The Christian Malford lagerstätte in the Oxford Clay
Formation of Wiltshire contains exceptionally well-preserved squid-like
cephalopods, including Belemnotheutis antiquus (Pearce). Some of these fossils preserve muscle
tissue, contents of ink sacks and other soft parts of the squid, including
arms with hooks in situ and the head area with statoliths (ear bones) present in
life position. The preservation of soft-tissue material is usually taken as
an indication of anoxic or dysaerobic conditions on the sea floor and within
the enclosing sediments. Interestingly, in the prepared residues of all
these sediments there are both statoliths and arm hooks as well as abundant,
species-rich, assemblages of both foraminifera and ostracods. Such
occurrences appear to be incompatible with an interpretation of potential
sea floor anoxia.
The mudstones of the Oxford Clay Formation may have been compacted by
70 %–80 % during de-watering and burial, and in such a fine-grained
lithology samples collected for microfossil examination probably represent
several thousand years and, therefore, a significant number of foraminiferal
life cycles. Such samples (even if only 1–2 cm thick) could, potentially,
include several oxic–anoxic cycles and, if coupled with compaction,
generate the apparent coincidence of well-preserved, soft-bodied,
cephalopods and diverse assemblages of benthic foraminifera.
Introduction
During the construction of the Great Western Railway west of Swindon in the
1840s “borrow pits” were excavated to provide material for the adjacent
railway embankment. The mudstones of the Oxford Clay Formation (Callovian,
Jurassic) yielded a large number of exceptionally well-preserved coleoid fossils
(Pearce, 1841; Owen, 1844; Mantell, 1848), many of which have been
redescribed by Donovan (1983), Page (1991), Page and Doyle (1991), and
Donovan and Crane (1992). The majority of these specimens can be attributed
to the Phaeinum Subchronozone (Athleta Chronozone, upper Callovian, Middle
Jurassic), and many contain fossilized soft tissues, muscle fibres and the
cell content of their ink sacks (Wilby et al., 2004, 2008; Hart et al.,
2016a). As these mudstones also contain species-rich and abundant
assemblages of microfossils that appear to indicate a normal, oxygenated sea
floor this would look to be incompatible with soft-bodied preservation, which
is often taken to indicate a lack of sea floor oxygen and rapid burial. This
research explores this apparent contradiction and looks at the issue of
taphonomy and the impacts of sediment compaction.
Locality map of Christian Malford (Wiltshire). The excavation site
was immediately to the south of the railway line, as near to the original
“borrow pits” as possible.
Materials and methods
The original excavations near Christian Malford and the subsequent discovery
of comparable material near Ashton Keynes (Wilby et al., 2004) stimulated
this reinvestigation of the Christian Malford lagerstätte. In 2006
exploratory drilling was undertaken close to the railway line south of the
village of Christian Malford in order to provide stratigraphical control for
the subsequent excavations (Fig. 1). In October 2007, the full-scale
excavation of a “pit” (surface area 32 m2) was undertaken but at
∼5 m depth became flooded, and fossil collecting was limited
to ∼240 t of spoil that had been arranged in approximate
stratigraphical order around the pit prior to flooding. This material was,
however, unsuitable for accurate micropalaeontological sampling and, from
the sites drilled in 2006, Core 10 was selected for processing (Fig. 2).
Core 10 was ∼5 m in length and preserved in six sections, all
of which had been studied for macrofauna, resulting in parts of the core
being fragmented and, in places, rendered unsuitable for very close
sampling. Each section was approximately 62–108 cm in length and, after
disregarding the top 10 cm of weathered mudstone and soil, 41 samples were
collected. After removing the disturbed outer surface of the core, the
samples were broken into small pieces, dried at <40∘C
and weighed. Once dried the samples were processed using the Brasier (1980)
white spirit method whereby each sample was soaked in the solvent which,
after ∼4 h, was then decanted and the sample immersed in
deionized water for <24 h (until disaggregated). Samples were
washed on a 63 µm sieve, filtered and dried in an oven at <40∘C. Samples were investigated in the >500,
500–250, 250–150 and 150–63 µm size fractions. All
fractions were weighed prior to picking and/or counting. In the case of
foraminifera a minimum of 250–300 individuals were counted in each size
fraction, but the ostracods, otoliths, statoliths (Hart et al., 2009, 2013,
2015a, 2016a; Clarke, 2003; Clarke and Hart, 2018) and arm hooks (Hart et
al., 2016a, 2019) were treated differently as, particularly in the case of
statoliths, there are no counting protocols to follow. Weighing samples to
determine foraminiferal numbers was undertaken but, as many of the specimens
are in-filled with pyrite, any calculations based on these figures would
probably be invalid. One major problem is that of adherent foraminifera,
which are abundant in these Middle Jurassic mudstones. While many specimens
of Bullopora, Vinelloidea and Nubeculinella have become detached, and could be counted alongside other
foraminifera, many remain attached to shell fragments (e.g. Hart et al., 2009,
Fig. 3), statoliths, otoliths and even other foraminifera. Specimens of
Bullopora, once detached from the host surface, often break into one, two or several
fragments, thereby making any counts almost meaningless.
Core 10 sediment log, lithostratigraphy and biostratigraphy. Gaps
in the log indicate no, or very reduced, recovery. Adapted from Hart et al. (2016a) with permission.
While the foraminifera and ostracods are typical of Callovian–Oxfordian
strata elsewhere in the UK and northern France (Cordey, 1963a, b; Coleman,
1974, 1982; Gordon, 1965, 1967; Barnard et al., 1981; Shipp, 1989; Morris
and Coleman, 1989; Henderson, 1997; Page et al., 2003; Oxford et al., 2000,
2004; Wilkinson and Whatley, 2009), the statoliths (Clarke, 2003; Hart et
al., 2015a, 2016a; Clarke and Hart, 2018) and arm hooks (Hart et al., 2016a,
2019) are both exceptional and relatively little-known. Formal taxonomy of
the statoliths is still in progress, and in all cases open nomenclature has
been used. At the present time the taxonomy and stratigraphical distribution
of the ostracods has not been completed.
Foraminifera
The benthic foraminifera recorded in Core 10 are dominated by calcareous
taxa, with relatively few agglutinated species (Fig. 3). The absolute
abundance of benthic foraminifera, when measured as number per gram is relatively
low (Hart et al., 2016a, fig. 8) even though the assemblages are quite
diverse and appear abundant. The heterogeneity (H) fluctuates throughout the
succession (average 0.5–1.0) while the dominance (as percentage) is highly
variable (Hart et al., 2016a, fig. 9). When the assemblage is subdivided
into agglutinated, aragonitic and calcitic taxa (Fig. 4), a number of
patterns emerge. As the agglutinated foraminifera are generally rare, the
graphs of aragonitic and calcitic taxa are, quite clearly, showing the same
variation. There are only two samples in which there was a very slight
increase in the agglutinated taxa (∼205 and
∼365 cm core depth). There appear to be no differences in
sediment type at these levels or evidence of any hiatus.
Illustration of some of the foraminifera recovered in the samples
from Core 10: (a)Verneuilinoides tryphera (scale bar 50 µm); (b)Verneuilinoides sp. 2 Morris and Coleman, 1989
(scale bar 50 µm); (c)Trochammina sp. (scale bar 100 µm); (d)Oolina sp. (scale bar
20 µm); (e)Oolina sp. (scale bar 100 µm); (f)Eoguttulina liassica (scale bar 100 µm); (g)Frondicularia irregularis (scale bar 100 µm); (h)Dentalina pseudocommunis (scale bar 100 µm); (i)Citharina flabellata (scale bar 100 µm); (j)Nodosaria hortensis (scale bar 100 µm); (k)Frondicularia franconica (scale bar 100 µm); (l)Lenticulina muensteri (scale bar
100 µm); (m)L. muensteri with no umbilical boss (scale bar 100 µm); (n)L. muensteri showing
uncoiling (scale bar 100 µm); (o)Lenticulina sp., showing distinct uncoiling and
thickened sutures (scale bar 100 µm); (p)L. muensteri, showing test deformation
(scale bar 100 µm); (q)Lenticulina sp., showing chamber deformation and, what
appears to be an additional chamber (scale bar 100 µm); (r, s)Epistomina regularis (scale bar 100 µm); (t)Epistomina stellicostata Bielecka and Pozaryski (scale bar 100 µm);
(u)Reinholdella lutzei (scale bar 100 µm); (v, w)Conoglobigerina sp., an example of a pyrite steinkern of
a planktic foraminiferid (scale bar 50 µm); (x)Bullopora sp. adherent on shell
fragment (scale bar 200 µm). Reproduced from Hart et al. (2016a) with
permission.
If the distribution of aragonitic taxa is subdivided into the component
species, then the dominance of Epistomina regularis becomes apparent (Fig. 5). The graph in
Fig. 5 shows a number of oscillations with “peaks” at 60–80, 140–150, 260–280, ∼375 cm and (perhaps) ∼425 cm. Oxford et al. (2004) have reported a similar variability in the
distribution of epistominids in the Oxfordian strata of the Dorset coast
east of Weymouth and suggested that this may be picking up the maximum
flooding “zones” of para-sequences. The distributions of both statoliths
(Hart et al., 2016a, fig. 4) and otoliths (Hart et al., 2016a, fig. 5) show
patterns with some features in parallel with the distribution of the
epistominids though the reasons for this are not obvious as both coleoids
and teleost fish are nektonic, rather than benthic, organisms.
Distribution of foraminifera (agglutinated, aragonitic, calcitic).
Adapted from Hart et al. (2016a) with permission.
Distribution of epistominids in Core 10, showing the “flood” of
Epistomina stellicostata at 60 cm downhole, and the potential cyclicity in the recorded numbers of
E. regularis. Adapted from Hart et al. (2016a) with permission
The “flood” of E. stellicostata is significant as, in the British Geological Survey pit, large slabs of slightly
fissile mudstone covered in foraminifera were also found (Hart et al.,
2016a, fig. 12). Some of these epistominids contain the chitinous inner
linings of chambers (Wilby et al., 2004, text-fig. 2) and clearly indicate
an unusual degree of preservation, possibly suggestive of dysaerobic or even
anoxic conditions on the sea floor or just below the water–sediment
interface. This may be the only evidence of a short-term event recorded by
our foraminiferal investigation. Winnowing does not seem to be an option, as
a mechanism to concentrate only one species of epistominid is impossible to
envisage.
Taphonomy
Throughout the samples from the excavations and the cores, many of the
macrofossils are compressed, and this includes both the ammonites (e.g.
Kosmoceras phaeinum) and the coleoid phragmacones. This is normal in Jurassic mudstones and
indicates a certain degree of post-depositional de-watering and compaction.
The most conspicuous macrofossils are the bivalves Bositra and Meleagrinella and the gastropod
Dicroloma. Though often appearing complete, when samples were processed, cracks caused
by compaction often caused specimens to collapse. Wilby et al. (2008) also
recorded levels in which there was a conspicuous absence of benthos and
interpreted this as indicative of inhospitable conditions on the sea floor.
This observation was also supported by the frequent occurrence of
microscopic juvenile bivalves (spat) that did not grow to maturity. As these
spat have been found in microfossil residues containing foraminifera and
ostracods, their failure to develop may be associated with other
environmental issues. There is also evidence of soft tissue preservation
which is often used as an indicator of dysaerobic or even anoxic conditions
on the sea floor (Wilby et al., 2004, 2008). However, all our samples
contain both foraminifera and ostracods, which is suggestive of oxygenated
conditions on the sea floor. The question is, therefore, how one resolves
this apparent disagreement in the interpretation of the sea floor conditions
that generate both abundant and diverse assemblages of foraminifera and a
world-famous lagerstätte for the soft-bodied preservation of coleoid
cephalopods (e.g. Belemnotheutis).
Modern environments
Modern samples from a range of locations in south-west England have been
investigated for foraminifera, including Plymouth Sound (Castignetti, 1997),
offshore Plymouth (Hart et al., 2016b), Fowey Estuary (Hart et al., 2014)
and Fal Estuary (Olugbode et al., 2005; Hart et al., 2015b, 2017). Samples
from all these locations were fixed with ethanol and, during processing,
were stained with rose bengal in order to identify the individuals that were
living at the time of collection. In all samples it was found that living
foraminifera are relatively rare, often comprising only ∼1 % of the total assemblage recovered. This indicates that most sea floor
samples contain large numbers of dead foraminifera, probably representing
several seasonal cycles, as well as any specimens brought in by wave or
storm activity (Hart et al., 2016b). Muddy samples, which often contain more
organic material, usually contain a more abundant and diverse assemblage
compared to sediments with a higher sand or silt component (e.g. Plymouth
Sound; see Oxford et al., 2004, fig. 7). The mud-rich samples from Stations
L4 and E1 of the Western Channel Observatory (Smyth et al., 2015; Hart et
al., 2016b) from water depths of ∼50 and ∼75 m respectively are the closest to the environments represented by the
Callovian mudstones of Christian Malford. Samples collected at these
locations and the nearby Hillsand Station (Hart et al., 2016b, fig. 1) all
contain <1 % living foraminifera even when collected in early
June. The greater part of the assemblage must represent life cycles from the
previous years as well as transported individuals. Samples from box cores at
<50 cm subsurface contain comparable assemblages of foraminifera
to the surface, but the clays are, even at this depth, beginning to
consolidate with reduced water content.
Jurassic mudstones
During compaction (Sarda and Yang, 1998; Yang and Aplin, 2010), as a result
of burial and de-watering, the clay-rich sediments will compact more than
those with a silt or sand component, perhaps by up to 75 %–80 % (Fig. 6).
With rising levels of compaction, the clay-rich samples become less porous
and permeable, eventually becoming an aquiclude. This suppresses water
movement and reduces the possibility of post-depositional dissolution.
Oxygen levels would also become reduced, and this would enhance the
preservation of aragonitic microfossils such as epistominid foraminifera,
otoliths and statoliths. The preservation of organic material within the
sediment would also be enhanced, leading to the preservation (Wilby et al.,
2004) of soft-bodied fossils that had – for various reasons – survived on,
and just below, the sea floor. The original sea floor assemblage of several
life cycles of foraminifera will be enhanced by the reduction in sediment
thickness, and any micropalaeontological work, even if based on samples of
only 1–2 cm vertical thickness, would clearly represent an unknown number of
life (i.e. annual?) cycles.
Model for the preservation of the microfossil assemblage and
compaction of sediments from the Oxford Clay Formation.
Horizons within these mudstones that may have been dysaerobic, or even
anoxic, for a short period of time could easily be “lost” within samples
that still yield a “normal” assemblage of foraminifera and other
microfossils. If the dysaerobic–anoxic horizons were temporary, then the
development of a characteristic agglutinated assemblage (Hart, 2018) may not
have had the time to develop or, being mixed with a normal assemblage from
the immediately adjacent sediments, would not be detectable after
processing.
An average sample thickness of 2–5 cm is, therefore, incapable of resolving
the foraminiferal assemblage to the accuracy required to detect temporary
anoxia and the preservation of a coleoid fossil with in situ soft parts. Even
sampling to a resolution of 1–2 cm would be incapable of resolving the
assemblage into seasonal cycles within a mudstone succession compacted by
75 %–80 % (or even less). This could mean that temporary anoxia, capable of
soft-bodied preservation, is masked by an apparently normal, benthic
foraminiferal assemblage.
One possible test of this concept would be to sample a thin (0.2–0.5 cm)
horizon exactly contiguous with an exceptionally preserved coleoid as well
as the sediment immediately below and above the fossil. This might detect
differences, although the resolution of such fine sampling may not be
enough. This would probably have to be done in the field, adjacent to a
fossil, as few museums would entertain sampling their valuable specimens in
such a way even if it were physically possible.
Summary
In the Oxford Clay Formation (Callovian–Oxfordian) of southern England
there is a species-rich, abundant assemblage of foraminifera and ostracods,
though the latter have not been fully investigated. These mudstones are
highly compacted as many macrofossils (ammonites, coleoid phragmacones,
etc.) are completely flattened. The presence of well-preserved members of the
superfamily Ceratobuliminidae (sensu Loeblich and Tappan, 1987) is indicative of
the assemblage not being impacted by dissolution caused by migratory fluids
and groundwater. As the sedimentation rate was probably quite low, rapid
burial was probably not a contributory factor in the preservation debate.
From the investigation of modern assemblages it is known that most sea floor
samples contain the record of several life cycles as well as the presence of
transported and re-worked foraminifera. If the mudstones have been compacted
by >70 %, then the average core sample of 2–5 cm thickness is
incapable of the identification of the temporary dysaerobic or anoxic
conditions required for the preservation of soft-bodied macrofossils such as
coleoids. This rare occurrence of pavements of epistominid foraminifera, some
containing the original organic chamber linings, points to such levels of
temporary anoxia. This interpretation may help to resolve the current
conflict between the presence of soft-bodied fossils (indicating potentially
anoxic conditions) and species-rich assemblages of foraminifera (indicating
oxic conditions).
Data availability
All samples and picked slides are in the collections of the School of Geography, Earth & Environmental Sciences, University of Plymouth.
Taxonomic notes on foraminifera
The species mentioned in the text are well known from Jurassic strata in the
UK and a full taxonomy is not presented. The species are listed in
alphabetical (not taxonomic) order.
Bullopora rostrata Quenstedt, 1857: p. 580, pl. 73, fig. 28.
Citharina flabellata (Gümbel, 1862) =Marginulina flabellata Gümbel, 1862: p. 223, pl. 3, fig. 24.
Dentalina pseudocommunis Franke, 1936: p. 30, pl. 2, fig. 20.
Eoguttulina liassica (Strickland, 1846) =Polymorphina liassica Strickland, 1846: p. 31, text-fig. b.
Epistomina regularis Terquem, 1883: p. 379, pl. 44, figs. 1–3.
Epistomina stellicostata Bielecka and Pozaryski, 1954: p. 71, pl. 12, fig. 60a–c.
Frondicularia franconica Gümbel, 1862: p. 219, pl. 3, fig. 13a–c.
Frondicularia irregularis Terquem, 1870: p. 125, pl. 4, fig. 12a, b.
Lenticulina muensteri (Roemer, 1839) =Robulina muensteri Roemer, 1839: p. 48, pl. 20, fig. 29.
Neogloboquadrina pachyderma (Ehrenberg, 1861) =Globigerina pachyderma (Ehrenberg) =Aristerospina pachyderma Ehrenberg, 1861: p. 276–277, 303,
but figured by Ehrenberg, 1873 (for 1872), pl. 1, fig. 4.
Nodosaria hortensis Terquem, 1866: p. 476, pl. 19, fig. 13.
Reinholdella lutzei Barnard, Shipp, and Cordey, 1981: p. 432, pl. 4, figs. 3, 7.
Verneuilinoides tryphera Loeblich and Tappan, 1950: p. 42, pl. 11, fig. 16a, b.
Author contributions
Fieldwork was carried out by MBH, KNP and GDP, stratigraphy by KNP, micropalaeontology and SEM imaging by MBH and CWS, and all were involved in discussion and editorial work.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
The authors wish to thank the British Geological Survey
(especially Phil Wilby) for access to the material from Christian Malford
and the two reviewers who prompted improvements to the original manuscript.
Review statement
This paper was edited by Thomas M. Cronin and reviewed by Laura Gemery and one anonymous referee.
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