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ROLLING BACK THE ‘MUDSTONE BLANKET’: COMPLEX GEOMETRIC AND FACIES RESPONSES TO
BASIN ARCHITECTURE IN THE EPICONTINENTAL OXFORD CLAY FORMATION (JURASSIC, UK)

 * March 2022
 * Newsletters on Stratigraphy 56(1)

DOI:10.1127/nos/2022/0685
Authors:
Mark A. Woods


Mark A. Woods
 * This person is not on ResearchGate, or hasn't claimed this research yet.



Jan Hennissen
 * British Geological Survey



Andrew J. Newell


Andrew J. Newell
 * This person is not on ResearchGate, or hasn't claimed this research yet.



Keith Duff
 * Independent Researcher



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1

Rolling back the ‘mudstone blanket’: complex geometric and facies responses to
basin architecture 1
in the epicontinental Oxford Clay Formation (Jurassic, UK) 2
Mark A. Woodsa*, Jan A. I. Hennissena, Andrew J. Newellb, Keith L. Duffc, Philip
R. Wilbya 3
aBritish Geological Survey, Keyworth, Nottingham, UK, NG12 5GG 4
bBritish Geological Survey, Maclean Building, Wallingford, UK, OX10 8BB 5
c 21 Bishops Walk, Barnack, Stamford, Lincs, UK, PE9 3EE 6
*Corresponding author: Email: maw@bgs.ac.uk 7
Abstract 8
Facies variability of mudstones is likely greater than generally perceived, with
important implications 9
for their behaviour in major civil engineering, energy and waste disposal
applications. Here, we 10
explore this variability for the UK Oxford Clay, a widely studied Middle/Upper
Jurassic mudstone. 11
Evidence from wire-line logs, geochemistry, sequence stratigraphy and biofacies
analyses are 12
combined to reveal heterogeneity within the Peterborough Member (Lower Oxford
Clay) and to 13
explore the extent to which it blanketed basin features or responded dynamically
to them. Thickness 14
modelling suggests that the Mid North Sea High, formed by Mid Jurassic thermal
doming, likely 15
influenced sediment pathways, favouring thick sediment accumulation in the
Wessex Basin, thinner 16
successions across the East Midlands Shelf, and sediment starvation in the Weald
Basin. Biofacies 17
patterns, determined using a novel combination of detrended correspondence and
cluster analysis, 18
vary significantly and suggest a complex patchwork of environments related to
local basin setting. 19
The Type Section of the Peterborough Member seems to represent only a narrow
range of 20
conditions that influenced its deposition, and cautions against developing
basin-scale models based 21
on a few well exposed and heavily researched outcrop successions. 22
Key Words: Callovian, mudstone, heterogeneity, sequence stratigraphy, XRFS
chemostratigraphy, 23
biofacies 24
1. Introduction 25
26
Recent research on the processes of mudstone deposition question the long-held
view that these 27
sediments can be understood as a ‘mud-blanket’ resulting from slow deposition
from suspension in 28
quiet water conditions (e.g. Scheiber et al. 2007, Macquaker and Bohacs 2007,
Birgenheier et al. 29
2017). Earlier work on both the Oxford Clay (Hudson and Martill 1991) and the
Kimmeridge Clay 30
(Wignall, 1989) suggested that more episodic depositional events, including
storms, may be 31
important factors in mudstone accumulation. Temporal and spatial variability of
facies in the lower 32
part of the Oxford Clay were mentioned by Hudson and Martill (1991), and wider
evidence of 33
mudstone facies heterogeneity has emerged from outcrop and borehole studies
(Macquaker 1994, 34
Macquaker and Howell 1999, Birgenheier et al. 2017). Here, we adopt a
multidisciplinary surface-to-35
subsurface approach to test the extent of facies variation at basin scale across
a range of contrasting 36
palaeogeographical settings for part of the Oxford Clay Formation (Peterborough
Member) – a mud-37
dominated unit that has historically been regarded as generally uniform
(Callomon 1968, Holloway 38
1985). Limited outcrop studies point to the presence of vertically stacked
lithofacies that can be 39
related to patterns of relative sea level change and sequence stratigraphy
frameworks (Partington et 40
al. 1993, Macquaker 1994, Norris and Hallam 1995, Macquaker and Howell 1999,
Hesselbo 2008). 41
Our work combines these localised outcrop data with an extensive subsurface data
archive,

42



2

FIGURE 1 43



3

providing a fully contextualised model-based understanding of the likely extent
of facies variability 44
across the entire preserved basin, allowing understanding of the role of
structural/bathymetric 45
features in controlling facies patterns. 46
Current knowledge of the Oxford Clay is strongly influenced by a few
well-exposed onshore 47
sites distributed across its outcrop (particularly the type sections of the
component members in 48
Cambridgeshire (Peterborough Member), Bedfordshire (Stewartby Member) and Dorset
(Weymouth 49
Member); e.g. Hudson and Martill 1991, 1994, Kenig et al. 1994, Macquaker and
Howell 1999, Page 50
et al. 2009; Figs 1, 2). These outcrops, however, provide an incomplete picture
of the Oxford Clay 51
because much of the formation is concealed beneath a thick (up to 1km) cover of
younger strata in 52
areas such as the Wessex-Channel Basin in southern England. Fortunately, a long
history of 53
hydrocarbons exploration in these deep basins has left a rich source of borehole
information. Here, 54
we make use of this subsurface information to elucidate the extent of facies
heterogeneity in the 55
lower part of the Oxford Clay (Peterborough Member) and its relationship to
basin architecture. To 56
achieve this we: 1) use borehole geophysical logs and biostratigraphical data to
understand patterns 57
of sediment thickness with respect to major palaeogeographical and basin
structural features; 2) 58
combine new geochemical data with existing knowledge to refine understanding of
the impact of 59
relative sea level change; and 3) explore variability in biofacies data for a
series of distinct basin 60
settings (shallow platform, shallow shelf, shallow flooded massif, intra-basinal
high, deep shelf, 61
faulted basin margin, distal basin) in order to resolve the extent to which
environmental signals are 62
reflected in the gross lithological character of local successions. 63
Key objectives of this study are to: 1) understand basin-scale facies
variability for the 64
Peterborough Member, and the extent to which it behaves as a ‘mud blanket’; 2)
develop 65
conceptual understanding of how basin architecture might influence sediment
fluxes and 66
environments across the basin, and how these relate to facies variability; and
3) understand the 67
extent to which any heterogeneity in the Peterborough Member might be replicated
in other 68
Jurassic mudstones. More broadly, our new digital model provides a resource for
future assessment 69
of regional physical property variation in mudstones like the Oxford Clay, of
particular relevance to 70
planned infrastructure development (National Infrastructure Commission 2018),
and their suitability 71
as a host rock for nuclear waste storage (Delay et al. 2007, Butler 2010; Norris
2017). 72
2. Geological Setting 73
Deposition of the Oxford Clay Formation (Callovian/Oxfordian) coincided with a
period of marked 74
crustal extension and fracturing associated with North Atlantic rifting (Wilhelm
2014). Mid Jurassic 75
(Late Toarcian – Bathonian) thermal doming of the North Sea region began to
subside in the Early 76
Callovian (Underhill 1998), although the dome flanks persisted as a positive
structural entity 77
(Bradshaw et al. 1992). These features added to a complex palaeogeographical
fabric in the UK 78
region, where an epicontinental sea, dotted with emergent island massifs and cut
by extensional 79
basins controlled by reactivated Variscan structures, formed the environment for
deposition of the 80
Oxford Clay Formation (Fig. 1). 81
The Oxford Clay Formation is subdivided into three parts: the Peterborough,
Stewartby and 82
Weymouth members (Fig. 2). The lowermost Peterborough Member is the most
organic-rich part of 83
the formation, comprising brownish-grey mudstones and silty mudstones with
typically 3 – 16 % TOC 84
(Kenig et al. 1994), plus occasional sandstone and concretionary limestone
units. Compositionally, it 85
is a mixture of mica, illite, mixed illite/smectite, kaolinite and quartz, with
calcite from shell beds, 86
foramininifera and nannofossils, and amorphous organic matter derived from
marine phytoplankton 87
(Kenig et al. 1994; Macquaker 1994; Norry et al. 1994). The highest organic-rich
mudstone unit 88
defines the top of the Peterborough Member. Younger parts of the formation
(Stewartby and 89
Weymouth members; Fig. 2) typically comprise massive, paler grey silty mudstone
(Cox et al. 1992) 90


4

FIGURE 2 91



5

and are less organic-rich, suggesting a more oxidizing depositional environment
(Kenig et al. 1994). A 92
thin limestone interval (Lamberti Limestone) or equivalent shell bed/siltstone
separates the 93
Stewartby and Weymouth members (Cox et al. 1992). In northern England
(Yorkshire), deposition 94
was strongly influenced by a shallow structural block (Market Weighton High;
Fig. 1) and the 95
Peterborough and Stewartby members are replaced by sandstones of the Osgodby
Formation (Cope 96
2006, Powell et al. 2018, fig. 13; Fig. 2). This facies transition and thinning
of the Oxford Clay across 97
eastern England into the Cleveland Basin was documented by Penn et al. (1986).
Across the rest of 98
southern England, thickness data for the Peterborough Member indicate around 17
m at 99
Peterborough on the East Midlands Shelf (Hudson and Martill 1994), ca. 10.5 m in
the Eriswell 100
Borehole (Bristow et al. 1989) on the flanks of the Anglo-Brabant Massif, ca. 15
– 18 m in boreholes 101
in the Weald Basin (Lake et al. 1987), and about 22 m near Weymouth on the
Dorset coast 102
(Callomon and Cope 1995). 103
Offshore, strata equivalent to the Oxford Clay are sandy and silty in the
Southern North Sea 104
(Seeley Formation; Lott and Knox 1994; Fig. 2), become deltaic and coal-bearing
in the Central 105
Graben (Møller and Rasmussen 2003), and return to marine, mud and silt-dominated
facies (Heather 106
Formation; Fig. 2) in the Moray Firth and Viking Graben (Underhill and
Partington 1993). Across 107
southern England and the adjacent offshore regions the top of the Oxford Clay is
widely conformable 108
with the overlying Corallian Group, except where the West Walton Formation has
eroded into the 109
top of the Weymouth Member (Penn et al. 1986), or where the Corallian Group has
been completely 110
removed by later erosion. 111
112
3. Materials and Methodology 113
Determining the likely range and extent of basin-scale facies patterns in the
Peterborough Member 114
requires broader knowledge of the depositional system that influenced those
patterns. Key factors 115
are basin architecture, sea level fluctuation, and knowledge of environmental
gradients (e.g. 116
oxygenation, wave energy) at different locations across the depositional
landscape. To meet these 117
requirements our multidisciplinary approach uses borehole geophysical,
lithological and biozonal 118
data to understand stratigraphical patterns and model sediment geometry;
geochemistry to 119
interpret sea level fluctuation and its impact on the length of sediment
pathways; and biofacies data 120
to infer environmental signals across contrasting basin settings. The large
archive of subsurface data 121
(borehole core, geophysics, biostratigraphy) for the Peterborough Member held by
the British 122
Geological Survey (BGS) provides both the spatial extent and stratigraphical
resolution required by 123
the scale of this study. 124
3.1 Regional geophysical log interpretation 125
The inflection patterns of borehole geophysical logs in the Oxford Clay are
usually sufficiently 126
distinctive to allow recognition of its component formations (Whittaker et al.
1985, Penn et al. 127
1986), with the elevated organic content of the Peterborough Member typically
corresponding with 128
a higher gamma log response. Stratigraphic picks for the Oxford Clay were made
mostly from gamma 129
ray and sonic logs for ca. 127 boreholes across southern, central and eastern
England and adjacent 130
areas of the Channel and Southern North Sea basins (Figs 1, 3 and 4). Where
present, stratigraphical 131
picks were made for the top of the Great Oolite Group (Cornbrash Formation),
Kellaways Formation, 132
Peterborough Member, Stewartby Member and Weymouth Member, as well as associated
133
unconformities and faulted contacts. An important first step in this process was
calibrating log 134
responses against known Oxford Clay stratigraphy in boreholes that, in addition
to a suite of 135
geophysical logs, had associated core (Callomon and Cope 1971, Whittaker et al.
1985, Penn et al. 136
1986, Bristow et al. 1989, 1995 fig. 36) and biostratigraphical data (Gallois
1979, Cox 1977, 1984, 137


6

FIGURE 3 138



7

FIGURE 4 139



8

1988, 1991, Gallois and Worssam 1983, Buckley et al. 1991). These reference
boreholes provided an 140
important constraint on extending stratigraphical interpretations into boreholes
where only 141
geophysical logs and cuttings information were available. Interpretations were
checked for internal 142
consistency using a grid of intersecting borehole correlation panels, and by
flattening correlations on 143
multiple horizons to explore stratigraphical and structural trends. 144
Stratigraphic picks were subsequently used to create a refined thickness model
of the 145
Peterborough Member and wider Oxford Clay Formation. Maps were interpolated
using Discrete 146
Smooth Interpolation (DSI) (Mallet 1989) in SKUA-GOCAD™ software from borehole
thickness data 147
that was corrected for both deviated borehole paths and variable structural dip.
148
3.2 Lithological data 149
Lithological information is provided by pre-existing boreholes in BGS archives,
supplemented by two 150
newly drilled boreholes near Christian Malford, Wiltshire (Fig. 1), as well as
information collated 151
from published records, BGS technical reports and unpublished borehole logs in
BGS data archives. 152
The boreholes at Christian Malford are continuously cored and provide an
important record of the 153
succession across the Coronatum/Athleta Zone boundary (Fig. 2) in an area where
data are sparse. 154
Pre-existing borehole data (boreholes 2, 4, 9 – 11 of Fig. 2) relate to
historical BGS work on the 155
Oxford Clay, and are represented by discontinuous core samples. These boreholes
were selected 156
from the BGS archive to optimise core sample density (typically 0.1 m or less)
and provide 157
representative stratigraphical and geographical coverage of the Peterborough
Member. Lithological 158
descriptions of discontinuous core samples were made using a binocular
microscope during 159
acquisition of biofacies data for these successions (see 3.4 below), and the
observations used to 160
construct synthetic graphical borehole logs, supplemented by information from
pre-existing logs and 161
reports where available and relevant. 162
3.3 Geochemistry 163
A newly drilled cored borehole (CM11; Fig. 1) in the higher part of the
Peterborough Member 164
(Coronatum & Athleta biozones; Fig. 2) at Christian Malford, Wiltshire, provided
material for 165
geochemical analysis (Fig. 5). Borehole CM9, drilled as part of a previous
investigation at Christian 166
Malford (Hart et al. 2016, 2019) and partially overlapping with CM11, provided
material for analysis 167
in the higher part of the Phaeinum Subzone. 168
Geochemical data were obtained from slabbed core lengths of CM11 using a
portable Niton 169
XLt 793 X-Ray Fluorescence Spectrometer (XRFS), fitted with a 40kV Ag anode
X-ray tube and using 170
the ‘Standard Soil Mode’. Measurements were made at 10 mm intervals and for 30
seconds along 171
the core length, with values checked for internal consistency against a
designated standard 172
reference sample. This procedure was repeated for the partially overlapping
succession in CM9 using 173
selected milled samples of core material, analysing for 120 seconds with the
XRFS in a static semi-174
automated configuration. 175
176
3.4 Biofacies & biozonal data 177
Biofacies data (Figs 5 – 9) characterise stratigraphical intervals according to
the types and 178
abundances of fossil material, and use understanding of palaeoecological
affinities of facies 179
components to infer environmental signals. The technique can reveal patterns of
environmental 180
change that can be compared with variability in the lithological character of
host sedimentary 181
successions. Previous work on the Peterborough Member (Duff 1974, 1975) has used
biofacies from 182
outcrop data for a limited part of the depositional basin to understand
stratigraphical shifts in 183
depositional conditions. This work expands application of these data to a
network of sites across the 184


9

wider depositional basin in southern and eastern England, selected to capture
responses in a 185
probable diverse range of environmental settings in contrasting
palaeogeographical and structural 186
settings. We analyse the distribution of biofacies components in borehole core
using 187
correspondence analysis and clustering to explore their relationships and
patterns in their 188
stratigraphical distribution. 189
Biofacies data for the Peterborough Member, comprising more than 2200
observations of 190
core samples, were compiled for the following 8 cored borehole sites (Fig. 1):
Christian Malford CM9 191
and CM11 (distal shallow marine shelf), Down Ampney 2 (proximal shallow marine
shelf), Combe 192
Throop (intra-basinal high), Kimmeridge 2 (deep marine shelf), Warlingham
(faulted epicontinental 193
basin margin), Ashdown 2 (distal marine epicontinental basin), Parson Drove
(shallow marine 194
platform), and Eriswell (shallow flooded massif margin). 195
For the Christian Malford (CM11) Borehole, biofacies data were collected from
half-core 196
samples (10 cm diameter) at 10 mm intervals, examining part and counter-part
bedding surfaces. 197
Biofacies components (Appendix 1) assessed as part of this study cover a range
of taxonomic 198
groupings, mostly genera but including some broader categories (e.g. ammonites,
foraminifera) 199
where appropriate, and also include the occurrence of features like wood and
coprolite. Relative 200
frequency of key biofacies components (Appendix 1) was assessed using a
semi-quantitative scale, 201
based on threshold counts (present; common, 2 – 4 specimens; abundant, ≥5
specimens; plaster, 202
with numerous specimens covering core surface). Biofacies data for the other
boreholes rely on 203
discontinuous core samples, for which observations of all surfaces of each
sample are combined into 204
a single record with a modified assessment of relative frequency (present; few,
2 specimens; 205
common, ≥3 specimens). Biofacies data for the Peterborough Member, collected at
sites in eastern 206
and central England (Duff 1974, 1975) and re-analysed as part of this work, are
fully quantitative 207
observations of aerially extensive bedding planes. 208
Statistical analysis of biofacies data uses a combination of Detrended
Correspondence 209
Analysis (DCA) and clustering (hierarchical clustering & Non-Euclidian
Relational Clustering (NERC)) 210
to explore: 1) the faunal composition of different samples; and 2) the grouping
of samples into 211
coherent biofacies. NERC clustering is typically more suited to analysis of
palaeontological data 212
which are incomplete (Vavrek 2016). However, because our data are from borehole
core, with an 213
implied age/depth relationship between consecutive samples, we combine both
hierarchical and 214
NERC clustering techniques in our results. 215
Primary data were conditioned by: removal of rare/ambiguous components that
might 216
otherwise distort the analysis; merging records of related components (e.g.
species within a genus); 217
coding sample composition, sample size and biozonal assignment, and
standardising scale of relative 218
frequency (see Supplementary Data for detailed procedure). All statistical
analyses were performed 219
in the open source environment R (R Core Team 2020) using the following
packages: vegan (Oksanen 220
et al. 2018) (DCA analysis based on upper quartile biofacies components);
NbClust (Charrad et al. 221
2014) (optimum number of clusters for each borehole based on analysis of primary
data); rioja 222
(Juggins 2017) (hierarchical clustering); ecodist (Goslee and Urban 2007)
(calculation of ecological 223
distance matrix); fossil (Vavrek 2016) (NERC cluster analysis). An ecological
distance matrix is 224
required for NERC clustering, and for our semi-quantitative data we adopt the
Sørenson dissimilarity 225
index (or Dice index), one of the most commonly used and effective
presence/absence dissimilarity 226
measures (Southwood and Henderson 2000, Magurran 2004). Compilations of
biofacies data for the 227
analysed boreholes and details of their statistical analysis are provided as
Supplementary Data. 228
The boreholes that are the source of our biofacies data are part of a larger
suite of cored 229
boreholes in the Peterborough Member across southern Britain, drilled in
connection with BGS 230
regional mapping programmes that date back to the late 1960s. Biostratigraphical
interpretations of 231
many of these successions, published in BGS memoirs and technical reports, were
compiled for this 232


10

study to provide precise stratigraphical understanding of geophysical log
signatures, to allow 233
accurate comparisons of biofacies data, and to provide additional insight into
patterns of thickness 234
variation. 235
Supplementary material: Geophysical log correlations, geochemical data and
detailed biofacies 236
data, methodology and results are available as Supplementary Data. 237
4. Results 238
Geophysical log data provide a highly resolved picture of thickness variation
for the Peterborough 239
Member across Southern Britain, with correlation panels and biostratigraphical
data compilation 240
emphasising regions that were the long-term focus of sedimentation and others
where deposition 241
was persistently restricted (Figs 3, 4; 10). Cyclical trends in sedimentary
geochemical data at 242
Christian Malford (Fig. 5) show a strong relationship with patterns of
sedimentation and biofacies in 243
borehole core. Across the basin, biofacies data show significant lateral
contrasts between sites (Fig. 244
6). The results for each category of data are discussed in turn below. Detailed
compilations of our 245
results are provided as Supplementary Data. 246
4.1 Geophysical log correlation and basin modelling 247
Along the northern margin of the Anglo-Brabant Massif (ABM), from the East
Midlands Shelf in the 248
NE, towards the Wessex Basin in the SW, there is a consistent regional trend of
thickening, for both 249
the Peterborough Member (<5 m to >55 m) and Oxford Clay Formation as a whole
(<10 m to ca. 180 250
m) (Figs 3, 4). Significant thinning of the Peterborough Member (to ≤ 10 m)
occurs on the flanks of 251
the ABM, and likely also the whole of the Oxford Clay, although erosion of the
succession prevents 252
confirmation. The succession remains thin into the Weald Basin to the south,
where the total Oxford 253
Clay Formation is about 80 m thick in the Ashdown 2 Borehole, with ca. 15 m
representing the 254
Peterborough Member (Fig. 3). Offshore thickness trends are consistent with the
onshore data, with 255
extremely thin (possibly incomplete) Oxford Clay inferred to occur in the
southern North Sea (where 256
the formation is generally not separated from the parent Humber Group), and
relatively thick 257
Peterborough Member (ca. 42 m) forming part of an eroded Oxford Clay succession
in the Channel 258
Basin. As well as basin-scale features (e.g. ABM), there is an apparent
association of local thickness 259
patterns with 1:1500000 -scale structural lineament data (Fig. 3A), seen in
parts of the Wessex Basin, 260
for example local thickening near Shrewton, and thinning across the
fault-bounded Norton Ferris 261
High (Chadwick and Evans 2005, figs 86, 88). 262
4.2 Lithology 263
Borehole CM11 proved a strongly cyclical succession of lithofacies that can be
matched with 264
geochemical data (4.3 below; Fig. 5). Three broad lithofacies are recognised in
borehole core: 1) pale 265
grey silty mudstone with abundant shell remains, dominated by nuculacean
bivalves, including 266
specimens with articulated valves; 2) massive or very weakly fissile, medium –
grey-brown silty 267
mudstone with scattered or sparse shell remains, but including occasional
bedding-plane plasters of 268
the bivalves Meleagrinella and/or Bositra and, 3) dark grey-brown, organic-rich
laminated mudstone 269
with abundant plasters of Bositra and/or Meleagrinella. The bases of shell beds
may be sharply 270
defined or gradational, and cycles comprise either: 1) sharp-based shell bed
with thin gradational 271
intervals of weakly fissile mudstone into dark laminated mudstone (Type 1; Fig.
5), or 2) weakly 272
fissile mudstone, with thicker gradational transition into a shell bed (Type 2;
Fig. 5). Type 1 cycles are 273
thin (ca. 30 cm) and occur in the middle part of the borehole succession, and
Type 2 cycles are 274
thicker (ca. 70 cm) and occur particularly in the higher parts of CM 11 (Fig.
5). 275
276


11

FIGURE 5 277



12

For sites across the rest of southern England, lithological data summarised on
synthetic 278
borehole logs (Fig. 6) show that in the Wessex Basin, conspicuously silty
mudstone dominates in the 279
lowest part of the Peterborough Member (Calloviense and Jason Zones) in the
Kimmeridge 2 and 280
Combe Throop boreholes. In the Kimmeridge Borehole this diminishes with the
appearance of dark 281
grey mudstone in the Coronatum Zone, but remains a persistent feature of the
Combe Throop 282
succession. Bioclastic mudstone is a feature of the Coronatum Zone in the CM 9
and CM11 283
boreholes, and extends north-eastwards, in both the Down Ampney 2 and Parson’s
Drove 284
successions. In the Phaeinum Subzone, the rhythmic mudstone in CM9 and CM11
cannot be traced 285
into the Down Ampney 2 succession. Here, this interval contains a sharp contrast
between darker 286
grey mudstone in the lower part of the subzone and much paler grey, biotubated
mudstone in the 287
upper part. This change occurs immediately above a ca. 2.5 m thick laminated
interval in the lower 288
part of the Phaeinum Subzone, that contains few fossils apart from wood,
foraminifera and bone (Fig 289
6), and is unique to this borehole succession. A similar colour change occurs
near the top of the 290
Peterborough Member in the Parson Drove succession on the East Midlands Shelf,
although here the 291
lithology is laminated and silty, with greater development of dark mudstone in
the underlying 292
Phaeinum Subzone. On the flanks of the ABM, the Coronatum Zone and Phaeinum
Subzone in the 293
Eriswell Borehole comprises thin units of distinctively pale and medium grey,
bioturbated mudstone 294
with abundant finely comminuted shell. The correlative interval thickens at the
edge of the Weald 295
Basin in the Warlingham Borehole, where the lithology is rather uniformly
bioclastic mudstone with 296
occasional laminated intervals, becoming distinctly silty in the upper part. The
sparse data for the 297
central Weald Basin, from the Ashdown 2 Borehole, suggest a very different
pattern of 298
sedimentation, with samples from the ?Coronatum Subzone represented by very dark
brownish-299
grey, hard, silty, pyritic calcareous mudstone. 300
301
4.3 Geochemistry 302
303
Titanium in the Peterborough Member shows a strong correlation with silica
(Norry et al. 1994), and 304
functions as a proxy for variability in the flux of hydrodynamically heavy
detrital components to the 305
depositional basin and/or winnowing. Figure 5 plots the variability of Ti
normalised to K (the optimal 306
proxy for understanding detrital fluxes in the absence of XRF values for Si, Zr
and Al) through the 307
CM11 and CM9 successions. The plot shows a strongly cyclic pattern, with an
overall shift from 308
relatively high values in the lower 2.5 m of the succession (Coronatum Zone), to
relatively lower 309
values in the middle part (interval above S4 and below S10 of Fig. 5), spanning
the latest Coronatum 310
and earliest Athleta zones. Shell concentrations are picked out by the Ca/K
plot, with those 311
characterising the Type 1 cycles in the CM11 succession forming four strong,
closely spaced peaks 312
(S4 – S7; Fig. 5). An inflection in the trend of Ti/K data, from sharply falling
to gradually increasing, is 313
coincident with the interval between two regionally extensive marker-beds in the
Oxford Clay, the 314
Comptoni Bed and the overlying Acutistriatum Band (Hudson and Martill 1994; Fig.
5). Above these, 315
peaks S9 – S15 define a stacked succession of Type 2 cycles, with progressively
increasing detrital 316
content and shell beds coincident with sharp peaks in the Ca/K curve. 317
318
4.4 Biofacies 319
Litho- and biofacies variation occurs on a very fine (lamina) scale in the
Oxford Clay (Macquaker and 320
Howell 1999). Our biofacies data resolves this fine-scale variation into
decimetre-scale trends, 321
represented by the stratigraphical distribution of biofacies clusters (Figs 5 –
9), with the composition 322
of each cluster summarised as relative proportions in bar charts below each
borehole log. The 323
number and composition of clusters is distinct for each site and determined by
the data available for 324


13

FIGURE 6 325



14

FIGURE 6 (continued) 326


15

each site. Attempts to develop a unified biofacies classification across all
sites forced removal of 327
components that were defining and common at individual sites, suggesting a
significant degree of 328
site-specific biofacies character. Consequently, stratigraphical analysis of our
biofacies data focuses 329
on identifying major contrasts in the arrangement and composition of biofacies
within borehole 330
successions, and examining the extent to which these correspond with analogous
or strongly 331
contrasting biofacies in correlative successions at other sites across the
basin. 332
DCA plots of samples and taxa provide insight into which taxa are important for
333
characterising samples. DCA analysis for CM11 (Fig. 8A) is consistent with
observations made during 334
borehole logging, and indicate two strongly contrasting end-members: 1) dark,
organic-rich, fissile 335
mudstone dominated by remains of the thin-shelled bivalve Bositra (with or
without Meleagrinella); 336
2) Shell beds associated with pale grey mudstone with abundant thick-shelled
nuculacean bivalves, 337
often associated with the gastropod Procerithium and sometimes also the serpulid
Genicularia. 338
Most biofacies clusters comprise mixtures of these end member components, and
are therefore 339
environmental composites that reflect the extent to which particular
environmental settings are 340
more or less dominant at the scale of individual laminations. 341
Differences between biofacies cluster composition for a given borehole can be
subtle or very 342
significant. Both the Warlingham and Parson Drove boreholes show a strong
pattern of 343
stratigraphical specificity in the distribution of biofacies clusters, but
several of the cluster 344
compositions are very similar, suggesting either a limited range of
environmental change at these 345
sites, or smearing of the ecological signal by external factors. It is notable
that the extremely shell-346
rich Warlingham succession is characterised by an incongruent mixture of
biofacies components 347
(e.g. Bositra, Meleagrinella, nuculacean bivalves, Genicularia; Fig. 6) that
characterise discrete 348
intervals elsewhere. 349
The successions in the Kimmeridge 2 and Combe Throop boreholes show a regular
repetition 350
of biofacies clusters, with a tendency for particular biofacies to be slightly
more dominant at some 351
levels (Fig. 6). Kimmeridge 2 also shows a pronounced stratigraphical and
compositional shift in 352
biofacies at the top of the core log. In the Down Ampney 2 succession there is
more marked 353
domination of particular intervals by particular biofacies clusters (Fig. 6),
and these are also more 354
compositionally distinct, a pattern that is even more pronounced in the CM11
succession. The 355
largest number of biofacies clusters are associated with the highly condensed
Peterborough 356
Member section at Eriswell (Fig. 6). Despite this, there is a strong unifying
compositional feature (a 357
high proportion of shell hash and relative paucity of ammonites) that is unique
to this site. Data for 358
Ashdown 2 are limited, but are notable for the extremely high proportions of
bone and the bivalve 359
Bositra in all biofacies clusters. 360
To explore the fidelity of our biofacies methodology and provide additional 361
palaeoenvironmental data, we have used the raw quantitative data collected by
Duff (1975) as part 362
of his biofacies interpretation of the Peterborough Member (at Norman Cross near
Peterborough, 363
Stewartby, Bletchley and Calvert) to generate biofacies classifications based on
NERC clustering. 364
Figure 7 shows the correlation of the sections and compares the biofacies
assignments of Duff 365
(1975) with those assigned using NERC. Both facies classifications show striking
similarity in the 366
broad pattern of biofacies subdivisions for different biozonal intervals. The
youngest and oldest 367
parts of successions show significant facies variability in both
interpretations, with less variation in 368
the intervening part (Jason & Obductum subzones). In some cases there is direct
correspondence of 369
biofacies recognised by Duff (1975) with biofacies clusters defined by NERC, and
in other cases the 370
NERC analysis appears to be detecting contrasts identified by Duff (1975)
although the boundaries of 371
these units are not necessarily coincident. These patterns, and the similarity
in the taxa that define 372
distinct nodes in the DCA sample distribution (e.g. Bositra, Meleagrinella,
nuculaceans, oysters; Fig. 373
374


16

FIGURE 7 375



17

9) to other localities examined as part of this study (Fig. 8), suggests our
combined NERC – DCA 376
methodology is robust in identifying meaningful environmental gradients. 377
NERC biofacies data for the localities described by Duff (1975) show that,
generally, the 378
oldest parts of the succession (Enodatum/Medea subzones) seen at Norman Cross
and Bletchley are 379
relatively rich in ammonites and oysters, but including at Norman Cross thin
intervals with sharply 380
contrasting facies, where these faunal elements are sparse and Bositra more
dominant. Higher in the 381
succession (Jason/Obductum subzones), facies at most localities are dominated by
a mixture of 382
Bositra, Meleagrinella and nuculacean bivalves, with the Obductum Subzone at
Norman Cross and 383
Stewartby having a relatively higher proportion of Bositra compared to the
corresponding interval at 384
Calvert. The distinctly more cyclic facies of the Grossouvrei Subzone contains
frequent shell beds in 385
the Duff (1975) facies classification, corresponding with NERC facies clusters
in which Meleagrinella 386
and/or nuculacean bivalves tend to be more dominant compared to Bositra. The
appearance of 387
more shell-dominated facies appears somewhat delayed at Stewartby, occurring
some distance 388
above the base of the Grossouvrei Subzone. Scattered through the succession are
relatively thin (ca. 389
1 m or less) organic-rich mudstone units identified by Duff (1975) as
Grammatodon-rich Bituminous 390
Shale, with apparently no consistent relationship to NERC biofacies clusters,
and containing 391
relatively low proportions of Bositra and Meleagrinella and high concentrations
of deposit-feeding 392
and infaunal suspension-feeding bivalves (particularly Grammatodon, nuculacean
bivalves and the 393
gastropod Procerithium). Apart from these horizons, Grammatodon is absent from
most of the 394
Peterborough Member. 395
Overall, the results suggest that there are strong site-specific factors
influencing the 396
composition and successions of biofacies, with stratigraphical persistence of
the distinct character of 397
sites suggesting that this pattern is not a consequence of chance variation in
palaeoecological 398
conditions. Detailed results of biofacies analysis for individual boreholes are
discussed below in the 399
context of environmental interpretations. 400
4.5 Biozonal data 401
Compilation of biozonal data for the Peterborough Member (Fig. 10) in boreholes
extending along 402
the same NE – SW alignment as our model thickness data reveals: 1) SW thickening
is largely driven 403
by expansion of biozones in the lower part of the Peterborough Member; 2) the
Eriswell Borehole, 404
located close to the ABM, shows maximum thinning of biozones, but expansion
north-eastwards 405
away from this structural feature and towards the East Midlands Shelf and
Southern North Sea is 406
muted, with the thickness of the Coronatum Zone in the Tydd St Mary and Parson
Drove boreholes 407
similar to Eriswell. 408
5 Discussion 409
To understand the degree and likely causes of facies heterogeneity in the
Peterborough Member 410
we: 1) use our modelled thickness data to resolve depositional geometry in the
context of basin 411
structure and palaeogeography; 2) review existing sequence stratigraphy
understanding for the 412
Peterborough Member in light of our new geochemical and regional
biostratigraphical data to 413
establish a framework for understanding sea level variability, and 3) combine
knowledge of (1) and 414
(2) to develop a conceptual basin model that explains contrasts and similarities
in biofacies patterns, 415
and the extent to which they reflect basin-scale trends in mudstone variability.
Finally, we make 416
comparisons with other well-known organic-rich Jurassic mudstone successions.
417
The constituent taxa of our biofacies have been the subject of previous work to
understand 418
patterns of ecological change in the Peterborough Member, which are primarily
controlled by 419
seabed oxygenation and/or substrate consistency (Duff 1975, Martill et al. 1994,
Kenig et al. 2004). 420
421


18

FIGURE 8 422



19

FIGURE 9 423



20

Since our biofacies carry an ‘averaged’ environmental signal, our ecological
conceptualisation of 424
them (below) is necessarily simplified, effectively dampening short-term
environmental “noise”. 425
Here, we follow previous authors (Kauffman 1981, Etter 1996; Caswell et al.
2009; Danise et al. 426
2015) in regarding Bositra and Meleagrinella as opportunist suspension feeding
taxa, tolerant of low 427
oxygenation. In the context of previous work by Kenig et al. (2004) using
geochemical data to 428
understand environmental signals in the biofacies of the Peterborough Member,
and the 429
relationship of biofacies to Ti/K data in the CM11 succession, end-members (4.4
above) represented 430
by (1) Bositra -rich organic mudstone and (2) nuculacean shell beds (+/-
Procerithium, Genicularia) 431
are interpreted to correspond with low oxygen (anoxic or dysoxic) and more
oxygenated (oxic) 432
settings, respectively. Meleagrinella-dominated successions appear to occupy a
position between 433
these two end members. 434
5.1 Basin geometry and palaeogeography 435
Modelled thicknesses reveal clear trends in the depositional pattern of the
Peterborough Member, 436
with distinctly wedge-shaped regional thickness geometries characterising the
unit and wider Oxford 437
Clay along the northern edge of the ABM, from the Southern North Sea into the
Wessex Basin (Fig. 438
3), as well as from the eastern Weald westwards (Fig. 4). However, the
palaeogeographical settings 439
of these two regions are strongly contrasting in terms of likely sediment
accommodation space (Fig. 440
1), suggesting that the similar geometries are a product of different processes.
Along the northern 441
margin of the ABM, north-eastward thinning closely corresponds with flanks of
the Mid North Sea 442
High, and the modelled thickness variation (Fig. 3A) seems consistent with a
palaeobathymetric 443
gradient from relatively shallow water conditions around this semi-emergent
feature to deeper 444
water conditions in the Wessex Basin. Certainly, there is evidence in the
younger Corallian Group for 445
the margins of the Mid North Sea High being associated with the development of
shallow-water 446
facies (Cameron et al. 1992), and it seems probable that prior to this the broad
area of crust affected 447
by North Sea doming (>1250 km diameter; Underhill and Partington 1993) extended
to the East 448
Midlands Shelf during deposition of the Peterborough Member. 449
Modelled thinning of the Peterborough Member and total Oxford Clay Formation
across 450
much of the Weald Basin is striking and consistent with biozonal data from cored
boreholes (Fig. 10), 451
including the Grove Hill [TQ 6008 1359] and Brightling No. 1 [TQ 6725 2182]
boreholes in Sussex 452
(Lake et al. 1987). This sharply contrasts with the significant thickening seen
in this area during 453
deposition of the prior Inferior and Great Oolite groups and subsequent Late
Jurassic succession 454
(Whittaker 1985). The conclusion from our modelling is that this area is likely
to have been relatively 455
sediment starved. 456
From the thick successions of Peterborough Member in the Wessex Basin there is
evidence 457
of a slight thinning trend into the Channel Basin, particularly seen in the
pattern of biozonal data for 458
Dorset (Fig. 10). This may reflect greater ability of the Wessex Basin to create
additional 459
accommodation space through sediment loading, with some differential compaction
across fault 460
lineaments potentially suggested by thickness data in the vicinity of the
Shrewton Borehole (Fig. 3C). 461
5.2 Cyclicity and Sequence Stratigraphy 462
New geochemical data for Christian Malford, and its relationship to lithological
features in borehole 463
core (Fig. 5), allows interpretation of likely patterns of relative sea level
change during this time. 464
Combined with our basin-wide synthesis of biostratigraphical data (Fig. 10), and
previously reported 465
observations of Mid/Late Callovian successions in the wider UK region (including
data for the Moray 466
Basin), we outline a sequence stratigraphy framework for the Peterborough Member
that can be 467
used to understand controls on the pattern of deposition and potentially also
decimetre-scale trends 468
in biofacies data. Whilst often considered as a separate depositional entity,
the succession 469


21

FIGURE 10 470



22

developed on the margin of the Moray Basin (at Brora) shows noted similarity
with that of eastern 471
England (Page 2002); there is broad coincidence in the stratigraphical horizon
of intervals showing 472
particular sequence stratigraphical responses (e.g. progradation, maximum
flooding), and 473
palaeogeographical evidence supports probable wider marine connectivity with the
Oxford Clay 474
depositional system (Davies et al. 1996). 475
5.2.1 Current knowledge framework 476
The oldest part of the Peterborough Member, and coeval parts of the Heather and
Brora 477
Argillaceous formations in the Moray Basin (Fig. 2), relate to marine flooding
at the base of the 478
Enodatum Subzone and regression in the overlying Jason Zone (Davies et al. 1996,
Nagy et al. 2001, 479
Hesselbo 2008). At the type Peterborough section, initial flooding and later sea
level fall corresponds 480
with the ‘Gryphaea and Reptile Beds’, comprising a thin (1.2 m) unit of fissile
pyritic mudstone rich in 481
the oyster Gryphaea, consistent with shallow water oxic conditions (Duff 1975,
Hudson and Martill 482
1994, Kenig et al. 2004). Maximum Flooding surfaces are usually associated with
thickening of the 483
associated stratigraphical intervals towards the basin margins where sediment
becomes ponded on 484
newly created shelf areas (Catuneanu et al. 2011). The thin Enodatum Subzone at
Peterborough 485
(compared to more basinal setting; Fig 10) suggests either a very proximal
setting at the maximum 486
extent of flooding, and/or significant erosion associated with later sea level
fall. 487
Thickening of the Jason Zone into the Wessex Basin (Fig. 10) is consistent with
relative sea 488
level fall focussing sedimentation towards available accommodation space in more
distal areas. In 489
this context, organic-rich facies in the higher part of the Jason Zone at
Peterborough (Bed 10 of 490
Hudson and Martill 1994), sandwiched between Gryphaea-rich intervals, probably
reflects the 491
development of anoxia in a relatively shallow water setting, possibly in
response to restricted local 492
circulation (ponding, boosting organic matter preservation) associated with sea
level fall. Sandstone 493
in the likely Jason Zone at Stewartby (Bedfordshire), ca. 60 km to the
south-west, is interpreted to 494
reflect winnowing or bypass of fine -grained sediment in a more proximal setting
(Macquaker 1994), 495
providing further evidence of limited local sediment accommodation space close
to the basin margin 496
at this time. 497
Relative sea level fall is inferred to have continued into the early Coronatum
Zone before 498
rising in the later part of the Zone (Hesselbo 2008). In the Moray Basin this
phase is represented by a 499
wedge of coarse, glauconitic sandstone (Nagy et al. 2001), and by stacked
parasequences in the 500
Peterborough succession showing a trend of increasing up-section silt content
(Macquaker and 501
Howell 1999). These successions may be a response to lack of accommodation space
caused by 502
relative sea level fall, and/or reflect the rapid advance of sediment into
limited areas of newly 503
created accommodation space during early transgression. 504
5.2.2 Completing the knowledge framework 505
The remainder of the Peterborough Member reflects rising relative sea level,
peaking in the lower 506
part of the Phaeinum Subzone (Hesselbo 2008) and consistent with a widespread
Maximum 507
Flooding Surface recognised in the Moray Basin (Davies et al. 1996, Nagy et al.
2001). Trends in the 508
Ti/K data from the Christian Malford Borehole CM 11 are a proxy for the delivery
of coarse detrital 509
components to the depositional site, and shed new light on the pattern of
environmental change 510
represented by the upper part of the Peterborough Member. We regard the regular
cyclical pattern 511
of Ti/K data in the Christian Malford succession (Fig. 5) as analogous to the
systematic trends in the 512
silt-content used by Mcquaker (1994) and Mcquaker and Howell (1999) to
understand fluctuations in 513
the length of sediment transport pathways between source and sink caused by
relative sea level 514
change. Individual cycles fine-up (Type 1 cycle) or coarsen-up (Type 2 cycle)
and can be grouped into 515
broader associations showing overall reductions in Ti/K or overall increases in
Ti/K. These broader 516


23

cycle trends, and the inflection points between these trends are used to infer
likely changes in 517
sediment accommodation space and to make interpretations of sequence
stratigraphy. 518
The decline in the Ti/K ratio in the lowest ca. 1 m of the CM11 succession
(Grossouvrei 519
Subzone; Fig. 5), terminating abruptly at a peak in the Ca/K ratio (‘C’ of Fig.
5) suggests that an 520
increase in accommodation space acted to reduce the flux of detrital material to
this site. The 521
conspicuous ammonite/foraminifera concentration at ‘C’ likely represents a
period of sharply 522
reduced sedimentation rate in response to sea level rise. Three cycles in the
Ti/K ratio (S1 – 3, Fig. 5) 523
in the overlying ca. 1.5 m of mudstone correspond with alternating paler and
darker grey mudstone 524
units, and show an overall upward increase in Ti/K. This trend through cycles S1
– 3 suggests 525
progressively enhanced delivery (progradation) of coarser-grade sediment to the
depositional site 526
associated with shortening of sediment pathways. These are analogous to the
parasequences 527
described in the lower part of the Peterborough succession by Macquaker and
Howell (1999). The 528
largest peak (S3, Fig. 5) in Ti/K marks the beginning of a sharp upward shift to
significantly lower Ti/K 529
values that persists through several metres of the overlying succession. This
peak is interpreted to 530
represent a major pulse of marine transgression associated with current
winnowing on newly 531
flooded areas and significant increase in available accommodation space. 532
The four closely spaced Type 1 cycles (see above) in CM11, corresponding with
peaks S4 – S7 533
(Fig. 5), are interpreted to represent a series of transgressive pulses,
initially marked by sediment 534
winnowing events, separated by periods with less current scour. The overall
reduction in the Ti/K 535
ratio through this interval is consistent with an increase in accommodation
space and lengthening of 536
sediment pathways. The major Ti/K peak (S4) associated with the lowest shell bed
suggests that this 537
was a particularly strong/prolonged pulse of transgression. It coincides with a
major shift in 538
biofacies, a sharp upward decline in the abundance of wood potentially
reflecting increased distance 539
from shorelines, and a decline in foraminifera that is a possible response to
increased current scour 540
(Fig. 5). Sharp/erosive (‘X’ Fig. 5) bases to the shell beds are consistent with
significant current scour 541
which was probably important for oxygenation and colonisation of seabed sediment
by the infaunal 542
bivalves that dominate these units. Minor Ti/K peaks (S5 – 7; Fig. 5) that cap
subsequent shell beds 543
suggest a phase of enhanced off-shelf movement of mobile sediment from newly
flooded areas by 544
wave scour following each transgressive pulse (including material swept across
the site into more 545
distal settings). The alternation of shell-rich units and intervening mudstone
is matched by a 546
pronounced oscillation in biofacies (Fig. 5). The less diverse fauna, dominated
by foraminifera and 547
the bivalves Bositra and Meleagrinella, in the dark, fissile units that cap the
cycles, suggests quieter 548
depositional conditions with less consistent sea bed oxygenation. The Comptoni
Bed (peak S8), 549
widely associated with a rolled and winnowed fauna (Hudson and Martill 1994), is
inferred to mark 550
the maximum extent of transgression, and therefore the Maximum Flooding Surface.
551
Above the Acutistriatum Band in CM11, the progressive cyclical build-up in Ti/K
(Fig. 5, S9 – 552
S14), indicates a major shift in basin evolution. Biofacies at and just above
the Acutistriatum Band 553
(and particularly between peaks S9 & S10) are indicative of dysoxic conditions
(enrichment in 554
Bositra, coprolite) and are consistent with low sedimentation rates (enrichment
in bone), and the 555
Acutistriatum Band is interpreted as a Condensed Section at the base of a
Highstand Systems Tract 556
(cf. Catuneanu et al. 2009). The stacked succession of Type 2 cycles (see above)
with progressively 557
increasing detrital content that form the remainder of the succession in CM11
and overlapping parts 558
of CM9 are interpreted to form part of a ‘normal regression’ (Catuneanu et al.
2011) during sea level 559
Highstand. Peaks S11 – 15 (Fig. 5), and likely represent the winnowed tops of
prograding 560
parasequences, represented by paler and generally more shell-rich mudstone units
in borehole core. 561
Reducing amplitude of Ca/K peaks upwards through the interval is probably a
response to increasing 562
dilution of shell by sediment influx, whilst the gradational bases of shell beds
(‘Y’ Fig. 5) might reflect 563
the ability of infauna colonising parasequences to progressively improve the
habitability of deeper 564


24

sediment layers by improving oxic water circulation. Decimetre-thick intervals
of dark, organic-rich, 565
fissile mudstones, that are either sparsely shelly or dominated by Bositra and
ammonites, represent 566
periods of deposition when creation of new accommodation space (e.g. from minor
sea level 567
fluctuation or basin subsidence) outstripped sediment flux, and current
circulation was less effective 568
at maintaining seabed oxygenation. Initially, the Type 2 cycles are poorly
defined by biofacies data, 569
but around the ‘Squid Bed’ (Fig. 5) and coincident with the horizon of the
Christian Malford 570
Lagerstätte (Wilby et al. 2008), the distinction of Bositra-rich intervals
suggests an increasing trend 571
towards anoxia/dysoxia. Plateauing of Ti/K values above S14 potentially reflects
over-extension of 572
sediment pathways, and diversion of material to adjacent regions with steeper
shelf to basin 573
gradients. 574
5.3 Conceptual basin model and mudstone heterogeneity 575
Modelled thickness data for the Peterborough Member, combined with knowledge of
the 576
palaeogeographical framework for the Mid Callovian (Figs. 1, 3), suggest that a
depth gradient from 577
the Mid North Sea High, and laterally contiguous areas, was likely significant
in focusing sediment 578
south-westwards towards the Wessex Basin, potentially augmented by sediment
flows via the 579
Worcester Graben. Palaeocurrent data for the Oxford Clay are sparse, but Hudson
and Martill (1991) 580
speculated that a large assemblage of belemnites seen at Peterborough (>300
specimens; Martill, 581
1985) with a N – S alignment might reflect the action of currents responsible
for removing sediment 582
from the East Midlands Shelf succession and depositing it in deeper parts of the
basin. In the Mid 583
Callovian, the Mid North Sea High formed an extensive semi-emergent area arcing
around the 584
southern North Sea Basin, with a coal-forming deltaic system in the Central
North Sea (Møller and 585
Rasmussen 2003), both providing significant potential for delivery of
fine-grained sediment to 586
offshore regions (Fig. 11). Thin and significantly condensed sedimentation on
the flanks of the ABM 587
(Figs. 3, 10) suggests that even if not fully emergent, it likely formed a
significant structural feature. 588
However, similarly thin Peterborough Member successions in the Weald Basin,
where sediment 589
accommodation space is unlikely to have been limited, suggest sediment
starvation, with limited 590
supply of sediment from the ABM itself and likely shielding by the ABM from
sediment sources 591
further north. 592
In the Wessex Basin, the thick Peterborough Member potentially includes material
fed via 593
the Worcester Graben (Fig. 11), which structural and regional gravity data
(Chadwick and Evans, 594
2005; Fig. 12) suggest was a long-lived conduit for Mesozoic sediments.
Published data for the 595
source of Oxford Clay sediments are lacking, but it is noticeable that
thickening of the Peterborough 596
Member occurs in geophysical log transects across the buried mouth of this
structure, located north 597
of the current outcrop margin (Fig. 12). Maintenance of accommodation space in
the Wessex Basin 598
was probably a response of the highly fractured basement to the extensional
stresses responsible for 599
North Sea rifting, coupled with greater potential for compactional subsidence of
the thick underlying 600
Triassic and Early Jurassic succession, in contrast to the shallow-buried
Variscan basement on the 601
East Midlands Shelf (Whittaker 1985, Map 3). 602
The inferred patterns of sea level change are reflected by variable litho- and
bio-facies 603
across the basin, that appears largely a response to local basin setting. Thus,
sea level fall in the 604
Jason Zone is associated with the development of organic-rich mudstone in
probable shallow water 605
settings on the East Midlands Shelf, whereas coeval strata at Combe Throop and
Kimmeridge 2, in 606
the Wessex Basin, contain relatively low proportions of Bositra, and high
proportions of oysters, 607
nuculacean bivalves and the deep burrowing bivalve Thracia (Fig. 6), indicative
of broadly oxic 608
conditions. Higher in the succession, sea level rise across the
Coronatum/Athleta Zone boundary is 609
reflected by a shift to more organic-rich facies at Christian Malford. Further
south-west in the 610
Wessex Basin, correlative strata at Combe Throop comprise silty laminated
sediments characterised 611


25

FIGURE 11 612



26

by high proportions of both Bositra and nuculacean bivalves (Fig. 6), suggesting
more rapidly 613
fluctuating oxic/suboxic/anoxic environments. This plausibly reflects the
position of the borehole on 614
the Hampshire – Dieppe High, and perhaps also contrasting conditions affecting
the fault-bounded 615
Mere Basin opening immediately to the north (Chadwick and Evans 2005, fig. 85).
616
In the Down Ampney 2 Borehole, 30 km NE of CM11, laminated mudstone facies in
the 617
lower part of the Phaeinum Subzone (Fig. 6) contains common wood, bone and
foraminifera, but 618
few other fossil remains. The abundance of wood and bone in laminated mudstone
facies suggests 619
low rates of sedimentation in a low energy, near-shore setting. The general
absence of biota might 620
indicate significant localised freshwater run-off affecting both salinity and
potentially also 621
oxygenation. This unusual and unique unit suggests significant influence of
local 622
basin/environmental factors. It occurs between two contrasting successions: dark
grey laminated 623
bioclastic mudstone in the early Phaeinum Subzone (with biofacies dominated by
Bositra, 624
Meleagrinella & nuculacean bivalves) suggesting an intermittently dysoxic marine
setting, and pale 625
grey, poorly laminated and conspicuously bioturbated silty and shelly mudstone
above (with 626
biofacies dominated by nuculaceans, Procerithium, foraminifera and subsidiary
Bositra), suggesting 627
more, oxic, open marine circulation. 628
In the Parson Drove Borehole on the East Midlands Shelf, biofacies clusters with
high 629
proportions of Bositra dominate much of the succession, suggesting a persistent
pattern of low 630
oxygenation (Fig. 6). Pale grey, silty mudstone facies that dominate most of the
Phaeinum Subzone 631
in the Down Ampney succession, only occur near the top of the Phaeinum Subzone
in the Parson 632
Drove succession. This interpretation is supported by the sparse record of
Genicularia in much of the 633
Pason Drove succession, seen also in the voluminous quantitative data of Duff
(1974, 1975) collected 634
from the East Midlands Shelf. Genicularia is an epifaunal suspension feeding
serpulid (Duff, 1975) 635
that our biofacies data show is predominantly associated with strongly developed
nuculacean shell 636
beds. On DCA data plots (Fig. 8) Genicularia is consistently distant from poles
defined by fauna linked 637
to dysoxic environments (e.g. Bositra, Meleagrinella), and Duff (1975) recorded
it as a dominant 638
component in his calcareous clay facies, characterised by a diverse fauna
including nuculaceans and 639
oysters and low Total Organic Carbon (TOC). With these characteristics, we
regard Genicularia in our 640
biofacies data as an indicator of some of the least dysoxic conditions in the
Peterborough Member. 641
In the Parson Drove Borehole, Genicularia is present towards the top of the
succession, coincident 642
with a shift to much paler grey, silty mudstone (Fig. 6), potentially presaging
wider regional 643
environmental change in the later part of the Athleta Zone. 644
The sporadic distribution of the Grammatodon-rich Bituminous Facies of Duff
(1975), and its 645
unusual fauna dominated by elements more typical of oxic conditions is
enigmatic. Duff (1974) 646
suggested that this facies was likely associated with a slight increase in
current activity, and many 647
modern arcid bivalves (like Grammatodon) are adapted to life in unstable
environments from which 648
they might be dislodged by currents (Thomas 1978). Such an environment seems
unfavourable for 649
the build-up of significant organic enrichment (up to 6.1% TOC; Duff, 1974),
unless this was prolific 650
and occurred in periods of relatively short duration (e.g. linked to disturbance
of redox boundaries 651
across the East Midlands Shelf by enhanced storm activity). 652
Significant contrasts with the East Midlands Shelf and Wessex Basin occur in the
facies at the 653
margin of the ABM at Eriswell, and at Warlingham on the edge of the Weald Basin.
At Eriswell, 654
biofacies are consistently rich in broken-up shell material and generally poor
in ammonites, where 655
current winnowing in a relatively shallow water setting likely shaped deposition
of the thin and 656
condensed succession. The large number of faunal clusters might reflect the
development of cryptic 657
omission surfaces separating units with subtly varying faunal composition. Here,
the relative rarity of 658
Genicularia is unexpected in a setting that evidence suggests was likely well
oxygenated, and may be 659
a response to the high energy marine setting. The main facies response to rising
sea level at the base 660


27

of the Athleta Zone in the Eriswell succession is a slight increase in the
frequency of ammonites as 661
marine deepening likely strengthened connectivity with open marine settings. In
contrast, 662
Genicularia is unusually abundant in the Athleta Zone in the Warlingham
succession (CL3, Fig. 6), 663
where the DCA plot and the composition of biofacies clusters (Fig. 6, 9B) show
that Bositra and 664
nuculacean bivalves are closely associated and present in high proportions
throughout the 665
succession. Here, inferred sea level rise across the Coronatum/Athleta Zone
boundary is marked by a 666
subtle change in biofacies composition, largely related to the disappearance of
the bivalve 667
Meleagrinella in the lower part of the Athleta Zone. A similar gap in the record
of this bivalve is 668
noticeable at other sites (e.g. Eriswell, CM11, Combe Throop), and is likely
driven by factors that are 669
not site specific. Thus, at Warlingham, the significance of this bivalve for
defining a change in 670
biofacies at the Coronatum/Athleta Zone boundary is largely an indication of the
unusual 671
compositional stability of other biofacies components through much of the
succession. High 672
concentrations of bone suggest low sedimentation rates and/or in situ sediment
winnowing 673
(Boessenecker et al. 2014) causing mixing of faunal components. This process may
have been helped 674
by export of winnowed sediment from the adjacent ABM, potentially aided by
Callovian syn-675
depositional normal faulting at the margin of the Weald Basin (Holloway 1985),
creating a steep 676
sediment pathway. Further out into the Weald Basin the conditions influencing
deposition are 677
unclear. The thin succession and evidence of pyritic and organic-rich
lithologies rich in Bositra and 678
bone seen in Ashdown 2 Borehole, suggest a poorly oxygenated, sediment starved
setting. 679
6. The Peterborough Member in context 680
Facies patterns in the Peterborough Member appear more laterally variable than
other organic-rich 681
Jurassic mudstone units in SE Britain, like the underlying Lias Group and
overlying Kimmeridge Clay 682
Formation (Wignall 1991, Taylor et al. 2001), in which depositional patterns are
predominantly 683
modulated by Milankovitch climate cycles (Weedon et al., 2004, Pearce et al.
2010, Xu et al. 2017) or 684
major oceanographic change (Toarcian Oceanic Anoxic Event (OAE); McArthur et al.
2008). The 685
evidence from this work is that basin palaeogeography produced a more laterally
variable response 686
of facies in relation to relative sea level change in the Peterborough Member.
This facies variability 687
may, at least in part, reflect the timing of deposition of the Peterborough
Member, which occurred 688
at a relatively early stage in a cycle of broader sea level rise following
rifting, potentially providing a 689
more dynamic and accentuated basin environment for its deposition compared to
other Jurassic 690
mudstones. The contrasting development of organic-rich mudstones in the lower
Phaeinum 691
Subzone at Christian Malford, compared to some more distal parts of the Wessex
Basin, might not 692
only reflect the influence of complex structure transecting the basin; it might
also indicate that 693
shallower regions of the basin margin more easily became thermally stratified
and anoxic. This is 694
somewhat analogous to transgressive nearshore black shales described by Wignall
and Newton 695
(2001) in the Kimmeridgian, and by Leonowicz (2016) in the Middle Jurassic of
Poland, although the 696
Christian Malford organic mudstone succession appears to represent deposition
during early 697
Highstand and also in a more distal, though not basinal, setting. 698
Although some previous workers have characterised the Callovian as an OAE
(Hautevelle et 699
al. 2006, Soua 2014), widespread deposition of organic-rich facies (Dromart et
al. 2003, Martinez 700
and Dera 2015) appears more strongly related to continental rifting (Robertson
and Ogg 1986) and 701
the creation of intra-shelf basins (Carrigan et al. 1995). These widely
distributed but more localised 702
tectonic settings, coupled with marine transgression and a nutrient supply fed
by humid climate 703
weathering, seem likely to have been controlling influences in both organic
matter accumulation in 704
the Callovian, and the demise of contemporary shallow-water carbonate platforms
(Hautevelle et al. 705
2006, Andrieu et al. 2016). 706
707


28

FIGURE 12 708
709
Termination of Callovian organic matter sequestration that defines the
Peterborough 710
Member coincides with evidence of southward migration of polar and sub-polar
waters across the 711
Eur-Russian area and a major shift in the Late Jurassic climate system (Dromart
et al. 2003, Dera et 712
al. 2015). This may have been a consequence of carbon-burial and CO2 draw-down
(Dromart et al. 713
2003), or potentially in response to the impact of rifting on patterns of marine
circulation. 714
7. Conclusions 715
Deposition of the Peterborough Member was likely strongly influenced by highly
variable basin 716
architecture, with a depositional gradient from the Mid North Sea High
channelling sediment 717
towards the Wessex Basin, and the Anglo-Brabant Massif acting to shield the
Weald Basin from this 718
sediment source. Deposition on the flanks of the ABM is thin and condensed, and
limited in the 719
Weald Basin, suggesting sediment starvation, despite the likely presence of
significant sediment 720
accommodation space. 721
The facies at given points in the basin reflect the impact of local basin
architecture and its 722
interplay with variable sea level. Spatial contrasts in facies across the basin
provide evidence of 723
environmental gradients that can be used to inform how these facies are likely
to be distributed and 724
transition across the basin. For example, organic-rich mudstones on the East
Midlands Shelf in the 725
Jason Zone, coincident with low relative sea level, correspond with biofacies in
the Wessex Basin 726



29

indicative of relatively greater oxygenation, with significant silt content, and
containing limited 727
lithological evidence for poor circulation. The facies response of the
Peterborough Member to 728
relative sea level rise at the base of the Athleta Zone is markedly variable. At
the edge of the Wessex 729
Basin there is a sharp transition into organic-rich mudstone with a sparse fauna
of infaunal bivalves; 730
further into the Wessex Basin the signal is much less stark, with no sustained
facies shift, but instead 731
evidence of rapid oscillation between more and less oxic facies; and at the
margin of the Weald 732
Basin the event occurs within a shell-rich interval, with components indicative
of a range of 733
environments, that may reflect the impact of steep (?fault-controlled)
depositional gradients. 734
Compared to other organic-rich mudstones, like the Kimmeridge Clay Formation,
the 735
Peterborough Member seems to be a product of a much more heterogeneous
depositional 736
environment. This may reflect deposition at an early stage in the cycle of
regional sea level rise 737
combined with the continued impact of earlier regional uplift, both potentially
acting to restrict 738
accommodation space (Macquaker, 1994) and accentuate the impact of basin
irregularity on facies 739
patterns. Given the broad and varied character of the successions investigated
for this work, the 740
location of the stratotype Peterborough Member appears unrepresentative of
conditions across the 741
wider depositional basin. It cautions against developing basin-scale models from
a few well exposed 742
and heavily researched outcrop successions, and emphasises the value of
multidisciplinary studies 743
for revealing the underlying depositional controls that shape the geometry and
complexity of 744
mudstone heterogeneity. 745
Acknowledgements: We thank J D Hudson (University of Leicester), M J Norry
(formerly University 746
of Leicester) and S G Molyneux (BGS) for useful discussion and comment, and K N
Page (University of 747
Exeter) and M J Barron (formerly BGS) for providing advice on stratigraphy and
correlation. We 748
particularly acknowledge the contribution of our former colleague B M Cox for
the wealth of data on 749
the fauna and stratigraphy of the Oxford Clay published in BGS Technical
Reports. We also 750
acknowledge the United Kingdom Oil and Gas Authority
(https://www.ogauthority.co.uk/) for 751
allowing use of borehole geophysical log data. This paper is published with the
permission of the 752
Executive Director, British Geological Survey, UKRI. 753
754
755


30

References 756
Andrieu, S., Brigaud, B., Barbarand, J., Lasseur, E., & Saucѐde, T. (2016).
Disentangling the control of 757
tectonics, eustasy, trophic conditions and climate on shallow-marine carbonate
production during 758
the Aalenian – Oxfordian interval: From the western France platform to the
western Tethyan 759
domain. Sedimentary Geology, 345, 54 – 84. 760
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FIGURE CAPTIONS: 1092
1093
Fig. 1. UK (Mid Callovian, E. coronatum Zone) palaeogeography for the Oxford
Clay Formation, and 1094
location of key outcrop and borehole data referred to in this study.
Palaeogeography based on 1095
Bradshaw et al. (1992, Map J8). Lines 1 & 2 are borehole correlation lines shown
in Fig. 3. Map 1096
references for localities are given in text and/or Supplementary Data. 1097
1098
Fig. 2. The stratigraphy of the Oxford Clay Formation and coeval geological
units in the Southern 1099
North Sea and Moray basins. Grey highlight indicates stratigraphy that is the
focus of this study. 1100
Boreholes are continuously cored successions drilled for BGS, or partially cored
successions drilled 1101
by others for hydrocarbons exploration (Ashdown 2, Kimmeridge 2) and held in the
BGS national 1102
borehole archive at Keyworth, Nottingham. Biozonal nomenclature used in the text
follows the 1103
conventions discussed by Cox (1990), in regarding Jurassic ammonite zones as
chronostratigraphical 1104
units, referred to by species name with an initial capital letter written in
non-italicized text. For 1105
clarity, we include the genus name on Figure 2, abbreviated by an initial letter
for Subzones where 1106
this is the same as the genus used for the corresponding zone. 1107
1108
Fig. 3. Modelled thickness and correlation of the Peterborough Member. (A) Gamma
ray log 1109
correlation: Southern North Sea - East Midlands Shelf - Wessex Basin - Channel
Basin (Line 1 of Fig. 1110
1). (B) Gamma ray log correlation: East Midlands Shelf - Anglo-Brabant Massif -
Weald Basin (Line 2 1111
of Fig. 1). (C) Interpolated thickness map showing faults (from Pharaoh et al.
1996) and borehole 1112
locations. Borehole numbering follows that used on Fig. 1. WWB (West Walton
Beds); MD 1113
(measured depth, metres); TVD (true vertical depth, metres); gAPI (gamma ray
American Petroleum 1114
Institute units). Cored boreholes (annotated) provide stratigraphical control
for interpretations. 1115
Fig. 4. Gamma log correlation of the Oxford Clay Formation and Peterborough
Member in the Weald 1116
and Wessex basins. Log interpretations are guided by the records from cored and
geophysically 1117
logged boreholes, published log interpretations (e.g. Winterborne Kingston; Rhys
et al., 1981), and 1118
related borehole data held in BGS data archives. 1119
1120
Fig. 5. Stratigraphy, geochemistry and biofacies of the CM9 and CM11 boreholes.
Geochemical data 1121
show patterns of enrichment in shell (red curve) and detrital material (blue
curve). The 1122
stratigraphical distribution of samples and their biofacies assignment is
represented by the pattern 1123
of short horizontal lines plotted for each defined biofacies cluster (Chm 1, 2
etc). The composition of 1124
biofacies clusters is given in bar charts showing relative proportions of the
key components, 1125
calculated by dividing the total number of records for each component by the
total number of 1126
samples. See Supplementary Data for full details. Contrasting detail of shell
beds at different levels in 1127


38

the succession are shown as core images for interval 'X' (sharp-based shell bed)
and for interval 'Y' 1128
(shell bed with gradational base). ‘C’ and S1 – S15 are geochemical peaks
discussed in the text. 1129
1130
Fig. 6. Lithology and biofacies of cored borehole successions in the
Peterborough Member. Biofacies 1131
clusters are unique to each site and based on NERC clustering. See caption to
Fig. 5 for explanation. 1132
NB: for clarity of other detail, shell hash not annotated on Eriswell log, but
it is common throughout. 1133
* denotes lithologically and faunally distinct interval in Down Ampney 2
Borehole characterised by 1134
concentration of wood, foraminifera and bone. 1135
Fig. 7. Correlation and biofacies classification of Peterborough Member
successions according to 1136
Duff (1974, 1975) compared with biofacies classifications assigned using DCA and
NERC clustering. 1137
The comparison shows broad similarity in the pattern of classification deduced
by the contrasting 1138
methodologies. The stratigraphical ranges of NERC biofacies clusters (Ca1, BL1,
St1, NC1, etc.) 1139
corresponds with quantitative data for individual beds within each succession.
The composition of 1140
biofacies clusters is given in bar charts showing percentages of key components
in each NERC 1141
cluster. 1142
Fig. 8. Detrended Correspondence Analysis (DCA) of fossil assemblages in the
Christian Malford CM 1143
11 (A) and Warlingham (B) boreholes, showing positions of samples and upper
quartile taxa. The 1144
highly contrasting geometry of sample points with respect to key taxa at
Warlingham compared to 1145
Christian Malford suggests a significantly contrasting relationship in the
association of different taxa 1146
that characterise biofacies at the two sites. The distributions at Warlingham
are inferred to be a 1147
mixing signal rather than an indication of altered palaeoecological
relationships between taxa (see 1148
text for details). The proximity of Bositra and nuculacean bivalves on the DCA
plot for Warlingham is 1149
reflected in their unusually close association in biofacies clusters throughout
this succession (Fig. 7). 1150
See Fig. 7 for key to biofacies components. DCA plots for all cored boreholes
forming part of this 1151
study are given in Supplementary Data. 1152
Fig. 9. Detrended Correspondence Analysis (DCA) for localities described by Duff
(1974, 1975) 1153
showing positions of samples and upper quartile taxa, with samples classified
according to NERC 1154
cluster assignment. See Fig. 7 for key to taxa. DCA plots for all cored
boreholes forming part of this 1155
study are given in Supplementary Data. 1156
Fig. 10. Biozonal correlation of cored boreholes and key outcrops in the
Peterborough Member. 1157
Borehole numbering follows that used on Fig. 1. Zn (Zone), Sz (Subzone), Gr
(Grossouvrei), Ob 1158
(Obductum). 1159
1160
Fig. 11. Conceptual basin model for deposition of the Peterborough Member,
showing key 1161
palaeogeographical and structural elements. 1: deltaic deposition in collapsed
graben along crest of 1162
Mid-North Sea High; 2: thin successions with organic-rich mudstone on East
Midlands Shelf; 3: highly 1163
condensed deposition with abundant shell hash on flanks of Anglo-Brabant Massif;
4: shell-rich 1164
mudstone at faulted margin of Weald Basin, fed by sediment from higher on flank
of Anglo-Brabant 1165
Massif; 5: sediment-starved basin with thin, organic-rich and pyritic mudstone
and limestone; 6: 1166
intra-basinal high with silty and sand-rich mudstone; 7: main depocentre
underlain by extensive 1167
network of east-west faults, fed by sediment from flanks of Mid-North Sea High,
East Midlands Shelf 1168
and pathways associated with the buried Worcester Graben. Note: vertical scale
exaggerated. 1169
Fig. 12. The Worcester Graben defined by regional gravity data, with significant
thickening of 1170
Peterborough Member occurring to the south and south-west, in line with the
mouth of this 1171


39

structure. Black arrows denote likely sediment pathways. Gravity data from Smith
and Edwards 1172
(1997), https://www.bgs.ac.uk/datasets/gb-land-gravity-survey/. 1173
1174
Appendix 1 – Biofacies components and their abbreviation used in statistical
analysis. Data list has 1175
been conditioned to remove species-level data (mainly applicable to data
originally collected by 1176
Duff (1974, 1975) 1177
1178
ammonite (AM) 1179
ammonite spat (AmS) 1180
Anisocardia (Aic) 1181
aptychus (AP) 1182
arcid (AR) 1183
Bathrotomaria (BA) 1184
belemnite (BL) 1185
Belemnotheutis (BT) 1186
bone (BN) 1187
Bositra (BO) 1188
bryozoan (BR) 1189
burrowing (BU) 1190
Camptonectes (Ca) 1191
Chlamys (CH) 1192
cirripede (CI) 1193
coprolite (CO) 1194
Corbicella (CL) 1195
Corbulomima (C) 1196
crinoid ossicle (Cr) 1197
Dicroloma (DI) 1198
Discomiltha (Dm) 1199
Echinoid spine (Es) 1200
Entolium (EN) 1201
foraminifera (FO) 1202
gastropod (juvenile) (GJ) 1203


40

Genicularia (GE) 1204
Grammatodon (GR) 1205
Gryphaea (GY) 1206
Shell hash (HS) 1207
Hooks (belemnoid) (HO) 1208
Isocyprina (IS) 1209
Isognomon (IG) 1210
Lingula (Li) 1211
Mastigophora (MA) 1212
Mecochirus (MS) 1213
Meleagrinella (ME) 1214
Mesosacella (MC) 1215
Modiolus (MO) 1216
Myophorella (MP) 1217
Nanogyra (NG) 1218
Neocrassina (NE) 1219
Nicaniella (NI) 1220
nuculaceans (NU) 1221
Ooliticia (OO) 1222
Ophiuroid (OP) 1223
Orbiculoidea (Orb) 1224
ostracod (OS) 1225
otolith (OT) 1226
Oxytoma (OX) 1227
oyster (OR) 1228
Parainoceramus (PA) 1229
Pecten (PN) 1230
Pholadomya (PM) 1231
Pinna (PIN) 1232
Plagiostoma (PG) 1233
Pleuromya (PL) 1234


41

?Praecoria (PRa) 1235
Procerithium (PR) 1236
Protocardia (PC) 1237
Pteroperna (PT) 1238
Quenstedtia (QU) 1239
rhynchonellid (Rh) 1240
Rollierella (Ro) 1241
scaphopod (SC) 1242
serpulid (SE) 1243
shell hash (HS) 1244
Solemya (SO) 1245
solitary coral (Sco) 1246
spat (SP) 1247
sponges (Spo) 1248
terebratulid (TE) 1249
Thracia (TH) 1250
trigoniid (Tr) 1251
wood (W) 1252
1253
Appendix 2 – Other abbreviations used in statistical outputs 1254
Athleta Zone (Az) 1255
Calloviense Subzone (Csz) 1256
Calloviense Zone (Caz) 1257
Combined Coronatum, 1258
Jason, Calloviense 1259
Zones (CJC) 1260
Coronatum Zone (Cz) 1261
Enodatum Subzone (Esz) 1262
Grossouvrei Subzone (Gsz) 1263
Jason Subzone (Jsz) 1264
Jason Zone (Jz) 1265


42

Medea Subzone (Msz) 1266
Phaeinum Subzone (Psz) 1267
Sample Size (Sz) 1268
1269



CITATIONS (0)


REFERENCES (86)




ResearchGate has not been able to resolve any citations for this publication.
Reconstructing the Christian Malford ecosystem in the Oxford Clay Formation
(Callovian, Jurassic) of Wiltshire: exceptional preservation, taphonomy, burial
and compaction
Article
Full-text available
 * Jul 2019
 * J MICROPALAEONTOL

 * Malcolm B. Hart
 * Kevin N. Page
 * Gregory D. Price
 * Christopher W. Smart

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
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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
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(even if only 1–2 cm thick) could, potentially, include several oxic–anoxic
cycles and, if coupled with compaction, generate the apparent coincidence of
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View
Show abstract
Radioactive Waste Confinement: Clays in Natural and Engineered Barriers –
introduction
Article
Full-text available
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 * Simon Norris

Extract There is general agreement internationally (Nuclear Energy Agency, OECD
2008) that geological disposal provides the safest long-term management solution
for higher-activity radioactive waste. Many countries (e.g. Canada, Finland,
France, Switzerland, Sweden, UK and USA) have chosen to dispose of all or part
of their radioactive waste in facilities constructed at an appropriate depth in
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Exceptional accumulations of statoliths in association with the Christian
Malford Lagerstätte (Callovian, Jurassic) in Wiltshire, United Kingdom
Article
Full-text available
 * May 2016

 * Malcolm B. Hart
 * ALEX DE JONGHE
 * Kevin Page
 * Christopher W. Smart

In the shell-rich, laminated clays of the Phaeinum Subzone (Athleta Zone, upper
Callovian, Middle Jurassic) of the Peterborough Member of the Oxford Clay
Formation, large numbers of statoliths and otoliths have been recovered. This
apparent mass mortality is associated with the Christian Malford Lagerstätte in
which there is exceptional, soft-bodied preservation of coleoid fossils.
Statoliths are the aragonitic 'stones' that are found in the fluidfilled
cavities (or statocysts) within the cartilaginous head of all modern and
probably many fossil coleoids. Jurassic statoliths are largely undescribed and
there are no known genera or species available to aid their classification.
Otoliths, which may be of somewhat similar appearance, are the aragonitic
stato-acoustic organs of bony (teleost) fish. These are more familiar to
micropaleontologists and have a better known, though limited, fossil record. The
abundance of statoliths in the Phaeinum Subzone at Christian Malford may
indicate a mass mortality of squid that extends over some 3 m of strata and,
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interpreted as a record of a breeding area (and subsequent death) of squid-like
cephalopods over an extended period of time rather than a small number of
catastrophic events.
View
Show abstract
Nearshore transgressive black shale from the Middle Jurassic shallow-marine
succession from southern Poland
Article
Full-text available
 * Apr 2016

 * Paulina Leonowicz

Five facies types are distinguished in the Middle Jurassic dark-grey mudstone of
the Częstochowa Ore-Bearing Clay Formation on the basis of sedimentary
structures, bioturbation intensity, and composition of trace fossil and benthic
fauna associations. Three of them, laminated mudstone (Ml), laminated claystone
(Cl), and alternating laminated and bioturbated mudstone (Ma), are varieties of
black shale. They formed in relatively shallow water, several tens of meters
deep, in an epicontinental sea, mainly during the early phase of Middle Jurassic
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narrow, proximal zone near flooded land, which delivered increased amounts of
organic matter and nutrients, triggering plankton blooms. Oxygen-deficient
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of the sea floor, and the occurrence of better-oxygenated time-equivalent facies
in more distal settings, indicates that these deposits represent nearshore
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correlation with minor transgressive–regressive cycles show that its formation
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relative sea-level changes within the basin.
View
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A comparison of clustering methods for biogeography with fossil datasets
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 * Matthew Vavrek

Cluster analysis is one of the most commonly used methods in palaeoecological
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all the methods performed well with clearly differentiated and well-sampled
datasets, when data are less than ideal the linkage methods perform poorly
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analysis, Unweighted Pair Group Method with Arithmetic Mean and neighbor joining
methods are less reliable with incomplete datasets like those found in
palaeobiological analyses, and the k-means and NERC methods should be used in
their place.
View
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R: A Language and Environment for Statistical Computing
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 * SEDIMENT GEOL

 * Simon Andrieu
 * Benjamin Brigaud
 * Jocelyn Barbarand
 * Thomas Saucède

The objective of this work is to improve our understanding of the processes
controlling changes in the architecture and facies of intracontinental carbonate
platforms. We examined the facies and sequence stratigraphy of Aalenian to
Oxfordian limestones of western France. Seventy-seven outcrop sections were
studied and thirty-one sedimentary facies identified in five depositional
environments ranging from lower offshore to backshore. Platform evolution was
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were identified on the entire western France platform and correlated with
European third-order sequences at the biozone level, demonstrating that eustasy
was the major factor controlling the cyclic trend of accommodation. The tectonic
subsidence rate was computed from accommodation measurements from the Aalenian
to the Oxfordian in key localities.Tectonism controlled the sedimentation rate
and platform architecture at a longer time scale. Tectonic subsidence triggered
the demise of carbonate production at the Bathonian/Callovian boundary while the
uplift made possible the recovery of carbonate platform from Caen to Le Mans
during the mid Oxfordian. Topography of the Paleozoic basement mainly controlled
lateral variations of paleodepth within the western France platform until the
mid Bathonian. A synthesis of carbonate production in the western Tethyan domain
at that time was conducted. Stages of high carbonate production during the
Bajocian/Bathonian and the middle to late Oxfordian are synchronous with low
δ¹³C, high eccentricity intervals, and rather dry climate promoting (1)
evaporation and carbonate sursaturation, and (2) oligotrophic conditions.
Periods of low carbonate production during the Aalenian and from the middle
Callovian to early Oxfordian correlate with high δ¹³C and low eccentricity
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Shell form and the ecological range of living and extinct Arcoida
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Arcoid bivalves occupy an intermediate position, in terms both of morphology and
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the arcoid hinge and ligament grow by the serial repetition of simple
structures, in contrast with the development of more specialized, complex
structures in other groups. These simple growth patterns place significant
mechanical constraints on the range of possible shell forms. Most arcoids live
in moderately unstable environments, where they are liable to be excavated or
detached from their substrates. Many employ recovery strategies, being adapted
to regain their life positions. However, a variety of specialized forms,
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dislodged in the first place. Thus, intrinsic growth patterns and substrate
relationships have been the major factors in the evolution of the Arcoida.
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Geology of the country around Ely. Memoir for 1:50 000 geological sheet 173 (
England & Wales).
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 * Jan 1988

 * R W Gallois

Traces the geological history of the district from the muddy seas of the
Silurian of 400 My, via the deserts of the Triassic and the shallow tropical and
subtropical seas of the Jurassic and Cretaceous, to the Quaternary glaciations
of 250 000 to 18 000 years ago when the district was covered by ice sheets up to
several thousand feet thick. Finally, it records the history of the period after
the retreat of the ice when temperate climates returned and the broad glaciated
hollow that was to become Fenland was infilled with muds and peats. The greater
part of the district is occupied by the Recent sediments of Fenland. These were
deposited during the past 10 000 years, during which period sea level has risen
by about 30 m. Through man's activities the fens have been converted to richly
fertile peat soils and the meres to calcareous marls. But in the process,
shrinkage and wastage of the soils due to the drainage works have caused large
areas of the underlying geology, including the Jurassic clays, Quaternary
gravels and Recent marine clays, to be revealed and have given rise to markedly
less fertile soils. Provides an authoritative account of all aspects of the
geology of the Ely district including descriptions of the rock types and their
faunas, their structure and geological history, and the relationships of these
features to past and present land use.-from Author
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The Kellaways Beds and the Oxford Clay
Article
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 * J.H. Callomon

View
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