Earth-Science Reviews 99 (2010) 31–49
Contents lists available at ScienceDirect
Earth-Science Reviews
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e a r s c i r ev
Environmental significance of foraminiferal assemblages dominated by small-sized
Ammodiscus and Trochammina in Triassic and Jurassic delta-influenced deposits
Jenö Nagy ⁎, Silvia Hess, Elisabeth Alve
Department of Geosciences, University of Oslo, P.O. Box 1047, Blinden, N-0316 Oslo, Norway
a r t i c l e
i n f o
Article history:
Received 13 March 2009
Accepted 8 February 2010
Available online 16 February 2010
Keywords:
foraminifera
low diversity
agglutinated
small-sized
low salinity
reduced oxygen
Triassic and Jurassic
a b s t r a c t
The sediment packages analyzed for benthic foraminifera consist of mudstones with interbedded sandstones
deposited in shallow delta-influenced shelf to deltaic environments. The sections are located in Spitsbergen,
the Barents Sea, northern North Sea and Yorkshire, and range in age from Late Triassic to Middle Jurassic.
Salient features of the foraminiferal successions are: (1) The assemblages consist entirely or dominantly of
agglutinated taxa. (2) The faunal diversities are extremely low. (3) The dominant genera are Ammodiscus
and Trochammina. (4) The species are generally of small size compared to usual dimensions within the
genera.
The features listed above suggest that the assemblages were adapted to restricted conditions (clearly
divergent from those of a normal marine shelf), where the main limiting factors were low salinity and
reduced amount of dissolved oxygen in unstable, storm-influenced environments. Evidence for
environmental conditions is obtained from modern analogues, although the large evolutionary changes
in foraminifera during post-Jurassic time make it difficult to find such analogues. Additional information is
derived from functional morphology, sedimentary features and paleogeography.
The analyzed sediment packages show close faunal similarities suggesting opening of a marine pathway,
which connected the paleo-Arctic Ocean with the western European shelf seas in Early Jurassic. A
depositional biofacies model of the small-sized Ammodiscus–Trochammina assemblages envisages a deltainfluenced shelf environment, where high freshwater influx would have created a density-stratified water
column with a tendency to develop hypoxic conditions in its deeper parts. The depth interval between fairweather and storm wave base (the offshore-transition zone) is indicated as the habitat of the small-sized
Ammodiscus–Trochammina assemblages. In this zone, benthic biota would have been stressed by
intermittent periods with moderate hypoxia combined with lowered salinity and storm impacts.
© 2010 Elsevier B.V. All rights reserved.
Contents
1.
2.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.
Background and purpose of study . . . . . . . . . . . . .
1.2.
Main features of Jurassic foraminiferal facies . . . . . . . .
1.2.1.
Normal marine shelf assemblages . . . . . . . . .
1.2.2.
Delta-influenced assemblages . . . . . . . . . . .
1.2.3.
Hypoxic shelf assemblages . . . . . . . . . . . .
Distribution of small-sized Ammodiscus–Trochammina assemblages . . .
2.1.
Knorringfjellet Formation at Festningen, western Spitsbergen
2.2.
Knorringfjellet Formation, central Spitsbergen . . . . . . . .
2.3.
Knorringfjellet Formation, Wilhelmøya . . . . . . . . . . .
2.4.
Ragnarok Formation of the Mjølnir structure, Barents Sea . .
2.5.
Rannoch Formation, Gullfaks Field, North Sea . . . . . . . .
2.6.
The Yons Nab Beds, Yorkshire Coast . . . . . . . . . . . .
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⁎ Corresponding author. Tel.: + 47 22 85 66 48; fax: +47 22 85 42 15.
E-mail addresses: jeno.nagy@geo.uio.no (J. Nagy), silvia.hess@geo.uio.no (S. Hess), elisabeth.alve@geo.uio.no (E. Alve).
0012-8252/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.earscirev.2010.02.002
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J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
3.
Environmental significance of assemblage features . .
3.1.
Main aspects of biofacies . . . . . . . . . . .
3.2.
The agglutinated faunal component . . . . . .
3.3.
The calcareous faunal component . . . . . . .
3.4.
Species diversities . . . . . . . . . . . . . .
3.5.
Dominant foraminiferal taxa . . . . . . . . .
3.6.
Occurrence of ostracods . . . . . . . . . . .
3.7.
Taphonomy of assemblages . . . . . . . . .
4.
Test size and shape of foraminiferal taxa . . . . . .
4.1.
Size distribution in the studied sections . . .
4.2.
Literature-based size distributions . . . . . .
4.3.
Size comparisons . . . . . . . . . . . . . .
4.4.
Test morphology and habitats . . . . . . . .
5.
Modern analogues . . . . . . . . . . . . . . . . .
5.1.
Drammensfjord . . . . . . . . . . . . . . .
5.2.
Aso-kai Lagoon . . . . . . . . . . . . . . .
6.
Triassic and Jurassic environments of small-sized taxa
6.1.
Delta-influenced shelf embayment . . . . . .
6.2.
Prodelta–delta front transition . . . . . . . .
6.3.
Interdistributary bay . . . . . . . . . . . . .
6.4.
A depositional biofacies model . . . . . . . .
7.
Conclusions . . . . . . . . . . . . . . . . . . . .
Acknowledgments . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
1.1. Background and purpose of study
Benthic foraminiferal successions heavily dominated by Ammodiscus
and Trochammina are known from several Late Triassic to Middle
Jurassic sediment packages along the Atlantic margin of northwestern
Europe, from the North Sea up to the Arctic (Nagy et al., 1990; Bremer
et al., 2003). Several of the successions are typified by small dimensions
of these and other taxa, extremely low species diversities and, with a
few exceptions, by consisting entirely of agglutinated forms. The present
study examines geographically widely spaced assemblages of this type,
recognized in several sediment packages as follows (Figs. 1 and 2): The
Knorringfjellet Formation of Spitsbergen studied at Festningen, in
central Spitsbergen and on Wilhelmøya; the Ragnarok Formation of the
Mjølnir impact crater in the western Barents Sea; the lower Rannoch
Formation of the Gullfaks Field in the northern North Sea; the Yons Nab
Beds on the Yorkshire coast of northeast England.
The above-listed sedimentary successions are composed mainly of
mudstones and sandstones interbedded in varying proportions. In the
literature, these successions are attributed to shallow shelf, deltaic
and coastal marine environments of restricted nature, based on
sedimentary features combined with foraminiferal biofacies. Until
recently, low salinity shallow water conditions were regarded as the
main restricting factor in these environments (Nagy et al., 1990),
although tendency to hypoxic conditions in a stratified water column
was suggested as an additional factor affecting depositional conditions of the Knorringfellet Formation (Nagy and Berge, 2008).
The objectives of the present study are: (1) Outlining the regional and
stratigraphic occurrence of small-sized Ammodiscus and Trochammina
assemblages. (2) Comparison of these assemblages in order to delineate
their common features. (3) An assessment of the environmental
significance of small-sized agglutinated assemblages. (4) To contribute
to the knowledge of delta-influenced and marginal marine foraminiferal
assemblages, which have received little attention in spite of the high
geological importance of shallow shelf to coastal marine deposits.
1.2. Main features of Jurassic foraminiferal facies
1.2.1. Normal marine shelf assemblages
Assemblages of this type consist entirely or dominantly of
calcareous foraminifera, belonging mainly to Nodosariacea, although
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agglutinated taxa can also form a significant component. High species
diversities are typical. There are numerous studies dealing with
assemblages of this type from various sedimentary successions e.g.:
Exton (1979), Pliensbachian and Toarcian of Portugal; Pietrzenuk
(1961), Sinemurian and Toarcian of northeastern Germany; Copestake and Johnson (1989), Hettangian to Toarcian of North Wales;
Barnard et al. (1981), Callovian and Oxfordian (Oxford Clay), England;
Norling (1972), Pliensbachian of Western Scania, Sweden.
The main faunal proxies for normal marine shelf assemblages of
the Northern North Sea Basin, were calculated by Nagy et al. (1990) in
two sediment packages. (1) The Ladys Walk Shale Member in the
Moray Firth Basin is of Late Sinemurian to Early Pliensbachian age. It
comprises 3 foraminiferal assemblage units with average alpha
diversities from 4.9 to 7.3 and proportion of calcareous taxa from 73
to 99%. (2) The Amundsen Formation of the Statfjord area includes 6
assemblage units with average alpha values ranging from 4.0 to 8.4
and frequency of calcareous taxa varying from 0.3 to 38%. In both
successions the dominant genera are Marginulina, Mesodentalina,
Lenticulina and Dentalina. The alpha diversities are generally well
above 5, corresponding to the values of modern normal marine
shelves.
1.2.2. Delta-influenced assemblages
Paralic foraminiferal assemblages dominated by large-sized
Ammodiscus were reported by Løfaldli and Nagy (1980) from the
Toarcian to Bathonian Kongsøya Formation sampled on Kong Karls
Land (easternmost part of the Svalbard Archipelago). Dominant
species are the robust Ammodiscus asper (Terquem, 1862) and the
medium-sized A. limitatus (Terquem, 1864). The diversities are
extremely low, with number of species per sample varying from 1
to 3. Several samples are barren and thin coal seams are present in the
section. The assemblages are attributed to shallow, strongly hyposaline, well-oxygenated waters in lagoonal or estuarine settings.
The varied foraminiferal succession of the Middle Jurassic Safa
Formation of Sinai has been analyzed by Ghandour and Maejima
(2007) who distinguished two agglutinated biofacies. (1) The
medium diversity Ammobaculites biofacies is referred to brackish
prodelta environments dominated by Trochammina and Verneuilinoides in addition to the nominate genus. (2) The low diversity
Ammodiscus–Glomospira biofacies is ascribed to brackish delta plain
and estuarine conditions dominated by Miliammina in addition to the
nominate taxa.
J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
33
Fig. 1. Paleogeographic map for Toarcian to Aalenian time showing position of sections containing low diversity agglutinated foraminiferal assemblages characterized by small-sized
taxa dominated by Ammodiscus and Trochammina discussed in this paper.
Map based on own interpretations and information from Ziegler (1982), Nøttvedt et al. (1992), van Veen et al. (1992), Torsvik et al. (2002), Riis et al. (2008).
Other low diversity, agglutinated, delta-influenced assemblages
are recognized in the Jurassic of the western and northwestern
European margin, in the 6 sections that form the basis of the present
study. As shown in the following chapters, hyposaline conditions are
here combined with varying degrees of hypoxia leading to small test
dimensions.
1.2.3. Hypoxic shelf assemblages
The western and northwestern European margin comprises several
sediment packages composed of organic-rich shales. The foraminiferal
assemblages of three of these are referred to hypoxic depositional
conditions and are outlined here. (1) The Black Stone Band, a subunit of
the Kimmeridge Clay Formation of Southern England, is strongly
dominated by agglutinated taxa with an extremely small calcareous
component (Jenkins, 2000). The alpha diversity is typically around 3
and the dominant genera are Reophax, Ammobaculites, Textularia and
Trochammina. (2) The Brora Shale Member (Callovian) on the
northwestern margin of the Moray Firth Basin is heavily dominated
by agglutinated forms with a strongly subordinate calcareous
component (Nagy et al., 2001). The average alpha value is 2.5 and
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J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
Fig. 2. Stratigraphic positions of sections containing small-sized foraminiferal
assemblages dominated by Ammodiscus and Trochammina. Intervals with poorly
defined stratigraphic age are stippled.
the dominant genera include Haplophragmoides, Gaudryina and
Ammobaculites. (3) The Lardyfjellet Member (Oxfordian) of the
Agardhfjellet Formation contains exclusively agglutinated assemblages (Nagy and Basov, 1998). The species diversity is extremely
low and the dominant genera are Trochammina, Recurvoides and
Thuramminoides.
The examples cited above reveal that the hypoxic black shale
assemblages are entirely agglutinated or contain only a very small
additional calcareous component. The species diversities in these
deposits are extremely low. The generic composition of the
assemblage is variable, although Trochammina seems to be the most
typical genus.
while the Teistberget comprises its Early Jurassic part. The intervening
hiatus is believed to represent the Late Norian to Early Pliensbachian
time interval.
The lower part of the Tverrbekken member consists of highly
bioturbated sandstones, while its middle and upper parts are
composed of mudstones with siderite-cemented sandstone interbeds.
The mudstones are generally dark grey in color, and their TOC content
ranges from 0.9 to 1.6%, while their calcium carbonate content varies
from 0.02 to 2.00%. The presence of indistinct lamination suggests a
very low degree of bioturbation. The sandstone interbeds are heavily
bioturbated.
The lower and middle parts of the Teistberget member consist
mainly of mudstones, while its upper part is dominated by
bioturbated sandstones (Fig. 3). The mudstones are light grey in
color, silty and weather to pieces of blocky appearance suggesting the
presence of diffuse bioturbation. The TOC content of these mudstones
ranges from 0.2 to 1.0%, while their calcium carbonate content varies
from 0.03 to 1.68%, except for the disconformity level at base of the
member where 33% carbonate was measured.
The foraminiferal assemblages of the Knorringfjellet Formation are
entirely agglutinated except for the occurrence of Eoguttulina liassica
Strickland 1846 in two samples and Astacolus sp. in a single sample; in
each case they account for less than 1%. The most common species
throughout the formation is Ammodiscus aff. yonsnabensis Nagy,
Løfaldli and Bomstad 1983, increasing in frequency upwards (Fig. 3).
Trochammina aff. eoparva Nagy and Johansen, 1991 is abundant in the
Tverrbekken member and occurs in reduced amounts in the
Teistberget member. Less common but characteristic species include
Reophax metensis Franke 1936, Bulbobaculites aff. vermiculus Nagy and
Seidenkrantz 2003 and Verneuilinoides subvitreus Nagy and Johansen,
1991. Three other species occur in more than 30% of the samples but
with frequencies less than 10%: Evolutinella sp.1, Thurammina sp.1 and
Trochammina sp.1.
The faunal diversity is low. This is shown by the number of species
per sample, with a mean value of 6.2 and a range from 1 to 10. There is
a general diversity decrease upwards through the Teistberget
member, leading to a monospecific assemblage at the top of the
member (Fig. 3). More details about the composition of these
assemblages are given by Nagy and Berge (2008).
2.2. Knorringfjellet Formation, central Spitsbergen
2. Distribution of small-sized Ammodiscus–Trochammina assemblages
2.1. Knorringfjellet Formation at Festningen, western Spitsbergen
Festningen is located close to the western coast of Spitsbergen (the
main island of the Svalbard Archipelago). Here, the low diversity
agglutinated assemblages dominated by small-diameter Ammodiscus
and Trochammina are typical of the 22.8 m thick Knorringfjellet
Formation of Late Norian to Toarcian age (Fig. 3). The formation
represents the upper part of the Kapp Toscana Group. The group
consists of shallow marine shelf to deltaic mudstones, siltstones and
sandstones, having a wide regional distribution in Spitsbergen and on
the western Barents Shelf (Mørk et al., 1982).
The Knorringfjellet Formation consists mainly of mudstones with
sandstone interbeds (Fig. 3). The occurrence of phosphatic horizons
and siderite-cemented levels indicate a condensed succession, and in
accordance with this, the presence of at least two major and several
minor hiatuses has been suggested (Mørk et al., 1982; Nagy and
Berge, 2008). Both the base and top of the formation are disconformities, marked by transgressive phosphatic conglomerates. In the
Festningen section the formation is informally subdivided into a lower
Tverrbekken member and an upper Teistberget member, separated by
a major disconformity (Pčelina, 1980; Mørk et al., 1999). The
Tverrbekken constitutes the Late Triassic part of the formation,
In addition to the Festningen section, similar low diversity
agglutinated assemblages dominated by small-sized Ammodiscus
and Trochammina are observed in three sites exposing the Knorringfjellet Formation in central Spitsbergen: (1) Detailed analyzes of a
section at Marhøgda showed that also here the dominant species are
T. aff. eoparva and A. aff. yonsnabensis, while the species diversity
varies from 4 to 11 around a mean of 8.8 (Nagy and Berge, 2008). (2)
Two analyzed samples from a section at Juvdalen revealed a high
dominance of A. aff. yonsnabensis and extremely low species diversity.
(3) A pilot study of a section located close to the front of Drønbreen
demonstrated a dominance of T. aff. eoparva and A. aff. yonsnabensis
once more associated with low species diversities.
2.3. Knorringfjellet Formation, Wilhelmøya
This island is located on the northeastern coast of Spitsbergen
where the Knorringfjellet Formation is thicker, more sandy, and
apparently more complete than in western and central areas. Two
samples were analyzed from the Wilhelmøya exposures; both
contain low diversity agglutinated assemblages of small-sized taxa,
among which T. aff. eoparva and A. aff. yonsnabensis are the most
common.
J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
35
Fig. 3. The Knorringfjellet Formation at Festningen, western Spitsbergen, showing lithology, number of species and distribution of the three dominant taxa.
2.4. Ragnarok Formation of the Mjølnir structure, Barents Sea
The Mjølnir structure is a crater-like depression with a diameter of
40 km seen in pre Cretaceous deposits located in the central Barents
Sea (Fig. 1). Interpretation of seismic data and drill-hole sediment
cores reveal that the structure was formed by the impact of a major
meteorite (Tsikalas et al., 1998; Bremer et al., 2003; Dypvik and Jansa,
2003). The impact has taken place in latest Tithonian time, when the
target area formed a deeper part of the paleo-Barents Sea shelf.
A cored drill-hole, 7329/03-U-01, located within the Mjølnir
crater, penetrated a 24 m thick succession of Ryazanian to Valanginian
post impact deposits. These rest on a 97 m thick impact-affected
sediment package, the Ragnarok Formation, of Late Triassic to Early
Jurassic age (Fig. 4). The formation is severely deformed by the
impact, but its lithology indicates that it originally formed a
succession of mudstones interbedded with sandstones. Thus, both
the age and lithology of the Ragnarok indicate that it is correlative
with the Kapp Toscana Group, including the Knorringfjellet Formation
of Spitsbergen.
The foraminiferal and palynomorph content of the Mjølnir drill
core 7329/03-U-01 are presented by Bremer et al. (2003) and the
following foraminiferal data are derived from that paper. The
foraminifera occurring in the Ragnarok Formation are entirely
agglutinated, small in size and form low diversity assemblages. The
dominant species are A. aff. yonsnabensis, Trochammina minutissima
Dain, 1972 and Evolutinella sp. 1 (Fig. 4). The number of specimens per
sample ranges from 3 to 12, with a mean of 6.1.
2.5. Rannoch Formation, Gullfaks Field, North Sea
The Rannoch Formation of Aalenian age belongs to the sanddominated deltaic Brent Group deposited in the northern North Sea
Basin in Middle Jurassic time (Nagy et al., 1990). The small-sized
Ammodiscus–Trochammina assemblages were recognized in the lower
10 m of the Rannoch in well 34/10-1 drilled on the Gullfaks Oil Field.
The lower Rannoch is a succession of silty mudstones containing thin
interbeds of sandstone (Fig. 5). A thin coal seam is present at the top.
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J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
Fig. 4. The Ragnarok Formation of the Mjølnir Structure, western Barents Sea, showing lithology, number of species and distribution of the three dominant taxa.
The TOC content of the lower Rannoch mudstones ranges from 0.1
to 2.6%. The calcium carbonate content varies from 0.0 to 3.8% in the
lower part of the interval, while in the upper part it attains 37%
although the values are strongly variable. The mudstone-dominated
succession of the lower Rannoch grades upwards into a thick package
of sandstones which comprise the main body of the formation,
interpreted to have been deposited as an advancing wave-dominated
delta front (Graute et al., 1987) including distributary mouth bar and
prograding shoreface sands.
The foraminiferal assemblages in the lower Rannoch deposits
consist almost entirely of agglutinated taxa (Nagy and Johansen,
1991). The dominant species are A. yonsnabensis, T. eoparva and
Reophax metemsis (Fig. 5). Other species that are locally common
include Trochammina aff. sablei Tappan, 1955, Trochammina sp. 1 and
Lagenammina aff. inanis (Gerke and Sosipatrova 1961). Calcareous
taxa are observed only in three samples, and then at frequencies b1%.
The calcareous species are Spirillina numismalis Terquem and
Bertehlin 1875 and Astacolus sp. The number of species per sample
ranges from 1 to 8 with a mean of 4.9.
lower and upper parts, and bivalve shell horizons in the middle part of
the succession (Fig. 6). Good exposures are present on the foreshore at
Yons Nab where the thickness of the unit is around 8 m.
At this site, the foraminiferal assemblages were analyzed in a
south-eastern section and a north-western section by Nagy et al.
(1983). The faunal composition in the two sections proved to be
closely similar, although the south-eastern section is generally richer
both in species and individuals. The data presented in this paper are
therefore those derived from the latter.
The foraminiferal assemblages of the Yons Nab Beds show low
diversities with an overall dominance of agglutinated taxa, although
calcareous species are also represented in significant amounts,
particularly within the middle part of the section and at the base
(Fig. 6). Spirillinids dominate the calcareous component but nodosariids are also significant. A range chart of all the species observed in the
section is presented in Nagy et al. (1983). The dominant species (in
decreasing abundance) are: A. yonsnabensis, Turrispirillina punktulata
(Trequem 1870), Conicospirillina pictonica (Berthelin 1879) and
Trochammina sp. The number of species per sample ranges from 1
to 8 with a mean of 4.1. Ostracod carapaces occur in most samples.
2.6. The Yons Nab Beds, Yorkshire Coast
3. Environmental significance of assemblage features
The Yons Nab Beds together with the underlying Millepore Beds
comprise the Cayton Bay Formation of Aalenian age (Cope et al.,
1980). This formation represents a major marine intercalation in the
Middle Jurassic deltaic succession of Yorkshire. The lithology of the
Yons Nab Beds is dominated by mudstones with interbedded
sandstones. Accessory lithologies include thin siderite beds in the
3.1. Main aspects of biofacies
As shown on the preceding pages, the foraminiferal facies of the
analyzed sections are characterized by the following main features:
(1) The assemblages are virtually entirely agglutinated, except some
J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
37
Fig. 5. The lower part of the Rannoch Formation in the Gullfaks Field, northern North Sea, showing lithology, number of species and distribution of the three dominant taxa.
samples in the Yons Nab Beds where calcareous taxa occur in varying
amounts; (2) the species diversities are low, showing values below
those typical for normal marine shelves; (3) the species are generally
of small dimensions; and (4) the dominant species belong to
Ammodiscus and Trochammina. The following is a closer consideration
of the environmental significance of these biofacies features.
3.2. The agglutinated faunal component
The foraminiferal assemblages of the central Spitsbergen sections
and the Mjølnir core (Fig. 4) are entirely agglutinated. The
assemblages of the Festningen and Gullfaks sections (Figs. 3 and 5)
are agglutinated except for a very few scattered samples containing
calcareous taxa that account for less than 1% of the assemblages. The
Yons Nab Beds are heavily dominated by agglutinated taxa, although
in the lower and particularly in the middle part of the section
calcareous forms also occur in significant amounts (Fig. 6).
In an overview of Jurassic foraminiferal facies in the northern
North Sea Basin, Nagy et al. (1990) demonstrated the occurrence of
agglutinant-dominated assemblages in Toarcian, Aalenin and Callovian strata, which they referred to brackish conditions in proximal to
distal deltaic settings, and to hypoxic conditions in organic-rich shelf
environments. These interpretations imply that Jurassic environments
divergent from those of normal marine shelves developed low
diversity agglutinated assemblages. The Rannoch Formation and the
Yons Nab Beds were included in that Jurassic study.
3.3. The calcareous faunal component
As shown above, the foraminiferal assemblages are composed
exclusively of the agglutinated suborder Textulariina in three of the
analyzed sites (central Spitsbergen, Wilhelmøya and Mjølnir). Two
other sites (Festningen and Gullfaks) are virtually agglutinated, and
provided only a few samples containing very small (b1%) calcareous
components belonging to nodosariids. These features are consistent
with the restricted nature of the assemblages typifying the five sites.
Nagy et al. (1990) have demonstrated the environmental
significance of calcareous assemblages in Early to Middle Jurassic
deposits of the North Sea Basin, where normal marine shelf strata
were found to be typified by high diversity faunas dominated by
nodosariids. Significant divergences from normal shelf conditions
result in low species diversity coupled with high dominance of
agglutinated forms or entirely agglutinated assemblages. The dominant calcareous group of the Mesozoic, the nodosariids, was
apparently not adapted to restricted conditions during that era, as is
also the case with the Cenozoic descendents of this superfamily,
although strongly reduced both in abundance and diversity in most
Cenozoic and modern environments.
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J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
Fig. 6. The Yons Nab Beds of the Cayton Bay Formation, Yorkshire Coast, showing lithology, number of species, distribution of the four dominant foraminiferal taxa and ostracod
occurrences.
Among the studied sections, only the basal and middle parts of the
Yons Nab Beds contain significant amounts of calcareous foraminifera.
The calcareous component here is dominated by spirillinids, while
nodosariids are subordinate. In modern faunas, spirillinids are
common in lagoons, living principally on seaweeds (Davies, 1970;
Brasier, 1975) while nodosariids are generally regarded to occur in a
much wider range of normal marine habitats. Significant amounts of
these calcareous components in the lower and middle part of the Yons
Nab Beds indicate marine ingressions, which resulted in temporary
increased salinity in an overall brackish development.
3.4. Species diversities
In all the studied sections, the number of species per sample is
generally low (Figs. 3–6). In accordance with this, both the alpha diversity
and H(S) values are strongly reduced (Fig. 7). The alpha diversities can be
summarized as follows: Festningen section, mean 1.05 (range 0.3–1.9);
Mjølnir structure, mean 1.26 (range 0.6–2.5); Gullfaks Field, mean 1.09
(range 0.4–1.7); Yons Nab section, mean 1.01 (range 0.2–2.0). Comparisons with the alpha values of modern faunas suggest that these low
diversity assemblages were formed under restricted conditions.
In modern faunas, alpha values below 5 are typical of environments where the restricting factor is low salinity, as in numerous
estuaries and hyposaline lagoons (Murray, 1973). However, similar
low alpha values are also found in assemblages from modern hypoxic
estuarine waters, as demonstrated by fjord environments in southeastern Norway (Alve and Nagy, 1986; Nagy and Alve, 1987; Alve,
1990). In accordance with this, strongly reduced diversities are also
typical for faunas occurring in organic-rich shales, as shown in the
Middle to Late Jurassic of Spitsbergen and Scotland (Nagy et al., 1988,
1990). Diversity diagrams where alpha is plotted against H(S) sow
typically low values in modern hyposaline environments as summarized by Murray (2006). Cross plots of these two indices reveal
comparable low values in the four sections analyzed in detail in the
present study (Fig. 7).
3.5. Dominant foraminiferal taxa
The most common genus in the analyzed sample material is
Ammodiscus (Figs. 3–6). It is represented by the small-sized A. aff.
yonsnabensis in the Knorringfjellet Formation of the Festningen,
central Spitsbergen, and Wilhelmøya sections, as well as in the
Ragnarok Formation of the Mjølnir structure (Plate 1, figs. 1, 2, 3). The
closely related small form, A. yonsnabensis, is dominant in the North
Sea Basin where it occurs in the Rannoch Formation of the Gullfaks
Field and in the Yons Nab Beds of the Yorkshire coast (Plate 1, figs. 16,
22, 23, 24).
The next most common genus is Trochammina, represented by
several small-sized species: T. aff. eoparva in the five Spitsbergen
sections, T. minutissima in the Mjølnir structure and T. eoparva in the
Gullfaks Field (Plate 1, figs. 6, 7, 18, 19, 20). In the Yons Nab section the
second most abundant species is the calcareous C. pictonica. Other
dominant species, all of comparatively small size, include: Reophax
metensis at Festningen and Gullfaks, Evolutinella sp.1 in the Mjølnir
Structure, and Turrispirillina punctulata and Trochammina sp.2 at Yons
Nab (Plate 1, figs. 5, 9, 25).
In the Festningen section, A. aff. yonsnabensis is associated with
low to extremely low species diversities. The percentage frequency of
this species reveals a gradual upward increase, which is inversely
related to the species diversity (Fig. 3). This development is portrayed
in more detail in Fig. 8, showing that the percentage of A. aff.
yonsnabensis is variable at lower diversities but increases strongly
J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
39
Fig. 7. Species diversities in four analyzed sections expressed by plots of the alpha index against H(S).
with further diversity reduction. At extremely low diversities the
frequency of the species is above 90%, with a single sample containing
a monospecific fauna. This relationship suggests that among the
species occurring in the Festningen section, A. aff. yonsnabensis is the
taxon most tolerant of restricting conditions that controlled the
distribution of foraminiferal species.
3.6. Occurrence of ostracods
A significant feature of the Yons Nab section is its content of
ostracods (Fig. 6). They are smooth-walled and occur in 11 of the 17
analyzed samples where they range in number from 3 to 179
carapaces per 100 gram sediment. These microscopic crustaceans
have a highly mobile mode of life (compared to foraminifera), which
implies a higher level of metabolism that requires increased
oxygenation of the water column. Consequently, ostracods are totally
absent or extremely rare in organic-rich shales. This is demonstrated
by their total absence from 50 samples of shales with high organic
content that were analyzed for microfossils from the Spekk Formation
(Late Jurassic) of the Mid Norwegian Shelf. Among several hundred
samples disintegrated form organic-rich shales of the Agardhfjellet
Formation (Middle to Late Jurassic) of Spitsbergen, only a single one
contained a few carapaces, while almost all samples produced
agglutinated foraminifera.
conditions may potentially dissolve calcareous tests if originally
present. Nevertheless, even if all calcareous species are lost through
dissolution, the remaining agglutinated assemblages may still hold
useful ecolocical information (Murray and Alve, 2000). In this context
it must be noted, however, that environments with extremely low
content of dissolved calcium carbonate might physiologically exclude
calcareous taxa.
Among the analyzed sections, only the Yons Nab Beds contain
significant amounts of calcareous fossils, including foraminifera,
ostracods, bivalves and crinoids (Fig. 6). No traces of dissolution are
observed on the foraminiferal and ostracod shells (the two groups
studied in detail), which thus excludes carbonate dissolution. Two of
the studied sections, Festningen and Gullfaks, contain a very small
calcareous foraminiferal component of b1% in a few scattered
samples, but no partially dissolved calcareous tests were observed.
The assemblages studied from central Spitsbergen, Wilhelmøya and
Mjølnir are entirely agglutinated.
Regarding carbonate dissolution in the present material, of
particular note is the presence of pyrite internal molds in many of
the non-compressed calcareous and agglutinated tests. Pyrite precipitation in chamber voids is a pre-compressional process, which
usually takes place immediately after burial. In the samples analyzed
in this study, pyrite internal molds without a covering shell have not
been observed, which suggests that no significant post-compressional
dissolution of carbonate has taken place.
3.7. Taphonomy of assemblages
4. Test size and shape of foraminiferal taxa
When interpreting fossil foraminiferal assemblages of agglutinated or mixed (calcareous-agglutinated) type, taphonomic dissolution
of calcareous tests is a relevant issue. Dissolution of calcium carbonate
can take place before burial (e. g. Murray and Alve, 1999), during
diagenesis and through modern weathering processes. Of particular
interest are stagnant environments, where low pH bottom water
4.1. Size distribution in the studied sections
The foraminiferal taxa occurring in the studied sections are
characterized by generally small test dimensions (Plate 1, figs 1–25).
This feature is exemplified by the common to dominant species: (1)
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J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
Plate 1. Common and characteristic foraminifera occurring in the small-sized Ammodiscus and Trochammina assemblages. 1, 2, 3: Ammodiscus aff. yonsnabensis, Knorringfjellet Fm
Festningen, sample 20, X180. 4: Trochammina sp. 1, Knorringfjellet Fm Festningen, sample 7, X180. 5: Evolutinella sp. 1, Knorringfjellet Fm Festningen, sample 10, X180. 6, 7:
Trochammina aff. eoparva, spiral and umbilical view (respectively), Knorringfjellet Fm Marhøgda, sample 7, X180. 8: Trochammina aff. eoparva, peripheral view, Knorringfjellet Fm
Festningen, sample 8, X180; 9: Reophax metensis, Knorringfjellet Fm Marhøgda, sample 2, X180. 10: Reophax sp. 1, Knorringfjellet Fm Festningen, sample 10, X180. 11: Bulbobaculites
aff. vermiculus, Knorringfjellet Fm Marhøgda, sample 9, X180. 12: Verneuilinoides aff. kirillae, Knorringfjellet Fm Marhøgda, sample 10, X110. 13: Verneuilinoides subvitreus,
Knorringfjellet Fm Marhøgda, sample 4, X180. 14: Lagenammina aff. inanis, Knorringfjellet Fm Festningen, sample 2, X180. 15: Thurammina sp. 1, Knorringfjellet Fm Marhøgda,
sample 9, X180. 16: Ammodiscus yonsnabensis, Rannoch Fm Gullfaks Field, sample 10, X180. 17: Bulbobaculites oviloculus, Rannoch Fm Gullfaks Field, sample 1, X180. 18, 19, 20:
Trochammina eoparva, spiral, umbilical and peripheral view (respectively), Rannoch Fm Gullfaks Field, sample 10, X180. 21: Glomospira aff. perplexa, Rannoch Fm Gullfaks Field,
sample 1, X200. 22, 23, 24: Ammodiscus yonsnabensis, Yons Nab Beds Yorkshire, sample 10, X190. 25: Turrispirillina punctulata, Yons Nab Beds Yorkshire, sample 7, X172.
J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
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Fig. 8. The percentage frequency of Ammodiscus aff. yonsnabensis plotted against H(S)
diversity. Diagram based on specimens from the Knorringfjellet Formation, Festningen
section. Number of overlapping plots given.
Ammodiscus aff. yonsnabensis, highly dominant in the Knorringfjellet
Formation of Spitsbergen and in the Ragnarok Formation of the
Barents Sea; (2) the closely related A. yonsnabensis, dominant in the
Rannoch Formation of the Gullfaks Field and in the Yons Nab Beds of
the Yorkshire Coast; (3) T. aff. eoparva, locally dominant in the
Knorringfjellet Formation; (4) the closely related T. eoparva, dominant
in the Rannoch Formation; (5) T. minutissima, dominant in the
Ragnarok Formation; (6) Trochammina sp. 2, common in the Yons
Nab Beds; and (7) Evolutinella sp. 1, common in the Ragnarok
Formation. In addition, the dimensions of most of the rare species
also appear to be reduced.
To quantify the size distribution of the two most common species,
the diameters of A. aff. yonsnabensis and T. aff. eoparva were measured
in assemblages from the Knorringfjellet Formation of the Festningen
section. Graphic presentation of the measurements reveals (Fig. 9)
that the diameter of A. aff. yonsnabensis has a mean value of 0.16 mm
within a range of 0.09 to 0.23 mm. Trochammina aff. eoparva attains a
mean diameter of 0.17 mm and varies from 0.12 to 0.24 mm.
The low diversity agglutinated assemblages typical for the other
sections are dominated by similarly small species, as shown in the
following by their mean values with ranges in parentheses. Ragnarok
Formation of the Mjølnir structure: diameter of A. aff. yonsnabensis,
0.14 mm (0.08–0.18 mm); diameter of T. minutissima, 0.15 mm (0.11–
0.17 mm). Rannoch Formation of the Gullfaks Field: diameter of A.
yonsnabensis 0.22 mm (0.16–0.32 mm); diameter of T. eoparva,
0.22 mm (0.20–0.26 mm). Yons Nab Beds of the Yorkshire coast:
diameter of A. yonsnabensis 0.19 mm (0.15–0.29 mm).
4.2. Literature-based size distributions
The size distributions of Ammodiscus and Trochammina, observed
in the studied sections, are placed in a wider regional and
stratigraphic context by comparison with the dimensions of the
same two genera recorded in the literature, covering an extensive
range of sedimentary successions located outside the area of the
present study. To obtain the most relevant data set, this literature
study is restricted to the Triassic and Jurassic time interval. It must be
noted, however, that from the Triassic only a moderate number of
relevant size measurements could be obtained, owing to the generally
small number of papers dealing with foraminifera of this period. The
database is then used for constructing the regional size distribution
diagrams displayed in Fig. 10, which are based on measurement data
extracted from 27 papers for Ammodiscus and from 28 papers for
Trochammina. Most of the information used in the diagrams originates
from the papers referred to in Tables 1 and 2.
Fig. 9. Diameter categories of Ammodiscus aff. yonsnabensis and Trochammina aff.
eoparva calculated as percentages from the total number of measured specimens (n).
Data from the Knorringfjellet Formation, Festningen section.
4.3. Size comparisons
The literature-based regional data set demonstrate that the genus
Ammodiscus is extremely variable in size, with reported diameter
measurements ranging from 0.09 to 4.69 mm (Fig. 10). The smallsized A. aff. yonsnabensis recorded in the present study, with diameter
of 0.09 to 0.23 mm, is restricted to the lowermost part of this regional
size range (Fig. 11). Similarly, the mean diameter of Ammodiscus in
the literature-based dataset is 0.35 mm, while that of A. aff.
yonsnabensis in the present samples is 0.16 mm.
The diameter range of the genus Trochammina found in the
literature search is from 0.07 to 1.26 mm (Fig. 10), while that of the
generally very small T. aff. eoparva in our material varies from 0.12 to
0.23 mm. In accordance with this, the literature-based main diameter
of Trochammina is 0.25 mm, while that of T. aff. eoparva in our material
is 0.17 mm (Fig. 11).
These comparisons demonstrate that the diameters of both A. aff.
yonsnabensis and T. aff. eoparva are significantly smaller than is
typically found for Ammodiscus and Trochammina in a wide range of
Triassic and Jurassic sedimentary successions (Fig. 11). We propose
that the reduced size is an adaptive response to restricted environmental conditions. It seems highly probable that the main restricting
factors were low salinity and low oxygen content, apparently in
varying combinations. In addition, rapid fluctuations in environmental
42
J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
Fig. 10. Diameter categories of various Triassic and Jurassic species of Ammodiscus and Trochammina calculated as percentages from the total number of measured specimens (n).
Data derived from published sources (see Tables 1 and 2).
conditions may have contributed to the overall stress. In more extreme
cases, destruction of the life faunas might have occurred, which was
followed by subsequent recolonization.
A special feature of the interdistributary bay Yons Nab Beds (on the
Yorkshire Coast) is their content of ostracods, which indicates
increased oxygenation of benthic habitats. Thus, the main restricting
factor during deposition of these strata was apparently low salinity,
particularly at stratigraphic levels where ostracods are abundant in
association with significant amounts of calcareous foraminifera.
Low diversity agglutinated assemblages dominated by small-sized
foraminifera are recognized in several Jurassic formations of organicrich shales ascribed to stagnant bottom conditions. Two examples
observed by the present authors are the Spekk Formation of the Mid
Norwegian Shelf, heavily dominated by the tiny Trochammina annae
Levina 1972, and the Brora Shale Member of northeast Scotland
dominated by the small Haplophragmoides aff. pygmaeus (Haeusler
1882). Low diversity agglutinated assemblages composed of smallsized taxa are also recorded from several localities as recolonization
faunas appearing immediately after the Cenomanian–Turonian
boundary anoxic event (Kuhnt, 1992; Coccioni et al., 1995).
4.4. Test morphology and habitats
Two of the sections included in the present study, the lower
Rannoch Formation and the Yons Nab Beds, have been analyzed for
foraminiferal morphogroups by Nagy (1992). The analysis applied a
system of 4 morphogroups and 7 subgroups based on test shape
combined with life position and microhabitat. Both of the sections are
heavily dominated by the flattened subgroup 4a. In the Rannoch
Formation this subgroup consists mainly of the planispiral A.
yonsnabensis and the low trochospiral T. eoparva. In the Yons Nab
Beds A. yonsnabensis is strongly dominant, and flattened trochospiral
spirillinids are common.
The foraminiferal succession of the Ragnarok and Knorringfjellet
formations appears to have a morphological composition closely
similar to those of the two North Sea Basin sections cited above. The
Ragnarok assemblages are dominated by A. aff. yonsnabensis, T.
minutissima and Evolutinella sp. 1, all belonging to morphosubgroup
4a. The same is the case with taxa dominant in the Knorringfjellet
Formation, A. aff. yonsnabensis and T. aff. eoparva. Based on
comparisons with modern analogues, the species composing
J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
Table 1
List of selected papers used to delineate the regional size distribution of Triassic and
Jurassic Ammodiscus.
Paper
Age
Region
Table 2
List of selected papers used to delineate the regional size distribution of Triassic and
Jurassic Trochammina.
Paper
Triassic Jurassic
Late
Ten Dam (1945)
Kristan-Tollmann (1964)
Copestake (1989)
Wicher and Bartenstein
(1962)
Azbel and Grigyalis
(1991)
Kaptarenko-Chernousova
(1959)
Gerke (1961)
Nagy and Johansen
(1991)
Tappan (1955)
Souaya (1976)
Early Middle Late
x
x
x
Caucasus, Siberia,
Pechora
Ukraine
x
x
North-Central Siberia
North Sea Basin
x
x
x
x
x
x
Wall (1983)
x
x
x
Løfaldli and Nagy (1980)
x
Wall (1960)
Bartenstein (1972)
Dain (Ed.) (1972)
Hedinger (1993)
Late
Netherlands
Austria
England
Germany
x
Age
Region
Triassic Jurassic
x
x
x
x
x
43
x
x
x
x
x
Arctic slope, Alaska
Canadian Arctic
Archipelago
Canadian Arctic
Archipelago
Kong Karls Land,
Svalbard
Saskatchewan, Canada
Eastern Indian Ocean
Western Siberia
Northern Territories,
Canada
subgroup 4a are regarded as mobile epifaunal forms, living on the
seabed surface or on marine vegetation in shallow brackish waters.
5. Modern analogues
This study demonstrates that low diversity agglutinated assemblages dominated by Ammodiscus and Trochammina appear to have
been widespread in Late Triassic to Middle Jurassic restricted environments along the northwestern and western European margin, from
Spitsbergen to the North Sea Basin. In spite of their extensive regional
and temporal ancient distribution, it seems difficult to find modern
analogues to these assemblages. The main reason of this circumstance
should be sought in the large time gap between Jurassic and Recent,
which includes an extensive evolutionary expansion of calcareous
foraminifera that lasted from Late Cretaceous to Recent time. During
this expansion, calcareous taxa became adapted to a wide range of
environments, replacing the originally agglutinated faunas, which
were typical and dominant in many habitats during the Jurassic.
A number of modern oxygen-depleted environments are dominated by small-sized mainly calcareous foraminiferal species, according to Bernhard and Sen Gupta (1999). They cite several small-sized
examples, but also mention that there are exceptions from this
development. The present study demonstrates that reduced dimensions are a feature of low diversity stressed assemblages, and the
authors support the widely-held view that this feature has a double
function. (1) Small size means an increase of the surface area relative
to body volume, enhancing oxygen uptake. (2) Small size reflects
rapid reproduction rate, which is a feature of opportunistic taxa
adapted to rapidly changing conditions typical for several paralic
environments where fast recolonization is an advantage. Two
examples of modern analogue assemblages are recorded below.
Kristan-Tollmann (1964) x
Copestake (1989)
x
Kaptarenko-Chernousova
(1959)
Gerke (1961)
Brouwer (1969)
Souaya (1976)
Said and Barakat (1958)
Ziegler (1959)
Lutova (1981)
Yakovleva (1984)
Nagy and Basov (1998)
Løfaldli and Nagy (1980)
Sharovskaya (1961)
Wall (1960)
Oesterle (1968)
Kuznetsova (1972)
Dain (Ed.) (1972)
Hedinger (1993)
Early Middle Late
Austria
England
Ukraine
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
North-Central Siberia
Northwestern Europe
Canadian Arctic
Archipelago
Sinai, Egypt
Bavaria, Germany
Middle Siberia
Pechora Basin
Spitsbergen
Kong Karls Land,
Svalbard
Northern Siberia
Saskatchewan, Canada
Switzerland
Eastern Indian Ocean
Western Siberia
Northern Territories,
Canada
estuarine stratification owing to the high freshwater discharge into
the head of the fjord by the river Drammenselva (Fig. 12). The
freshwater influx shows large yearly fluctuations changing the
composition of the upper water layers in particular. The salinity of
these layers drops to minimum values in the spring months and
attains peak values during the winter. The presence of a sill 20 km
down from the head of the fjord reduces water exchange with the
more open Oslofjord, contributes significantly to the development of
stagnant deeper water. The water depth is mainly around 10 m at the
river mouth, but increases to a maximum of 124 m close to the sill.
The depth of the sill itself is 10 m.
The upper water layer in Drammensfjord is nearly fresh to
brackish, with a salinity that varies between 1 and 20‰ with the
seasons. A pronounced halocline located around 15 m separates this
highly brackish and unstable upper layer from the underlying
5.1. Drammensfjord
A modern low diversity Ammodiscus-dominated foraminiferal
fauna has been recorded by Alve (1995) from Drammensfjord in
south-eastern Norway. The water column in this fjord reveals a typical
Fig. 11. The size ranges (diameters) of Triassic and Jurassic Ammodiscus and Trochammina
based on published data, compared with the size ranges of Ammodiscus aff. yonsnabensis
and Trochammina aff. eoparva measured in the Knorringfjellet Formation, Festningen
section. Mean values of dimensions are also indicated. Based on datasets used in Figs. 9
and 10.
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J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
Fig. 12. Depth section from Drammensfjord, south-eastern Norway, showing distribution of salinity, dissolved oxygen, alpha diversity index and percentage of the small-sized
Ammodiscus gullmarensis.
Based on data from Alve, 1995.
moderately brackish transitional layer, which extends down to about
38 m and shows a salinity increase from 24 to 30‰. Below this depth,
the salinity distribution is fairly constant at 30.0 to 31.5‰. The amount
of dissolved oxygen in the water column decreases from 8.5 ml/l at
the surface to 4.4 ml/l at the halocline (Fig. 12). Through the
transitional layer the oxygen content decreases further to 1.7 ml/l at
38 m. Below this water depth, anoxic conditions might develop. The
TOC content of the sediments in the transitional zone is on average
0.8%.
According to Alve (1995), the shallowest and highly brackish
waters of the fjord are dominated by the agglutinated form
Miliammina fusca (Brady 1870). The depth interval between this
and the top of the transitional layer (at 15 m) is dominated by the
calcareous species Elphidium excavatum (Terquem 1870) and E.
albiumbilicatum (Weiss 1957). The strongly hypoxic stable deeper
water from 38 to 50 m is dominated by the agglutinated taxon
Spiroplectammina biformis (Parker and Jones 1865) and the opportunistic calcareous form Stainforthia fusiformis (Williamson 1858).
In the moderately brackish transitional zone (15–38 m) the
foraminifeal assemblages are highly dominated by agglutinated taxa
that account for 60 to 100% (mostly 90%) of the assemblages, while
the alpha diversity ranges from 1.9 to 3.9 (average 2.9). The species
most commonly dominant is the small-sized Ammodiscus gullmarensis
Höglund 1948, comprising 21 to 58% (average 37%) of the fauna. Other
dominant species in the transitional zone include the agglutinated
forms Eggerelloides scabrus (Williamson 1858) and Adercotryma
glomerata (Brady 1878), as well as the calcareous form E. excavatum.
Below the halocline, the salinity and oxygen distribution of the
fjord waters shows only minor seasonal fluctuations. The environment is nevertheless subjected to major long term changes, as
demonstrated by the recorded occurrence of a long period (N5 years)
with increased anoxia, when the redox boundary moved upward to a
water depth of 30–35 m (Alve, 1995). This event was followed by
increased oxygenation leading to recolonization by S. fusiformis and S.
biformis, followed by A. gullmarensis.
As shown above, the foraminiferal distribution in Drammensfjord
is controlled by two main restricting factors, low salinity in shallow
waters and hypoxia combined with reduced salinity in the transitional
interval (Fig. 12). In addition, changing conditions create a stressed
habitat. At greater water depths the restricting effect of hypoxia
increases strongly. The foraminiferal fauna occurring in the transi-
tional zone in Drammensfjord shows considerable similarities to the
Triassic–Jurassic assemblages presented here, by virtue of its low
diversities coupled with high dominance of small-sized Ammodiscus.
Calcareous species occurring in this depth zone of the fjord belong to
the abundant genera Elphidium and Stainforthia, and to the less
common Ammonia and Haynesina. However, all these genera have
evolved since the Eocene, according to stratigraphical ranges given by
Loeblich and Tappan (1988).
5.2. Aso-kai Lagoon
This lagoon is located on the western coast of central Japan, and its
benthic foraminiferal assemblages were recorded by Takata et al.
(2005). The lagoon is partially separated from the Sea of Japan by a
sand barrier, and communicates with offshore waters through a
narrow channel. The lagoonal water column is maximum 15 m deep,
brackish (mostly in the shallower layers) and oxygen-poor (particularly at deeper levels) owing to limited water exchange with the
open sea. The hydrography of the water masses reveals large seasonal
changes, with highest temperatures and lowest oxygen values during
the summer.
In winter time, the salinity of the surface waters is around 23‰ and
increases to 29‰ at a halocline located between 3 and 4 m depth
(Fig. 13). From this level and downwards (to the deepest measurements at 12 m) there is a smooth salinity increase to 30‰. The oxygen
content of the lagoon water is highest in February, when the upper
3.5 m of the water column contains 11.0 to 12.6 mg/l oxygen. From
here, there is a rapid drop in oxygen content to 3 mg/l at a depth of
5 m. The underlying depth interval down to 12 m is characterized by
low values, varying from 2 to 4 mg/l.
The foraminiferal diversities of the Aso-kai Lagoon are generally
low but spatially highly variable, with alpha values ranging from 1.0 to
7.9 (average 3.1). The lowest values are found in the deeper parts of
the lagoon (Fig. 13), although also here with large local changes. Most
of the samples from shallow waters were collected along the seaward
margin of the lagoon. In these samples, the assemblages are strongly
dominated by the small-sized Trochammina cf. japonica Ishiwada
1950, attaining 60%, followed by species of Rosalina, Ammonia,
Elphidium, Haynesina and Miliolina.
In the deepest, central parts of the lagoon, the frequency of
Trochammina cf. japonica is strongly reduced. The assemblages here
J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
45
Fig. 13. Depth section from the Aso-kai Lagoon, central Japan, showing distribution of salinity, dissolved oxygen, alpha diversity index and percentage of the small-sized
Trochammina cf. japonica.
Based on data from Takata et al., 2005.
are highly dominated by Stainforthia fragilis (Grindell and Collen, 1976),
an opportunistic species particularly adapted to low oxygen conditions.
It is of interest to note that high proportions of T. cf. japonica are
positively correlated with slightly increased alpha diversities. This is in
contrast to several other brackish Japanese lagoons with a tendency to
hypoxia, where T. cf. japonica is among the most dominant species, as
also exemplified by Lake Saroma (Takata et al., 2005).
The foraminiferal distribution of the Aso-kai Lagoon is controlled by
two main restricting factors, low salinity and reduced oxygen content,
as is the case in Drammensfjord. The common features of the
foraminiferal faunas of the lagoon, and of the analyzed Triassic–Jurassic
assemblages, are the low diversities and the dominance of Trochammina. Also in this case the most obvious difference is that the modern
lagoon contains comparatively large amounts of calcareous taxa,
belonging mostly to post-Jurassic genera, particularly: Virgulina evolved
in the Early Cretaceous, while Rosalina, Ammonia, Elphidium and
Haynesina developed during the Tertiary (Loeblich and Tappan, 1988).
6. Triassic and Jurassic environments of small-sized taxa
The application of modern faunal features to the interpretation of
Triassic–Jurassic foraminiferal facies is complicated by the fact that
modern hyposaline and hypoxic environments can be heavily
dominated by either agglutinated or calcareous taxa, or contain a
balanced mixture composed of both groups, features demonstrated in
several papers (e.g. Alve and Nagy, 1986; Alve, 1990) and by the two
examples presented here.
Low diversity, however is, a common feature of modern and
ancient (e.g. Mesozoic) faunas. As mentioned previously, the large
differences in taxonomic composition are explained by major
evolutionary changes that have taken place in the order Foraminiferida, mainly during Cenozoic times. When interpreting Mesozoic
foraminiferal facies, it is therefore important to integrate faunal data
with sedimentary features, paleogeography and transgressive–regressive relationships.
6.1. Delta-influenced shelf embayment
The Knorringfjellet Formation of Spitsbergen (Fig. 3) and its facies
equivalents in the Barents Sea, the Ragnarok (Fig. 4) and Stø
formations, were deposited in an extensive, shallow marine shelf
embayment covering the western Barents Sea platform. It is assumed
that this embayment received a large freshwater supply from
surrounding land areas, with a particularly massive discharge from
the southeast, as shown by extensive delta outbuilding from this
direction (Van Veen et al., 1992; Riis et al., 2008). Delta outbuilding is
also assumed from the northeast, connected with the ancient
Lomonosov Ridge terrain (Fig. 1).
This paleogeography, involving a large fluvial influx, would explain
the brackish conditions in the shallow Barents Shelf embayment. The
high freshwater input implies the formation of density-stratified
water masses, which potentially would have led to hypoxia in the
lower part of the water column below fair-weather wave base to
storm wave base.
An example of modern shallow water continental shelf hypoxia
was presented by Rabalais et al. (1991) from the inner and middle
shelf of the Gulf of Mexico, off the Louisiana coast. In this area,
development of a stratified water column is mainly controlled by the
freshwater discharge (from the Mississippi and Atchafalaya rivers),
adequate marine current regime and regional wind fields. The
hypoxia develops seasonally, during midsummer, within the 5 to
60 m depth interval.
The sandstone interbeds of the Knorringfjellet Formation are thin
(less than 50 cm), and show sharp base and top contacts (Fig. 3). They
are believed to have been deposited in the offshore-transition zone by
episodic major storm events punctuating the background offshore
mud sedimentation of quite periods. Storm-generated sedimentary
structures have not been observed in these sandstones, obviously
because they have been erased by the intensive bioturbation. This
development of bioturbation is in accordance with increased bottom
water oxygenation during storm events.
The calcium carbonate contents of the Knorringfjellet Formation
are extremely low in the Festningen section (averaging 0.76% in the
Tverrbekken and 0.61% in the Teistberget member), and similar values
were also obtained in central Spitsbergen. Such low values, close to
the detection limit, are expected with deposition in hyposaline
waters. The absence of unequivocal normal marine salinity indicators
(such as ammonites, belemnites and echinoderms) points in the same
direction.
The TOC content of the sediments forming the Festningen section
is intermediate (averaging 1.3% in the Tverrbekken and 0.8% in the
Teistberget member), and the same is the case in central Spitsbergen
(average 1.0%). These rather modest values are lower than those of
typical hypoxic deposits. This does not, however, exclude seasonal
hypoxia since an originally high organic influx might have been
diluted by massive input of silt and clay from active deltaic sources.
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J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
6.2. Prodelta–delta front transition
In the northern North Sea (Gullfaks Field), small-sized Ammodiscus–
Trochammina assemblages occur in the lower part of the Rannoch
Formation (Fig. 5) deposited in the East Shetland Basin, a marine paleoembayment between the East Shetland Platform and Norway. During
the Middle Jurassic this embayment was the site of extensive delta
progradation resulting in the deposition of the Brent Group delta system,
the lower part of which is made up of the Rannoch Formation (Fig. 1). As
shown by its coarsening upward lithology, the lower Rannoch
succession of sandy mudstone with subordinate sandstone interbeds
represents a transition from prodelta to delta front conditions in a distal
delta front setting (Parry et al., 1981; Nagy and Johansen, 1991).
The lower Rannoch sandstone interbeds are thin, bioturbated and
are typified by abrupt basal and top contacts. They have been
interpreted as intermittent storm deposits by Brown et al. (1987).
Consequently, the depositional setting of the lower Rannoch was the
shelf-transition zone, above storm wave base, with fair-weather mud
deposition.
During the outbuilding of the Rannoch delta, extensive fluvial
discharge from the south created hyposaline conditions in the
prodelta–delta front area. The carbonate content in the lower part
of the analyzed interval is extremely low (0.0 to 3.8%) and accords
well with hyposaline conditions. In the upper part of the interval, the
carbonate values are strongly variable (0 to 37%) but it is possible that
the carbonate here is siderite or ankerite.
The TOC content of the sediments is intermediate (average 1.8%),
and accords with extensive influx of organic matter to a delta frontprodelta area where the siliciclastic depositional rate was high.
Intensive freshwater supply to this area might have created a salinitystratified water column with hypoxic conditions in near-bottom
waters. The strongly variable foraminiferal assemblage composition
suggests changing hydrographic conditions (Fig. 5).
6.3. Interdistributary bay
The Yons Nab Beds (Fig. 6) together with the underlying Millepore
Bed comprise a transgressive–regressive cycle that has inundated a
delta plain of the Middle Jurassic deltaic series of Yorkshire (Fig. 1).
The main transgressive phase is represented by the Millepore Bed of
calcareous sandstones, which were deposited in shallow water
marine sandwave environments. The regressive phase is formed of
the Yons Nab Beds of mudstones and sandstones, deposited in shallow
water interdistributary bay environments (Hancock and Fischer,
1980). This regressive stage is succeeded by a shale-sandstone
package, including freshwater swamp deposits.
The relatively coarse lithologies of the Yons Nab Beds with distinct
vertical changes are in accordance with the shallow water marginal
marine nature of the environment. Thinner sand beds with sharp base
and top contacts probably represent storm deposits, while thicker
ones with upwards-coarsening base are regarded as minor mouth bar
progradations (Fig. 6). The interdistributary bay depositional setting
of these strata implies hyposaline depositional conditions, although
major changes in sediment influx and salinity are suggested by the
lithology and foraminiferal fauna. The calcium carbonate content is
generally low (mainly below 2%, maximum 12%).
The foraminiferal succession of the Yons Nab Beds is dominated by
agglutinated taxa (mainly Ammodiscus), but the lower and particularly the middle parts of the section contain rather large amounts of
calcareous forms. The calcareous group is mainly composed of species
belonging to spirillinids (Fig. 6) but species of nodosariids are also
significant. Levels with increased frequencies of calcareous taxa
represent intermittent ingressions of marine salinities into an overall
brackish environment with low species diversities. The marine impact
culminates in the middle part of the section, signaled by peak values
of calcareous foraminifera and associated shell beds and crinoid
stems.
As mentioned previously, the Yons Nab Beds contain ostracod
shells in 65% of the samples although in strongly varying abundance.
The presence of these highly active biota in significant amounts can be
taken as indicative of well-oxygenated bottom waters at certain
horizons in the shallow water interdistributary bay succession.
Likewise, intervals with an absence of ostracods but high frequencies
of Ammodiscus imply hypoxic bottom water conditions in a salinitystratified water column.
6.4. A depositional biofacies model
The scenario for the Late Triassic to Early Jurassic delta progradations in the North Sea–Mid Norway–Barents Sea region is an
extensive, north–south trending seaway (Fig. 1). It was widely open
in the north towards the paleo-Arctic Ocean, and continued
southwards between Greenland and Norway as a narrow inlet. In
the Late Triassic, the southern termination of the inlet was located
outside Mid Norway (Nagy and Berge, 2008). During Early Jurassic
time, this seaway extended further southward, and connected the
paleo-Arctic Ocean with the western European shelf seas. This
southwards extension provided a marine pathway for migration of
foraminiferal faunas between deltas of the Barents Shelf, northern
North Sea and Yorkshire.
The main factors controlling the habitats of the small-sized
Ammodiscus–Trochammina assemblages were low salinity combined
with reduced access to oxygen in a density-stratified water column, as
suggested by the main faunal features and analogies with modern
assemblages recorded above. A depositional biofacies model delineating the environments of these assemblages envisages extensive
but shallow prodelta shelf to delta front areas (Fig. 14), or an
interdistributary bay in the case of the Yons Nab Beds. The salient
feature of the model is the extensive influx of river water from deltaic
sources. In such settings fluvial discharge creates salinity-stratified
water masses, with a tendency to hypoxia below fair-weather wave
base and particularly below storm wave base. This development
implies rapidly changing bottom water properties creating stressful
conditions for benthic biota.
According to the prodelta–delta front model (Fig. 14), the
Ammodiscus–Trochammina assemblages occupied the offshore-transition zone (located between fair-weather and storm wave base)
where a moderate degree of hypoxia is combined with reduced
salinity. As demonstrated by the modern Drammensfjord, a combination of these factors favored the development of low diversity
Ammodiscus-dominated faunas in the depth interval between the
fresh to highly brackish surface water layer and the strongly oxygendepleted deeper water (Fig. 12).
The depositional setting of the Ammodiscus–Trochammina assemblages in the offshore-transition zone is supported by the sedimentary
succession of the analyzed sections, consisting mainly of mudstones
with interbeds of thin sandstones typically developed in the
Knorringfjellet and Rannoch formations (Figs. 3 and 5). The sandstone
beds are interpreted as storm sands interrupting the background
sedimentation of offshore muds (containing the foraminifera). This
bathymetric setting implies periodic mixing of the upper water layers,
leading to intermittent increase of oxygenation during storm events.
Emplacement of storm sands would wipe out the foraminiferal faunas
of the area, which would be recolonized in subsequent calm periods
by low diversity small-sized assemblages.
Another but principally related environmental setting for these
assemblages is a deltaic interdistributary bay, which is suggested by
the stratigraphic and paleogeogtaphic position of the Yons Nab Beds
of Yorkshire (Fig. 6). Presence of upwards-coarsening sandstone beds
of supposed distributary mouth bar origin is consistent with such a
J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
47
Fig. 14. Depositional biofacies model of the small-sized Ammodiscus–Trochammina assemblages, developed particularly for the Knorringfjellet Formation of Spitsbergen with its
correlative units in the Barents Sea, and for the lower Rannoch Formation of the North Sea.
depositional environment. Storm impacts ate signaled by thinner,
sharp-bounded sandstones.
7. Conclusions
The analyzed sections from Spitsbergen, the Barents Sea, North Sea
and Yorkshire span the time range from Late Triassic to Middle
Jurassic and display sedimentary successions consisting of mudstones
and sandstone deposited in shallow shelf to deltaic environments. The
sections contain mainly agglutinated (benthic) foraminifera, commonly in high abundance. Distinctive compositional features of the
foraminiferal successions are explained by adaptation to restricted
environmental conditions, which means conditions divergent from
those of a normal marine shelf. Salient features of the assemblages are
outlined below:
1) The assemblages consist exclusively or almost entirely of agglutinated taxa, except for segments of one of the studied sections
(Jons Nab). These agglutinated assemblages are attributed to
restricted environments where the controlling factors were low
salinity combined with low amounts of dissolved oxygen in storminfluenced temporarily unstable habitats. Absence of calcareous
foraminifera from such settings is explained by the presumption
that Triassic and Jurassic calcareous taxa (mainly nodosariids)
were not adapted to restricted conditions, as is still the case with
their modern descendants.
2) The faunal diversities are expressed by the number of species, alpha
index and the Shannon–Weaver index H(S). The measured values
are low and accord with restricted conditions; the alpha indices are
clearly below 5 which is the lower diversity limit displayed by
modern normal marine shelf assemblages.
3) The test dimensions of foraminifera are generally smaller than
usual. This has been quantified by measuring the diameter of the
two dominant species of the Knorringfjellet Formatuin, A. aff.
yonsnabensis and T. aff. eoparva, and comparing these data sets
with dimensions of Triassic and Jurassic Ammodiscus and
Trochammina extracted from the literature. The comparison
demonstrates that the average diameter of A. aff. yonsnabensis is
46% of that usual for the genus Ammodiscus, while the average
diameter T. aff. eoparva is 68% of that usual for Trochammina.
4) Ostracods are absent from the analyzed sections, except in the Yons
Nab Beds where they are most common at horizons containing
significant amounts of calcareous foraminifera. The presence of these
highly active crustaceans indicates better-oxygenated bottom
waters, at least if they occur in adequate abundance.
Modern analogues to the small-sized Ammodiscus–Trochammina
assemblages are difficult to find owing to the large time gap, which in
addition includes a major evolutionary expansion of calcareous
foraminifera that radically changed assemblage compositions through
time. A close analogue occurs in the modern Drammensfjord
(Norway) where the small-sized A. gullmarensis is dominant in a
low salinity oxygen-depleted interval of the density-stratified water
column. Another cited analogue is the Aso-kai Lagoon (Japan) where
Trochammina cf. japonica dominates in low salinity oxygen-poor
waters with density stratification.
The facies features and depositional setting of the studied sections
reveal useful criteria for recognizing the habitat of the small-sized
Ammodiscus–Trochammina assemblages: (1) The Knorringfjellet
Formation of Spitsbergen and correlative strata of the Barents Sea
(the Ragnarok and Stø formations) were deposited in the same
extensive shallow shelf embayment with a high deltaic influx
creating a brackish, salinity-stratified water column with a tendency
to develop bottom water hypoxia. (2) The lower Rannoch Formation
of the northern North Sea was deposited at the prodelta to delta front
transition in a wide shelf embayment receiving high deltaic
discharge leading to a brackish, stratified water column potentially
oxygen-depleted in the lower parts. (3) The Yons Nab Beds of
Yorkshire are referred to a delta plain interdistributary bay setting
with hyposaline and hypoxic conditions intermittently modified by
open marine influence.
48
J. Nagy et al. / Earth-Science Reviews 99 (2010) 31–49
The foraminiferal assemblage occurring in the studied sediment
packages reveal close similarities, which indicate marine faunal
communications between the various sedimentary basins. The
pathway of these communications were establishes in the Early
Jurassic along Greenland and Norway, and connected the paleo-Arctic
Ocean and the Barents Shelf with the western European shelf seas.
A depositional model envisaging the main environment of the low
diversity small-sized Ammodiscus–Trochammina assemblages applies
a combination of hyposaline and hypoxic conditions formed in a
gravity-stratified water column created by high deltaic discharge
(Fig. 14). The occurrence of these assemblages in mudstone packages
with thin sandstone interbeds, suggests deposition in the offshoretransition zone, between fair-weather and storm wave base. This
setting implies large temporal variations of ecological factors, such as
salinity and dissolved oxygen, increasing the stress level of the
habitats of benthic organisms. Storm impacts contributed to the
instability of these environments.
Acknowledgments
The project was supported by the StatoilHydro-VISTA program,
which covered general research expenses. The University Centre of
Svalbard is gratefully acknowledged for making field work at
Festningen possible. We are also indebted to Henning Dypvik for
giving access to samples from the Mjølnir structure. Special thanks are
extended to Merethe G. A. Bremer and Sigrid H. Berge for inspiring
discussions. We much appreciate the comments of Sorin Filipescu and
Adrian Read improving the manuscript.
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