (white); a = green/red ratio; b = blue/
yellow ratio (Russell 1984). The colour index correlates not only reasonably well
with the visual identification of lithotype colour, but, as shown in Fig. 5.22,
differentiates it even further. An alternative quantitative assessment of lithotype
appearance is by remission measurements (diffuse reflectance), which are carried
out under the microscope on freeze-dried coal samples. An intensity of 8% for the
remitted light determined at a wavelength of 660 nm in reference to a barium
sulphate standard is taken as the boundary between light and dark lithotypes
(Jacob 1958b, 1967; Wolff-Fischer 1989).
There has been an ongoing discussion in the literature about the nature and
origin of the colour banding in brown coals. Jurasky (1928), Teichmiiller (1950,
1958, 1989), Pflug (1952), Teichmiiller and Thomson (1958), Thomson (1950,
1951, 1954, 1956), Blackburn (1981) and Kershaw and Sluiter (1982) found good
correlation between colour-based lithotypes and their floristic content in Tertiary
brown coals, but Heinhold (1909), Gothan (1924), Pietzsch (1925), Walk (1935),
Jacob (1955), Hiltmann (1976), Hagemann and Hollerbach (1979), von der Brelie
and Wolf (1981a), Dehmer (1989b) and others dispute such a relationship for the
Rhenish brown coals and regard the colour banding as the result of different
biochemical conditions of peat formation from essentially similar phytogenic
precursors. The light brown and pale coals have been variously interpreted as:
°
1. rheotrophic, limnotelmatic "reed moors" with anemophilous pollen and spores
from extrapaludal sour.ces (Jurasky 1928; Teichmiiller 1950, 1958, 1989; Pflug
1952; Teichmiiller and Thomson 1958; Thomson 1950, 1951, 1954, 1956);
2. largely unvegetated, open water, lacustrine environments with mainly
allochthonous accumulation of reworked peat (Jacob 1968; Hiltmann 1976) and
plant detritus (Luly et al. 1980; Kershaw et al. 1982; Klein-Reesink et al. 1982;
Minnigerode and Riegel 1983; Gloe 1984);
3. the residues of aerobic decomposition of (ombrotrophic) peats consisting of
relatively stable components, such as liptinite (Heinhold 1909; von der Brelie
and Wolf 1981a; Hagemann and Wolf 1987);
4. terrestrial, i.e. relatively dry, wooded mire environment (Hiltmann 1976);
5. the dry end-members of repeated Gycles of groundwater oscillations between
high and low positions (Mackay et al. 1985).
The Australian brown coal lithotypes included in Fig. 5.23 bear little resemblance
in composition to either the bituminous coal lithotypes or to the Okefenokee peat
types of Fig. 5.21, both of which represent topogenous (rheotrophic) environments.
This assumption is based on the present setting of the Okefenokee Swamp and, in
the bituminous coals, on the high ash content, the occurence of several epiclastic
stone bands in the seams and on the frequency of seam splitting. In contrast, the
brown and subbituminous coals illustrated in Fig. 5.23 contain little ash and hardly
any stone bands, which would suggest an ombrotrophic origin. The gelification
indices of their lithotypes are very high (see Fig. 5.20), which is not a function of their
210
Coal Facies and Depositional Environment
70
60
SO
40
30
20
10
0
80
70
60
SO
40
30
20
10
0
70
60
SO
40
30
20
10
0
70
60
SO
40
30
20
10
0
60
SO
40
30
20
10
0
i
1Hf.
6713A
1111
1~~~
TV
D+GV
:i:
'iO.
!I!I::
In
Lipt Min
I
TV D+GV
Dk
In
Lipt Min
TV
D+GV
In
Lipt
Fig. 5.23. The maceral composition of the lithotypes of two brown coals and one subbituminous
coal (all Tertiary). Left column. Hauptfloz, Niederrheinische Bucht, Germany (after von der Brelie
and Wolf 1981b); central column Yallourn Seam, onshore Gippsland Basin, Victoria, Australia,
as in Table 4.4 (after George 1981); right column. Seam A and B in Fortescue A-3 Bore (after
Palmer 1986). TV telovitrinite; D + GV detro- + gelovitrinite; In inertinite; Lip liptinite; Min
minerals
rank, since Palmer (1986) reported similar GI values from stratigraphically
equivalent subbituminous coals at 3.3 km depth in the offshore Gippsland Basin.
The high GI readings correspond to generally low TPI values, which is due to
the predominance of humodetrinite (detrovitrinite), as shown in Table 4.4 and Fig.
5.23. The varying admixtures of some humotelinite (telovitrinite) result in limited
gradation in tissue preservation from dark to pale lithotypes. According to Table
4.4 the proportion of humotelinite (telovitrinite) decreases from 32% in the dark
Lithotypes as Palaeo-Environmental Indicators
211
to 7.3% in the pale lithotypes. A similar decline is shown by humocollinite
(gelovitrinite) and by densinite, the more gelified member of the humodetrinite
(detrovitrinite) subgroup. These trends are balanced by an increase of less or
non-gelified macerals within the light-coloured lithotypes, such as attrinite, plus
liptinite, in particular liptodetrinite and sporinite. The latter survives well in
subaqueous, reducing conditions, as is evidenced by its widespread occurrence in
oil shales and other sediments but it has also been found. to possess a good
preservation potential in oxidising environments (von der Brelie and Wolf 1981a),
provided they are not too alkaline (Pfaffenberg 1953/54; Taylor and Warne 1960;
Dehmer 1988).
While the presumably ombrotrophic Victorian brown coal shows little
resemblance to the rheotrophic examples in Fig. 5.21, its lithotype composition,
particularly the mostly high proportions of detro- and gelovitrinite, as well as the
compositional trends between different lithotypes, correspond well with German
Tertiary brown coals from Helmstedt and the Niederrheinische Bucht (Lower
Rhine Embayment), as reported by Winkler (1986) and von der Brelie and Wolf
(1981b), respectively. The composition of the latter is illustrated in the left column
of Fig. 5.23. The lithotypes have not been identified by colour, as in the Victorian
brown coal illustrated in the central column, but by their pollen content. According
to von der Brelie and Wolf (1981 b), an upward change from wet to moist conditions
is indicated by the combination of decreasing tissue preservation and palynological
variations. Samples 67/3A to 60/6B contain relatively high proportions of
telovitrinite (humotelinite), which is associated with a pollen assemblage consisting
of Sequoiapollenites polyformosus, Disaccites, Sciadopityspollenites serratus and
Cyrillaceaepollenites megaexactus in varying proportions. Because of the high
proportion of the latter, von der Brelie and Wolf (1981b) refer to this assemblage
as the wet "megaexactus forest", in which wood tissues were preserved because of
a high groundwater table and relatively rapid subsidence. The upward deterioration
in tissue preservation, as indicated by the shift from telo- to detrovitrinite +
gelovitrinite in samples 67/5 and 58/2, is accompanied by a change in the pollen
assemblage: Disaccites, Sciadopityspollenites serratus and Cyrillaceaepollenites
megaexactus decline in proportion and are replaced by Quercoidites microhenrici.
Apart from Sample 67/3A in Fig. 5.23, which with its high degree of tissue
preservation shows more resemblance to some of the rheotrophic peats or the
banded bright and banded bituminous coal lithotypes of Fig. 5.21, there is
considerable similarity between the maceral content of the wet "megaexactus forest"
and the dark and medium dark lithotypes of the Victorian brown coal illustrated
in the central column of Fig. 5.23. Also, the drier "microhenrici forest", represented
by Samples 67/5 and 58/2, shows considerable resemblance to the pale and light
lithotypes. As indicated above, the offshore portion ofthe Gippsland Basin contains
at depth sub-bituminous coals in stratigraphically similar position to the brown
coals in the onshore portion ofthe Latrobe Valley. Palmer's (1986) maceral analyses
of some of the subsections of two such seams are illustrated in the right column
of Fig. 5.23. As expected, they can be readily correlated with George's (1982)
colour-coded lithotypes in the central column. Corresponding to their rank close
to the boundary between biochemical and physico-chemical coalification, the
212
Coal Facies and Depositional Environment
sub-bituminous coals have lost their brown coal habit but have not yet developed
full contrast between bright and dull bands. However, Samples A/5 to A/3, which
according to their relatively high detrovitrinite content correspond to the medium
dark (M-d) and medium light (M-1) brown coal lithotypes, are more distinctly
laminated into banded bright (Bb) and banded coal (BD) than Samples B/4 and
B/3, which appear more massive and matt. These observations support the genetic
relationship indicated in Fig. 5.18 by the coalification track leading from the two
perhydrous (because of high liptinite and lack of biochemical gelification) pale
brown coal lithotypes (Pa) to the likewise perhydrous banded dull coal (Db) and
the hydrogen-rich banded coal (BD). It should be noted that the inertinite content,
which hardly varies in the Victorian brown coal lithotypes, is higher in the slightly
matt sub bituminous lithotypes (Samples B/3 and B/4 in Fig. 5.23). It is assumed
that with increasing coalification more of the attritus, at this low rank still included
in detrovitrinite, might develop into low reflecting inertinite.
Because of the low soft brown coal rank (60 to 65% bed moisture) of the
Australian samples, the relatively high concentration of gelovitrinite (humocollinite) in the dark lithotypes cannot be related to epigenetic gelification but is due
to advanced syngenetic humification with high bacterial activity. The latter is also
suggested by Dehmer (1988) for the dark lithotypes in the brown coal from the
Niederrheinische Bucht in Germany. In a comparative'study of the origin of colour
banding in peat and brown coal, she has found light and dark peat types in both
ombrotrophic and rheotrophic mires, the latter including limnotelmatic settings.
For example, the Mariscus peat from the Everglades in Florida is dark but the
likewise limnotelmatic Nymphaea peat from the Okefenokee Swamp is lightcoloured. Similar differences occur in ombrotelmites, such as a dark sedge peat
from Soos, CSR and a light Combretocarpus peat from Palangkaraya in Kalimantan.
Also when occuring in a raised bog setting, Cohen et al. (1989) find Nymphaeadominated peat to be least decomposed ( = fibric) and lightest in colour compared
with the other peat types they identified in the Panamanian Changuinola deposit.
The opposite is shown by the swamp forest peat, which is dark and rather
decomposed (= sapric).
While Cohen et al. (1989) regard persistence of wetness and accessibility of
oxygen as a controlling factor in peat type, Dehmer's (1988, 1989) work points
towards the hydrogen ion concentration in the peat-forming environment and the
pH-related microbial activity as being of paramount importance. Evidence for this
is seen in the high proportion of bacteria generated biomarkers, such as
homohopanes and picenes (see Chap. 5.5.2) found in dark peat types in contrast
to their paucity in light-coloured peats. In reference to the recent ombrotelmite
from Kalimantan, Dehmer (1988) concludes that the dark lithotypes of the brown
coal from the Lower Rhine area indicate weakly acid to neutral pH and a relatively
good supply of nutrients (eutrophy), whereas the pale lithotypes were formed under
oligotrophic and rather acid conditions with little input by bacteria. The result is
humification under more sterile conditions, which might account for the predominance of ungelified attrital tissue fragments in the pale lithotypes. The likewise
relatively ungelified pale lithotypes in the Tertiary brown coals of the Gippsland
Basin, Australia, support this interpretation, as well as the above-mentioned
Lithotypes as Palaeo-Environmental Indicators
213
observation that low Eh and pH conditions favour the preservation of cellulose
and unmodified lignin.
As has been discussed above, biochemical gelification raises the aromaticity
ofthe peat. It is therefore not surprising that the largely ungelified and liptinite-rich
(albeit much of it in very fine dispersion resulting in high fluorescence) pale
lithotypes contain a high alipathic/aromatic ratio (Johns et al. 1981) and plot well
in the perhydrous field of Fig. 5.18. According to Klein-Reesink et al. (1982) the
likewise perhydrous nature· of the pale lithotypes in the Eocene brown coals of
the Borken deposit near Kassel in Germany is due to the high proportion of
bituminite in the strongly fluorescing groundmass of the lithotype. This bituminite
is thought to be of algal origin and has been inherited from the lacustrine
environment in which the pale lithotypes accumulated. It is interesting to contrast
this conclusion with Winkler's (1986) observation that the brown coals from
Helmstedt in Germany contain alginite only in the dark lithotypes, whereas fungal
remains are more frequent in the light lithotypes.
Markov chain analyses by Mackay et al. (1985) have led to the recognition
of many depositional cycles in the Morwell Seam, Gippsland Basin, Australia, an
example of which is illustrated in Fig. 5.25. The cycles commonly begin over a
sharp basal contact with dark lithotypes, which may represent a rheotrophic phase
with influx of nutrients and low acidity, followed by more restricted oligotrophic
and possibly ombrotrophic conditions as the lithotype colour lightens upward.
Also Schneider (1986), based on palaeo botanic research, recognises a systematic
vertical change in seam development from eutrophic to oligotrophic conditions
in the Miocene brown coals of the Oberlausitz (Lusatia) in Saxony. A similar
genetic development was recognised by Dehmer (1989a) in Miocene brown coal
from the Oberpfalz in Bavaria, where the lowest ash contents of 4.3% (ad) are found in
the un banded pale lithotypes, which are interpreted as indicative of drier conditions.
Even stronger evidence for the capacity of ombrotrophic settings to produce light
coloured lithotypes has been reported from China by Lu and Zhang (1986) and
Jin and Quin (1989). The Pliocene brown coals of the Jinsuo Basin in Yunnan
Province contain a number of "yellowish brown" bands, (greyish white when
weathered), which consist of almost pure Sphagnum coal. The chemical composition
of these bands is not unlike that described by Johns et al. (1981) from the pale
bands of the Australian Gippsland Basin coal, i.e. they are characterised by high
extract bitumen, high H/C and aliphatic/aromatic ratios and low ash basicity.
Conversely, Dehmer (1988) finds higher proportions of alkanes, aromatics and
heterocompounds in the dark lithotypes of the Lower Rhine brown coals, compared
with the extract composition of their light-coloured bands.
As mentioned above, the ombrotrophic interpretation of the pale and
light-coloured bands has not found general acceptance. Blackburn (1981),
investigating megafossil/lithotype correlation in the Yallourn Seam of the
Gippsland Basin in Victoria, presents an almost reverse interpretation to that of
Mackay et al. (1985) by regarding the dark lithotypes as the "dry" termination
rather than the "wet" beginning of each depositional cycle, and Teichmiiller (1989)
has only recently reiterated her interpretation of the pale bands as being the
products of limnotelmatic accumulation. In the light of this controversy Dehmer's
214
Coal Facies and Depositional Environment
(1988) petrographic and geochemical results from the Lower Rhine brown coal
and both ombrotrophic and rheotrophic peats from various parts of the world
are particularly interesting, because they suggest that the colour banding is not
primarily a matter of wet or dry formation but one of acidity, bacterial activity
and oxygen supply, which can be almost as effective in aerated water (Jacob
1968) as in air. This notion is supported by Jacob's (1952a) earlier work on the
hydrogen ion concentration in brown coal lithotypes, which showed that lightcoloured humus with a high residual tissue content shows a lower pH than dark
humus with low tissue preservation. The dark colour of the latter is related to its
higher proportion of bacteria generated colloidal humic substances. In the light
lithotypes the at least partial presence of oxygen and high acidity not only suppress
anaerobic bacterial activity and thus the formation of humic colloid, but the latter
are also actively destroyed by fungi. Most ofthese tolerate low nitrogen availability
and acidities ranging in pH between 3.5 and 5.5, whereas the majority of anaerobic
bacteria prefers a pH between 5.5 and 7.5 (Mohr and van Baren 1959; Zeichmann
1980) and a higher nutrient supply, including nitrogen (Flaig et al. 1975).
On the available evidence it can be concluded that pale bands are the product
of partial oxygenation, oligotrophy and its concomitant high acidity, which can
be achieved in raised bogs as much as in topogenous settings with limited nutrient
supply (low rheotrophy or minerotrophy).Since the question of pH has not been
addressed in the above mentioned rather contrasting interpretations (Items 1 to 5)
of the origin of light and pale lithotypes they may not be as exclusive of each
other as it seems at first sight. This conclusion encompasses the likewise controversial question of phytogenic input. The various states of preservation of the
xylite plotted in Fig. 5.18 demonstrate that identical source materials can yield
different products, but the ecologic specificity of many plants and their sensitivity
to environmental changes (Connell and Slatyer 1977) assures that differences in
the position of the groundwater table, nutrient availability and pH will also affect
floral distribution in the mire. Different plant communities differ in their composition, such as their cellulosejlignin ratios. Given that under most peat-forming
conditions lignin is more likely to survive longer than cellulose (Hatcher et al. 1989a),
some influence of vegetal type on coal type cannot be denied.
5.4 Optical Properties as Palaeo-Environmental Indicators
As discussed in Chap. 3.2 and illustrated in Fig. 3.28 to 3.30, the fundamental
differences in physical and chemical properties of coal macerals become increasingly
obliterated during the physico-chemical stage of coalification. Few attributes of
coal demonstrate this convergence more clearly than the optical properties of
reflectance and fluorescence, which is the reason for their capability of being used
as rank indicators. However, even within such a relatively homogeneous maceral
as telocollinite, small but measurable differences in reflectance and fluorescence
intensities persist into advanced coalification. While some of these residual
Optical Properties as Palaeo-Environmental Indicators
215
variations may have been inherited from different vegetal sources, the discussion
below will show that others can be traced back to differences in the depositional
environment.
5.4.1 Vitrinite Fluorescence
Carbon atoms are joined with each other and with atoms from other elements by
covalent bonds, i.e. by pairs of electrons, in which one electron of each pair has
been donated by each of two adjacent atoms. Outer shell electrons are also shared
by adjacent atoms, but varying degrees of mobility are retained in conjugate double
bonds (Bertrand et al. 1986; Lin and Davis 1988a, b), as in the case in unsaturated
hydrocarbons, such as aromatics (e.g. in lignin), substituted aromatics (in various
plants), isoprenoids (e.g. in bacterial lipids) and carotenoids (high in algae).
According to orbital theory, the cause of the fluorescence is the recovery of
part of the irradiation energy as the excited electrons of an atom or molecule, which
had been elevated to higher energy orbitals by absorption of energy, return to the
ground state (Lin and Davis 1988a, b). Because a portion of the excitation energy
had been dissipated, the energy recovered by the return of the promoted electrons
to their original positions is of a lower level, i.e. of longer wavelength. As illustrated
in Fig. 5.24, when a substance that is capable of fluorescing, is irradiated with
ultra-violet light, the result will be fluorescence in the green band of the wave
spectrum. Alternatively, irradiation with blue light, which has been used in the
analyses discussed below, will result in likewise longer wavelength fluorescence. The
relatively mobile electrons that can be promoted to higher energy orbitals, are
referred to as n-electrons, and the chemical groups, molecules or their nuclei that
contain excitable n-electrons and therefore have fluorescent properties, have been
called fluorophores by Lin and Davis (1988a, b).
UV-radiation
365-f--=-=-=-=-=O_~
400
o
substance capable
of fluorescing
0::0::
°0°000:.0
"energy loss·
500
555
600
Fig. 5.24. Schematic diagram illustrating
the energy loss between high energy
excitation and longer wavelength fluorescence. (After Holz 1975)
700
- - - - - - - .....- - -. . .
(green = visible)
fluorescence
emission
Coal Facies and Depositional Environment
216
Although fluorescence properties of different spectral intensity are displayed
by most macerals (Fig. 5.25), the sensitivity and varied responses of cell tissue to
the conditions to biochemical coalification make vitrinite a particularly suitable
object of research in this field. As shown schematically in Fig. 5.26, in wood, peat
and brown coal the original plant tissue displays a strong primary fluorescence,
which is largely based on the biopolymers cellulose, lignin and various lipids (van
Gijzel 1975; Teichmiiller and Wolf 1977; Teichmiiller 1982; Teichmiiller and
Durand 1983; Russell 1984; Stout and Bensley 1987). With increasing biodegradation during humification, fluorescence intensities decrease, until they reach a
minimum before the onset of large scale repolymerisation at the beginning of the
physico-chemical stage of coalification (Teichmiiller 1982; Ottenjann et al. 1982;
Wolfet al. 1983b; Wolf and Wolff-Fischer 1984; Stout et al. 1989; Black 1989); Jin
17
16
15
14
13
12
J
~
ill
:f
.E
7
~
J~
xo
6
5
4
o
:g
~
l
't.
h6
0.6
0.2
•
+ +.r
-0.6
:~
Iv
.~6~~
-t\,
\..
• •
+.r
.:.
2
•
•
••
.",
....
A.·•
•••
•
y"-2.62x+l.41
~::: :"~::!ocollinit
•
\.+~
.~. +
••
3
B
-0.2
\
6~ •
• Semlfus.+Fus.
1.0
~\;
0\0
'V
+ Makrlnlt
1.4
~~
6
6
x Desmocollinlt
o Telocolllnit
1.8
:\
8
r"-0.917
2.2
11
9
y"-3.87 x+3.19
3.0
2.6
'V
10
Ro max Telocollinit-l.28"
3.4
;...
•
6
Telocollinit
- 1 . 8 . Makrinit
Semifus.+Fusinit
-2.2
Ro max Telocollinit 0.80"
.
+-r---r-,.------,r---.-..---,--.""--,--..."L
+
~
-0.6-0.4-0.2
••
~+~
•
•
+ •• ~.
••
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4
In Ro max
..
y-~~
~ o~'-"-'-r'-"-"-rl"--r'-"',-r,~,~.~,--',~~~,~,~':;'~'~'~+=;'~~~I·~,=;~~,~~~~,=T,=?i·~,-r"
0.5
1.0
" Ro max
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Fig. 5.25 A, B. Correlation of fluorescence and reflectance intensities measured in various
macerals of two coals from the Sydney Basin, NSW, Australia. The left curve refers to the Great
Northern Seam, a high volatile bituminous coal of 0.80% mean Romax for telovitrinite; the right
curve represents the Bulli Seam, a medium volatile bituminous coal of 1.28% mean Romax for
telovitrinite. Real numbers have been plotted in A, whereas in B natural logarithms have been
plotted for the purpose of linear regression. (After Diessel and McHugh 1986)
217
Optical Properties as Palaeo-Environmental Indicators
\
....
>.
en
c:
....c:
(I)
(I)
0
c:
\
\
\
\
\
\
\
\
\
.(1)
0
\
\
en
\
(I)
....
0
*
:.----------
\
:>
IJ..
\
Primary
0
% Rort
\
"1
..............
--
"-
Secondary Fluorescence
0.5
1.0
"-
1.5
Fig. 5.26. Schematic representation of the correlation of micro-fluorescence intensities of
vitrinite/huminite with coal rank expressed as random telovitrinite reflectance. The dashed line
applies to a measuring wavelength of 546 nm, the solid line refers to 650 nm. (After Ottenjann
et al. 1982; Wolfet al. 1983b; Wolf and Wolff-Fischer 1984; Diessel1985a; Hagemann et al. 1989)
and Qin 1989). This decline in primary fluorescence properties as an expression
of the degradation of the biopolymers offers a possibility to quantify photometrically degrees of humification (Jacob 1973). Its highest degree is reached
when depolymerisation is largely complete and fluorescence has almost ceased at
a vitrinite/huminite reflectance of approximately 0.5%. With increasing coalification
the depolymerised nuclei begin to combine again to form organic geopolymers,
in the course of which a condensed aromatic network and a mobile phase are
generated. By the time the rank of high volatile bituminous coal has been reached,
a secondary fluorescence has developed (Wolf et al. 1983b; Lin et al. 1986; Quick
et al. 1988), which, according to Lin and Davis (1988b), is carried mainly by the
fluorophores contained in the intervening mobile phase. Beyond the rank of high
volatile bituminous coal the mobile phase is increasingly degraded by thermal C-C
bond cracking and is partly released from the coal as fluids and gas, while the
cross-linking and condensation ofthe remainder results in the decline and eventual
disappearance of the fluorescence properties in semi-anthracite.
Marine influence on peat formation raises the pH, thus increasing bacterial
activity which, on the one hand, results in biodegradation and loss of biomass,
while on the other hand, bacteria-derived lipids are added to the humic degradation
products. Particularly active in the formation of some lipids are species of the
anaerobic bacterium genus Clostridium, which, according to Belyaev (1981), converts
cellulose into fatty acids. Because these bacteria share the same environment and
are often associated with the sulphate-reducing bacteria Desulphovibrio desulfuricans and Clostridium desulfuricans (Degens 1965), syngenetic pyrite is often
found in coalified cell tissue (Given and Miller 1985). The resulting coal shows
low tissue preservation and a high proportion of detrovitrinite commonly with
some relative enrichment of the more hardy components, such as detrital inertinite
218
Coal Facies and Depositional Environment
fragments, and/or liptinite. The latter is quite resistant in acid mires, but will
decompose under neutral to slightly alkaline conditions. This leads to the formation
of dispersed liptodetrinite, which occurs as submicroscopic impregnations in humic
compounds, often too fine to be resolved by the optical microscope (see reference
to the aliphatic/aromatic ratio above), although they were readily identified in
TEM (transmission electron microscope) studies by Taylor and Liu (1987, 1989).
On a molecular scale the incorporation of bacterial lipids and absorbed and
otherwise finely dispersed liptinitic material into the variously humified precursors
of vitrinite increases the ratio between interstitial (intermicellar) material with low
aromaticity and the condensed aromatic clusters (micelles). The result is enhanced
development of the above mentioned mobile phase during physico-chemical
coalification, and a lowering of the rate of cross-linking and condensation of the
aromatic framework. In terms of optical properties the consequences are a
reduction in reflectance and an increase in fluorescence intensity of practically all
humic degradation products.
At this stage it is uncertain whether a genetic link exists between the strongly
fluroescent vitrinites described here, and the likewise highly fluorescent huminites
recently described by Jin and Qin (1989) from Mid-Pleistocene (!) brown coals
and younger peats in the Tengchong Basin in the western Yunnan Province of
China. Two seams of soft brown coal, the oldest dated 600 ka BP, occur within
240 m of what appears to be alternating fluvial and lacustrine sediments. In addition
to ordinary huminite (vitrinite) macerals, the coal contains members of this group
with unusually high H/C ratios and fluorescence. In these and other properties the
strongly fluorescent macerals appear to occupy a position between normal huminite
and liptinite, although their microscopic habit is that of huminite. Jin and Qin
(1989) refer to them as liptohuminite and distinguish the subgroups liptohumotelinite and liptohumocollinite. Both appear to have derived from Alnus glutinosa
wood, which is significant in view of the preference of this species for very wet,
including periodically flooded locations in eutrophic, topogenous mires of the
temperate zones of Europe and northern Asia. Jin and Qin (1989) found liptohuminite
to be commonly associated with "sclerotinite-like" phlobaphinite, which, together
with some of the liptohumotelinite tissue has been interpreted as mycorrhiza (root
nodules), i.e. a symbiotic association of intergrown root cells and fungal hyphae
producing lipid-rich tissue compared with unaffected roots.
Detrovitrinite shows generally higher fluorescence intensities than telovitrinite,
which can be related to three main causes:
1. Probably the main reason is the difference in aromaticity. As has been discussed
in Chap. 5.1.2, given similar conditions of biochemical coalification, telovitrinite
would form preferentially from lignin-rich (e.g. woody) progenitors, whereas
cellulose-rich herbaceous plants would be the preferred phytogenic sources of
detrovitrinite. At the beginning of physico-chemical coalification, the lignin-based
aromaticity, and this includes aromatic complexation (micelles) and crosslinking, is therefore inherently higher in telovitrinite than in detrovitrinite. The
latter contains more intermicellar mobile phase and thus potentially more
fluorophores.
219
Optical Properties as Palaeo-Environmental Indicators
2. Since under conditions of marine influence cellulose can be partly converted
into lipids by Clostridium, within the cell tissue, the higher proportion of cellulose
in the precursors of detrovitrinite, once again results in increased fluroescence
compared with telovitrinite.
3. Because the capacity to absorb other substances increases with the degree of
degradation and colloidal transformation of the host material, pre-detrovitrinites may have absorbed more lipid-rich humic fluids than pre-telovitrinite. The
possibility of humic and other solutions being able to impregnate cell tissue
has been mentioned in Chap. 4 and is further supported by Dehmer's (1988)
discovery of presumably migrated angiosperm biomarkers (e.g. picene, a
polyaromatic triterpene-(see discussion of biomarkers in Chap. 5.5.2) in
humified conifer xylite.
In spite of its lower fluorescence response to irradiation, telovitrinite is the preferred
material on which fluorometric measurements are carried out, because its intensities
vary less widely than that of detrovitrinite, which commonly contains varying
amounts of submicroscopic liptodetrinite admixtures and, depending on the degree
offragmentation and colloidal transformation, accommodates fluctuating amounts
of fluorophores in its humic phases as well.
When plotted in relation to coal rank, the fluorescence intensities of telovitrinite, detrovitrinite and inertinite, measured at a wavelength close to their maximum
spectral intensity (A max), approximate normal Gaussian distribution curves with
varying kurtosis. The highest intensities occur at the transition from high to medium
volatile bituminous coal, after which the fluorescence deteriorates due to the
thermal destruction of the fluorophores during advanced coalification. An example
of the correlation of fluorescence properties and coal rank is illustrated in Fig. 5.27
which is based on freshly mined Permian Australian coals, all of which have been
18
16
o
0
0
0° ~
.
.
..
12
o ....
•
•
~
10
8
0
00
...
00
00
••••
.•
..~.:o
0
6
•
• "'1:+ ....++ +
;;:: 4
0
•
l{)
2
~
0
.4
\l)
0
0
0
14
;. a •
eO
++
+
+ ++ + 1:
re
8
* + :j: ... +
+ ...
.6
.8
1.2
1.4
i
i
~
+
+ +
1.6
,
1.8
% Rort
Fig. 5.27. Correlation of mean fluorescence intensities measured in Australian Permian coals in
water immersion at a wavelength of 650 nm (I 650 w) of telovitrinite (dots), detrovitrinite (open
circles) and inertinite (crosses) with coal rank, expressed as mean random reflectance oftelovitrinite
in oil immersion (% Rort). (After Diessel and McHugh 1986)
220
Coal Facies and Depositional Environment
obtained from deep mines. The latter is important since open cut coal samples
often yield lower fluorescence values than would be expected on the basis of their
rank (McHugh 1986). Each entry in the diagram represents a mean of mostly 50
individual readings. In accordance with previous practice (Diessel 1985a),
measurements on inertinite macerals have been restricted to those with a
fluorescence intensity exceeding 0.5%. Data acquisition in fluorescence mode using
water immersion and blue light excitation of 450 to 490 nm wavelength has been
carried out as described by Diessel (1985a), Diessel and McHugh (1986), and
Diessel and Wolff-Fischer (1987).
The fluorescence intensity distribution illustrated in Fig. 5.27 is based on
ordinary humic coals without any marine or other unusual influence. As mentioned
above, coal seams which have been affected by marine conditions, either because
they carry a marine roof or because they were formed in a coastal setting, are
commonly characterised by excessive fluorescence intensities for both vitrinite and
inertinite. An example of this is illustrated in Fig. 5.28, using a comprehensive set
16
12
• ••
•
••
.....
".
• •
10
8
•
•
~ 4 ....
r--
•
•
•
6
g 2
c
•
B
A
14
..
=-
•
=
•
y
9.64 + 29.80x + 12.86x 2
Y = 4.59 + 1O.73x - 6.41x 2
y 14.55 + 1.77x - 4.58x 2
O~~--~--~~--~--~--~--~~--~~~~~--~--~~--~--r--
.6
c:
060
B
% Rort
1.2 1.4 1.6
.6
.8
0
1.2 1.4 1.6 .6
E
.
..
.8
1.2 1.4 1.6
1.8
F
-40
-60
.6 .8
% Rort
1.2 1.4 1.6
.6
.8
1.2 1.4 1.6.6
.8
1.2 1.4 1.6
1.8
Fig. S.28A-F. Mean fluorescence intensities measured on telovitrinite in three sets of
Carboniferous coals from the Ruhr Basin in water immersion at a wavelength of 700 nm (I 700 wt).
A Normal humic coals formed on alluvial and upper delta plains; B coals with fresh-water
lacustrine to brackish roof sediments; C coals with marine roof sediments; D to F residuals as
normalised deviation (in %) from fitted curve in A. The two dashed bars in F and the corresponding
data points in C refer to the Greta Seam in the Sydney Basin of New South Wales
Optical Properties as Palaeo-Environmental Indicators
221
of Carboniferous coals from the Ruhr Basin. In this case, intensity measurements
were carried out at a wavelength of 700 nm, after comparative measurements on
identical coals under different analytical conditions demonstrated that 700 nm gave
the most even intensity distribution about the mode. The distribution curve of
Fig. 5.28A also refers to normal humic coals without any noticeable marine or
other unusual influence. Conversely, the distribution in Fig. 5.28B has been
constructed from coals which carry a lacustrine to brackish roof. Identification
of these conditions was based on the occurrence of the respective fossils in the roof
sediments, mainly bivalves, worm burrows and feeding traces (e.g. Planolites
ophtalmoides). In the lower coalification range the position of many data points
obtained from coals with strong bioturbation in the roof is well above the normal
distribution curve in Fig. 5.28A, whereas most of the seams with fresh-water
bivalves in the roof plot more closely to the latter. In the upper coalification range
all values converge with the normal distribution. Figure 5.28C gives the fluorescence intensity values for the marine influenced coals. They all represent wellknown marine horizons in the European Carboniferous System including Aegir,
Domina (L Seam), Katharina, Wasserfall, Plasshofsbank, Girondelle and Wasserbank. Because they do not contain low rank examples, two samples from
the Greta seam from the Australian Sydney Basin have been added. The seam is
strongly marine-influenced and carries a rich brachiopodal fauna in its roof. The
lower rank coals display considerably higher fluorescence intensities than the
normal coals, and several samples plot also above the brackish-influenced coals.
In order to highlight differences between the three palaeo-environmental
settings, normalised residuals of the measured fluorescence intensities are illustrated
in Fig. 5.28D to F. The use of normalised residuals has the advantage that they
are independent of actual fluorescence values and thus allow comparison between
different instruments and methods of intensity determinations. In all three diagrams
the zero line represents the regression curve for ordinary humic coals displayed
in Fig. 5.28A, whereas the bars extending into the positive and negative regions
above and below zero indicate the deviation in percent of the measured values
from the fitted values in accordance with the regression equations indicated in
Fig. 5.28A to C. In Fig. 5.28D the positive and negative variations are more or
less in balance, although some very low values occur in the lower rank range.
They have been measured in stratigraphically high Westphalian C coals which are
situated not far below the Late Carboniferous unconformity and have been affected
by Permian weathering. According to Australian mining experience (Diessel,
unpubl. data) vitrinite fluorescence intensity (as well as Gieseler Fluidity) may be
suppressed to a depth of up to 150 below the present surface. As shown in Table 5.5
this has resulted in a slightly negative arithmetic mean. Diagrams E and F of
Fig. 5.28 display strong positive trends of the residuals, particularly the marine
influenced coals. Negative deviations occur in some high rank marine-influenced
coals, but they are likewise artefacts due to oxidation, having been sampled in
defunct open cuts.
Further applications of fluorescence measurements to coals from different
geological environments have been indicated in Table 5.5 together with arithmetic
means of the normalised residuals displayed in Fig. 5.28D to F, plus mean total
Coal Facies and Depositional Environment
222
Table 5.5. Comparison of mean deviation (normalised residuals in %) of measured fluorescence
values (at 700nm) from fitted regression curve displayed in Figure 5.28 with mean total sulphur
(db) and liptinite contents for five palaeo-environmental groups of Carboniferous Ruhr coals.
Mean liptinite refers to coals with less than 1.25% mean random telovitrinite reflectance
Coals having the following characteristics:
Ordinary
humic coals
With
densospores
Fresh-Brackishinfluenced
Marineinfluenced
Partly
sapropelic
% Deviation
n
Standard error
-3.6
86
2.201
+8.0
43
2.315
+ 10.8
28
2.913
+26.7
17
6.107
+30.9
14
5.742
% Sulphur (db)
n
Standard error
1.62
59
0.113
1.39
29
0.114
1.35
26
0.119
3.13
9
0.513
2.24
14
0.345
% Liptinite
n
Standard error
12.3
56
0.753
11.6
33
1.231
7.8
20
0.968
7.5
4
0.289
19.6
14
1.923
sulphur and liptinite contents of the coals used in the construction of Fig. 5.28. In
addition, two new palaeo-environmental categories have been included in Table 5.5.
These refer to rlensosporinite-bearing coals and those with a conspicuous proportions of sapropelic components in the whole coal sample. The former includes
samples which have also been used in the compilation of Fig. 5.28, where coals
were divided into the three illustrated palaeo-environmental categories on the basis
of their roof sediments irrespective of their spore content. Since samples containing
densospores occur in ordinary humic coals and those with fresh-water and brackish
roof sediments, the 43 samples with densosporinite listed in Table 5.5 do not
represent a new suite of samples but have been drawn from a combination of
these environments. This does not apply to coals with sapropelic influence, which
have not been previously included in Fig. 5.28.
The five palaeo-environmental categories listed in Table 5.5 differ in their mean
deviation of measured from fitted fluorescence intensities between - 3.6% for
ordinary humic coals to + 30.9% for coals with strong sapropelic influence, mainly
in the form of transitions to cannel coal. As previously noted, the negative mean
value is related to unusually low fluorescence intensities of coal seams situated
close to the Permian/Carboniferous unconformity. Likewise, some individual
negative values recorded among the marine influenced coals are similar artefacts,
the reduced fluorescence being related to the prolonged residence of the respective
coal seams (= splits of the Wasserbank Seam) under shallow cover beneath the
present day earth's surface. If these are subtracted from the listed figure, the new
mean for marine influenced coals increases to 31.8%, i.e. it exceeds the high value for
the partially sapropelic coals.
The fluorescence values of fresh-water to brackish-influenced coals differ only
insignificantly from those containing densosporinite. The reason for this is the
listing of several samples in both categories, i.e., a number of seams in which
densospores occur, are also overlain by either fresh-water or brackish sediments,
Optical Properties as Palaeo-Environmental Indicators
223
as, for example, is the case with the many splits of the Zollverein Seam. Indeed,
in the roof of Zollverein 1, the uppermost split of this seam, foraminifera (Michelau
and Tasch 1958; Rabitz 1966) and high boron contents (Ernst et aL 1960) have
been found in several parts of the Ruhr Basin, thus suggesting a weakly marine
rather than brackish influence. This situation is not unlike the marine cover on
the densosporinite-bearing Lower Kittanning coal of western Pennsylvania (Habib
1966; Habib and Groth 1967; Ting and Spackman 1975; Rimmer and Davis 1988;
Ting 1989).
While the fluorescence intensities of the Zollverein 1 Seam are well within the
range of those measured in strongly marine-influenced coals, a decline in vitrinite
fluorescence is noticed in coals with decreasing marine influence. This is due to
the suppression of bacterial activity under increasingly acid conditions, which
reduces both the microbial transformation of liptinite into protobitumina and the
contribution to fluorescence by bacteria-generated submicroscopic lipids. The high
fluorescence intensities recorded for coals with (presumably lacustrine) sapropelic
influence in Table 5.5 might therefore be surprising, but in these cases the supply
of liptinite is so large that even under less than optimum conditions of bacterial
activity a high amount of fluorescent liptodetrinite is produced and absorbed by
the humic degradation products. In this context note the mean liptinite content
of 19.6% listed in Table 5.5 for coals with sapropelic influence, which is almost twice
that indicated for normal or ordinary humic Ruhr coals. This is in sharp contrast
to the reduced liptinite percentages in the brackish and marine influenced coals,
which has been previously noted by Stach and Michels (1955/56) and Teichmiiller
(1962). The notion that pH has a controlling influence on the preservation of
sporinite and other liptinites is supported by Pfaffenberg's (1953/54) work on
Recent limnic peat deposits. Well-preserved pollen grains and cuticles were always
found in strongly acid peats but their state of preservation deteriorated sharply
with increasing alkalinity.
The highest values in Table 5.5 for total sulphur (db) occur in the marine
influenced coals, but even in the ordinary humic coals the arithmetic mean is still
elevated due to the inclusion of some exceptionally high sulphur concentrations
in the stratigraphically uppermost coal seams. Apart from the observation that
the sulphur contents of both marine and freshwater influenced coals are generally
higher in the warm-climate Carboniferous coals than in equivalent cold-climate
Gondwana coals, the sulphur concentration in these coals is related to their specific
stratigraphic position. As has been mentioned above, they are situated not far
underneath the Carboniferous/Permian unconformity above which the Permian
system begins with the saline Zechstein deposits from which the mainly pyritic
sulphur in the coal has been derived by seepage of sulphate complexes and
subsequent reduction.
5.4.2 Vitrinite Reflectance and Other Rank Parameters
Over the past 30 years vitrinite reflectance has become the most widely used
parameter in assessing coal rank (Murchison 1958, 1987; Teichmiiller and
224
Coal Facies and Depositional Environment
Teichmiiller 1966a; Davis 1978; Neave11981; Bustin et al. 1985; Teichmiiller (1987)
including maturation levels in dispersed organic matter (DOM), also referred to
as kerogen (Bostik 1973, 1979; Murchison et al. 1985; Robert 1981, 1988). Its
usefulness in this field is due to its precision, excellent repeatability, satisfactory
reproducibility, and the possibility to make numerous low cost assessments on
very small sample sizes and quantities. In addition to its primary role as rank
indicator, vitrinite reflectance can also be employed to reveal small variations in
photometric responses of isometamorphic vitrinites due to differences in source
material and palaeo-environmental conditions. When used in this role, it is important
that the measurements are carried out on comparable macerals (e.g. telocollinite
only), and that the changing pattern of vitrinite reflectance with increasing rank
is taken account of. The latter aspect refers to the relatively high dispersion and
slow increase in vitrinite reflectance in low rank coals up to the level of high
volatile bituminous coal. At this low rank interval, characterised by a low degree
of condensation of the aromatic molecular fabric, reflectance depends on changes
in refractive index, which proceed slowly. This changes in the more advanced
stages of physico-chemical coalification, when increased condensation and
cross-linking of the aromatic clusters results in increased molecular ordering such
that absorption becomes the dominant fundamental optical property causing a
more rapid increase in reflectance (Murchison 1987).
The changing rate of reflectance with increasing coalification means that
reflectance variations in isometamorphic vitrinites due to differences in source
material -and depositional conditions are large in low rank coals but become
gradually eliminated with increasing coal rank. An example of this is illustrated
in Fig. 5.29, in which the mean random telovitrinite reflectance of coal seams
encountered in two deep diamond drill holes sunk into the Upper Carboniferous
(Westphalian A to C) strata of the Ruhr Basin in Germany has been plotted against
present depth of burial. The relatively low reflectance values in KB Specking 1
display a considerably larger scatter compared with the tighter correlation of
reflectance with depth of burial shown by the higher rank coals encountered in
KB Bergbossendorf 1. Although a great deal of low rank reflectance scatter may
be rather unspecific, persistent deviations from an average reflectance/depth of
burial curve, particularly at higher rank levels, have probably a good chance of
being traced to a palaeo-environmental cause. Among these are coals with sapropelic
and marine influence, which have been indicated by crosses and open circles in
Fig. 5.29. In both bores they consistently take up positions at the low reflectance
side of the data distribution. These observations agree well with the fluorescence
behaviour discussed above and also support the previously mentioned suppression
of vitrinite reflectance values in coals that contain alginite (Hutton and Cook 1980;
Hutton et al. 1980; Wolf and Wolff-Fischer 1984; Kalkreuth and Macauley 1984,
1987; Price and Barker 1985) or large amounts of other liptinite macerals
(Kalkreuth 1982; Goodarzi 1985b; Correa da Silva et al. 1985; Wenger and Baker
1987; Correa da Silva 1981, 1989). The last-mentioned author, as well as Correa
da Silva and Wolf (1980), also considered facies changes to be the reason for the
vertical changes in vitrinite reflectance and other rank parameters observed in
Brazilian coal seams.
Optical Properties as Palaeo-Environmental Indicators
900
225
1000
I.
1100
+••
•
•
-,...
...
1200
+.
0
1300
++
'J
•
.....
•
•
•••
1400
Fig. 5.29. Correlation of mean random
telovitrinite reflectance (% Rort) of coal
seams measured in KB Specking 1 (left) and
Bergbossendorf 1 (right), two diamond drill
holes sunk into the Upper Carboniferous
(Westphalian A to C) strata of the Ruhr
Basin, with current depth of burial. Crosses
coals with sapropeJic influence; open circles
coals with marine influence. (With data
kindly supplied by Ruhrkohle AG)
.
••
+
1600
•
••
+
••
0
:.
•
E
::;; 1700
••
•
1800
0.6
0.8
,
•••••
•
0
4-'
c(J)
•
•••
1500
0
••
•••
1.0
1.2
14
•
1.6
% Rort
Although most authors, following Jones and Edison (1978), Teichmiiller (1982)
and Kalkreuth (1982) and others, have regarded absorption oflipids or bituminous
substances as the main reason for the increased fluorescence and suppressed
reflectance of vitrinite, this notion has been challenged by Wenger and Baker
(1987). They subjected powdered high-TEOM (total extractable organic matter)
samples with suppressed vitrinite reflectance (compared to associated humic coals)
to 48 h of Soxhlet extraction with chloroform, during which a large proportion
of the contained bitumens was removed. A comparison of vitrinite reflectance
values measured before and after Soxhlet extraction showed no significant
difference. Wenger and Baker (1987) conclude therefore that the suppression of
vitrinite reflectance is not so much a consequence of actual absorption of low
reflecting matter, but that both are concomitant effects of anoxic depositional
environments, which caused this vitrinite to be lower in carbon and to advance
in physicochemical coalification at a lesser rate than a vitrinite formed under more
oxic conditions.
Another example of the influence of depositional facies on vitrinite reflectance
is the general trend towards lower values in vitrinites occurring as dispersed organic
matter (DOM) in predominantly inorganic sediments compared with isometamorphic vitrinite measured in associated coals. According to Damberger (1968), Jones
et al. (1972) and Goodarzi (1985b), the reflectance of dispersed vitrinite depends
on the thermal conductivity of the rock matrix, its permeability, and underground
water circulation. Different enclosing rock types have different effects on vitrinite
reflectance. In some sediments, such as shale, a uniform trend towards lower
reflectance than that recorded in associated coal has been found, while in others,
226
Coal Facies and Depositional Environment
such as limestone, opposing trends were reported from different localities (Timofeev
and Boguliubova 1970; Kuenstner 1974; Bostick and Foster 1975; Goodarzi 1985b).
In a comprehensive study over a wide rank range in the Carboniferous coal
measures of the Ruhr Basin, Scheidt and Littke (1989), found a consistent decline
of isometamorphic vitrinite reflectance from coal to sandstone, while mudstone
and siltstone occupied an intermediate position. Although the authors considered
the possibility of bias towards telocollinite in the coal samples, they regard
enhanced microbial activity due to decreasing acidity gradients from coal to
sandstone environments as a reasonable explanation for the lowered vitrinite
reflectance.
Variations in vitrinite composition have likewise been held responsible for the
considerable differences in vitrinite reflectance encountered in some isometamorphic coals from the West Coast of New Zealand's South Island (Newman 1985a, b).
These coals are of Cretaceous to Eocene age, they are quite thick (in some cases
over 10m) but of variable lateral extent. The coals of the non-marine Paparoa
Coal Measures (for stratigraphic details and depositional setting (see Chap. 9.2)
are the thickest but are discontinuous and rarely exceed a few kilometres laterally.
Seams in the overlying marine-influenced Brunner Coal Measures are thinner but
can be correlated over a much larger area (Newman 1985b). Coal ash contents
are low to very low, occasionally averaging less than 1% in the raw coal, which
may contain in excess of 90% total vitrinite. Coalification is variable, but when
of bituminous rank, the coals exhibit extraordinarily high swelling and fluidity
values.
29
23
•
27
25
••
•
: f'
23
~
•
••
24
3"
l:
>
•
•
20
-w.
22
90
80
70
" Inertinite (mf)
00
50
10
30
20
Fig. 5.30. Diagram showing the partial dependence of the volatile matter yield of Australian
isometamorphic coals (Rort = 1.22 ± 0.1 %) on
their inertinite content (Y = - 7.6x + 243.6). (After
Diessel and WoltT-Fischer 1987)
Optical Properties as Palaeo-Environmental Indicators
227
Wellman (1952) and Suggate (1959) were among the first to realise that in the
West Coast coals of New Zealand the conventional rank parameters, such as
elemental carbon and volatile matter, displayed variations which are explained
neither by differences in thermal maturity (physico-chemical coalification), nor by
contrasting petrographic composition. An example of the latter's influence on
volatile matter yield is illustrated in Fig. 5.30, but such extreme differences do not
occur in the New Zealand coals referred to. As mentioned above, vitrinite,
particularly detrovitrinite, is their predominant constituent, whereas inertinite
occurs only in small proportions, although liptinite contents are quite variable.
Unless vitrinite contains a high proportion of submicroscopic liptinite, the strong
variations in vitrinite reflectance, volatile matter yield and other properties found
in isorank West Coast coals must be related to vitrinite chemistry. In order to
reduce the influence of extraneous effects, Suggate (1959) adjusted volatile matter
yields to a "dry mineral matter andl sulphur free" (dmmSf) basis:
VM (dmmSf) = 100[VM (db) - 0.1 ash (db) - sulphur (db)] .
100 - 1.1 ash(db) - sulphur (db)
(5.7)
Following detailed investigations of ash composition, Newman (1986) and Newman
(1987a) changed the above correction method to read:
VM (dmmO.5Sf) =
100[VM (db) - a (db) - 0.5sulphur(db)]
100 - ash (db) - a (db) - sulphur (db) + As (db) + Afe(db)
(5.8)
where a = ~0.40[K20 x ash (db)]/lOO (illite)
0.35[Al 20 3' x ash (db)/I00 (kaolinite), where A1 20 3' = Al 20 3- 2.5K 20)
0.78[CaO x ash (db)/100(CaC0 3)
1.10[MgO x ash (db)/100(MgC0 3)
0.55[Fe20 3 x ash (db)/100(Fe 20 3);
and As = (S03 x ash (db)/l00 = sulphate (in ash) factor
Afe = 0.1 (Fe 20 3) x ash (db) = iron (in ash) factor.
By using the above formula (5.8), Newman (1986) updated Suggate's (1959) volatile
matter/moisture relationship as illustrated in Fig. 5.31. The curve refers to an
average coal type and samples which plot in the fields above or below the average
are inferred to have been derived from either stagnant, poorly oxygenated or
relatively well-drained mires, respectively. By combining this concept with the
tissue preservation index (TPI) discussed above, Newman (1987b) was able to
distinguish between various peatland types including raised bog (low TPI, VM
deficient, also low in ash), wet brackish (low TPI, above average VM, also high
pyrite content), and frequently flooded (high TPI, variable but close to average
VM, also higher in ash) conditions.
Since vitrinite reflectance correlates well with volatile matter yield it is not
surprising that Newman and Newman (1982) and Newman (1985a, b, 1986,
228
Coal Facies and Depositional Environment
45
40
Q
1fJ
35
In
c:i
E
E 30
~
~ 25
tfi
0
2
4
6
8
10
% Moisture (ash freel
Fig. 5.31. The "average"
volatile matter/moisture
relationship for New
Zealand West Coast coals.
(After Newmann 1986)
1987a, b) found an inverse relationship between the two rank parameters in isorank
coals of different type. They concluded that the coals with the highest volatile
matter, which tend to have lower than normal rank-related reflectance, formed in
poorly drained, ponded swamps, whereas the lowest volatile coals with relatively
high reflectance, accumulated in raised bogs.
5.5 Geochemical Palaeo-Environmental Signatures
Coal contains a large variety of major, minor and trace elements, some of which
have been inherited from its vegetal progenitors, while others have been introduced
from outside sources. These elements have been combined to form many different
compounds in response to source material and depositional setting. It should
therefore be expected that such a rich reservoir of elements and their compounds
contain chemical signatures of the physical, chemical and biological conditions of
peat formation. These chemical signatures can be discussed on an elemental, as
well as a compound level. When employed as palaeo-environmental indicators,
the form of occurrence of a particular element is not an important consideration,
which is different when studying compounds. The latter do not necessarily contain
special elements and, when of organic origin, consist mostly or rather common
elements, such as carbon, hydrogen and oxygen. However, the interest in this
group is related to the manner in which the common elements have been combined
to form organic compounds of unique biochemical significance, which makes them
analogous to index fossils in biostratigraphic reconstruction.
5.5.1 Elements of Palaeo-Environmental Significance
Because the elemental composition of coal and its ash is one of the quality
parameters frequently determined in routine analyses for a variety of practical
purposes, it was the generally high concentration of sulphur in coals with marine
229
Geochemical Palaeo-Environmental Signatures
roof sediments that was first noted to be of palaeo-environmental significance
(Mackowsky 1943; Stach 1949; Edwards and Baker 1951; Petrascheck 1952; Brooks
1954; Teichmiiller 1955; Balme 1956; Degens 1958; Suggate 1959; Diessel 1961;
Bailey 1981; and others mentioned elsewhere in this text). Subsequent studies by
Ernst et al. (1958, 1960), Keith and Degens (1959), Potter et al. (1963), Eagar and
Spears (1966) on the distribution of boron and other trace elements in coal and
clay minerals extended further the scope of identifying marine influence on coal
and fossil-free sediments. It is not surprising therefore that much of the geochemical
interest in the elemental composition of coal has been directed towards the
identification of palaeosalinity indicators.
While most of the early work was carried out on whole-coal samples, Alpern
and Quesson (1956) used autoradiography for the detection of inorganic elements
in individual macerals. Since then the comparison of elemental spectra of vitrinites
from different sources by Chen et al. (1981), Minkin et al. (1987), Morelli et al.
(1988) and Lyons et al. (1984b, 1987, 1988a, b, c) has opened new avenues for the
microchemical analysis of coal constituents made possible by the development of
new microprobe techniques (Dutcher et al. 1964; Augustyn et al. 1976; Boateng
and Phillips 1976; Stanton and Finkelman 1979; Finkelman 1981; Minkin et al.
1979, 1982, 1983; Chen et al. 1981; Gaines and Page 1983; Wolf et al. 1983a;
Makjanic et al. 1983; Thorne et al. 1983; Dobell et al. 1984; McIntyre et al. 1985;
Martin and McIntyre 1985; Palmer and Wandless 1985; Martin et al. 1986;
Creelman et al. 1986; Hamilton and Salehi 1986; Salehi and Hamilton 1986;
Corcoran 1989; Salehi et al. 1989), among which laser micro mass spectrometry
(Denoyer et al. 1982; Morelli et al. 1987, 1988; Lyons et al. 1984b, 1987, 1989a, b, c)
holds particular promise for future palaeo-environmental coal research. Another
new technique for the detection of trace elements is instrumental neutron activation
analysis (INAA), which has been used by van Berkel and Filby (1988) to determine
nickel and vanadium contents in porphyrins of Green River oil shales.
The elemental composition of coal has been inherited from two sources: the
bulk ofthe coal consists of carbon, oxygen, hydrogen and nitrogen, which have been
contributed by the coal's phytogenic precursors, mainly in the form of cellulose,
lignin, proteins and lipids. In addition to these organic compounds plants contain
small amounts of inorganic matter which they have extracted from the substratum
on which they grew. A list of the elements essential in plant growth is given in
Table 5.6. List of elements necessary for healthy plant growth. (After Weier et at. 1974, Nicholas
and Egan 1975, and Warbrooke 1981)
Carbon
Hydrogen
Oxygen
Nitrogen
Phosphorous
Sulphur
Boron
C
H
0
N
P
S
B
Potassium
Sodium
Calcium
Magnesium
Iron
Manganese
K
Na
Ca
Mg
Fe
Mn
Zinc
Molybdenum
Copper
Cobalt
Vanadium
Chlorine
Zn
Mo
Cu
Co
V
Cl
230
Coal Facies and Depositional Environment
Table 5.6. According to Weier et al. (1974) and Warbrooke (1981) these elements
are utilised by plants in mainly four different modes:
1. The bulk elements C, Hand 0 form the structural components of the plant
body and its organs.
2. Some of the extracted elements become part of organic molecules affecting
various forms of metabolism, such as Mg and N in chlorophyll, P in adenosine
triphosphate, Nand S in proteins etc.
3. Several kinds of trace elements are contained in enzymes, which catalytically
determine the course of many physiological functions, e.g. B in carbohydrate
breakdown, Mn as activator in anaerobic respiration, Co in bacterial nitrogen
fixation etc.
4. Some alkalis (K, Na) are contained in the cell sap in ionic form where they
assist in maintaining osmotic balance.
In view of the important functions of trace elements in plants it is not surprising
to find similar trace element spectra in peat and coal (Palmer and Cameron 1988),
although their molecular association may be very different. Apart from the higher
proportion of bioliths in peat and brown coal than in bituminous coals, many of
their inorganic elements are bound to humic and other derivatives of organic acids
(Miller and Given 1978) or occur in amorphous and poorly crystallised minerals
(Cohen et al. 1989), only to be re-arranged during coalification (epigenetic
recrystallisation).
Apart from the essential elements listed in Table 5.6, most plants absorb
additional elements that happen to be dissolved in their intake water (Weier et al.
1974). The presence of such non-essential elements in vegetal matter is often an
indication of their concentration in the substratum and is therefore used in the
geochemical prospecting for ore-bodies. It is likely that regional differences in the
distribution of non-essential trace elements in identical coal macerals reflect similar
variations in the elemental uptake by the parent plants in response to different
geochemical mire settings.
The primary reservoir of elements concentrated by the coal's vegetal progenitors
is supplemented by a large variety of elements contributed to the coal from the
depositional environment in the form of adventitious minerals. By definition of
the term coal the total ash content must be less than 30%, i.e. the contributed
mass from this secondary source of elements has to be relatively small, but the
various introduced elements may exert a strong influence on the chemical
conditions of peat accumulation and thus on the composition of the organic
compounds as well.
The concentration of a non-detrital inorganic element in coal is subject to
several constraints, the first of which is its availability in the depositional environment. The second constraint is its solubility in water. Apart from carbon and
oxygen, which terrestrial plants extract from the air in the form of carbon dixoide
by photosynthesis, all other elements which have been chemically incorporated in
peat either organically or as authigenic minerals, were once dissolved in water.
Water is the continuous phase that acts as solvent, transmitter and donor of
231
Geochemical Palaeo-Environmental Signatures
elements, and thus constitutes an indicator for mire chemistry, irrespective of
whether it occurs above ground or is confined in the substratum. In Table 5.7
average concentrations of a number of elements contained in modern river water
and sea water are compared. The listed enrichment factors, although they must
be regarded as general guides only and are bound to vary from case to case,
indicate a considerable degree of contrast between the two environments, which
are likely to affect the chemical milieu of any mire under their influence. Similar
contrasts are likely to have affected the peatlands of past geological periods, since
ocean water appears to have reached more or less its present composition well
before the onset of large scale coal formation (Rankama and Sahama 1950; Kramer
1965; Krauskopf 1967).
The third constraint is its precipitability in immobile form, which depends
on the presence of other suitable elements or compounds with which the element
can combine in order to form a stable, water-insoluble compound. In reference
Table 5.7. The mean distribution of some elements with contrasting concentrations in modern
fresh- and sea water of 3.5% salinity. (After Wedepohl 1969 and Warbrooke 1981)
Element
Concentration (ppb) in
Sea water
River water
Ag
AI
As
B
Ba
Ca
Co
Cr
Cu
Fe
Ga
0.28
1
2.6
4450
21
411000
0.39
0.2
0.9
3.4
0.03
392000
0.003.4
170
1290000
0.4
10
670
10800000
6.6
88
0.03
90400
2900
8100
1
3.3
1.9
0.013
5
0.026
K
La
Li
Mg
Mn
Mo
N
Na
Ni
P
Pb
S
Si
Sr
Ti
U
V
Y
Zn
Zr
Enrichment factor in
Sea water
River water
0.39
360
1.7
13
11
15000
0.19
1
3
670
0.09
2300
0.2
3.3
4100
6
1
230
63000
0.3
19
7
3733
6113
50
2.7
0.06
0.9
0.07
25
2.61
1
360
2
342
2
27
2
170
52
315
5
3
197
3
59
15
10
3
1714
22
5
233
242
2
162
3
55
2
5
5
100
232
Coal Facies and Depositional Environment
to Table 5.7 this means that a high enrichment factor of an element, such as
shown by sodium in sea water, is oflittle consequence ifthe element has a preference
for mobile and, in the case of sodium, water-soluble compounds. Conversely,
sulphur and boron are very successful palaeosalinity indicators because they form
stable compounds in spite of their rather modest enrichment factors compared
with sodium.
The fixation of an element can take several forms, including organic
complexation by living plants, as well as by humus colloids. Many minor elements
are absorbed into the co"al by the formation of organo-metallic complexes during
biochemical coalification (Zubovic 1966; Cooper and Murchison 1969). Also
sorption on clays, and reaction with other dissolved elements followed by
precipitation as authigenic minerals are common modes by which inorganic
elements become part of the coal ash. The precipitation of authigenic minerals is
commonly related to changes in chemical equilibrium which destabilise the
dissolved element. Such changes may involve the redox potential, hydrogen ion
concentration and water temperature (Warbrooke 1981). Additional changes may
be brought about by variations in salinity due to the flooding of a fresh-water
swamp by sea water, or the dilution of brackish swamp water by an influx of
fresh water. Another source of change in the elemental concentration of peat water
depends on the activity of sulphate reducing bacteria.
From the above discussion follows that in palaeo-environmental analysis it
is desirable to know the origin of the elements contained in coal in terms of the
relative contribution made by the various inherent and adventitious sources.
Emphasis in this chapter is not on the major elements C, H, 0, and N, but on the
minor and trace elements, that are commonly concentrated in the ash.
Elements whose proportion correlates positively with the total ash content of
the host coal are of adventitious origin, having been added to the organic matter
either by authigenesis or as detrital minerals. In the coal seams of New South Wales,
Slansky (1985) found the percentages (by weight) ofSi, AI, Ti, K, and Mn to increase
with ash yield, which can be accounted for by the high clay content ofhigh.:ash coals.
An interesting method of elucidating the elemental origin has been applied
by McCarthy et al. (1989) to the peat deposits ofthe Okavango Swamp by comparing
the elemental composition of peat with the average composition of the source
plants. In their case the source plants were not only known but also extremely
low in species, which made the task of obtaining an average composition of the
feedstock relatively easy. The principle of the method is explained in Table 5.8
and Fig. 5.32, where the total ash (db) in peat (or coal) is plotted on the abscissa
(X-axis), while the proportion (normalised to total ash) of plant ash is plotted on
the ordinate (Y-axis). McCarthy et al. (1989) found the amount of inherent plant
ash to average 5%, which therefore marks the starting point of Column 2 in
Table 5.8. If there is no other source of ash (e.g. allochthonous minerals) and the
inherent ash has not been increased, for example, by excessive oxidation or other
removal of organic matter, the 5% plant ash corresponds to the total peat ash, as
indicated in Column 1. In view of the inevitable loss of biomass, this correlation
is not strictly correct, but considering that also some inorganic matter is lost
during humification, it gives a minimum value. In relative terms, the plant ash
233
Geochemical Palaeo-Environmental Signatures
B
A
100
90
60
70
.J::
60
Ul
-<{
.J::
50
-<{
....,
40
Ul
Ul
"o
<IJ
30
:;:;
:;:;
c
<IJ
.J::
20
"0
~
10
~
C
L
E
<IJ
>
-<{
....... .j............~-~~~~~~~~
o·.I-~~~~~~:::=~*=-
o
10 20 30 40 50 60 70 60 90
% Total Ash
0
10 20 30 40 50 60 70 60 90 100
% Total Ash
Fig. 5.32 A, B. Diagrammatic representation of the proportion of inherent (A) and adventitious
ash (B) in total ash. (After McCarthy et al. 1989)
Table 5.8. Calculation of the changing proportion of plant ash (inherent) and its components in
total ash content of peat (or coal) with varying degrees of mixing between inherent and adventitious ash. (After McCarthy et al. 1989)
Total ash
in peat (%)
1
Plant ash in
total ash (%)
2
Relative % plant
ash in total ash
3
Relative % of component n
in total ash
5
4
5
10
20
30
40
50
60
70
80
90
100
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
100.00
45.00
20.00
11.66
7.50
5.00
3.33
2.14
1.25
0.55
0
60.00
27.00
12.00
7.00
4.50
3.00
2.00
1.28
0.75
0.33
0
20.00
9.00
4.00
2.33
1.50
1.00
0.67
0.43
0.25
0.11
0
constitutes therefore 100% of the total ash content of the peat. A reverse situation is
indicated at the bottom of each of Columns 1 to 3. The total ash content is 100%
(db), which means that the material is totally adventitious (e.g. a stone band) and
does not contain any organic matter. The proportion of plant-derived ash in total
ash is therefore zero. Between the two extremes, Columns 2 and 3 give the
incremental changes in the contribution of plant ash to total ash in absolute and
relative percentages, respectively. A graphic display of the relative distribution of
inherent plant ash is given in Fig. 5.32A, while the opposite relationship of the
relative proportion of adventitious ash in total ash is illustrated in Fig. 5.32B.
If the plant ash were to consist of one element only, the relative proportion
of the latter would coincide with the trace of the former illustrated in Fig. 5.32A.
Coal Facies and Depositional Environment
234
70
B
A
60
50
{i 40
-<{
c
.~
...,
Zone of Enrichment
30
c
~ 20
o
0E 10 Zone of
o
u
~
Depletion
o~~~~~~~~~--~~~~~~~~~~~~
o 10 20 30 40 50 60 70 80 90
0
% Tot1l1 Ash
% Total Ash
10 20 30 40 50 60 70 80 90 100
Fig. 5.33 A, B. Diagram illustrating the proportion of a wholly plant-derived component n in total
ash under the conditions of incremental mixing with adventitious ash. In A the concentration of
n in pure plant ash is 60%. Succesive dilution with adventitious ash reduces the proportion of n
in the total ash content along the illustrated curve (mixing line). In B pure plant ash is assumed
to contain between 20 and 30% of component n. The single mixing line of A is therefore replaced
by a mixing band constrained by the upper (60%) and lower (20%) curve. (After McCarthy et aL
1989)
In practice this is unlikely to happen, but a similarly shaped curve can be drawn
for each elemental percentage in peat, if its proportion in the original plant ash
and the total ash content of the peat are known.
Another hypothetical example is given in Table 5.8, Column 4 and Fig. 5.33A,
where it is assumed that a component "n" makes up 60% of the total ash and has
been inherited exclusively from the source plants without any contribution from
external sources, such as allochthonous minerals or the like. Two entries can be
made in the diagram, one at 100% total ash, where n is zero, and one at 5% ash,
where its proportion is 60%. Increasing admixtures of adventitious ash to the pure
plant ash produce a trace, called "mixing line" by McCarthy et al. (1989) between
the two entries, which is obtained by calculating 60% of each of the entries in
Table 5.8, Column 3 in reference to their corresponding values in Column 1. As
indicated in Fig. 5.33A, any analysis result for component n that plots above the
mixing line is either partly adventitious, or has been concentrated in some other
way, for example, by oxidation of the organic matter. Conversely, any entry below
the mixing line indicates depletion of component n by postdepositional processes,
e.g. leaching. Where source plants with different ash contents have been recorded,
an upper and lower mixing line can be used resulting in a "mixing band". An
example of this is illustrated in Fig. 5.33B, where component n has been assumed
to range in plant ash between 60 and 20%. The results of the respective calculation
are given in Column 5 of Table 5.8. It is obvious that the hypothetical data plot
will conform to the configuration of the mixing line illustrated in Fig. 5.32B, when
the source of the element is totally adventitious.
When applying this method to coal, considerably more assumptions will have
to made. Since neither the composition of the source plants nor their ash content
Geochemical Palaeo-Environmental Signatures
235
is known, both have to be estimated from the coal itself. This can be done by
either hand picking a sample of clean vitrain, or by crushing and washing the coal
to the density of clean coal. The density of mineral-free coal depends on its maceral
distribution and rank. Because devolatilisation in the early stages of physicochemical coalification affects the relatively heavy oxygen more than hydrogenbearing compounds, many macerals drop in density untill hydrogen becomes
increasingly part of the degassing process beyond the rank of medium volatile
coal. The pathways of maceral densities in relation to rank are illustrated in Fig. 5.34.
The ash contents of various density fractions of the Dudley Seam, a high volatile
bituminous coal from the Newcastle Coalfield in eastern Australia, are illustrated
in Fig. 5.35. Also included are three samples of hand-picked vitrain from the same
seam plus the respective analysis results obtained from four samples of clean
ombrotrophic peat. The results indicate that at a density of 1.3 g/cm 3 the coal has
reached an average ash content of3% (by mass), which can be regarded as a reasonable approximation of the inherent ash, because the value is very close to the
density of 1.28 g/cm 3 for clean vitrain for this coal rank (see Fig. 5.34), and further
washing or hand picking does not seem to alter the result significantly. The value
of 3% ash, which represents the mean value for the three vitrain samples from the
Dudley Seam, has been used in the calculation of the mixing line for the elements
2.0
1.9
1.8
1.7
1.6
1.5
Fusinite
Semifusinite Macrinite
'" 1.4
E
o
....Cl
Vitrinite
>- 1.3
l-
e;;
Z
UJ
o 1.2
Fig. 5.34. The density distribution of some macerals
and maceral groups in relation to coal rank. (After van
Krevelen 1961; Dyrkacz
et al. 1984; Diessel and
Wolff-Fischer 1986)
Liptinite
1.1
+------.--------y------,-70
80
90
i
0.3
0.4
0.5
I
0.7 0.9
Iii
1.2
1.0
100 %C
2.0
1.5
4.0
3.0
%Rort
6.0
Coal Facies and Depositional Environment
236
••
•
60
..,
SO
Q)
c.
.".;:
c
u
:<=
c.
40
!::;
0
!::;
0
I...
1:l
E
0
.0.
30
.c
20
.,
1:l
U
~
~
0
1.1
•
•
•
(f)
I
.,c
Q)
-<{
•
-'"
u
c
.r:: 10
•
I
Q)
1.3
1.2
1.4
1.5
1.6
>1.6
Washing Fraction (g/cm 3)
Fig. 5.35. Diagram illustrating
the ash contents of various
density fractions of the Dudley
Seam, Newcastle Coal Measures,
Sydney Basin, NSW, Australia,
plus the respective values for
some peat samples. (Calculated
from analyses by Naucke 1980;
Ward 1980; Warbrooke 1981)
listed in Table 5.9. As an example of the application of the method, the results
obtained from the various washing fractions ofthe Dudley Seam and some adjacent
coals of similar composition, are illustrated in Fig. 5.36 for the two compounds Si0 2
and Fe 2 0 3 . The silica values follow a pathway not unlike the one illustrated in
Fig. 5.32B for adventitious matter, whereas iron is more randomly distributed with
more values plotting close to the mixing line. As expected, the greatest departure
from the mixing line occurs for all analysed elements in the highest density fractions,
where the largest proportion of detrital admixtures occurs.
The analyses of the Dudley Seam, for which a topogenous setting is assumed,
revealed no unequivocal evidence for element depletion, which would have been
indicated by the respective results plotting below the mixing line. In contrast, considerable depletion should be expected in coals of extremely low ash content. Examples
are some of the above mentioned West Coast coals from New Zealand, as well as
Table 5.9. Calculation of the changing proportion of inherent ash in total ash content of a high
volatile bituminous coal and some of its constituting elements with varying degrees of mixing
between inherent and adventitious ash. (Based on analysis results reported by Warbrooke 1981)
% Coal
ash
Vitrain ash
absolute %
Vitrain ash
relative %
%
%
%
%
3
lO
20
30
40
50
60
70
80
90
lOO
3.0
2.7
2.4
2.1
1.8
1.5
1.2
0.9
0.6
0.3
0
lOO.OO
27.00
12.00
7.66
4.50
3.00
2.00
1.29
0.75
0.33
0
61.65
16.64
7.40
4.72
2.77
1.85
1.23
0.79
0.46
0.20
0
19.51
5.27
2.34
1.49
0.88
0.58
0.39
0.25
0.15
0.06
0
5.370
1.450
0.644
0.411
0.242
0.161
0.107
0.069
0.040
0.018
0
0.360
0.097
0.043
0.028
0.016
0.011
0.007
0.005
0.003
0.001
0
SiOz
Al 2 0 3
Fe z0 3
PzOs
237
Geochemical Palaeo-Environmental Signatures
100
80
•
. ..
•
60
..
.•
40
N
0
Fig. 5.36. Two diagrams
illustrating the proportion
of silica (top) and iron
oxide (bottom) in various
density fractions of the
Dudley Seam and related
coals from the Newcastle
Coal Measures, Sydney
Basin, NSW, Australia.
The drawn-out trace represents the mixing line (after
McCarthy et al. 1989) of
each of the two elements if
they were wholly plantderived.
20
<h 0
8
6
~
..
..
4
2
0
N
CII
u. 0
0
20
40
60
80
100
% Ash in cOZll
the brown coals from the Gippsland Basin in Victoria, Australia, and others. The
total ash content of some of these coals, determined on raw coal, is less than 1%,
which is lower than the plant ash of their vegetal progenitors. There are two possible
reasons to account for the depletion: one is due to rain- and groundwater leaching,
while the other results from a nutrient conservation strategy, whereby essential
elements are recycled from senescent and dead vegetable matter to living plants.
McCarthy et al. (1989) found evidence of this strategy operating in the topogenous
and rheotrophic Okevango Swamp, but it would be a more common mechanism
of sustaining healthy plant growth in an ombrotrophic setting.
Although plant-derived elements frequently form chelate complexes and other
organic compounds (Kiss and King 1977, 1979; Kiss 1981, 1982; Given and Spackman
1978; Miller and Given 1978) whereas adventitious matter occurs commonly in
inorganic compounds, both groups of elements can enter into a variety of different
bonds. The determination of the various modes of molecular associations of
elements in coal has been based mainly on statistical correlations, for example, by
Gluskoter et al. (1977). A more recent study of the occurence of elements contained
in coal in organic, inorgainc or both phases was carried out by Warbrooke and
Doolan (1986), who established a theoretical distribution pattern for an organically
and inorganically bound element in five different density fraction of washed coal.
Figure 5.37 A shows the distribution of a wholly organically bound element in five
density fractions of a hypothetical coal expressed on dry (db), dry organic matter
free (domf) and dry mineral matter free (dmmf) bases. According to Warbrooke
Coal Facies and Depositional Environment
238
3.5
3
01
2.5
01
::J.
2
III
1.5
......
....c:
E
A
III
W
0.5
0
800
700
-III
400
III
300
B
01 600
01
~
c:
E
W
500
200
100
0
Fl.30
Fl.40
Fl.50
Fl.60
S 1.60
Fig. 5.37 A, B. Two diagrams illustrating the distribution of an element in
five density fractions of a
hypothetical coal. The element has been calculated
to three different bases: dry
(db) = dots; dry organic
matter free (domf)= triangles; and dry mineral
matter free (dmmf) = squares. In A the element is
assumed to be organically
bound, in B it is considered
to be restricted to the
mineral impurities of the
coal. (After Warbrooke
and Doolan 1986)
and Doolan (1986), the configuration of the three resulting curves is unique for
the organic affiliation of the element, which is characterised by a strong dependence
on the density (i.e. ash) of the coal for the (dom/) base, weak dependence for the
(db) base and no dependence for the (dmm/) base. In contrast, the inorganic
affiliation of an element illustrated in Fig. 5.37B, shows constant concentration on
a (dom/) base, because the diluting effect of the organic matter has been eliminated,
while the same element calculated to (db) and (dmm/) base increases with increasing
density.
The two distribution models obtained by the above method are then compared
with actual analyses of the respective washery fractions by statistical correlation
analysis. The results, which display the changing preferences of a large number of
elemets analysed in six major coal seams of the Sydney Basin (Australia) are given
in Table 5.10 together with some general seam characteristics. The listed elements
are divided into 5 groups in order of increasing inorganic affiliation. Group 1
elements have the strongest organic and Group 5 elements the strongest inorganic
affiliation. Individual codes have been numbered in the same manner and, although
the numbers assigned to each code will mostly reflect the group in which the
element has been listed, small variations occur in response to the other encoded
properties. Alpha coding is as follows: 0 = organic bond, I = inorganic bond,
239
Geochemical Palaeo-Environmental Signatures
Table 5.10. General coal characteristics and elemental bonding in some coals of the Sydney Basin
in eastern Australia. (After Warbrooke and Doolan 1986)
Seam:
Big
Ben
Borehole
Victoria
Tunnel
Australasian
Great
North.
Wongawilli
Ash (db)
V.M. (db)
Total S
Vitrinite
Liptinite
Inertinite
Romt
17.1
33.2
0.92
68.S
9.6
21.9
0.80
24.1
29.6
0.48
80.4
6.6
13.0
0.91
37.6
24.4
0.30
84.6
S.O
10.4
0.90
36.9
24.8
0.36
83.9
1.9
14.2
0.83
14.S
29.6
0.34
46.3
6.S
47.2
0.80
24.0
22.6
0.57
74.1
O.S
2S.4
1.19
Group 1 elements
Carbon
Hydrogen
Nitrogen
Oxygen
Sulphur (org.)
Germanium
ION
ION
ION
1 OL
2 ON
1 OL
ION
ION
ION
1 OL
2 ON
3 BN
ION
ION
ION
ION
2 ON
10M
ION
ION
ION
1 OL
ION
1 OL
ION
1 OL
ION
1 OL
1 OL
2 ON
ION
1 OL
1 ON
ION
1 ON
3 BN
Group 2 elements:
Molybdenum
Nickel
1 OL
4 IN
S 1M
2 ON
10M
1 OL
10D
2 ON
3 BN
1 OL
3 BN
3 BM
3 IN
3 BM
3 IN
3 BM
2 BN
3 BN
3 BN
3 BM
3 BD
3 BD
3 BD
4 IN
3 BM
3 BD
3 IN
3 BD
31M
4 IN
3 BD
4 IN
4 IN
3 BD
4 IN
S ILD
41LD
3 BN
10M
3 BD
3 BD
4 IN
4 IN
10M
1 OL
1 IN
4 IN
3 BN
4 IN
3 BN
4 IN
S ILD
S IN
S ILD
S BD
4 IN
4 IN
4 IN
4 IN
4 IN
4 BN
4 IN
10D
4 IN
41N
4 IN
4 IN
4 BN
10M
4 IN
SID
41LD
4 BN
SID
4 IN
S ILD
S ILD
S ILD
S ILD
4 IN
S ILD
4 IN
41LD
4 IN
4 IN
4 BN
3 BN
4 IN
4 IN
3 BN
4 IN
41LD
S ILD
4 IN
4 IL
41LD
S IL
4 BN
S IN
SIM
S IN
S IN
S IL
SID
Group 3 elements:
Beryllium
Bromine
Chromium
Copper
Mercury
Scandium
Selenium
Tungsten
Vanadium
Yttrium
Group 4 elements:
Arsenic
Boron
Cerium
Gallium
Lanthanum
Lead
Silver
Strontium
Sulphur (inorg.)
Thallium
Titanium
Uranium
Zirconium
Group S elements
Aluminium
Antimony
Barium
Cadmium
SID
SIM
SID
S IL
SID
S IN
SIM
4 IN
3 BM
3 BM
10D
4 IN
4 IN
3 BN
3 BN
4 IN
3 BN
3 BN
4 IN
4 IN
3 BD
4 IN
S ILD
4 IN
4 IN
41N
41LD
4 IN
4 BN
4 IN
S IN
4 IN
41N
4 IN
3 BD
4 IN
4 IN
4 IN
4 IN
4 IN
3 BM
3 IN
SID
SID
S ILD
S 1M
SID
3 BN
S IN
4 IN
(continued)
Coal Facies and Depositional Environment
240
Table 5.10. (Continued)
Seam:
Big
Ben
Borehole
Victoria
Tunnel
Australasian
Great
North.
Wongawilli
Calcium
Caesium
Fluorine
Iron
Magnesium
Manganese
Phosphorous
Potassium
Silicon
Sodium
Tin
Thorium
Zinc
5ILD
5ID
5ILD
5ILD
51LD
5ID
5 IN
5ILD
5 1M
51LD
4 IN
5ID
4 IN
5ILD
5 IL
5 IL
51LD
5 IN
5ILD
3 BN
5ID
5ID
5ID
4 IN
5ID
5ID
5 IN
5ILD
5ILD
5 IN
5ID
5ID
5ILD
51LD
51LD
5 ILD
4 IN
5 1M
5ID
5 ILD
5 IL
5 IL
5ILD
5 ILD
5 1M
4 IN
4 IN
5 1M
51LD
4 IN
5ID
5ID
5 IN
4 IN
4IN
3 BD
5 IN
5 IN
5 IN
5ID
5ID
5 ID
4 IN
5 1M
5 IN
5 IN
5 IN
41M
5 IN
5 1M
5ID
5 IN
5 IL
5ID
5ILD
B = both organic and inorganic bonds. The concentration of inorganically bound
elements in the light, middle and dense washery fractions is expressed by the letters
L, M and D, respectively, whereas N stands for non-fractionated. The results
demonstrate that the occurrence of elements in coal in organic or inorganic bond,
or both, follows broad patterns of preference. However, in some cases significant
differences between seams have been recorded, which may reflect local conditions
of the accumulation of peat and its post-depositional history.
This above results demonstrate the considerable contrast in the percentage
distribution of elements obtained for the different analysis bases used for the
calculation of the results. The same element concentration in the coal yields very
different numerical values depending on whether the analysis results are expressed
on a db, dmmf or domf basis. The difference in information obtained from the
post-analysis treatment of the results has also been stressed by Glick and Davis
(1987) on the basis of a regional study of inorganic element distribution of US
coals. By using cluster analysis techniques they found that in spite of some local
variations between the major elemental composition of coal seams, a well-defined
partition into regional/stratigraphic clusters consisting of eastern, interior and
western provinces was recognised. This provincialism lends support to Finkelman's
(1981) notion that positive statistical relationships between elements indicate a
common source. Work by Minkin et al. (1982), Slansky (1985) and Lyons et al.
(1989c) likewise support the notion of strong provincialism in elemental distribution.
5.5.1.1 Sulphur
The distribution of sulphur and sulphur-bearing minerals in coal has been variously
referred to in previous chapters, the following discussion therefore emphasises the
Geochemical Palaeo-Environmental Signatures
241
origin of sulphur and its characteristics. Based on similarities between the
occurrence of this element in Recent peats and some u.s. coals of Carboniferous
age, Casagrande (1987) concludes that all syngenetic sulphur in coal can be
accounted for in the peat stage. The main source of sulphur in low sulphur coals
« 1% total sulphur) appears to be the peat-forming plants themselves and their
saprophytes, where it occurs mainly in amino acids, cuticles and exines (Brooks
1954; Berner 1971). Although most of this sulphur is organically bound, the work
of Altschuler et al. (1983) suggests that small amounts of pyrite can be formed by
bacterial reduction of organic sulphur compounds. Sulphur isotope studies of
Smith and Batts (1974) and Price and Shieh (1979) demonstrate that the sulphur
isotope ratios 4 Sj 32 S) of low sulphur peats and coals are characterized by a small
relative enrichment in the heavier isotope and plot consistently within a narrow
band of a34 s = + 4.6 to + 7.3 permil. Casagrande's (1987) sulphur isotope ratios
obtained from low sulphur fresh-water peats of the Okefenokee Swamp are slightly
higher and range from + 9 to + 13 permil. These figures correlate well with the
isotope ratio of + 10 permil obtained from the peat-producing Nymphaea vegetation, where sulphur is contained mainly in proteins. The reason for the comparatively high a34 s is the small supply of sulphur in non-marine environments, which
forces sulphate-reducing bacteria to make use of all available sulphur in spite of
their preference for the lighter isotope (Price and Shieh 1979; Westgate and Anderson
1984; Lyons et al. 1989b).
In contrast to the narrow spread of positive values for a34 s found in coals
with a low total sulphur content (most of this is organic sulphur), the isotope ratios
of high sulphur coals (> 1% total sulphur) are more variable. This is a reflection
of the greater abundance of sulphates in sea-water, which allows preferential
reduction of the 32S-isotope (Lyons et al. 1989b). The combined values reported
by Smith and Batts (1974), Price and Shieh (1979) and Love et al. (1983) range
from - 6.49 to + 24.4 permil, whereas Casagrande's (1987) analyses of marine
influenced Florida peats with sulphur contents up to 10% (much of it in the form
of pyrite), yield only negative values ranging from - 8 to - 30 permil, while Lyons
et al. (1989b) quote a range of - 17.8 to 28.5 permil for high-sulphur Carboniferous
U.S. coals. These results are in agreement with the findings of Kaplan et al. (1963),
Kaplan and Rittenberg (1964) and Price and Shieh (1979) that hydrogen sulphide
formed by the bacterial reduction of sulphates is enriched in 32S thus giving 15
to 40% lower a34 s values than those obtained from the original sea-water sulphate,
currently at a a34 s ratio of + 20 permil (Casagrande 1987). The sulphur isotopic
ratio of sea water sulphate is a reflexion of the sea water ratio. which, for past
periods, is commonly obtained by analysing the isotope ratios of evaporates. On
this basis the isotopic ratios of sea water seem to have fluctuated during the earth's
geological history, although no unanimity exists about the precise values. For
example, Holster and Kaplan (1966) give a a34 s ratio of + 18.5 permil for Late
Carboniferous sea water, whereas Claypool et al. (1980) suggest +15 permil. For
Permian sea water, which is of special interest for Gondwana coals, Claypool et al.
(1980) give a a34 s ratio of + 10 to + 15, whereas Smith and Batts (1974) suggest
+ 20 to + 24 based on sulphur isotope determinations on massive pyrite
concretions in the Greta Seam of New South Wales.
e
Coal Facies and Depositional Environment
242
The reduction of sulphates contained in sea water by the anaerobic bacteria
Desulphovibrio desulfuricans and Clostridium desulfuricans results in the oxidation
of organic matter. According to Berner (1971), the following reaction applies to
carbohydrates:
(5.9)
The generated H 2 S reacts either with organic matter to produce organic sulphur
or with ferrous iron resulting in the syngenetic precipitation of pyrite (Casagrande
1987) or any of the other modifications of FeS 2 • There is a preference for pyrite
to form under neutral to slightly basic conditions, whereas marcasite prefers a
more acid environment according to Tarr (1927), Edwards and Baker (1951),
Rosenthal (1956), Rickard (1975) and Littke (1985). In particular, the work of
Sweeney and Kaplan (1973), Berner (1970), Berner et al. (1979) and Cecil et al.
(1979) has shown that framboids of pyrite cannot be generated at low pH conditions, which is supported by Littke's (1985) observation of low framboidal pyrite
counts in coal seams or parts thereof assumed to have been formed under acid
conditions. Conversely, marine influenced coal seams are generally high in their
content of framboidal pyrite. Iron, being a very common and widespread element,
appears to be freely available under peat-forming conditions, probably from within
the mire. Because of its frequently observed concentration in the vicinity of dirt
bands and near roof and floor sediments, it has been suggested (Reidenouer et al.
1967; Klare 1983; Littke 1985a) that some iron has been transported into the
swamps. The consequences of marine influence on sulphur distribution in coal will
be further explored in Chap. 8.
While the bulk of sulphur in coal is either organically bound or contained in
Fe-sulphides, other metal sulphides have been listed by Mackowsky (1968).
Additional sulphur has been found in coal as sulphate, although many such
occurrences have been interpreted as artefacts resulting from sulphide oxidation
during sample preparation (Ward 1989). As a natural coal component sulphate
has been reported from arid zones, such asLeigh Creek in South Australia (Kemezys
and Taylor 1964), and from Utah, U.S.A., by Ward (1986a). In both cases, the
mineral concerned was epigenetic gypsum, possibly precipitated from percolating
groundwater (Ward 1989).
5.5.1.2 Boron
As indicated in Table 5.7, boron shows a significant enrichment in sea water, where
it occurs mainly as boric acid and its dissociation products (after Kemp 1956):
(5.10)
Boron owes its use as a palaeosalinity indicator to its concentration in seawater
(Table 5.7) and tight fixation in clay minerals, notably (in order of preference) illite,
montmorillonite and kaolinite (Harder 1961; Couch 1971). Its initial absorption
Geochemical Palaeo-Environmental Signatures
243
appears to be followed by metasomatic substitution, since boron is considered to
be an ideal replacement for aluminium, particularly in the tetrahedral position. In
reference to mica, Degens (1965) calculated the theoretical limit of tetrahedral
aluminium substitution by boron to amount to a maximum boron content of 2.9%.
Since the highest analysed boron content amounted to only 0.2%, he concluded
that substitution affects only the outer aluminium positions and ceases once they
have been replaced by boron.
Because the fixation of boron in clays involves reactions between fluid and
solid phases, the effectiveness of the fixation depends on the total surface area of
the solids (clay minerals), the concentration of boron in the fluid (water), its
temperature, and the reaction time. Analysed boron concentrations vary therefore
not only between different depositional environments but also within similar
settings in response to variations in the above reaction parameters. Illite-rich
marine clays have been found to contain boron concentrations ranging between
100 and 200 ppm compared with 10 to 50 ppm in fresh-water clays (Degens 1965).
However, Spears (1971) believes that salinity is not the only controlling factor in
boron fixation, and suggests that in many cases boron is allochthonous, having
been transported into the mires with clay minerals from physiographically mature
hinterlands.
Because ofthe particular mode of its fixation in clay most palaeo-environmental
studies involving boron were targeted on coal measure shales and the clay-rich ash
concentrations of coal. However, from a comparison of the distribution of elements
in different density fraction of Newcastle coals from the Sydney Basin, Warbrooke
(1981) concluded that in addition to its fixation in clays, a large proportion of
boron is organically bound. Since its proportion is higher than can be attributed
to the boron content of the parent plants, it appears to be absorbed by the humic
degradation products during peat formation. This conclusion concurs with the
high boron content of 400 to 450 ppm analysed by Lyons et al. (1989c) in vitrinite
concentrates of marine-influenced coals from Illinois. These boron concentrations
far exceed those found by Bohor and Gluskoter (1973) in illites (200 ppm maximum)
from the Illinois Basin. Since the boron in vitrinite does not correlate positively
with either ash or silica, Lyons et al. (1989c) conclude that it is organically bound.
Apart from being a valuable palaeosalinity indicator in its own right, boron
has also been used in ratios with other elements, such as boron/lithium and
boron/gallium. However, none of this work was carried out on the coal itself but
either on interseam sediments (Degens et al. 1957; Ernst et al. 1958) or on sediments
unrelated to coal (Walker and Price 1963; Reynolds 1965).
5.5.1.3 Other Elements
In addition to the two classical elemental palaeosalinity indicators, sulphur and
boron, there are some other elements, which have been associated with marine
influence on coal. One of them is phosphorous which according to Table 5.7 has
a fairly low enrichment factor in sea water but has been found to be concentrated
in some marine influenced coals (Mackowsky 1968). In spite of its occurence as
244
Coal Facies and Depositional Environment
an essential elemental in living plants, phosphorous does not appear to be bound
to organic matter in coal (Brown and Swaine 1964), but occurs mainly as
fluor-apatite (Brown et al. 1959; Cook 1962; Durie and Schafer 1964; Kemezys
and Taylor 1964; Ward 1978; Corcoran 1979), although, as mentioned in
Chap. 4.4.2.4, Diessel (1961) reported hydroxyl-apatite and phosphorite together
with conodonts and possible fish teeth from the sapropelic upper portion of
Katharina Seam in the Ruhr Basin. Other phosphorous-bearing minerals, such as
goyazite, an aluminium phosphate of the crandallite group, have been reported
from Australian coals by Ward (1974, 1978, 1989) and form the U.S.A. by Palmer
and Wandless (1985).
Among the common carbonate forming metals, calcium and magnesium are
enriched in sea water, and iron in river water (Table 5.7). The frequent occurrence
of dolomite in marine and of siderite in fresh-water-influenced coals bears out this
relationship. However, the mineralogical distinction is less clear on the elemental
level, where marine influenced coals appear to contain more iron than fresh-water
coals because of their high pyrite concentration (A. Bailey 1981).
Another element, which has been mentioned as a possible palaeosalinity
indicator, is manganese (Spears 1964), which according to Table 5.7, shows a small
amount of enrichment in sea water. Warbrooke (1981) found manganese in
Australian coals to be equally distributed between organic and inorganic phases,
but none of the common manganese minerals have been found (Frazer and Kinson
1972). According to Brown and Swaine (1964), it occurs mainly in siderite, calcite
and dolomite, where it replaces iron and some of the alkaline earths.
Nitrogen has been associated with both increasing (Stach et al. 1982) and
decreasing (Suggate 1959) marine influence. Warbrooke (1981) found the nitrogen
content of Australian Sydney Basin coals to decrease with increasing departure
from marine conditions. However, in the same direction the coals changed from
predominantly bright to dull coals, which suggests that nitrogen, which commonly
occurs in organic bond, has a preference for vitrinite, rather than a particular
depositional environment.
Additional elements that have been found to be enriched in marine influenced
shales, include nickel, cobalt, vanadium and copper (Potter et al. 1963; Tourtelot
1964; Swaine 1967), although A. Bailey (1981) reports an enrichment in fresh-water
coals for the latter. According to Swaine (1967), lead also appears to be slightly
enriched in non-marine coal seams. Concentrations of these and other elements
are often overly dependent on the geochemical spectrum of the mire setting to be
universally useful as palaeosalinity indicators. Better palaeo-environmental resolution may be obtained by establishing ratios of several elements, as has been done
for both carbonaceous and non-carbonaceous sediments (Ernst 1970). When used a
ratio, even soluble elements, such as sodium and potassium may be useful indicators.
For example, in the Ostrava-Karvina Coalfields of Czechoslovakia Kessler et al.
(1967) found the Na/K ratio to show positive correlation with the degree of marine
influence.
Finally, the depletion in cerium has been mentioned as a possible palaeosalinity
indicator by Lyons et al. (1989c). These authors found particularly low Ce-contents
in vitrinite concentrates in marine influenced coals from the Illinois Basin, which
suggested to them leaching by sea water, which is depleted in cerium.
Geochemical Palaeo-Environmental Signatures
245
5.5.2 Organic Geochemical Characteristics
Although most vegetal constituents undergo profound changes during coalification,
some organic molecules found in coal can be traced back to their phytogenic
precursors, because they either remained unchanged or were altered only
insignificantly, mainly by loss of hydroxyl, carboxyl and other functional groups
leaving the carbon skeleton intact (Tissot and Welte 1978). For this reason they
have been variously tagged with such terms as "biological markers", "biomarkers",
"biological fossils", "geochemical fossils" or "chemofossils", because they provide
information about the phytogenic sources of the coalified biomass and thus about
the depositional environment. Seifert and Moldowan (1986) have listed the
following qualities demanded from a chemical compound before it can fulfill its
role as a biomarker or chemofossil:
1. It should have a high degree of specificity towards its source material;
2. it should be relatively abundant;
3. it should be detectable with precision in a complex mixture;
4. it should have a wide distribution in different basins with varying thermal
histories.
Most of the early studies of these compounds were directed towards the
identification of the biological progenitors and maturity levels of crude oil, but
over the last decade increasing use has been made of geochemical signatures in
palaeo-environmental studies of coal as well. This rapidly developing branch of
organic geochemistry has become so complex that only a brief overview can be
given in this chapter. Biomarkers that are either mainly found in crude oil or
merely indicate derivation from higher plants, which may be of interest in the
search for the biogenic precursors of oil and natural gas, are not considered here,
because detailed knowledge of the vegetal sources of humic coal can be obtained
much better from their spore and pollen content, as well as from other associated
plant fossils. Emphasis will therefore be on those chemofossils in coal which can
provide specific information about peat-forming environments. For a more detailed
treatment of the subject the reader is referred to the specialised literature or some
comparatively recent summaries provided by Brooks and Welte (1984), Philp
(1985), Johns (1986) and Yen and Moldowan (1988).
Geochemical biomarkers have two principal sources consisting either of
molecules synthesised by living plants, or of the metabolites generated by
saprophytes (bacteria and fungi) feeding on the plants after their death. Lipids,
carbohydrates, amino sugars and acids formed in this manner were found in peat
by Given and Dickinson (1973) and others, which gives testimony of the important
contribution made by microbes to the products of humification and subsequently
to coal. Because higher plants, which have been the main contributors to peat,
are relatively poor in lipids (Tissot and Welte 1978), concentrations of the latter
in coal correspond therefore to a high input of lipid-rich algal plant material, or
to enhanced bacterial activity during the peat stage. As has been variously referred
to, the latter activity is related to the biochemical milieu of the peat-producing
246
Coal Facies and Depositional Environment
environment and its depositional setting. Although concentrations of lipids in coal
are indicated by high fluorescence or can be detected in transmission electron
microscopy (Taylor and Liu 1987, 1989), their identification and correlation with
the phytogenic precursors responsible for their presence requires the application
of chemical methods.
The two kinds of chemofossils, i.e. source plant- or microbe-derived, differ in
their respective usefulness for the palaeo-environmental enquiry. In geologically
young deposits, source plant-derived biomarkers can be compared with those from
living representatives of the same genera or species, whose environmental preference
is known. With increasing age of the coal, fewer 'Plants with living representatives
will have contributed to the original peat, which increases the degree of uncertainty.
An additional problem arises from the higher degrees of thermal degradation
frequently suffered by the organic molecules contained in more ancient coals due
to the possibility of prolonged residence time at elevated temperatures. The range
of available chemofossils and their usefulness decrease therefore after the coal has
entered the physico-chemical stage of coalification.
The identification of the geochemically interesting organic constituents is
carried out by different methods. Non-destructive techniques: such as Fourier
transform infra-red spectroscopy (FTIRS), and various modifications of 13C
solid-state nuclear magnetic resonance spectroscopy (NMR) have been applied to
whole coal samples (Zilm et al. 1979; Barron and Wilson 1981; Russell and Barron
1984; Hatcher et al. 1989a, b), but destructive tests, in which a dissolved or pyrolised
extract of the coal is investigated, have been more commonly applied to the search
for biomarkers. Various techniques have been used, ranging from semi-quantitative,
relatively low-resolution Rock-Eval kerogen typing (Espitalie et al. 1977; Espitalie
1979) to gas chromatography (GC), mainly in combination with mass spectroscopy
(GC-MS), as discussed by McFadden (1973), Hayatsu et al. (1978), Littke and Ten
Haven (1989), and others.
Since the raw samples commonly consist of rather complex mixtures of organic
compounds, it is necessary to seperate the target constituents of the coal into
analytically manageable groups of related components without damaging them
beyond recognition. However, having survived coalification, the resilience of the
geochemically interesting biomarkers in coal is such that they can go through the
conventional extraction methods without much alteration (White et al. 1977; Philp
and Gilbert 1986). The fractionation is usually carried out by one of two methods:
The first one is by dynamic pyrolysis, of which different methods exist (Horsfield
1984), which are all based on the principle that a sample can be fractionated by
heating it under controlled conditions to a predetermined temperature range, e.g.
400 to 600 0c. A carrier gas is used in order to transport the pyrolysis gas to the
detector system employed in the analysis (Horsfield et al. 1989). Some recent
examples include pyrolysis-gas chromatography plus mass spectroscopy (Py-GS-MS)
by Philp et al. (1982), van de Meent et al. (1980), Larter and Senftle (1985), Philp
and Gilbert (1986), Hatcher et al. (1989a, b), Jin and Qin (1989), Bertrand (1989)
and others.
The second method of preparing samples for analysis employs a solvent
extraction process. A variety of methods has been applied to the fractionation and
247
Geochemical Palaeo-Environmental Signatures
different solvents have been used in the various solvent extraction processes, among
them dichlormethane, chloroform, acetone, methanol, either singly or in various
combinations. An example of the main steps and intermediate products in the
analysis of organic chemofossils is illustrated in Fig. 5.38. Following the analysis
procedure of Piittmann et al. (1989) and Hagemann et al. (1989), the pulverised
coal sample is first treated in a Soxhlet apparatus for 24 h with dichlormethane
(CH 2 CI 2 ). This separates the feed coal into a soluble fraction and an insoluble
residue. The yield of the soluble organics is dependent on both coal type and rank
and varies slightly in accordance wih regional differences. According to Radke et
al. (1980), maximum soluble yield is obtained from high volatile bituminous coals
between 0.9 to 1.0% random vitrinite reflectance or between 80 and 85% Carbon
(daf). The example from the Ruhr Basin illustrated in Fig. 5.39 supports this notion.
The soluble coal extract is fractionated by column chromatography using
pre-washed silica gel and with methanol (CH 3 0H) as an eluent for the heterocyclic
compounds, n-hexane for the saturated hydrocarbons and dichloromethane for
aromatic hydrocarbons. Subsequent analyses of the fractions were carried out as
above by GS and GS-MS methods, which have been variously described under
such terms as capillary gas liquid chromatography (LC), discussed by Henderson
et al. (1969), McFadden (1980), Piittmann et al. (1986), Dunham et al. (1988)
Hagemann et al. (1989) and others; or high-performance liquid chromatography
Pulverised Feed C02l1
Residue
I
COLUMN CHROMATOGRAPHY
with Silica gel followed by elution
with dichlormethane and methanol
I
1
Heterocyclic Compounds ~ porphyrins
Satur!lted Hydrocarbons
I
Alkanes ~ n-, iSo-, cyclo-paraffins;
J Aromatic Compounds
(fatty acids)
phenanthrene; nephthaline
Fig. 5.38. Flow chart showing the main steps and intermediate products in the ,analysis of
organic chemofossils. (After Hagemann et al. 1989 and Dehmer 1988)
..
r
Coal Facies and Depositional Environment
248
u
• •
•
18
~ 16
o
0> 14
--...
•
0>
E 12
c
...... 10
•
u
b 8
~
•
,.
6
<lJ
•
,,-
~
4
~
2~~~~--~--~--~--~
o
78
')
•
80
82
% Carbon (daf)
84
86
88
90
Fig. 5.39. Correlation between the proportion
of coal (in reference to total organic carbon)
soluble by Soxhlet extraction for 24 h in dichIormethane and coal rank (expressed as total
carbon) for a set of Ruhr coals. (Calculated
from data published by Hagemann et aI. 1989)
(HPLC), employed by Dark et al. (1977) and others for separating the tlUld coal
extract into molecular classes, followed by gas chromatography-mass spectroscopy-computer (GC-MS-C) techniques for molecular identification (Winkler 1986;
Piittmann et al. 1986; Philp and Gilbert 1986; Chaffee et al. 1986; Dehmer 1988;
Hagemann et al. 1989; and others). This may be preceded by the recently developed
tandem mass spectrometry (MS-MS), which appears to be a useful tool for the
preliminary screening of the target material (Philp and Johnston 1988; Quirke
et al. 1988).
5.5.2.1 Alkanes
Long-chain unsaturated aliphatic molecules (e.g. n-alkenes) are rather unstable
during coalification (Chaffee et al. 1986), although Meuzelaar et al. (1984) obtained
strong alkene peaks from Curie-point mass spectrometry ofliptinite concentrations.
Conversely, alekanes are often well preserved and have therefore been the subject
of intensive geochemical investigations. This group of saturated open-chain or
acyclic hydrocarbons constitutes the paraffin series, which is characterised by the
general formula C n H 2n + 2' The members of this group are termed saturated because
the carbon atoms are linked by single bonds only while the remaining valences
are taken up by hydrogen. A distinction is made between n-alkanes, the prefix
"n-" indicating the presence of "normal", unbranched carbon chains, and
iso-alkanes, in which the carbon chain is branched. Alkanes of low molecular
weight (n-C 1 to n-C 6 ) are gaseous at room temperature, they are liquid from n-C 7
to n-C 21 , and solid from n-C 22 to n-C 62 . The alternative term paraffin implies
that these compounds are relatively inert and are therefore able to withstand
postdepositional degradation better than many other organic molecules, although
they, too, are subjected to thermal cracking during advanced coalification.
The proportion of n-alkanes in coal t:xtracts is usually less than 1% (Allan
and Douglas 1977; Bartle et al. 1978; Radke et al. 1980) and it shows negative
correlation with tissue preservation (Hagemann and Hollerbach 1979). In spite of
their small proportion, n-alkanes are of geochemical interest, particularly those
Geochemical Palaeo-Environmental Signatures
249
which have been synthesised by plants, for example, in cuticular waxes, spore and
pollen exines, and seeds. According to Koons et al. (1965) these range in carbon
number from n-C 7 (heptane) to n-C 62 (dohexacontane), whereby each plant and
group of plants are characterised by a limited number of alkane species, although
there is much overlap between different groups. Algae, for example, have a relatively
narrow range from n-C 14 to n-C 32 , among which n-C 15 and n-C17 are dominant,
whereas terrestrial higher plants show a wider range between n-ClO and n-C 40
with strong dominance of.n-C 23 , n-C 2S ' n-C 27 , n-C 29 , n-C 31 , n-C 33 and n-C 3S
(Tissot and Welte 1978). Spectra of the distribution of n-alkane species are obtained
by gas chromatographic analysis of solvent extracts from plants, peats and coal.
As in the two examples above, they frequently demonstrate a clear preference of
many plant groups for n-alkane molecules with odd-numbered carbon atoms,
referred to as OEP (= odd-over-even predominance) by Chaffee et al. (1986).
According to Bray and Evans (1961), this can be expressed numerically as the
carbon preference index (CPI), which is the mass ratio of odd to even molecules
within the range:
CPI = ,![C 25 +C 27 +C 29 +C 31 +C 33 + C 2S + C 27 + C 29 + C 31 + C 33
2 C 24 +C 26 +C 28 +C 30 +C 32
C 26 + C 28 + C 30 + C 32 + C 34
J.
(5.11)
Alternative but basically similar ratios have been defined by Philippi (1965), Scalan
and Smith (1970), and Allan and Douglas (1977). Since in contrast to terrestrial
vegetation, algae and other marine plants have little odd-carbon number preference
above n-C 18 (Koons et al. 1965) CPI values are commonly small in extracts
obtained from marine sediments (Powell and McKirdy 1973) compared with those
found in peat and coal (Welte 1967; Leythaeuser and Welte 1969; Brooks 1970).
The latter, having been derived from terrestrial plants, not only contain a higher
proportion of total n-alkanes than algal matter, but most of them are of high
molecular weight with a preference for odd-carbon numbers. Microbial activity
on the one hand destroys the n-alkanes contained in the source plants, but also
adds new ones to the degraded biomass from the bacteria-generated lipids, particularly in the form of long chain paraffins without any obvious odd/even predominance (Tissot and Welte 1978).
Care should be exercised in the palaeo-environmental interpretation of alkanes,
since oxidative biodegradation reduces not only the total amount of n-alkanes in
the peat (Dehmer 1988), but also the proportion of odd-numbered species can be
increased during humification by the stripping of functional groups (e.g. COOH)
from even-numbered alcohol-derived acids (by oxidation), fatty acids and esters.
Conversely, in strongly reducing environments the reduction of alcohols and acids
may increase the proportion of even-numbered n-alkanes (Tissot and Welte 1978).
Much of this reduction appears to take place in the catatelm during diagenesis
and is independent of the conditions prevailing at the peat surface (acrotelm). This
is based on Winkler's (1986) observation of a fairly even proportion of C 16 -, C 18 -,
and C 20 - n-alkanes in all brown coal lithotypes analysed by her, which she
Coal Facies and Depositional Environment
250
1.3
•
1.25
1.2
1.15
•
•
1.1
1.05
•
.95
•
r =0.765
Y =-0.02x + 2.96
f:J .9
•
•
•• •
•
•
.85+--~-~-~~~~--~
78
80
82
84
% Cllrbon (dllf)
86
88
90
92
Fig. 5.40. Correlation between the carbon
preference index (CPI) and coal rank (expressed as total carbon) for a set of Ruhr coals.
(Calculated from data published by Hagemann
et al. 1989)
interprets as late stage reduction by anaeorobic bacteria either of even-numbered
b-fatty acids or of suberin from deep-seated roots.
Another factor which influences the carbon preference index is coal rank. The
work of Radke et al. (1980) has shown that the maximum yield of n-alkanes
coincides with the solubility maximum in high volatile bituminous coal referred
to above and illustrated in Fig. 5.39. These and other authors (e.g. Moldowan
et al. 1985) also found a decrease with increasing coal rank in the proportion of
high-molecular weight alkanes and the carbon perference index. An example of
the latter trend is illustrated in Fig. 5.40.
As mentioned above, in addition to the normal (n-) alkanes there are other
alkane groups, in which the chains are branched. These iso-alkanes (from isomeric)
of the 2-methyl and the anteiso-alkanes of the 3-methyl series are associated with
n-alkanes and, although less frequent, follow a similar pattern of occurrence but
with a tendency toward an even-number preference (Chaffee et al. 1986). Saturated
paraffins in which the carbon chain is closed to form a single ring structure are
called cyclo-alkanes or cyclo-paraffins. They follow the general formula C nH2",
and as in the normal alkanes, the number of carbon atoms in the molecule determines
the name of the compound after the prefix cyclo- (Degens 1965). They appear to
have been targeted more for geochemical research of petroleum and oil source
rocks than for coal environments.
5.5.2.2 Fatty Acids
The lipids of plants and animals consist largely of oils, waxes and fats (= esters
of glycerol), which upon hydrolysis yield chain-like fatty acids or n-aliphatic
monocarboxylic acids with the general formula C nH 2n + 1 COOH. The low carbon
members of the group are colourless, water-soluble, corrosive liquids (e.g. acetic
acid = CH 3 COOH) having a pungent smell, those with intermediate carbon
numbers (e.g. butyric acid = CH3[CH2J2COOH) form slightly water-soluble,
Geochemical Palaeo-Environmental Signatures
251
greasy fluids with a strong sweaty smell, whereas from C lO onwards they lose their
smell and they become solids, which are insoluble in water but soluble in alcohol
and some other organic solvents.
In this context only the high-carbon numbered fatty acids are of interest. They
show some similarity to n-alkane, which, as has been mentioned above, can be
derived from fatty acids by decarboxylation during humification. The reason for this
affinity is that fatty acids can be thought of as having been formed from paraffins
by the substitution of one hydrogen atom by one carboxyl group as:
(5.12)
While the influence of the carboxyl group on the properties of fatty acids with
low carbon numbers is strong (pungent smell etc.), its effect weakens with increasing
carbon number, when the rest of the molecule grows into a long chain of increasing
molecular weight, which apart from the one attached carboxyl behaves much like
a normal alkane. As in the latter, the molecular chains may be normal (n-fatty
acids) or branched (= iso-fatty acids). Even-numbered n-fatty acids ranging from
C 24 to C 32 appear to be concentrated by terrestrial plants in limnic environment
and have been found in the montan wax fraction of brown coals from the Gippsland
Basin in Victoria, Australia (Brooks and Smith 1969) and Czechoslovakia (Wollrab
and Streibl 1969). Lower molecular weight n-fatty acids ranging from C I4 to e 22
with preference for C 16 , CIS and (occasionally) C 22 have probably been produced
by algae of marine origin (Tissot and Welte 1978). Chaffee and Johns (1985) observed
n-fatty acids up to the rank of high volatile bituminous coal, although at much
reduced chain length and with less pronounced even-carbon number preference
(Pederson and Lam 1975; Lam and Pederson 1978).
5.5.2.3 Isoprenoids
The two geochemicaIIy important members of this group are terpenes and steroids.
They are common plant products consisting of multiples of isoprene, which has
the general formula (CsHS)n. The value of n forms the basis of the classification
and nomenclature of terpenes, which constitute such varied plant products as
essential oils, rubber, vitamins and pigments. The molecular structure of isoprene
is shown below:
Isoprene:
(5.13)
Because of its two double bonds, the molecule is quite reactive and capable of
polymerising into both chain and cyclic isoprenoids. Two isoprene molecules
combine to form terpene or monoterpene, "mono"-because the basic molecule can
be looked upon as representing a "hemiterpene" CsHs. Monoterpenes are volatile
substances and are associated with essential oils.
252
Coal Facies and Depositional Environment
Three isoprenes form sesquiterpene (C 1S )' which occurs in cyclic and chain-like
(acyclic) configuration. They are widespread components of plant resins and
essential oils (Simoneit 1986). They have been found in low rank coals associated
with resins (Streibl and Herout 1969; Douglas and Grantham 1974), predominantly
of gymnosperm origin for which they can be regarded as biomarkers (Grantham
and Douglas 1980).
Isoprenoids with four isoprenes constitute the various modifications of
diterpene (C 20 ). Cyclic structure with two or three rings are common, for example,
in gymnosperm resins (Thomas 1969; Hollerbach 1980; Grantham and Douglas
1980; Noble et al. 1985). Of greater biological and geochemical significance are
acyclic diterpenes, which are based on the phytol nucleus. This is an important
constituent of chlorophyll-a, where it forms a side chain (-CO-O-phytyl).
attached to the main molecule. Although bacterial sources of some isoprenoids
have been found (Volkman and Maxwell 1986), the quantitative contribution of
chlorophyll-based phytol to coal probably outweighs any bacterial input.
Depending on either transient oxidising or reducing depositional condition (Welte
and Waples 1973), phytol is thought to be transformed into either pristane (C 19 )
by phytol oxidation and decarboxylation or into phytane (C 20 ) by dehydration
and hydrogenation (Brooks et al. 1969; Powell and McKirdy 1973; Murchison
1987). Both products constitute the most frequent isoprenoid hydrocarbons found
in coal (White et al. 1977). According to Powell and McKirdy (1973) and Didyk
et al. (1978), reducing, anoxic depositional environments are characterised by
(pristane/phytane) < 1, whereas ratios exceeding unity are typical for oxic conditions.
In support of this notion the pristane/phytane ratio has been correlated with the
tissue preservation index and the detrovitrinite content, respectively. The results,
compiled for high volatile bituminous coals (random vitrinite reflectance between
0.78 and 0.97%) from the Ruhr and Saar Basins, are illustrated in Fig. 5.41A and
•
A
12
•
10
•
8
•
•
•
•
Q)
•
c
...,'" 6
• •• •
>.r::
0..
Q:;
•
4
c
BIJl
T:
r = 0.701
Y = -4.1 x + 11.9
2
•
B
•
•
••
••
•
•
•
. •••
•• •
r = 0.706
Y = 0.2x - 2.7
0..
.2
.4
TPI
.6
.8
1 1.21.4 1.6 1.8 2 2.22.425 30 35 40 45 50 55
60
65 70
75
Oetrovitrinite + Liptinite
Fig. 5.41 A, B. Correlation between the pristane/phytane ratio and tissue preservation index (TP/)
in A and with detrovitrinite (desmocoIIinite) + Iiptinite in B for high volatile bituminous coal
ranging in random vitrinite reflectance from 0.78 to 0.97% from the Ruhr and Saar Basins in
Germany. (Calculated from data published by Hagemann et al. 1989)
253
Geochemical Palaeo-Environmental Signatures
B. An inverse relationship exists between the pristane/phytane ratio and TPI,
partly because of the negative correlation with telo-inertinite (fusinite and
semifusinite) which is part of the tissue preservation index and indicates occasional
dehydration and oxidising conditions, including fire, during peat formation.
Conversely, a high detrovitrinite content suggests tissue destruction in the acrotelm
followed by prolonged residence under anoxic conditions in the catotelm, with
which the pristane/phytane ratio correlates quite well, particularly, after liptinite
had been added to the diagram in Fig. 5.41B.
The relatively high pristane/phytane ratio of 1 to 3 in brown and subbituminous coals and from 4 to 10.2 in high volatile bituminous coals (Brooks et al.
1969; Brooks 1970; Connan 1974; Powell and McKirdy 1975; Radke et al. 1980)
can therefore be explained by the initially oxidising nature ofhumification, followed
by a further increase of the ratio with coal rank, which is due to the generation of
pristane during coalification by the decarboxylation of phytanic acid (Tissot and
Welte 1978). According to Brooks et al. (1969) and Radke et al. (1980), the pristane/
phytane ratio increases during coalification until it peaks at the rank of high
volatile bituminous coal between 83 and 85% carbon (daf). Further coalification
beyond this rank appears to initiate thermal degradation and aromatisation
(Hayatsu et al. 1979), resulting in a reduction of the pristane/phytane ratio in
higher rank coals. An illustration of this trend based on Hagemann et al.'s (1989)
analyses of Ruhr coals is illustrated in Fig. 5.42. In comparison to Fig. 5.39 it is
interesting to note that the peaks for both pristane/phytane ratio and carbon
solubility occur at the same coal rank.
Six isoprene molecules combine to triterpenoid (C 30) mainly of pentacyclic
configuration. According to Tissot and Welte (1978), they consist either of
6-membered rings only or of four 6-membered rings and one 5-membered ring.
The corresponding hydrocarbons are called triterpanes (e.g. hopane) when
saturated, or triterpene, when unsaturated. The latter may become aromatic, when
one or several rings acquire three conjugated double bonds.
Among the many pentacyclic isomers, many of which are common plant
constituents, the hopanoids are of particular interest for their high concentration
14
12
10
8
<l!
Fig. 5.42. Correlation between pristanejphytane
ratios and coal rank
(expressed as total carbon)
for a set of Ruhr coals.
(Calculated from data published by Hagemann et al.
1989)
~
....,
6
0..
"-
4
>-.c
i
G
2
(J)
C
0..
...
.---
,.~
<l!
C
.~
2
o~--~----~--------~----~--------~
78
80
82
84
86
88
90
92
% Carbon (daf)
254
Coal Facies and Depositional Environment
in bacteria and blue-green algae (Rohmer and Ourisson 1976). Triterpenic hopanes
form three isomeric series consisting firstly, of (17a, 21/3) H-( = homo-) hopane,
commonly shortened to a/3 hopane, secondly of (17/3, 21a) H-hopane or /3a hopane
and thirdly, of (17/3, 21/3) H-hopane=/3/3 hopane (van Dorsselaer 1975; Ourisson
et al. 1979). Chaffee (1981) and Johns et al. (1984) have found significant differences
in the hopanoid composition between different brown coal lithotypes in Victoria,
Australia. Their findings have been supported by Hagemann and Hollerbach (1980),
Winkler (1986) and Dehmer (1988), who used the high proportion of a/3 hopane
and hop-17 (21 )-ene, an unsaturated pentacyclic triterpene, found in dark peat and
brown coal lithotypes to associate their formation with strong bacterial activity
under mildly acid conditions in contrast to the paucity of these triterpanes in the
light lithotypes. As mentioned before, this was interpreted as the result of a low
level of microbial activity due to increased acidity. Further geochemical evidence
for acid conditions during the formation of pale lithotypes is seen by Dehmer
(I 989b) in the presence of tetra cyclic steroids, which are of similar composition to
triterpenes and can be derived from them by loss of methyl groups.
Steroids are a complex group of polycyclic hydro-aromatic compounds which
contain a perhydrocylcopentan-phenanthrene nucleus and are common constituents of animal and plant tissues, for example algae. Some are alcohols and are
referred to as sterols, while the corresponding saturated and unsaturated hydrocarbons are called steranes and sterenes, respectively. Polyunsaturated sterenes
may become aromatic compounds, their names depending on the number of aromatic
rings in the molecule, e.g. monoaromatic steroid. Up to 30 different steranes have
been identified in some coals (Gu et al. 1988) with varying palaeo-environmental
significance. Earlier, Huang and Meinschein (1978, 1979) had established a threefold
palaeo-environmental relationship between C 27 , C 28 and C 29 sterols allocating
them to marine planktonic, lacustrine and terrestrial (higher plants) sources,
respectively. The derivation of C 29 sterols from land plants has been supported
by Hoffmann et al. (1984) and Philp and Gilbert (1986), while the specificity
of the two other sterols has been disputed (Volkman 1986). Because steranes
are not widely distributed in bacteria (Ensminger et al. 1973), their presence in
peat and low rank coal is taken as an indication of reduced bacterial activity in
spite of their frequent occurrence in marine-derived organic matter.
Another geochemically interesting group of pentacyclic triterpenes consists of
the oleannane, ursane, lupane and related families, including the polyaromatic
picene referred to in Chap. 5.4.1, which have been derived from squalene, a
polyunsaturated isoprenoid with the general formula (C30HSO)' This group is of
evolutionary interest, because of its apparent restriction to angiosperms as source
plants. Its occurrence in coal is therefore confined to post-Upper Cretaceous
deposits (Chaffee et al. 1986).
Tetraterpenes (C 40 ) consist of eight isoprenes, which combine to form a large
number of isomeric compounds. They comprise many plant pigments, such as
chlorophyll (green) or the carotenoids, which form red and yellow pigments in the
fruits of many plants (e.g. carrots, tomatoes) as well as in many algae and in spore
exines (Brooks 1971; Shaw 1971). Their molecular preservation during coalification
is poor, but unsaturated derivatives persist into high volatile bituminous coalifica-
Geochemical Palaeo-Environmental Signatures
255
tion causing high fluorescence intensities (van Gijzel1971). Higher molecular weight
polyterpenes have been found in plant gums (e.g. caoutchouc and gutta) and in
spore and pollen membranes (Brooks and Shaw 1972), which are well preserved
during coalification and form important constituents of land-derived vegetation.
5.5.2.4 Heterocyclic Compounds
This group is characterised by ring systems, which contain other elements beside
carbon. Sizes range from three to ten members per ring, but most frequent are
five- and six-membered heterocycles, the latter often of aromatic derivation.
Elements substituting for ring carbons are called hetero-atoms, among which
oxygen, nitrogen and sulphur are of particular importance. Porphyrins constitute
a biologically significant group of heterocyclic compounds, which occupies a special
place in organic geochemistry, because it was the first group of organic compounds,
where a link between biological source and fossil product was established by Treibs
(1934a, b, 1935a, b, 1936). Their structure is based on the five-membered ring of
the pyrrole molecule, which has the following structure.
HC~CH
Pyrrole:
II
II
HC~CH.
\/
(5.14)
NH
Four pyrrole nuclei linked by me thine (=CH--) groups form double bonded
porphin, from which a variety of biologically important members of the pyrrole
group, such as chlorophyll (complexed with Mg) and hemoglobin (complexed with
Fe), are obtained by substitution. On account of its conjugate double bonds
hemoglobin is a fully aromatised porphyrin, while chlorophyll, having one double
bond less, belongs to the subclass phorbide (Zelmer et al. 1988). Fossil porphyrins
appear to have been derived mainly from chlorophylls by phorbide to porphyrin
aromatisation. Different plants produce different kinds of chlorophylls, which differ
slightly in their molecular architecture and are designated chlorophyll-a, -b, -c etc.,
each of which can be divided into further isomeric forms. For a detailed discussion
of this highly complex subject the reader is referred to Baker and Louda
(1986).
Many fossil porphyrin varieties are known, among which the two most
common series are the desoxophylloerythroetioporphyrins, or DPEP-series, with
an isocyclic ring in addition to the tetrapyrrolic nucleus and mesoetioporphyrins,
or ETIO-series, without the isocyclic ring (Tissot and Welte 1978). Both series
have been found by Treibs (1935a, b) to occur in alginite-rich sapropelic coals, but
porphyrins found in humic coal consist predominantly of the latter series where
it is confined to members below C 32 (Palmer et al. 1982; Baker and Louda 1986)
and is commonly complexed with gallium, iron and manganese (Bonnett and
Czechowski 1980, 1981; Bonnett et al. 1984). In Chinese brown coals (Lower
Tertiary) form the Fushun Basin, Yang and Cheng (1988) found a DPEP/ETIO
256
Coql Facies and Depositional Environment
ratio of 0.3 (C 32 -ETIO being most abundant in a range C 2S -C 36 ) compared with
4.5 for oil shales (C 32 -DPEP being most abundant) and between 2.2 and 3.4 for
crude oils from various Chinese basins, with C 31 - D PEP being the most abundant
free porphyrin. Higher rank Cretaceous coals form the Uinta region, Colorado,
have yielded even lower DPEP/ETIO ratios of 0.2 for a subbituminous and 0.03
for a high volatile bituminous coal, respectively (Gu et al. 1988), which suggests that
DPEP-porphyrins are preferentially degraded during coalification.
Several chlorophyll derivatives, such as pheophytin-a from vascular plants
and bacteriopheophytin from bacteria, have been discovered in Recent peat
deposits by Sanger and Gorham (1973), Pratt and Gorham (1977), Hatcher et al.
(1977) and Palmer et al. (1982). Because of its biosynthesis under reduced
conditions, bacteriopheophytin is considered to have a better preservation
potential than pheophytin-a, which tends to suffer oxidative biodegradation in
the acrotelm before reaching the catotelm. Simoneit (1978) found porphyrins to
correlate with low pristane/phytane ratios, which would suggests an anoxic depositional environment (Volkman and Maxwell 1986), but it is not clear whether they
consisted of bacteriaopheophytin.
5.5.2.5 Aromatic Compounds
Although coal, particularly vitrinite and inertinite (Meuzelaar et al. 1984), contains
large amounts of aromatic carbon (see discussion of aromaticity in Chap. 3) only
a small proportion of this is extracted by conventional organic solvents. Some of
the soluble aromatics have already been referred to above as having been derived
from the aromatisation of unsaturated polycylic isoprenoids and heterocompounds. The reason for the poor solubility of much of the aromatic carbon
in coal is its concentration in condensed multi-layered ring systems (Fig. 3.32)
which become increasingly cross-linked and ordered during coalification and
acquire graphitoid crystallinity. Solubility is therefore restricted mainly to the
mobile intermicellar phase and to the plate edges of the aromatic stacks, where
they are flanked by functional groups and linked by hydroaromatic and methylene
bridges (Tissot and Welte 1978).
In contrast to the large number of ring structures present in the macromolecular
framework left behind in the insoluble residue, the mobile phase aromatic nuclei
found in coal extracts consist of only a small number of condensed benzene rings
per molecule. According to Chaffee et al. (1986), the most common isomeric species
range from mono- to hexa-aromatic, many of them probably formed by the
aromatisation of cyclic isoprenoids. Others, including the phenols found in peat
and brown coal extracts have probably been derived directly from remnant lignin
and tannin nuclei inherited from the vegetal progenitors (Fischer and Schrader
1922). Lignin in particular is a common constituent in the walls of wood and some
other plant cell (including those of Sphagnum and other mosses), where it is packed
between the bundled cellulose fibrils for the purpose of strengthening the tissue.
Chemically, a phenol can be regarded as hydroxybenzene, i.e. it consists of one
or several benzene rings to which OH- groups have been attached. Lignin can
be thought of as high molecular weight polyphenols, which are synthetised in
Geochemical Palaeo-Environmental Signatures
257
plants by dehydration and condensation of aromatic alcohols (Tissot and Welte
1978). Reference was made in Chap. 5.3.1 to the loss in OH- from catchol-like
structures during coalification, which, according to Hatcher et al. (1989a), resulted
in an increasing concentration of phenols in gymnosperm wood up to subbituminous
coal rank. Further dehydroxylation during subsequent coalification to bituminous
coal rank will affect the phenols with the result that their proportion decreases
with increasing coal rank, as shown by the decline in phenol concentration in the
pyrolysate obtained from flash pyrolysis of vitrinite (van Graas et al. 1981; Allan
and Larter 1983). Since Yarzab et al. (1979) had previously found a linear relationship
between the rate of dehydroxylation and increasing coal rank, it is not surprising
that Senftle et al. (1986) and Mrazikova et al. (1986) found the relationship between
coal rank and reduction in phenol extraction to be linear, too. According to Larter
(1989) the declining phenol yield of vitrinite pyrolysis as a function of vitrinite
reflectance can be expressed as:
Phenol concentration = (159-Romv)/0.19
r=0.949.
(5.15)
Phenol concentration refers to the proportion of phenols in the pyrolysate. It is
measured in standardised units, whereby one unit corresponds approximately to
0.25% by weight of the original sample. Romv stands for mean maximum vitrinite
reflectance in oil.
Specific correlations between biological source compounds and their geochemical derivatives have been established by Cheng (1988), who found aromatic extracts
from Cretaceous to Tertiary humic source rocks from China to be rich in phenanthrene, pimanthrene, retene, tetrahydroretene and chrysene. Conversely, sapropelic
source rocks proved to be high in alkylphenanthrene and alkylnaphthalenes.
Assuming that pimanthrene has been derived from diterpenes formed in conifer
resin, he regards the ratio of pimanthrene/total alkylphenanthrenes (Pi/LP) as a
relative measure of the contribution made by conifers to the biomass. In applying
the ratio to his humic and sapropelic source rocks, Cheng (1988) obtained
consistently higher values from the former. The same applies to retene/total
alkylphenanthrene (Re/LP). Retene, an aromatic hydrocarbon derived from
tricyclic diterpenoids via abietic acid (Simoneit 1977), is also regarded as a
derivative of higher plants thus resulting in low values for the Re/LP ratio in
sapropelites, while those obtained from humic source rocks are generally higher
but not as consistently so as the Pi/LP ratios.
Ratios of aromatic compounds have been applied to coalification studies by
Hagemann et al. (1989), who correlated C 4 -naphthalene/phenanthrene ratios with
coal rank in the Ruhr and Saar Basins of Germany and, apart from regional
difference, found a general decrease of the ratio with increasing coalification.
5.5.2.6 Amino Acids
Most coals contain one or two percent of nitrogen, which, at least partly, have
been derived from amino acids, the building blocks of proteins. They are organic
258
Coal Facies and Depositional Environment
acids in which hydrogen has been replaced by one or several NH 2-groups. Amino
acids account for most of the nitrogen found in all organisms (Tissot and Welte
1978), but Casagrande and Given's (1980) investigation of the distribution of these
compounds in Recent marine influenced peat deposits in Florida, and the study
of McCarthy et al. (1989) of the downstream changes in the elemental peat
composition in the Okavango Delta revealed fungi or bacteria to be their main
sources. According to Chaffee et al. (1986), high nitrogen levels suggest extensive
microbial reworking of the peat-forming vegetation, which was also suggested by
Bajor's (1960) correlation of high concentrations of amino acids with marine
influenced portions of the Tertiary brown coals of the Lower Rhine brown coals
from near Cologne, Germany. With increasing rank the proportion of free amino
acids declines sharply due to thermal degradation.
5.6 Epiclastic Minerals and Palaeo-Environments
The minerals referred to in previous chapters as being useful indicators of particular
palaeo-environments were exclusively authigenic in origin and mostly syngenetic
in reference to peat accumulation. However, a consideration of epiclastic detrital
minerals contained in coal can likewise provide useful information about the nature
and setting of the mire.
Apart from indicating a particular depositional environment, epiclastic
minerals have been used to delineate the water catchment and drainage pattern
of former swamps. An example of this, based on the Hoskissons Seam from the
Gunnedah Basin in New South Wales, is illustrated in Fig. 5.43 and Fig. 5.44. In
Fig. 5.43A the isopachs of the Hoskissons Seam are illustrated, which indicate that
over much of the central portion of the Gunnedah Basin the seam is between 5
and 8 m thick but increases in thickness to 18 m in the north and 16 m in the southeast. Total ash content in percent is illustrated in Fig. 5.43B, which reveals very high
ash figures in areas of reduced seam thickness. A comparison with Fig. 5.43C, which
illustrates the distribution of stone bands as percentage of their aggregate thickness
in relation to total seam thickness, suggests that a large portion of the ash is
concentrated in these bands. According to Hamilton (1985), the areas in which peat
accumulation has been repeatedly interrupted by deposition of clastic sediments,
are loci of accelerated subsidence due to differential compaction of the underlying
platform. However, the ash content remains relatively high also where fewer stone
bands occure in the seam. This is evidenced by the distribution of the disseminated
ash illustrated in Fig. 5.43D. This ash content is obtained by washing the coal at
1.60 density, which effectively removes high mineral concentrations, such as stone
bands, without any significant loss in coal. The 1.60 float fraction in Fig. 5.43D
refers therefore only to the ash which is finely disseminated in the coal, but this ash
also remains quite high.
The source of the ash in the Hoskissons Seam has been investigated in reference
to the molar Si0 2/ Al 20 3 ratio of the 1.60 float fraction (Tadros 1988b). This
Epic1astic Minerals and Palaeo-Environments
259
--~j.'
ir- :::-8
~,
""--,
-IYONIl
1"
NEW E"GlAIrIID
FOL.D BEL T
",
'",
30
Fig. 5.43 A-D. Four maps illustrating coal properties of the Hoskissons Seam in the Gunnedah
Basin of New South Wales. A Seam isopachs in metres. B Total ash in percent. C Aggregate
thickness of stone bands in percent of total seam thickness. D Ash content (in percent) of the
1.60 density float fraction. (After Tadros 1988a)
260
Coal Facies and Depositional Environment
'0
SoCAU
10
30
.0 ....
Fig. 5.44 A-D. Four maps illustrating coal properties of the Hoskissons Seam and its roof strata
in the Gunnedah Basin of New South Wales. A Regional variation of the molar Si0 2 /AI 2 0 3
ratio of the 1.60 density float fraction. B Isopachs of the upper Black Jack Formation. C Isolith
map of the aggregate thickness (in metres) of quartzose sandstones derived from the western
foreland. D Isolith map of the aggregate thickness (in metres) of conglomerates derived from the
eastern New England Fold Belt. (After Tadros 1988a, b)
Summary of Palaeo-Environmental Indicators
261
technique had been originally employed by Shibaoka (1972), who found proportionality between the ratio and the percentages of quartz and clay in the coal. The
original variation in the silica/alumina ratio illustrated in Fig. 5.44A shows a strong
increase in the western portion of the Gunnedah Basin, while low values prevail in
the east. This distribution reflects well the changing influence of the main sediment
source areas at the time the Hoskissons Seam was formed. The Permian sediments
of the Gunnedah Basin wedge out to the west against an old foreland from which
predominantly mature quartz-rich sediments have been derived, whereas to the
east the New England Fold Belt was rising at the time shedding immature and,
to a large extent, volcaniclastic sediments into its foredeep. Figure 5.44B is an isopach
map of the upper part of the Black Jack Formation, i.e. it comprises the remainder
of the coal measures above the Hoskissons Seam. Fig. 5.44C is an isolith map (in
metres) of the western sandstones from this section, while Fig. 5.44D is an isolith
map of conglomerates from the eastern source.
There are many examples of this kind in the coal geological literature, which
reveal the close links between the composition of coal ash and the geological
setting of coalifields. The matter will be further discussed in Chap. 9.
5.7 Summary of Palaeo-Environmental Indicators
The preceding discussion has shown that there is a considerable variety of coal
properties which carry palaeo-environmental signatures. Individually, their
analytical resolution is mostly too low to lead to a specific depositional setting, but
when several such indicators are combined to form a composite response with
good internal consistency, they constitute a powerful tool in palaeo-environmental
analysis. The selection of coal properties for this purpose is usually constrained by
budgetary limitations and the practicalities of exploration, mining and coal testing,
which often disallow the application of either expensive or time consuming analysis
procedures. With few exception the short list of coal facies indices with palaeoenvironmental significance presented in Table 5.11 is therefore restricted to those
properties which are either routinely tested for or can be analysed within most
coal research establishments. Naturally, the tabulation of the influencing factors
on coal formation conveys a false image of simple relationships. It is stressed
therefore that the table is not more than an incomplete list of properties, which
are the result of a complex interaction of various agencies and hitherto not well
understood processes.
A distinction is made in Table 5.11 between five settings of bituminous humic
coal formation. Three of these refer to topogenous mires and two to ombrogenous
raised bogs. Both settings are further subdivided on the basis of moisture availability,
hydrogen ion concentration, nutrient supply and bacterial activity. In the first two
columns a continuously wet, telmatic topogenous setting is assumed due to a mostly
high groundwater table under a minerotrophic or rheotrophic regime. The main
difference is in the hydrogen ion concentration, which is low in the freshwater
environment inferred in Column 1, but near neutral or weakly alkaline under the
Rare
5-10
Common
Coal characteristics:
% Coal ash (excl.
stone bands):
Stone bands:
% Sulphur content:
VM yield:
Atomic HIC
Vitri. reflectance:
Vitri. fluorescence:
TPI (high rate
of subsidence):
TPI (low rate
of subsidence):
GI:
Hopanoids
<1
Low
Mostly low
Moderate
Low
High
High
High
Moderate
Low
Low
Average-high
Average-low
Moderate
Moderate to high
Moderate
Rare pyrite
Low
High framb.
pyrite
High
High
Low
High
Moderate
>2
5-20
Common
Moderate-low
Moderate
High-moderate
4-6
Variable
watertable
3
Moderate
Moderate
pyrite
Average
Average
Average
Average
High
<2
5-20
High-moderate
Moderate
Moderate
Nutrient supply:
Bacterial activity:
Fungal activity:
a34 s:
% Syngenetic FeS 2 :
High
High
Low
4-7
pH
6-8
High
watertable
2
Topotelmites
I
High
watertable
Peat
characteristics
Table 5.11. The conditions of peat accumulation as inferred from the properties of humic bituminous coal
Low
Low
High
Average-low
Moderate to low
Low
Low
Moderate
Low
High
Low
High
Rare
<0.5
<5
Very rare
Low
Low-moderate
High
3-5
lntermittently dry
5
High
Some
marcasite
High
Average-high
Average
Average
Moderate
<0.5
<3
Very rare
Low
Low
High
3-5
Continuously wet
Ombrotelmites
4
0-
a'"
a
m
::s
::.
0"'
::s
e:..
::s
g.
f!l.
"0
'"0
0
0-
::s
~
'on"
O.
~
"TJ
n
0
e:..
N
N
Summary of Palaeo-Environmental Indicators
263
brackish to marine mire setting assumed in Column 2. The variable position of
the water table indicated in Column 3 applies again to a fresh-water environment,
for example a flood plain, which is more frequently subjected to dry periods, than
the flood basin of Column 1. Due to the proneness to flooding of these three
settings there is ample nutrient supply (eutrophy), particularly in the nearshore
environments of Column 2, but in some cases flood waters might have lost much
of their nutrient supply before reaching distal swamps (mesotrophy). Similar
mesotrophic conditions will ensue in mire settings transitional to the raised bogs
inferred in Columns 4 and 5. The first (Column 4) is probably most typical of a
tropical humid to perhumid climate, such as the raised bogs of Southeast Asia,
but any climate which prevents excessive drying of the peat surface would fit the
listed conditions. In contrast, Column 5 depicts a raised bog subjected to pronounced seasonal changes, which include a dry period. Although these differences
lead to different peat and coal types, all ombrotelmites are formed under rather
acid conditions and low nutrient supply. The various mire settings have a strong
influence on bacterial activity, which increases with the pH of the peat water.
Due to the frequency of flooding of rheotrophic mires, coals formed from
topotelmites are characterised by relatively high ash contents, although the actual
percentages on washed coal (density = 1.5 to 1.6 g/cm 3 to exclude stone bands but
retain high recovery) vary over a wide range. Apart from the climatic influence the
latter depends on the size and elevation of the river catchment draining into the
swamp, as well as on the proximity of flood-prone feeder channels to the sites of
peat accumulation. The highest ash contents of 20% are recorded when the position
of the water table fluctuates, because the increase in inherent ash due to the loss
of biomass is added to adventitious minerals deposited during flooding. Higher
ash figures have been reported, but they are excluded because Table 5.11 refers only
to humic coals. The respective ash percentages for ombrotelmite-derived coals are
considerably smaller, because their main source is the inorganic matter contained
in the phytogenic progenitors of the coal. Those formed in continuously wet
raised bogs, where the loss of biomass is lowest, have the smallest ash percentages
of all coals, rarely exceeding 3% and, under a regime of nutrient recycling, may
be showing less than 1%, as demonstrated by some of the West Coast coals of New
Zealand. Slightly higher ash figures are encountered in raised bog coals which
were subjected to intermittent drying, either in a monsoonal climate or due to
lack of rain and possibly freeze-drying under cool conditions with strong seasonal
changes. The higher oxidative decay of some of the organic matter compared with
the continuously wet ombrotelmites will lead to an increase in coal ash.
The consideration of stone bands is restricted to epiclastic deposits left behind
after the peat had been flooded. Inorganic deposits, which are unrelated to the
depositional setting of the mire, such as pyroclastic sediments and their weathered
derivative claystones are not considered; As expected, the distribution of epiclastic
stone bands follows a pattern similar to the ash content, although some variations
are indicated in the table, such as their comparatively rare occurrence in the
brackish and marine influenced coals of Column 2. These coals are mostly related
to marine transgressions, which tend to push the strandline inland thus forcing
rivers to alluviate further upstream. As a result less sediments is spread over the
coastal mires when they are flooded.
264
Coal Facies and Depositional Environment
The distribution of sulphur and pyrite is inversely related to the acidity of the
mire, which accounts for the high concentration, mainly of framboidal pyrite in
marine influenced coals. Conversely, the low pH found in ombrotelmites prevents
any widespread sulphur enrichment in raised bogs, although some marcasite can
be found in high volatile coals derived from continuously wet ombrotelmites.
Moderate concentrations may be found in eutrophic mires because of the dilution
and neutralisation of acids by alkalis and alkaline earths, when the mire is flooded
with nufrient laden waters. Because of the preference of sulphate reducing bacteria
for 32S, the 34SP2S isotope ratio is low in the marine or brackish influenced coals
formed from low acid to alkaline topotelmites, whereas others are characterised
by high ratios.
The coal quality parameters volatile matter yield, atomic H/C ratio, vitrinite
reflectance and vitrinite fluroescence intensity are all described in terms of average,
high or low in reference to the rank of the coal in which they have been determined.
Being interrelated rank parameters themselves, their descriptors vary little from
each other, although their sensitivities towards type variations differ quite
considerably. For example, bulk properties such as volatile matter yield are more
affected by changes in maceral group proportions in the feed coal than by the
absorption of lipids in vitrinite, which influence very strongly its fluorescene and,
to a lesser extent, reflectance. It is therefore possible for a highly sensitive parameter
(e.g. vitrinite fluorescence) to act as a palaeo-environmental type indicator in
correspondence with a less sensitive parameter (e.g. reflectance), which takes the
role of a rank indicator, as has been done in Fig. 5.27 and 5.28. The assessment of
these properties in terms of average, high, or low will gain further from a correlation
of the analysis results either with another independent rank parameter, such as
depth of burial, as in Fig. 5.29, or with results obtained from other sources. For
example, an evaluation of the atomic H/C ratio, which is also related to wetness
and lipid absorption, can be obtained by plotting the mass percentages obtained
for hydrogen and carbon into Seyler's Chart or its modified version illustrated in
Fig. 5.18. A position in the perhydrous field above the bright coal band indicates
a higher than average H/C ratio, whereas a plot in the subhydrous area below
the bright coal band relates to a lower than average H/C ratio.
The tissue preservation index appears twice in Table 5.11 depending on the rate
of subsidence in relation to the rate of peat accumulation. A relatively high rate of
basin subsidence results in better tissue preservation compared with a low rate.
Consequently, moderate amounts of residual plant tissue in the form of telovitrinite
and/or telo-inertinite are still found in coals formed either under adverse conditions
(of tissue preservation), or from herbaceous vegetation with low preservation
potential. High TPI values based on telovitrinite are typical of the coals formed
under the conditions inferred in Column 1, mainly, because trees and woody plants
in general constitute the bulk of their biogenic feedstock. Conversely, a low rate
of subsidence will even reduce the TPI of these coals, although not as much as in
the others, which are further affected by either high bacterial activity (Column 2)
or frequent oxidation (Columns 3 and 5). Since the gelification index reflects the
consistency of moisture availability, it can be lowered either by frequent or
prolonged drops in the groundwater table of topogenous mires (Column 3), or in
response to the seasonal drying of raised bogs (Column 5).
6 The Relationship Between Coal
and Interseam Sediments
Coal seams and their surrounding strata share several spatial and genetic
relationships, some of which are common to all sediments, while others are specific
to the transition from primarily inorganic to organic sedimentation and vice versa.
Although coal seams and their enclosing inorganic sediments differ in many aspects,
a mutual influence on each other's composition and structural relationships is often
observed in the vicinity of their contacts. Examples are the distribution of elements
and minerals, which may be quite uniform in a vertical seam section, but undergo
considerable changes in concentration near the sediment/coal interface (Nicholls
1968; Gluskoter et al. 1977; Pareek and Bardhan 1985). The observation of Dorsey
and Kopp (1985) of a gradual upward decrease in elemental concentration in the
Pewee Seam of the Wartburg Basin in Tennessee, U.S.A., followed by a sharp
reversal of the trend (increase in Si, AI, Ti, K, Mg) below the seam roof is probably
not an isolated occurrence. The authors regard the gradual upward element
depletion as an indication of the decreasing influx into the Pewee swamp of
terrigenous minerals, which was followed by renewed flooding and abundant
sediment supply, thus terminating peat accumulation. While this interpretation is
probably correct, an additional factor in the upward elemental depletion may be the
recycling of essential elements, when plants cannot obtain sufficient nutrients from a
deeply buried and water-logged soil, or because of the cessation of nutrient supply
by flood waters.
Systematic upward changes in coal composition are particularly striking in coal
seams which carry a marine roof (White and Thiessen 1913; Petrascheck 1952;
R. Teichmiiller 1955b; Diessel1961; M. Teichmiiller 1962; Williams and Keith 1963;
Smith and Batts 1974; Goldhaber and Kaplan 1974; Boctor et al. 1976; Casagrande
et al. 1977; Parrat and Kullerud 1979; Pearson 1979; Price and Shieh 1979; Harris
et al. 1981; Smith and Batts 1984; Shim oyama 1984; Cohen et al. 1984; Casagrande
1987). As has been referred to in Chaps. 4 and 5, the sulphur content of coal seams
increases strongly as marine roof sediments are approached. This aspect will be
further discussed in Chaps. 7 and 8.
A reverse effect of coal seams on their roof and floor rocks can likewise be
observed. The seepage of humic and other organic acids from peat into underlying
sediments has frequently resulted in the leaching of alkalis, alkaline earths and other
elements from basal soils (seat earths, underclays), while the high compactability of
peat may exert a strong influence on the structural relationship between coal seams
and their roof rocks.
266
The Relationship Between Coal and Interseam Sediments
Coal/floor couples are generally concordant or conformable with each other, i.e.
their principal surfaces of deposition (Sp-planes) have similar attitudes. In many
cases the same is true for coal/roof couples, but exceptions occur where portions of
the seam have been eroded or where rapid loading and subsequent compaction of
the seam has led to angular discordances between the coal and its roof rocks. It is
mainly these aspects that will be explored in this chapter following some general
remarks about the nature of coal measure sediments.
6.1 Some Characteristics and Properties of Interseam Sediments
The dependence of peat formation on either high rain fall or a high groundwater
table also puts constraints on the range of depositional environments for the
associated coal measure sediments. Neither desert nor deep sea deposits are
consanguinous with coal, but sediments which are formed on low-lying terrain or in
shallow water are frequently encountered between coal seams because their
respective depositional environments coexisted and were laterally contiguous.
The aim of this discussion is to present an inventory of the various lithologic
elements that constitute coal measure sequences and to summarise the essential
features by which they can be recognised. Although such a presentation will
encompass several general sedimentological aspects, it is intended to emphasise
those rock characteristics which are commonly found in coal measure sediments.
Among the latter there is a predominance of epiclastic deposits, whose sedimentation elements can be arranged in a hierarchy, as proposed by Potter and Pettijohn
(1963). Following their lead, Allen (1967) stressed that the hierarchical order found
in a sediment is not merely a convenient arrangement to classify its various
components and geometrical domains, but that it is the lithological expression of a
likewise hierarchical order of sedimentary processes and forces. For example, a body
of water moving in a river channel constitutes a force field of high magnitude. It can
be subdivided into flow vectors of lesser magnitude and high frequency which will
produce specific responses in a sediment formed within such a flow vector system.
Sedimentation elements therefore range from those with low magnitude and high
frequency (e.g. single particles) to complex and are ally extensive lithofacies
associations, of which a basin may contain a limited number only (low freqency, high
magnitude). Indeed, the sedimentary basin itself may be regarded as forming the
apex of the hierarchical pyramid to which all other sedimentation elements,
including lithosomes, sedimentary structures, depositional fabric and the like are
subordinate.
The position of a sedimentation element within the hierarchy has a strong
bearing on its analytical value in the solution of many coal geological problems. For
instance, in coal exploration and in the planning of the layout of a mine, knowledge
of the syn- and post-depositional drainage pattern is of considerable advantage, in
order to locate areas of potentially difficult mining conditions due to erosion and
seam deterioration. The necessary palaeocurrent analyses can be based on either
Some Characteristics and Properties of Interseam Sediments
267
single particles, fabric elements, primary structures or even lithosomes. Lithosomes,
being responses to whole flow vector systems, reflect more faithfully a drainage
pattern than sedimentation elements of lower magnitude which correspond to
smaller and often local vector fields within the system. Flow directions derived from
low ranking sedimentation elements may be at variance to the main trend but in
view of their high frequency and the poor preservation potential (Allen 1967) of
whole channels, most palaeocurrent analyses are based on sedimentary textures and
structures. In order to yield meaningful results, low order sedimentation elements
require a larger sample population than is necessary in domains of high magnitude.
Even then, variances about mean values are often high.
6.1.1 Single Particles
The smallest petrographic entities of a rock consist of single particles which are
characterised by very low magnitude and high frequency. They are studied by a
variety of methods and for many different reasons because both the composition and
texture of the grains composing a sediment carry a record of its origin, mode of
transportation and deposition, as well as the palaeo-environment of the depositional
site. Furthermore, single particles are the basis on which sediments are classified.
6.1.1.1 Origin and Composition of Single Particles
Most epiclastic sediments are composed of the products of weathering of preexisting rocks which themselves may be of sedimentary, igneous or metamorphic
origin. The weathering products occur within the sediment in several forms and
stages of degradation which can be linked to the source rock in the following way:
Source Rock ~ rock fragments
~ primary detrital or residual allogenic minerals
~ weathered derivative minerals
~ authigenic minerals.
In the above scheme the components are listed in order of decreasing influence of the
source rock on the composition ofthe components involved. It has been implied that
rock fragments are the results of incomplete physical weathering in the form of block
disintegration. Thermal stresses caused by insolation together with a low heat
capacity and conductivity of the source rock are effective means of physical
weathering. In addition there are mechanical stresses, such as frost wedging,
crystallisation pressures etc. The products of these processes can become part of an
epiclastic sediment at any stage during their development but there are considerable
differences in the potential of the various source rocks to form sedimentary clasts.
Both particle size and their resistance to weathering put constraints on the
268
The Relationship Between Coal and Interseam Sediments
suitability of a source rock to form rock fragments. A number of dense and resistant
source rocks are frequently encountered as detritus even in fine-grained clastic
sediments. Those of igneous origin usually consist of extrusive rocks, rich in volcanic
glass, or of aphanitic and microcrystalline texture. Many lithic wackes and sandstones contain such detritus, particularly those formed in an orogenic setting.
Because of their large crystal size, high grade metamorphic rocks are restricted
to coarse-grained sediments in the same way as plutonic rocks are. Only relatively
dense phyllites, slates and some contact-metamorphic rocks, like hornfels, occur in
sandstones. Sedimentary rocks themselves can be recycled but many are poorly
cemented and disintegrate during weathering and transportation. Most sedimentary clasts are either rich in lime or silica or both, such as silicified limestones.
Among the most resistant of all rock fragments are the many varieties of cherts and
radiolarites.
In the course of prolonged physical weathering and frequently aided by varying
coefficients of expansion between different minerals, the grain fabric of a source rock
will eventually disintegrate, thus leaving a residual detritus which consists of
individual minerals. When incorporated in a clastic sediment, this fraction forms the
primary detrital or residual allogenic minerals.
Close to the sediment source almost any mineral may become a detrital
component, but with prolonged transportation, unstable minerals will be successively eliminated from the maturing sediment. Indeed, immature polymict sandstones whose components have been transported over short distances reflect
faithfully the composition of their (igneous or metamorphic) source rocks. Sediments which have been deposited some distance away from their sources contain
fewer mineral species as detritus than are found in their parent rocks. The
elimination of minerals follows a regular pattern beginning with olivine, pyroxenes
and other species which have been formed early in a cooling magma. The most
resistant primary minerals are white micas, orthoclase and, particularly, quartz.
Being of late magmatic formation, these minerals are more stable in the low
temperature hydrous environment on the earth's surface than the minerals formed
under high temperatures and pressures. Minerals can therefore be arranged in a
sequence of decreasing resistance (stability series) which is the reverse of the order in
which they crystallise from a cooling magma (Rosenbusch-Bowen series). It follows
that of the hundreds of minerals that are potentially represented as clasts in
sediments, only few are really common. These are quartz and chalcedony, acid
feldspars, white mica, calcite, and a variety of heavy minerals.
The weathered derivative minerals are still detrital in the sense that they have
been transported and deposited by physical means but they show the effects of some
chemical alteration. Chemical weathering is accelerated by the increasing fragmentation and decreasing particle size of the detritus resulting from prolonged physical
weathering and the wear the grains are subjected to during transportation. A
decreasing particle size leads to an increase in the total grain surface area of the
remaining sediment, and since the speed of chemical reactions between solids (e.g.
minerals) and fluids (e.g. water and air) is largely controlled by the available surface
area, a large number of small particles is more prone to chemical changes than a few
large ones. In the initial stages, chemical weathering leads to hydration, followed by
Some Characteristics and Properties of Interseam Sediments
269
hydrolysis and leaching of some elements, mainly alkalis and alkaline earths. Such
processes are usually accompanied by structural lattice adjustments and the
formation of clay minerals which comprise the most common weathered derivative
minerals (secondary detrital) in coal measure sediments. An example is the
kaolinitisation of feldspar and mica discussed in Chap. 4.4.2.1.
The substances leached out of the original source rock and minerals form
colloids or hydrous solutions. Most of the elements dissolved during chemical
weathering ultimately accumulate in the sea, where they become part of the salt
content and also generate the wide range of chemical and biochemical sediments so
typical of marine environments. The minerals precipitated as part of the sediment
from hydrous solutions or colloids, with or without the aid of organisms, are called
chemical or authigenic minerals. They form cements in clastic rocks, concretions,
infillings in vugs, joints and the like, or build up chemical sediments. On the basis of
timing of authigenesis in relation to the deposition of the host rock, a distinction is
made between syngenetic (early) and epigenetic (late) precipitates. As discussed in
Chap. 4.4.2, the two can be distinguished by the amount of compaction that has
occurred around the authigenic formation (Fig. 4.43).
6.1.1.2 Particle Size
One of the fundamental parameters in the classification of all rocks is the size of their
constituting particles as crystals or clasts. But this is not the only reason for
considering particle size. In epiclastic sediments, with which most coal seams are
interbedded, particle size can be useful indicator of palaeocurrent directions and it is
also a relative measure of the energy level that prevailed in the depositional
environment. The principle can be illustrated by Hjulstrom's Diagram (Fig. 6.1),
which demonstrates that the energy required to erode a fine clay is as high as the one
needed to start motion in cobbles. However, once transportation has begun, a clay
requires only minimal inputs of energy to remain in suspension, which is in sharp
contrast to large particles. In this context, high energy means fast-flowing water and
turbulence. The latter is necessary to lift the particles against the gravitational pull.
1000
100
:Wl
~A1
"
G)
'"
E
"~
10
FIELD OF
TRANSPORT A TlON
>f-
(3
Fig. 6.1. Diagram showing the kinetic
relationship between particle size (in mm)
and flow velocity. (After Hjulstrom 1939).
A - A' erosion threshold; B - B' deposition
threshold
o...J
UJ
>
0-1
100
10
PARTICLE DIAMETER
0-1
B'
(){)1
0001
270
The Relationship Between Coal and Interseam Sediments
Both degree and geometry of turbulence result from flow velocity and bed forms.
Large bedload clasts in a channel present obstacles to the current flow resulting in
the formation of individual turbulences around each particle. With diminishing
grain size the turbulences decrease accordingly until the particles reach clay size
where individual turbulences cannot be generated. The clay bed then acts like a
smooth surface whose resistance to erosion is increased by the cohesive forces
between the clay particles.
In the process of deposition, particles of diminishing size will be dropped
successively as water velocity decreases. This means that a down-stream sorting
takes place in response to the decreasing competence of the flow system as river
gradients flatten away from the elevated source area. This effect is superimposed on
the reduction in grain size due to fragmentation and wear during transportation.
The down-slope decrease in particle size necessitates changes in sedimentary
nomenclature. There is no unanimity about the best classification for epiclastic
rocks, but the most widely used system is the Udden-Wentworth Scale listed in
Table 6.1. Its basic unit is a particle with a diameter of 1 mm. Successive size classes
are obtained by either multiplying or dividing by 2.
Rocks which consist of one size class only are said to be very well sorted. But few
conglomerates contain only rudaceous particles, and rarely does a sandstone consist
Table 6.1. The particle size classification of clastic sediments according to the Udden-Wentworth
scale. Also included is the Phi-scale, which gives the negative logarithms of the actual
diameters
MM
PHI
Particle name
>256
256
128
64
32
16
8
4
<-8
-8
Boulder
2
-1
-7
-6
-5
-4
-3
-2
0
Cobble
Granule
Very coarse
Medium
1/4
2
1/8
3
1/16
1/32
1/64
1/128
1/256
< 1/265
4
5
6
Silt
8
>8
Clay
7
Rudite
(e.g. conglomerate)
Pebble
Coarse
1/2
Principal rock
Arenite
(e.g. sandstone)
Fine
Very fine
Lutite
(e.g. mudstone, shale)
Some Characteristics and Properties of Interseam Sediments
271
of arenaceous detritus only. Many conglomerates contain up to 50% sandy matrix
and yet they are identified as conglomerates. None of the particle size classes is free
from admixtures by either finer or coarser grains, which is expressed in such terms as
sandy conglomerate, or pebbly sandstone, sandy shale and the like. These terms
describe hybrid rocks of poor sorting in which the various particle sizes are well
mixed and evenly distributed. An alternative example of hybrid rocks is represented
by laminated sediments in which different particle sizes are concentrated in distinct
layers, too thin to be logged separately. Depending on whether the emphasis is on
either compositional or textural aspects, different sediment classifications have
evolved. For details see Pettijohn (1963), Pettijohn et al. (1972), Greensmith (1978),
Ruby et al. (1981), Selley (1982), Adams et al. (1984), and Fiichtbauer (1988) for
general aspects, and Ferm and Melton (1977), Mallett and Ward (1982), Conze
(1984), and Ward (1984) for specific classification of coal measure sediments.
6.1.1.3 Particle Shape and Roundness
Both shape and roundness of detrital particles are influenced by the rigor and
duration of transportation and the shape of the original grains, whether mineral
or rock fragment. In sediments which have been transported over short distances
only, the clasts are hardly worn and have retained much oftheir initial configuration
before the onset of erosion. With increasing distance of transportation, edges and
corners become abraded and the particles approach a spherical shape. However,
complete sphericity is rarely established in anisotropic mineral grains because
anisotropy is not only a.n optical quality but includes other physical properties such
as hardness. Quartz, for example, is slightly harder along the crystallographic C-axis
and therefore forms clasts which tend to remain somewhat oblong even after
prolonged transportation.
The shape ofa rock fragment is likewise affected by its original shape and grain
fabric. Because coarse grains suffer heavy collisions in relation to their strength, they
show signs of wear after a few kilometres of travel. However, small particles have to
be transported over larger distances before they become affected in the same way.
Often hundreds of kilometers of transportation are necessary before sand grains
become rounded. The highest degree of roundness is usually acquired in environments where the particles are in constant motion, such as on beaches and in shallow
seas. Indeed, particles of silt size and below will rarely become rounded even in high
energy environments because the collisions they suffer are small with respect to their
material strength. The threshold below which rounding stops is close to the sand/silt
boundary in water transportation, whereas air-borne particles can become rounded
down to 0.02 mm diameter because in aeolian environments the cushioning effect of
water is lacking (Pettijohn 1963).
It is necessary to make a clear distinction between shape and roundness.
Particle shape can be defined in various ways, a frequently used parameter is
sphericity which, after Wadell (1935), can be defined as:
Sphericity = d/a .
(6.1)
The Relationship Between Coal and Interseam Sediments
272
D
D
I
0
ANGULAR
r------------j
!
!
1
1
L ____________ J
(-------~
I
0-15
~--- ...--~
01.
030
SUB ANGULAR
L_ _ _ _ J ~-~ 0
(-------"\
'
:
(----\
~-.-.~
030
050
SUBROUNDED
(--~
I
'
(--~
~..-~
050
070
ROUNDED
q
070
10
WELL -ROUNDED
Fig. 6.2. Comparison of similar coefficients of roundness in particles of different shape. The
numerical values for the roundness grades are after Russel and Taylor (1937)
a = maximum particle diameter; d = nominal particle diameter, i.e. the diameter of
a sphere having the same volume as the particle.
Roundness refers to the degree of wear on edges and corners of a particle
independent of its shape as shown in Fig. 6.2, in which the roundness scale has been
defined as:
Coefficient of roundness = ~)/nR.
(6.2)
The formula is designed to handle two-dimensional images, e.g. photographs or
traces of the particles, whereby R = radius of the maximum inscribed circle; r = radii
of circles fitted into edges and corners; n = number of radii measured.
6.1.2 Depositional Fabric
In this chapter, the components of a sedimentary rock are regarded not just as single
grains but as part of a depositional fabric. This fabric is primary in the sense that the
manner of aggregation of its elements, i.e. the particles, is the result of the
depositional process and not of tectonic or any other secondary overprinting. In
view of the predominance of epiclastic sediments in most coal measure sequences,
these fabrics will be emphasised.
6.1.2.1 Classification of Fabric Elements
As mentioned in the discussion of particle size (Chap. 6.1.1.2), it is rare for a sediment
to consist of one size class only. Usually, the degree of sorting varies but even in wellsorted sandstones or conglomerates, small particles often occupy the voids between
the larger particles. The larger, more obvious particles are referred to as the
phenoclasts (i.e. the visible fragments) whereas the smaller interstitial detritus
273
Some Characteristics and Properties of Interseam Sediments
constitutes the groundmass or matrix. In the example illustrated in Fig.6.3A, the
phenoclasts are shown to be in contact with each other, the fabric is therefore said to
be clast- or framework-supported. Conversely, in poorly sorted sediments, there is
no distinct size partitioning of phenoclasts and matrix (Fig. 6.3B). In such case, the
rock type boundaries of the Udden-Wentworth Scale are used to distinguish
between phenoclasts and matrix. A pebble conglomerate, for example, would have a
sandy matrix, and in a sandstone, silt and clay-sized particles constitute the matrix
fraction. In poorly sorted sediments the proportion of matrix is commonly so large
that it prevents the phenoclasts from touching each other. Such a rock is therefore
said to be matrix-supported. In addition to the detritus, lithified sediments also
contain a cement which consists of authigenic minerals that have been precipitated
in the voids between clasts or have partly replaced them.
The volumetric proportion between phenoclasts and matrix and their respective composition is used in classifying rudites into para- (matrix-supported) and
orthoconglomerates (framework-supported) and arenites into the lithotypes listed
in Table 6.2. There is no unanimity about sandstone classification, so the
Fig. 6.3 A, B. Cartoon comparing the differences between phenoclasts (white) and
matrix grains (spaced stipples) in framework-supported (A) and matrix-supported clastic rocks (B). Cement is densely
stippled
Table 6.2. Classification of arenites on the basis of fabric and composition
Composition of
phenoclasts
Low matrix
content < 15%
High matrix content> 15%
Leucocratic
Melanocratic
> 90% Quartz +
other detrital SiOz
Quartz sandstone
Quartz wacke
> 25% Feldspar
Arkose
Feldspathic wacke
Feldspathic
grey wacke
> 50% Rock
fragments
Lithic sandstone
Lithic wacke
Lithic
greywacke
274
The Relationship Between Coal and Interseam Sediments
organisation and terminology used in Table 6.2, is merely one of several possibilities.
However, most authors agree on 15% matrix separating high and low matrix, i.e.
between clean (ortho-) sandstones and wackes. The reason for this is that less than
15% small detritus can be accommodated in the pore space between the phenoclasts
of a reasonably well-sorted, framework-supported clastic rock. When the matrix
fraction exceeds 15% the phenoclasts begin to lose contact, sorting deteriorates and
the fabric becomes matrix-supported. The matrix-supported arenites may be further
divided into wackes wjth a light-coloured, leucocratic matrix and greywackes
having a dark, melanocratic matrix. In leucocratic arenites the matrix consists of
clay micas, kaolinite, quartz and other light-coloured debris, whereas dark ferromagnesian minerals and their chloritic derivatives constitute much ofthe matrix of a
melanocratic arenite, giving the rock a dull greenish grey colour.
The light-coloured wackes are common in molasse-type coal measures formed
in foredeep settings. They usually derive from the destruction of an adjacent fold
belt. Greywackes, on the other hand, are rare as interseam sediments but occur most
commonly as flysch-type deposits within the orogenic belt itself. Originally
emplaced by turbidity currents on the ocean floor, they have been derived mainly
from basic to intermediate volcanic debris produced near converging plate margins.
Because the source rocks contain very little free quartz, there is little possibility of
quartz greywacke. Further subdivisions and combinations can be made on the basis
of accessory minerals and the kind of cement.
6.1.2.2 Types of Aggregation
The spatial arrangement of particles with respect to each other results from their
sizes and shapes, as well as the orientation and sorting imposed on detrital grains by
gravitation, current direction, current strength and, occasionally, earth magnetism.
Four types of aggregation can be distinguished:
1. A homogeneous isotropic fabric (Fig. 6.4A) consists of equidimensional grains
(e.g. spheres) either all of the same size or if not, of randomly distributed sizes.
Examples are oolitic limestone (oosparite or oomicrite), some dune and beach
sands and some gravels. The rocks are homogeneous because the fabric consists
of similar elements even if only relatively small parts of the rock are considered.
The rock is also isotropic in that its mechanical and other physical properties are
equal in all directions.
2. A homogeneous statistically isotropic fabric (Fig. 6.4B) is still homogeneous in
the sense mentioned above but the particles are not equidimensional any more.
However, because oftheir random orientation, large enough portions of the rock
react like isotropic materials. Mudstones are typical examples.
3. A homogeneous anisotropic fabric (Fig. 6.4C) differs from the two examples
above by the manner in which its mechanical properties depend on the direction
from which a load is applied. A typical example is shale which, because of its
fissility, splits with greater ease parallel to stratification than normal to it.
275
Some Characteristics and Properties of Interseam Sediments
A
_-~_-~_-_-_-_-_-_-_-_-
-----------r---------------------r_-_-_-_-_-_-_-_-_-_-_-_
Fig. 6.4 A-D. Four different types of epiciastic fabric.
A Homogeneous isotropic. B Homogeneous statistically isotropic. C Homogeneous anisotropic. D Heterogeneous anisotropic. (After Diessel and Moelle 1965)
c
.........................................
4. A heterogeneous anisotropic fabric (Fig. 6.40) displays a distinct separation into
alternating coarse and fine laminae. There is therefore an even stronger
dependence of its properties on the direction of force application. All laminated
sediments are good examples of this fabric type.
Although mechanical properties have been emphasised in order to explain the
differences between the various kinds of depositional fabric, other features, such as
acoustic, electric and optical properties are likewise affected.
In addition to its palaeo-environmental significance, coal measure fabric is of
special interest because it affects the performance of interseam sediments as roof and
floor rocks during mining. The more homogeneous and isotropic, the more
predictable is the rock's response to an applied mechanical force field or the
disturbance of the existing stress field. The extreme heterogeneous and anisotropic
nature oflaminites makes both rock mechanics calculations and scaled-down model
tests very difficult, mainly because of the existence of numerous planes of mechanical
discontinuity between the different lithotypes along which bed separation occurs
very easily. For this reason, laminites make mechanically poor and rather
unpredictable roof rocks in coal mines.
The manner of aggregation is usually slightly modified by diagenetic processes.
These are post-depositional sensu stricto and therefore oflesser concern here, but as
The Relationship Between Coal and Interseam Sediments
276
Fig. 6.5. Cartoon showing different grain
contacts in framework supported clastic
rock: p point; st straight; c concave/convex;
su sutured contacts
diagenesis is part ofthe sedimentary cycle and somewhat related to the depositional
environment, these processes require some consideration. Depending on the degree
of diagenesis, compaction of the sediments increases resulting in a change of the
type of contact between particles, particularly in framework-supported fabrics.
Because of increasing pressure solution at grain contacts the following changes have
been observed by Taylor (1950) and Fiichtbauer (1967a):
Point (p) ---+ straight (st) ---+ concave/convex (c) ---+ sutured (su).
Figure. 6.5 gives an illustration of the various grain contacts, from which the degree
of compaction can be calculated as:
· d .
P + 2st + 3c + 4su
P a k mg
enslty = = - - - - - - - - p + st + c + su
(6.3)
Data acquisition for the determination of packing density is normally carried out by
counting the types of contact along traverses in a thin section.
6.1.2.3 Symmetry Relationships
Clastic particles have various shapes which can be described as disks, prisms,
pencils, cubes and spheres. The two latter may be characterised by different packing
densities but they cannot have preferred orientation. All other shapes can possess
preferred orientation to varying degrees. For example, the disk-shaped clay micas in
a shale are usually oriented parallel. to each other, thus forming a homogeneous
anisotropic fabric. The parallelism is a consequence of mainly gravitation, which
acted upon the particles as they settled out of suspension. If gravitation is
supplemented by a tangentional force, as is the case when particles are deposited by
a current (Potter and Pettijohn 1963), they form an imbrication fabric in which the
plates overlap like roof tiles. The symmetrical relationships found in the depositional fabric are a result of the kinematic and, less directly, the dynamic conditions
of transportation and deposition, which can be measured and analysed statistically
Some Characteristics and Properties of Interseam Sediments
277
CONCENTRATION %
•
>30
30·20
§lIl
20-10
[IIJ
10-2
§
2-1
Q
<
A
D
I
B
• •
>25
> 15
25-15
15-10
§lIl
§lIl
15- 5
10- 5
[IIJ
[ill
5-2
5-2
§
§
2- I
...
D
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2- I
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< I
D
Fig. 6.6 A-D. Four diagrams representing current-influenced directional fabric elements measured
in the roof sandstone of the Dudley Seam of the Newcastle Coal Measures, New South Wales.
A Rose diagram of 44 articulate stems and Glossopteris leaves at 10° interval. B Polar stereogram
(Schmidt Net) of 100 foreset beds. C Polar stereogram ofthe AB planes (maximum projection) of
100mica platelets measured in top set beds (principal surface of deposition, Sp). D Polar
stereogram of the AB planes of 100 mica platelets measured in foreset bed, but referred to Sp as
datum. (After Diessel 1966)
or by graphical methods. The results of a fabric analysis involving platy particles
are shown in Fig. 6.6C and D by means of polar stereograms of mica measured
in a fluvial sandstone above the Dudley Seam in the Newcastle Coal Measures of
New South Wales. Stereogram C, based on the orientation of mica in horizontally
bedded sandstone, shows the expected up-current dip of the mica flakes. Stereogram
D is based on measurements carried out in the plane of a foreset bed (Sf) where
the micas dip down-current when referred to the principal bedding plane (Sp) as
datum. However, normal up-current dip is established also in this case, when mica
imbrication is referred to the tilt-corrected foreset as datum.
Pencils and prismatic grains also display variations in orientation depending on
whether they have been deposited under a gravitational force field only or in one
278
The Relationship Between Coal and Interseam Sediments
that contained a tangentional force, such as a current as well. In the first case, the
elongated particles (minerals, rocks, fossils) will be randomly arranged on the
depositional surface, whereas in the second case they acquire a parallel orientation
with respect to each other. An example is shown in Fig. 6.7, which shows a slab of
laminite from the roof of the Bulli Seam, New South Wales. The rock contains a
large number of Glossopteris and other plant leaves, most of which are oriented
parallel to current flow (SE) indicated by the northeasterly trending crests and
troughs of current ripples. The results of a fabric analysis on such material can be
illustrated either in two-dimensional rose diagrams (Figs. 6.6A and 6.8D) or in threedimensional polar stereograms, as in Fig. 6.8B and C. From a two-dimensional
representation (Figs. 6.6A and 6.8 D) only a line of movement can be obtained but
no actual flow direction. For example, without the azimuths obtained from the dip
direction of the foreset beds in Fig. 6.6B, or the mica imbrication in Fig. 6.6C and D,
it could be assumed that the orientation of the plants in Fig. 6.6A corresponds either
to a northwesterly or a southeasterly current. Only when the three - dimensional
stereo grams are considered as well, do the measured parameters define a northwesterly current.
While in Fig. 6.6A the plant fossils are oriented parallel to the other sedimentation elements, in Fig. 6.8D they are arranged almost at right angles to the
other current indicators. Indeed, an orientation of elongated particles normal to
current flow is quite common in traction transportation, when particles roll along
the accretion plane. If there is a sudden drop in current velocity, for instance due to
breaching of levee banks and stream avulsion, this movement picture can be
preserved in the lower reaches ofthe channel, which have suddenly been deprived of
their water supply. An orientation parallel to current direction of elongated particles
Fig.6.7. Slab of laminite from the roof of the Bulli Seam, Illawarra Coal Measures, New South
Wales, showing plant fossils in perpendicular orientation to the crests and troughs of current
ripples
279
Some Characteristics and Properties of Interseam Sediments
N
w
N
E W
CONCENTRATION
_>40%
§
40-10%
0<10%
N
D
EW
Fig. 6.8 A-D. Four diagrams illustrating directional features measured in the roof of the Upper
Homeville Seam, Greta Coal Measures, New South Wales. A Polar stereogram (Schmidt Net) of73
foreset beds. B Stereogram (Schmidt Net) of the long axes of 600 pebbles. C Stereogram of 65 tree
trunks more than 1 m long. D Rose diagram 291 plant leaves. (After Diessel 1984)
is commonly obtained from gradually waning currents when it is still possible to
push the particles into positions where they offer least resistance to flow even though
their transportation has ceased. Additional fabric elements showing a parallel
orientation to current direction and other palaeocurrent indicators are tree trunks
whose roots are dragged along the river bed.
Bed configuration is another reason why elongated particles can be at variance
with the orientation of other directional sedimentation elements. In relatively
tranquil flow, small plant debris most commonly accumulates in the trough of ripple
marks and sand waves, where it orients itself parallel to the trough line but normal to
the current trend.
280
The Relationship Between Coal and Interseam Sediments
6.1.3 Coal Measure Structures
While the previously discussed sedimentation elements, the single grains and their
fabric, are classified as sedimentary textures, the term structure encompasses the
larger-scale attributes of sedimentary beds, some ofthem are listed in Table 6.3. The
primary mechanical structures are of particular interest because of their diagnostic
value in both facies and palaeocurrent analyses. This kind of geological enquiry
plays an important part in coal exploration and mining mainly for the purpose of
predicting changes in roof and floor rocks of coal seams, the trend of washouts,
erosion surfaces and other sedimentary features which might be detrimental to
mining. The primary mechanical structures will therefore be discussed in some detail
together with some of the secondary mechanical structures which may have an
adverse effect on mining conditions (e.g. clastic dykes). For a more comprehensive
treatment see Potter and Pettijohn (1963,1964), Gubler (1966), and Conybeare and
Crook (1968).
6.1.3.1 Stratification
Among the most basic primary structures of sediments are the partitions between
individual strata. These stratification planes have many different origins, but in the
predominantly clastic coal measures, changes in particle size from one stratum to the
other are the most common reason for their occurrence. Various terms are used in
order to indicate the spacing between adjacent stratification planes and the
thickness of the enclosing lithotypes. An example is given in Table 6.4 in which the
term bedding is used for stratification involving strata more than 1 cm thick,
whereas the term lamination applies when they are thinner.
In coal measure sediments all the kinds of strata listed in Table 6.4 occur.
Naturally, the thickly and very thickly bedded sediments are confined to coarsegrained clastics, such as conglomerates, some fluvial sandstones, and some deltaic
and other nearshore deposits. Such relatively thick and coarse inter seam sediments
are commonly found close to strong sediment sources, e.g. near the orogenic
margins of molasse foredeeps (foreland basins). In such settings the coarse clastics
Table 6.3. Classification of sedimentary structures
Primary
Secondary
Mechanical
Stratification, bed undulations,
cross-stratification, sand
lineation, flute casts, tool marks
Weather marks, load casts, ball and
pillow structures, flame structures,
convolute bedding, slump folds and
breccias, clastic dykes and sills, gas
heave and fluid escape structures
Chemical
Oolites, sinter, stalactites and
stalagmites
Concretions, druses, amygdules
281
Some Characteristics and Properties of Interseam Sediments
Table 6.4. Classification of sedimentary strata according to their thickness. (After Ingram
1954)
Spacing of stratification planes
(cm)
Very thickly bedded
Thickly bedded
Medium bedded
Thinly bedded
Very thinly bedded
Thickly laminated
Thinly laminated
>100
30-100
10-30
3-10
1-3
0.3-1
<0.3
form wedge-shaped deposits which distally become finer and thinner so that
laminated sediments comprise the majority of the interseam deposits.
Among the many types of laminites which have been described only relatively
few are frequently encountered in coal measures. These are listed below:
1. Sandstone laminite is a sandstone which contains laminae of differently sized
arenites.
2. Shale-laminated sandstone is a sandstone which is composed of more than 60%
sandstone and between 40 and 10% shale laminae. The term shale is meant to
include both silt and clay fractions in approximately equal proportions.
3. Shale/sandstone laminite contains both lithotypes in approximately equal (i.e.
between 40 and 60% respectively) proportions in the form of laminae.
4. Sand-laminated shale is a lutite containing between 40 and 10% arenaceous
laminae.
5. Clay-laminated siltstone is a lutite in which 10 to 40% clay laminae alternate with
at least 60% silt laminae.
6. Silt-laminated claystone is a lutite in which 10 to 40% silt laminae alternate with at
least 60% clay laminae.
7. Coal-laminated shale or coaly shale contains 10 to 40% coal laminae interstratified with lutite. Carbonaceous shale has a similar coal/shale ratio but the
two components are not segregated into separate laminae.
Most of the laminites occurring in coal measures can be accommodated within the
above system, which is based primarily on particle size differences. However,
occasionally, additional qualifiers may be applied on the basis of colour differences
or in order to account for genetic changes, such as the alternation of laminae of epiand pyroclastic sediments.
The origin oflaminites varies but in coal measures the bulk of this material has
been formed as overbank sediments on flood plains and in flood basins within a
fluvial setting or in interdistributary bays of delta environments.
Both laminae and beds often display vertical variations in particle size in the
form of graded bedding. In most cases, this involves a decrease in grain size in an
upward direction (fining upward) but, occasionally, particle sizes increase in an
282
The Relationship Between Coal and Interseam Sediments
upward direction (coarsening upward). Fining upward may be the result of a waning
current in a fluvial sediment, a seasonal change in sediment supply to a lake (e.g.
varved shales), or the change in the particle size may be related to the emplacement
pattern of a turbidite. Upward coarsening results mostly from depositional
progradation of deltas.
6.1.3.2 Bed Undulations
There are several primary sedimentary structures which occur as undulations (bed
forms) on the upper bounding surface of sedimentary strata, whereas others are
better observed at their undersides. In the following discussions emphasis is put on
such surface undulations as illustrated in Fig. 6.9, which can be divided into smallscale ripples and megaripples (including sand waves and dunes). When formed by a
current they have a gentle upstream slope or stoss-side and a steeper downstream
slope or lee-side except for the so-called antidunes where the slope angles are
reversed.
Small-scale ripples (Fig. 6.10) are the smallest bed forms which usually develop
as periodic undulations on the surfaces of silt or sand deposits in response to wind or
water movements. Their sizes are commonly restricted to wavelengths (measured
from crest to crest or trough to trough) between 5 and 50 cm with a ripple height
from trough to crest ofless than Scm. A distinction can be made between oscillation
or wave-formed ripples, in which both slope angles and slope lengths measured from
the crest to the trough on each side of the crest are equal, and asymmetrical ripples
which have been formed in response to a current. Under conditions of low flow
velocity, the crest lines of current ripples are usually straight and normal to the flow
direction. Lee-side angles are up to 30 ° provided the clay content of the sediment is
Fig.6.9. Photograph of megaripples (emphasised by dashed lines) in sandy facies of the Munmorah
Conglomerate, south of Newcastle, New South Wales
Some Characteristics and Properties of Interseam Sediments
283
Fig. 6.10. Ripple marks with internal cross-lamination from the roof of the Bulli Seam, Illawarra
Coal Measures, New South Wales
low. Clay tends to lower lee-side angles in water but increases them in wind-formed
ripples (Jopling 1966).
With increasing flow strength the lee-side angles decrease to below 20° and the
ripple crests become first sinuous, then catenary, linguoid and lunate. This morphological response of ripples to changing flow patterns renders them suitable for
palaeo-environmental analysis. The indices most commonly used in order to
identify environments of ripple formation are (after Tanner 1967):
1. The ripple index is the ratio ripple length to ripple height. Wave-formed ripples
have ripple indices usually exceeding 15, which is similar to those formed in the
swash zone. All other ripples formed in water have ripple indices ofless than 15,
provided ripple crests have not been re-shaped by planning-off or differential
compaction.
2. The ripple symmetry index is the length of the stoss-side divided by the length of
the lee-side. In current ripples, this index usually exceed 4, whereas in waveformed ripples both flanks are approximately equal in length giving an index of
around 1.
3. The continuity index is the ratio of the distance between beginning and end of a
ripple measured along its crest, and the spacing of ripples (ripple length). Indices
above 15 are common for wind-formed ripples but they overlap with waveformed oscillation ripples. Values below 10 are common to current ripples
formed in water and values below 4 are exclusive to them.
4. The straightness index is measured by dividing the length of curvature measured
along the ripple trend by the radius of the curvature. Indices above 15 are
common to wind- or wave-formed ripples. Indices of less than 4 are typically
found in water-generated current ripples.
284
The Relationship Between Coal and Interseam Sediments
Apart from the above-mentioned morphological features of ripples, their environments of formation can be deduced from their association with non-rippled
sediments. An example commonly found in inter-distributary bays, estuaries, and
other depositional sites of coal measures influenced by tidal action are flaser, wavy
and lenticular bedding. These terms describe ripple bodies consisting of fine to
medium sand which have migrated across a muddy sub-strate (Blatt et al. 1980), but
differences exist in the relative proportions of sand and mud within the intertidal
sequence. In flaser bedding, sand dominates and the lutite fraction is restricted to the
ripple troughs in the form of incomplete mud laminae, usually with an upward
concave curvature. Frequently this type of bedding forms in tidal channels or the
middle portion of the intertidal flats where currents are swift and mud is swept from
the sand flats except for the discontinuous mud flakes trapped in ripple troughs.
In lenticular bedding, mud predominates such that sand occurs only in the form
of isolated, so-called starved ripples without forming continuous sheets of sand.
Such structures develop close to protected shore lines where the tidal currents are
too weak to transport much coarse material into the mud flats which dominate the
low energy environment. Wavy bedding occupies a position transitional between
flaser and lenticular bedding in both morphology and depositional site.
Finally, ripples are among the few sedimentary structures which permit a
quantitative assessment of the relative contribution made to the formation of a stratum by traction transportation and fall-out from suspension, respectively. If no
material is added to a ripple body it will migrate across a substratum bystoss-side
erosion and accretion on the lee-side. Under conditions of ample sediment supply by
traction transportation, the ripple will be drawn out to a thin bed because material
Fig.6.11. Photograph ofthe downcurrent termination of a mega ripple, which has been drawn out
by'lee-side addition to form a continuous cross-stratified bed extending to the right of the central
field of view. The intervals on the scale are 10 cm. Waratah Sandstone at top of Tomago Coal
Measures, New South Wales
Some Characteristics and Properties of Interseam Sediments
285
will be added to the lee-side without being eroded frolT' t1::te stoss-side. Previous leeside positions will be preserved as cross-laminations. By using a drawn-out
megaripple as an example, the result of this process is illustrated in Fig. 6.11.
Ripple trains will form successive sheets of drawn-out ripples which are separated by stratification planes more or less parallel to the principal surface of
deposition. When traction transportation is supplemented by addition of material
from suspension, the surfaces over which the lee-sides advance, i.e. the preceding
drawn-out ripples, are constantly raised in response to the fall-out accretion. The
result is a set of climbing ripples (Fig. 6.12) in which the angle of up-current dip of
the surfaces separating the drawn-out ripples is dependent upon the proportion
between sediment addition by traction and out of suspension, respectively. The
higher the contribution of sediments carried in suspension, the steeper the climbing
angle. Climbing ripples are formed mainly at the waning stages of floods at the
tops of point bars and in overbank deposits and in turbidites.
It has been mentioned above that the change from straight-crested ripples to
sinuous, catenary and the other curved varieties is related to increasing flow
strength. This is true only as long as water depth and particle size remain constant.
Given such limitations, a further increase in flow velocity would eventually destroy
the ripples and replace them with megaripples, which would tend to repeat the
change from straight to markedly curved crests. Further increase in velocity leads to
a replacement of the bed forms by plane beds and a renewal of megaripples in the
form of antidunes. A final stage would be the development of shutes and pools.
Following the work of Simons and Richardson (1961), the various bed forms can be
arranged into a lower and upper flow regime, respectively. In the lower flow regime
bed forms have no or only minor influence on the configuration of the water surface.
If ripples or waves occur on the water, they are not in phase with the bed form at the
Fig. 6.12. Climbing ripples in 20-cm-thick layer above key (encircled) in the upper portion of a
point bar sandstone above the Yard Seam, Newcastle Coal Measures, New South Wales
286
The Relationship Between Coal and Interseam Sediments
bottom of the water column. The lower flow regime encompasses not only the
various ripple types, but also sand waves and dunes (megaripples). Both are larger in
scale than ripples but differ in their ripples indices. Sand waves are very wide in
comparison to their height. The latter ranges from more than 5 cm to several metres
but the ripple length may be over 100 m, resulting in ripple indices of > 50 which is
in contrast to the < 15 commonly found in water-formed current ripples. Dunes are
also large scale but their morphology is closer to that of ripples. When formed in
water, ripple length rarely exceeds 10 m (Blatt et al. 1980) which results in ripple
indices generally below 50.
The lower flow regime ends when the megaripples are planed off due to
increased flow velocities, after which transportation continues by sheet flow, i.e. by
whole layers of sand grains travelling together in more or less continuous layers. The
result is a sequence of flat beds which are transitional to the upper regime but are
conventionally included in this regime because their origin is hydrologically
different from the flat-bedded sequence formed at the beginning of the lower flow
regime, when the current is still too weak to produce any bed forms at all,
particularly in coarse sands. Typical bed form of the upper flow regimes are the
antidunes, which are in phase with the waves generated on the water surface (Fig.
6.13). Indeed, the water waves are practically an image of the underlying bed forms.
Accretion takes place on the stoss-side which often results in up-current migration of
the antidunes.
Fig. 6.13. Antidunes and
standing
waves in storm water channel on a New
South Wales beach
287
Some Characteristics and Properties of Interseam Sediments
200
AnJdunes
175
/
150
/ ,/
125
100
75
:I
Fig. 6.14. The
relationship between flow
parameters and bed
forms. F Froude Number.
(After Simons and
Richardson 1961 and
Reineck and Singh 1975)
E
u
50
.5 25
/
/ v/
/'
/
II;
V
~
/ V5
Plan bed or
Standing Waves
VV
~
15
/Transition
! egaripPITs~
V-
-----
10
/F.
./
V
I/V V
V
VF'13~
../'-0.40
~LriPPles
___ F· 0.30
r-~
20
Small Ripples
t
F·0.15
Plane Bed
25
30
35
Depth in em
Because of the interaction between flow velocity, water depth and particle size,
no absolute flow velocities can be given for the generation of the various bed forms.
A common approach is to combine velocity and water depth into a single parameter
such as stream power (mean flow velocity times shear stress) or the Froude Number
(F). The latter is defined as:
V
Froude Number =---;-,
-y gh
(6.4)
where V = flow velocity, g = gravitational constant and h = the height of the water
column above the depositional interface ( = water depth). It is obvious that identical
Froude numbers can be obtained from different flow velocities provided that the
water depth changes accordingly. This means that a particular bed form can be
maintained either in fast-flowing deep water or in shallow water of reduced flow
velocity. An illustration of the concept of flow regimes in relation to the controlling
parameters is given in Fig. 6.14.
6.1.3.3 Cross-Stratification
Most of the bed forms discussed above display an internal cross-stratification which
results from the accretion of sediments on the lee-sides in current-generated
288
The Relationship Between Coal and Interseam Sediments
undulations, on the stoss-side in antidunes and on one or both flanks in waveformed ripples. A more complex internal organisation is found in longitudinal and
interference ripples which are rare in coal measures. In bed forms which have been
drawn out to form continuous strata, as illustrated in Fig. 6.11, the kind of crossstratification contained in them in the form of obliquely dipping foreset beds (Sfplanes) sandwiched between two principal surfaces of deposition (Sp-planes) is often
the only indication of their mode of origin. A study of cross-stratification is therefore
of palaeo-environmental significance and, by measuring the azimuth of the dip
direction offore-set beds, the direction offlow at the point of origin can be obtained.
The first comprehensive classification of cross-stratification was presented by
Allen (1963), which is followed here with minor modifications. The classification is
based on the following six criteria:
1. Solitary versus grouped sets of cross-strata. A solitary set consists of a crossbedded (or -laminated) stratum which is over- and underlain by non-crossstratified sediments. A grouped set consists of two or more co-sets in direct
contact with each other.
2. Magnitude is an aspect which follows closely the classification of bed undulations. Cross-stratified units with a bed thickness ofless than 5 cm are referred to
as small scale, which equates to small-scale ripples in bed forms. Originally, crossstratified units of more than 5 cm thick were regarded as large scale, but following
suggestions by Crook (1965) and McCabe (1977), a distinction is now made
between medium scale = 5 cm to 1 m, large scale = 1 to 3 m, and giant> 3 m
between upper and lower bounding surfaces.
3. The lower bounding surface is erosive if a portion ofthe substratum was removed
before the deposition of the cross-stratified unit. Alternatively, the lower Sp-plane
is non-erosive or even gradational if the lower bounding surface is not really a
plane but consists of a narrow zone in which the Sf-planes of adjacent sets
align themselves tangentionally in a Sp-direction. This is the case in climbing
ripples.
4. The contact angle between foreset beds and lower bounding surfaces may be
concordant or discordant. The first case occurs in some trough cross-beds in
which the original hollow has been filled by suspension fall-out such that the
foresets are parallel to the scour surface. A discordant-relationship exists as a
result of traction transportation but variations occur due to flattening of dip
angles near the toes of foreset beds.
5. Shapes oflower bounding surfaces vary. Most bed forms produce either irregular
(erosive) or planar (erosive or non-erosive) surfaces. Cross-beds occurring in
erosion hollows may be either cylindrical, scoop-shaped or trough-shaped.
6. Lithologic differences occur when there are variations in the particle size of the
sediment supplied. In such instances, the cross-stratified unit is heterogeneous. In
the case of constant sediment supply homogeneous cross-strata result.
On the basis of the above criteria, Allen (1963) distinguished 15 different types of
cross-stratified units which he identified by letters of the Greek alphabet. In Fig. 6.15
the types of cross-stratification are illustrated which are of special interest in
Some Characteristics and Properties of Interseam Sediments
289
Fig. 6.15. Types of crossstratification commonly
found in coal measures.
(After Allen 1963)
palaeo-environmental studies of coal measures. Closely related cross-bedding types
will be discussed together:
1. Alpha (a), Beta (f3) and Gamma (y) cross- bedding is found in solitary sets of medium
to large scale. Foreset beds are discordant with the lower bounding surface but
tangential alignment is common. In a transverse section normal to bedding and
parallel to the direction of transport (ac-plane) foreset beds are either planar or
upward is slightly concave. Lithologically, the three types are homogenous. They
differ only with respect to the lower bounding surface, which is planar and nonerosional in alpha, planar but erosional in beta and irregular and erosional in
gamma. This suggests that all three types have resulted from the migration of
solitary megaripples (Fig. 6.11), commonly with either straight or only slightly
sinuous crests. They belong to the middle to upper portion of the lower flow
regime and are listed in the order of increasing energy. Their depositional
environments are fluvial and tidal channels, delta distributaries, proximal
crevasse splays and shallow seas.
2. Epsilon (6) cross-bedding is usually large scale and often not recognised as crossbedding in small outcrops because the dip angles of the foreset beds are often
quite shallow. Although Allen (1963) included this structure in his cross-bedding
types, in most cases it represents accretion surfaces on point bars, formed in
meandering channels either of fluvial or tidal origin. The lower bounding surface
is therefore commonly erosive but may be planar or irregular. Depending on the
strength of the floods during which point bar accretion occurs, this type is
lithologically heterogeneous with either straight or upward convex surfaces.
290
The Relationship Between Coal and Interseam Sediments
3. Theta (8) and Iota (1) cross-bedding both have a trough-shaped lower bounding
surface. They are solitary and medium to large scale. The troughs probably are
cut into the substratum by short-lived eddies which rotate around a vertical axis.
As they move down-stream they scour into the bed, then lift off and dissipate.
subsequently, the cut is filled either by traction (theta) or from suspension (iota).
Theta is common in fluvial deposits.
4. Kappa (K) and Lambda (2) cross-stratification are small scale and occur in grouped
sets. They have been formed from climbing ripples which have sinuous crests in
kappa and straight crests in lambda. Both types appear identical in a vertical
section cut in the direction oftransport (ac-plane) but are dissimilar in a section cut
parallel to the ripples (be-plane). In the latter case, kappa shows a pinch-and-swell
pattern whereas lambda displays parallel lamination. Both types belong to the
lower portion of the lower flow regime and are frequently formed in the waning
stages of a flood when suspension fall-out is added to traction transportation.
5. Mu (Ji) and Nu (v) cross-stratification are also small scale and grouped and have
been formed from migrating ripple trains. However, addition of material was by
traction only. Mu has been formed from straight and nu from sinuous or
otherwise curved ripples. Both types are frequent in overbank deposits,
interdistributary bays, intertidal sediments and on the flanks of megaripples.
6. Omikron (0) and Pi (n) cross-bedding are the medium to large scale equivalents of
the mu and nu types. They rank therefore higher in the lower flow regime and
have been formed from straight (omikron) or curved (pi) sand waves and dunes
migrating down-stream in rivers, across flood plains or the floor of shallow seas.
Being the product of grain-by-grain transportation, cross-bedding is a useful
structure in both palaeo-environmental and palaeocurrent analysis. Both aspects
will be further discussed in Chap. 7.
6.1.3.4 Surface and Sole Markings
The stratification planes of many clastic sediments are covered with markings, most
of which have been formed as indentations in a soft sediment surface after which they
have been preserved by the infilling of the moulds by the overlying sediment. Both
formation and preservation are best when the substratum is a lutite and the
overlying sediment is an arenite. In this case, moulds are filled with sand casts which
adhere to the sole ofthe covering bed when the two strata separate after lithification.
Except for the markings formed by organisms, surface and sole marks can be
divided into three groups: firstly, those formed by running water, secondly, subaerial
weather marks and, thirdly, load casts which will be discussed together with
deformational structures. Only those of the first group represent primary structures,
all of which have directional significance. A list of the following structures of this
group is given in Table 6.5:
1. Current or parting lineation is found on the surfaces of some sandstones in the
form of parallel small ribs and furrows and also as elongated patches of sandstone
Some Characteristics and Properties of Interseam Sediments
291
Table 6.5. Surface and sole markings of primary and secondary origin found in coal measures.
(partly after Potter and Pettijohn 1964)
Process
Name
Sheet flow
Erosion
Current lineation
Flute casts
Current crescent casts
Rill marks
Tool marks (e.g. drag marks
or groove casts)
Saltation marks (e.g. prod,
bounce, brush and skip marks)
Rain and hailstone imprints
Mud cracks
Ice crystal imprints
Load pockets (or casts)
Ball and pillow structures
Flame structures
Convolute bedding
Slump folds and breccia
Clastic dykes and sills
Gas/ water escape structures
Markings made by
moving objects
Precipitation
Insolation
Frost action
Graviation
Gravitation plus
tangentional transport
Fluidisation (e.g. thixotropic
liquefaction) plus injection
laminae which adhere to either surface when a sandstone is split parallel to
stratification (Fig. 6.16). The structure is caused by the orientation of sand grains
in the flow direction.
2. Flute casts are spoon-shaped protrusions usually found at the underside of
sandstones (Fig. 6.17). The up-current end is sharp and narrow, whereas downcurrent they broaden and become aligned with the principal surface of
Fig. 6.16. Current lineation on the bedding plane of a fluvial sandstone above the Kimberley Seam
at Four Mile Point, Joggins, Nova Scotia. The line of movement is parallel to the pocket knife
292
The Relationship Between Coal and Interseam Sediments
Fig. 6.17. Sketch of flute
casts (F), current crescent
casts (c) and tree trunks (P)
at the base of the Coal
Cliff Sandstone overlying
the Bulli Seam at South
Clifton Colliery, New
South Wales. (After Diessel
et al. 1967)
deposition. In plan view flute casts have a bilateral symmetry and range in size
from a few millimetres to several decimetres. They have been formed as infillings
of small erosion scours produced by water turbulence. Internal cross-lamination
is therefore quite common.
3. Current crescent casts (Fig. 6.17) are crescent-shaped protrusions which occur at
the soles of silt- and sandstones. Their convex part points up-current whilst
down-stream they are concave. Current crescent casts form by a combination of
Fig. 6.18. Current crescent casts formed around pebble (diameter = 5 cm) in beach sand on the
New South Wales coast
Some Characteristics and Properties of Interseam Sediments
293
water bank-up and turbulence on the up-stream side of an obstacle, such as
pebble on a beach (Fig. 6.18) or river bed. As the water flows around the object it
produces the crescent-shaped scour which is preserved when being infilled by a
covering sediment. These structures are commonly found in fluvial sediments arid
on flood plains but also along strandlines and on intertidal flats.
4. Rill marks are small branching and often anastomosing water channels, usually
only centimetres wide, which form when water trickles over a subaerially exposed
bed (Fig. 6.19). A fossil example is shown in Fig. 6.20, which was formed by water
flowing laterally into a channel above the Bulli Seam, New South Wales. The rill
marks stop abruptly approximately 30 cm above the base of the channel, thus
suggesting that the channel was only one quarter full when the rill marks were
formed.
5. Drag marks or groove casts appear on the underside of some sandstones as
straight and narrow protrusions often several metres long and between some
millimetres to several centimetres in relief (Fig. 6.21). Occasionally, they occur in
parallel sets in which case they may have been formed by the same object, such as
the roots of a tree dragged along a river bed. Other tools are pebbles, ice, bones
and shells. The tools can sometimes be found at the down-stream end of the
groove casts.
6. Saltation marks are formed by tools which have only intermittent contact with the
substratum but move for most of the time in suspension.
All the current-formed surface and sole markings mentioned above are primary
structures which can give useful information on the drainage pattern that prevailed
at the time of their formation. The following subaerial weather markings are
secondary structures because they have been imprinted on an existing sediment.
They are also listed in Table 6.5:
Fig.6.19. Recent rill marks in beach sand on the New South Wales coast
294
The Relationship Between Coal and Interseam Sediments
Fig. 6.20. Rill marks found on the western
side of a washout cut into roof laminite of
the Bulli Seam at Mount Kembla Colliery,
New South Wales. The channel is 2.5 m
wide. (After Diessel et al. 1967)
Fig.6.21. Groove casts occurring on the underside of a fluvial sandstone from the Joggins section
in Nova Scotia
7. Rain and hailstone imprints (Fig. 6.22) can be found as small impact craters on the
surface of some lutites and arenites. Occasionally, it is possible to determine the
wind direction at the time of their formation because the craters become tilted
into the prevailing wind direction and resemble prod marks. They have been
observed in overbank deposits.
8. Mud cracks (Fig. 5.17) have polygonal shapes and extend up to several decimetres
into the substratum. In coal measures they are found in overbank deposits where
they have been preserved by infilling with sandy sediments. Due to differential
compaction, the infillings display a zig-zag pattern in cross-section by which the
degree of compaction can be determined.
9. Ice crystal imprints develop when thin sheets of water freeze. The result is a
skeletal growth of ice crystals which can be preserved when covered by a sediment
filling the voids left after the ice has thawed. An example from the Greta Coal
Measures, New South Wales, is shown in Fig. 2.11.
Some Characteristics and Properties of Interseam Sediments
295
Fig.6.22. Rain imprints in modern beach sand at New South Wales coast
The following group of deformational structure comprises the results of modifications to a sediment which happened after deposition but before diagenetic induration. These soft sediment deformations depend on several factors, among which
the type of sediment affected, its cohesion, particle size and shape are of some
importance. Table 6.5 gives a list of the various structures in this group, which
commonly occur in coal measures:
to. Load casts are commonly listed under sole marks because they are found at the
underside of sandstones, which are underlain by shales or other lutites. They
appear as bulbous downward convex protrusions of circular to polygonal shape
ranging in size from a few centimetres to several decimetres. They are the result
of rapid deposition of sand on a water-logged mud which, because of its plastic
properties, deforms easily, causing the sand to sink into the mud.
11. Ball and pillow structures are ball-, pillow-, and kidney- shaped bodies of sand
enclosed in a matrix of clay or silt. Usually they are slightly elongated and
convex downward and range in size from some centimetres to several
decimetres. Their origin is similar to that ofload casts except that the downward
sinking of sand into a soft mud led to a complete separation of the protrusion
from the sand layer above. In some instances, the formation of ball and pillow
structures may have been triggered by seismic shocks. Overbank deposits
formed on flood plains and in interdistributary bays have been found to contain
these structures.
12. Flame structures (Fig. 6.23) develop by the combined effects of gravitation and
tangential creep on a load casted deposit. The mud ridges between the load casts
push into the overlying sand and become drawn out by the differential
movement of sand and mud. The depositional environment is as for (10) and
(11).
296
The Relationship Between Coal and Interseam Sediments
Fig.6.23. Polished ac-plane of silt-laminated fine sandstone showing flame structure consisting of
carbonaceous shale. Roof of Bulli Seam, IIIawarra Coal Measures, Mount Kembla Colliery, New
South Wales
13. Convolute stratification is a form of intraformational folding which involves a
limited number of strata. This structure is common to siltstones and fine sandstones which, when water-logged, show hydroplastic behaviour.
14. Slump folds and intraformational breccia are both the result of gravity sliding of
one or several dislodged beds which had been deposited on an inclined surface.
Depending on the degree of deformation, the slumped sediment is folded,
sheared or brecciated.
15. Clastic dykes and sills are injections of fine to coarse-grained sediments into a
host sediment including coal. Not all clastic dykes represent true injections from
a source below the host rock but the term also covers infillings of fissures from
above. Figure 6.24 illustrates several generations of clastic dykes in the Greta
Seam of New South Wales. Frequently these structures are parallel to the
tension joints of the affected area (Courel 1987), which means that such joints
have been formed early, when the injected material was still soft and had a high
water content. There may be several possibilities for the formation of these
structures but a prerequisite for the true injections is rapid burial and enclosure
Fig. 6.24. Sketches of three
generations of clastic dykes
occurring in the Greta Seam
at Hebburn No.2 Colliery,
New South Wales. The coal
seam is 3 m thick. (Diessel
1984)
Some Characteristics and Properties of Interseam Sediments
297
Fig. 6.25. View of the bedding plane of a slightly carbonaceous siltstone with Permian plant fossils
(upper left) and worm burrows (light-coloured circular outlines). Tomago Coal Measures, New
South Wales
of sands in impermeable muds. With a thickening overburden and increased
lithostatic load a high pore fluid pressure develops in the sand which is released
by fissuring, for example, during an earthquake and injection of the liquefied
sand.
16. Gas heave and fluid escape structures are volcano-like or dish-like structures
ranging from several centimetres to decimetres in size from which mud or sand is
extruded through a central pipe. The reason for the extrusion is either ascending
gas or water. The first case is common in coal measures as the result of rotting
vegetation which has been rapidly buried.
17. Bioturbation refers to the disturbance of stratification in sediments by plant
roots and burrowing or feeding organisms. The degree of disturbance ranges
from the occurrence of occasional worm burrows, as in Fig. 6.25, to the complete
destruction of stratification.
There are some other structures which have been formed occasionally in coal
measure sediments such as coal and cinder dykes. The first example probably of
similar origin as the clastic dykes except that hydro plastic peat was mobilised.
Cinder dykes, on the other hand, contain semicoke which was mobilised during
carbonisation of a coal seam in contact with an igneous intrusion.
6.1.4 Coal Measure Lithosomes
In accordance with the hierarchical arrangement of sedimentation elements, the
next group ranking above sedimentary structures are lithosomes, i.e. rock bodies of
relatively uniform composition. Most stratigraphic columns contain several or
298
The Relationship Between Coal and Interseam Sediments
many different kinds of rock bodies which have been formed in response to specific
depositional conditions, and it is the identification and recording of these lithofacies
in bore cores and outcrops which form the basis of bore logs and the various kinds of
geological maps. In areas where the targeted coal measures form outcrops, field
mapping, supplemented by air and satellite photo interpretation (Shepherd et al.
1981), lithofacies studies and palaeocurrent analysis are the first steps in developing
a depositional model. The aim of the geological mapping is not only the
identification and cartographic display of the various rock and time-rock units
occurring on the surface of the investigated area but it also entails a statement on the
likely continuation and spatial attitudes of rocks and structures below ground level.
Field mapping therefore also assists in formulating a strategy of extending the
investigations into the subsurface.
In regions where coal-bearing strata are either deeply weathered or concealed
by younger deposits, as is the case in many parts of Australia (Ward 1982),
subsurface investigations are the main source of information, although even then
some reconnaissance mapping of the surface geology should be carried out, if only
for the purpose of selecting bore locations and traverses for geophysical surveys.
Improvements made in the last two decades to survey techniques and instrumentation have led to an increasing application of geophysical methods to coal
exploration. Refraction seismic (Peck and Yu 1982), more importantly, high
resolution reflection seismic have become important tools, not only in the
construction of structure contour maps and cross-sections (Taylor 1965; Ringis et al.
1967; Elliott 1979; Peace 1979; King 1979; King and Greenhalgh 1981; Ziolkowski
1979; Ziolkowski and Lerwilll979; Buchanan and Jackson 1986; Palmer 1987), but
also in the delineation of washouts and other impediments to mining (Harman and
Rutter 1979; Harman 1981, 1983, 1984; Hanes et al. 1989). To date, most seismic
reflection surveys of this kind present the results as two-dimensional profiles of the
strata beneath the survey lines, but current work is aimed at developing a threedimensional seismic technique for the coal industry (Hatherly et al. 1989). These
surface methods may be supplemented by in-seam seismic surveys (Ruter 1979;
Hatherly and Holt 1984; Doyle 1987; Doyle and Poole 1987), ground radar (J. Cook
1975; Coon et al. 1981; Turner and Yelf 1989), and radio imaging methods (Williams
and Thomson 1989). Although the in-seam methods are primarily designed to detect
tectonic discontinuities or assess stress patterns in coal seams by tomographic
imaging of p-waves and Love channel waves (Poole and Downey 1989), they can
also be employed in the mapping of washouts and other forms of roof rolls. In
addition, surface resistivity measurements have been used to trace boundaries of
shallow oxidised coal, concealed igneous rocks and nearsurface faults (Johnson
1977). Magnetic and gravity surveys made at the surface and underground
(Henderson 1967), as well as air- and shipborne magnetic plus gravity surveys
(Chamberlain 1948; Duffin 1970) have been employed in coal exploration for the
purpose of detecting areas of coal deterioration due to igneous intrusions.
In spite of the considerable amount of information that can be gathered from
the surface about subsurface coal measure architecture, any such projections carry a
low level of confidence if not backed up by a well-planned and executed drilling
campaign (Leblang and Svenson 1977; Whitby and May 1987). Two kinds of drilling
Some Characteristics and Properties of Interseam Sediments
299
are generally applied to coal, namely relatively cheap open-hole or non-core and
more expensive core-drilling (Price and Svenson 1977). In open-hole drilling the
only physical evidence of the penetrated lithosomes are the rock cuttings brought up
by the lubricant, which is either air or water. Considerably more insight into the
composition of the strata at depth is gained by coring the drill hole. The core
constitutes a permanent record of the subsurface at the drill site and it can be logged
in a similar manner as a continuously outcropping section on the surface, i.e.
litho somes and their textures and structures can be observed and identified and coal
lithotypes can be assessed. The order of superposition of the penetrated strata will
yield information concerning the depositional environment of the coal measures
from which a facies model can be either derived or updated, and thus may form the
basis for the positioning and spacing of subsequent bores. The information potential
can be further improved by obtaining oriented core samples which can provide
likewise oriented thin sections or polished blocks for petrofabric and other analyses.
The core-orienting device is set in non-magnetic rods attached to the inner core
barrel and can be activated at will from the surface through an impulse system and
retrieved with the core by wire-line (Price and Svenson 1977).
As has been mentioned above, coal measure sediments are classified in different
ways depending on whether the emphasis is on either compositional or textural
aspects. It is natural therefore that the same sets of parameters which are used in
classifying rocks, plus some additional features, are also employed in their identification. The basic parameters used by the field or well geologist in distinguishing
between different rock types are particle size, shape and roundness, depositional
fabric, type of stratification, composition, and the presence or absence of fossils.
While rock textures and structures are often revealed by direct observation and
require little more instrumental aids than a hand lens, the small size of many matrix
or cementing minerals makes it difficult to identify them outside the laboratory.
Field geologists overcome this problem by inferring mineral composition from such
observations as rock colour, streak, feel, weight (specific gravity), fracture pattern,
friability or coherence, hardness, smell and even taste, plus some simple chemical
tests directed to reveal the presence of carbonate. In bore-core logging some
additional observations are employed which concern the condition of the core, e.g.
loss, breakage, dilatation and others. In modern drilling operations the abovementioned observations are often supplemented or, in open holes, replaced by a
wide range of geophysical wireline tests. Indeed, most of the current techniques of
geophysical well logging are lithosome-oriented, i.e. they are capable of distinguishing between broad lithofacies categories.
The application of geophysical well logging to coal exploration became
widespread in the 1970s, when oil companies extended their activities into the search
for coal. In the application of geophysical techniques to bore logging in coal
exploration, the existing tools required little alteration as far as interseam sediments
were concerned, except that their size had to be reduced to about 5 cm (or less) outer
diameter in order to cope with the slim holes commonly used in coal exploration. To
be applicable to the specific characteristics of coal, the range of measurements had to
be geared to its physical properties. Moreover, it was necessary that the logging
equipment was light-weight and mobile and offered a cost advantage over coring
300
The Relationship Between Coal and Interseam Sediments
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Fig. 6.26. Comparison of a geophysical wireline log and a geologist's measured section (in m)
from the German Creek Coal Measures, Bowen Basin, Queensland. (Original prepared by
McElroy Bryan Geological Services Pty. Limited)
Some Characteristics and Properties of Interseam Sediments
301
because the latter would probably be preferred at equal costs. The relative merits
may be arguable, because it has been claimed (Lavers and Smits 1976; Ward 1982),
that geophysical logs provide a more detailed picture of coal seams and interseam
sediments than can be achieved with core descriptions by a geologist. However,
often they measure other properties than those determined in the laboratory (Till
1987). On the other hand, a bore core can yield substantially more information than
a log, apart from constituting a sample which can be analysed in the laboratory.
Most of the geophysical logging techniques which are particularly useful in
discriminating between coal measure sediments, including varying coal qualities,
make use of either natural or induced radioactivity. According to Mumme (1987)
they can therefore be divided into:
1. Passive methods which involve measurements of gross and spectral characteristics of naturally occurring gamma-ray emission, and
2. Active methods which invoke response through the use ofinterrogating radiation
based on reactions between either gamma-rays or neutrons and formation nuclei.
The brief discussion given below includes those wire-line logging techniques which
are particularly applicable to coal exploration. For more information see Reeves
(1976, 1979), Jackson (1981), Renwick (1982), Groves and Bowen (1982), Weber
(1982), Haigh and Edwards (1982), Mumme (1987). An application of some of the
techniques to a section of Australian coal measures is illustrated in Fig. 6.26.
6.1.4.1 Natural Gamma-Ray Log
This is a passive method which measures the gamma-ray emission from the wall
rocks as the counter is lowered into the borehole. Its importance in coal exploration
is based on the extremely low radioactivity of clean coal compared with all other
rocks encountered in coal measures. Of the three natural gamma radiation
emmitters, uranium, thorium and potassium, the latter, in the form of isotope 4°K, is
commonly contained in mica and illitic clay mica. The gamma log is therefore a
measure of the clay and mica content of rocks and, since the various rocks are
characterised by varying clay fractions, gamma logs are good discriminators
between different lithosomes. Because marine shales contain a higher proportion of
illite than lutites of terrestrial or freshwater origin, they give a particularly high
response. The same is true for tuffs and many tuffaceous claystones which is
particular useful in coal measure correlation (Agnew and Bayley 1989). Conversely,
clean sandstone records almost as low gamma-ray deflections as coal (Reeves 1976,
1979), while intermediate readings are obtained from shaly coal and coaly shale, as
well as from sandstone/shale transitions. This means that a positive identification
can be made of shale, including some estimation of its depositional setting, but no
clear discrimination between either clean or impure sandstone and coal is possible
with response from the commonly used gross count gamma ray tools alone. They
record a broad energy spectrum, but it is possible to supplement them with a
gamma-ray spectrographic system which allows the measurement of the intensities
302
The Relationship Between Coal and Interseam Sediments
of discrete narrow bands of energy, thus permitting discrimination between different
sources of radioactivity and thereby extracting more information about the penetrated rocks (Mumme 1987). However, more commonly the additional information
is obtained from the active nuclear logging techniques discussed below.
6.1.4.2 Density (Gamma-Gamma) Log
Because the density of coal is considerably lower than that of most other rocks
encountered in a logging operation, positive identification and a distinction between
coal and sandstone with good boundary definition is possible with the aid of density
measurements. This applies also to shaly coals and sandstones with clay matrix,
which may record similar gamma ray readings but still differ considerably in their
respective densities. The instrument consists of a gamma radiation source at one end
of the tool and a detector, such as a Geiger-Muller counter, a small distance away.
The tool is shielded such that the only radiation from the source which reaches the
detector is that deflected back from the wall rock by Compton scattering. Its
response depends on the electron concentration in the rock which is proportional to
its density (Dobrin 1960). Depending on the spacing between radiation source and
detector, the resolution of the density tools can be varied. The three types most
commonly used yield long spacing density logs (LSD) with a spacing of 48 cm, high
resolution density logs (HRD) with a spacing of 24cm, and bed resolution density
logs (BRD) with a spacing of 15 cm (Lavers and Smits 1976). In the multifunction
probe described by Weber (1982) the respective spacings are 40,20 and 9.7 cm.
6.1.4.3 Neutron-Neutron Log
The high energy with which neutrons generated by the neutron logging tool enter
the wall rock is gradually reduced by collisions of the neutrons with formation
atoms. For the common geological materials hydrogen is the most efficient agent for
moderating the incoming neutrons, followed, to a lesser extent, by carbon (Mumme
1987). The high proportion of both hydrogen and carbon in coal gives it a fairly high
response, commonly higher than shale, in which the hydrogen of the hydration
water and the hydroxyl groups contained in clay minerals respond to the tool (Anon
1981). However, the main property revealed by the neutron log is porosity. The
higher the porosity of a rock, the higher the moisture content and the more the
neutron energy is attenuated by the pore water. The log is therefore useful in
distinguishing high porosity from low porosity rocks, e.g. sandstone from dense
limestone, which are not easily distinguished by other tool responses. As mentioned
above, some difficulty is experienced with coal, which is commonly of low porosity
but is high in hydrogen and carbon, thus indicating an anomalously high porosity.
6.1.4.4 Caliper Log
The actual diameter of boreholes varies with the nature of the strata penetrated by
the drill. Diameter measurements are carried out by the caliper tool, which consists
Some Characteristics and Properties of Interseam Sediments
303
of either one or three equally spaced spring-loaded arms. While in the latter case the
three arms centre the probe in the hole, the purpose of a single-arm caliper is to push
the tool and its sensors against the wall of the borehole in order to assure close
contact (Weber 1982). Firm and well-lithified sediments will form a solid wall rock,
whereas soft or partially soluble strata will be more affected by the drill and tend to
break out. A caliper log may show therefore considerable detail which allows
competent and incompetent strata to be separated and measured with reasonable
accuracy. Dull coals usually remain close to the diameter of the drilling bit, the
brittleness of vitrinite causes bright coals to show some abrasion, while cavities
occur frequently at the contacts between coal and interseam sediments.
Apart from indicating lithologic variations themselves, caliper logs are needed
in conjunction with density logs. Particularly high resolution and bed resolution
logs are adversely affected by variations in borehole diameter for which corrections
ha ve to be made.
6.1.4.5 Sonic Properties
Coal is characterised by very low acoustic velocities which tend to vary somewhat
with compaction and density. Reeves (1979) quotes sonic'velocities of 160 ms/ft for
subbituminous coal and 120 ms/ft for bituminous coal. The acoustic velocities for
interseam sediments are commonly much higher, which means that sonic logs are
effective not only in identifying coal seams but, within limits, also as indicators of
coal rank. Moreover, combinations of sonic and density logs have been used in order
to show variations in rock strength.
6.1.4.6 Resistivity Log
The electrical properties of most sediments are governed by their formation water
and its salinity. On this basis most coals would be characterised by high resistivity,
whereas dirt hands composed of hydrated clays display increased conductivity. The
developments of focussed resistivity tools has improved the measurement of true
formation resistivity. This has important implications in coal seam analysis (Reeves
1976), but the diagnostic properties are not consistent and can vary locally (Anon
1981). Careful calibration of the log and frequent checking with measured sections
from the same area appear to be necessary.
6.1.4.7 Spatial Attitude
Both accurate measurement of true seam thickness and assessment of the degree of
tectonic deformation of a concealed coalfield require good knowledge of the
structural attitude of the beds penetrated by the drill. This can be measured by a
dipmeter which is based on a centralised slimline sonde with a maximum outer
diameter of 5 cm, equipped with three miniature focussed resistivity sensors. An
304
The Relationship Between Coal and Interseam Sediments
array of two level cells and three magnetometers in the upper section of the sonde
provides continuous hole orientation data. The resistivity sensors, mounted on a
caliper mechanism and kept in a known geometrical relation to the body of the tool,
create a continuous record of formation resistivities recorded at extremely high
resolution and changing with lithology (Anon 1981).
6.1.4.8 Combination Tools
In order to speed up the logging operations and improve their quality, various tools
have been developed which combine up to four different measurements into one
instrument. They are variously referred to as multifunction probe (Weber 1982) or
combination sonde (Anon a). The main improvement compared with single-purpose
tools is not only the considerable time saving achieved by simultaneously recording
four properties with logging speeds around 10 m/min in interseam sediments or
overburden and at 2 m/min in coal. An equally important benefit of a combination
sonde is the simultaneous recording of several variables, which overcomes problems
of lining up different logs that have been recorded separately (Reeves 1979).
Four measurements are carried out by the combination sonde (Anon a) which
are commonly arranged in three different presentations:
1. The coal lithology log is based on a combination of gamma ray, side-wall LS
density and caliper logs, usually run at scales of 100: 1 or 200: 1 to facilitate
lithologic correlation. The position of coal seams, shale and sandstones can be
accurately determined, but more complex rock combinations may require
additional data from neutron, sonic and focussed resistivity measurements.
2. The coal quality log is based on a combination of gamma ray and side-wall LS
density run slowly on an expanded scale. The density of the coal can be read
directly in gm/cm 2 thus allowing ash content evaluation while the gamma log
gives a cross-check on shale content. Very thin seams cannot be fully resolved,
because LS density is a deep penetration tool which lacks fine resolution.
3. The seam thickness log is a combination of side-wall BR density and caliper log.
Because of the high resolution of the BR density, even thin coal seams are fully
resolved. The caliper log plays an important role because BR density is a shallow
penetration tool and requires careful correction measures when the hole diameter
varies.
Other multiple tools, for example the multifunction probe described by Weber
(1982), combine different sensors including natural gamma rays, self-potential (SP),
single point resistance, neutron, and hole deviation. In summary, coal, and most
clastic sediments can be processed from LS density and gamma logs with a direct
computer print-out. The appearance of limestone or igneous rocks in succession
would make the use of additional sonic and neutron logs desirable. By establishing
the relationship between coal bulk density and ash content for a given area, ash
content values can be obtained from a combination of LS density and gamma logs.
Seam thickness and stone band distribution, provided the latter are at least several
Some Characteristics and Properties of Interseam Sediments
305
centimetres thick, can be achieved by BR density combined with caliper logs. For
more detailed information see the BPB Coal Interpretation Manual (Anon 1981),
which lists the following (modified) steps in the interpretation of coal measure
lithosomes:
1. Search out low gamma-ray and low density responses, which indicate coal.
2. Draw a shale line through consistently high gamma-ray peaks.
3. Mark the highest gamma-ray peaks beyond the shale line. These indicate marine
(or if present, uranium) shales.
4. Draw a sand line through consistently low gamma-ray but high density (and
porosity if available) peaks.
5. The tool responses between the sand and shale lines are likely to indicate
mixtures of gradations between the two lithofacies.
6.1.4.9 Data Management
An important part oflithofacies analysis is the recording, processing and evaluation
of surface and subsurface (including bore data) information. The signals generated
by down-hole geophysical tools are electronically stored and processed by a truckmounted computer (Weber 1982) or at base, which enables them to be displayed as
fully calibrated logs (Haigh and Edwards 1982). The description of rock types in
outcrop sections, drill cores and cuttings is commonly recorded in log form.
Frequently the logs are compiled manually using full descriptive language but this is
time-consuming and expensive. Moreover, there is sometimes a lack of consistency
in the observations recorded in such logs, especially when more than one geologist is
engaged on a project. The shortcomings of manually transcribed logs become more
evident where a large number of measurements and observations are required to
evaluate a deposit. Computers afford an ideal means of handling and manipulating
the data, thereby avoiding the laborious and expensive method of manually
compiling the logs in full descriptive language (Goscombe et al. 1977).
The repetitive nature of the main rock and coal beds encountered in rock
sequences typical of particular coal measures has led to a structured form of
description for the main features which has enabled these features to be coded for
computer assimilation (Melton and Ferm 1978; Lehmann 1978; Pauncz and Holt
1982). As discussed by Mallett and Ward (1982), the formats range from simple
prompt lists to tabulated questionnaires with option boxes and mnemonic
descriptors. Supported by standard photographs and reference charts, it is possible
to make a concise, systematic description of a particular rock by making use of a
limited number of salient textural, structural and compositional descriptors (Ward
et al.1984). From the tabulated coding sheets the gathered information is entered
into the data bank of a computer (Mallett and Bonner 1981; Gill 1982), but it is also
possible to record field data directly by a portable and rugged microcomputer
(Ward 1986b). Following the development of appropriate software (Stoddart and
Wood 1987; Miller and Huntington 1987), the latter approach was found to have
many benefits which include:
306
The Relationship Between Coal and Interseam Sediments
1. Elimination of the paper coding sheets in the field and therefore once only entry
of data, i.e. elimination of errors during data transferal.
2. Storage of all valid entry codes or numerical codes in a dictionary which
automatically traps simple data input errors.
3. Prompt for simple data entry of predominantly alphabetical type (electronic log
sheet) with automatic generation of default responses, but allowing for flexibility
where an answer is not known at the time of logging, or where non-coded
comments need to be included.
4. Rapid availability of processed data to the field geologist at the time of logging
thus permitting alterations to the log if necessary.
As a result of this development, a wide range of computer-based data processing
packages have been developed (Goscombe et al. 1977; Thomson 1979; Grimstone
et al. 1982; Miller and Huntington 1987; Moule et al. 1987; Stoddart and Wood
1987), which enable borelogs, structure contour, lithofacies and coal quality maps,
as well as cross-sections and three-dimensional models of the deposit to be drafted
from an automatic plotting device, and it is further possible to programme the
computer to translate the coded input from the log into a written print-out.
6.2 Coal Seams and Their Floor Rocks
In the following discussion, the subject of coal measure lithosomes will be focussed
on the specific relationships between coal and interseam sediments beginning with
the floor rocks of coal seams. Geological interest in floor rocks dates back to the
beginning of modern coal science in the early 19th century, when the widespread
similarity in their composition and the abundance of roots in them was used by the
supporters of in-situ i.e. autochthonous coal formation to challenge the idea of a
drift origin of coal (Bennett 1964).
The floor sediments of many peat and coal deposits consist of deeply weathered
soils, particularly when they were formed in a warm tropical climate. They
constitute seat earths, which are often penetrated by numerous plant roots resulting
in the loss of much of the original stratification, although most seat earths, which
appear non-bedded when fresh, show some vague bedding when weathered, a
Carboniferous example of which is illustrated in Fig. 6.27. In contrast, coals having
developed under cool to cold conditions, such as most Gondwana coals, are
associated with poorly developed soils, although Vertebraria (roots of Glossopteris)
may be abundant and deep-reaching. Figure 6.28 shows Vertebraria penetrating a
silt-laminated sandstone with almost undisturbed stratification and few signs of
weathering and soil formation. Leaching is commonly restricted to the contact zone
with the overlying coal, which is particularly obvious when the coal seam rests on a
pyroclastic floor, which is frequently the case in the Newcastle Coal Measures of
New South Wales. An example of the compositional changes is illustrated in
Fig. 6.29 by means of two sets of differential thermal analysis (DT A) curves which
Coal Seams and Their Floor Rocks
307
Fig. 6.27. Photograph of Stigmaria appendices in seat earth of the Katharina Seam, Ruhr Basin,
Germany
Fig.6.28. Vertebraria roots in bedded sandstone underneath Borehole Seam, Newcastle Coal
Measures, New South Wales
are described below:
1. Low temperature range (0 to 250 °C). All samples display a strong endothermic
peak due to dehydration at 100°C and above. In this range most of the absorbed
water is lost from the interlayer space of clays, particularly from montmorillonite, which tends to display a double peak.
2. Medium temperature range (250 to 800 °C). The shift in the endothermic
dehydroxylation temperature from 500 and 600 °C in the zone penetrated by
308
The Relationship Between Coal and Interseam Sediments
. , , - - - - - - - 2.30 m Coal - - - - - - _ + _
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K - Kaolinite (dominant)
k -Kaolinite
M- Montmorillonite (dominant)
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m- Montmorillonite
i- Illite
s - Siderite
Fig, 6.29. The changing composition of the seat earth of the Wallarah Seam, Newcastle Coal
Measures, as analysed in two different localities by DTA techniques. For details see text
plant roots (Samples lc and w to 3c and w), to 600 and 700 °C in the underlying
samples, indicates a change from kaolinite as the dominant constituent to
montmorillonite with some associated kaolinite and illite. The low peak
temperature for the kaolinite dehydroxylation suggests poor crystallinity, which
was verified by XRD and microscopy. Small exothermic reactions in the 300 to
400 °C temperature range (mainly in Sample lc) indicate burning of coal
inclusions.
3. High temperature range (800 to 1000 0q. The main feature is an exothermic peak
between 900 and l000°C, which in some samples is preceded by a small
endothermic reaction. It indicates the rearrangement of the degraded crystal
lattice and the formation of'}' Al 2 0 3 or mullite. This peak is commonly associated
with kaolinite but is also found in other clay minerals.
While the clay minerals and the small amounts of siderite found in the above
claystone have been derived from the devitrification of volcanic glass, the seat earth
also contains dispersed grains of quartz, feldspar, biotite and accessory zircon, all of
which have been inherited from the pyroclastic history of the deposit. Both biotite
and feldspar (orthoclase and plagioclase) have been affected by partial kaolinitisation, which is responsible for the small kaolinite signature occurring in the DT A
traces throughout the deposit. Kaolinite is dominant and has replaced montmorillonite and montmorillonite/illite mixed layer clays in its upper portion, where
the volcanic ash was affected by plant roots and the acid peat water laden with CO 2 ,
Coal Seams and Their Floor Rocks
309
Fig. 6.30. Photograph of the Carboniferous Bottom Hutton Seam, in coastal
outcrop north of Tynemouth, U.K., displaying concordance with roof and floor
rocks. Root traces in seat earth to the left
of the hammer have been emphasized with
white chalk. Roof consists of alternating
sand- and siltstone
The seat earths of Carboniferous and other coals formed under warm
conditions often consist of well-developed fossil soils (palaeosols), which have been
leached of alkalis and alkaline earths. When fine-grained, they form underclays or
fire clays, the latter term referring to their refractory properties, because of their high
kaolinite content. Their colour is usually pale grey to dark and bedding is commonly
absent, although near the coal contact a thin transitional zone consisting of
alternating coal and shale laminae (coaly shale and shaly coal) can be found in some
seams. Seat earths are frequently slickensided and may contain pyrite and/or siderite
nodules and coal inclusions in addition to the dense network of rootlets. The
frequency of slickensided surfaces increases with both clay and root content, and
the branching of the commonly vitrinitised roots may determine the position and
orientation of the slickensided surfaces. Slickensides are less frequent in sandy seat
earths, among which leaching of alkalis and alkaline earths may have caused
enrichment in silica. The resultant hard and brittle rock, known as ganister in British
coalfields consists, according to Eden et al. (1957) and Jessen (1961), of almost 100%
silica and has been used commercially as refractory material.
Although floor rocks are mostly planar and have a concordant relationship
with the overlying coal (Fig. 6.30), undulations and protrusions of floor material
into the coal are not uncommon. In English-speaking countries miners have
variously referred to them as floor-rolls, stone-rolls, roll-stones, swillies, razor backs,
310
The Relationship Between Coal and Interseam Sediments
hogbacks, or horsebacks. These structures consist oflong (often tens of metres) and
rather narrow (up to a few metres) ridges of floor rock which often occur in
subparallel swarms and protrude upward into the coal thus reducing the seam
thickness above them. Their mode of formation is very controversial and the
explanations offered may be summarised as follows:
1. Expansion folds due to swelling of floor shale on hydration (Woolnough 1910,
1933).
2. Small compression folds resulting from post-sedimentary tectonic deformation
(Edwards et al. 1944; Moore 1913, 1940).
3. Bulges of floor shale associated with minor syn- and possibly post-sedimentary
disturbances (Hills 1963).
4. De-watering structures resulting from the forceful injection under lateral pressure
into the seam of floor material in a quasi-liquid rheotropic state (Agrali 1987).
5. Aapamire (string-bog) structures of perm a-frost origin related to frost heave
(Conaghan 1984).
6. Ridges in the floor prior to peat deposition with drainage channels ( = swallows)
between them (Harper 1915).
7. Rather than constituting ridges between channels the stone-rolls themselves are
silted-up drainage channels, which drained the mire before and in the early stages
of peat accumulation until they became defunct and overgrown (Diessel and
Moelle 1970).
The geographic distribution of floor-rolls is very uneven. They are concentrated in
only a few coalfields scattered throughout the world, including Pennsylvania, U.S.A.
and in Australia at W onthaggi in Victoria and in the southern part of the Sydney
Basin, N.S.W.
While the organic acids percolating from the peat into the floor strata affect
their composition, a reverse influence of the seam floor on both geometry and
composition of the overlying coal is also evident. The palaeotopography of the
peat's depositional base controls the drainage pattern of the mire and thus many
aspects of peat accumulation, particularly in peatlands where a strong palaeorelief
of the seam floor predated the onset of peat accumulation. Examples of this kind
have been reported from the Maules Creek Subbasin of the Gunnedah Basin in
New South Wales (Brownlow 1981a; Butel et al. 1983; Thomson and Flood 1984;
Thomson 1986) and the Highveld Coalfield in South Africa (Winter 1986).
According to the latter author, coal is either absent on palaeo topographic highs or
very thin, and is rich in inertinite and high in ash due to considerable degradation
resulting from periodic subaerial exposure. Although more peat accumulated in
palaeotopographic lows, the latter also attracted fluvial channels, which caused
erosion and seam deterioration due to high adventitious ash. Most of these effects
are concentrated in the lower portions of coal seams, since with sustained
accumulation, palaeo relief became progressively subdued and consequently had less
effect on peat composition.
In the above mentioned Maules Creek Subbasin the Late Carboniferous/Early
Permian Boggabri Volcanics form the base of the coal measures. According to Butel
Coal Seams and Their Roof Rocks
311
et al. (1983) the volcanics exhibit a relief in excess of 200 m over very short distances.
The basement highs have resulted in localised non-deposition of lower coal
members and the abrupt convergence of lower seam portions into complex
composite horizons, followed by marked differential compaction.
6.3 Coal Seams and Their Roof Rocks
The contacts between coal and its overlying strata may be abrupt or gradual, in
common with any other boundary between adjacent sediments. In some cases the
coal and its roof sediments are in direct contact without any intervening beds, which
may be due either to a sudden change in depositional conditions, accentuated by
slow sedimentation, or to the removal by erosion of intervening strata. In this
situation probably no strong genetic link exists between the petrographic nature of
the coal and the superincumbent sediments.
Transitional contacts result from a gradual change in depositional conditions,
for example in a eutrophic mire when an increasing rate of subsidence leads to a
rise in water level and the expansion of open water (ponds, lakes) in the peatland.
Originally relatively dry portions become wetter, resulting in a change in vegetation
and the conditions of biochemical coalification. Before the peat finally drowns, it may
produce the precursors of "wet" lithotypes, such as sapropelic (boghead and cannel)
coal, and dull coals rich in hypautochthonous or allochthonous inertodetrinite,
sporinite and perhaps alginite. The increased flooding of the peat surface may raise
the mineral content of the coal and lead to intercalations of stone bands until
lacustrine or marine shale facies become dominant.
In the opposite case of a falling water level, the evidence is not always preserved
because the ensuing exposure of the peat leads to its partial destruction and possible
erosion. Nevertheless, the peat will respond to the change to drier conditions well
before peat formation ceases by undergoing more severe humification, desiccation
and possibly combustion, so that "dry" lithotypes, i.e. fusain and dull coal rich in
autochthonous inertinite, become concentrated near the seam roof. This kind of
genetic relationship involves primarily compositional aspects of coal/roof couples,
but Jones et al. (1972) also found that vitrinite reflectance in the underlying coal also
varies with the nature of the roof sediments.
Where the coal seam and its roof sediments are genetically linked, the roof/coal
contact is commonly concordant, i.e. the former depositional surfaces, as represented by the stratification planes in both coal and overlying sediments, are parallel to
each other. In many instances the same spatial relationship applies also to abrupt
contacts, but angular discordances between coal/roof couples are not rare, and some
of them are unique to coal and are not recorded in other stratiform deposits. The
following discussion is therefore divided into two sections, the first concentrates on
some palaeo-environmental implications of concordant coal/roof relationships,
while the second highlights the genetic significance of discordant coal/roof
contacts.
312
The Relationship Between Coal and Interseam Sediments
6.3.1 Concordant CoaljRoof Couples
The brief examples given above show that transitional contacts between coal and
its roof and floor strata reflect a gradual shift in depositional conditions from
predominantly organic to inorganic sedimentation. Even where coal seams are in
abrupt contact with their roof sediments, the depositional environments of the latter
may have affected the underlying coal. An example of this effect would be where a
marine environment transgresses rapidly across a coastal platform, in the course of
which fresh-water peat is inundated by sea water, which in turn will penetrate into
the underlying peat and, by lowering acidity, change the pattern of the subsequent
biochemical coalification in the affected portion. Although in this case marine
conditions were established after peat accumulation had ceased, the effects on coal
composition are not dissimilar to an increasing marine influence during peat
formation. This aspect will be further explored in Chap. 8.
6.3.1.1 Abrupt Contacts Between Coal and Roof Rocks
Most contacts between adjacent lithosomes are conformable, i.e. there is no angular
discordance between beds and no indication of a prolonged pause in sedimentation.
Nevertheless, conformable contacts may be quite abrupt, which occurs in regions
experiencing very slow sedimentation. The stratigraphic columns found in such
areas are so condensed that gradual transitions from one lithosome to the other
cannot be accommodated within a relatively short time frame (Krumbein and Sloss
1963). Also, in areas of low topographic relief even limited vertical base level
variations may bring about rapid and considerable lateral shifts of facies boundaries. These may proceed at a rate which causes different sediments to occur in close
superposition wihtout any transitional or passage beds between the contrasting
lithosomes. The sharp contacts between coal and the roof limestone found in the
North English coalfields are examples of how sensitively facies boundaries in regions
of low relief and proximity to the sea have responded to even minor vertical
fluctuations in sea level.
Abrupt contacts between coal and roof sediments often indicate rapid
palaeo-environmental changes and possibly a sudden, even catastrophic end to peat
formation. The events which change the depositional environment so severely that
peat formation cannot recover may take several forms, such as inundation by fresh
water due to river avulsion, or flooding of a coastal mire by sea water following the
collapse of beach barriers by a combination of cyclonic storm activity and high tides.
In any of these cases, subsequent sedimentation will tend to fill the erosion scours,
which have been excavated into the peat by the preceding violent events, resulting in
some of the discordant roof/coal contacts discussed in Chap. 6.3.2.2.
A catastrophic event, which is capable of terminating peat accumulation
without causing substantial erosion of the peat surface, is the emplacement of a thick
cover of volcanic ash (tephra) on the mire surface. Vulcanicity shares with some
other catastrophic events complete genetic independence from mire development,
Coal Seams and Their Roof Rocks
313
which means it can terminate peat formation at any stage in the generation of a
peatland by choking youthful and senescent plants alike with volcanic debris. In
contrast to many epiclastic roof sediments, which were deposited on the peat after
the mire had become defunct, pyroclastic roof rocks cover former peat surfaces
which had remained active to the moment the peat-forming plants were killed by the
volcanic ash.
Examples of volcanic activity are the intra-seam tonsteins discussed in
Chap. 4.4.2.1. In addition to these relatively thin layers of pyroclastic material
contained within coal seams, some coalfields contain substantial quantities of
interseam tuffs. Striking examples of these are found in New South Wales, Australia,
where approximately 20% of the 500-m-thick Late Permian Newcastle Coal
Measures consist of rhyolitic to rhyodacitic tuff and tuff-derived material (Diessel
1980a, b, 1985c). Also the composition of many epiclastic deposits in this setting
may have been strongly influenced by penecontemporaneous vulcanicity, which
was situated outside but close to the orogenic basin margin and repeatedly caused
pyroclastic material to be shed into the Sydney Basin. These tephra layers range in
NAMED PYROCLASlliS
--------~,..,..o+- VALES POINT
COWPER TUFF - - - MANNERING PARK TUF~
SWANSEA CLAYSTONE
MOON ISLAND
BEACH
BOORAGUL TUFF ---_.--.GREj~T
AWABA TUFF
UPPER PI LOT
REIDS MISTAKE FORM.
~G}-LOWER PILOT
-
BOOLAROO
HARTLEY HILL
IAN
HILLSBOROUGH TUFF - -
ADAMSTOWN
---.. FERN VALLEY
Fig. 6.31. Stratigraphic
column of the Newcastle
Coal Measures of New
South Wales highlighting
the distribution of
interseam tuffs. (Diessel
1985c)
NOBBYS TUFF - - - -
----- VI (TOR I A TUNNEL
LAMBTON
314
The Relationship Between Coal and Interseam Sediments
Fig. 6.32. Photomicrograph of vitric tuff with volcanic glass shards from the Reids Mistake
Formation, Newcastle Coal Measures, New South Wales. Transmitted light. Top one polariser,
bottom crossed polarisers, actual length of the field of view = 15 mm
thickness from less than 1 mm to 25 m, of which the stratigraphic column in Fig. 6.31
indicates the position of the major pyroclastic horizons only. The tuffs range in
particle size from coarse crystal-vitric varieties, as illustrated in Fig. 6.32, with
occasionallapilli to dense ashstones, the latter often altered to bentonitic claystone.
Quartz, biotite, plagioclase, orthoclase, volcanic rock fragments and unwelded glass
shards occur in varying proportions in the tuff layers, which often display normal
grading from coarse crystal tuff at the bottom through vitric tuff to fine ashstone
within a thickness of only a few centimetres.
According to Ziolkowski (1978), many of the de vitrified fine-grained tuffs
display a microcrystalline intergrowth of authigenic chalcedony and analcime in
315
Coal Seams and Their Roof Rocks
coal and bands
grey cherty
claystone
coal and bands
contorted crysta
and vitric tuff
c:
o
'"
massive crystal
and lithic tuff
E
(;
lL
"""
as
.!!!
::.
".0;a:"'
parallel bedded
vitric and crystal
tuff
Fig. 6.33. Schematic
representation of the
pyroclastic zonation
found in the Reids
Mistake Formation,
Newcastle Coalfield.
(After Diessel 1985c)
coal and bands
Q. E
""'!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!l!!!!!!!!!!!!!!==!!! soft c I a y s ton e
- as 55
"o "
---
----
coal and bands
~cJ)
soft claystone
...J
coal and bands
irregular, almost cloudy concentrations, or they occur as metasomatic replacements
of glass shards. Angular quartz grains show marginal resorption and precipitation
of chalcedony around the rims. Plagioclase is often fresh or only weakly sericitised,
whereas biotite which occurs in very high concentrations in some tuffs (Fig. 4.38),
usually shows some alteration to kaolinite or montmorillonite/illite random mixed
layers (Loughnan 1966).
Montmorillonite is the earliest authigenic phase resulting from the hydrolysis of
fine-grained volcanic glass as rain water percolated through the tuff layer after it had
settled on the ground. The same process leached alkalis, silica and iron from the
glass, which increased the pH of the descending fluids. Zeolites formed as an
intermediate metastable phase, which broke down as the sodium concentration
increased and, together with montmorillonite, transformed into analcime and silica
(opal and chalcedony) near the bottom of the pyroclastic pile (Hay 1977; Hay and
Sheppard 1977). The silica formed in this manner impregnated both plants and the
uppermost peat layers, thus preserving the mire surface as it was at the time of the
volcanic event.
A common characteristic of all pyroclastic deposits in the Newcastle Coal
Measures is their considerable lateral persistence, although thickness variations
occur between inter- and intraseam tuffs. As discussed in Chap. 4.4.2.1, intraseam
tuffs have been transformed into claystones which retain their thickness over a large
area. Interseam tuffs consist of stacks of tephra, which differ from each other in
grain size (coarse pyroclasts to fine ash), colour (white, pink, green, cream), fabric
316
The Relationship Between Coal and Interseam Sediments
(massive, cross-bedded, laminated), different grades of secondary silicification and
other forms of authigenesis resulting in contrasting weathering patterns. These piles
of pyroclastic lithosomes, while covering many hundreds and in some cases
probably thousands of square kilometres, vary considerably in their composite
thickness. The multitude of individual beds and laminae contained in a thick
pyroclastic column can usually be grouped into a relatively small number of units
with similar features. An example of this is given in Fig. 6.33, based on the Reids
Mistake Formation of the Newcastle Coal Measures. Its stratigraphic position is
indicated in Fig. 6.31. Within its thickness of 7 m it is possible to distinguish four
major units (from Diessel 1985c):
Unit 1 in Fig. 6.33 forms the immediate roof of the Lower Pilot Seam. It is composed
of parallel bedded vitric to crystal tufTs which consist of 2- to 20-cm-thick strata.
These are cross-laminated and show surface undulations, such as ripple marks and
minor cut-and-fill structures indicating lateral movement from the northeast.
Unit 2 displays extensive dune and antidune development, which has been
accentuated in many places by the ramping-up of subsequent deposits against
Fig. 6.34. Photograph of some of
the depositional units of the Reids
Mistake Formation indicated
in Fig. 6.33. 2 Unit 2; 3 Unit 3; 4
Unit 4; 5 Upper Pilot Seam.
Movementfrom right to left
Coal Seams and Their Roof Rocks
317
Fig. 6.35. Cartoon illustrating various modes formation of pyroclastic deposits. (After Diessel
1985c)
earlier bedforms. The bedding surfaces of this unit show a distinct hummock-like
pattern. Pinch-and-swell structures and cross-bedding are common.
Unit 3 consists of a 0.5- to 1.5-m-thick massive and coarse pyroclastic interval with
irregular lower but relatively smooth upper bounding surfaces (Fig. 6.34). The
coarsest portion including fragments of aphanitic volcanic rock occurs near the top
of this unit.
Unit 4 is similar to Unit 2 with wavy to contorted bedding, hummock-like surfaces,
dunes, antidunes, ripples, cross-bedding, pinch-and- swell structures (Fig. 6.34) and
climbing megaripples. Thin beds with pronounced grain size separation are draped
over bed forms, and accretionary lapilli occur in the upper portion of this unit.
In accordance with the work of Sparks and Walker (1973), Fisher (1979), Self
and Sparks (1980), Walker (1980), Lipman and Mullineaux (1981), Fisher and
Schmincke (1984) and others on modes of pyroclastic transportation and emplacement, three mechanisms can be considered as being responsible for the origin of the
Reids Mistake and other tuff sequences in the Newcastle Coal Measures, namely
pyroclastic fall, flow and surge. They have been schematically illustrated in Fig. 6.35
and are discussed below (after Diessel 1985c):
1. Pyroclastic falls comprise ash particles which have been explosively ejected from
a vent, for example, as part of a Plinian column. They fall through the air and
settle on the ground as an ash fall deposit. A lateral component may be imposed
318
The Relationship Between Coal and Interseam Sediments
on the settling trajectory by wind drift, lateral expansion of the ash plume, and
initial lateral velocity resulting from the shape of the vent. An extreme case of the
latter was the directed blast which proved to be so destructive in the Mount St.
Helens explosion of 18th May 1980. Although the main feature ofa directed blast
is the formation of a pyroclastic surge, turbulence within the blast cloud lifts ash
particles high into the air, from which they will also settle as an ashfall. The same
happens when a ground surge and flow develop following the gravitational
collapse of a Plinian ash column. The resulting ash fall deposits are crystal-poor,
display mantle bedding, good to moderate sorting, almost exponential decrease
in bed thickness and particle size with distance from the vent and, when waterflushed, accretionary lapilli may be common. Multiple falls show pronounced
parallel bedding.
2. Pyroclastic flows (ash-flow tuffs) are volcanically produced hot, gaseous,
particulate density currents (Fisher and Schmincke 1984). They can be generated
in various ways (Wright et al. 1980), among which the collapse of an explosion
column seems to be a frequent cause. Other modes of origin include dome
collapse, lateral blast and a "boiling over" the crater rim without the formation of
a vertical column (Fisher and Schmincke 1984). The flows constitute a fluidized
system in which the continuous medium is hot gas and the particle/gas ratio is
high. According to Sparks and Walker (1973), they are the pyroclastic equivalent
of mud flows in that they are concentrated particulate flows but unlike mud flows
(lahars) they are more mobile. Their high temperature is shown by the welding of
glass shards in proximal ignimbrites and by pyrolitic affects on plant remains for
tens of kilometres beyond the zone of welding. Flow deposits are poorly sorted,
non- or poorly bedded and high in crystal content. Lapilli and rock inclusions
occur throughout but mainly in the upper portion of thick proximal flow units,
which distally thin in an irregular manner due to the development of a lobe-andcleft configuration resulting from a vortex-like lateral expansion of the flow
(Taylor 1958; Fisher 1979). In view of their relatively high density, pyroclastic
flow deposits tend to fill depressions in their path, which results in irregular lower
but relatively even upper bounding surfaces.
3. Pyroclastic surges differ from pyroclastic flows in their lower solid/gas ratio. The
pyroclasts are carried laterally entrained in turbulent gas as a ground-hugging
dilute pariculate flow (Walker 1980). The deposits formed by pyroclastic surges
consist of relatively thin, laminated units with good separation into different
particle sizes. Stratification may be parallel, wavy, lenticular and includes lowangle cross-lamination (Fisher and Schmincke 1984). Foresets may have
erosional basal contacts. In view of the rapid build-up and decline of high
velocities in passing pyroclastic surges, antidunes cannot only be formed but are
also preserved. Particle sizes decrease and sorting improves with distance from
the source. Three types of pyroclastic surges have been distinguished: (1) the base
surge, which follows a steam explosion, (2) the ground surge, and (3) the ash cloud
surge.
In Fig. 6.35 ground surge deposits have been attributed to two modes offormation.
To the left of the vent a directed blast situation is depicted which relates to the 18th
Coal Seams and Their Roof Rocks
319
May eruption of Mount St. Helens. According to Lipman and Mullineaux (1981),
the resulting surge velocity reached 300 mis, which within minutes extended over 25
km and downed all trees in its path. From the blast cloud a pyroclastic surge deposit
was formed which can be divided into a thick and coarse basal unit covering an area
of 140 km 2 up to a distance of about 14 km from the vent. This ground surge deposit
is overlain by a fine upper unit which covers an area of 600 km 2 up to 30 km away
from the vent and is better sorted, more thinly bedded and consists of superimposed
tabular cross-bedding sets that resemble migrating straight-crested dunes (Moore
and Sisson 1981). This fine-grained upper unit, which probably represents an ash
cloud surge deposit, is overlain by the kind of ash fall deposits mentioned above.
As indicated in Fig. 6.35, both ground surges and ash cloud surges can also be
generated in conjunction with ash flows from collapsing eruption columns (Fisher
1979). In this case a low-density ground surge formed from the margin of the
collapsing eruption column precedes the high density flow, which originated from
the heavier inner portions ofthe column. This separation results in the deposition of
fine-grained, well-stratified and often cross-bedded tuff in front of and below the
thicker, massive and coarse-grained flow deposit.
Apart from the turbulent ash cloud surge which accompanied and extended
beyond the ground surge of the Mount St. Helens eruption, ash cloud surges have
been found to elutriate and segregate from the turbulent tops of pyroclastic flows
which they override and leave behind. In the Bandelier Tuff of New Mexico, Fisher
(1979) observed that the first ash cloud surge deposits appear on top of flow deposits
several kilometres from the vent as discontinuous lenses, approximately 2 to 5 cm
thick and about 0.5 to 1 m long. Distally the lenses thicken to 35 cm and combine to
form continuous beds with internal lamination 0.5 to 3 mm thick consisting of
alternations of crystal-rich and crystal-poor laminae. Dunes, unidirectional lowangle internal cross-stratification, and pinch-and-swell structures unrelated to
buried topography are all characteristic structures of ash cloud surge deposits.
All the features mentioned in the above description of the various pyroclastic
deposits occur in the tuffs of the Newcastle Coal Measures. The variety in textures
and structures, as well as the composition shown by the thick interseam tuffs and
claystones, indicate successive eruptive episodes and different modes of emplacement. Many parallel bedded tuffs, such as those of Unit 1 in Fig. 6.33, are ash fall
deposits but some reveal evidence of lateral transportation by their internal crossstratification, which could be due to wind drift or water transportation although
there is no reason to believe that the latter was involved. Judging by the large
number of downed trees contained in Unit 1, it is possible that it also comprises some
pyroclastic surge deposits.
Unit 2 appears to have been formed as a ground surge preceding the overlying Unit 3, which was formed by a pyroclastic flow. However, both bedforms and soft sediment deformation structures suggest very violent conditions of
tephra emplacement, not unlike those illustrated and described by Fisher and
Schmincke (1984) for base surges. These result from explosions caused by sudden
steam generation, when either hot ash and gas or magma come into contact with
groundwater. In the first case the resultant violent eruption is called phreatic and in
the second case phreatomagmatic. Both come under the more general term of
320
The Relationship Between Coal and Interseam Sediments
hydroexplosions, which also include steam explosions caused by lava or pyroclastic
flows covering surface (vadose) water (Fisher and Schmincke 1984).
Since base surges are areally restricted to within a few kilometeres of the site
of the explosion, it is unlikely that Unit 2 originated from a phreatic or phreatomagmatic eruption. There are no indications of Permian volcanic activity within
the coalfield, as the uniformly low coal rank indicates an undifferentiated low heat
flow in the subsurface. Base surges generated by phreatomagmatic activity outside
the basin would not have reached the coalfield because the onshore basin margin,
which gives the minimum distance to the volcanic source, is 40 km away from the
locality of the Reids Mistake Formation, although magnetic anomalies some 15 km offshore from Newcastle could indicate a closer origin of the pyroclastic deposits.
Much greater distances can be bridged by pyroclastic surges and flows, which
could have originated from beyond the basin margin and caused localised steam
explosions as they travelled across the wet and often ponded mire surface. Indeed,
in some instances steam explosions may have been responsible for either the
formation or modification of lakes, now represented by so-called "wants" in coal
seams.
The term want is used by Australian miners for an enclosed area, in which a coal
seam deteriorates and is replaced by inorganic sediments. According to Warbrooke
(1981) these zones of seam deteriorations are mainly circular or oval in shape with a
maximum diameter of 1 km. The sediments filling the wants are generally finegrained (e.g. laminated claystones and siltstones) and, although their lateral
contacts with the surrounding coal are commonly sharp and often quite steep, some
interbedding with coal occurs around the edges while several tuffaceous clay
markers can be traced from within the coal for a few metres into the want.
Wants are irregularly distributed and, although our knowledge of this aspect
appears to be partly a function of the intensity of mining, more wants have been
found in the Victorial Tunnel Seam than in any other equally well-exposed coal. As
shown in Fig. 4.41, this up to 3 m thick coal seam contains a large number of lightcoloured tuffaceous claystone bands, some of which are up to 20 em thick and mostly
bentonitic in composition with a high siderite content. The wants, whose known
distribution is illustrated in Fig. 6.36, also contain a high proportion of tuffaceous
material, and both coal and sedimentary fill display a high intensity of soft sediment
deformation, all of which is consistent with hydroexplosive activity. Because the
Lower Pilot Seam underlying the Reids Mistake Formation is not mined, no wants
have been recorded in this coal, but the seemingly violent structures displayed by
Unit 2 could be related to steam explosions.
Unit 3 has all the hallmarks of a pyroclastic or ash flow deposit, indicated by the
massive nature ofthe rock, some inverse grading and its composition. It consists of
small amounts of very dense volcanic rocks and approximately equal proportions of
crystals (quartz, plagioclase and biotite) and glass shards. As shown in Fig. 6.32, the
latter are not welded, which suggests the distal nature of the flow. According to
Fisher and Schmincke (1984), pyroclastic flow deposits (ignimbrites, ash flow tuffs)
have been observed to radiate well over toO km away from their sources, but the
limit of welding does not seem to extend further than approximately 50 km (Smith
1979).
Coal Seams and Their Roof Rocks
321
CHARLES TOWN
.
./ OUlcrop of
\...coal me t lures
~ Wanl • observed
' ''' )
Want
a
projected
W
.. t
~B EL MON T
Fig. 6.36. Chart of the
coastal portion of the
Newcastle Coalfield of
New South Wales showing
the distribution of wants
in the Victoria Tunnel
Seam. (After Warbrooke
1981)
MN
o
4km
'---'---'---'---'---"
322
The Relationship Between Coal and Interseam Sediments
Unit 4 is interpreted as an ash cloud surge deposit. It is finer in particle size than
the other units, internally laminated with dune structures occurring directly on top
of the underlying flow deposit (Unit 3), and it is capped by pockets of accretionary
lapilli (Fig. 6.37). These features are consistent with the notion that fine ash has been
elutriated out of the turbulent ash flow moving below. The dunes are mantled by
finer laminated material and in several cases, megaripples have been observed with
a ripple height of up to 50 cm. Lateral changes from rippled to flat-bedded structures
are probably due to regional variations in flow regimes at the time of deposition.
A unique feature of the pyroclastic units overlying coal seams in the Newcastle
Coal Measures is the occurrence of a variety of coal inclusions, which were entrained
as peat or plant fragments and incorporated mainly into the surges and flows.
Although coal clasts have been reported from epiclastic rocks (e.g. Teichmiiller and
Teichmiiller 1950, 1952; Mackowsky and Kotter 1962; Krausse et al. 1979; Dorsey
and Kopp 1985; Nelson et al. 1985; Scheidt 1986), few investigations have been
carried out on coal inclusions in pyroclastic deposits. Apart from brief references in
the older literature to such occurrences by Geikie (1902) in Scotland and David
(1907) in eastern Australia, it seems that only Hamilton et al. (1970), Allan et al.
(1975) Raymond and Murchison (1988), and Raymond et al. (1989) have actually
carried out analyses on coal inclusions in pyroclastics. The latter authors combined
the techniques of optical and transmission electron microscopy (TEM) with organic
geochemistry (mainly GS-MS) in the study of Carboniferous wood contained in a
lithic tuff from the Midland Valley, north of Edinburgh, Scotland. Apart from the
occurrence of unsubstituted polyaromatic hydrocarbons (PAH), which have been
attributed to combustion, the wood does not give any indications of thermal effects.
Indeed, its low vitrinite reflectance of 0.39% suggests a suppression of coalification,
possibly as a result of partial impregnation of the wood with mineral solutions
generated from the surrounding tuff.
The coal inclusions in the Reids Mistake Formation and other pyroclastics of
the Newcastle Coal Measures range in size from a few micro metres to several metres,
the latter consisting of tree trunks. The majority of inclusions occurs in the
millimetre to centimetre range, and unlike the upright stumps and downed logs,
which are concentrated near the contact with the underlying coal, the smaller
fragments are either scattered throughout a flow or surge unit or, more rarely, occur
in defined bands. An example of the latter from Unit 3 in the Reids Mistake
Formation is illustrated in Fig. 6.38. The inclusions in this and other pyroclastics
consist of two kinds:
1. Inclusions offormer peat clasts. As shown in Fig. 6.39, the inclusion contains all
the macerals common to a high volatile bituminous coal, except that the
specimen has been thoroughly impregnated by silica in the form of common opal,
and, to a lesser extent, some other minerals. The degree of petrification dominates
the white light image, but, as illustrated in Fig. 6.39, fluorescent mode reveals that
little replacement of organic matter has actually occurred. Most of the impregnating silica either fills desiccation cracks or occupies the central cavities of
spores and pollen. In a normal coal sample cut perpendicular to bedding, as is the
Coal Seams and Their Roof Rocks
323
1
I
Fig.6.38. Coal inclusions (black) in the
Reids Mistake Formation of the Newcastle Coal Measures, New South Wales
em
case in the illustrated coal inclusion, spore and pollen grains would appear
flattened. In Fig. 6.39 this is not the case, because silica solutions, mobilised by the
early breakdown of the surrounding volcanic glass, infiltrated the relatively
uncompacted peat clasts and precipitated in all available voids.
2. Inclusions of peat derivatives. David (1907) referred to the occurrence of shiny
black specks on fresh surfaces of Nobbys Tuff (see stratigraphic column in
Fig. 6.31) and regarded them as resin droplets. However, when viewed under the
microscope, these inclusions, examples of which are illustrated in Fig. 6.40, do not
appear to be resin. They display flow structures and are not unlike some hydrogenation residues variously referred to as vitroplasts (Mitchell et al. 1977),
coagulants (Guyot and Diessel 1979, Diessel and Guyot 1984), hydroplasts
(Shibaoka and Russell 1981), primary vitroplast (Shibaoka 1981), or plasticoal
(Shibaoka et al. 1982). Both white light reflectance and fluorescence intensity of
the material vary. In the field of view shown in Fig. 6.40, reflectance values range
from 0.40 ("normal" coal = 0.75%) to 1.15% Ror, while the respective fluorescence
intensities measured in water immersion range from 6.5 to 1.8% at 546 nm, from
7.1 to 3.5% at 650 nm, and from 4.8 to 1.7% at 700 nm.
324
The Relationship Between Coal and Interseam Sediments
Fig. 6.39. Photomicrograph taken in water immersion of an almost uncompacted peat (now coal)
inclusion in ash flow tuff from the Reids Mistake Formation, Newcastle Coal Measures. Left
Incident white light, dark areas represent impregnating silica, inertodetrinite is white; right incident
fluorescence mode showing bright liptinite, including uncompacted spores; actual length of each
field of view = 0.22 mm
Coal Seams and Their Roof Rocks
325
Fig. 6.40. Photomicrographs taken in water immersion of heat affected peat (now coal) inclusion in
ash flow tuff from the Reids Mistaken Formation, Newcastle Coal Measures, showing lateral
variations in both reflectance and fluorescence intensities. Left incident white light, dark areas
represent impregnating silica; right incident fluorescence mode; actual length of each field of view
=O.22mm
326
The Relationship Between Coal and Interseam Sediments
Fig. 6.41. Tree stump in growth position
extending from the coal top into overlying pyroclastic deposits. (Diessel
1980b)
Although the precise mode of formation of these inclusions is not known, it
can be assumed that softening and pyrolysis of vegetable matter and peat were
involved in their formation. Both white light and fluorescent mode images show
signs of heat effects in the form of an uneven distribution of reflectance and
fluorescence intensity values.
Transitions between both types of inclusions can be found in practically all ash flow
tuffs and surge deposits of the Newcastle Coal Measures. This means that volcanic
effects on the inclusions range from nil to severe, which, in conjunction with the lack
of welding of glass shards, suggests that differential heating occurred before
deposition. This notion is supported by the lack of any systematic decrease in
reflectance towards grain centres and by the fact that inclusions with weak and
strong thermal effects occur adjacent to each other. The most likely mechanism of
fragmenting peat is by steam explosion. If the fragments were subsequently
entrained in a water-rich base surge, they would not have suffered any heat effe·cts.
However, if a ground surge had caused a chain of steam explosions and the
subsequent ash flow had picked up the dislodged peat fragments, variable heat
transfer could have taken place during entrainment.
More conspicuous thatl the small coal inclusions are the many coalified and
partly silicified tree trunks, which extend from the coal seams up to 4 minto
overlying tuff deposits. Figure 6.41 is an illustration of a tree stump embedded in tuff,
while an example of the lateral distribution of the petrified forest from the base of the
Reids Mistake Formation, supplemented by measurements of David (1907) is shown
in Fig. 6.42. Also included is the position and a rose diagram of the measured
azimuths of downed trees. They were rooted in the Lower Pilot Seam and most of
them penetrate for 0.5 to 1 m into the overlying vitric tuff. At that level many of the
trunks snapped off and became embedded in the volcanic ash. The mean thickness of
327
Coal Seams and Their Roof Rocks
N
-1
w+-----~~~--_+
25.
1~ .25 .25
5 ·.::;;.//020
40't'
.10
_35
10,
-30
25.
30-
20-
22_
Fig. 6.42. Chart of the
position of tree stumps and
downed trees at the contact
between the Lower Pilot
Seam and the Reids Mistake
Formation at Swansea Head.
The figures next to the stumps
give their diameters in centimetres; A is a polar stereogram (Schmidt Net) of the
intersection ofthe stump axes
with the lower hemisphere; B
is a rose diagram of the azimuths of downed tree trunks.
(Supplemented after Diessel
1985c)
..t60
30-
A
s
-
25.
__ 25.
15.
.45
40_
·10
20_20
.10
·15
s
the trunks above the root system is 25.4 cm (s = 13.9, n = 42) and mean spacing
is 2.90 m (s = 1.40, n = 30). Most of the downed trees point in a southwesterly
direction (mean azimuth = 260°, n = 65) and some are still attached to the stumps
on the ground.
The wood ofthe fossil trees is commonly both coalified and petrified, mainly by
silica and iron carbonate. The remaining coal in the interior ofthe stems consists of
telovitrinite with a maximum reflectance of 0.75%, which is normal for this
stratigraphic level. However, close to the surface the wood is often charred and the
bark is partly torn off the trunk. Similar examples of trees felled by pyroclastic flows
have been reported by Froggatt et al. (1981) and Wilson and Walker (1981) from
New Zealand, where the Holocene Taupo eruption downed trees over an area of
15000 km 2 and up to 45 km away from the vent. More recent examples of this kind
are well known from the 1980 eruption of Mount St. Helens.
Apart from the trees which broke off a short distance above their base, some
stems extend for several metres into the overlying tuff. Many of these trees are
328
The Relationship Between Coal and Interseam Sediments
Fig. 6.43. Photograph of Seiaginella, a herbaceous lycopsid, embedded in tuff at the contact
between the Fassifern Seam and the overlying Awaba Tuff, Newcastle Coal Measures, New South
Wales
markedly tilted upward to the southwest, which is also illustrated by the stereogram
shown in Fig. 6.42. At this locality the mean plunge angle of 11 stems is 52° in a
direction of 30° (intersection of the tree trunk axes with the lower hemisphere of
Schmidt Net), while 4 km further south mean plunge of 18 trees is 68° at an azimuth
of 54°. In addition to the tilt, several trees show signs of abrasion on their
northeastern sides in the form of flattening and missing portions of annual growth
rings (Diessel 1985c).
Of considerable palaeo-environmental significance is the observation that in all
11 instances in the Newcastle Coal Measures in which thick tuff beds overlie coal
seams, trees extend from the coal into the roof. In all cases humic coals are involved,
but their composition varies widely from the very bright, vitrinite-rich Nobbys
Seam, which is overlain by the up to 25-m-thick Nobbys Tuff (for stratigraphic
position see Fig. 6.31), to the inertinite-rich, dull coals, which occur in the upper
portion of the stratigraphic column. Both sets of coals have relatively high tissue
preservation indices (TPI), which in the bright coals is due to the dominance of
telovitrinite and in the dull coals results mainly from a high semifusinite content.
Only the Wallarah Seam (for stratigraphic position see Fig. 6.31), the last economic
Permian coal seam to form before sedimentation became exclusively coarse clastic
terrestrial, shows low TPI values due to an increase in inertodetrinite. Since trees
extend also from this seam into the overlying pyroclastics (Cowper Tuff), the decline
in tissue preservation is not due to a reduced contribution of wood to the peat but is
a function of more severe biodegradation of the wood. As thicknesses of the tuffcovered coal seams vary from several decimetres to several metres, peat formation
must have been terminated at many different stages of mire development, yet in all
cases the peat surface was forested and carried undergrowth of herbaceous lycopsids
(Fig. 6.43), not unlike the Selaginella harrisiana which Townrow (1968) described
Coal Seams and Their Roof Rocks
329
from the Illawarra Coal Measures of New South Wales. The only noticeable
difference is in tree size. The stems which protrude from the brighter coal seams in
the lower portion of the Newcastle Coals Measures into their overlying roof rocks
show larger average (20 to 30cm) and maximum (approximately 1 m) diameters
compared to those rooted in the stratigraphically higher dull coals (10 to 20 cm
average and approximately 50cm maximum). However, the .avaihible sample
population is small and does not allow for more than a faint suggestion that
the intertinite-rich coal s(!ams might have been affected by somewhat stunted tree
growth.
6.3.1.2 Gradational Contacts Between Coal and Roof Rocks
Gradational contacts between adjacent litho somes suggest a slow change in depositional conditions. Examples common to many coal measures are back-swamp
deposits, in which the upward change from coal to shaly coal, coaly shale and shale,
followed by flood plain laminites might indicate the encroachment of a sediment
source, e.g. in the form of a meandering river. As the channel approaches, it sheds
progressively coarser sediments over the adjacent mire surface during floods, which
increasingly interfere with peat accumulation, until it ceases. In interseam sediments
gradual contacts can be formed even in relatively short depositional episodes. In
fluvial sediments it is often difficult to distinguish between the top of a point bar and
the base ofthe levee or overbank deposits because no sharp break separates the two
kinds of deposits.
Gradual contacts may occur in the form of mixed transitions, i.e. both
litho somes are connected by a zone of mixed composition of the two end-members,
or in the form of continuous transitions, in which case the two lithologies are
connected by a zone of intermediate particle size. An example of the first case would
be a sandy shale between a sandstone and a shale, and in the second case, a siltstone
between a sandstone and a shale.
The gradual contacts commonly found in coal measures consist of the mixed
type in which the end-members of two rock types alternate with each other in a
transitional zone. Typical examples are laminites, which occur frequently between
adjacent lithosomes. In this case, the contact between laminae within the
transitional zone may be quite sharp, and it is largely a matter of scale whether one
regards the whole laminite section as one heterogeneous transitionallithosome or
whether each lamina is taken as a homogeneous lithosome in its own right. In the
latter case the succession is characterised by numerous closely spaced abrupt
contacts.
6.3.2 Discordant Coal/Roof Couples
A variety of geological events can produce angular discordances between adjacent
sediments whereby both the contacts and hiatus between them reflect in their
respective magnitudes the time spans required for their formation. Examples from
330
The Relationship Between Coal and Interseam·Sediments
either end of the spectrum would be a regional angular unconformity bracketing
a tectonic event, and the discordant contact between a set of horizontal beds and
an overlying set of inclined foresets. Neither of these two extremes is specific to
coal measures, but there are some cases of angular discordances which are either
restricted to, or particularly well documented from the contacts between coal seams
and their roof sediments.
6.3.2.1 CoaljRoof Discordance Due to Unequal Loading
Differential compaction due to unequal loading is found in peatlands subjected to
the encroachment of bedload channels of both braided and meandering types. In
braided channels the high width/depth ratio of coarse bedload streams prevents
accommodation oflarge volumes of clastic debris within the shallow channels which
leads to rapid lateral fanning of the fluvial depositional environment if down-slope
transportation cannot remove the incoming clastics. In a coal-forming environment
braid-plain deposits may then spread laterally across peat which depending on its
state of pre-compaction, will respond in a variety of ways to the loading.
The Newcastle Coal Measures in New South Wales contain several conglomerates which overlie coal seams showing a variety of angular relationships. In some
cases, the principal bedding planes of both lithosomes have similar attitudes
irrespective of the occurrence of irregular erosional scours at the conglomerate base.
In other couples, a marked angular discordance exists between the bedding planes
(Sp) of the two units, and there are many transitions between the two types.
An example of the discordant conglomerate type is shown in Fig. 6.44 by the
Redhead Conglomerate, named after a prominent headland on the coast south of
Newcastle, N.S.W.1t forms a clastic wedge between the two splits ofthe Fern Valley
Seam (for stratigraphic position see Fig. 6.31) and consists of interbedded conglomerate and sandstone lenses stretching inland from the coast in a belt approximately
Fig.6.44. View of the Lower Fern Valley seam (centre) discordantly overlain by the Redhead
Conglomerate, and concordantly underlain by flood plain deposits of the Kotara Formation.
North of Redhead, New South Wales
Coal Seams and Their Roof Rocks
331
6 km wide and of considerable but unknown length. In spite of the relatively
restricted areal extent of the sequence the conglomerate reaches a maximum
thickness of almost 40 m at Redhead (Diessel and Moelle 1988).
The Redhead Conglomerate consists of mostly 10- to 30-cm-thick layers of
granule to pebble conglomerate alternating with up to 2-m-thick beds of coarse to
pebbly sandstone. The average phenoclast diameter of the conglomerate is 3 to 5 cm
but, in most cases, grain sizes decrease in an upward direction. These beds are
laterally persistent and often massive, although cross bedding is not rare. In the 10to 30-cm-thick layers the majority of the foresets is straight and planar with
relatively steep angles of repose. Markedly curved (upward concave) and tangentially aligned trough cross-beds are more common in thicker scour fills of coarse to
pebbly sandstone. Coalified plant material occurs throughout the deposit, consisting offlat lenses of bright coal (vi train) which has probably been derived from wood
and bark. Most clasts are subrounded to rounded and of medium high (0.40 to 0.55)
sphericity. The coarser portions of the Redhead Conglomerate are all clast- or
framework-supported but with decreasing particle size and in conglomerate/
sandstone transitions the fabric becomes more matrix-supported.
In contrast to the often sheet-like conglomerates, the sandy intercalations are
more confined to cut-and-fill structures. Several large sand-filled channels, approximately 40 m wide and, at the centre 2 m deep, contain abundant, large-scale,
heterogeneous cross-bedding and many current scours carrying pebble lag at their
bases. Elsewhere, erosional contacts are quite frequent and trough cross-bedding is
ubiquitous. Most sets are 20 to 40 cm high. Some of them show ripple drift
lamination and contain clay ironstone nodules.
The most conspicuous feature of the Redhead Conglomerate is the strong
angular discordance of its principal bedding planes (Sp) with bedding in the
underlying coal seam. The coal follows the regional dip of 6 to the southwest
(azimuth = 230°), whereas the lowermost conglomerate layers and lenses rest on the
coal with northesterly dips up to 45°. In several localities, the clastics protrude into
the coal as load casts. The large angle between the coal seam and its roof strata is
presumably related to the coarse nature of the overlying sediments. Where in similar
geological situations the seam roof consists of steeply dipping shales and sandstones,
as is the case in the Wittingham Coal Measures of New South Wales, a tangential
alignment between coal and roof strata is commonly observed near the point of
contact (Britten et al. 1975; Marchioni 1982). In the Redhead Conglomerate no such
tangential alignment has been found in the coarse portion (Fig. 6.45), and even in the
more sandy fractions it is restricted to the immediate seam roof, as illustrated in
Fig. 6.46.
In an upward direction the dips of the bedding planes of the Redhead
Conglomerate decrease so that from approximately 10 m above its base the
principal surfaces of deposition flatten and approach regional dip. An illustration of
the upward change in the spatial attitude of the lithosomes is given in the measured
section of Fig. 6.47, while Fig. 6.48 is a polar stereogram of 35 principal bedding
planes measured mostly in the lower half of the Redhead Conglomerate.
At first sight, the large inclined bedding planes of the Redhead Conglomerate, as
well as of similar occurrences elsewhere in eastern Australia, appear like giant foreset
0
332
The Relationship Between Coal and Interseam Sediments
Fig.6.45. Steeply dipping (45°) coarse fraction of the Redhead Conglomerate without tangential
alignment with the coal contact. North of Redhead, New South Wales
Fig. 6.46. Weak tangential alignment
of the pebbly sand fraction of the Redhead Conglomerate with the underlying
Lower Fern Valley Seam. Note the
cross-bedding in the granule conglomerate in the upper part of the frame
with an apparent dip of 45°! North of
Redhead, New South Wales
Coal Seams and Their Roof Rocks
333
.coal
f..=-=:I Claystone
Fig. 6.47. Measured
section through the
Lower Fern Valley Seam
and the overlying
Redhead Conglomerate
north of Redhead, New
South Wales. The scale is
in metres
r:: ::>·1 Sandstone
~Planar } Cross-bedding
~ Trough
.... ::: : .: ... ......
'.
~.
',
.
','
'.'
beds or like rather steep slipfaces of a prograding fan delta or Gilbert-type delta, and
they have, indeed, been interpreted as such (Parbury 1976; Conaghan 1981;
Galloway and Hobday 1983; Hunt and Hobday 1984), or as large-scale foresetsof a
crevasse splay or subdelta (Flood 1985; Flood and Brady 1985). However, a close
inspection of the Redhead Conglomerate reveals the above-mentioned abundant
internal cross-bedding with fore sets dips of up to 55 between the inclined large
stratification planes. There is no doubt that at the time of deposition the large planes
acted as principal surfaces of deposition (Sp) and that the fore sets (Sf) must have been
considerable flatter than they are now. A polar stereogram constructed from 60 tiltcorrected foresets is illustrated in Fig. 6.49. The calculated vector mean is 126 0 with
a moderately low variance of almost 5000.
0
334
The Relationship Between Coal and Interseam Sediments
X -5S'
Fig. 6.48. Polar stereogram (Schmidt
Net) of 35 Sp-planes measured in the
lower portion of the Redhead Conglomerate, north of Redhead, New
South Wales. Interval concentration
= 50
H
Fig. 6.49. Polar stereogram (Schmidt
Net) of 60 tilt-corrected foreset beds (Sf).
Interval concentration = 5 o. North of
Redhead, New South Wales
Fig. 6.50. Cartoon showing the compaction pattern of the Lower Fern Valley Seam responsible for
the inclined Sp-planes in the Redhead Conglomerate. The mean azimuth of dip of Sp = 55 gives
the direction of lateral channel migration which contrasts with the direction of channel flow
indicated by the mean azimuth of dip of Sf-planes = 126
0
0
Coal Seams and Their Roof Rocks
335
It appears that the conglomerates have been deposited mainly as gravel sheets,
banks and bars in a braided river system. Sandstone channels dominate, although
the occurrence of some trough cross-bedding suggests that some channels were filled
by conglomerate. However, sandstone channels are more common. The southeasterly flow direction is almost at right angles to the present northeasterly dip of the Sp-planes. As illustrated in cartoon form in Fig. 6.50, this is
interpreted as a compaction feature resulting from the weight ofthe sand and gravel
loaded on the water-logged peat when the braid plain extended laterally in a
northeasterly direction across the Fern Valley swamp. As the peat responded to the
loading by progressive compaction, the concomitant subsidence created more space
near the surface, which attracted more sediments in a similar way as has been
suggested by Mallett (1983) and Mallett and Durnbavan (1984) for the tilted fluvial
channels in the German Greek and Rangal Coal Measures of the Bowen Basin
in Queensland.
The compaction of the peat underneath the accumulating load caused some
peat to become squeezed upward and to move like a bow wave in front of the
laterally migrating braid plain. The peat bulge protected the adjacent mire surface
from being covered by overbank sediments but some suspension load has been
deposited in ephemeral ponds formed behind the peat bulge from the excess water
draining from the compacting peat. Some siltstone and silt-laminated fine sandstone
lenses which occur concordant with the coal roof have probably been formed in this
manner.
The sediments deposited at the peat margin on the right hand side of Fig. 6.50
show the steepest inclination because they formed the leading edge of the laterally
spreading braid plain and therefore follow the full 3: 1 to 4: 1 compaction the peat
suffered under a 12-m overburden which is the height of the sediment column above
the seam at which bedding planes begin to flatten. Sediments deposited to the left of
the peat margin are less inclined because they do not onlap uncompacted peat but
offiap sand and gravel overlying increasingly compacted peat. As has been reported
from similar cases ofload effects of roof sediments on coal by Britten (1972), Britten
et al. (1975), Krausse et al. (1979) and Marchioni (1976, 1982), it is common in such
situations for the original depositional shape of the litho somes to become deformed
by the effects of differential compaction and gravity sliding. The finer the particle size
of the rocks involved, the stronger the compaction and deformation. Since the
Redhead Conglomerate consists mainly of competent coarse clastics, the soft
sediment deformation suffered during compactional tilting is comparatively mild
and restricted to minor slump folding near the coal contact.
The significance of the compactional features associated with the Redhead
Conglomerate is that they provide evidence for a close temporal coexistence of peat
accumulation and clastic sedimentation. McCabe (1984) has argued strongly in
favour of a temporal separation of peat accumulation and clastic deposition but,
while this may be so in many instances, the emplacement of the conglomerate on
comparatively uncompacted peat of the Lower Fern Valley Seam suggests that epiclastic deposition terminated peat formation in the split portion of the Fern Valley
Seam. In the unsplit Fern Valley Seam peat continued to accumulate contemporaneously with clastic deposition of the split portion and was able to overgrow the
336
The Relationship Between Coal and Interseam Sediments
sands and gravels of the split without any noticeable time delay once the influx of
clastic material had either ceased to function or the conveying channels had been
deflected into other parts of the Fern Valley swamp. Since compaction features of
this and related kinds have been described from other Australian coalfields by
Britten (1972) and Britten et al. (1975) for vertically aggrading, and by Burgis (1975),
Marchioni (1976, 1982), Mallett (1983), Mallett and Durnbavan (1984) for laterally
accreting channel systems, time continuity between coal and roof sediments is
probably quite common.
From the above di'scussion it follows that conglomerates and other fluvial
sediments whose principal bedding planes do not display a steep angular discordance but are concordant with the underlying coal probably have been deposited
on an already partly compacted peat. In many cases this could be due to the effects of
loading by a previously deposited sediment which was subsequently removed by
erosion.
6.3.2.2 Coal/Roof Discordance Due to Erosion
In contrast to the laterally onlapping and largely non-erosive nature of the braid
plains discussed in the previous chapter, many coal measures have been formed
under a regime in which partial removal of previous deposits was an integral part of
the sedimentation process. A meandering river, for example, has to cut its bed into
the underlying ground, as is the case with any other channellised flow system of
reasonable stability. Since fluvial transportation and deposition play an important
part in the construction of many coal measures, erosional contacts between fluvial
and underlying sediments are a common feature of interseam deposits.
Depending on the kind of flow system (meandering, braided, anastomosing) the
fluvial sediments consist mostly of point bar or channel fill deposits which occur in
a variety of spatial relationships with associated coal seams. Illustrations and
discussions of a variety of coal/channel relationships are given by Elliott (1979).
Some have been formed much later than the coal on which they now rest (Fig. 6.51)
or which they have subsequently eroded. A spectacular example of the latter is the
Anvil Rock Sandstone in Illinois, U.S.A., which developed as a fluvial channel in the
Lawson Shale, between the Conant and Bankston Fork Limestones. In many places
Fig.6.51. Sketch of a washout above the Bulli Seam, New South Wales, which has cut through
several metres of roof sediments without eroding into the coal. (After Diessel etal. 1967)
Coal Seams and Their Roof Rocks
337
Fig.6.52. Photograph of a cross-section (be-plane) through a fluvial channel within the Nobbys
Seam, Newcastle Coal Measures, exposed at Burwood Beach, New South Wales. The section is
continuous without break from south (upper left) to north (lower right). The encircled back-pack is
60 em high
the channel has cut down to the underlying Herrin (No. 6) Coal Member and
replaced it with sandstone (Krausse et al. 1979). Other fluvial channels were formed
penecontemporaneously with the surrounding peat which accumulated on the flood
plains and temporarily stabilised islands between a network of anastomosing river
channels. These deposits form relatively narrow but elongated belts of lithification
within coal seams which in cross-section are compacted around the more resistant
stone bulges, as illustrated by Nelson et al. (1985) from the basal Pennsylvanian coal
measures in Indiana, U.S.A. An Australian example of this type is shown in Fig. 6.52
from the Newcastle Coal Measures.
While the erosive parts of channels have usually cut into the underlying strata
such that their walls are mostly relatively smooth, co-depositional contacts between
channel fill and coal or other surrounding sediments are frequently intercalated, as
illustrated in Fig. 6.53. This relationship is best seen in channels in which vertical
aggradation exceeded lateral accretion. Such channels may show a differentiation
between a lower erosive and an upper co-depositional portion.
Another channel type is represented by the large Walshville Channel, which
extends almost diagonally through the Illinois Basin, U.S.A. It developed in and coexisted with the Herrin (No.6) Coal with which its sandstone fill is intercalated along
the channel margin. However, it outlived peat accumulation by extending high into
the seam roof, where it is marginally interbedded for some 7 m above the seam with
338
The Relationship Between Coal and Interseam Sediments
Fig. 6.53. Photograph of the eastern flank of a 200-m-wide fluvial channel in the Illawarra Coal
Measures near Bylong in New South Wales. Note the intercalated margin of the channel with the
dark laminated overbank deposits. Encircled, is a person for scale
Fig.6.54. Contact between the roof of the Great Northern Seam and the overlying Teralba
Conglomerate, Newcastle Coal Measures, at Catherine Hill Bay, New South Wales
Coal Seam Splitting
339
its own overbank deposits represented by the Energy Shale of the Carbondale
Formation. The Conant and Brereton Limestones, which have been cut by the Anvil
Rock Channel referred to above, overlap the Walshville Channel with reduced
thickness, due to the effects of differential compaction within and outside the
channel influence (Krausse et al. 1979).
Since peat accumulation appears to have been frequently terminated by stream
avulsion, it is not uncommon to find the erosional bases of fluvial sandstones and
conglomerates forming the immediate roof of coal seams (Fig. 6.54). In most such
cases there is little evidence 'of significant loss of coal, but occasionally channels have
been cut into the coal which are usually referred to as washouts or rolls. The latter
term is rather non-specific, as it is also used for a variety of other elongated
protrusions of clastic sediments into the coal, including the flocr-rolls discussed in
Chap. 6.2, clay dykes and associated compaction faults (Damberger 1970, 1973), as
well as load structures, which were squeezed as soft sediment into the upper peat
layers (Krausse et al. 1979).
Considering the high frequency offluvial sediments overlying coal seams, it may
be surprising not to find more washouts in them. The reason for their comparative
scarcity is the resistance of peat to erosion. Particularly fibrous and woody forest
peats are strengthened by interlocking branches and tree trunks, which require
considerable force to overcome the tough "doormat effect". For the same reason it is
also common to find much later-formed river deposits resting on top of a seam after
they have cut through several metres of roof sediments. However, the sketch
illustrated in Fig. 6.51 shows that even without much loss of coal, mining conditions
may be difficult in the vicinity of the mechanical discontinuities along the erosional
contacts. It is therefore advantageous for the purpose of mine planning to have prior
knowledge of the occurrence and extent not only of washouts but of any erosional
contacts, which might adversely affect the seam roof conditions during coal extraction. An example of a palaeocurrent analysis carried out in a portion of an
Australian coal mine is shown in Fig. 6.55.
6.4 Coal Seam Splitting
The concept of depositional base level, which will be discussed further in Chap. 8,
defines a hypothetical threshold below which a deposit is considered to have a good
chance of being preserved in the sedimentary record. Application of the concept is
intimately linked to the scale of the investigation. If this is concerned with the
formation of coal macerals or coal lithotypes, depositional base level will correspond to the position of the groundwater table, and the vertical fluctuations which
may bring about changes in coal composition are in the centimetre to decimetre
range. Conversely, if the investigation encompasses a whole depositional basin or
part thereof, the base level concept assumes a much larger role and, in a paralic
setting, may be equated with sea level. Vertical variations, which bring about
The Relationship Between Coal and Interseam Sediments
340
, , ,
'? ......... ¥
; ~D~D
.
;:
~
~
" ..... "" - -
'"
j
~ ..... 0
.... 0
2"
'
.
I
~ , :;C ~ 0:;
I~~
-
'0
Q.~
!~
0
-
~
0
-_
i!~,
" ~DD
c
I 0';'
I~O
IZ
UJ
:::IE
a...
o
-.J
UJ
:>
UJ
c
UJ
Z
':;
!
..
~
.5
\ 1
( I
N
0
c
~
g
~~2
!
Coal Seam Splitting
341
lithological changes, increase accordingly and are measured in the metre to decametre range. The extreme scales of base level changes may grade into each other
through an intermediate order of magnitude in the decimetre to metre range, which
is large enough to affect peat accumulation very severely, at least in some parts of an
extended mire system.
Under conditions of falling water table, areas of the mire will dry out and large
portions of the peat surface will be oxidised if they remain uncovered. Also a rising
water table in parts of apeatland will severely impede plant growth and peat
accumulation in the affected area. Low-lying parts of the mire will be drowned, thus
attracting deposition of clastic sediments while peat accretion continues in the nonaffected portions. The result of this development is seam splitting, which typically
occurs in rapidly subsiding rift valley and foredeep settings, but it is comparatively
rare in stable cratonic environments. Because of this link with the tectonic
environment, a correlation also exists between seam splitting and coal measure
lithology (Elliott 1979). Permian coal seams associated with coarse clastic interseam
sediments, such as those found near the orogenic margin of the Sydney Basin in New
South Wales, are more frequently subjected to splitting (Branagan and Johnson
1970; Connolly and Ferm 1971) than coals interbedded with a high proportion of
marine sediments, for example in the Lower Carboniferous Limestone Coal group
of Scotland (Elliott 1979).
From the discussion in Chap. 6.3.2.1 it follows that the different compaction
rates between coal and the various interseam sediments may result in a distortion of
the attitude of the interseam sediments such that they dip towards the split axis or
line of union between the parent and the two daughter seams. Naturally, the greater
compaction ratio of the coal compared with the clastic split wedge exerts an
influence on the subsequent sedimentation pattern and may initiate further seam
splitting at higher stratigraphic levels.
At the expense of some simplification, seam splitting can be attributed to either
differential subsidence or autosedimentational causes, but one mechanism has been
reported by Staub and Cohen (1979) from the Snuggedy Swamp of South Carolina
which does not fit any of the common causes. The authors refer to it as "fire splays"
because it is initiated during drought periods when the peat catches fire and burns
down to below the average water level of streams cut into the peat. On the return of
normal conditions, the burned-out portions of the peat will be filled with water from
which silt and clay is deposited on the fusinite-enriched residual peat. Eventually the
area affected by the fire will be reclaimed by the swamp through terrestrialisation,
leaving a small seam split of fine clastics behind.
The kind of mechanism responsible for any particular case of seam splitting can
be deduced from such factors as basin geometry, lateral and vertical changes in
Fig.6.55A-E. Palaeochannel delineated by isopachs of a carbonaceous shale between the roof of
the Bulli Seam and an overlying channel sandstone at Appin Colliery, New South Wales. The
following directional features were measured in the channel fill: A and C rose diagrams of the
orientation of 190 (A) and 265 (C) plant fossils, respectively. B, D and E Polar stereograms (Schmidt
Net) of 100 (B) and 200 (E) mica platelets, and 42 foresets (D). (After Diessel 1966)
The Relationship Between Coal and Interseam Sediments
342
I: : :1
POST'PERMIAN
~ MAITLAND GR. : : ,=" -= _.;,_-- - =
E:::::3
.......... .
If=~j NEWCASTLE C. M.
IZJGRETA C. M.
F.7l
~ TOMAGO C. M.
~ DALWOOD GR. : ~ ~ •
/:;' -
!2;~5 SINGLETON S. GR.
em
• ••••••
..
•
. . .. .
MORIS SET' • •
::::: PRE-PERMIAN • • • • • ,
•
o
10Km
-'1
1...'_ _. . . . . . ._ _
Fig.6.56. Simplified geological map of the Newcastle Coalfield, New South Wales, with the
traverse of coal seam splitting illustrated in Figs. 6.57 and 6.58. The coalfield is situated in the
Macquarie Syncline, which is bordered to the west by the Lochinvar Anticline between Maitland
and Branxton
interseam sediments, and the properties of the affected coal seams. With respect to
the latter, Warbrooke (1981) and Warbrooke and Roach (1986) found contrasting
trends, depending on whether the splits were caused by tectonically induced differential subsidence or were autosedimentational. The examples discussed below are
based on the above mentioned author's work in the Newcastle Coalfield of New
South Wales, approximately along the transverse A, B in Fig. 6.56.
Coal Seam Splitting
343
6.4.1 Seam Splitting Due to Differential Subsidence
The first example (Figs. 6.57, bottom and 6.58, left) results from an increase in
subsidence towards the depositional centre of the basin or any other region of
increased subsidence within a sub-basin. The axis of splitting therefore marks a
boundary between areas of different subsidence rates or different subsidence/
sedimentation ratios. In paralic settings, this kind of splitting may be genetically
connected with a cycle oftransgression-immersion- regression which was brought to
3-0 ..
2·0 ..
'-5..
1-0 ..
o
' - - - -1... 0 -5 m
1Icm
'-----.J
Horiz:onlal 'H al.
N1393
3·5m
3 -0 ..
2-5 m
2·0m
- ' -_ _ _.L
0
, , -_ _ _
1-5m
--'111m
Hori'Zonta l "cclt!
Fig. 6.57. Two examples from the Newcastle Coal Measures of New South Wales showing the
effects on seam thickness of autosedimentational (top) and tectonic (bottom) seam splitting.
Numbers beginning with N identify diamond drill holes. (After Warbrooke 1981)
The Relationship Between Coal and Interseam Sediments
344
30 m
20
10
E
W
12
m
NW
SE
~.
Borehole Seam
West
~
Borehol
Seam
•
0
15
10
Coal
D Fluvial Channels
0
km
2j~---"'
km
__..
~
.~
1
Coal Thickness
I
20
.~.
•
\~~.~,
10
40
,:
.
Ash (CF 1.60)
...
~~ ~.,~.
1-----.
Q",rt,
'"j
10~:
~---~
j
-------~______• '" .
40
I
I
~.
L====r====~--~----
I
60
i ._._.__.__j)C=___.
50i . - - - - - \
30
: ===-~:::::~.. ::1 ~_____ ~n
i
............... Liptinite.
I-----.-----r---.,.-=~'--
10
15
km
_.-.-.-.
i
____ •
•
i
___ •
...
__
n_.:
'" .
I
I
4
6
. ...•
km
Terrestrial
Limnotelmatic
Fig. 6.58. Two coal seams from the Newcastle Coal Measures of New South Wales showing the
results of different kinds of seam splitting and their effects on compositional characteristics. Left
Splitting caused by tectonically initiated differentional subsidence; right autosedimentational
splitting. (After Warbrooke 1981; Warbrooke and Roach 1986)
345
Coal Seam Splitting
c:
Q)
Q)
'"
C/)
c:o
Q)"O
-en
~:v
en
E
Ol
en
Q)
0
~
~E
.<::
;;:t:L:
!D
~§
....
....
....
0
"0
Ol
Q)
c:
'"
c.?
2!
:e
...J
'c:0"
0
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0
.<::
0
~
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;;:: S.E.
UJ
Z
UJ
()
o
::;:
UJ
Z
UJ
500
Erft Basin
~',""'.'.'.''''.'.'.'.'.'.
()
o
c.?
...J
o
1000m
o
30km
~!----~----~----~,
Fig.6.59. Longitudinal section through the Tertiary coal measures of the Niederrheinische Bucht
(Lower Rhine Embayment) from near Cologne to Holland. Marine sands are dotted. (After von der
Brelie et al. 1962)
a halt at the axis of splitting although lithotypes within the seam may still further be
affected. An example of this is the marine/estuarine seam split reported by Britten
(1987) from the Rathluba Seam in the Tomago Coal Measures of New South Wales.
Seam splitting of this kind is commonly the result of a combination of several
influencing factors, including eustatic sea-level changes, whereas differential
compaction of subsurface strata (Elliott 1969) and a variety of tectonic causes can
lead to similar results. If the basin contains a tectonic hingeline, split axes are usually
situated within its region of influence, as shown in Fig. 6.59 by a longitudinal section
through the paralic lignite deposits of the Lower Rhine Embayment (Niederrheinische Bucht) extending from Germany into the Netherlands.
The Young Wallsend Seam illustrated both with daughter and with associated
coal seams in Fig. 6.57 (bottom) and 6.58 (left) is an examples of seam splitting in
response to tectonically induced differential subsidence on the flank of the
Lochinvar Anticline, which is positioned on an old basement high and forms the
western limit of the Newcastle Coalfield, as illustrated in Fig. 6.56. From the
anticlinal flank an up to 2-m-thick seam of high volatile bituminous coal, the West
Borehole Seam, splits basin ward successively into several daughter seams, and in
each case, the aggregate coal thikness in both split and conjoined portions ofthe coal
is at maximum near the split axes. Concomitantly the coal's disseminated ash, i.e. the
ash content of the cumulative floats at 1.6 density, decreases basin ward. Likewise,
there is a decrease in the silica/alumina ratio which is due to a drop in quartz and
expanding clays combined with an increase in the amount of kaolinite contained in
the coal (see Fig. 6.58).
Among the coal lithotypes the proportion of bright coal increases towards the
split at the expense of dull coal, which in Fig. 6.58 is indicated by the increase in
346
The Relationship Between Coal and Interseam Sediments
vitrinite and decrease in inertinite. The reason for this development is seen by
Warbrooke and Roach (1986) as reflecting the faster rate of subsidence towards the
split axis. Up-slope, in the conjoined portions of the seam, relatively dry peatforming conditions prevailed, leading to a higher rate of plant decay and ablation,
which resulted in a thinner seam with higher concentration of inertinite-rich dull
coal ("dry durain") and inherent ash than near the split axis. In the vicinity of the
split axis, the environment was wetter and more conducive to the preservation of
biomass, producing bright coal and, because little plant material was lost to
oxidation, the amount· of inherent ash remained more diluted. This notion is
supported by the distribution of microlithotypes which, when plotted in the coal
facies triangle modified from Hacquebard and Donaldson (1969), indicate a
transition from dry forest swamps (terrestrial zone) to wet forest swamps (telmatic
zone) in the direction of splitting (Fig. 6.58, bottom left).
Another measure of the improved preservation of biomass under the optimum
subsidence regime in the vicinity of the split axis, is the shoot/root ratio found in
fossil peats petrified by dolomite in coal balls referred to in Chap. 5.1.2. An example
is the Union Seam in the Lancashire Coalfield of the United Kingdom, which splits
into the Upper Foot and Lower Mountain Seams to the southwest, i.e. on the
down throw side of the northwest trending Deerplay Fault. According to figures
given by Phillips et al. (1985, Table 1), at Bankhall Colliery near Rowley, a distance
of 4.5 km up-slope from the line of splitting, the shoot/root ratios of two lycopodrich (93.0 and 93.3%) peat samples are 0.69 and 0.86, respectively, while their fusain
content is 10.3 and 10.2%. At the Hill Top Drift Mine, 1.5km northeast of the split
axis the shoot/root ratio has increased to 0.87 m (lycopod content = 95.3%), while
the fusain content has dropped to 7.7%. Right on the split axis, at Hapton Valley
Colliery (lycopod content = 91%), the shoot/root ratio has further increased to 1.16
and the fusain content is down to 7.5%. Also in this case the aggregate coal thickness
of the parent coal (Union Seam) and its two splits (Upper Foot and Lower Mountain
Seams) is at maximum near the split axis. This characteristic was already recognised
last century by Aitken (1866), and reconfirmed by Broadhurst et al. (1968), who
comment that peat-forming conditions were particularly favourable in a narrow belt
up-slope from the split axis. These authors also noted many instances of down-slope
overfolding in the roof of the downwarped Lower Mountain Seam, as well as tree
trunks, which apparently had rolled down the developing slope and are now
oriented parallel to the split axis.
6.4.2 Seam Splitting Due to Autosedimentational Causes
The most common forms of autosedimentational seam splitting seem to be related
to fluvial action either by stream avulsion, by rapidly covering a portion of the low
lying peat surface with a crevasse splay deposit, or by the slow encroachment on part
of the peatland by a migrating river channel and its associated overbank deposits
(Britten 1972; Burgis 1973; Marchioni 1976, 1982). The temporary spreading of
Coal Seam Splitting
347
Fig. 6.60. Autosedimentational seam splitting caused by the encroachment of a river channel on
the Bayswater Seam in the Wittingham Coal Measures in New South Wales. B Bayswater Seam;
C channel sandstone; 0 overback deposits; R Ravensworth Seam. (Diessel 1984)
Fig. 6.61 A- B. Cartoon
illustrating the
development of an
autosedimentational seam
split. For explanation, see
text
348
The Relationship Between Coal and Interseam Sediments
lacustrine or lagoonal conditions across a swamp environment may likewise cause
seam splitting of this kind.
In the case of an approaching meandering river or a system of laterally
migrating braided channels, overbank sediments are often shed onto the adjacent
peat surface during successive floods. An example of a seam split caused by a thin
wedge of fine overbank deposits associated with a coarse fluvial sandstone is illustrated in Fig. 6.60, while a model of the mechanism is shown in Fig. 6.61. Similar
splits have been reported by Britten (1987) from the Big Ben and Donaldson Seams
of the Tomago Coal Measures in New South Wales. The weight ofthe encroaching
overbank wedge causes compaction of the underlying peat, thus attracting more
surface sediments; however, peat accumulation is slowed by the presence of less
compactable clastic sediments in the area adjacent to the river (Warbrooke and
Roach 1986). If the fluvial event affects the swamp as a whole, peat accretion will
terminate and the seam will be covered by a primary roof of laminated overbank
sediments, followed by an erosional contact overlain by drawn-out point bar or inchannel sediments. Depending on the relationship between the rates of basin
subsidence and channel migration, the primary seam roof might be removed
altogether, whereby in high energy systems, the upper portion of the underlying coal
might also be affected. However, the presence of a seam split indicates that at some
stage, the direction of channel migration was either reversed or the channel became
defunct, after which peat accumulation recommenced (Fig. 6.61, bottom). In some
coalfields of eastern Australia renewal of peat formation above the seam split shifted
fluvial (or tidal) activity to parts ofthe mire not previously affected in this way thus
producing the en-echelon stacking of anastomosing channels between zig-zag seam
splits described by Britten (1972) and Britten et al. (1975).
Seam splits related to autosedimentational events, such as the encroachment of
a river channel on a back swamp, tend to display the features illustrated in Figs. 6.57
(top) and 6.58 (right). Because of the increasing interference of normal peat accretion
by flood deposits, less peat is formed as the river approaches, which results in a
decrease in aggregate coal thickness towards the split axis and beyond. Concomitantly, the ash content of the coal increases in the form of stone bands (ash of coal
plus bands) and disseminated clay and silt (ash of cumulative floats at 1.6 density).
Whereas the increase in ash content in the parent seam away from the
subsidence-induced splitting is due mainly to a relative increase in inherent ash due
to oxidation and peat ablation, the fluvial system responsible for the au to sedimentational splitting is also the main source of adventitious minerals in the adjacent
coal (Warbrooke and Roach 1986). This is evidenced by the increase in the percentage
of CF (cumulative floats) 1.6 ash and of detrital quartz in the direction of the split
(Fig. 6.58). The increase in expanding clays, i.e. illite, montmorillonite and mixedlayer clays, is probably a peculiarity of the stratigraphic setting within the Newcastle
Coal Measures. As discussed above, they contain a large proportion of inter- and
intraseam tuffs whose bentonitic and other devitrification products were often
redistributed by rivers.
7 Coal-Producing Sedimentary Environments
Genetically related litho somes constitute the integrated response of a sedimentary
environment to the depositional process. Sedimentary environments are therefore
integral parts of the hierarchy of sedimentation elements, since they consist of
combinations of lower ranking sedimentation elements by which they can be
identified. Palaeo-environmental analysis makes use of diagnostic combinations of
these in many different forms, ranging from the superposition of characteristic
deflections in geophysical logs to the regional changes observed in a fossil
assemblage. In some cases palaeo-environmental conclusions can be reached only
after the careful study of many different aspects of a vertical profile, while in others
the genetically based depositional assessment becomes part of the logging process,
whereby either outcrops, bore cores, geophysical signals or a combination of these
are used (Fisher and McGowen 1967; Weber et al. 1984; Hamilton and Beckett 1984;
Hamilton 1986).
Sedimentary environments are commonly referred to in reference to geomorphologic terms, such as fluvial, glacial, deltaic and the like. Such physiographical
attributes define parts of the earth's surface which are physically, chemically, and
biologically distinct from adjacent areas (Selley 1982). By extending the Sydney
Basin environments previously referred to in Table 5.3 to include the shared
subenvironments, Table 7.1 gives a list ofthe most common depositional settings for
coal seams plus some of their characteristics. In most large coal basins several such
environments would have coexisted and replaced each other laterally, as well as
vertically.
The strong influence of rivers on peatlands and coal measures is demonstrated
by the terminology used to describe coal-forming environments. Based on the
relationship between bedload (mainly sand and gravel) and suspended load (mainly
silt and clay), Schumm (1963) distinguishes between three types of channels:
1. Suspended load chamiels which carry less than 15% bedload.
2. Mixed load channels which transport 15 to 35% bedload.
3. Bedload channels which transport 35 to 70% bedload.
There exist strong relationships between the kind ofload conveyed in a river and its
configuration. Bedload channels have a high width/depth ratio, i.e. they are wide
with respect to their shallow depth, which tends to raise the Froude Number
[Eq. (6.4)] and leaves behind bedforms of the upper flow regime and the upper
portion of the lower flow regime. With decreasing bedload, the width/depth ratio
350
Coal-Producing Sedimentary Environments
Table 7.1. The main coal-forming sedimentary environments
Environments
Subenvironments
Coal characteristics
Gravelly braid plain
Bars, channels, overbank
plains, swamps, raised bogs
Bars, channels, overbank
plains, swamps, raised bogs
Channels, point bars, flood
plains and basins, swamps
fens, raised bogs
Delta front, mouth bar, splays,
channels, swamps, fens and
marshes
Off-, near-, and backshore,
tidal inlets, lagoons, fens,
swamps, and marshes
Mainly dull coals, medium to low TPI,
low GI, low sulphur
Mainly dull coals, medium to high
TPI, low to medium GI, low sulphur
Mainly bright coals, high TPI,
medium to high GI, low sulphur.
Sandy braid plain
Alluvial valley and
upper delta plain
Lower delta plain
Backbarrier strand
plain
Estuary
Channels, tidal flats, fens and
marshes
Mainly bright coals, low to medium
TPI, high to very high GI, high sulphur.
Transgressive: Mainly bright coals,
medium TPI, high GI, high sulphur;
regressive: Mainly dull coals, low TPI
and GI, low sulphur
Mainly bright coal with high GI and
medium TPI
decreases, i.e. the channels deepen and bedforms of the lower portion of the lower
flow regime become more common. Likewise, the sinuosity, i.e. the ratio between
thalweg and valley length (Leopold and Wolman 1957), or the sinuosity index, i.e.
the ratio channel length/length of meander belt axis (Brice 1964), increase
downstream as bedload channels grade into meandering mixed and suspended load
channels (Miall 1977). An example of the change in channel geometry downpalaeo slope is the Lower Freeport Sandstone from the Upper Carboniferous
(Pennsylvanian) coal measures of eastern Ohio, U.S.A. Near the southern source
area it consists of coarse detritus which was deposited in low sinuosity channels and
relatively narrow alluvial plains, while to the north the channels are deeper and their
meander belts become wider (Flores 1979). This suggests increasing sinuosity, which
is also indicated by the increasing variability in cross-bedding directions.
While on subsiding ground braided channels accumulate sediment by vertical
aggradation and downslope progradation, the tendency to form deposits by lateral
accretion on point bars increases with increasing sinuosity. However, the discussion
of coal/roof discordances due to unequal loading in Chap. 6.3.2.1, as well as presentday examples (e.g. Kosi River in India) have shown that low sinuosity braided
streams are also capable of lateral migration, and that this mechanism was
responsible for the formation of very wide braid plains in some coal measure
environments. In contrast, there are some very high sinuosity channels which show
little evidence of lateral migration. Such relatively stable systems of channels, which
bifurcate around semi-permanent and commonly vegetated islands, are referred to
as anastomosing (Schumm 1968). According to Smith and Smith (1980), anastomosing channel systems grow predominantly by vertical aggradation, although lateral
accretion, i.e. incipient point bar formation, may occur on channel bends. This
feature was also observed by Rust and Legun (1983) in the Carboniferous coal
Coal-Producing Sedimentary Environments
351
measures (Clifton Formation) of New Brunswick, by Flores and Hanley (1984) in the
Tertiary Fort Union Formation of the Powder River Basin, U.S.A., and by
Cassyhap and Tewari (1984) in the Lower Permian coal measures of the SonMahanadi Valley and Koel-Damodar Valley Basins in east-central India.
The islands between the channels are raised above fair-weather water level but
may be inundated during floods, when they receive new sediments, like the
remainder of the flood plain. The key to the stability of such a river system is the
reinforcing riprap effect of the vegetation growing on both flood plain islands and
channel embankments. Erosion tests carried oyt at a flow rate between 1.5 and
1.6 m/s by Smith (1976) in the Alexandra River, an anastomosing tributary of the
Saskatchewan River in Alberta, Canada, have yielded the following inverse
relationship between erosion rates and root penetration of channel embankments:
y = 52.26 (0.79)X,
(7.1)
whereby x = percent of vegetation roots in sediment and y = erosion rate in cm/h.
The embankment sediments tested consisted of approximately 70% silt and 30% fine
sand plus clay, while their root content ranged from 0 to 22% (by vol.).
Based on the channel sinuosity of Leopold and Wolman (1957) and the braiding
parameter (= number of braids per mean meander wavelength), Rust (1978a)
defines the following four channel types:
1. The single-channel. high-sinuosity system consists of a meandering channel with
braiding parameter < 1 and sinuosity> 1.5.
2. The multi-channel. low-sinuosity system is commonly referred to as braided with
braiding parameter > 1 and sinuosity < 1.5.
3. The single-channel. low-sinuosity system is represented by a straight channel with
braiding parameter < 1 and sinuosity < 1.5.
4. The multi-channel. high sinuosity system consists of anastomosing channels with
braiding parameters> 1 and sinuosity> 1.5.
Because of the comparatively high channel gradients often observed in braided
rivers, Rust et al. (1984) regard transitions between anastomosing and meandering
channels to be more common than anastomosing-braided transitions. However, in
both Recent and ancient fluvial successions anastomosed and braided river deposits
have been found to replace each other vertically (Rust and Legun 1983), as well as
laterally, the latter in the Coopers Creek system of Central Australia (Rust and
Nanson 1986; Nanson et al. 1986). According to Eq. (7.2), braiding can occur at very
low slope angles, provided bankfull discharge is high. Because floods are comparatively infrequent, it is likely that between periods of flooding low hydraulic energy
conditions prevail which, according to Smith (1983), are required for the stability
of an anastomosing flow system. The anastomosing fluvial system Rust et al. (1984)
recognised in the lower 500 m of the Upper Carboniferous Cumberland Group at
the Joggins Section of Nova Scotia in Canada, is lithologically chracterised by an
352
Coal-Producing Sedimentary Environments
Fig. 7.1. Traces of trough cross-stratification (arrowed) on ac-planes in thick
channel sandstone above Kimberley
Seam, Cumberland Group, at Four Mile
Point, Joggins Section, Nova Scotia
overbank and a channel association, in which the fluvial sandstones share a number
of characteristics with braided channels, while the overbank sediments are similar to
those of the alluvial valley and plain association. Similarly to the sandy channels of
braid plains, the up to 12.5-m-thick sandstones of the Joggins Section lack lateral
point bar accretion planes but instead have accumulated by vertical aggradation,
which commonly was halted suddenly by stream avulsion, resulting in sharp upper
bounding surfaces of the channel sandstones. They contain trough cross-bedding
(Figs. 7.1 and 7.2) and subhorizontal erosion surfaces and stepped base as their
principal primary structures. Unlike braid-plain environments the fluvial channels
at Joggins have very limited lateral extent and therefore a comparatively low
width/depth ratio, and they are bordered by thick, principally lutaceous, overbank
deposits. These contain numerous sheets of splay sandstone, mostly less than 1 m
thick. Their large number is related to the humidity and frequency of flooding at the
time, and to the multi-channel nature of anastomosing systems, which infers that no
part of the flood plain was ever far away from a chanelised sediment source (Rust
et al. 1984). Channel density is also influenced by climate and is considerably lower
in arid regions. For example, Rust (1981) found that in arid and semiarid Central
Australia only 3% of the alluvial plain of the anastomosing Coopers Creek system
are covered by channels.
Coal-Producing Sedimentary Environments
353
Fig. 7.2. View of ab-planes in transport
direction (arrowed) of trough crossstratification in thick channel sandstone
above Kimberley Seam, Cumberland
Group, at Four Mile Point, Joggins
Section, Nova Scotia
In spite of the controlling influence of fluvial systems on coal-forming
environments, the scope of the discussion will not be restricted ot actual river
deposits but will, in fact, concentrate more on the associated overbank deposits
which will be divided into flood basin and flood plain sediments. The former are
mainly subaqueous deposits which may contain coal formed under very wet to
allochthonous conditions including sapropelites (e.g. boghead and cannel coal), but
they may also consist of inorganic sediments formed in comparatively stable
lacustrine environments. Flood plain deposits are the products of floods which leave
behind fine-grained sediments in ephemeral lakes and water courses which dry up
when the flood waters retreat into the channels. Coals may likewise by formed in
poorly drained flood plain environments, usually with a stronger autochthonous
signature than shown by the flood basin coals and, when low in ash, ombrotrophic
elements may be included as well. In the case of well-drained flood plains, l~terally
persistent rheotrophic (topogenous) mires cannot be maintained and peat formation, if it occurs at all, is restricted to isolated pockets of poor drainage. This
results in a patchy distribution of coal, as has been described by Cavaroc and Flores
(1984) from the Upper Cretaceous, river-dominated Bartlett Member in the Gallup
Coalfield of New Mexico, U.S.A.
354
Coal-Producing Sedimentary Environments
7.1 The Braid Plain
The coarsest coal-bearing sediments are commonly found in close VICllllty to
mountainous terrains from where large quantities of clastic debris are washed down
from the uplands and are deposited in broadening alluvial valleys between
mountain ranges, which subsequently merge with proximal piedmont plains in a
foreland setting. The sediments consist of coarse, immature, molasse-type, clastic
wedge deposits, which accumulate rapidly as the waters draining the mountainous
hinterland leave the confines of the valleys and begin to fan out. Due to human
interference, free-running braided streams are becoming increasingly rare, although
some splendid examples can still be found on the Indogangetic Plain, the forelands
of the Rocky Mountains, the Canterbury Plains in New Zealand and eleswhere. A
relevant current example of gravelly braid plains in an intra- and/or intermontane
setting is the Altiplano in the South American Andes.
Each valley exit acts as a point source for an alluvial fan (Fig. 7.3) and,
depending on the number of rivers leaving the mountains, the various deposits may
merge with one another thus forming a linear belt of overlapping fans adjacent to the
mountain front. Alluvial fans which extend beyond the coastline or prograde into a
lake form fan deltas (Nemec and Steel 1988), while on land the proximal fans which
prograde by pulsations of debris flow and sheet floods ("diffuse gravel sheets" of
Hein and Walker 1977), give way to first gravelly, then sandy braid plains in which
channelised flow becomes increasingly dominant (Zaitlin and Rust 1983). Concurrently the azimuth of the drainage pattern changes from being initially transverse to
the mountain front to a longitudinal flow, i.e. parallel to the main axis of the foreland
basin. This pattern, which is common to modern mountain chains and their
Fig.7.3. Example of a small alluvial fan in Banff National Park, Canadian Rocky Mountains
355
The Braid Plain
"Ljubljana
Fig. 7.4. Drainage pattern of the European Alps as an example of the contrast between the
transverse tributaries and the longitudinal trunk streams (Danube and Po) collecting and removing
the outflowing water
forelands (an example is illustrated in Fig. 7.4), appears to have also been a frequent
arrangement in ancient fold belt/foredeep couplets, as, for example, shown by the
palaeodrainage patterns of the Late Permian and Early Triassic molasse sediments
of the New England Fold Belt in the Sydney Basin of New South Wales (Diessel and
Moelle 1970), and the Early Cretaceous molasse of the Rocky Mountains in western
North America (McLean 1977). Conditions of the upper flow regime are maintaint<d
only briefly when large stretches of the lowlands are inundated by flood waters. Coal
forming conditions may not seem to be ideal but considerable quantities of Recent
peat (e.g. in the Bavarian lowlands adjacent to the European Alps) and many fossil
coal deposits bear witness of the productivity of an environment in which both high
and low energy conditions coexist in close proximity to each other.
The rivers which build the braid plains of the piedmont environment are
physiographically youthful, oflow sinuosity, and characterised by high width/depth
ratios, often in excess of 300. With increasing departure from its source, the sinuosity
of a river increases when the channel enters a terrain of low slope gradients and
changes from a relatively straight to a meandering configuration. However, slope
gradient is not the only factor that determines whether a stream will be braided or
meandering. Based on wbrk by Leopold and Wolman (1957), Miall (1977) makes a
Coal-Producing Sedimentary Environments
356
numerical distinction between braided and meandering rivers by relating the slope
angle (S) and bankfull discharge (Qb in m 3/s) according to:
S = 0.013 Qb -0.44.
(7.2)
Given a certain discharge, an actual slope angle that is higher than the calculated
slope angle will result in the formation of braided channels, whereas smaller slope
angles will produce meandering streams. Some modifications of this relationship
may result from the influence of additional factors, such as the amount of sediment
transported, bed roughness, and channel size. Nevertheless, the higher the discharge
the lower is the minimum slope angle required for the formation of braided
streams.
The highest discharge occurs at flood peaks, during which the whole braid plain,
including channels and raised flood plains, is inundated. The flood waters' high
competency (the ability to shift large clasts) and high capacity (the ability to carry
large quantities) facilitate the transportation by traction of coarse sediments, mainly
sand and gravel. According to Leopold and Wolman (1957), braiding is initiated
when the flood waters start leaving behind the larger paritcle sizes and begin to
recede into their shallow and wide bedload channels. The rapid build-up of bars in
the channel causes it to bifurcate and flow around these bars with the result that
most braid plains are transsected by many individual channels, although according
to Rust (1972), one or a very limited number of these are dominant. Since they will
carry most of the coarse load, fossil braid plains can be illustrated by sand/shale
ratio maps. An example from the Newcastle Coalfield in New South Wales is shown
in Fig. 7.5. The bars are subaerially exposed at low water conditions (Fig. 7.6), they
therefore migrate only during peak floods resulting in an imbrication fabric in coarse
gravel (Fig. 7.7), and in planar cross-bedding in the more distal fine gravels and
gravelly sands (Rust 1979). Although a large number of bar types has been
established (Smith 1978), only three types are frequently referred to (Miall 1977)
which, in order of decreasing energy conditions are:
1. Mid-channel longitudinal bars consisting of crudely stratified, but often im-
bricated gravel sheets.
2. Mid-channel transverse or lingoid bars which are composed of cross-bedded
sand and gravel formed by avalanching of tracted particles on down-current slip
faces.
3. Point or side bars protruding from the margin into the channel following
coalescence of bedforms.
An additional bar type, called diagonal bar, has been defined by Smith (1974). Its
internal organisation may be a combination of the above types 1 and 2, but its
main characteristic is the diagonal orientation with respect to the channel trend. In
any of the above-mentioned bar types 1 to 3, varying rates of water discharge
during floods and their duration are reflected in particle size variations between
superimposed bar deposits and their thickness, an example of which is illustrated in
Fig. 7.8.
The Braid Plain
357
r-------.-~~~~~~-
~,
o
0.5
Fig.7.S. Sand/shale ratio map of the Tighes Hill Formation, Newcastle Coal Measures, New
South Wales. (After Warbrooke 1981)
Between floods, which are relatively short-lived events, water flow and sediment
transportation are restricted to the braided channels, although considerable
volumes of water may also be conveyed through gravels and sands below bars.
Because fair-weather discharge rates are small, many sands below bars. Because fair
weather discharge rates are small, many channels have a tendency to form meanders
and migrate laterlly across the braid plain. The bed forms generated by this
channelised flow include lingoid megaripples resulting in trough cross-bedding.
358
Coal-Producing Sedimentary Environments
Fig. 7.6. Channels and bars in the Rakaia River, southwest of Christchurch, New Zealand
Fig.7.7. Pebble imbrication in gravel bar on a raised terrace of the Rakaia River, southwest of
Christchurch, New Zealand
These often reflect the greater variability in channelised flow directions as opposed
to the slope azimuths obtained from the imbrication of pebbles and other
components of channel bars, which only move at flood peaks. According to Rust and
Jones (1986), another source of variance between current directions obtained from
vertically stacked channel deposits is due to channel avulsion following the sudden
abandonment of an earlier channel complex and the re-establishment of a new
channel system above an erosional basal contact.
The Braid Plain
359
Fig. 7.8. Particle size variation between
stacked longitudinal gravel bars in Teralba Conglomerate, Newcastle Coal
Measures, New South Wales
On the basis of such physical properties as particle size, lithosome geometry,
sedimentary structures and other features listed in Table 7.2, Miall (1977)
distinguishes four types of braided stream deposits (later expanded to six in Miall
1978), which he named after modern rivers typifying the depositional environments.
Two of these, the Donjek type with 10 to 90% gravel, named after the Donjek River
in the Yukon Territory and referred to as GIll lithotype by Rust (1978b), and the
South Saskatchewan type ( < 10% gravel), after the so named river in Saskatchewan
(Su lithotype of Rust 1978b), appear to be most commonly represented in the coalbearing braid plain deposits of eastern Australia. A third variety, called Scott type
(> 90% gravel) after the Scott Glacier outwash river in Alaska, and called G u
lithotype by Rust (1978a), is represented by two massive conglomerates (Teralba and
Bolton Point) in the northern (proximal) portion of the Newcastle Coal Measures,
although distally they grade successively into Dongjek and South Saskatchewan
types.
Coal-Producing Sedimentary Environments
360
Table 7.2. The lithological properties of braided streams. (After Miall 1977 and Rust 1978a)
Facies
identifier
Lithofacies
Gm
Massive, or crudely
bedded; some sand, silt or
clay lenses
Bedded gravel
Gp
Gt
St
Sp
Sr
Sh
Ss
Fl
Fm
Features
Imbrication in gravel,
cross-bedding and ripples
in sand
Planar cross-bedding,
graded bedding
Bedded gravel
Broad, shallow trough
cross-bedding
Medium to very coarse,
Solitary or grouped
pebbly sand
trough cross-bedding
Medium to very coarse,
Solitary or grouped
pebbly sand
planar cross-bedding
Very fine to coarse sand
Micro-cross-lamination
and all kinds of ripple
marks
Very fine to coarse sand
Horizontal bedding,
parting lineation
Fine to very coarse,
Sand overlying basal
pebbly sand
erosional surfaces
Very fine sand to silt and Parallel and micro-crossclay, may include coal
lamination, bioturbation,
ripples
Mud, silt and clay, may
Massive appearance,
include coal
Rootlets, desiccation
cracks
Sedimentary
interpretation
Longitudinal bars, lag
deposits
Mainly linguoid bars
Channel fill
Minor channel and scour
fill
Linguoid bars and sand
waves
Small-scale ripples
Deposits of sheet
(mostly) or tranquil flow
Sand-filled broad, shallow
scours
Deposits of waning floods
and in overbank settings
Deposits formed in pools
of standing water
7.1.1 The Gravelly Braid Plain
Depending on climate and elevation of the mountainous sediment source, proximal
braid plains are usually composed of gravel sheets, which distally grade into sandy
braid plains. As will be discussed more fully in Chapter 9, ancient coal-bearing
examples of this composition are found either in the lower portion of rift and pullapart basin, or in the upper portion of molasse foredeeps (foreland basins). In both
cases the formation of the gravel sheets is possible only because of the proximity of
the depositional sites to the elevated sediment sources, which puts them into
tectonically unstable positions, often in close vicinity to major faults. The coarsest
portions of coal measures, particularly when located near the border thrusts of
foreland basins, are therefore subject to destruction by subsequent tectonic
movements affecting proximal basin settings. It seems that this is a major reason for
the relative scarcity of gravelly braid-plain deposits among coal measures. Examples
of coal formation in association with gravelly braid plains have been reported from
the Anthracite Coal Basins in eastern Pennsylvania (Weller 1930; Pedlow 1979), the
base of the upper Carboniferous Morien Group in the Canadian Sydney Basin of
Nova Scotia (Rust and Koster 1981; Rust et al. 1987), and the Cantabrian Zone of
The Braid Plain
361
northwestern Spain (Colmenero et al. 1988). According to Fernandez et al. (1988),
the Upper Carboniferous (Westphalian D) Spanish braid-plain coals are part of a
5000-m-thick sequence of proximal coarse clastic and distal marine sediments which
have been attributed to the development of several large fan deltas in the Central
Coal Basin southeast of Oviedo. Coal seams are thought to have been formed at the
outer fringe of fan lobes and on the surface of abandoned lobes. In the first case the
seams are thin, discontinuous and are very high in ash, due to frequent flooding,
but the second group consists of generally thicker coal seams which also are low
in sulphur, although their ash content may still be high.
For the purpose of relating the composition of coals to their position within the
braid-plain complex, an example from eastern Australia will be used, where thick
and coarse conglomerates interbedded with coal seams dominate the upper portion
of the Newcastle Coal Measures in the Sydney Basin of New South Wales
(Ziolkowski 1978). A stratigraphic section of the Newcastle and (underlying)
Tomago Coal Measures is illustrated in Fig. 7.9A to D. In the conglomerates, large
and occasionally sandy trough cross-beds (Gt facies) alternate with remnants of
gravel bars and massive conglomerate sheets (Gm facies) which form compound
lithosomes up to 50m thick. They contain isolated thin lenses of either flat
bedded or massive sandstone (Sh facies, as in Fig. 7.10) or pebbly sandstone
with internal crossbedding (Fig. 7.11), but lutaceous overbank deposits (FI and
Fm facies) are rare. Williams and Rust (1969) suggest an origin on bar tops or
in abandoned channels for mudstone units in similar braided stream deposits
in Canada. Allochthonous coal lenses, probably incorporated as eroded peat,
are present in the Teralba Conglomerate (835-868 m in Fig. 7.9D) testifying to
the rapid rate of transportation and deposition. Occasional layers of bright coal
(vitrain) have been derived from the wood and bark tissues of uprooted and
transported trees (Fig. 7.12). Contacts between litho somes are abrupt and
commonly erosive with channels cutting into each other, as illustrated in
Fig. 7.13.
As Fig. 7.14 shows, the trough cross-beds occurring in conglomerates (Gt facies)
are quite large, occasionally exceeding twenty metres in width and five metres in
height with foreset angles up to 30°. Coalified tree trunks are often oriented normal
to the trough axes, indicating rolling down the slip faces of the gravel masses choking
the channels. The latter are confined to relatively narrow zones within the 5- to
20-km-wide braid plains and, in proximal setting along the northeastern margin of
the coalfield, they are separated by the quantitatively more important composite
gravel sheets of the Gm and Gp facies.
In reference to Miall's (1977, 1978) braided stream models, some of the
conglomerates of the Newcastle Coal Measures (e.g. Charlestown Conglomerate
between 652 and 690m in Fig. 7.9C) resemble the Scott type whereas others, e.g.
Teralba Conglomerate between 835 and 868 m in Fig. 7.9D and the Bolton Point
Conglomerate located outside the section line of Fig. 7.9D, are transitional to the
Donjek type.
The middle portion of the Newcastle Coal Measures, contains several examples
of interbedded conglomerates and sandstones of which the Croudace Bay Formation (771-807 m in Fig.7.9D) is a good example. Its conglomerates and
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366
Coal-Producing Sedimentary Environments
Fig. 7.10. Thin sandstone lenses occurring within the Teralba Conglomerate at Catherine Hill Bay,
New South Wales
Fig.7.11. Part of a cross-bedded sand
lens in the Bolton Point Conglomerate
at Catherine Hill Bay, New South Wales
The Braid Plain
367
Fig. 7.12. Illustration of lenses of bright
coal (B), representing stranded drift
wood, on the sandy flank of a channel in
the Teralba Conglomerate at Catherine
Hill Bay, New South Wales
Fig. 7.13. Erosive contact between two channels in the Bolton Point Conglomerate at Catherine
Hill Bay, New South Wales
368
Coal-Producing Sedimentary Environments
Fig.7.14. Photograph showing parts of large cross-beds (arrows) in the Bolton Point Conglomerate, Newcastle Coal Measures. See hammer (encircled) for scale
sandstones form the top of an upward-coarsening sequence which begins with coal
(Upper Pilot Seam from 763 to 765 m in Fig. 7.9D), followed by several metres of
redistributed volcanic ash. With increasing sand content, the latter grade first
into coarse, then medium braid-plain sandstones and conglomerates of Miall's
(1977) Donjek type (Ziolkowski 1978). There are also similarities to the "pebbly
sandstone assemblage" described by Rust et al. (1983) and Gibling and Rust (1984)
from the South Bar Formation in the Pennsylvanian Morien Group of the Sydney
Basin, Nova Scotia. The pebbly sandstone assemblage has been interpreted by Rust
et al. (1987) as a mid-braid-plain setting, in which sand was conveyed in channels by
sinuous-crested dunes migrating across a pebbly lag. While in the Canadian
example this sequence contains only allochthonous coal as intraclasts, there are
some thick autochthonous coal seams associated with this facies in the Newcastle
Coal Measures. Also the rudite content is slightly higher and more concentrated in
distinct bands representing bar deposits. As illustrated in Fig. 7.15, trough crossbedded pebbly sandstones (St facies) and solitary pebble conglomerate channel bars
(Gm facies) are the main lithosomes. The trough cross-beds may be up to 6 m wide
and 2 m in height. They occur either as solitary sets (theta type of Allen 1963), or as
cosets, similar to Allen's pi type. Both have upward concave foresets. In addition to
the large sets, small to medium scale cross-beds are common in small cut-and-fill
channels (Ss facies).
In cross-bedded conglomerates the foresets are commonly steep and have
discordant basal contacts, whereas in the sandstones the foreset angles are more
variable but are usually shallow and may be tangentially aligned with the lower
bounding surface. Heterogeneous cross-beds are quite common and show a
steepening of the foreset angles with increasing grain size. Coalified tree trunks are
frequent and often oriented normal to the trough axes.
The Braid Plain
369
Fig. 7.15. Trough cross-bedded pebbly
sandstone (St) alternating with gravel
bars (Gm) in the Croudace Bay Formation of the Newcastle Coal Measures,
New South Wales
Fig. 7.16. View of part of the outcrops of the Merewether Conglomerate, Newcastle Coal
Measures, at Shepherds Hill, Newcastle, N.S.W.
370
Coal-Producing Sedimentary Environments
The internal organisation of many conglomerate lithosomes is somewhat
haphazard. An example is the Merewether Conglomerate of the Newcastle Coal
Measures, which is listed in Fig. 7.9C between 558.8 and 569.3 m. The main body of
the Merewether Conglomerate displays considerable particle size variation ranging
from pebbly gravel to silt. As illustrated in Fig. 7.16, the various particle sizes are
concentrated in lenticular beds which are rarely continuous but alternate with each
other in an abrupt manner. Erosional contacts between beds are common, resulting
in the destruction of graded bedding but some bedforms, such as megaripples, are
still visible. Similarly to the other conglomerates in the Newcastle Coal Measures, it
seems that the sandy fraction has been formed in channels while the gravel fraction
consists largely of remnants of channel bars.
Some ofthe gravel-dominated lithosomes are matrix-supported, whereas others
are clast-supported. The latter are between 10 and 20 cm in thickness and may
exhibit weak pebble imbrication. This is lacking in the matrix-supported beds which
are also considerably thicker (up to 2-3 m). These also contain the largest pebbles
(up to 60 mm in diameter) in spite of the sandy matrix, whereas the clast-supported
thinner beds commonly consist of a more evenly grained finer pebble fraction. The
distribution of mean maximum particle size (mean of longest diameters of the ten
largest clasts) measured at 5-m intervals is illustrated in Fig. 7.17, which shows some
upward fining for the whole conglomerate, although the upper contact with the
lood plain sediments underneath the Fern Valley Seam is usually abrupt.
Lateral contacts between the conglomerate bodies and adjacent flood plain
sediments vary. Down-palaeoslope there is a general increase in the proportion of
sandy channel facies at the expense of gravel bars. Proximal sandstone channels
retain a high proportion of rudaceous bedload, which distally diminishes, as does
the particle size of the bars. From approximately 10 to 20 km basin ward (SW) of
their most proximal outcrops in the northeastern portion ofthe Newcastle Coalfield
the continuous conglomerate and sandstone sheets begin to break up into separate
25
22.5
•
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OJ
15
]
12.5
~
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~
7.5
5
•
•
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Fig. 7.17. Distribution of mean maximum particle size measured at 5-m intervals through
the Merewether Conglomerate at Shepherds HilI, Newcastle. Based on measurements by
A. Kohlrusch, The University of Newcastle, N.S.W.
371
The Braid Plain
NORTH OF REDHEAD - S . 3 K m - LITTLE REDHEAD - - 4 K m -
COi l
Flood Pl,ln
flood Basin.
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SHEPHERDS HILL-MEREWETHER (Section trends N20E)
FERN VALI.. EV SEAM
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./
./
./
s.
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-
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VICTORIA TUNNEL SEAM
Fig. 7.18. Three sections ofthe Kotara Formation with the Merewether Conglomerate, Newcastle
Coal Measures, measured between Newcastle (Shepherds Hill) and Redhead along the New South
Wales coast. For geographical orientation refer to Fig. 6.56
channel and overbank facies. Laterally, conglomerate sheets tend to interdigitise
with their adjacent overbank facies. An example from the southern margin of the
above-mentioned Merewether Conglomerate is illustrated in Fig. 7.18 by three
measured sections over a distance of 9.3 km from Newcastle to Redhead.
7.1.2 The Sandy Braid Plain
The lithofacies associated with this environment is transitional with the gravelly
braid plain and is similar to Miall's (1978) South Saskatchewan type of braided
stream development, as well as to the "sandstone assemblage"described from Nova
Scotia by Rust et al. (1983,1987) and Gibling and Rust (1984), except, once again, for
the better developed coal content. Many examples ofthis type are represented in the
Newcastle Coal Measures, for instance, by the Warners Bay Formation (735-749 m
in Fig. 7.9C). It consists of a basal trough cross-bedded sandstone (St facies)
commencing with a very coarse intraformational bedload and a sandstone layer (Sh
372
Coal-Producing Sedimentary Environments
Fig.7.19. Remnant of silt-laminated
shale (above person on ladder) in the roof
of the Great Northern Seam below the
Teralba Conglomerate, Newcastle Coal
Measures, near West Wallsend, N.S.W.
Fig.7.20. Intraformational shale clasts in the roof sandstone of the Wallarah Seam, Newcastle
Coal Measures, N.S.W.
The Braid Plain
373
facies}. As indicated by the narrow ranges of foreset azimuths at 737 and 749 m in
Fig. 7.9C, channels are mostly of low sinuosity but some point bar development
suggests the beginning of meandering.
In all facies types conglomeratic channel bar deposits and sandstone lenses are
bounded by erosion surfaces. Patches of overbank shale (FI facies) above coal seams
(Fig. 7.19), show concordant seam/roof relationships and suggest substantial removal of an originally more widespread primary roof shale, whereas in discordant
relationships a lack of extensive floodplain deposition before the encroachment of
braid-plain conditions is indicated. Where a primary lutaceous roof sediment was
eroded, its remnants are sometimes seen to occur as intraformational bedload clasts
(intraclasts) in the overlying braid-plain deposits (Fig. 7.20).
~ Gravelly Braidplain
o
~
Sandy Braidplain
o
:
5
10
Km
~ Peatiand
~ Lake
NEWCASTLE
_ CATHERINE HILL BAY
Fig. 7.21. Sketch map illustrating the regional distribution of palaeo-environments in the
Newcastle Coalfield, N.S.W., at the time the Bolton Point Conglomerate was formed. The actual
time slice represented can be located in Fig. 7.23 as a horizontal surface at -150 m. The section line
indicates sample localities for coal facies indices given in Fig. 7.25. (After Bocking et al. 1988)
374
Coal-Producing Sedimentary Environments
Sandy braid plains are the result of the distal reduction of phenoclast size, so
that down-palaeo slope the proportion of conglomerate is reduced to thin lag
deposits overlying basal erosional contacts. An excellent example is the abovementioned Bolton Point Conglomerate (below the Awaba Tuff at 834 m but not
encountered in the measured section of Fig. 7.9). Bocking et al. (1988) have traced its
channel system in cored diamond drill holes for more than 40 km downstream, in
which direction the sediments grade from pebble to sand size, and the width of the
depositional area changes from several kilometres in the coarse northern portion to
an at least 20 km wide sandy braid plain with local meander belts. As shown in
Fig. 7.21, lacustrine (flood basin) shales and laminites occur in the southernmost
(i.e. distal) portion of the mapped area.
7.1.3 The Coals of the Braid Plain
The high energy release and depositional instability of the braid-plain environment
does not seem to be conducive to widespread peat formation, but the evidence for a
temporal coexistence of peat formation and coarse clastic deposition is strong. A
Holocene example are the mires of the subalpine lowlands of Bavaria. Before most
ofthem were transformed into agricultural land, they occupied areas of sedimentary
bypassing between the low-sinuosity streams draining the Bavarian Alps towards
the Danube River. In many cases peat and coarse clastics accumulated concurrently
and in close lateral proximity, whereby moisture was supplied to the mires from
both ombrotrophic and rheotrophic sources, resulting from heavy precipitation in
the Alpine foreland and by ponding of subterranean watercourses precolating
through Pleistocene gravel sheets. It appears that before the onset of human
interference peat formation used to be terminated mainly by lateral channel
migration over adjacent mires. The effectiveness oflateral migration is, for example,
demonstrated by the westward displacement of the Kosi River of 100 km in
230 years across the Indogangetic Plain (Gole and Chitale 1966). While the speed of
this lateral migration may be an extreme case, Wolman and Leopold (1957) found
figures of several tens of metres per year to be quite common.
Given suitable conditions, the passing of a braid plain across a mire would be
followed by the recolonisation of the abandoned channel system by peat-forming
plants resulting in a close superposition of deposits with rather contrasting energy
requirements. Another example of this is the splitting of the Fern Valley Seam by the
Redhead Conglomerate in the Newcastle Coal Measures discussed in Chapter
6.3.2.1. A different example of the change from high energy braided stream
conditions. to low energy mire development can be found along the coast of the
Canterbury Plains on New Zealand's South Island, where it is related to the postPleistocene eustatic sea-level rise. Similar to the Bavarian lowlands, the Canterbury
Plains are transsected by a network of high energy channels. However, in contrast to
Bavaria, where the transverse channels draining the mountain chain feed into a
longitudinal trunk stream, the Danube River, the channels draining the New
The Braid Plain
375
Fig. 7.22. The colonisation of gravel sheets and bars by marsh vegetation near the mouth of the
Rakaia River, South Island, New Zealand
Zealand Alps, feed directly into the sea. In response to the Late Holocene sea-level
rise, the lower reaches of some of the New Zealand rivers have come under tidal
influence. The gravel bars close to the shore have been converted into pebbly
beaches, while inshore they are now vegetated and carry some peat (Fig. 7.22).
By using the Awaba Tuff (from 818.5 to 834 m in Fig. 7.9D), a thick and wellcorrelated layer of volcanic ash in the Newcastle Coal Measures, as a datum, the
interbedding between thick coal seams and conglomerates is illustrated in Fig. 7.23.
It shows en echelon arrangement of conglomerate bodies and coal seams not unlike
the geometric distribution of sand bodies in the Allegheny rocks of West Virginia
described by Ferm and Cavaroc (1979). In the area covered by the cross-section, the
Bolton Point Conglomerate is up to 8 km wide and over 50 m thick (Bocking et al.
1988). It is completely enclosed by the Fassifern Seam (from 808.7 to 813.2 m in
Fig. 7.9D) within which it occurs as a seam split (not shown in Fig. 7.9D) and with
which it is also laterally interbedded. This suggests that the Bolton Point
Conglomerate was formed as a succession of stacked braid plains which coexisted
with the Fassifern peat. Although individual channels had some degree of freedom
to move within the braid plain, the latter must have remained confined to more or
less the same position by the surrounding peat for several tens ofthousands of years.
Apart from a narrow marginal zone of interbedded coal and clastics, the Fassifern
Seam contains only few dirt bands and has a moderate ash content. In accordance
with Eq. (7.1), the vegetation and peat surrounding the Bolton Point braid plain
must have therefore constituted an effective barrier against excessive contamination
with mud and silt carried by flood waters.
According to McCabe (1984, 1987), the above scenario is likely to occur in
places where rates of peat accumulation exceed rates of inorganic overbank
sedimentation. As at the time of peat formation, the Gondwana climate of eastern
376
Coal-Producing Sedimentary Environments
w
E
50
o
DMD2
DMC
- --------- __
-50
EE31
---
\
\
EEll
EE108
EE65DME2
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HW4EE310 EE87EE10~
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WALLARAH SEAM
~~~~~~~~~~~~~~;;;~GREAT
_
-150
o
•
AWABA TUFF
NORTHERN
CHAIN VALLEY
SM.S.
•
-200
-250
Me'res
o
km
Fig. 7.23. The relationship between the conglomerates and coal seams in the Moon Island
Beach Subgroup of the Newcastle Coal Measures, New South Wales. The Awaba Tuff has been
used as a datum. The section extends west of Swansea across the map illustrated in Fig. 7.21.
(After Bocking et at. 1988)
Australia was cool to cold, the analogy to the Donjek River, referred to above in
reference to the conglomerates coexisting, for example, with the Fassifern peat, may
be taken a step further. The Donjek is a tributary of the Yukon River in
northwestern Canada, where likewise cool summers and cold winters occur. In the
braid plain of the Donjek River, Williams and Rust (1969) recognised the following
four topographic levels ranging from channel bottoms to overbank islands:
1. The lowest level accommodates the channels whose bars are devoid of vegetation
and are exposed at low water conditions.
2. The second level is sparsely vegetated and inundated during floods but is
otherwise dry, except for a small number of deep channels.
3. The third level is affected only by major floods, it carries a low but continuous
vegetation cover.
4. The fourth level is densely vegetated and rarely affected by floods.
The considerable intercalation of such coals as the Fassifern, Great Northern and
Wallarah Seams and conglomerates illustrated in Fig. 7.23, indicates a collateral
relationship of peat formation and braid-plain activity. In view of the considerable
resistance of peat and plant roots to erosion, the vegetated third and fourth
topographic levels of Williams and Rust's (1969) Donjek model, modified to extend
into lowlands situated on abandoned braid plains, could be considered as a possible
peat-forming environment. Since the surface of active peat accumulation can be
The Braid Plain
377
raised above ground level only if the groundwater table can be raised as well, an
additional source of water, independent of the normal groundwater table, is
required. In view of the cool, moist climate at the time and the position of the
Newcastle Coalfield in the piedmont setting not more than 50 km away from the
then rapidly rising New England Fold Belt, summer rain and winter snow with low
evaporation seem to be the logical source of the required additional water,
supplemented by ground discharge and ponding of subterranean water which
flowed down from the northern highlands through the permeable gravel sheets.
The coals of the braid-plain environment are inertinite-rich and characterised
by dull lithotypes. As referred to in Chap. 5, such coals have been associated with
ombrotrophic peat forming conditions. For example, Esterle and Ferm (1986), relate
the lateral zonation of modern raised bogs, ranging from large trees near the bog
margin to stunted vegetation in the centre (Anderson 1964, 1976, 1983; GrosseBrauckmann 1969; Moore and Bellamy 1974; Dreissen et al. 1979; Gottlich 1980;
Clymo 1983, 1987), to the vertical succession of coal lithotypes in the Upper Hance
Seam of southeastern Kentucky. At its full thickness of approximately 1.5 m, ash,
sulphur and vitrinite are lowest, while the inertinite content is relatively high. From
this area, which covers only few square kilometres, the seam thins and splits away in
the manner described in Chapter 6.4.1 (seam splitting due to differential subsidence),
i.e. vitrinite content increases at the expense of inertinite, while in the split portion
both ash and sulphur contents increase. Esterle and Ferm (1986) conclude that the
inertinite-rich coal (highest value = 28%) found in the unsplit seam profile reflect the
stunted growth in the nutrient-depleted central portions of an ombrogenous raised
bog, although in a later paper, Esterle et al. (1989) cast some doubt on the validity of
the tropical raised bog model as an explanation of the lithotype succession found in
Carboniferous coals. Since only marine-influenced coals exceed 1% sulphur in
Australia, the main difference between the Upper Hance Seam and the braid-plain
coals of the Newcastle Coal Measures, apart from their greater thickness and larger
lateral extent, are their higher ash content (around 20% excluding stone bands) and
the high frequency and close spacing of both coalified and petrified tree trunks which
extend from the dull coal (e.g. Fassifern Seam) into the roof. Indeed, the discussion in
Chap. 6.3.1.1 of coal seams with tuffaceous roof rocks in eastern Australia has shown
that in all instances where peat accumulation was violently terminated by a volcanic
eruption, the peat-forming vegetation was arborescent over many hundreds of
square kilometres, irrespective of whether bright or dull lithotypes dominated the
coal. The trees extending from the Fassifern Seam into the overlying Awaba Tuff
average about 15 to 20 cm in thickness (mean spacing is approx. 1.5 m) which is not
as much as in some other coal seams of the Newcastle Coal Measures, but it does
indicate reasonable growth conditions. Similar variations in tree size have also been
found in vitrinite-rich bright coals. The relatively high tissue preservation index, due
to the large proportion ofsemifusinite found throughout the seam, also suggests a
high input from woody sources.
Based on the high ash content of the braid-plain coals of around 20~~ (excluding
stone bands), it is concluding therefore that rheotrophy, possibly in the form of
ground discharge and occasional flooding, was an important factor in the water
budget. This notion is further supported by the conclusion that the Fassifern peat
Coal-Producing Sedimentary Environments
378
(plus other seams of similar setting and composition) was quite densely wooded.
This is an important point in view of the observation that ombrotrophic bogs of cool
temperate to cold climatic zones carry little arborescent vegetation, and strong
evidence for at least cool conditions with marked seasonal changes at the time of
Permian peat formation in the Sydney Basin. It appears therefore that the braidplain coals were formed from a mixed, at the most only slightly oligotrophic peat in
which the normal groundwater table was raised by a combination of both
precipitation and ground discharge. In view of the frequent occurrence of tuffs and
tuffaceous tonsteins in the Newcastle Coal Measures, it is also possible that plant
nutrition was further supplemented by airborne volcanic ash, as is the case in some
Holocene raised bogs of Sumatra and Java.
Peat growth and braid-plain aggradation, both probably seasonal in response
to the cool climate, were in balance with basin subsidence, resulting in the vertical
stacking of the gravel sheets. As the latter grew in thickness, imbalances in
compaction ratios between the Bolton Point gravel and the surrounding Fassifern
peat caused upstream avulsion and the build-up of a new braided channel system,
now represented by the Teralba Conglomerate, adjacent to and above the Bolton
Point Conglomerate. This development was repeated several times and has led to a
vertical and lateral stacking ofthe conglomerate lenses and intervening coal seams,
as illustrated in Fig. 7.23.
The coal seams formed in coexistence with gravelly braid-plain environments in
the Newcastle Coal Measures have a strong petrographic signature which, in terms
of the coal facies indices illustrated in Fig. 7.24, is expressed by a medium to low
-decreases
200.0
TREE DENSITY
Increases-
te/mafic
limno-te/matic
100.0
50.0
'"
'"OJ
E
0
10.0
In
.
'0
..,."
."
c:
0
0
5.0
~
>
.
Q>
.Q
'"
Q.
::0
Q>
•
1.0
•
•
•
E
"E
::;;
'""
.. • -
0.5
terrestrial
GI
TPI
0.5
1.0
1.5
2.0
2.5
Fig. 7.24. Bivariate plot of
the tissue preservation
and gelification indices of
whole coal samples from
the seams illustrated in
Fig. 7.23 in"close association with conglomerates
in the upper portion
(Moon Island Beach Subgroup) of the Newcastle
Coal Measures. (Extended
after Diessel 1986a)
379
The Braid Plain
tissue preservation index (TPI) and a very low gelification index (GI). Macroscopically they are dull, i.e. rich in autochthonous durain and clarodurain. It
seems that periodically the peat surfaces were subjected to drying and oxidation,
including freeze-drying (Taylor et al. 1989), which restricted the formation of
telovitrinite.
A gradual rise in the groundwater table would result in a shift towards telmatic,
i.e. wet forest conditions would will facilitate a gradual replacement oftelo-inertinite
by telovitrinite. This will affect the gelification index more than the tissue preservation index because one group of structured macerals is replaced by another.
If, on the other hand, a drop in groundwater table leads to severe humification and
prolonged exposure of the peat, the proportion of structured macerals will be
reduced by oxidation, thus shifting both TPI and GI towards very low values before
destroying all remaining organic matter. In cases of advanced humification, the
normally dominant telo-inertinite (mainly as semifusinite) is replaced by autochthonous detro-inertinite (mainly as inertodetrinite). This is coupled with an
increase in inherent ash and liptinite macerals, such as sporinite, resinite and
cutinite. Although these have been traditionally regarded as particularly resistant to
biodegradation (Thiessen and Johnson 1930; Waksman 1938; Alpern 1960; Taylor
and Liu 1989), the various liptinites appear corroded when associated with other
highly humified macerals.
In accordance with the trasitional position of the sandy braid plain, its coals
range in composition between those associated with the gravelly braid plain and the
meandering river-dominated alluvial plain. An example is illustrated in Fig. 7.25
which shows the down-slope change in coal facies indices of the above-mentioned
Fassifern Seam along the section line indicated in Fig. 7.21, as the associated
gravelly channel sediments grade distally into sand. As mentioned above, the coal
has been derived from forest peat in which woody tissue formed the main
2
Fig. 7.25. Diagram illustrating
the downslope change along
the section line in Fig. 7.21,
from a proximal gravelly to
a distal sandy braid-plain
environment, in the tissue
preservation and gelification
indices to composite whole
coal samples from the
Fassifern Seam. Coal samples
for facies analyses from bores
25,34 and 35 have been kindly
made available by the State
Electricity Commission of
New South Wales
35
1.8
25
34
25
34
1.6
1.4
35
~ 1.2
Z
D
.8
0
5
Distance in km
10
15
20
25
30
35
40
380
Coal-Producing Sedimentary Environments
component. The resultant coals display little variation in the proportion of
structured macerals, which gives the coals a moderately high tissue preservation
index along the 35 km distance between the proximal (Z) and distal (34) sample.
However, the relative uniformity in TPI is contrasted by increase in the gelification
index, which corresponds to a shift from telo-inertinite (mainly semifusinite) to telovitrinite (as both telinite and telocollinite). This shift is a function of the
establishment of a less fluctuating and more consistently high groundwater table
further into the basin, wh~re the rate of subsidence is higher and the Fassifern Seam
has been divided into several discrete splits.
7.2 The Alluvial Valley and Upper Delta Plain
The transition from alluvial valleys and plains through which mature rivers flow in
meanders of increasing sinuosity towards the river mouths located on protruding
deltas has traditionally been associated with coal formation (Moore 1958, 1959; Fisk
1960). A distinction is commonly made between an upper and lower delta plain
depending on the degree of marine influence on sedimentation. According to Saxena
and Ferm(1976), Saxena (1979), and Coleman and Prior (1980), the upper delta plain
occupies the subaerial portion of the delta and merges imperceptibly with. the
alluvial valley. The lower delta plain is defined as the zone which marks the updip limits of tidal inundation. In ancient delta deposits, this zone is represented by
alternating low sinuosity channel sandstones and interdistributary bay shales which
may contain brackish to marine fossils, seat earths with rootlets and interspersed
coal (Horne et al. 1979a). An additional depositional model being transitional
between upper and lower delta plains has been distinguished by Horne et al. (1978),
but this practice is not followed here.
A delta (the term refers to the triangular shape of the deposit resembling the
respective Greek letter) is commonly formed when a river carries more sediment into
a lake or the sea than can be dispersed along the coast by tides, wave action or
longshore drift. However, the term has also been applied to other geomorphological
features, where the receiving body of water is restricted to the delta itself. An
interesting example with bearing on coal formation is the previously mentioned
(Chap. 5.1.1.1) Okavango Delta, also called Okavango Swamp, in Botswana. It
appears like a hybrid between a classical birdfoot delta and an alluvial fan. The
Okavango River and its tributaries feed seasonal floodwaters from the Lunda Ridge
in Angola into a fault-bounded depression on the northern margin of the Kalahari
Basin (McCarthy et al. 1989). The delta occupies an area of 18000km 2 , which is
divided into some 6000 km 2 of perennially and between 7000 and 12000 km 2 of
seasonally flooded ground (McCarthy et al. 1989). On account of its generally
shallow water of approximately 1.5 m (UNDP 1977), the permanently flooded
protion represents a limno-telmatic marsh environment in which a high rate of peat
accumulation of up to 5 cmja (McCarthy et al. 1986) is sustained mainly by two
dominant herbaceous plant species, Cyperus papyrus L. in the proximal portion and
The Alluvial Valley and Upper Delta Plain
381
Miscanthus junceum Stapf. in the distal portions (Smith 1976). The contribution of
biomass from trees is small and restricted to islands and higher ground within the
papyrus marsh. It should be noted that the location of the Okavango Swamp on the
fringe of the Kalahari Desert precludes any significant ombrotrophy and that the
very substantial peat accumulation is purely by rheotrophic means.
Legun and Rust (1982) regard the seasonally flooded portion of the Okavango
Swamp as. a modern analogue for parts of the Westphalian Clifton Formation in
New Brunswick, Canada. In the lower succession of Member B of this formation,
thin coals alternate with seat earths and reddish mudrocks containing pedogenic
calcareous nodules and hardpans, as well as desiccation cracks with calcareous
coatings. According to Legun and Rust (1982), this indicates periodic emergence of
the flood basin and exposure to semi-arid conditions, followed by submergence and
peat accumulation not unlike the marginal portions of the Okavango Swamp, in
which deep surface desiccation, colour mottling and incipient calcrete formation in
the subsurface are indicators of repeated droughts and periodic encroachments of
the surrounding desert.
In more conventional delta settings along the present sea shores it is sometimes
difficult to make a clear distinction between upper delta plains and their alluvial
hinterlands. The distinction between ancient upper delta and alluvial plain
association is even more difficult, because they produce similar lithofacies. For this
reason they will be treated together. Furthermore, it is not necessary for the alluvial
plain to be linked to a delta, but it may be part of a coastal plain, commonly situated
landward ofthe backbarrier strand plain, or it may have no connection to the sea at
all. The reason for linking alluvial and upper delta plains in this chapter is based on
the frequency with which alluvial valleys form the up-slope continuation of upper
delta plains, as well as the similarity in both coals and interseam sediments produced
by the two closely related associations. If a distinction between them is required, the
evidence has to come from the geological setting and the nature of the underlying
sediments. This problem will be discussed further in conjuction with the lower delta
plain setting in Chap. 7.4.
The alluvial and upper delta plain environments conform largely to the
"alternating sandstone and mudstone assemblage" of Rust et al. (1983, 1987) and
Gibling and Rust (1984), in which several subenvironments can be distinguished,
consisting of river deposits and overbank units. Examples of both are very common
in most coal measures, including the stratigraphic sections illustrated in Fig. 7.9B
(490-523 m) and 7.9C (523-538 m). The facies of meandering river deposits depends
on whether they have been formed in active or passive channels. In the first case they
consist of drawn-out point bar deposits which are the products of active channel
migration and include any surviving bedforms of in-channel deposition. The
resultant sediments are clearly distinguished from those formed within inactive
channels, i.e. abandoned meanders of oxbow lakes which are part of the overbank
association and have been filled with organic debris and lutaceous sediments that
were carried into the ponded water in suspension.
The overbank association encompasses the sediments and coals of flood basin
and flood plain environments. The lateral extent of both coal and flood plain
deposits is related to the size and sinuosity of the controlling river channel(s) which
Coal-Producing Sedimentary Environments
382
commonly meander(s) back and forth across the width of the alluvial plain leaving
behind a blanket of point bar and related overbank deposits. Horne and Ferm (1978)
report from the Upper Carboniferous coal measures of eastern Kentucky upper
delta plain fluvial deposits which range in thickness between 15 and 25 m, and are
between 1.5 and 11 km wide. An Australian example of a meandering fossil river of
relatively low sinuosity is illustrated in Fig. 7.26 from the Upper Permian Newcastle
Coal Measures in New South Wales. The fluvial and associated overbank deposits
constitute the Dewey Point Member which forms a seam split within the Borehole
Seam, listed without the split between 477.6 and 479.8 m in Fig. 7.9B. On either side
ofthe area covered by the Dewey Point Formation in Fig. 7.26, the Borehole Seam is
complete, while within the split only the relatively small portion of the seam, which
"0
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~
0
0
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0
0
0
0
0
0
0 0
0
0
0
0
0
0
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o
0
0
0
0
0
0
0
0
Q!
0
0
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0
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0
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o KILLINGWORTH 0
0
0
0
0
0
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00
0
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°
0
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0
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0
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t
,,'l:
~
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0
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1
,
4Km
,
o
~
>1
[J "0.5
-:-
I:-~ OJ 0.5-0.'
I=---j 0.'-0
o
Shaft or bore
\ .../
Outcrop of Borehole Seam
,.-J Limit of seam splitting
(Dewey Point Member)
Fig. 7.26. Distribution and sand/shale ratios of the Dewey Point Member, a fossil river deposit
within the Borehole Seam at the base of the Newcastle Coal Measures of New South Wales. The
highest sand/shale values indicate the meandering thalweg within the otherwise relatively low
sinuosity fluvial environment. (After Warbrooke 1981, Diessel and Warbrooke 1987 and Diessel
et al. 1989)
The Alluvial Valley and Upper Delta Plain
383
was time-equivalent to the intervening clastic sediments, is missing. The relatively
youthful meanders of the river dissecting the Borehole swamp are indicated by both
the shape of the narrow alluvial plain and the distribution of sand/shale ratios, the
highest values of which delineate the controlling channel within the alluvial plain.
With increasing maturity river sinuosity will increase which, according to Leopold
and Wolman (1960), will lead to the development of meanders with a curvature ratio:
(7.3)
rmfW = 2 to 3
where rm = mean radius to centre line of channel, and W = bankfull width of
channel. According to Hey (1976) the radius (r) of curvature of a mature meander is
related to channel width as:
r=2.4 W.
(7.4)
Meander wavelength (Lm) corresponds to the bankfull width of high sinuosity
channels as follows (after Leopold et al. 1964):
(7.5)
According to Carlston (1965), meander wavelength, channel width (W) and width of
the meander belt (Wm) are related to mean annual discharge (Qm) as follows:
Lm = 106 QmO. 46
(7.6)
W=7QmO. 46
(7.6)
Wm = 65.8 Qm°.47.
(7.8)
Alternative methods for the calculation of channel geometry and its use in the
estimation of channel characteristics have been provided by Schumm (1972). He
makes use ofthe parameter M which relates channel morphology to the transported
load, and is defined as:
M = [(Sc) (w) + (Sb) (2h) ]/(w - 2h).
(7.9)
Sc = %lutite in channel fill, which can be determined by point counts of thin sections,
Sb = %lutite in channel banks, which is obtained from point counts of channel wall
rocks, and h = bankfull depth of channel. By making use of the M-parameter, the
following relationships are obtained (after Woodward and Posey 1941; Barnes 1967;
Cotter 1971; Schumm 1968, 1972):
W/h = 225 M-l.0 8
(7.10)
P = 0.94 M-O. 25 , or = 3.5 W-o. 2 /h
(7.11)
Qm O.38 = WM O.37/37
(7.12)
Lm = 1890 QmO.34/Mo.74
(7.13)
Coal-Producing Sedimentary Environments
384
S = 60 M- 38 Qm- O. 32
(7.14)
(7.15)
Vm=Qm/A,
where P = sinuosity, S = slope of channel (m/m), Vm = mean current velocity (m/s),
and A = channel profile (W * h) in m 2 . According to Leeder (1973), the width of high
sinuosity channels (> 1. 7) is approximated from channel depth by the relationship:
log W = 1.54 log h + 0.83,
(7.16)
where the standard deviation for h = 0.35 log units, or from:
(7.17)
Collinson (1978a) relates bankfull channel depth (h) to meander belt width (Wm)
and meander wavelength (Lm), and obtains the following correlations:
Wm = 64.6 h1.54
(7.18)
(7.19)
While it is commonly not possible to determine the width of ancient high sinuosity
river channels from their laterally accreted deposits, channel depth can frequently be
estimated from the thickness of their point bars, in particular the vertical extent of
point bar accretion planes, as is discussed below. Another problem concerns the
estimation of sinuosity of ancient channels. Lack of suitable measurements might
prevent the application of Eq. (7.11), in which case it may be possible to use the
following alternative relationship (after Miall 1976):
..
Smuosity =
1
1 + (8/252)
2
(7.20)
The angle Theta is calculated from the 10 point moving average of weighted foreset
azimuths measured on bars in a vertical section through a channel deposit. This is
based on the assumption that the range of foreset azimuths is a reflection of changes
in channel orientation which, according to Langbein and Leopold (1966), is related
to the degree of meandering or channel sinuosity.
Naturally, the empirical nature of all the above equations, based on the study of
Recent rivers, demands caution in their application to ancient fluvial systems.
Collinson (1978b) considers these equations as approximate but probably of the
right order of magnitude, although channel abandonment, erosion, vertical stacking
of point bars and other processes may add complications to the relatively simple
relationships presented above.
The Alluvial Valley and Upper Delta Plain
385
7.2.1 The Point Bar
When in flood, meandering rivers shift their beds laterally by undercutting and
eroding the steep, concave meander banks and by depositing part of the eroded
material, together with extraformational upland sediments, on the convex, accretionary banks, called point bars, further downstream. The lateral accretion planes
separating successive flood deposits are gently inclined in bedload streams of high
width/depth ratio, but steepen when the width/depth ratios decrease in mixed and
suspended load streams. An example is shown in Fig. 7.27, which illustrates a
compound sandstone body above the Dudley Seam in the Newcastle Coal Measures
of New South Wales. The lower portion of the sandstone has been formed by lateral
point bar accretion, which is clearly visible (dipping to the right), including the
upward fining in the middle portion. The upper half of the sandstone bedding is
predominantly horizontal, presumably as a result of vertical aggradation.
Although, according to Eqs. (7.3) and (7.4), meander curvature of mature
channels is characterised by a fixed relationship to channel width, not all channels
Fig. 7.27. The cliff section at
Susan Gilmore Beach below
Shepherds Hill, Newcastel, N.S.W.,
showing the Dudley Seam (from
512.4 to 516m in Fig.7.9B) in
centre (D), overlain by lacustrine
and flood plain laminites. Above a
distinct erosion contact (arrow)
follows a fluvial sandstone with
inclined (approx. 15°) point bar
accretion planes in lower and
horizontal bedding in middle
portion
386
Coal-Producing Sedimentary Environments
are mature which leads to a wide range in rjW, which also affects the shape of point
bars. In reference to tidal creeks Barwis (1978) found the following relationship
between rjW and point bar shape:
1. Tight meanders (rjW < 2.5) produce small point bars with steep flanks. They are
fully attached to the channel banks and do not contain chutes.
2. Point bars of intermediate meanders (2.3 < rjW < 3) may be multilobed and
complex. Chutes are commonly present, as well as ripples, dunes and sand waves.
They may be partly detached from channel banks.
3. Point bars generated by gentle meanders (rjW > 3) are elongated and relatively
narrow. They contain bedforms as in (2) and are partly, occasionally completely,
detached from the inner bank.
4. Very gentle meanders (rjW> > 3) are linear, narrow and fully attached to the
bank.
The variable spacing of point bar accretion planes and the heterogeneous particle
size distribution of the intervening sediments is related to the changing strength of
the successive floods responsible for the growth of the point bars. Since the accreting
flood deposits cannot grow above high water level, the thickness of a single story
point bar deposit indicates the depth of a channel in flood and is equivalent to the
thickness of the removed overbank deposits. A size relationship exists also between
river depth and width of its meander belt [Eq. (7.18)J, which gives some indication of
the transverse extent of a point bar sequence in relation to its thickness. However, as
reported by Rust et a1. (1987), the ratio between point bar thickness and meander
belt width established by Collinson (1978a) does not apply to laterally amalgamated
channel sandstones. Fluvial deposits occurring in upper delta plain environments of
the Upper Carboniferous coal measures of eastern Kentucky and West Virginia
have been formed in 1.6- to 8-km-wide meander belts from laterally accreting rivers
with a depth ranging from 15 to 23 m (Horne and Ferm 1978). According to
Eq. (7.17), the width (W) of individual channels would have ranged between
440 and 850 m.
The transported sediments respond to systematic velocity gradients in the flow
system by a likewise systematic vertical particle size gradation from coarse to fine.
The upward decrease in particle size is matched by a likewise reduction in the size of
sedimentary structures (Walker and Cant 1979). Point bar deposits therefore display
a more regular shape and internal organisation than braided stream deposits, which
is illustrated in Fig. 7.28 and will be discussed below:
1. The Bedload Zone (A in Fig. 7.28) after Visher (1965a), or State A (conglomerate
facies) after Allen (1970) occurs at the base of the point bar sequence above a
mostly planar or only slightly irregular erosional contact with the underlying
sediments (Fig. 7.27). It consists of the coarsest particles moved by the current
close to the thalweg, and the mode of transportation is mainly by traction.
Intraformational conglomerate consisting of fragments eroded from the channel
bed or its walls, is often mixed with extraformational conglomerate of smaller
clast size. Crude horizontal bedding and particle imbrication are the most
The Alluvial Valley and Upper Delta Plain
387
Fig. 7.28. Cartoon illustrating the composition of a point bar
common features in this zone, which is rarely thicker than 1 m and many be
missing altogether.
2. The Megaripple Zone (B in Fig. 7.28) after Vis her (1965a), or State Bl (crossbedded-sandstone facies) of Allen (1970) is up to several metres thick. It is usually
composed of well-sorted, upward-fining sandstone which displays lithologically
heterogeneous point bar accretion planes, mostly as epsilon cross-stratification
after Allen (1963). An example oflarge-scale, planar point bar accretion planes is
illustrated in Fig. 7.27 above the basal erosion contact. Bedforms of the upper
part of the lower flow regime are also common, including solitary or grouped
planar and trough cross-bedding (alpha, beta, gamma, pi, and omikron types
after Allen 1963). Their azimuths, which are indicative of palaeoflow, are
commonly at variance with the laterally accreting point bar slopes or epsilon
crossbedding. Levey (1978) reports from the Upper Congaree River in South
Carolina, U.S.A. the occurrence of megaripples with tabular and trough
cross-beds in the middle portions of coarse sandy point bars. Their point bar
accretion surfaces are further cut by chute channels and bars, produced at rapid
flow conditions (McGowen and Garner 1970), while their down-stream margins
near the channel thalweg are modified by transverse bars, mostly with tabular
foresets. In multistory point bar sandstones successive units of this zone may be
telescoped into each other. Alternatively, a transitional interval may occur
between the bedload zone and the accretion planes, as has been reported by
Gibling and Rust (1987) from the upper portion .of the Upper Carboniferous
Morien Group in Nova Scotia, Canada. This interval consists of trough crossbedded, fine to coarse sheet sandstone, up to 5 m thick, which underlies the finergrained epsilon cross-bedded wedges of the laterally accretionary point bar.
Horne and Ferm (1978) refer to a similar fluvial sequence above the Upper
Carboniferous Hazard No.6 Coal near Hazard North, Kentucky, where a trough
cross-stratified unit is sandwiched between a pebbly bedload zone and overlying
point bar accretion beds.
3. The Laminated Zone (C in Fig. 7.28, see also Fig. 6.27) after Visher (1965a), State
B2 (flat-bedded sandstone facies) of Allen (1970) is a horizontally bedded unit of
fine sand or silt ranging from several centimetres to several metres. Parting
lineation is common, suggesting deposition from traction carpets in shallow
388
Coal-Producing Sedimentary Environments
water near the top of the point bar, analogous to the plane-bed condition in
shallow water decribed by Harms and Fahnestock (1965). The laminated zone is
formed when shortly after flood peak, current velocities are still high but water
levels are falling sufficiently to push the Froude Number into the transitional
region between the upper and lower flow regimes.
4. The Ripple Cross-bedded Zone (D in Fig. 7.28) after Vis her (1965a), or State B3
(cross-laminated sandstone facies) of Allen (1970) is not always developed. It is
composed of fine sandstone interlaminated with shale, and results from a
combination of traction transportation and deposition from supension. This
leads to frequent occurrences of Allen's (1963) kappa and lambda type crosslamination, whose climbing angles are shallow upstream but steepen downstream in response to an increasing amount of fine sediment dropping out of
suspension in the waning stages of a flood.
As has been pointed out by Walker(1981), there are many modifications ofthe above
succession 1 to 4 by ripples forming lower and dunes and sand waves (megaripples) higher in the sequence, and by parallel lamination, due to transitions to
upper flow regime conditions, occurring almost anywhere in the section. Vertical
stacking and telescoping of several point bars, as well as terracing, adds further
complications (Jackson 1978). Field observations suggest that relatively thin point
bar units adhere more closely to the above succession 1 to 4, whereas according to
Walker (1981) the vertical profile oflaterally accreted sand bodies exceeding 3-4 m is
often more complex.
7.2.2 The Flood Plain
Active channels occupy only a relatively small proportion of the area covered by
most alluvial plains. The largest part is taken up by inter-channel or overbank
environments which can be further divided into several subenvironments in
response to the proximity and influence exerted by nearby rivers on them. There is
no unanimity about the classification of overbank environments, in particular, the
terms flood plain and flood basin have been variously interpreted. As used here, the
flood plain occupies low lying stretches of terrain within the meander belt, which
may be submerged at flood peaks but are normally vegetated and not covered by
water except for abandoned meanders (oxbow lakes) and creeks feeding into the
main streams. The sediments accumulating in this environment are supplied by
flood waters and, adjacent to active channels, form deposits with distinctive
geometry and internal organisation and correspond to Elliott's (1969) lateral
developing clastic succession, which spread over the peat surface during floods.
They consist of laminated shales and siltstones which are deposited when the
channel has either overtopped or breached its levee banks. Tree stumps and fallen
trees are common and, when preserved, appear either petrified (Fig. 7.29), commonly by carbonate or sillica, or as flat lenses of bright coal. Ripple marks and
The Alluvial Valley and Upper Delta Plain
389
Fig. 7.29. View of the bedding plane of a silty flood plain deposit with small petrified tree trunk
below Lower Fern Valley Seam, Newcastle Coal Measures, north of Redhead, New South Wales
mud cracks, the latter indicating the drying-out that follows the inundation, are the
most common mechanical sedimentary structures found in the overbank deposits.
Chemical structures are commonly restricted to concretions, mainly in the form of
sideritic clay-ironstone nodules. Where overbank lutites have been subjected to a
marine influence, dolomite nodules may also occur. Close to the controlling rivers,
the natural levees and crevasse splay deposits form an important part of the
proximal flood plain.
Natural levees are elevated above all other topographic features in the alluvial
landscape (Rust et al. 1984), but they are so intimately associated with fluvial
sedimentation that sometimes they cannot be distinguished with certainty from the
ripple cross-bedded zone at the top of the point bar. Indeed, Belt et al. (1984) found
levee bank deposits to be the most difficult to recognise in their study of Tertiary coal
measures in the Williston Basin of North Dakota, U.S.A. The sediments which
constitute natural levees are formed as spill-over deposits during flood peaks when
the water volume conveyed through the river channels exceeds their holding
capacity. They appear therefore as stacked wedges of laminated shales and fine
sandstones which may have steep slopes towards the channel but pinch out laterally
over a distance of several metres or tens of metres, depending on the size of the
controlling river and the sediments conveyed in it. Horne and Ferm (1978) quote a
thickness of 4.6 to 9 m and a width of up to 3.2 km near active channels in upper delta
plain coal measures of eastern Kentucky compared with thinner (1.5 m) and more
narrow levees in lower delta plain environments. Although particle sizes are variable
in levee bank deposits and commonly decrease rapidly away from the controlling
channel, Rust et al. (1984) found upward-coarsening to be well represented in the
Upper Carboniferous (Pennsylvanian) Cumberland Group, South of Joggins, Nova
390
Coal-Producing Sedimentary Environments
Scotia, whereas Riegel et al. (1986) found distinctly wedge-shaped levees in the
Wealden deposits of Osterwald east of Hannover, Germany, to be upward-fining
with sideritic clays near the top. Common sedimentary structures are climbing
ripples (ripple drift), mud cracks and bioturbation in soil horizons due to root
penetration. Intraformational clasts consisting of desiccated clay peels and clay
ironstones are likewise frequently found. Slump structures occur mainly on the sides
facing the channel due to instability of the steep and water-logged slopes when the
river is in flood.
Because of their setting close to the source of river-borne nutrients, levees are
commonly richly vegetated. In the Joggins section of Nova Scotia levees carry
petrified tree trunks in growth position (Rust et al. 1984), an example of which is
illustrated in Fig. 7.30. Sandstone-filled casts of trees rooted in levee banks have
been reported from the Upper Carboniferous Clifton Group in New Brunswick,
Canada (Legun and Rust 1982). Also Horne and Ferm (1978) refer to upright
Calamites stems in Upper Carboniferous levee deposits from West Virginia, while
the palaeobotanical studies of Riegel et al. (1986) in the above-mentioned Wealden
deposits have revealed marked contrast between a rather diverse flora which
occupied the leveee banks and an almost monotypic flood plain vegetation.
Fig.7.30. Upright Sigillaria stem rooted
in levee bank silt-and sandstone of the
Cumberland Group (Upper Carboniferous) at the Joggins section, Nova Scotia
The Alluvial Valley and Upper Delta Plain
391
Crevasse splay deposits are the result of breaches of levee banks during flood
periods, followed by the spreading of sediment-laden flood waters away from the
breach in the levee bank. As reported by Duff et al. (1982), Gibling and Rust (1984)
and Rust et al. (1987), palaeocurrent directions in splay deposits are usually more or
less normal to the directions obtained from the main fluvial sandstones. Depending
on a number of influencing factors, such as the frequency of flooding, the sediment
types carried by the trunk stream, its topographic relationship to the adjacent flood
plain and other palaeogeographical considerations, breaches in levees may either be
plugged quickly or they may remain open for a considerable time span. In the first
case the resulting overbank sediment consists of single sheets of fine sand or silt
which represent discrete depositional events of relative short duration. Many of
these blanket deposits are less than a few decimeters thick. In proximal positions
they are laminated (upper flow regime) and distally they are rippled (lower flow
regime). However, the thickness and areal extent of these flood deposits is obviously
related to the size of the trunk streams, and may reach several metres in the vicinity
of the levee breaches, in which case it may be difficult to distinguish them from
channel sands, when outcrops are limited.
A guide to the identification of thick crevasse splay and/or avulsion deposits is
the occurrence of both uprooted and upright trees. Uprooted trees and other
transported vegetation are commonly found in channelised as well as in crevasse
splay deposits, but the occurrence of tree stumps in growth position requires special
circumstances. They normally do not grow in fluvial channels and, although they do
so on flood plains, they are rarely preserved unless they are entombed in a rapidly
emplaced rock, for example, by burying the forest in volcanic ash, as was discussed in
Chap. 6.3.1.1. Epiclastic sediments can have a similar effect, when deposited by a
likewise catastrophic event, such as a flood surge close to a massive breach in the
levee bank of a large river. In its course many trees might be downed, but others will
remain upright and become partly buried in the resulting deposit. For example,
poorly bedded sandy mudstones containing upright fossil trees at Roaring Creek
Mine in Indiana, U.S.A. have been interpreted as a crevasse splay by Eggert and
Phillips (1982) and Nelson et al. (1985). An example of this kind is illustrated in
Fig. 7.31 from the Wittingham Coal Measures of the Sydney Basin in New South
Wales. The illustrated tree occurs above the Whybrow Seam at Saxonvale Mine in
the Hunter Valley. This tree and others occurring in the same horizon are rooted in
50 cm of grey, slightly laminated shale. While the inner parts of the tree trunk are
partly petrified, its bark is preserved as bright coal (vi train). Macroscopically there
does not appear to be any difference between coalified bark from the stem and the
primary root branches, although their respective microscopic images are quite
different. Both bark types consist of telovitrinite, but a sample taken from the stem is
composed of telinite, whereas the respective root sample consists mainly of
telocollinite. A slight erosion contact occurs between the top of the shale horizon
and the overlying splay sand, whose thickness of 6 m coincides with the height of the
preserved part of the tree. The conically inclined beds towards the stem shown in
Fig. 7.32 mark erosion scours within the splay deposit, probably caused by the
swirling of flood waters around the tree. The upper contact of the sandstone is
abrupt and, while coalified drift wood and other vegetable matter occur sparsely
392
Coal-Producing Sedimentary Environments
Fig. 7.31. A stem of partly coalified/petrified Dadoxylon wood entombed in a 6-m-thick layer of
crevasse splay sand (light grey)
exposed on the high wall at Saxonvale Mine, New South Wales.
Its stratigraphic position is at
400 m in Fig. 7.70 above the Whybrow Seam, Wittingham Coal
Measures
throughout the sandstone, they are more concentrated near the top of the deposit.
Within the opencut, i.e. over a distance of several hundred metres, the sandstone
thins by several metres, thus suggesting proximity to the levee breach.
If breaches in levee banks remain open for years, or tens of years, the resulting
overbank deposits appear like fans or satellite deltas prograding over the adjacent
flood plain, into distal flood basins and backswamps, or, on lower delta plains, into
inter-distributary bays. Close to the crevasse the channelised sandy splay deposits
contain cross-bedding and climbing ripples above an erosional base. As illustrated
in Fig. 7.33, and discussed by Rust et al. (1984), these channels are relatively narrow
and grade laterally into splay sheets. With increasing distance from the source the
splays fan out into thin (centimetres to decimetres), but laterally extensive sheets of
fine sand to silt, which constitute the largest portion of the deposit (Fig. 7.34). In the
Joggins Section of Nova Scotia, many of the splay sheets contain plant roots near the
top, while internally grouped ripples are common, which display considerable
variation in forest azimuths between co sets (Rust et al. 1984). Also climbing ripples
occur, particularly in upward-fining sand sheets. Similar characteristics have been
observed in the alluvial plain deposits of the Upper Westphalian portion of the Ruhr
The Alluvial Valley and Upper Delta Plain
393
Fig.7.32. Lower portion of the tree
trunk illustrated in Fig. 7.31 showing the
tilt of turbulent splay bedding towards
the stem near the centre of the illustration. See hammer for scale
Coal Measures, in which a large number of mostly less than 3-m-thick currentrippled splay sands are set in flood plain laminites between major channel sands,
which are commonly more than 10m, and often more than 20m thick. Typical
examples are indicated in Fig. 7.35B between the Iduna 1 and Hagen 1 Seams.
The more distal flood plain deposits consist of alternating layers of fine sand and
silt, many of them laminated and grading from sand-laminated shales to shalelaminated sandstones. Ripple marks and mud cracks are common structures and
soil horizons with roots and thin layers of coal are widespread. Occasionally, the
continuity of the sediments is interrupted by small washouts which represent creeks
that traversed the flood plain.
7.2.3 The Flood Basin
Lar~ rivers such as the Colorado, Yukon, Missouri and Mississippi average a
lateral accretion of several tens of metres per year, whereas even in substantial floods
their associated overbank deposits barely reach a few decimetres (Visher 1965b).
394
Coal-Producing Sedimentary Environments
Fig. 7.33. Crevasse
channel
arrowed associated with splay
sands in the Cumberland Group
(Upper Carboniferous) at the
Joggins section, Nova Scotia
Fig. 7.34. View of shaly overbank deposits containing numerous splay sheets mostly composed of
fine sandstone. Cumberland Group (Upper Carboniferous) at the Joggins section, Nova Scotia
The Alluvial Valley and Upper Delta Plain
395
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Coal-Producing Sedimentary Environments
396
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The Alluvial Valley and Upper Delta Plain
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CONGLOMERATE
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Fig.7.35G
402
Coal-Producing Sedimentary Environments
Although overbank deposition is generally slow, a gradation exists in sedimentation
rates which decrease away from the trunk stream. The result is that the levee bank
and crevasse splay deposits close to the fluvial sediment source build up the
proximal flood plain faster than the more distal portions, which receive only small
amounts of sediments during major floods. The concentration of sedimentation in
the vicinity of active channels in conjunction with the constraints imposed on
meander belt width by the difficulty to erode into silted-up abandoned meanders
(clay plugs), leads to the formation of an alluvial ridge (Happ et al. 1940; Fisk 1952a;
Allen 1965; Schumm 1977), which causes the whole width of the meander belt and
flood plain to rise above the level of the sediment-starved distal flood basins
(Fig. 7.36). The instability resulting from the difference in elevation between the
raised flood plain on the alluvial ridge and the surrounding low lying parts of the
alluvial plain is periodically relieved by avulsion, i.e. by breaching the banks of the
raised channel and by shifting the meander belt to a lower position (Collinson
1987b). Flood basins are therefore preferred locations for the reconstruction of the
river system after avulsion has occurred, which is one of the main causes for the
occurrence of fluvial sandstones to occur in direct contact with underlying coal
seams.
Flood basins constitute the low lying, poorly drained areas adjacent to the
raised flood plains. Their setting is often distal with respect to channel activity, and
they are reached only by small quantities of fine outwash sediments. Flood basin
deposits are therefore often lacustrine in origin and contain much organic material
either in the form of algal deposits or as dispersed organic matter redistributed from
nearby peatlands. Ancient lakes of this kind varied in size and reached quite large
dimensions. By integrating bio- and lithostratigraphic relationships Flores and
Hanley (1984) were able to distinguish between small ( < 1 km in diameter) and large
(approx. 3 km) flood basin lakes in the Tertiary coal measures (Fort Union
Formation) ofthe northern Powder River Basin in Wyoming, U.S.A. The small, low
energy, shallow lakes were characterised by few mollusc genera which were
disseminated in marls, whereas in the larger lakes multiple mollusc habitats were
Alluv ial Ridge with Flood Plein
Flood Bas in
Not to :scele
Fig. 7.36. Cartoon of alluvial ridges separated by a flood basin
Alluvial Ridge
with Flood Plein
The Alluvial Valley and Upper Delta Plain
403
differentiated on the basis of water depth and lime content of the substrate. Flood
basin lakes of much greater dimensions have been reported from the Carboniferous
coal measures of the UK. Based on the methods of Komar (1974) and Allen (1979,
1981) of calculating wave fetch from oscillatory ripple parameters, Fielding (1984)
obtained a lateral extent of 20 km for Westphalian lakes in upper delta plain settings
of the Durham Coalfield. Thicknesses recorded for the respective lacustrine deposits
suggest a water depth of 8 m. Also the Carboniferous lakes commonly contain freshwater (algal) limestone and fossils, such as bivalves, gastropods, ostracods and fish,
all of which have been reported by Masson and Rust (1983, 1984) from mudstones in
the Sydney Mines Formation near Sydney, Nova Scotia. Other occurrences of
Carboniferous fresh-water faunas in flood basin deposits have been discussed by
Schwarzbach (1949), Jessen et al. (1952), Jessen (1956b; 1961), Paproth (1955,1962),
Fiebig (1960, 1972), Rabitz (1966a,b), Betekhtina (1970), Bless (1970), Guion (1984),
Eagar (1970, 1985), Vasey (1985), Fulton (1987) and others.
Peat-producing backswamps are part of both the flood plain and flood basin
associations, whereby flood plain peat may have a higher huminosity grade and may
be slightly more oxidised than peat formed in flood basins. The influx of clastic
Fig. 7.37. View of the cliff section at
Susan Gilmore Beach, N.S.W., similar to
Fig. 7.27 but giving a clearer differentiation between the fine-grained and dark
lacustrine (flood basin) sediments (F)
above the Dudley Seam (D) and the
coarser and lighter coloured splay deposits (S) of the flood plain below the
basal erosional contact (E) of the fluvial
sandstone
404
Coal-Producing Sedimentary Environments
detritus decreases in proportion to the distance from the nearest active channel,
which in the coal is indicated not only by a decrease in the occurrence of stone bands
but also by a decline in the disseminated ash content from proximal flood plain to
distal flood basin.
In the lower reaches of many rivers, in particular near the boundary between
upper and lower delta plains, the width of alluvial ridges and flood plains decreases
so that flood basins occupy much ofthe overbank terrain and are separated from the
controlling channels by relatively narrow levee banks only. On the basis of the
subaqueous character of Carboniferous seat earths and their frequent penetration
by Calamites roots, Teichmiiller (1955b, 1956) and Roeschmann (1962) regard such a
lim no tel rna tic flood basin setting as the most common scenario for the beginning of
coal seam formation in the Ruhr Basin.
Coal measures of the alluvial and upper delta plain associations are often
characterised by an upward gradation from coal to lacustrine flood basin sediments,
followed by flood plain sediments and, after an erosional contact, by fluvial
sandstone. As illustrated in Fig. 7.37, this order of superposition leads to a general
increase in particle size from the coal to the base ofthe fluvial sandstone, after which
grain size decreases again in accordance with the model given in Fig. 7.28. The
reason for the upward coarsening in,the overbank portion is the approaching river,
whose flood deposits bury the peat under an increasingly thick and coarse pile of
splay sediments until the river erodes into the top of the overburden and replaces it
with coarse point bar deposits.
7.2.4 The Coals of the Alluvial Valley and Upper Delta Plain
According to Weimer (1976), alluvial and upper delta plains contain three coal
producing subenvironments: (1) back-levee swamps, (2) abandoned channels, and (3)
flood basin swamps. The first two are part of the flood plain environment, in which
rivers dominate depositional patterns and thus also influence both shape and
composition of the coals associated with them (Cassyhap 1970; Galloway and
Hobday 1983). Upper Cretaceous and Eocene upper delta plain coals of Alberta,
Canada and Texas, U.S.A., respectively, are described by Nurkowski and Rahmani
(1984) and Rahmani (1984) as forming basinward dip-elongated belts between
bifuracted networks of similarly trending sandstones. Kaiser et al. (1978) find in
Eocene coal measures in Texas, U.S.A., a positive correlation between sand and
brown coal content in deltaic settings but a negative correlation in fluvial settings.
Peat is formed in backswamps on flood plains and in flood basins between
rivers, and the peatlands are separated from the river channels by levee banks, the
occasional breaching of which spreads silt and mud over the peat margins. In flood
plain coals these deposits appear as stone bands of which the Australasian Seam
displayed in the stratigraphic section of the Newcastle Coal Measures in Fig. 7.9C
(690-698 m), as well as the autosedimentational seam splitting discussed in
Chap. 6.4.2 are good examples. Lakes and ponds which interrupt the generally dense
The Alluvial Valley and Upper Delta Plain
405
vegetation cover are receptacles for both organic and inorganic detritus which may
lead to the formation of sapropelic coal surrounded by humic coal, or to shale
lenses and other kinds oflithification in the seam (want). Conversely, after siltation,
the above-mentioned lakes in the Durham Coalfield have often acted as platforms
for peat accumulation (Fielding 1984).
While the predominantly dull coals associated with braid-plain environments
indicate fluctuating water supply and occasional drying ofthe peat surface, the flood
plains and basins associated with meandering and/or anastomosing rivers carry
forested peatlands, which tend to be continuously wet and lack extended periods of
drying. Exceptions to this rule are indicated by the colour of the interseam
sediments, which changes from the usual grey to red or brown, as is the case in the
uppermost Pennsylvanian coal measures of Nova Scotia (Rust et al. 1983, 1984;
Gibling and Rust 1984, 1987). Because rates of subsidence in relation to sediment
supply are often high, the accumulating vegetable matter passes faster through the
oxidising zone (acrotelm) than in the braided stream environment. This results in a
better preservation of biomass, shown by the dominance of woody peat, which
converts into telovitrinite-rich coal. Its macroscopic expression is a predominance of
bright lithotypes, such as vitrain and elarain, or bright, banded bright and banded
coal, which together can reach in excess of 75% of all lithotypes (Diessel 1965a;
Marchioni 1980). Both tissue preservation and gelification indices are therefore
relatively higher than those of the braid-plain coals illustrated in Fig. 7.24. The
coals with the lowest GI values in Fig. 7.38 are probably flood plain coals which
show a slight increase in the proportion of telo-inertinite (fusinite and semifusinite)
at the expense of telovitrinite (telinite and telocollinite), i.e their coal facies indices
overlap with the coals of the sandy braid plains.
In Fig. 7.38 a distinction has been made between Permian upper delta and
alluvial plain coals from the Sydney Basin, N.S.W. (full circles), and Carboniferous
coals from the Ruhr Basin (open circles). Both groups of samples share the same
region within the diagram but a large portion of the Carboniferous coals is lower in
TPI and higher in GI values, with some affinity to the lower delta plain coals. The
reason for the higher TPI values in some of the Permian coals is their considerably
higher proportion of telo-inertinite, which together with their moderate telovitrinite
content more than balances the higher telovitrinite percentage in the Carboniferous
coals.
Although optimum biomass preservation in the upper delta/alluvial plain
environment is shown by the overall bright, i.e. telovitrinite-rich nature of their
coals, differences occur in the vertical grouping of lithotypes. As discussed in
Chap. 5.1, a characteristic order of superposition begins with shaly coal plus some
telo-inertinite near the base, followed by bright and banded bright lithotypes of the
vitrinite-fusinite facies and, near the top of the seam, by the dull and banded dull
lithotypes of the densosporinite facies. This frequently observed vertical succession
may be the product of a lateral shift in the location of a major river or river
system, or it may be the response to a larger-scale scenario involving the interaction
of basin subsidence and eustatic sea-level variations, i.e. basin-wide transgressions
and regressions. In the first case of autosedimentational processes, clastic sediment
supply to the shaly coal facies diminishes as the trunk stream migrates away, thus
Coal-Producing Sedimentary Environments
406
TREE DENSITY
-decreases
increases-
200.0,------------------IImno-te/matic
fe/mafic
100.0+------------------50.0-+-------------------
'"
'"OJ
E
0
10.0
III
..
."
.."
0
5.0
o
co
.."
.Q
"0
•
....
o
c
e· Q~ 0W • • •
eit-0
gco • 00
00
•
•
o
. • tP
0
1.0
•
w•
Q •••
• •
•
•
0
•
•
•
0
•
0
;;;
•
>
Q;
"'"
Il.
E
=>
E
><
OJ
:;
0.5
terrestrial
GI
TPI
0.5
1.0
1.5
2.0
2.5
Fig. 7.38. Plot of the tissue
preservation and gelification indices of whole coal
samples from upper delta
to alluvial plain environments. Full circles refer to
the Permian Newcastle
and Wittingham Coal
Measures of New South
Wales (extended after
DiesseI1986a), open circles
to the Upper Carboniferous coal measures of the
Ruhr Basin. (After analyses
by Littke 1985a; Strehlau
1988; Diessel, unpubl.)
lowering the ground level until it approximately coincides with the position of the
groundwater table. The shaly coal facies will therefore be replaced by the precursors
of the vitrinite-fusinite facies of Strehlau (1988, 1990) which, given a sustained
balance between basin subsidence and peat accumulation, will form the thickest part
of the seam, although in many coals it is restricted to its lower portion (Smith 1962;
Hacquebard et al.1967; Hacquebard and Donaldson 1969; Hacquebard and Barss
1970; Smyth and Cook 1976; Cameron 1978; Marchioni 1976, 1980; Hower and
Wild 1982; and others). The vitrinite-fusinite facies is equivalent to Smith's (1962,
1968) Lycospore phase. While this facies is only seldom reached by distal crevasse
splays, maximum biomass production is possible because the swamp is still supplied
with nutrients carried in solution by flood waters which periodically inundate the
peat.
Further away from the fluvial sediment source, the supply of dissolved nutrients
diminishes and may even cease temporarily, which may result in oligotrophic
limnotelmatic conditions, a reduction in total peat production, and an increase in
the proportion of hypautochthonous peat components. Fluctuating water tables
and relocation of maceral precursors may cause oxidation and inertinite generation,
as has been observed by Corvinus and Cohen (1984) in the upper peat strata of the
Okefenokee Swamp. The alternative model given in Chap. 5.1 envisages the
termination of eutrophic conditions to be due to the raising of the bog surface above
the groundwater table and the beginning of ombrotrophic peat formation, which
has also been associated with the formation of inertinite-rich dull coal, for example
by Smith (1962, 1964), Littke (1985a, b, 1987), Esterle and Ferm (1986), and Fulton
(1987). However, as mentioned above, Esterle et al. (1989) have questioned the
suitability of the ombrotrophic model to explain the "dulling up" of many coal
The Alluvial Valley and Upper Delta Plain
407
seams. Based on their petrographic investigations of domed peat deposits in
Sarawak, Malaysia, they suggest that modern tropical raised bogs are more likely to
produce overall bright coals consisting of a detrovitrinite matrix which incorporates
bands of telovitrinite.
If sediment supply is small and meander belts are widely spaced, extensive flood
basins situated between relatively small alluvial ridges will provide suitable
conditions for both topogenous and ombrotrophic peat formation. In either case the
ash content of the resulting coal will be low, but a topogenous setting would be
indicated by widespread transitions into sapropelic coal, whereas ponding on raised
bog surfaces can produce relatively small pockets of sapropelic coal only.
A high rate of sediment supply and a closely spaced drainage pattern will leave
little room between broad alluvial ridges for flood basins to develop. Meander belts
are therefore connected by flood plains in which the groundwater table rises above
the ground surface only during floods and in which shaly coal and the vitrinitefusinite facies are dominant. In the stratigraphic column this development is
paralleled by an increase in both frequency and thickness of fluvial sandstones, as is
the case in the Westphalian Band C of the Ruhr Coal Measures illustrated in
Fig.7.3SA-C. In palaeogeographic terms this trend suggests a shift of the
depositional site away from a coastal towards a more inland setting (Strehlau 1988;
David 1989), i.e. from the marine-influenced delta plain to the more terrestrial
alluvial valley environment.
In addition to the effects on coal facies of autosedimentational mechanisms,
seam development can also be influenced by more global events such as marine
transgressions and regressions generated by eustatic sea-level variations. The
upward trend from relatively dry to relatively wet facies types would be indicative
of a marine transgression, even though the sea need not reach the swamp because
the concomitant rise of the groundwater table is sufficient to bring about the
necessary shift in peat-forming environments. A carefully controlled study of the
effects of a rising water table on coal composition has been conducted on the Lower
Kittanning and Clarion coals in Clearfield County, Pennsylvania, by Reidenouer
et al. (1979). Both coals overlie disconformably an' undulating floor topography
which affected peat formation such that basement lows contain more than twice the
coal thickness than basement highs. In the Lower Clarion coal at Krebs mine
Reidenouer et al. (1979) distinguished eight petrographic zones in the thick seam
section formed in a basement low. On approaching the adjacent topographic high
the lower six petrographic zones wedged out in upward succession, leaving only the
upper two zones to comprise the reduced seam above the basement high.
Reidenouer et al. (1979) conclude that peat accumulation began in areas now
represented by basement lows, and that with time, it was extended successively to
topographically higher parts of the terrain. Before on- and overlapping the flank of
the basement high the petrographic zones, numbered 1 to 8 from top to bottom,
changed laterally from basement low (left column) to basement high (right column)
in the following manner:
1. Clarain > no change
2. Claystone> bone coal ( = shaly coal) > durain
408
Coal-Producing Sedimentary Environments
3. Clarain > no change
4. Durain > clarain
5. Claystone> durain
6. Clarain > no change
7. Claystone> no change
7. Clarain > no change
The above comparison shows that the changes which occurred in petrographic
zones (2), (4) and (5) were directed from lithotypes formed in basement lows under
relatively wet conditions to comparatively drier lithotypes near basement highs.
Reidenoutlr et al. (1979) also included inorganic coal constituents in their study,
which demonstrate that pyritic sulphur was highest in durains which laterally or
vertically grade into claystone. The latter does not possess a particularly high
sulphur content because it is the product of open, wave-agitated and oxygenated
water, in which the anaerobic sulphate-reducing bacteria cannot function. Near the
basement highs, where plant growth and allochthonous sedimentation of liptinitic
and detro-inertinitic components mixed with the outer clay facies protected
limnotelmatic shorelines, a low redox potential, anaerobic conditions and
sufficient iron supply favoured bacterial pyrite formation. Further shoreward, toxic
conditions due to decreasing water circulation in the peat lowered bacterial activity
again resulting in reduced pyrite (and sulphur in general) contents in the coal, but it
improved tissue preservation in the form of higher clarain contents. This and other
topics related to the effects of systematic variations in groundwater levels on coals
and coal measure sedimentation will be explored further in Chap. 8 in the context of
the interaction between basin subsidence and eustatic sea-level changes.
Vertical profiles showing a systematic trend from predominantly bright
lithotypes in the lower and middle portion of the seam section to mainly dull
lithotypes near the top of the coal are common not only to 75% of Carboniferous
coal seams in the Ruhr Basin (Hoffmann 1933), but they are frequently observed in
coals of other ages as well. Studies by Smyth 1967, 1970 and Smyth and Cook 1976)
have shown that some Australian Permian and Triassic coal seams follow a similar
pattern of vertical succession in their composition beginning with vitrinite-rich coal
at the base and grading upward into vitrinite-poor coal. Other coal seams appear to
possess a more random distribution oflithotypes. An example is the Liddell Seam in
the Foybrook Formation of the Wittingham Coal Measures in New South Wales,
which has been interpreted by several authors to have been formed in an alluvial
plain environment (Britten 1972; Herbert 1976; Marchioni 1976, 1980). By applying
Gingerich's (1969) method of Markov chain analysis to the Liddell Seam, Marchioni
(1980) found three types of significantly non-random upward (left to· right)
transitions:
1. Shale> coaly shale> shaly coal> dull coal
2. Banded bright> banded dull coal
3. Bright coal> banded dull coal.
Transition (1) is virtually identical to the landward transition from claystone into
bone coal (= shaly coal) into durain of Reidenouer et al. (1979), but Marchioni's
The Lower Delta Plain
409
(1980) preferred transitions do not combine to lead to any systematic trend in the
gross seam profile. Also Smyth's (1972) Markov chain analyses of microlithotype
successions in the Liddell and associated coal seams revealed a preference for
oscillatory transitions between vitrite- and clarite-dominated coal plies without any
significant trend.
7.3 The Lower Delta Plain
Upper and lower delta plains form the planar capping of the delta complex which
houses a wide range of subenvironments, including point bar and fluvial channel,
flood plain and basin, paludal and lacustrine on the upper delta plain; fluvial and
intertidal channel, lagoonal, bay, estuarine and paludal on the lower delta plain; and
beach, bar, subtidal channel, shoreface and others on the delta front. Because of the
hybrid nature of the active delta between fluvial and regressive marine processes
(Visher 1965a), few of the mentioned subenvironments are specific to the delta
setting but are common also to other related environments, which makes a clear
distinction between them difficult. The problem of separating delta from purely
fluvial environments has been mentioned in the discussion of the alluvial
valley/upper delta plain association, and it is obvious that such physiographic
features as lagoons, bays, bars, tidal flats etc. present problems when trying to
distinguish them from the back barrier/strand plain and estuarine settings discussed
in Chaps. 7.4 and 7.5 respectively. For example, the physical attributes of the
sediments deposited as distributary mouth and distal bars on the delta front are
quite similar to those constituting the seaward-migrating shoreface of a barrier
beach, but the larger sediment supply and thus thicker pile of detritus and the
association with fluvial sediments distinguishes the delta from the barrier beach and
adjacent lagoons or back barrier swamps. Even more diagnostic is the shape of the
shoreline and nearshore sediments. Given a reasonable number of suitable
observation points (outcrops, boreholes, electric logs etc.), it should not be too
difficult to identify the presence or absence of protruding delta lobes by the construction of isopach (Walker 1981), sand/shale and net sandstone (Hamilton 1985)
maps. Also the identification of deltaic subenvironments, for example, by the shape
of the framework sandstones of the delta complex (Kaiser et al. 1978; Galloway and
Hobday 1983) can assist in proving the presence of an ancient delta. A palaeogeographic reconstruction of the distribution of coal-bearing Tertiary (Fort Union
Formation) deltas in the Powder River Basin of Wyoming and Montana, U.S.A., has
been presented by Ayers and Kaiser (1984). Based on outcrops and approximately
1400 geophysical logs in the Tongue River Member, they produced percentage and
net thickness maps of the main coals and sandstone litho somes, which reveal the
Palaeocene delta architecture in the Powder River Basin. Because of the diagnostic
importance ofthe sediments associated with the delta complex the discussion of the
lower delta plain and its coals is preceded by a brief overview of adjacent
environments. They are illustrated in Fig. 7.39.
In contrast to the upper delta plain, whose depositional environments are
dominated by the rivers which supply the sediments that build the delta complex, the
410
Coal-Producing Sedimentary Environments
Fig. 7.39. Cartoon illustrating the size relationship between some of the subenvironments of the
delta complex
lower delta plain may be less affected by fluvial action but owes much of its
sedimentation pattern to the physical characteristics of the basin into which it
progrades. A corollary of the waning fluvial influence is the change from high to low
sand/shale ratios in the transition from upper to lower delta plains (Horne and Ferm
1978), but the fluvial influence may still remain dominant if the delta pro grades into
a large lake or inland sea devoid of tides and strong wave action. An example is the
above-mentioned Powder River Basin, in which the distributary sandstones of the
Palaeocene Tongue River Member have retained a strongly dendritic birdfoot delta
pattern (Ayers and Kaiser 1984). According to Elliot (1978), the characteristics of the
receiving basin which affect delta development include water salinity, basin shape,
size, depth, energy regime, sea-level fluctuations, subsidence and tectonic activity.
Several of these features are interdependent, for example, tide-dominated delta
fronts and lower delta plains occur preferentially in embayments which are open to
the ocean. Such settings are affected by particularly high tidal ranges but they cause
wave diffraction and often lack longshore currents, which means that wavedominated deltas with beach-barrier shorelines protrude more frequently from
straight coastlines.
7.3.1 The Prodelta
Because prodelta sediments are deposited in the most distal parts of the delta
complex, they occupy the basal portion in a stratigraphic column through a
prograding delta sequence. When formed in a marine setting, the prodelta is
transitional to the shelf environment and, according to Coleman and Gagliano
(1965a, b), sediments of the two can be distinguished only when the associated delta
The Lower Delta Plain
411
complex is known either in vertical section or plan view. The distinguishing feature is
the larger proportion of silt and clay the prodelta receives from the rivers feeding
sediments into the adjacent delta but this can only be established in comparison with
the sedimentation rates away from the delta influence.
Up-slope microcross-Iamination and current ripples are common, and the rock
types consist mainly of sand-laminated siltstone and shales (proximal prodelta).
Down-slope laminae become thinner and particle sizes decrease. Silt-laminated
clayshales prevail and, in many places, the lutites display only colour lamination
(distal prodelta). Shell remains and dispersed organic matter (DOM) may be
common. Bioturbation is likewise widespread but because oJ the higher sedimentation rate it is somewhat diluted compared with ordinary shelf sediments.
7.3.2 The Delta Front
Compared with the prodelta, both sedimentation rates and slope angles are higher
on the delta front, which begins with shale at its base and may terminate with clean
sand and gravel at the top. An example of upward-coarsening delta front sediments
in illustrated in Fig. 7.40. The delta front begins seaward with distal bar deposits
consisting of alternating sand-laminated shales and shale-laminated fine sandstones, often displaying flaser bedding. According to Coleman and Gagliano
(1965a, b), dilution of sea water by fresh water along with nutrient-laden currents
make this a favourable environment for burrowing organisms. Current ripples,
scour-and-fill structures and erosional truncations reflect the closer proximity to
high energy conditions and the stronger currents the delta front is subjected to, while
the increased slope angles may lead to soft sediment deformation. Periodical floods
are indicated by an increase in both particle size and the thickness of frontal splay
deposits.
Up-slope the distal bar gives way to the distributary mouth bars which are
formed where river waters leave the confines of the distributary channels and dump
most of their load as they lose velocity on entering the sea. The high sedimentation
rate leads to shoaling and subaerial exposure of the mouth bar at low tide, and the
combined effects of fluvial, tidal and longshore currents, as well as wind and wave
action, remove and redistribute many of the fine particle fractions. The result is that
the distributary mouth bar contains the coarsest and most mature sediments of the
delta front. Structures commonly associated with distributary mouth bars are
current and wave ripples, various types of cross-bedding, fluid escape and gas heave
structures, the latter resulting from gas released by rotting vegetation which has
been buried rapidly during floods. Autochthonous fossils consist of burrowers or
thick-shelled benthos situated between occasional layers of allochthonous plant
material.
While laterally distributary mouth bar sands grade into interdistributary bay
shales and laminites, their lower contacts are gradational or abrupt with the distal
bar deposits of the lower delta front. The coarse-grained upper portion of
412
Coal-Producing Sedimentary Environments
Fig. 7.40. Upward-coarsening
delta front sediments, from darkcoloured proximal prodelta at
bottom ofsection to light-coloured
clean distributary mouth bar
sands near right hand top in
Cretaceous coal measures at
Drumheller, Alberta, Canada
distributary mouth bars may be overlain by distributary channels and their levee
deposits or by interdistributary bay sediments and/or coal. As illustrated in
Fig. 7.39, the area covered by the different subenvironments of a prograding delta
display a systematic variation in size within the complex. This is an important
consideration in the exploration of a delta sequence because it means that the
likelihood of drilling a distal bar in a bore is considerably greater than intersecting a
distributary mouth bar, even when several distributary mouth bars have coalesced
into a delta front sheet sand (depicted in Fig. 7.39), as is typical for river-dominated,
lobate deltas (Walker 1981). Even smaller in areal distribution are individual
distributary channels.
The coarseness, thickness, and areal extent of distributary mouth bars vary over ,
a wide margin, depending upon the rate of sediment supply, tectonic setting and the
wave and current conditions of the coast (Saxena 1979). In rapidly subsiding areas,
distributary mouth bars are thick and of limited lateral range, whereas in more
stable regions the deposits are thinner and more widespread. In either case the
mouth bar sands have a flat conical shape, being widest at their base (Horne 1979).
The distributary mouth bars described by Baganz et al. (1975) from the Upper
The Lower Delta Plain
413
Carboniferous (Pennsylvanian) coal measures of eastern Kentucky are between 1.5
and 5 km wide and range in thickness between 15 and 25 m.
Also the sediments overlying the distributary mouth bars are affected by
tectonic controls. Where subsidence is rapid, the rate of sedimentation high, and
wave energy low, distributaries bifurcate around their own mouth bars. One of the
bypass channels becomes eventually abandoned while the preferred one forms
another mouth bar further seaward. At the same time, the old bar is starved of
sediments and becomes vegetated, or is flooded to become the floor of an
interdistributary bay. In the case of slow subsidence, the distributaries will rework
their mouth bars, which results in the formation of strong erosional contacts
between the distributary mouth bars and the overlying channel sands (Saxena 1979).
7.3.3 The Distributary Channel
The distributary channels of the lower delta plain are commonly isolated, straight,
and often filled with fine-grained material, although the composition of the channel
fill may vary in response to the proximity and strength of the sediment source. An
impression of the variety of sediments conveyed through distributary channels can
be obtained from a comparison between the silts and fine sands of the Mississippi
delta and the coarse sands and gravels of the Rhone delta. In rapidly prograding
deltas, the distributary channels of the lower delta plain are short-lived and
frequently undergo avulsion. Often they carry finer-grained sediments than the
trunk streams further up-slope, which leads to a lowering oftheir width-depth ratios,
for instance, from 1000 for the alluvial channels of the upper to around 50 for the
distributary channels of the lower Rhine and Rhone delta plains (Oomkens 1974).
Distributary channels ("Type III sandstones") mapped by Marley et al. (1979) in
lower delta plain environments of the Upper Cretaceous Blackhawk Formation of
Utah, U.S.A., average 7.3 m in thickness and 92 m in width, which gives them a
width/depth ratio of 12.6.
The shape of the distributaries varies also in relation to the sinuosity of the
channels. Under conditions of low wave action and small tidal range, distributary
channels are shallow and exhibit a branching pattern (Coleman and Wright 1975).
Such conditions are though to have existed in the coal-bearing Ecca Group of the
Karroo Basin in southern Africa (Hobday 1987), where closely spaced, bifurcating
and mutually truncating distributaries, between 8 and 60 m wide and up to 4 m deep,
overlie mouth bar sandstones and bay-fill deposits. The sandstone-filled channels
contain both solitary and grouped trough cross-beds. In the river-dominated lower
delta plain of the basal Tomago Coal Measures of New South Wales (Fig. 7.9A, B)
the mostly straight channels display lenticular outlines in transverse cross-section.
They are set in upward-coarsening interdistributary bay sediments with which they
may form gradational contacts, although more often they are separated from them
by irregular basal erosion surfaces, as illustrated in Fig. 7.41. The upper contacts
between distributary channel sands and overlying interdistributary bay deposits are
414
Coal-Producing Sedimentary Environments
Fig.7.41. Cross-sectional photograph of the base of the distributary channel illustrated in the
Tomago Coal Measures west of Buchanan Tunnel, near Newcastle, N.S.W., between 175.5 and
181.5 m in Fig. 7.9A. Note the irregular basal erosion above alternating interdistributary bay,
marsh and crevasse splay deposits
also frequently abrupt, which results from the sudden cessation of sediment supply
when active channels are abandoned due to river avulsion. Abandoned channels
which were not filled during their active period become receptacles of fine mud and
organic detritus settling out of suspension (Saxena and Ferm 1976). Once the water
level is low enough for plants to grow, peat formation may conclude the filling
process.
Commensurate with their coarse nature, the channel sands in the Tomago Coal
Measures of New South Wales are usually not thicker than 3 to 6 m but their
proportion in the coal measures is high and, in some localities, they show en echelon
arrangement, probably due to frequent channel switching. Solitary and grouped sets
of medium to large-scale cross-bedding are common in the lower portions of the
channels, whereas near upper limits numerous tree trunks occur which are well
aligned parallel to the channel axes, as, for example, indicated in the upper portion of
a distributary channel at 171.6 m in Fig. 7.9A.
Because of the thin levee banks separating distributary channels from the
adjacent interdistributary bays, meandering of the channels draining the lower delta
plain is not widespread. If it occurs, it indicates absence of a strong proximal
sediment source and appears to be part of a tide-dominated delta complex. The
resulting point bars are generally finer-grained than those of the upper delta plain,
and their sands and silts, formed by traction transportation, are interbedded with
mud drapes deposited by suspension settling during slack periods on point bar
accretion planes, dunes (megaripples), sand waves and other bedforms. Depending
on whether the channel is set in the intertidal or subtidal zone, single or double mud
drapes (tidal bundles) occur as illustrated in Figs. 7.42 and 7.43. According to Visser
The Lower Delta Plain
415
Fig. 7.42. Single and double mud drapes deposited on foreset accretion surfaces in tide-affected
fluvial sandstone during slack water at the turn of high tide. Above Gottessegen Seam (Namurian
C) at Steinbruch Rauen, southern Ruhr Valley, Germany. For stratigraphic position see Fig. 7.35G
Fig. 7.43. Tidal bundles on point bar
accretion planes in a tide dominated
channel in the Cretaceous Bear Paw
Formation near Drumheller, Alberta,
Canada
416
Coal-Producing Sedimentary Environments
(1980) tidal bunbdles are produced in four steps:
1. A foreset or accretion surface is deposited by the dominant current which, in an
estuary often coincides with the ebb-current because of its augmentation by river
flow.
2. During the subsequent slack water stage a thin mud layer is draped over the
foreset or accretion surfaces.
3. The subordinate reverse current which in an estuary often coincides with the
flood-tidal current, covers the first mud drape with a thin layer of sediment
similar to but finer than the foreset bed formed by the dominant current. Partial
erosion of the foreset and its mud drape may form a reactivation surface.
4. During the second slack water period another mud layer is formed which covers
both the fine sand or silt and the reactivation surface, which makes it larger than
the underlying first mud drape.
Another characteristic of tidal bundles is their grouping into distinct sets. Such
segregation may be in response to neap and spring-tidal cycles which separate the
bundles into sets of 14 (diurnal) or 28 (semidiurnal), respectively (Visser 1980;
Boersma and Terwindt 1981). Other variations in spacing between sets of tidal
bundles may be related to fluctuating sediment supply.
The internal organisation of tidally influenced meandering channels is similar to
that of alluvial channels. Shell fragments can be found as part of the bedload, as has
been the case in the tidal channel indicated between 1400 and 1410 m in Fig. 7.44
above the Katharina Seam in the Ruhr Basin. Dip directions offoreset beds in tideaffected channels are often bimodal in response to the changing directions (Oomkens
and Terwindt 1960; de Raafand Boersma 1971; Terwindt 1971, 1981; van den Berg
1981), which results in herringbone cross-bedding as illustrated in Fig. 7.45.
However, flow separation can result in some channels or parts of a channel being
preferred by the flood- and others by the ebb-tide thus giving the impression of a
seemingly unimodal flow direction in a restricted outcrop. An additional feature of
tidally influenced distributaries is the occurrence close to channel mouths of linear
sand ridges which probably are caused by alluviation as flood tides interact with
river currents (Elliott 1978).
7.3.4 The Interdistributary Bay
Bay deposits consist mainly of shales which are rich in organic matter and often
highly bioturbated. An example of bioturbation caused by burrowing worms is
shown in Fig. 7.46. A frequently observed upward increase in particle size is caused
by an increasing emplacement of splay deposits as a distributary channel progrades
into the bay, which is a mechanism not unlike the upward coarsening of fluvial
overbank deposits discussed above. In river-dominated deltas, the main source of
interdistributary bay sediments is excess discharge during flood periods when
sediment-laden water is diverted from the channels into the interdistributary bays,
The Lower Delta Plain
417
Prodelta
136o
Peatland on
Flood Plain
12
~
-v-
Distributary
r....-:-"":'-":~......~ Gs
St
Channel
.....,.....-.......--:"'0<,.<:..1<:;:,
FI
-v0
LAURA 1
Peatland
InterDistributary
Bays with 137 o
Splays
LAURA 2
Peatland on
Flood Plain
VICT~ 1jO
""\I"
0
Inter-
d
0
Distributary
139o
Bay with
FI
-v-
Crevasse
140
o~~t:"
Inter-Tidal
Channel
InterDistributary
Bay
141 o
Root Horizon 0 n
Crevasse Spla y
Interdistr. Bay
Distributary142 o
Mouth Bar
Stacked
Distal Bars
and Frontal
Splays
143
Proximal
Prodelta
-- -
°mm
0
/Sh I'"
Sf -v"V"'
:
0FI
-vSp,s 0
FI
Fm
-v-
~
0
s;- F>
FI
-v-
0
0
Sf --u-
--FT;r
./SS
jO
"I'"
R.r .."..jO
sr
Fm
Peat land
f"
.QL
St
FI
Stacked Spla
and InterDistributary
Peatlands 13
Peatland
InterDistributary
Bay
13
Flood Plain
Peatland
Distributary
Mouth Bar
Stacked
13 20
Distal Bars
and Frontal
Splays
,..
0
12 80
Peatland and
Flood Plain 12 90
f'
"""IT"
Sr
-v-
FI
cop
Sr
-v-
jO
-v-
I'"
Fl,r
10
Sr
-v-
-v-
6
-v-
SubProdelta
Fm
13 40
Distributary
Mouth Bar
13 50
Distal Bar
KATHARINAHERMAN-GUSTAV
jO
ZOLLVEREIN 7/8
13 30
0
134 o
145 o
SubProdelta
"V""
Fl,r
-v-
St
Distributary
Mouth Bar
jO
-.r
Distal
Prodelta
12 10
..EL
jO
~
138o
Splays
ZOLL VEREIN 6 UBK
6
Distal Bar
.::5
.::5
Sl,r
6
-v6
SI
6
jO
-v-
Fig. 7.44. Part of the Ruhr section of Fig. 7.35C and D with palaeo-environmental interpretation
of the marine to tidal interval above the Katharina Seam. The scale is in IO-m intervals numbered
as measured from top (upper right) to bottom (lower left). For legend see Fig. 7.35G
418
Coal-Producing Sedimentary Environments
Fig. 7.45. Herringbone cross-stratification in a tide-affected channel in the Cretaceous Bear Paw
Formation near Drumheller, Alberta, Canada. See hand-brush in upper right corner for scale
Fig. 7.46. Bedding plane showing bioturbation in interdistributary bay sediments of the Tomago
Coal Measures, west of Buchanan Tunnel, near Newcastle, N.S.W.
flood basins, marshes and swamps. The processes by which sediment is transferred
from the distributaries to the bay are the same as those operating on alluvial and
upper delta plains, namely, according to Dawson et al. (1989), overbank flooding,
crevassing and avulsion. In tide-dominated deltas sediments may also be supplied
by tidal currents. At high tide, they enter the distributary channels and spill over into
the interdistributary bays (Elliott 1978). Except for their setting, frequent bioturbation by worm burrows, and the occurrence of calcite and siderite concretions, the
interdistributary bay sediments are not fundamentally different from the flood basin
The Lower Delta Plain
419
sediments ofthe upper delta plain and like them, they may grade laterally into flood
plain deposits. Poorly sorted clay, silt and fine sand are common size fractions, and
are mostly interlaminated, although stratification is often disturbed by bioturbation.
The thickness of interdistributary bay deposits varies between decimetres and
several tens of metres. Baganz eta!. (1975) quote 15 to 55m as a usual thickness
range for these deposits in eastern Kentucky, where they extend laterally from 8 to
110 km. In common with similar deposits elsewhere, these sediments display an
upward gradation from fine to coarse which, according to Horne (1979), begins with
dark grey to black clay shales, occasionally with le nticular limestone and siderite
nodules, and finishes with silty or fine sandstone. In the coarser upper portion of the
bay fill oscillation and current ripples are common sedimentary structures with the
addition of medium and large scale cross-bedding in current-affected sediments.
A common feature, particularly of river-dominated deltas, is the high proportion in the interdistributary bay deposits of crevasse splay sands. According to
Horne et a!. (l979a), crevasse splay deposits fan out from the breached levee banks
into the bay with the coarsest and thickest sediments being deposited close to the
channel above a basal erosional contact. More distally, both sediment thickness and
particle size decrease rapidly, the basal contact becomes non-erosive, and the shape
of the deposit changes from a proximal wedge to a distal blanket of fine sand, only a
few centimetres or decimetres thick, like the thin sandstone bands in the
interdistributary bay sediments underneath the distributary sandstone of Fig. 7.4l.
The larger area covered by distal crevasse splays results in their more frequent
intersection in bores and outcrops. For example, Marley et a!. (l979) report from
their study of the Upper Cretaceous Blackhawk Formation near Emery in Utah,
U.S.A., II occurrences of crevasse channels ("Type II sandstone") averaging 2.1 m in
thickness, in contrast to 72 tabular splay deposits ("Type I sandstone") with a mean
thickness of 0.7 m.
The frequency of crevassing increases from the upper to the lower delta plain
because of decreasing depositional stability and because the higher and wider levee
banks flanking the channels in the upper delta plain are not as easily breached as the
lower and more narrow levees separating the distributaries of the lower delta plain
from the surrounding bays and marshes. If breaching of a levee bank occurs during a
major flood on the upper delta plain or in an alluvial plain setting (e.g. alluvial
ridge), the opening is usually plugged when the waters recede. In more distal settings
breaches are kept open much longer thus giving rise to the formation of subdeltas
adjacent to the main channel (Coleman and Gagliano I965b). During their
constructive phase, subdelta lobes push into the interdistributary bays in a similar
manner as the whose delta complex pro grades into the main basin. The original
upward- coarsening pattern ofthe delta front is therefore repeated on a smaller scale
(2 to 10m according to Elliott 1974) as often as interdistributary bays are filled by
the products of prograding subdeltas. An ancient example of a large crevasse splay
which prograded into an open bay above the Middle Kittanning coal has been
described by Cavaroc and Saxena (l979) from northwestern Pennsylvania and and
northeastern Ohio. The splay deposits consist of four facies:
9
I. Major channels broadening downward into distributary mouth bars which in
vertical profile appear as crevasse delta/channel couplets.
420
Coal-Producing Sedimentary Environments
2. A set of minor channels which extend from the major channels and occupy a
peripheral belt.
3. Interchannel silt cones occurring as overbank deposits on the flanks of the major
channels.
4. Chernier-barrier deposits consisting of reworked splay sediments at the distal
edge of the splay system.
E.xamples of mouth bar/crevasse channel couplets similar to those listed under (1)
are indicated in the stratigraphic column of Fig. 7.9A between 125 and 164 m, while
an outcrop of part of a subdelta from a similar stratigraphic level but situated outside
the section line of Fig. 7.9A is illustrated in Fig. 7.47. An extended vertical profile of
the interval is shown in Fig. 7.48, which has been interpreted as follows (after Diessel
et al. 1985):
Stage I. At the base of the section occurs a quartz- and feldspar-bearing sandstone
which is of medium grain size and contains both tabular and trough cross-bedding.
It has been interpreted as the upper portion of a distributary channel.
Stage I I. The delta distributary phase gives way to overbank deposits in an
interdistributary setting. Mudstone and coal horizons alternate, thus indicating a
peatland environment which produced the rather dirty and pyrite-rich Lower
Rathluba Seam. Rootlets and other plants remains, including occasional tree
stumps are common in the mud- and siltstones.
Fig. 7.47. Photograph of a minor mouth bar (A)jcrevasse channel (8) couplet between the Upper
(C) and Lower Rathluba Seam (covered by rubble at bottom of photograph) in the Wallis Creek
Formation of the Tomago Coal Measures at Thornton Brick Pit, near Newcastle. N.S.W. See
person (encircled) for scale
The Lower Delta Plain
80
~
6
421
M.tr.s
. . ..
30
•••
~
2130
••
I
Delta Distributary Phase
IX
Pea \land Phase
VIII
I
GAP
Delta Distributary
20
VII
~
Phase
2140
Interdis trib u tary
Peatland Phase
VI
Delta Distributary
V
~
Phase
214~ 26 -F--,-,7,'-,---.r,'-.':".+. .:=?'
0
10
~
197 0
Distributary
Mouth Bar
Distal Bar
Pro(Crevasse Splay) Delta
>-
Q)
en
, :;:; r-'"
a.
c :e
~
Q)
~
en
0
>-
IV
'"
CD
5=~=~-!_!l=!!=!r:::::::==:::::~T~r:a~n:s~g~re:s:s:io~n::p~h~as~e~=======jlll
Rathluba
Seam
Interdistributary
Pea !land Phase
~
26~
II
Delta Distributary Phase
Fig,7,48, Measured section of the minor mouth bar/crevasse channel couplet between the Upper
and Lower Rathluba Seam illustrated in Fig. 7.47. Thefigures next to the arrows to left of the section
indicate mean azimuth of cross-bedding, the encircledfigures refer to the number of readings. (After
Diessel et al. 1985)
Stage I I I. A transgressive phase begins with a thin shale band which overlies the
lower portion of the Rathlula Seam.
Stage IV. After the drowning of the Rathluba peat, subaqueous deposition
continued in a sheltered interdistributary bay environment (A in Fig. 7.47). The
lower 2 m of sediments consist of laminated shales and siltstones rich in worm
422
Coal-Producing Sedimentary Environments
burrows and micro cross-lamination. The reference to a prodelta environment in
Fig. 7.48 is not meant to infer a setting in front of the main delta complex, but on the
frontal slope of a subdelta prograding into a larger interdistributary bay. This finegrained sequence is interrupted by a 0.5-0.8-m-thick medium to coarse proximal
crevasse splay sandstone with an irregular basal erosional contact. This sandstone
sheet is overlain by more bioturbated laminites, whose particle size increases
upward to form a clean, horizontally bedded sandstone representing a distributary
mouth bar. In its lower portion landward directed (mean azimuth = 26°) mediumsized foreset beds appear to have been formed by wave-driven flat megaripples.
Stage V. The distributary mouth bar is deeply but irregularly eroded by a trough
cross-bedded splay channel sandstone up to 3 m in thickness (B in Fig. 7.47). It is
approximately 15 m wide and contains both at its base and within lower channel
walls, laminite intraclasts up to 2.5 m in length and 30 cm thick. They are the result of
bank collapse and are followed initially by very coarse, then upward-fining
sandstone. The southward directed foreset azimuths are consistent with the
northern basin margin, but they are opposite to those of the wave-driven bedforms
in the underlying mouth bar. The highly irregular and erosional basal contact,
and, in particular, the presence of the large, locally derived intraclasts suggest
that the channel is the product of a sudden high energy event, such as the rapid
emplacement of a crevasse channel following breaching of the levee bank of a trunk
stream. The intraclasts consist of sand-laminated shale and shale-laminated
sandstone, which may have been derived from the undercutting and collapse oflevee
banks in a similar manner as suggested by Gibling and Rust (1984) for the large
mudstone intraclasts occurring near the floor and margins of Upper Carboniferous
braided channels in the Morien Group near Sydney, Nova Scotia.
Stage VI. Overbank siltstone is followed by the upper split of the Rathluba Seam
illustrated in Fig. 7.47 (C) in the upper portion of the outcrop. Numerous plant roots
extend from the coal into the silty floor.
Stage VII. The upper split of the Rathluba Seam is overlain by a 6-m-thick channel
sandstone which is coarse at its base but becomes fine- grained and laminated in its
upper portion. Trough cross-bedding with basin ward directed foreset azimuths is
common.
Stage VIIl. Following a stratigraphic gap of a approximately 5 m a third split of the
Rathluba Seam occurs below another channel sandstone. In Fig. 7.48 they have
been referred to as Stages VIII and IX, respectively.
In contrast to the above examples of a mouth bar/channel couplet there are also
cases where the minor mouth bar is not capped by a crevasse channel, as, for
instance, illustrated above the Katharina-Hermann-Gustav Seam in Fig. 7.35C and
D, or at 1420m in Fig. 7.45. There may be two reasons for the absence of a crevasse
channel:
The Lower Delta Plain
423
Fig.7.49. Starved ripples in tide-affected interdistributary laminite from the Tomago Coal
Measures at Buchanan Tunnel near Newcastle, N.S.W.
1. Following upstream avulsion either of the trunk stream or of the crevasse
channel, the subdelta was abandoned before channel sedimentation had started
in the reference area.
2. Similarly to the distributary mouth bars of the main delta lobes which cover a
larger area than their associated distributary channels, the crevasse channels too
are not as widely distributed as their mouth bars, with the result that bars are
more frequently encountered in bores and outcrops than channels.
As shown by the above examples, crevasse splay and subdelta deposits are overlain
by a variety of sediments depending on the rate of subsidence during their decaying
stage once the breach has been plugged or their feeding channel has been
abandoned. Their subaerial portions often become vegetated and if the growth rate
of the plants is not exceeded by the rate of subsidence, peat formation may
commence on the splay deposit.
In tide-dominated deltas the interdistributary areas are submerged at high tide.
They consist of bays, marshlands, lagoons, and intertidal flats dissected by tidal
creeks (Elliott 1978). Silt- and shale-laminated sands, as well as sand-laminated
shales often displaying flaser bedding and starved ripples (Fig. 7.49) are commonly
formed in these subenvironments, as well as mud drapes and other tidal indicators.
7.3.5 The Coals of the Lower Delta Plain
Lower delta plain marshes and swamps occupy interdistributary positions on the
delta surface. In contrast to the bare, tide-swept mud flats, their most noticeable
feature is the abundance of plant life which covers the raised levees of distributary
424
Coal-Producing Sedimentary Environments
channels, as well as the silted-up portions of the interdistributary bays (Fisk 1960).
Coupled with the proximity of the watertable, this affords suitable conditions for the
accumulation and preservation of plant material in the form of peat, the
accumulation of which is interrupted by the introduction of fine clastics during
flooding, thus adding mineral impurities to the peat and subsequent coal. On the
other hand, flooding also disperses nutrients and spreads fresh water across the peat,
both of which provide for better growth conditions than prevail in settings away
from fluvial influence. The tidal effects on the peat leave their mark in the form of a
relatively high iron sulphide content which results from the reduction of sulphates
contained in sea water by bacteria, as discussed in Chaps. 4, 5 and 8.
The lower delta plain constitutes an environment in which the position of
depositional base level is maintained very close to mean sea level by a fine balance
between basin subsidence, sediment int1ux, subsurface compaction and eustasy.
Variations in any of these factors will bring about changes to sedimentation
patterns. The lower delta plain of an active, prograding delta is therefore not an
environment in which thick coal seams are likely to be formed, unless the influencing
factors remain stable for a long period oftime. Marley et al.'s (1979) observation in
Cretaceous coal measures from Utah U.S.A. of relatively greater coal thickness and
lateral seam persistency in lower delta plain environments compared with upper
delta plains cannot be generalised. According to Rahmani (1984), the lower delta
plain coals associated with the Upper Cretaceous Ferron Sandstone in Utah occur
as narrow belts land wards of and parallel to the delta front sandstone, whereas in the
Lower Cretaceous Falher Member of northwestern Alberta, Canada, a similar
pattern is broken up by transecting contemporaneous distributary channels. Coal
thickness, seam frequency and distribution in the coal measure profiles illustrated in
Fig. 7.9A, B, from the Australian Sydney Basin and in Fig. 7.35E, F, G from the German
Ruhr Basin, respectively, as well as reports from other areas, including the U.K.
(Fielding 1984) and South Africa (Holland et al. 1989) indicate considerably better
coal-forming conditions in upper delta plain environments than under lower delta
plain conditions. Also Horne and Ferm (1978) report that the lower delta plain coals
occurring in the eastern Kentucky portion (Pocahontas Basin) of the Appalachian
foreland basin system are widespread but thin. Modern examples of landward
gradation from thin, lower delta plain salt-marsh peat into thick, upper delta plain
fresh-water swamp peat have been described from coastal Louisiana by Frazier and
Osanik (1979), including the lateral coalescence of several splits of coastal marsh
peats into a thicker portion of inland swamp peat.
In the Permian coal measures of eastern Australia, coal seams formed in the
interdistributary areas of active lower delta plain settings, including those illustrated
in the stratigraphic column of Fig. 7.9A between 37 and 190 m, are also usually thin
and rarely exceed a thickness of 1 m. The marine influence is indicated by a high
pyrite content, whereas the wet conditions of peat formation are demonstrated by
low inertinite counts and a high gelification index, as illustrated in Fig. 7.50. This is
based on a high proportion in these coals of detrovitrinite (desmocollinite), probably
due to a high contribution to the peat by soft-tissued plants and the high rate of
biodegradation under elevated pH conditions. The tissue preservation index is
therefore also low, and there is evidence of a high contribution to the coal of cuticles.
The Lower Delta Plain
425
100.0
50.0
10.0
r--.
..
"
"co...
0
5.0
TREE DENSITY
-decreases
200.0
IImno-te/matic
..
•
••
te/matlc
..""
E
••
~
Ii • •
•
co
Increases-
0
•
m
•
"0
c:
0
;0
>
."
"
n."
.Q
Fig. 7.50. Correlation between tissue preservation and
gelification indices of whole
coal samples from interdistributary lower delta plain
environments of the
Permian Tomago Coal
Measures of New South
Wales. (Extended after
Diesel 1986a)
U)
E
"
.§
1.0
.."
::;:
0.5
terrestrial
GI
TPI
0.5
1.0
1.5
2.0
2.5
These have been partly corroded by the brackish water so that the recorded cutinite
percentage probably does not fully represent its original input. The combination of
unstructured vitrinite, chemically corroded cutinite and the apparent paucity of
wood-derived macerals in the lower delta plain Gondwana coals of Fig. 7.50 might
point to an origin in a cold-climate marsh environment, analogous to present-day
high latitude settings.
In the course of delta progradation the tree-less marshes will be replaced by
forested peat swamps, as indicated by a reduction in sulphur content and a rise in
TPI, because more wood-derived macerals with a better preservation potential will
contribute to the coal. Because of intermittent dry conditions, the proportion of
semifusinite and fusinite will increase, thus lowering GI values which leads to the
upper delta and alluvial plain coals illustrated in Fig. 7.38. Conversely, should the
already high water table rise further, either hypautochthonous or allochthonous
conditions will be established which may result in the deposition of pyrite-rich
sapropelites. Alternatively, organic sedimentation may cease altogether.
According to Scruton (1960), the history of a delta can be divided into a
constructional phase of progradation and a destructional phase following abandonment. Abandonment of an old and construction of a new delta lobe follows upstream avulsion ofthe feeder channnel, which is seeking a shorter and steeper route
to the coast, when its gradient has been excessively reduced by the preceding delta
progradation. Because of the termination of sediment supply, delta progradation
ceases and the destruction phase begins, during which part of the delta plain and
upper slope may be subject to marine reworking. This results in delta switching,
i.e. the construction of a new delta (marine regression), while the inactivated lobe is
426
Coal-Producing Sedimentary Environments
gradually flooded (marine transgression). A modern example is the Cenozoic
Mississippi Delta where many such switching events have been recorded (Coleman
and Gagliano 1965b; Frazier 1967; Gould 1970), the most recent of which began late
last century with the increasing diversion of Mississippi water into the Atchafalaya
River. So far, complete avulsion has been prevented only after human intervention
(Fisk 1952b). A mature delta complex consists therefore of a stack of partially
overlapping lobes which carry a record of the repeated phases of delta construction
and abandonment. One of many ancient examples is the coal-bearing Ludlow
Member of the Fort Uriion Formation in the Williston Basin of North Dakota,
U.S.A., which, according to Belt et al. (1984) contains at the base a lower delta plain
association, consisting of stacked subdelta lobes, which originated from fused
crevasse splay deposits.
As the delta complex grows, the ratio between inactive and active delta lobes
increases and, although abandoned lobes are partially reworked and subject to
marine transgression, the area covered by abandoned delta lobes increases as well,
unless there is a strong eustatic sea-level rise. According to Frazier (1967) and
Penland et al. (1988), approximately 75% of the present delta surface of the
Mississippi River consists of abandoned lobes currently undergoing transgression.
The drowning of the seaward margin of the abandoned delta lobe (= transgressive submergence of Penland et al. 1988) may be accompanied by the up-dip
formation of extensive blankets peats which cover much of the inactive delta surface
and many extend into the upper delta plain. The wide areal extent of the blanket
peats of the Mississippi delta complex differs markedly from the vertically and
laterally more restricted high ash peats formed between active delta distributaries
(Coleman and Smith 1964), and, according to Kaiser et al. (1978), Tewalt et al.
(1982), and Hamilton (1985), the contrast between a progradational "interdistributary type" and an "abandonment type" of lower delta plain peat is also reflected in
ancient coal deposits. According to Ayers and Kaiser (1984), the spreading of
interdistributary mires across abandoned delta lobes in the Powder River Basin of
Wyoming and Montana, U.S.A. has produced thick Tertiary coal seams, which are
parallel to the basin margin and also cover interdeltaic coastal plains.
The relatively thin and laterally discontinuous coals discussed above formed in
active interdistributary settings of the constructional phase. They carry the
hallmarks of depositional instability, which sets them apart from the more extensive
coals formed on defunct delta plains. Examples of delta abandonment coals from
eastern Australia are the Melvilles Seam in the Upper Permian Black Jack
Formation of the Gunnedah Basin (Hamilton 1985) and the Mid-Permian Greta
Seam in the Hunter Valley portion of the Sydney basin. The Greta Seam is part of a
deltaic molasse wedge which separates two thick marine sequences along the
northeastern margin of the Sydney Basin. The Greta Seam was formed on an
abandoned delta surface towards the end of a terrestrial interlude when the clastic
sediment supply from the nearby New England Fold Belt waned and marine
conditions began to spread again. The seam is underlain by conglomerate which
probably represents a gravelly delta plain and front. The seam averages 3 m
(maximum 7 m) in thickness and is in direct contact with the overlying marine
sediments from which, in some places, worm burrows extend into the coal. Being
The Barrier Beach/Strand Plain System
427
largely bypassed by sediments, the Greta Seam and other coals of this kind are
relatively low in ash and contain no or only few epiclastic stone bands. Because of
the enhanced microbial action due to their close contact with seawater in the course
of the marine transgression, these coals are rich in sulphur, detrovitrinite and
dispersed liptinite. This also affects other coal quality parameters, which will be
discussed in Chap. 8.
7.4 The Barrier Beach/Strand-Plain System
Shoreline morphology is largely a function ofthe ratio between sediment supply and
coastal energy. Where the ratio is high, the combined effects of wind, wave, tidal and
other current energy are insufficient to dissipate the volume of sediment supplied to
the shoreline, and a delta will be formed at the point of sediments delivery, usually a
river mouth. With decreasing sediment supply, wave and current activity will
distribute an increasing proportion of the incoming sediment over the adjacent
coast, resulting in a reduction of delta size and the number of distributaries
(Coleman 1981), until an elongated shoreline is formed in which the beach
constitutes the barrier between the land and the sea. In cases of very low ratios, a
negative delta, i.e. an estuary, will be formed.
Deltas and broad coastal plains behind barrier beaches are not mutually
exclusive (Ward 1984). Flanking barriers may be formed by the reworking and
longshore dispersal of sand from abandoned delta lobes (Penland et al. 1988), but
sand barriers may also occur on the edges of wave-dominated active deltas as
reformed distributary mouth bars and levees, and by the wind-driven accretion of
longshore bars onto the beach (Curray and Moore 1964). This results in the
formation of coast-parallel elongated sand bodies of considerable extent and
textural, as well as compositional maturity, although the latter may vary with source
material and rate of sedimentation. A maximum length of 64 km has been reported
by Englund and Windolph (1971) for an individual barrier sand body in eastern
Kentucky. Horne and Ferm (1978) quote a maximum thickness range of 11 to 26 m
and a width of up to 8 km for individual barrier systems from the same area.
Notwithstanding the preference of many coal geologists for the delta environment as a model for coal formation, indications are increasing that a high
proportion of coal has been formed on strand plains behind barrier beaches either
within delta complexes, between deltas (interdeltaic coastal plains of Englund 1974;
Vaninetti 1978; Ayers and Kaiser 1984), or completely separate from any deltaLc
influence. This new awareness is probably due to the realisation that the delta model
has often been invoked without much geological evidence. If Walker's (1981, p. 4.10)
demand that "the term delta cannot be applied to prograding beach sequences unless
one had plan view geometry in the form of isopach maps" had been rigorously
applied in the past, the delta model of coal formation would probably not have
today's prominence in the geological literature.
Foremost among the reasons for considering coastal plains outside the delta
complex as suitable coal-forming environments is the wave dominance of many of
428
Coal-Producing Sedimentary Environments
the shoreline sediments involved, as shown by the upper Fernie and lower Kootenay
Formations in western Alberta, Canada (Hamblin and Walker 1979) and the
examples cited below, and the extremely high ratio between areal extent and
thickness of some nearshore sediments and their associated coal seams. Some strandplain coals (as used here, the strand plain, which is the product of shoreface
accretion, incorporates the lower or seaward portion of the coastal plain between
the shoreline and the upper coastal plain of more alluvial affinity) cover very large
areas without any obvious interruption, which is a feature they share with the delta
abandonment coals referred to above. Indeed, there appears to be little fundamental
difference in the conditions of peat accumulation between the gradual drowning of a
coastal plain or an abandoned delta lobe. However, if delta abandonment is such
that it allows peat accumulation at all, the resulting coal will usually carry a
transgressive signature and any re-activation of the abandoned lobe will lead to
renewed progradation and change from blanket peat to disconnected interdistributary peat. Because the methods of sediment supply and shoreline accretion are
different in strand-plain settings, there is no such geometric contrast between peats
formed under transgressive or regressive (shoreline progradation) conditions, and
both trends may produce laterally very extensive coals. For example, Langenberg
et al. (1989), Kalkreuth et al. (1989), and Kalkreuth and Leckie (1989) report on up to
12-m-thick Early Cretaceous strand-plain coals in Canada which rest on 15- to 35-mthick shoreface sands and gravels of the Gates Formation, that can be traced along
strike for up to 230 km and down depositional dip for up to 90 km (Leckie and
Walker 1982). Similar geometric relationships have been reported from eastern
Australia, where several thick, low ash coking coals of the Moranbah and German
Creek Coal Measures can be traced over a distance of 240 km along the western
flank ofthe Bowen Basin in Queensland (Godfrey 1985). Th.e coal-bearing strata are
little more than 100 m thick and are sandwiched between marine sediments.
Athough several authors have postulated a delta setting for the German Creek Coal
Measures and their lateral equivalents (Godfrey 1985; Phillips et al. 1985; Falkner
and Fielding 1990), the possibility to follow individual coal seams along strike for
over 200 km with almost constant seam thicknesses over much ofthis distance (e.g. 6
to 9m for the Goonyella Middle Seam, according to Staines and Koppe 1980 and
Johnson 1984) requires considerable uniformity of depositional conditions over a
very large coastal section. The lack of lobate delta front sands, the occurrence of
lagoonal sediments and texturally mature sandstones with beach lamination in the
floors of some coal seams, and other features, suggest that some of the stratigraphically lower coals might have been formed in a strand-plain setting which both
landward and upward graded into the kind of river-dominated alluvial plain coal
measures discussed by Johnson (1984). Another set of backbarrier strand-plain
coals has been described by Martini and Johnson (1984) from the Collinsville Coal
Measures in the northern portion of the Bowen Basin, where the authors were able
to identify shoaling-upward, prograding sandy barriers, lenticular and flaser
bedded, bioturbated tidal flats, herringbone cross-bedding, lagoonal deposits
associated with coal seams formed on wide, flat coastal plains. It is therefore
intended to discuss first some of the general aspects of barrier beach complexes,
followed by some relevant coal measure examples.
429
The Barrier Beach/Strand Plain System
OFFSHORE
TRANSITION
I
SHOREFACE
Mean Fairweather
Wave Base
Swell Zone
Vertical profile of
prograding non-
Breaker
Line
Shoaling Zone
BACK SHORE
FORE-I
SHORE
Dune
Berm
'"
;
~
Runnel
N
o
AEOLIAN
N
5
DUNES
HWL--
o
FORESHORE
UPPER
-5
LOWER
Landward directed lunate megaripples
Landward directed asymmetric ripples and bioturbation
with HCS-sands
-10
-
,
Ww
0:0
0",
ILL
(/)
' - ' OFFSHORE
-«'
TRANSITION
~
Fig. 7.51. Cartoon correlating the main physiographic features of a micro- to mesotidal, wavedominated, non-barred shoreline with a vertical profile of sedimentary structures produced by
them under a regime of coastal progradation. (After Clifton et al. 1971; Hunter et al. 1979)
"~
OFFSHORE
TRANSITION
a;
::;;
Mean Fairweather
Wave Base
10
NEARSHORE
BAR
SHOREFACE
Breaker
Line
w
0:
OI
I (!I
(/):l
(!1O
zo:
01...J
~I
BACKSHORE
(/)
~ Berm
oLL
Runnel
Vertical profile of
prograding barred
shoreline
AEOLIAN
5
DUNES
o
-5
-
HWL---
- LWL----
FORESHORE
-~~=---~~~~~~~~~~T~R~OUGH
landward directed lunate megaripples
f""--~~.:>i LOWER
SHOREFACE
-10
landward directed asymmetric ripples and bioturbation
with HCS-sands
OFFSHORE
TRANSITION
Fig. 7.52. Cartoon correlating the main physiographic features of a micro- to meso tidal, wavedominated, barred shoreline with a vertical profile of sedimentary structures produced by them
under a regime of coastal progradation. After Clifton et al. 1971; Hunter et al. 1979)
The strand plain is a terrain of low topography that occurs landward of the
beach. In suitable climates it may be occupied by peatlands which either abut
directly on the inner portion of the barrier beach (backshore) or are separated from
the latter by coast-parallel lagoons. These may be connected to the open sea by inlets
which divide the barrier beach into elongated barrier islands. Two cartoons of
barrier beach morphology and sedimentation are illustrated in Figs. 7.51 and 7.52
together with the respective columnar section expected to form under conditions of
coastal progradation, mainly due to longshore accretion. They are based on studies
of Holocene shorelines by Clifton et al. 1971 and Hunteret al. 1979 and it is assumed
Coal-Producing Sedimentary Environments
430
I
~
SHOREFACE
Metres
/
10
SUBTIDAL
CHANNEL
I
U
",
----r-- / ~
\
5
"-
-HWL-- -
o
-10
FLOOD TIDAL DELTA
EBB
""TIDAL
DELTA
""--/~
"
---------.::
\
I /
~
'\ '
"\
...--
.'"
-
"----
SPIT
MIGRA TlON
"
~
LAGOON
"
SPIT
PLATFORM
'--./
-LWL-- - - -
-5
BACK BARRIER
. ~TIDAL
~I
,,~ \ I N L E T ' : ,
~
"-
l
~< ~/~~
/I !
f1'
~r:z
BARRIER
SHALLOW
CHANNEL
--
DEEP
CHANNEL
Mean Fairweather
LOWER EBB TIDAL
DELTA FRONT
LOWER
SHOREFACE
Wave Base
landward directed ripples. HCS-sand
and bioturbation
-v-~
OFFSHORE
-«\\ -V- TRANSITION
Fig. 7.53. Cartoon correlating the main physiographic features of a wave-affected meso- to
macrotidal shoreline with a vertical profile of sedimentary structures produced by them under a
regime of coastal progradation. (After Clifton et al. 1971; Hunter et al. 1979)
that the shoreline is wave-dominated under a micro- to meso tidal regime. A third
model is illustrated in Fig. 7.53 in which a stronger tidal influence is envisaged.
Although there is no unanimity about the terminology describing shoreline
deposits, it is commonly agreed that the shoreface consists of the subtidal zone
extending from low water level to mean fair-weather wave base. In terms of wave
formation the shoreface comprises the shoaling, breaker, and surf zones and,
depending on whether the shoreline is barred or non-barred, a variety of bedforms
is generated, as indicated in Figs. 7.51 and 7.52. Landward from the shoreface
follows the beachface or foreshore which comprises the intertidal zone between low
and high water levels, the latter being situated close to the beach berm. The foreshore
accommodates most of the swash zone and is bordered by the supratidal backshore
and subaerial dune system (Reinson 1979).
7.4.1 The Offshore Transition Zone
The three examples illustrated in Figs. 7.51 to 7.53 are similar in the offshore region
(below mean storm wave base) and offshore transition zone which is situated
between mean storm and mean fair-weather wave base. According to Elliott (1978)
wave base, i.e. the line near the coast where approaching waves "feel" bottom, is
situated where:
Water depth = 1/2 Wavelength.
(7.21)
The Barrier Beach/Strand Plain System
431
Fair-weather wave base is accompanied on the shoreward side by a transitional
oscillatory wave zone which gives rise to wave alignment and the formation of
symmetrical ripples on the sea bed. The sediments formed in the offshore transition
zone consist of bioturbated muds and silt-laminated fine sands with occasional
small-scale current ripples which may be landward-directed. Interspersed are sheets
of up to several-decimetre-thick clean sands which have been eroded from the beach
and upper shoreface during storms and redeposited seaward. They display a sharp,
Fig. 7.54. Photograph of hummocky (HCS) and swaly (SCS) cross- stratification measured in the
Waratah Sandstone below the Borehole Seam at 465 m in Fig. 7.9B at the Waratah Pistol Club,
Newcastle, N.S.W.
Fig.7.55. Detail of hummocky cross-stratification along strike from locality illustrated in Fig. 7.54
432
Coal-Producing Sedimentary Environments
erosional base and an undulating top, referred to as hummocky cross-stratification
(HCS), two examples of which are illustrated in Figs. 7.54 and 7.55. Up-slope, i.e.
near the fair-weather wave base, HCS is replaced by swaly cross-stratification (SCS)
due to the planing-off of the upward convex hummocks by shoaling fair-weather
waves, as has been reported by Leckie and Walker (1982). Further offshore, i.e.
beyond the reach of storm wave base these sands have a tendency to change into
thin, upward-graded turbidites. According to Hamblin and Walker (1979), both
HCS-bearing sands and turbidites have the same origin in a storm- generated surge
tide which pushes water against the coast. When the storm abates, a density current
is formed by the backwash, part of which may be deposited and moulded into HCS
above storm wave base, while another portion is transported further down-slope
and becomes a turbidite. Storm activity is an important agent of deposition in the
offshore region which receives only small volumes of sediment during fair-weather
periods. As has been stressed by Elliott (1978), the reverse is true for the upper
shoreface and foreshore, which aggrade during fair weather but are stripped of
sediments during storms.
7.4.2 The Shoreface
The shoreface is conventionally divided into a lower and upper portion. The lower
shoreface includes the wave build-up or shoaling zone between mean fair-weather
wave base and the breaker line whereas the upper shoreface extends from the latter
across the surf zone to the beginning of the swash zone (Fig. 7.51). Shoaling leads to a
progressive shoreward-directed asymmetry and eventual breaking ofthe waves. The
effects of wave energy on bottom sediments increase towards the shore, which is
indicated by a change from predominantly mud and silt to medium and coarse sand
as well as a change in wave-driven bedforms. With increasing wave build-up the
small ripples found in the offshore transition zone give way to flat but large
shoreward-directed lunate megaripples, which increase in size near the upper limit of
the lower shoreface and on nearshore bars during high tide. A recent example of
shoreward-directed megaripples from the Dutch North Sea coast is illustrated in
Fig. 7.56, while similar fossil examples from the basal portion of the Carboniferous
Ruhr Coal Measures are given in Fig. 7.57.
The seaward end of the upper shoreface is sometimes distinguished from the
other two units and referred to, e.g. by Reinson (1980), as the middle shoreface, which
is a zone of particularly strong wave action on either side of the breaker line. In
barred shorelines (Fig. 7.52) the energy pattern of this zone is complicated by the
occurrence of one or several long- or nearshore bar and trough couples (Fig. 7.58).
The latter convey considerable volumes of water and sediment parallel to the coast
before they are redirected seaward through rip channels (Fig. 7.59), as discussed by
Hunter et al. (1979). The result is an intricate set of ripples in which shore-parallel
and seaward directions prevail. Since during coastal progradation the nearshore bar
is eroded by the longshore trough (Fig. 7.52), the strongest indication for a former
The Barrier Beach/Strand Plain System
433
Fig. 7.56. Flat shoreward,directed lunate megaripples situated on the surface of a nearshore bar,
exposed at low tide along the barred shoreline near Zandvoort, Holland
Fig.7.57. View (ac-plane) of flat (approx. 15 cm high) southward, i.e. shoreward to right, directed
lunate megaripples in lower shoreface sand below the Namurian Grenzsandstein at the
Hohensyburg, Ruhr Valley, Germany. The measured azimuths of the foresets are indicated at the
base of the section in Fig. 7.35G
434
Coal-Producing Sedimentary Environments
Fig. 7.58. View of a longshore trough (centre) between a nearshore bar (left) and the foreshore
(right) exposed at low tide along the barred shoreline at Zandvoort, Holland
Fig. 7.59. View of a ~ip channel (middle distance) connecting a longshore trough (extreme right)
with the sea (upper left) across It nearshore bar (centre left foreground) at low tide along the barred
shoreline at Zandvoort, Holland
The Barrier Beach/Strand Plain System
435
barred deposit in a vertical profile is the occurrence of an erosion surface between the shoreface and the foreshore. If the longshore trough responsible for the
erosion is well developed, it contains the coarsest detritus available along the
shoreline. This lag deposit will overlie the erosion surface, probably followed by
shore-parallel current ripples within a thin sequence of upward-fining trough
sediments below the foreshore deposits.
Also in the upper shoreface, variable ripple directions, as well as planar beds,
can be found. In the lower portion of this zone (surf zone) the prevailing direction is
still landward (Figs. 7.51 and 7.52) but with increasing proximity to the swash zone
the backwash from the beach produces seaward-directed ripples (transition zone in
Fig. 7.51).
7.4.3 The Foreshore
The foreshore is subjected to constant changes from wave swash to backwash runoff,
as illustrated in Fig. 7.60. Because the water is shallow but often of high velocity,
upper flow regime sheet flows are common, resulting in the formation of mature
sands with planar lamination representing seaward sloping accretion planes called
beach or (more specifically) foreshore lamination (Thompson 1937; McKee 1957;
Davies et al. 1971). Storm erosion and subsequent build-up result in the formation of
low angle discordances between successive sets of accretion surfaces. Since the
foreshore always slopes basinward, although beach cusps and coastal embayments
may cause minor modifications, beach lamination is a very useful tool in
Fig. 7.60. Illustration of a foreshore with swash zone (the wet, sloping portion to the left) at Dixon
Park Beach, Newcastle, N.S.W.
436
Coal-Producing Sedimentary Environments
Fig.7.61. Photograph of gently southward (basin ward) inclined beach lamination (below hand) in
the upper portion of the Waratah Sandstone below the Borehole Seam at 477 m in Fig. 7.9B at the
Waratah Pistol Club, Newcastle, N.S.W.
Fig.7.62. View ofthe foreshore and spit platform portion of the Waratah Sandstone (between 474
and 477 m in Fig. 7.9B) at the Waratah Pistol Club, Newcastle, N.S.W. The outcrop shows welldeveloped foreshore accretion planes (beach lamination) over a basinward distance of 10 m. For
better identification one example has been arrowed. The (weathered) Borehole Seam is covered by
the vegetation behind the person
The Barrier Beach/Strand Plain System
437
palaeo-environmental analysis. Coal measure examples are Facies 5 of TavenerSmith (1982) in the Vryheid Formation of the Karroo Basin near Durban, South
Africa, or the shallow basinward-dipping foreshore lamination McHugh (1984)
found associated with heavy mineral zones and landward directed megaripples in
the Watt Sandstone at the base of the Permian Wollombi Coal Measures in the
northern Sydney Basin of New South Wales. Also the stratigraphic equivalent ofthe
Watt Sandstone, the Waratah Sandstone at the base of the Newcastle Coal
Measures, displays in its upper portion distinct foreshore lamination, which is
illustrated in Fig. 7.61 and at 477 m in Fig. 7.9B below the Borehole Seam. Its
southern basinward dip is in agreement with its palaeogeographical position near
the northeastern margin of the Sydney Basin and, by showing an opposite azimuth
to the foreset beds between 468 and 471 m in Fig. 7.9B, the dip of the foreshore
lamination is also consistent with the above-mentioned interpretation of this
cross-bedding as having been derived from wave-driven, landward-migrating
megaripples.
While in small outcrops and in bore core it may be possible to misidentify
beach lamination and mistake it for HCS or SCS, such diagnostic difficulty does not
arise when the exposure is sufficiently large. An example is illustrated in Fig. 7.62,
where the regularity and extent of the preserved foreshore slopes can only be
matched by some point bar accretion planes. However, as discussed above, point
bars possess other diagnostic features which exclude the possibility of a
misdiagnosis.
7.4.4 The Backshore
The backshore is entirely supratidal. It is separated from the foreshore by the berm
crest which forms the ridge between the seaward-sloping foreshore and the flat or
landward dipping back shore. As shown in Fig. 7.63, washover water collects in
backshore channels or runnels from where it is removed either by surface runoff,
evaporation, or subsurface percolation. The latter is mostly prevented by mud
drapes on the runnel floors, which results in the formation of elongated pools of
shallow water. Flat bedding (backshore lamination) is a common feature which is in
some contrast to the large-scale cross-bedding found in the subaerial dunes which
are often part of this environment. A variety of colonising plants and their roots can
be found among the dune sediments which have a low preservation potential and are
not commonly found in ancient stratigraphic columns.
Well-preserved berm crests or beach ridges and back shore lamination occur in
the above-mentioned Waratah Sandstone at the base of the Newcastle Coal
Measures. The relief difference between berm crests and intervening swales is
commonly less than 1 m with the result that unlike the Lower Kittanning and
Clarion coals discussed in Chap. 7.2.4, the overlying Borehole Seam shows little
variation between berm and swale positions. An example of a flat berm crest with
overlying Borehole Se~m and both fore- and backshore lamination is illustrated in
438
Coal-Producing Sedimentary Environments
Fig.7.63. View of the berm crest (centre) with foreshore and swash zone to the left. The backshore is
to the right, followed by back shore channel (runnel), and the dune belt in extreme upper right with
houses. Dutch North Sea coast near Zandvoort at high tide
Fig. 7.64. View of the section of the Newcastle Coal Measures between 476 and 481 m in Fig. 7.9B
showing the top of the Waratah Sandstone with the overlying Borehole Seam at Alnwick Street
quarry, Waratah, N.S.W. The encircled hammer has been positioned at a berm crest, and both fore(to left) and backshore (to right) slopes are mirrored by preserved accretion planes (arrowed)
Fig. 7.64, although only some of the accretion planes present in the sandstone are
actually visible in the illustration. For comparison, a Recent berm crest and its
associated fore- and backshore lamination is shown in Fig. 7.65. A comparison
between ancient and modern fore- and backshore lamination suggests that the latter
is more limited in extent, less varied and frequently shows lower slope angles.
The Barrier Beach/Strand Plain System
439
Fig. 7.65. View of a modern berm crest cut by a stormwater channel at Catherine Hill Bay near
Newcastle, N.S.W. Both fore- and backshore lamination are visible (enhanced). Note the similarity
to the Waratah Sandstone in Fig. 7.64
7.4.5 The Tidal Inlet
As illustrated in Fig. 7.53, in the vicinity of tidal inlets the shoreface and adjacent
zones are extensively modified by landward- and seaward-directed tidal currents.
Indeed, the whole barrier complex, being detached from the hinterland, is affected by
hydraulic and sedimentary conditions quite different from those operating without
a strong tidal influence. Depending on the availability of sediments and the
prevailing current conditions, ebb- and flood-tidal deltas may be constructed on
either side of the tidal inlet. In most cases sediment supply is uneven, so that the one
or the other delta becomes dominant. Where wave energy is high, tides are low and
rivers entering adjacent lagoons carry only small quantities of sediments, flood-tidal
deltas are common. Opposite conditions prevail in areas where the coastal lagoons
are supplied with large quantities of sediments from the hinterland, part of which is
picked up by tidal currents and used to construct ebb-tidal deltas. The latter build
out on the shoreface, which together with the fore- and backshore, is eroded into by
the channels connecting the lagoon to the sea. Next to sediment supply a major
controlling factor in the formation of tidal deltas is tidal energy and range. On the
basis of tidal range Davies (1964) established the following three categories of
shorelines:
1. Microtidal
2. Mesotidal
3. Macrotidal
< 2 m tidal range.
2-4 m tidal range.
> 4 m tidal range.
440
Coal-Producing Sedimentary Environments
These tidal ranges have an inverse effect on wave energy which along macrotidal
shorelines is less concentrated and spread over a wider area than on microtidal
shores. Microtidal coasts are therefore mostly wave-dominated, they favour the
formation of long and narrow barriers with washover fans, widely spaced inlets
through which flood-tidal deltas extend into open lagoons. According to Hayes
(1975,1979) and Barwis and Horne (1979), more water is conveyed through barriers
lining meso tidal shores, which results in closer inlet spacing and therefore shorter
barriers. Backbarrier lagoons are often replaced by tidal mudflats, salt- and brackish
water marshes. These are commonly dissected by tidal creeks feeding into the inlets
on the seaward side of which large ebb-tidal deltas are produced. Macrotidal
shorelines occur mainly in trumpet-shaped embayments, where tidal waves are
funnelled and amplified towards the centre of the bay. This results in a constantly
changing water depth which, in conjunction with the diffracted water wave pattern
in the coastal re-entrant weakens the effects of wave action and causes the shoreline
to be tide-dominated.
Since flood- and ebb-currents use the same channel, the opposing flow
directions are mirrored by bedforms, an example of which is the herringbone crossstratification illustrated in Fig. 7.45. This structure consists of grouped but opposing
sets of ripple cross-lamination formed in response to the ebb-flood cycle. Herringbone cross-stratification is found not only in tidal channels but also on intertidal
flats, in particular the sand flats occurring between the upper and lower mud flats.
An example of this kind is indicated in Fig. 7.35F below Niveau Geitling 2 of the
Lower Westphalian A (Witten Beds) from the Ruhr Basin. Because megaripples and
sand waves usually do not react fast enough to the diurnal change in flow direction,
herringbone cross-stratification is typically found is small bedforms. Larger
bedforms show diurnal reversals only at very high tidal flow velocities, for example
Terwindt and Brouwer (1986) quote peak depth-averaged velocities in excess of
0.85 mls for the reversal of inter-tidal sand waves in the Westerschelde estuary on
the Dutch North Sea coast.
Examples of bimodal azimuths in megaripples are common in the marineinfluenced lower portion of the Ruhr coal measures, as indicated in the channel
sandstones below the Wasserbank 1 Seam in Fig. 7.35F and in Fig. 7.35G below the
Neufl6z, the Gottessegen Seam, and in the Namurian C Grenzsandstein. The latter
is conventionally taken as the base of the Ruhr coal measures. It is a compound
shoreline sandstone, which is overlain by lagoonal or interdistributary bay
sediments and grades downward into lower shoreface sediments containing frontal
splays with hummocky cross- stratification and shoreward-driven lunate megaripples generated by shoaling waves. The upper portion of the Grenzsandstein shows
the westward-directed unimodal trough cross-stratification typical of many fluvial
sandstones in the coal measures. Its middle portion is horizontally bedded and
contains beach lamination, followed downward by a trough cross-bedded sandstone with bimodal azimuths, presumably of tidal origin. In the basal portion of the
Grenzsandstein horizontal bedding is repeated with the addition of some swaly
cross-stratification.
The absence of bimodal azimuths of foreset beds does not prove lack of tidal
influence, since in many inlets a flow separation occurs into either flood- or ebb-
The Barrier Beach/Strand Plain System
441
Fig. 7.66. Basal portion of an inferred tidal channel in the Waratah Sandstone at the Pistol Club
quarry in Waratah, N.S. W. The hammer is positioned at the irregular basal erosional contact, which
protrudes from the rock face because of the more resistant coarse channel fill. The foreset beds in
the upper part of the illustration dip to the north
Fig.7.67. View of the tidal inlet into Lawrencetown Lake at the coast of Nova Scotia, Canada. In
the foreground is the accretionary, convex end of the spit platform, in middleground is the inlet, and
in the background is the concave embankment of the inlet followed by old beach ridges and
swales now undergoing erosion
442
Coal-Producing Sedimentary Environments
dominated currents. This is particularly common in meandering tidal channels of
gentle meander radius. As discussed by Barwis (1978) and mentioned above, their
point bars may be partly or wholly detached from the inner channel bank, thus
dividing the channel into a commonly ebb-dominated main portion and a smaller
flood-dominated part situated between the detached point bar and the inner bank.
An ancient example of an inlet with unidirectional flow occurs in the beforementioned Waratah Sandstone at the base of the Newcastle Coal Measures. The
lower portion of its inlet channel is illustrated in Fig. 7.66. As indicated between 471
and 474.8 m in Fig. 7.9B, flow directions have varied widely but a northerly, i.e.
shoreward trend towards a flood tidal delta prevailed, and there is no biomodality
present in the foreset azimuths. These bedforms, which belong to the lower flow
regime, indicate that the tidal channels, when in flood, may have been several
metres deep. Shallow channels, which may be found on microtidal shores, are
indicated by flat lamination transitional to the upper flow regime (Reins on 1979).
Tidal inlets migrate along the shore by erosion and accretion in a similar
manner as meandering rivers across a flood plain. Inlet deposits therefore display
basal erosional contacts followed by channel lag consisting of shell debris, coarse
sand, pebbles and mud balls (Barwis and Hayes 1979). Lateral erosion on the
concave side of an old barrier is balanced by deposition on the convex side of a
longitudinally extending spit platform. Similarly to a point bar, the accretion planes
slope gently away from the centre of curvature and merge on the outer side with the
seaward-dipping beach lamination. Imbalances in the rate oflateral accretion across
the leading edge of the spit may cause the inlet channel to assume an oblique
position with respect to the shoreline, examples of which occur along many present
coasts. A ground view of a laterally migrating tidal inlet from Nova Scotia, Canada,
is illustrated in Fig. 7.67.
7.4.6 The Backbarrier
The back barrier area, being a low energy environment, produces sediments which
are in sharp contrast to the predominantly clean sands and gravels of the high
energy barrier beach and tidal inlet environments (Reins on 1980). Muddy facies, rich
in organic material is dominant, and only in places where washover sand, flood-tidal
deltas, or other shoreline features extend into back barrier lagoons, the prevailing
dark sediment colours give way to the lighter hues of high energy conditions. In
vertical profiles the contact between the commonly bleached strandline sand and the
overlying lagoonal and other back barrier deposits is sharp and marked by an
abrupt change from light to dark colours. Ancient examples are the contact between
the Upper Cretaceous coal-bearing Blackhawk Formation and the underlying Star
Point Sandstone in Utah, U.S.A. (Marley et al. 1979), many examples from the
Appalachian Upper Carboniferous coal measures (Horne and Ferm 1976, 1978;
Hobday and Horne 1977), the above-mentioned Grenzsandstein and its overlying
lagoonal deposits from the German Ruhr Basin (Fig. 7.35H), as well as the Permian
The Barrier Beach/Strand Plain System
443
Borehole Seam-Waratah Sandstone boundary in the Newcastle Coalfield of New
South Wales (Diessel et al. 1989).
Lagoons are stretches of shallow water situated between the shoreline and the
hinterland. Lagoonal configuration, water depth, salinity and depositional pattern
depend largely on the tidal range. Microtidal shores, such as those of western
Holland and northwestern Denmark (Hayes 1975), or along the Gulf of Mexico
(Hayes 1979), contain few tidal inlets, which in humid climates results in low
lagoonal salinities due to the high influx offresh river water. Because the latter also
carry terrestrial sediments, river deltas will form on the landward margin of the
lagoon.
Another important source oflagoonal sediments along wave-dominated coasts
are wash over fans. They are thin sheets of sand which extend from storm breaches in
the barrier into the lagoon where they interdigitise with the laminated muds and silts
which represent the normal fair-weather sedimentation from suspension. In proximal
setting they are often flat-bedded, having been emplaced from high velocity traction
carpets, but distally they show bedforms of the lower flow regime, including
landward-dipping ripple cross-lamination (Schwartz 1982).
The influx of sediments from both landward and seaward margins of lagoons
may lead to their filling and transformation into peat-bearing strand plains. An
example of the colonisation of a partially silted-up lagoon by Spartina marsh is
shown in Fig. 7.68 from Lawrencetown Lake on the Atlantic coast of Nova Scotia.
Vegetated strand plains extend also over former beaches which have become defunct
by the seaward progradation of the active beach front. An example, also from
Lawrencetown, is illustrated in Fig. 7.69, which shows remnants of former beach
Fig.7.68. View of Spartina marsh colonising the silted-up portions of Lawrencetown Lake, a
coastal lagoon on the Atlantic seaboard of Nova Scotia, Canada
444
Coal-Producing Sedimentary Environments
Fig.7.69. Beach ridge plain near Lawrencetown, Nova Scotia, with wooded berm crests (beach
ridges) and intervening swales containing up to 1 m peat
ridges near the left and right margins of the illustration. A comparison with Fig. 7.67
shows opposing vegetation patterns. The berms (beach ridges) ofthe more youthful
coastal beaches illustrated in Fig. 7.67 are still so high above the groundwater level
that they are too dry to support arborescent vegetation. Trees and shrubs are
concentrated in the swales between the ridges, where the groundwater is more easily
tapped. In contrast, the older, more landward-situated beach ridges of Fig. 7.69 are
much closer to the groundwater table due to basement subsidence. In this setting the
swales, which carry up to 1 m peat (R. Boyd, pers commun.), are too wet for the trees,
which are therefore restricted to the elevated ridges.
By filling the coastal lagoons with sediment, barrier islands become connected
to the hinterland, and rivers which formerly terminated in the lagoon become
extended and issue their load directly into the sea, as is the case with barrier beaches
devoid of any lagoons. Moreover, by transforming former lagoons into strand
plains, new ground is prepared for further peat accumulation. It follows that the
floors of strand-plain coals consist of either beach and associated deposits, or of
lagoonal sediments, whereby lateral transitions from one to the other are common.
7.4.7 Marine Transgression (Barrier Retrogradation)
Although in the stratigraphic column a marine transgression is indicated by the
superposition of seaward on landward deposits, a reasonably intact sequence of this
kind is possible only when the transgression is caused by a rapid rise in sea level in
order to avoid extensive erosion and reworking of shoreline sediments (Kraft 1971).
The Barrier Beach/Strand Plain System
445
However, very few marine transgressions proceed without any coastal erosion and
often the landward retreat of a wave-cut scarp has been accompanied by
considerable obliteration of the shoreface and beach complex (Bartberger 1976),
except when the transgression affects a delta complex, in which case part ofthe delta
may be preserved, initially, as a retrograding barrier shoreline and later as a shelf
sand (Penland et al. 1988). Conversely, transgressive straight shorelines are poorly
preserved and are often represented only by thin ravinement deposits in the form of
residual lag, whereas the m~jority of the overlying marine sediments already belong
to the subsequent regressive stage (Klein 1974). An excellent example ofthis kind is
the marine split (Archerfield-Bulga Formation of Uren 1985) between the Wynn
Metres
400
FI Sh Flood Plain with
~
Crevasse Splays
SI
..I
Point Bar
FI Flood Plain
390
S.
Whybrow Seam
(No.25)
FI
380
Back-Swamp
with
=>
0
a:
Crevasse Splay
Flood Plain
FI
$I
IlL
L-
Vaux Seam
(No.17)
FI
Sh
'=>"
aI
Seam
'"z Broonie(No.f6)
'"
3
'">a:
SI
SP,Gs
370
360
FI
Stacked Point
Bars
Flood Plain
Sp
Stacked Point
350
Redbank Creek Seam
(No.24)
FI
..
1 ,.
Bars
Volcanic Ash
Back-Swamp
Flood Plain
340
Sl,Ss Distributary
Sp
Channel
330
Distributary
!!!.... Channel
Stacked Point
Sr.FI Bars with
Back-Swamps
FI
Back-Swamp
(No.15)
130
Sh
..
Wambo Seam
(No.23)
=>
a:
0
"=>
..
300
Ss St Stacked
Sr'
Channels
-FI
290
Sp,Sh Crevasse Splays
FI
Gh
Ss
Stacked Point
Bars
~r
Flood Plain
Sh
St,Ss POint Bar
Volcanic Ash
260
Saxonvale Claystone
Glen Munro Seam
(No.21)
Back-Swamp
250
"'f
240
10m No outcrop
Unnamed coal and bands
210
Piercefield Seam
200
(No.tS)
Falrford Claystone
Flood Basin
$I
270
Mt.Arthur Seam
(No.19)
Back-Swamp
with
Volcanic Ash
=- =_=.: _
....
Flood Plain
FI,Sr Back-Swamp
Flood Basin
with Crevasse
Splays
Sh
Stacked
Channels
POint Bar
Plain
FI Flood
Back-Swamp
Flood Basin
FI.Sh with Crevasse
Splays
_
~ Back-Swamp
POint Bar
Flood Plam
Back-Swamp
with
FI
Volcamc Ash
.
5'P~
c:
~
""
280
Woodland Hill Seam
(No.2 0)
a:~",
w=>"
~
aI
~
w",O
ii:(!Jj::
320
310
'"z
'"
5
'">a: Blakefield
Seam
(No.22)
9
Bayswater Seam
z
(No.14)
Wynn Seam
(No.13)
No.12 Seam
No.11 Seam
Flood Plain
Flood Basin
Back-Swamp
§L
Stacked Point
Bars
FI,r
Flood Plain
Sh
POlOt Bar
FI
Flood Plain
IlL
POint Bar
Fl,r
Flood Plain
Crevasse Splay
FI
Sh
FI
Gs
c:
Flood Plain
"
""
...:?l
;ll
Flood Basin
Gs
$I
I
Pomt Bar
Wash over Fan
Back Barrier
Swamp
Washover Gravel
>
II
>
1
Z
~
,,>
""
"'"
SI
Stacked Barner
;~
Beaches
0
SI
z
FI.r Shoreface
-==--MARINE TRANSGREsSiON
FI
Flood Plain
Back-Swamp
S,
glth Crevasse
plays
Sm
I
POint Bar
...:?l>
rut.
>
FI
II
Z
!ij
;ll
"
Flood Basin
~>
Sh
.
">
r
Z
POint Bar
=>
a:
0
Ss
"=>
aI
'"w No. fa Seam
>
"' No.9 Seam
Z
FI
s;;- POint Bars
FI
PeaUand
Ss
Sh
Dlstnbutary
Channels
!!!....
NO.8 Seam
No 7 Seam
FI
Sh
NO.6 Seam
No 5 Seam
Interdistributary
Bay with
Crevasse Splays
Sh
FI
s;-
Interdistrlbutary
Bay with
Peatlands and
Crevasse Splays
+
Crevasse Splay
Interdlstnbutary Bay
Dlstnbutary Channel
Peatlands In
Interdlstnbutary Bay
0
~
"
~>
;2
>
Z
I
Stratigraphic Bottom of Sechon
Fig.7.70. Stratigraphic section of part of the Wittingham Coal Measures in the Upper Hunter
Valley, Sydney Basin, New South Wales. (After Diessel and Stoddart 1986)
Coal-Producing Sedimentary Environments
446
and the Bayswater Seams in the Wittingham Coal Measures ofthe Sydney Basin, or
its stratigraphic equivalent, the Arkarula Sandstone, in the Gunnedah Basin
(Britten and Hanlon 1975; Hamilton 1985). The relevant portion ofthe stratigraphic
section is shown in Fig. 7.70 between 110 and 140 m. Before the onset ofthe marine
transgression at 110m in Fig. 7.70, the lower delta plain of the underlying Vane
Formation appears to have been replaced by an upper delta plain or alluvial valley
setting which was to continue throughout the deposition of the Jerrys Plains
17771 Area (approx.)
LLL..:I of un split seam
· . Area of
O
. . . Archerfield-Bulga
NEW ENGLAND
FOLD BELT
Formation and
correlatives
____15 Isopachs
LACHLAN
FOLD BELT
N.T.
I
I
~- __ -.i. ...., BOWEN
,
SA
! BASIN
f-- __
GUNNEDAH
a
500
~
km
a
100
I
I
km
Fig.7.71. Sketch map of the distribution and isopachs of the regressive marine Archerfield-Bulga
Formation and its stratigraphic correlatives in the Sydney and Gunnedah Basins. Also indicated
are the regional names of the first coal seam overlying this marine horizon. A-B gives the position
of the section illustrated in Fig. 7.72. (After Hunt et al. 1986)
447
The Barrier Beach/Strand Plain System
Formation. Within this sequence, the combined Archerfield-Bulga Formation
indicates a short-lived marine transgression of wide distribution represented by only
a few metres thick, bioturbated shoreface and foreshore deposits, in which McHugh
(1984) recorded elevated boron contents, compared with the over- and underlying
strata. The transgression occurred apparently in response to a eustatic sea-level rise
(Hamilton 1985), although Brakel (1984) also suggests a tectonic influence. It flooded
peatlands as far inland as the Gunnedah Basin 200 km to the north, and also affected
parts of the Bowen Basin in Queensland, before retreating into the more rapidly
subsiding eastern sector of the Sydney Basin as indicated by the isopach distribution
shown in Fig. 7.71.
Near the split axis illustrated in Fig. 7.72 the thin pebbly ravinement deposit
called "basal conglomerate" constitutes the whole of the transgressive portion of the
combined Archerfield-Bulga Formation. It forms a lag which comprises the
remnants of beach deposits which, in the course of the marine transgression, have
been successively destroyed and rebuild up-palaeoslope. The underlying erosional
contact with the Wynn Seam, possibly formed on an abandoned and partially
rewor·ked delta platform in a back barrier setting, rises in the direction of the split
axis, which is clearly indicated by its position relative to two bands of volcanic ash
(Wynn Tuff 2 and 6). The overlying bioturbated siltstone and laminated sandstone
UPPER BROONIE SEAM
•
o
coal
___ erosion
shale
E2l
bore No
v
worm burrows
~ siltstone
D".. fine}
o
medium
0.70
0.41
FI
Channels
Flood plain
TPI (loft)
GI (right)
~ conglomerate
l
SI
Channel
FI
Flood plain
St
Channel
sandstone
~coarse
a
Flood plain
FI
tuff
Sh
Channel
OM
Sh
Channels
O.
BAYSWATER SEAM
,"M
ST.
E7
E21
E"
E2.
E22
E3.
Fig. 7.72. Section through the northeastern terminus of the Archerfield-Bulga Formation along
the section line A (left)-B (right) in Fig. 7.71. Note the tissue preservation indices (TPI) and
gelification indices (GI) to the left and right of the coal columns, respectively. (After Uren and Diessel
1986)
448
Coal-Producing Sedimentary Environments
of the lower shoreface (Bulga Formation) grades upward