Sedittietitology ( I 98 1 ) 28, 487-504
Anoxic events during sedimentation of a Palaeogene diatomite in Denmark
GUNVER KRARUP PEDERSEN
Geologisk Museum, Ostervoldgad~5-7, DK-1350, Kpfbenhavtr K , Diwnark
ABSTRACT
A diatomite, about 60 m thick, of late Palaeocene-early Eocene age crops out in northern Jutland,
Denmark. The diatomite is locally termed 'Moler'. Frustules of marine diatoms constitute c. 65
(by weight) of the diatomite, and clay minerals, chiefly montmorillonite, make up the remainder.
Slight variations in the relative supply of diatom frustules and clay minerals are preserved undisturbed
in laminated diatomite, while lamination is partly destroyed by burrowing organisms in weakly
laminated diatomite and obliterated by total bioturbation in structuretess diatomite; these three
facies alternate throughout the sequence.
The presence or absence of infaunal burrowing organisms is interpreted as a record of the content
of dissolved oxygen in the water above the sediment-water interface and hence of the position of the
redox potential discontinuity.
Interspaced in the diatomite are 179 identifiable layers of volcanic ash. These ash layers provide a
means of precise lateral correlation. They show that levels of laminated diatomite may be followed
throughout the basin and therefore that changes between anoxic and oxic conditions occurred
simultaneously across the area. The laminated diatomitesmayconsequently be interpreted as representing short-term anoxic events, of which twelve have been recognized.
'I,,
INTRODUCTION
Black, laminated, organic-rich sediments normally
reflect deposition under anoxic conditions. Anoxic
events are distinguished where anoxic conditions
alternate, more o r less isochronously, with oxic
conditions.
Oceanic anoxic events are recognized in deepwater sediments of Cretaceous age (Schlanger &
Jenkyns, 1976; Fischer & Arthur, 1977; Thiede &
van Andel, 1977); stagnant episodes are traced in
Quaternary sediments in the eastern Mediterranean
(Ryan, 1972; Ryan & Cita, 1977; Thunell, Williams
& Kennett, 1977; Stanley, 1978); and black laminated shales predominate within certain stratigraphical levels in Jurassic shelf sediments (Hallam,
1978; Hallam & Bradshaw, 1979).
The present study concerns a Palaeogene diatomite
(Hansen, 1979) deposited in a shelf area. The 60 m
0037-0746/81/0800-0487 $02.00
0 1981 International Association of Sedimentologists
thick sequence is uniform in general lithology with
small variations in fauna. The sedimentation ratewas
low but breaks in sedimentation have not been
observed. The recognition of anoxic conditions is
based on sedimentary structures and not on the
content of organic matter. As the diatomite is interspaced with numerous isochronous marker horizons
(layers of volcanic ash), a unique opportunity exists
to study the lateral and vertical extent of the anoxic
conditions. Detailed sedimentological logs of the
diatomite measured a t the different exposures form
the basis of the present investigation, which may
contribute to current investigations of the depositional environments of laminated, black, organicrich shales alternating with bioturbated shales in
epicont inental seas.
The above-mentioned s'tudies of anoxic events
have shown that climatic conditions favouring the
development of a strong oxygen-minimum layer are
488
G. K. Pedersen
MIOCENE
OLIGOCENE
EOCENE
@ YOUNGER
PALEOCENE
DANIAN
@ UPPER
CRETACEOUS
DIATOMITE EXPOSURE
x
DIATOMITE OCCURRING
I N A WELL
H A R H 0 J 7 NAME AND NUMBER
OF
LOCALllY WHERE A
SEDIMENTOLOGICAL LOG
HAS BEEN MEASURED
Fig. 1. Locality map of the area studied. The two squares delineate the areas shown in detail on the geological
map (Fig. 1, inner square) and the structural map of the Base Upper Cretaceous reflector (Fig. 2, outer square).
The geological m a p of the pre-Quaternary of northwest Jutland is taken from Schierling & Jensen
(1974). Exposures of the Moler diatomite are shown together with the names of those exposures where sedimentological logs have been measured (Fig. 1 I ) . The occurrence of diatomite in wells is registered at the
Geological Survey of Denmark.
Sedimentation of a diatomite
a likely contributing factor in the development of
ocean-wide synchronous deposition of organic-rich
shales. A circulation pattern which is only partially
determined by bottom topography (Stanley, 1978)
may attain great importance in a marginal sea like
the Mediterranean. On the continental shelf, bottom
topography is thought to be a factor which exerts
considerable influence on the deposition of laminated
sediments (Hallam & Bradshaw, 1979).
GEOLOGICAL SETTING
During the Palaeogene, the DanishSub-basin formed
part of the extensive shelf areas flanking the Central
Graben of the North Sea (Ziegler, 1975, fig. 1). It
was characterized by subsidence throughout the
Tertiary accompanied by a general regressive movement of the coastline from east to west. Mesozoic
subsidence was controlled by movements along the
faults in the Fennoscandian Border Zone and the
faults flanking the Ringkerbing-Fyn High (Baartman
& Christensen, 1975; Michelsen, 1978; Rasmussen,
1978).
The structural map of the base of the Upper
Cretaceous (Fig. 2) is compiled from seismic profiles
and shows large topographical variations which
recur in the top Cretaceous reflector (Section AB,
Fig. 2). The map clearly shows the influence of salt
diapirs, but the major trends in the distribution of
structural highs and lows are probably to some extent
controlled by older, regional faults. The diatomite
was deposited (Fig. I ) on the northern palaeoslope
of the basin (Section AB, Fig. 2).
The diatomite is exposed as floes, dislocated and
folded by glaciers during the Quaternary, but there
is no evidence of transport over considerable
distances (Gry, 1940).
Upper Palaeocene-Lower Eocene sediments in the
Danish Subbasin are fine-grained, montmorilloniterich clays (Tank, 1963; Dinesen, Michelsen &
Lieberkind, 1977). Diatoms are commonly found in
them and in contemporaneous sediments in northern
Germany (Benda, 1965), but true diatomites, such
as the one which is the subject of this paper, are
restricted to north-west Denmark.
489
logs of Fig. 11 are located in Fig. 1. The sediments in
the formation are described under three headings :
(1) volcanic ash layers, (2) diatomite, and (3)
calcareous concretions, which coctain both diatomite
and layers of volcanic ash.
Volcanic ash layers
The thickness of the numbered ash layers ranges
from 1 to 20 cm, but several thinner ash layers also
occur (Berggild, 1918). The ash layers are mostly
black, and are composed of glass particles with an
average grain size within the very fine sand and silt
fractions.
All the ash layers have graded bedding, and no
sedimentary structures showing transport and redeposition by currents on the sea-floor have been
observed. Water escape structures have been
described from some layers(Pedersen &Surlyk, 1977).
The ash layers were formed from airborne ash
particles that settled grain by grain through the water
column; their depositional rate was high. The
amount of volcanic glass dispersed in the diatomite
is negligible. which shows that deposition of volcanic
ash only took place immediately after a major
eruption, and that re-working was negligible.
Andersen (1937) showed that individual volcanic
ash layers may be recognized throughout the Danish
Sub-basin. The volcanoes are thought to have been
in the Skagerrak (Fig. I ) : Berggild (1918), Andersen
(1937) and Am (1973). The petrology of the ash has
recently been described by Pedersen, Engell &
Rransbo (1 975).
The ash layers are of great stratigraphical importance for the following reasons: they were airborne,
sedimented vertically within a very short time, were
not redeposited and thus constitute isochronous
horizons. The thickness of the individual ash layers
varies but each has a constant thickness through the
diatomite basin. The thickness of diatomite between
two ash layers showsonly minor variations. Therefore
the ash layers are identifiable and a detailed tephra
chronology with 179 numbered ash layers has been
established by Berggild (1918).
Diatomite
THE S E D I M E N T S
Diatomite exposures are accessible in coastal cliffs
and disused parts of quarries. The sedimentological
The diatomite is a non-calcareous sediment in which
the opaline frustules of marine diatoms (Benda,
1972) constitute c. 65 yo by weight (Pedersen, 1978).
The diatom frustules vary considerably in size.
G . K. Pedersen
491
'
B
om,'+
1000
A
1
YT
-m-o-o - -
BUG
TC Top Crelaceous
B U G Base upper Cretacequs
Fig. 2. Structural map of the Base Upper Cretaceous seismic reflector. Sketch map drawn from
seismic material stored at the Geological Survey of Denmark. The line of Section (AB) shows that where a
T o p Cretaceous reflector may be followed this is roughly parallel to the B.U.C. reflector. This indicates
that a substantial part of thesubsidence ofthe central structural low area took place in the Tertiary. The
diatomite (compare Fig. I ) was deposited on the slope between the northern stable area and the southern
subsiding area.
Seclimentatioti of a diafotnife
49 I
Fig. 3. Laminated diatomite. There is no recognizable pattern in the alternations between light and dark
laminae. Scale in centimetres.
Stephanopyxis dominates among small forms (2040 pm), while Cosciriodiscirs is the dominant large
form (100-200 pm).
The clay mineral content of the diatomite has
been calculated from chemical analyses of bulk
samples, and X-ray diffraction measurements of the
fraction < 2 pm. Montniorillonite dominates the
clay mineral assemblage throughout (Pedersen,
1978). The total clay mineral content of about 35 1;)
is fairly constant through most of the diatomite
sequence, but somewhat higher in the lowest and the
highest parts.
The diatomite is divided into three facies: laminated, weakly laminated and structureless. Even
lamination i s the only primary sedimentary structure
observed, which indicates that the fine-grained sediment was deposited below wave-base and not eroded
and redeposited o n the sea-floor. Furthermore the
absence of larger slump structures suggests that the
bottom topography was smooth.
O n weathered surfaces the colour of both laminated and structureless diatomite is white with
greyish, pinkish o r yellowish tinge. Colour variations
which might be attributed to primary variations in
the content of marine or terrestrial organic matter
o r pyrite d o not distinguish the facies. Therefore,
and because sufficiently fresh ssniples are generally
lacking, only two samples have been analysed for
their total carbon content ( = organic carbon). The
six measurements of each sample were made in a
Leco Analyser : laminated diatomite, 1.54 & 0.13 ( l o
C ; structureless diatomite, 1.33 +O.I6(,!,,C.
Lamination in the diatomite
Description
The lamination is seen as altcrnating white, whitish
and brownish milliinetre-thick laminae composed of
diatom frustules and clay (Fig. 3). The relative
density of diatoms varies from lamina to lamina, in
no recognizable pattern (Fig. 4). Three types of
laminae have been distinguished:
Type 1 : laminae almost exclusively composed of
diatom frustules usually dominated by a single
species, either Coscinodiscus sp. (Fig. 6a, b) or
Sky~hanopyxissp. (Fig. 6c). The laminae are white
and more than 800,:)are less than 0.5 niin thick; they
constitute 1-13% by volume of the laminated
diatomite (Fig. 5 ) .
492
G . K . Pedeuseri
Type 2: laminae containing both diatoms and clay
minerals, with thicknesses from 0.25 mm to more
than 4 mm (Fig. 4). The diatoms are evenly distributed within the laminae, but their density may
vary from one lamina to the next, and several species
of diatom are present (Fig. 6d, e). The colour is
yellowish to pale brown. Several type 2 laminae, distinguished by subtle differences in colour and content of diatom frustules, may occur in succession
before they are interspaced by laminae of types
1 or 3. Type 2 laminae constitute 80-90y0 of the
laminated diatomite (Fig. 5 ) .
Type 3: clay-rich laminae with few diatom
frustules (Figs 4, 6f). More than 50% of such
laminae are 0.25 mm or thinner, and they are usually
light coloured or white. Generally type 3 laminae lie
between pairs of type 2 laminae; they constitute
1-1 1 yo of the laminated diatomite (Fig. 5).
Discussion
The type 1 laminae may have been produced either
by large blooms of particular species of diatoms o r ’
through selective dissolution of the frustules. In
present-day diatom floras, selective dissolution
accounts for differences observed between assemblages in the upper part of the water column and in
the sediment. Up to 90% of the frustules may be
dissolved depending on water depth or whether the
frustules are incorporated in faecal pellets (Calvert,
1966b; Schrader, 1971, 1972). Differential solution
can produce diatom-rich layers dominated by a
single species (Mikkelsen, 1977).
In the present case, however, the various diatom
species seen in type 2 laminae must all be solutionresistant. Since only a few of these species occur
abundantly in type 1 laminae, it is inferred that these
are the results of large blooms and have not been
produced by differential solution.
Extraordinary supplies of clay minerals from rivers
may have produced the type 3 laminae, as there are
no signs of formation through resuspension and
resedimentation of type 2 laminae. Similar types
of deposit have been described from the Santa
Fig. 4. Section through laminated diatomite, drawn from
thin-section. Lamina types are indicated by the numbers
to the right of the column. The density of diatom frustules
shown reflects the variations seen in the thin-section. The
matrix consists of clay minerals and diatom fragments A
that are too small to be identified under the microscope.
Sample 32433,20 cm below ash layer - 17, Skarrehage.
Way u p of the sample is not known.
DIATOMS 20-40pm IN D I A M E T E R
DIATOMS 150-200pm IN D I A M E T E R
493
Sedimentation of a diatomite
Lamina type (vol. %)
1
3
2
Sample
0
7
Stratigraphical position)
Between +22 and +35 Calc. concr.
Below 16 Calc. concr.
Below + 16 Diatomite
2 m above - 17 Diatomite
20 crn above - 17 Diatomite
260 cm below - 17 Diatomite
275 cm below - 17 Diatomite
+
0
oo
0
0
Thickness
55
13
7
13
131
92
103
50
74
108
82
5
91
1
6
11
6
81
88
86
Few thin laminae
Thin laminae
Dominant
Dominant
Several thin laminae
Few thin laminae
1
8
Fig. 5. Distribution of the lamina types within samples of laminated diatomite, within or outside calcareous
concretions. I t is evident that laminae of type 2 dominate all the samples. Type 1 laminae composed of
Coscinodiscus sp. are usually found in the lower part of the sequence, and Stephanopyxis sp. dominate type
1 laminae in t h e upper part of the sequence. The preparation of the two lowest samples was unsuccessful
so only rough estimates of the lamina distribution could be obtained.
Barbara Basin, California by Fleischer (1972). Type
3 laminae could also represent levels of increased
diatom dissolution.
The relatively thick and heterogeneous type 2
laminae are interpreted as representing continuous
‘background’ sedimentation through long periods of
time. This was irregularly interrupted by the deposition of types 1 and 3 laminae, which are interpreted as representing unusual events during diatomite deposition.
Finely laminated sediments which have been
interpreted as varves are all characterized by a
regular alternation of two lamina types (Seibold,
1958; Hulsemann &Emery, 1961 ; Gross et al., 1963;
Calvert 1966a; McLeroy & Anderson, 1966; Wilson,
1977). Bonde (1974) interprets the diatomite described herein as predominantly laminated. He
regards the light laminae as diatorn-rich layers
deposited during the summer, and the darker laminae
as more clay-rich and deposited during the winter.
The diatomite was thus interpreted as composed of
varves with an average thickness of 1-2 mni. No data
were, however, presented in support of this hypothesis.
I t is clear from the present investigation that the
diatomite does not possess such a regular alternation
of lamina types (Fig. 4). Furthermore, the white
laminae (types I and 3) may be either enriched o r
depleted in diatoms with respect t o the darker
laminae with which they are intercalated. From the
present interpretation of the lamina types it is not
necessary that the events producing the laminae
types 1 or 3 occurred once a year. Bonde’s (1974)
interpretation of the diatomite as varved thus cannot
be supported by the present investigation.
Diatomite facies
Laminated diatomite
This facies is characterized by distinct lamination
(Fig. 3). Layers of volcanic ash possess sharp upper
and lower bounding surfaces (Fig. lOC, I). Wellpreserved fishes and land-derived insects are found;
benthic organisms are virtually absent although a
few ophiuroids are recorded from one exposure (C.
Heinberg, 1977, personal communication). Laminated diatomite constitutes 3 8 % of the diatomite
sequence below ash layer + I , but only 40/1 of the
sequence above (Fig. 11). In vertical sequence,
laminated diatomite passes transitionally into weakly
laminated diatomite o r abruptly into structureless
diatomite.
The preservation of the thin primary laminae in the
slowly deposited diatomite, and the presence of wellpreserved very delicate fossils leads to the conclusion
that it was deposited a t times when scavengers and all
benthic infaunal organisms were absent. Benthic
epifaunal organisms, othef than scavengers, may have
been present at times.
-
Fig. 6. Lamina types in laminated diatomite.
(A) Lamina type 1 with abundant large diatom valves, Coscinodiscus sp.
(B) Lamina type I , detail, showing a matrix of small or fragmented diatom frustules and clay between the
large valves of Coscinodiscus sp.
(C) Lamina type 1 with abundant small diatom frustules, Stephunopyxis sp.
(D) Lamina type 2. Coscinodiscrissp. scattered in a matrix of clay, diatom fragments and small diatom species.
The density of Cuscinon'iscus sp. valves is much lower than in the type I lamina (A).
(E) Lamina type 2. Trinacriu sp. in a matrix of mainly fragments of diatom fi-ustules. A few silicoflagellarec
are also seen.
(F) Possible type 3 lamina with fragments of diatoms, clay and complete specimens of small dicitonis,
Scepfrorieis sp. and Sfephonopyxis sp.
Sedimentation of' a diatomite
Weakly laminated diatomite
This transitional facies may be developed as millimetre-thin, laterally continuous laminae interspaced
with homogeneous layers up to 1 cm thick (Fig. 7).
It may also consist of diatomite with an evenly
distributed, vague, discontinuous lamination. Ash
layers may be bounded by planar-gradational contacts, or ash-filled burrows may occur (Fig. 10B, E,
H, K). Weakly laminated diatomite comprises 15yo
of the diatomite sequence below, and 404 above, ash
layer 1 (Fig. 1 I).
+
Structureless diatomite
Primary structures have not been detected in this
facies either in the field, in thin-sections or by Xradiography. The sediment is a homogeneous
mixture of clay minerals and diatom frustules (Fig.
8). The ash layers within this facies reveal trace
fossils belonging to the ichnogenera Teichichnus,
Planolites, Chondrites and Taenidium (Pedersen,
1978). Similarities in diameter and the existence of
transitional forms between Planolites and Teichichnus
suggest that they were produced by closely related
animals. The trace fossils are usually seen as ashfilled burrows extending down from the ash layers
(Figs 8,9). Trace fossils passing from diatomite down
into volcanic ash may also be seen (Fig. 10D, I, K).
Under favourable conditions trace fossils may be
seen within the diatomite. The trace fossils are
fodinichnia produced by infaunal deposit feeders
(Hantzschel, 1975), which in the literature are most
often reported from silty/clayey, organic-rich sediments (Fursich, 1975; Baldwin, 1977; Pickerill, 1977).
The trace fossils therefore would be expected to be
more common in the diatomite than in the ash layers,
whereas the reverse is observed. The trace fossils are
most common between ash layers 36 and 1 18.
Only a few impressions of pelecypod and gastropod shells are known from the structureless diatomite, although in some calcareous concretions from
the level below ash layer - 13 numerous gastropod
shells are preserved as calcium carbonate.
Structureless diatomite constitutes only 449; of
the diatomite sequence below, but 84:
above, ash
layer + 1 (Fig. 11).
It is assumed that diatotnfrustules and clay through
the whole diatomite sequence were deposited at the
varying rates seen in the laminated diatomite. Therefore the homogeneity of the structureless diatomite is
interpreted as the result of total bioturbation.
+
+
495
Teichichnus and Taenidium may be observed to
have burrowed to a minimum depth of 8-10 cm. The
intense bioturbation, however, is attributed to other
organisms living in the upper millimetres of the
sediment, because even thin layers of volcanic ash
are preserved undisturbed in the structureless
diatomite.
Discussion
Fenchel & Riedl (1970) defined the redox potential
discontinuity (RPD) as the layer above which dissolved oxygen is present even if in small amounts,
and below which H,S is contained in the pore fluids
in the sediment. The RPD layer thus represents an
equilibrium surface between oxygen consumption
and oxygen supply; it is normally located close to
the sediment/water interface in fine-grained sediments in low-energy depositional environments
(Fenchel & Riedl, 1970).
If the position of the RPD layer in these diatomaceous sediments was determined by grain size, the
deposition of the coarse-grained layers of volcanic
ash should be expected to shift the RPD layer downwards and make colonization of the sediment
surface by burrowing benthos possible. Fig. 10 shows
schematically the possible transitions between the
diatomite facies and the ash layers. The absence of
possibilities F, G and J (Fig. 10) indicates that the
position of the RPD layer was not governed by grain
size. It seems that the ash layers only serve. by way of
their lithologic contrast, to outline the trace fossils,
and are not directly involved in their formation.
Rhoads & Morse (1971) have shown that contents
of dissolved oxygen in the bottom water of less than
0.1 m10, I-' inhibit the presence of burrowing
animals. At contents of 0.3-1 .0 ml 0, I- infaunal
organisms without calcareous shells may exist, while
calcareous benthos are usually found where the
content of dissolved oxygen exceeds 1.0 m l 0 , I-'.
The absence of infaunal organisms in the laminated diatomite is ascribed to concentrations of
dissolved oxygen less than 0.1 ml 0, [ - I , at which
state the RPD layer would nearly correspond to the
sedimentlwater interface. The very rare presence of
ophiuroids in the laminated diatomite shows that the
bottom water at times contained small amounts of
dissolved oxygen which were restrictive to burrowing
benthos but not to a sparse epifauna. This might
correspond to the model proposed by Kauffman
(1978) for the Posidonia Shale, however, the laminated diatomite generally presents a picture of a sea-
49 6
G . K . Pedersen
Fig. 7. Weakly laminated diatomite. The sample has been stained with linseed oil to increase the contrast
between laminae. 300 cm below ash layer - 13, Skarrehage.
floor so devoid of life that it seems likely that the
bottom water was often poisoned by H,S, and that
the RPD layer lay so high above the sediment surface
that it was not disrupted by deposition of volcanic
ash (Fig. ]OF, G and J).
In the structureless diatomite the presence of
burrowing benthos is solely ascribed t o a sufficiently
high content of dissolved oxygen in the water above
the sediment/water interface. Therefore the structureless diatomite reflects oxic conditions when the
content of dissolved oxygen probably exceeded
0.3 nil 0, I-' but seldom exceeded 1.0 O2 1-l, as
indicated by the extreme scarcity of calcareous
benthos. The trace fossils indicate that the diatomite
may have been inhabited by a relatively high number
of organisms belonging to few genera and that the
assemblage of trace fossil-producing animals did not
change through time. This suggests that the diatomite
was deposited in a high-stress environment, and it is
proposed that the limiting factor was the content of
dissolved oxygen. The sedimentological features and
trace fossil content of the structureless diatomite are
the same where thin beds of this facies are interbedded within laminated diatomite as in thick
homogeneous beds. This may indicate that the content
of dissolved oxygen was continually low.
Calcerous concretions
Calcareous concretions are found at certain levels
in the diatomite (Fig. 1 1 ) . The close control pro130 reveals
vided by ash layers between + 1 and
that concretions are restricted to particular levels in
the sequence throughout the basin. Within these
levels, however, the size and number of the concretions may vary. Below ash layer + 1 the concretions are probably confined to distinct levels too,
but this is difficult to prove as the ash layers are
fewer and more widely spaced. The concretions are
usually ellipsoids with a maximum size of 1 x 0.5 m,
but continuous layers occur locally.
+
497
Sedimentation of a diatomite
Fig. 8. Structureless diatomite below ash layer
Teichichnus. Silstrup.
+ 36. The ash filled burrows belong to the ichnogenus
The concretions are generally found in Iaminated diatomite containing no ash layers or a few
thin ones (Fig. 11), with the concretions in ash
layers + 101/102 as a notable exception.
The diatomite outside the concretions is only
slightly more compacted than within them. The
relative age and the mode of formation of the concretions have not been studied in detail. Baggild
( I 91 8) suggested that basin-wide deposition of
calcareous microfossils, now dissolved, may have
made certain stratigraphical levels more favourable
for the formation of concretions.
The concretions described here resemble those
occurring in the Monterey Formation (Bramlette,
1946) except that there calcareous microfossils are
preserved within the concretions as well as in
adjacent beds.
MICROFOSSILS
Ninety species of diatoms are known (Benda, 1972)
and they are the main sediment component in the
formation. Radiolarians, silico-flagellates and dinoflagellates are present and have been used in strati-
graphical studies (Perch-Nielsen, 1976; Hansen,
1979) while calcareous microfossils are absent.
MACROFOSSILS
The diatomite contains remains of marine planktonic and nektonic organisms, as well as landderived fossils.
The fossils are extremely rare but their state of
preservation is generally good especially within the
calcareous concretions. Fossils in thecollections have
to a large extent been obtained from loose concretions lying at the foot of the cliffs, so that only part
of the fossils can be referred to exact stratigraphical
levels. The collections may be further biased through
the fact that calcareous concretions formed predominantly in laminated diatomite.
Among the invertebrates Ophiuriu furiae and two
very rare asteroids are known (Rasmussen, 1972),
the gastropods Fusinus sp. and Spirutella mercinensis
are fairly common while pelecypods are rare (Bonde,
1979). All these invertebrates are found almost
exclusively in the structureless diatomite (Bonde,
49 8
G. K. Pedevsen
+36
(Bonde, 1966). The remains of three turtles (Nielsen,
1959, 1963) and 14 land birds are described by Hoch
(1 975).
The relatively high proportion of land-derived
fossils indicates that the diatomite was deposited at
no great distance from the coast. Bonde (1974)
suggests that the Fennoscandian Border Zone constituted a land area approximately 150 km away.
DURATION OF DIATOMITE
DEPOSITION
+35
.
.. ; . ; ,’:,,.-I<.
.,..
%>,,.
....
,,
......
\,
.
. . .
. .
. . . ..,‘,:
. . . . , ’ . . . ,
~
,......
O . . ’ . . , ,I .
,
.
, -33
Fig. 9. Below ash layer +35 the diatomite is laminated,
and volcanic ash layers 34 and 35 are sharply delineated
by plane surfaces. Above ash layer 35 the diatomite is
+
structureless and contains numerous ash-filled burrows
(dominantly Teichchnus). The lower boundary of + 3 6 is
well defined because of the contrast in both colour and
grain size, but the upper boundary is gradational.
Silstrup. Drawn from photograph.
1979), which supports the interpretation of the
laminated diatomite as devoid of benthos.
Plants and insects are relatively frequent (Larsson,
1975). Besides rarer well-preserved trunks, branches
and leaves, small fusain fragments of terrestrial plant
material are present in all facies. In the insect fauna
wingless forms and larvae are completely lacking,
while very good flyers and small winged insects are
under-represented, and relatively large and heavy
insects dominate. This suggests that the insects were
transported by wind to the place of deposition, and
that a sorting of the original insect assemblage took
place. The preservation of the insects indicates that
scavengers were very rare or absent both on the seafloor (anoxic bottom water) and in the water
column, where they may have been killed by ‘red
tide’ poisoning (Larsson, 1975, p. 199).
The diatomite has yielded many vertebrate fossils.
Fishes are relatively common, especially a little
argentinoid (Bonde, 1979). Most fishes are fisheating, oceanic forms living in moderately deep water
(50-500 m), while fishes living near the bottom or
forms feeding on shelled invertebrates are absent
Sharma (1969) found three magnetic reversals in the
volcanic ash layers, and on the basis of the average
length of a normal or a reverse polarity calculated
that the sedimentation of the diatomite probably
lasted 3 m.y. This estimate was supported by a
stratigraphical study (Perch-Nielsen, 1976) but it has
Iately been challenged by Bonde (1974, 1979), who
favours a much shorter period of diatomite deposition.
Indirect evidence in support of more than one
million years of diatomite deposition exists in the
volcanic ash layers. The similarities between the ash
layers in thickness and grain size, as well as isopach
maps of individual ash layers (Andersen, 1937)
strongly suggest that all the ash originated from the
same volcanic region (in the Skagerrak) though not
necessarily from the same volcano.
Pedersen ct a/. (1975) found that four stages of
volcanism produced the ash layers. The stages were
characterized by different magmatic compositions
and varying intensities of volcanic activity. Stage
two resembles the products of continental rift
volcanism, while stage four is represented by more
than 110 layers of tholeiitic composition produced
by a regional upwelling of magma, probably caused
by a small mantle plume. The fourth stage reflects
intense volcanic activity and may have been produced within a relatively short period of time. It is,
however, highly improbable that the four distinguishable stages of volcanic activity should have
succeeded each other within tens of thousands of
years; a period of one or a few million years is far
more likely (A. K. Pedersen, 1978, personal communication). As breaks in sedimentation of the
diatomite have not been observed the average rate of
deposition must have been low.
Bonde (1 974) interpreted the diatomite as composed of varves 1-2mni thick and assumed that
laminated diatomitewas the dominant facies through-
499
Sedimentation of a diatomite
Structureless diatomite
+
Weakly laminated diat.
++
Laminated diatomite
Volcanic ash
++
+/ -
r6
Burrows
The illustrated cases are:
+/-
++
+
-
+
t
+/-
frequent
occasional
rare
absent
Fig. 10. Examples of existing and hypothetical relationships between ash layers and diatomite facies.
A-F illustrate ash layers within one diatomite facies, while G-K illustrate situations where ash layers
separate two different facies. The frequency with which the illustiated cases occur in the measured sections
is indicated.
A illustrates that intensive bioturbation may be ascribed to organisms living close to the sediment surface
(cf. D).
A, B and C show that all diatomite facies may be bounded above or below by ash layrrs, with no sign
of bioturbation, and such !ayers may also separate any two different facies. Therefore only facies transitions
that might conceivably correspond to a bioturbated ash layer are illustrated in G-K. Non- or slightly
bioturbated diatomite rarely overlies bioturbated ash layers (G, H); this, together with the evidence of I ,
J and K , indicates that ash layers were burrowed by benthos inhabiting the overlying diatomite.
That the presence or absence of burrowing benthos was determined not by grain size but by the content
of dissolved oxygen is deduced from: ( I ) the fact that possibilities C, F and 1 are frequent, absent and occasionally occurring respectively, and (2) from the contrast between I, J, K and D.
out the deposit. He then calculated by extrapolation
that the c. 60 m of diatomite was deposited within
60,000 y. No data were presented to substantiate this
hypothesis. The present investigation does not
support the hypothesis that the lamination represents Val-ves, and shows that laminated diatomite is
not the dominant facies. The calculations of Bonde
must accordingly be rejected.
ANOXIC EVENTS DURING
DIATOMITE SEDIMENTATION
The most complete section through the diatomite
sequence was obtained by combining thc intervals
between ash layers - 34 and 118 at F u r Knudeklint
+
32
and between +118 and +140 at Silstrup Sydklint
(Fig. f I ) . Exposures of diatomite below ash layer
- 34 are not available for study.
The ash layers form isochronous marker horizons
separating diatomite intervals cf varying thicknesses.
In order to compare the lateral and verrical changes
in diatomite facies, the variations in thickness of the
diatomite from one exposure to the next are compensated for by adjusting the distances between any
two a.sh layers to the distance between the same ash
layers measured in F u r Knudeklint or Silstrup
Sydklint. The intervals of laminated or structureless
diatomite are adjusted proportionally (Fig. I I ) .
The same sequence of vertical facies transitions
can be recognized from one exposure to the next.
Deposition of laminated diatomite thus characterizes
S I . 1)
2s
1
+ 51
:%-
+
0
-
I
4
~
. ....................
u-
1
-"x,?.,
,
3
,
~~.~
"
il
A
9
...
I
-
Fig. 11. For legend see opposite.
5
L.-J
-
6
9
l---
10 rn
LOCALITY NUMBER
ASH LAYER
CALC. CONCRETION
0
+I
OXlC
A N O X I C EVENT
DWEAKLY
ANOXIC
BOTTOM WATER
0STRUCTURELESS
WEAKLY LAMINATED
LAMINATED
DIATOMITE
0
0
VI
501
Sedimentation of a diatomite
certain stratigraphical levels throughout the basin
and is accordingly interpreted to reflect basin-wide
anoxic events.
The existence of isochronous ash layers in the
diatomite allows the recognition of the following
details about the anoxic events.
( 1 ) The change between oxic and anoxic conditions
happened simultaneously throughout the basin.
(2) Laminated and structureless diatomite predominate over transitional, weakly laminated diatomite, indicating that the change from anoxic to
oxic conditions was relatively swift.
( 3 ) The rate of diatomite deposition is seen to have
varied throughout the basin, independently from
the distribution of the diatomite facies. The
oxygen content of the bottom water thus seems
to have been less affected by varying rates of
deposition (varying organic production) than by
basin-wide, long-term fluctuations.
Twelve basin-wide anoxic events are recognized,
each represented by 40-400 cm of laminated diatomite.
Based on the estimate that the diatomite was
deposited during 3 m.y. (Sharma, 1969), the length
of anoxic events is calculated to have varied between
22,000 and 126,000 years with an average of 75,000
years. These values are maxima, as 3 m.y. probably
is the maximum estimate of diatomite deposition.
caused by rising of salt diapirs and differential subsidence rates (Fig. 2) is suggested to be the feature
that influenced water circulation and created local
conditions of upwelling, leading to high biological
production.
Deposition of a diatomite requires a high production of diatoms, as up to 99% of the diatom
frustules may be dissolved before sedimentation
(Calvert, 1968). Consequently, large amounts of
organic matter are decomposed through oxygenconsuming processes in the water column and at the
sediment/water interface. The generally low contents of oxygen in the bottom waters during the
deposition of the diatomite probablywerecaused both
by decomposition of organic matter and by slow
circulation of bottom water. Anoxic conditions are
strongly favoured by a restricted circulation of the
bottom waters (Hallam & Bradshaw, 1979). The
development from mostly anoxic to more oxic
conditions through the diatomite sequence (Fig. 1 I )
corresponds to the gradual filling of a depression in
the shelf, according to the model proposed by Hallam
(1978).
The early Eocene climate was generally warmer
than the present (Buchardt, 1978) as is also indicated
by the fossils in the diatomite (Hoch, 1975). These
climatic conditions may also have favoured the
development of oxygen-poor bottom waters, through
the processes discussed by Fischer & Arthur, 1977.
THE DEPOSITIONAL
ENVIRONMENT
DISCUSSION
The diatomite was deposited in a shelf sea (Ziegler,
1975, Fig. 1 ) relatively close to a land area (Bonde,
1974, 1979). The fossils indicate moderately deep
water and the fine grain size and lamination suggest
low-energy conditions.
Bonde (1974, 1979) has presented a general model
of upwelling for a large part of the North Sea Basin,
but the diatomite deposition presuniably was
restricted to a much smaller area (Fig. 2), in which
the biological production was much higher than in
the surrounding areas. A submarine topography
Detailed descriptions of Cret?ceous sediments in
DSDP cores (sites 356, 357 from the South Atlantic,
Thiede & van Andel, 1977; and off the west coast of
Africa, site 367, Gardner, Dean & Jansa, 1978)
mention cyclic variations from black, laminated
sediments to green or brownish bioturbated shales.
Similar variations are reported from sites 146, 153,
249, 305 and 364 by Ryan & Cita (1977), and it
seems that the oceanic anoxic events are only an
extreme example of a constantly changing degree of
Fig. 11. To the left is a composite sedimentological log through the Moler diatomite. The sequence between
ash layers -34 and + 118 was measured at Fur Knudeklint (9 o n Fig. I), and the’sequence above + 118 at
Silstrup Sydklint ( I ) . The most important ash layers are shown, as well as the most prominent levels with
calcareous concretions. In the other logs the distance between any two ash layers is adjusted to the distance
in the composite log.
The distribution of the three diatomite facies is shown in each fog. The laminated diatomite is found at the
same levels throughout the basin.
32-2
G. K. Pederseti
502
oxygenation of the bottom waters. Gardner et at.
(1978) estimate the length of the cycles to be approximately 50,000 years and propose that climatic
variations may produce fluctuations in the amount of
organic material supplied to the sediment.
In their account of Jurassic epicontinental bituminous sediments Hallam & Bradshaw (1979)
mention (p. 160) that ‘regular alternations of
bituminous and non-bituminous shale units, each
ranging u p to about a metre in thickness’ (c. 40,000
years of deposition) may be observed.
The duration of the anoxic events during deposition of the Palaeogene diatomite is here calculated
as tens of thousands of years, and is thus of the same
magnitude as the figures given by Gardner el a/.
( I 978) and Hallam & Bradshaw (1979).
The sapropels in the Pleistocene sediments in the
Mediterranean were caused by climatic variations
that drastically changed the circulation pattern
(Ryan, 1972; Ryan & Cita, 1977). The study of the
Palaeogene diatomite shows that in a shelf deposit,
laid down during stable climatic and circulationpattern conditions, isochronous changes in oxygen
content may also be observed. If these were caused
by climatic fluctuations, the fluctuations must have
been slight, because they are not apparent in the
fossil assemblage.
If slight climatic changes with a periodicity of tens
of thousands of years are traceable in Jurassic
epicontinental shales (Hallam & Bradshaw, 1979), in
Cretaceous to Tertiary oceanic sediments (Gardner
et a/., 1978) and in Palaeogene shelf deposits (this
paper), such changes may always have occurred. The
reason that they are seldom observed may be that the
variations are so slight that special sedimentological
o r palaeontological conditions are required for their
recognition.
CONCLUSlONS
The Moler diatomite affords an ideal opportunity to
study anoxic events because closely spaced, individually recognizable ash layers make correlation between
exposures possible, and because most of the factors
controlling sedimentation were constant : the clay
content varies little, the same diatom species are
present, and the lamination, where preserved, is of
uniform type. The only difference between the three
diatomite facies is their degree of bioturbation. The
laminated diatomite was deposited during absence of
burrowing benthos, though a very sparse epifauna
may have existed at times. The weakly laminated
diatomite reveals the presence of occasional burrowing organisms. The structureless diatomite was
deposited when a low diversity fauna of burrowing
benthos existed. Land-derived insects and plants are
found in all facies, together with pelagic marine
fossils.
The diatomite was deposited slowly though at
slightly varying rates, but breaks in sedimentation
have not been observed. Deposition took place
under warm, stable, equable climatic conditions and
probably stable patterns of water circulation. LOW
contents of oxygen characterized the diatomite
environment throughout, and were probably a result
of rich biological production, combined with
influence of bottom topography on water circulation
and possibly favoured by the warm, equable climate.
Lastly the relationship between sediment grain
size, bottom water and infaunal benthos can be
studied, because the ash layers are sandy while the
diatomite is silty/clayey. The presence o r absence of
burrowing benthos cannot be shown to be dependent
o n sediment grain size (Fig. lo), and therefore the
presence of burrowing benthos is correlated with the
content of dissolved oxygen.
The laminated diatomite represents anoxic conditions. The content of dissolved oxygen was below
0.1 ml O2 I-’ (Rhoads & Morse, 1971), if the water
wsa not actually poisoned by dissolved H2S. The
structureless diatomite represents oxic conditions
and the content of oxygen was above 0.3 nil O2 I
but seldom above 1.0 nd 0, I - l (Rhoads & Morse,
1971).
With the aid of the ash layer correlation it can be
shown that anoxic conditions were independent of
sedimentation rate, and the uniform content of
fusain wood indicates that anoxic conditions were
not caused by increased influx of terrestrial plant
material. Thus transitions between oxic and anoxic
conditions reflect changes in the oxygen content of the
bottom water produced by an external mechanism,
which is supposed to be climatic.
The hypothesis of Fischer & Arthur (1977), that
periods with warm equable climates may provide the
frame within which anoxic events occur, may be
supported. The development towards fewer, thinner
and more widely spaced intervals of laminated
diatomite in the upper part of the section supports
the hypothesis of Hallam (1978) that gradual filling
of depressions in the shelf improves circulation, as it
may be inferred that an area of rapid subsidence
originally existed and promoted anoxic conditions
as proposed by Hallam & Bradshaw (1979).
Sedinientotion of a diatomite
The complete diatomite sequence may represent an
analogue to one oceanic anoxic event in duration (cf.
Schlanger & Jenkyns, 1976), in the climate prevailing
and in the fact that fluctuations between anoxic and
low-oxygen conditions were observed in the diatomite as they have been in D S D P cores.
Within the diatomite twelve comparatively short,
anoxic events may be distinguished, each characterized by a uniform depositional environment and
comparable t o the anoxic events in the eastern
Mediterranean in the Quaternary o r to the individual
units of laminated black shales in the Jurassic
Kimmeridge Clay.
Thus it seems that thelarge-scale, global, somewhat
heterogeneous oceanic anoxic events a few million
years in length may comprise several smaller scale,
clearly defined, local anoxic events that lasted tens of
thousands of years.
ACKNOWLEDGMENTS
This paper is based on part of a thesis written a t the
University of Copenhagen under the supervision of
Dr F. Surlyk, who is thanked for support and helpful
criticism throughout the study.
An earlier draft of this paper was read by Professor
A. Hallam (Birmingham), D r H. J . Hansen (Copenhagen), D r H. Jenkyns (Oxford), Professor S. 0.
Schlanger (Hawaii) and Professor J. Thiede (Oslo)
who all offered many helpful suggestions. The paper
was revised by D r M. Bradshaw (Aston) and J. D.
Hudson (Leicester), who greatly improved the text.
I direct my best thanks to the above-mentioned
persons.
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