and south of it in the Warlingham borehole, Surrey, are remarkably similar (Callomon & Cope 1971, fig. 2, table 5). Old borehole records in Sussex and Kent ...
Journal of the Geological Society The Peterborough Member (Callovian, Middle Jurassic) of the Oxford Clay Formation at Peterborough, UK J. D. HUDSON and D. M. MARTILL Journal of the Geological Society 1994; v. 151; p. 113-124 doi:10.1144/gsjgs.151.1.0113
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© 1994 Geological Society of London
Journal ofthe Geological Society, London, Vol. 151, 1994, pp. 113-124, 7 figs, 2 tables. Printed in Northern Ireland
The Peterborough Member (Callovian, Middle Jurassic) Formation at Peterborough, UK
of the Oxford Clay
J . D. HUDSON & D. M . MARTILL Department of Geology, University of Leicester, Leicester LE1 7RH, U K Abstract: King’s Dykebrickpit,Whittlesey,nearPeterborough,Cambridgeshire,providesacontinuous section through the Peterborough Member (formerly known as the Lower Oxford Clay)of the Oxford Clay Formation, and is here described as its type section. It also shows parts of the underlying Kellaways Formation and of the overlying Stewartby Member of the Oxford Clay Formation. Other sections in the Peterborough district are discussed and related to the type section. The Peterborough Member is developed throughout southern England and aspects of its stratigraphy, palaeogeography and burial history are briefly reviewed.
Company’s pit at King’s Dyke, near Whittlesey (national grid reference [TL 248 9671) (Figs 1 & 2). This pit is worked by shale planer so the face is steep, and not as convenient for collecting as in some of theother pits. It is more amenable to measurement, however, and the pit is likely to have an extended life; part of the face should be conservable when working eventually ceases. Theupper part of the Kellaways Formationand the lower part of the Peterborough Member, up to Bed 13 (of Callomon 1968; Fig. 3 herein), is exposed in drainage pits in the floor of the pit. Bed 13 can be exposed by minor excavation at the base of the main face, which exposes the PeterboroughMember from Bed 14 upwards. The StewartbyMember (formerly Middle Oxford Clay) forms the upper part of the face with a distinct lithological change to massive calcareous clays at its base; this is marked by prominent slips that terminate at this level (Fig. 2).
The lower part of the Oxford Clay from around Peterborough, Cambridgeshire, was the source of raw material for the first modern bricks manufactured in Britain acentury ago, the village of Fletton giving itsname to a process that revolutionized the industry. The exposures in the extensive brick-pits have made the Oxford Clay a classic subject for stratigraphicaland palaeontological research. Two outstanding feats of collecting will always be associated with Peterborough: the assembly of the largest collection of marine reptiles ever made, by the Leeds brothers in the late nineteenth and early twentieth centuries (Andrews 1910-13; Leeds 1956), and the statistical study of the evolution of the ammonite Kosmocerus by Brinkmann (1929). Since the advent of mechanical methods of working the clay, opportunities for fossil collecting have somewhat diminished, but well-preserved skeletons of reptiles and fish are still being discovered at the rate of several per year, and the ammonitescontinue to providea standardfor Middle Callovian bio- and chronostratigraphy (Callomon 1968; Page 1991). Martill & Hudson (1991) give a review of Oxford Clay palaeontology, and Hudson & Martill (1991) discuss its palaeoenvironmental setting. Cox et al. (1993) have revised the nomenclature of the Oxford Clay Formation, defining the unit traditionally known as the LowerOxford Clay as thePeterborough Member, in accordance with current requirements of stratigraphicalnomenclature (Whittaker et al. 1991). We herein give adetaileddescription of itstype section at King’s Dyke Pit, Whittlesey. The succession in the several brick pits that are currently worked near Peterborough, or that have been worked since the 195Os, is fairly constant, and generalized sections for the area are usually employed (Callomon 1968). However, thereare variations in detail, especially in the lower beds, and some of the sections that have proved most fossiliferous in the past areno longer available. It is therefore important to record these sections, especially as collections from them have been used for geochemical studies (Williams 1988; Anderson et a l . , Kenig et al., this volume).We also discuss aspects of regional correlation,palaeogeography and palaeoenvironment that are relevant to the papers forming the thematic set included in this volume.
Graphic sections. Figure 3 is a log of the section based on examination of the beds in situ; key beds are indicated on the photograph (Fig. 2). Figure 4 includes a log of the condensed lower beds. The equivalents of these beds have yieldedmost of the large vertebrate fossils recordedfrom the Peterborough district (Martill 1985, 1986) and are characterized by shell beds with abundant Gryphaeadilobotes Duff; we refer to them informally as the ‘Gryphaea and Reptile Beds’. A further informal marker bed is Bed 18, the ‘Dark Bed’, which shows up conspicuously in the lower part of the worked face, as its equivalents do in other brickpits (e.g. Dogsthorpe, Horton et al. 1974, plate 8b). Two contiguous beds long recognized by their lithology and by ammonites, the ComptoniBedand the AcutistriatumBed (formerly Band), are important in correlation from Dorset to Humberside. The ammonitezonation in Fig. 3 is after Callomon (1968) and Page (1991). The biofacies are from Duffs (1975) analysis of the benthic faunas, supplemented by our own observations. Bed numbers. A generalized section for the Peterborough district was published by Callomon (1968), based in part on Brinkmann (1929), and has been the basis for all subsequent work. Duff (1978) compiled a section at Norman Cross Pit [TL 186 9161, 8 km SW of Whittlesey, adding lithological and fauna1 detail to Callomon’s account. In Callomon’s section the bed numbering starts within the Cornbrash and
Type section The best and most continuous section currently available in thePeterborough Member is that at theLondon Brick 113
J . D . HUDSON & D . M. MARTILL
114
f
N
I
\\
Norman Cross
1
Fig. 1. (a) Sketch map of the Oxford Clay Formation outcrop in England, showing localities mentioned in the text. D, Dogsthorpe; 0, Orton; W, Whittlesey (all in the Peterborough district; see Fig. lb); S, Stewartby; B, Bletchley; C, Calvert; A, Ashton Keynes; CM, Christian Malford. (b) Map of the Peterborough brick manufacturing area, showing recently-active pits mentionedin the text (Dogsthorpe, King’s Dyke, Bunting’s Lane borrowpit, Orton, Norman Cross), the site of the Whittlesey borehole, and some disused pits (lined).
includes the Kellaways Formation; the base of the Oxford Clay is his Bed 4. In this account (Table 1) we start the bed numbering at the base of the Peterborough Member and of the Oxford Clay Formation, which we define above the last silty sand of the Kellaways Formation (Callomon 1968; Cox et al. 1993). This is the level chosen by Brinkmann (1929) as number addilevel 0 in his centimetricmeasurement.We tional beds in the basal part of the succession. Our bed numbers coincide with Callomon (1968) and Duff(1978) from Bed 8 to the main part of Bed 14, the most important part of the section for fossil collecting. The Duff and Callomon sections diverge above Bed 15; Duff employs a more detailed subdivision up to and including the Acutistriatum Bed. Our numbering is similar butnot identical to his Norman Cross section. Duffs section does not extend above the AcutistriatumBed and Callomon’s lacks detail;our numbering is new.
Other sections in the Peterborough district As noted by Callomon (1968) these can in general be readily correlated with one another, and a common bed numbering system applied. Difficulties arise principally in the condensed lower beds.
Dogsthorpe. The brick pit atDogsthorpe [TF 210 0201, which closed in 1990, was for many years a prolific source of fossils, especially vertebrates. It was worked down to Bed 10 andexposed fresh faces up to and including Bed 15. The generalPeterborough section given by Callomon (1968) applies. The shell beds 11 and 13 contain abundant
Gryphaea. A section in the lower beds was exposed in a temporary excavation in 1990, and the section shown in Fig. 4 recorded. Orton. Orton Pit [TL 1609401 exposes a similar section to Dogsthorpe; above Bed 19 the sequence is highly weathered and devoid of calcareous fossils. It is worked by dragline and is currently the best pit for collecting, especially from Bed 10. A distinctive feature is the occurrence of septarian concretions in Bed 4, lower than that recorded by Martill (1986). A section of the lower beds from a drainage sump is included in Fig. 4. Bunting’s Lane. The temporary exposure at Bunting’s Lane [TL 200 9581, also known as Farcet Pit, was a borrow pit for part of the Peterborough Eastern by-pass thatexposeda complete section from the top of the Cornbrash, through the Kellaways Formation intothe lower part of the Peterborough Member. It afforded the best collecting in recent years from the beds below Bed 10. A summary section is included in Fig. 4. Norman Cross. The former brick pit at Norman Cross [TL 1869161was the site of the detailed collecting for Duffs (1975) palaeoecological studies. His section of the lower beds is shown in Fig. 4. Whittlesey core. Theauthors were presented with acore from a borehole sunk by the London Brick Company near
PETERBOROUGHMEMBER,OXFORDCLAYFM
115
Discussion of the Peterborough sections
Fig. 2. Photograph of the face at King’s Dyke brick pit, Whittlesey, Cambs., showing partsof the Peterborough and Stewartby Members of the Oxford Clay Formation (cf. Fig. 3). The bottom of the exposed face is near the base of Bed 14; Beds 18 (the ‘Dark Bed’), and 38 (the Acutistriatum Bed) indicated. The topmost arrow marks the topof Bed 55 of the Peterborough Member and the base of the Stewartby Member. Note the slipin the steep face, terminating at this level.
Whittlesey [TL 251 9601. Agraphic log of thecore is presented in Fig. 5, inwhich bed numbers are assigned by correlation to the nearby type section. There are some short breaks in the core signified by rotary fractures, so the log is somewhat approximate.However thecore providedfresh material for geochemical analysis (Kenig et al. this volume), whichwas particularly valuable for the higher and lowest parts of the section that could not be sampled at Dogsthorpe.
Peterborough Member. The ‘Gryphaea and Reptile Beds’. The lowest part of the Peterborough Member is of particular interest because it is stratigraphically condensed (Hudson & Martill 1991) and is a prolific source of fossils. It may be divided into two parts. Beds 1 to 7, and to a lesser extent 8 and 9 also, are variable across the Peterborough area (Fig. 4). The correlations shown are in part tentative. Beds 10 to 13 are rather constant within the region. Beds 1 and 2 of the Oxford Clay are shaly clay, with the olive-brown colour typical of the main part of the Peterborough Member, belonging the Enodatum to Subzone. The lower silty part (Bed 1) is more distinct in some sections than in others (Fig. 4). Above this, a variable development of Gryphaea shell-beds, alternating with shales, is characteristic of beds 3 to 7 (Medea Subzone). The shell-beds are thought to represent periods of winnowing on the sea floor, followed by colonization by several generations of Gryphaea (Duff 1975; Hudson & Martill 1991). Gryphaea shells are frequentlyfound overturned. The variability in shell-bed number and type across the area (Fig. 4) presumably reflects minor variations in the intensity of winnowing. The Medea Subzone, including also beds 8 and 9, is less than 0.5 m thick, implying an extremely slow net sediment accumulation rate (Hudson & Martill 1991). Bed 8 is more constant, being developed in most sections as an organic-rich, dark, fissile shale with abundant ammonites. It is the lowest bed to have yielded articulated vertebrates (Martill 1985, 1986). However Duff (1978) describes it as ‘fairly massive, slightly silty clay’ at Norman’s Cross. Bed9 is a thin shell-bed in most sections;at Dogsthorpe and Orton it is an ammonite plaster overlain by abundant Gryphaea, many with xenomorphic‘ornament’ showing attachment to ammonites. Grarnrnatodon, Pinna and Chlurnys alsooccur. At Norman Cross, Duff(1978) recorded only ‘dark olive-green clay with many shells’; again, the degree of winnowing presumably varied. Bed 9 marks the top of the Medea Subzone. Bed 10 is the most distinctive in the entire section. It is a fissile, organic-rich shale (up to 14% total organic carbon, TOC), with ovoid, calcareous, septarian concretions up to l m or more across and typically about 20cm thick; away from concretions, the whole bed is only 15-20cm thick. Within concretions, the pelletal nature of thesediment, prior to the compaction that has obliterated it elsewhere, is revealed (Hudson & Martill 1991; Martill et al. this volume). At Dogsthorpe (but not at Orton), the lower few centimetres are less fissile and less organic-rich than the central part of the bed. The upper few centimetres are riddled with cm-scale burrows, filledwith greenish clay brought down from above, and now appearinghorizontal because of compaction. The bed splits easily along particular bedding planes, especially where fossils are abundant, displaying varied faunas always dominated by crushed, mainly whole Kosrnoceras, including microconchs with lappets.Ammonites may cover 20% of beddingsurfaces; fragmented specimens, probably the result of predation, are sometimes common. Cadoceras and Paracenoceras are rare. Belemnites, mainly Cylindroteuthis, althoughcommon by most standards,are far subordinateto ammonites, and Belemnotheutis also occurs. Some bedding planes are virtually devoid of benthos, but others show Bositra plasters, composed mainly of tiny individuals (cf. Oschmann
J . D .H U D S O N
116
Metre Scale
Munsell Shell-bed Colour Ammonite zones No. Biofacies 5Y- Fissility
& D .M .M A R T I L L
Bed
type
Lithostratigraphy W
8 subzones
STEV ME1
18
17
DFS?
16
54 52
15
50
CC?
DFS?
-
ATHLETA:
FSICC?
PHAEINUM
48 CC?
14 46
DFS? FSICC?
13
42
11-411
25 -
44 DFS?
312
CC?
411
12 40 11
38 36
10
34 32 30
9
28 26 24
8
7
22 20
DFS? FRS
D FS D FS -
f & CII
FRS u-cc
CORONATUM: GROSSOUVREI
FRS FRS
FRS P d 7 L
6
18
16 5
GRS
m
-
4 CORONATUM:
3
14
OBDUCTUM
2
1
12 10
a
1-5
JASON: CALLOVIENSI JASON ENODATUM JASON: MEDEA CALLOVIENSE: CALLOVIENSE
I
KELLAWAYS I
Fig. 3. Graphic log of the type section of the Peterborough Member of the Oxford Clay Formation atKing’s Dyke brick pit, Whittlesey, Cambs. Biofacies and shell-bed type afterDuff (1975): CC, calcareous clay; DFS, deposit-feeder shale; FRS, foraminifera-rich shale; GRS, Grammatodon-rich shale;G , Gryphaea shell-bed; Gr,Grammatodon shell-bed; M, Meleagrinelk shell-bed; N, Nuculacean shell-bed.
PETERBOROUGH MEMBER, OXFORD CLAY King’s Dyke
Dogsthorpe Lane Bunting’s
Orton
10
FM
Norman Cross
117
[ X I
Septanan concretion
F
Fissile. organic-rich shale
j
m
Moderately fissile shale
[-l v]
9
Less fisslle shale h mudstone
8
7
- l
“(1 0
J
Shell -bed (Gr,Grammatodon; G, Glyphaea)
o
Sandstone (KellawaysFormatlon)
Silly mudstone or shale o
0
6
5
4
3
2 1
Fig.4. Comparative sections in the basal part of the Peterborough Member at localities near Peterborough (see Fig. l b and the text). Measured by the authors except Norman Cross(Duff 1975). Bed numbers of the present account at the left; in brackets to the right are the bed numbers used by Callomon (1968). Septarian concretions occurin Bed 10 at all localities but were included in the section measured only at Bunting’s Lane. Concretionsin Bed 4 have only been recorded from Orton. Beds 1-2 are in the Enodatum Subzone, Beds3-9 in the Medea Subzone and Bed 10 in the Jason Subzone (Callomon 1968). 1993), or flat-lying Pinna and Parainoceramus. Occasional oysters, showing incipient grypheate coiling, occur attached to ammonites. Other byssate bivalves recorded byDuff (1975) are Meleagrinella, Oxytoma and Chlamys. Burrowing forms are rare.Atwo-hoursearch atOrton yielded one Thracia, immediately above Bed 9, one Protocardia and a few Dicroloma. Duff(1975) additionallyrecords Grammatodon and Trautscholdia. No nuculaceanswere found, nor are they recorded by Duff. Thus the fauna is dominated by nekton, which is exceptionally abundant, and most of the rest of the fauna consists of Bositra, regarded as planktonic by Oschmann (1993). Duffs (1975) pendent bivalves, if interpreted as benthos rather than pseudoplankton (Wignall 1993), are epifaunal. The sediment must have been exceptionally inhospitable to preservable, burrowing benthos, probably because it was both ‘soupy’ (Hudson & Martill 1991) and overlain by dysoxic bottom water. According to Wignall (1993) the latter is indicated by the absence of nuculacean bivalves in the assemblage, together with the presence of sparse shallow infaunalsuspension feeders such as Protocardia and Grammatodon . Bed 10 has also been by far the most prolific source of large, articulated vertebrate fossils from Peterborough (Martill 1986). It is or was exposed near the floor of the pits at Orton,Dogsthorpe, Yaxley and Norman Cross (it is below the floor in the Whittlesey pits), and the question has arisen as to whetherthisexplains the large number of skeletonsrecoveredfrom it. However our data show that the nature, abundance and preservation of the fauna are all genuinely exceptional, as is the geochemistry of the organic matter (Kenig et al. this volume).
Bed 11 is a shell-bed, representing in part the winnowing of ‘Bed 10’ sediment; this is shown by the occurrence of an articulated partial skeleton in Bed 10, surmounted in Bed 11 by disarticulated but only slightly dispersed bones from the same carcass (Martill 1986). Bed 11 is sometimes floored by an ammonite plaster in which many of the ammonites show evidence of predation,probably by hybodontsharks and chimaeras whose teeth and dental plates are common (Martill 1986). At Dogsthorpe it contains abundant Gryphaea, also small flat oysters that may bejuvenile Gryphaea, Grammatodon, Trautscholdia, Protocardia, Dicroloma, Meleagrinella and Bositra. Many of the Gryphaea noted were well preserved and in life orientation, suggesting little disturbance after death. Local variation is suggested because at Norman Cross Duff (1978) records no Gryphaea, only flat oysters. Bed 12 is a fissile, dark, organic-rich shale in its lower part, where it has yielded articulated fish (Martill 1986), becoming paler and less fissile upwards. It is unique in containing an abundance of Protocardia, in place of nuculaceans, and Dicroloma, together with pendent bivalves (Duff 1978); possibly it represents unusually dysoxic conditions, as with Bed 10. Bed 13 is a well-developed shell-bed at Dogsthorpe, with fragmentary ammonites and abundant belemnites in a clay matrix. Some of the belemnites are conspicuously corroded possibly by passage through the digestive system of fish and reptiles. Otoliths (sacculiths) and statoliths areabundant, whereas fish bones are not present in proportion tothe otoliths. This is the last bed in the Peterborough Member with common Gryphaea. It also contains nuculaceans,
J . DM . A HR U TDISL MOL. N & D .
118
Table 1. Description of the King's Dyke, Whitllesey, section, measured by the authors in 1992-3 Bed
Bed .m
Oxford Clav Formation. Stewartbv Member ~
~~
Mudstone, calcareous, blocky weathering. 2.00
Genicularia, Gryphaea, Procerithium, partiallyarticulated ichthyosaur (cf. Ophthalmosaurus); seen tn more Shale, fissile, weathers white Shale, hard, fissile, prominent bed Mudstone, calcareous, hard, blocky; Procerithium, nuculaceans, Meleagrinella Shale, slightly fissile, pyritized compressed ammonites. Meleagrinella, nuculaceans, Mesosaccella,
34 Shale, fissile, burrows top. at Kosmoceras 33 Meleagrinella shell-bed.Muchshellhash.Gradesinto0.06 bed above. Nuculaceans 32 Clay, blocky, darker than bed below and less 0.30 fossiliferous
grey,greenish pale 31 Clay, 0.20 below, bedfissile than nuculaceans 0.30 30 Meleagrinella shell-bed, Burrowed, weathers pale. 0.10 Shell-hash, Procerithium, nuculaceans, large 3.42 Erymnoceras at top 29 Clay, greenish. Articulated nuculaceans, Thracia, 0.40 Meleagrinella, Genicularia. Burrows from base 28 Shale, deposit-feeder shale facies, burrows from bed Procerithium above filled with green clay. Nuculaceans Mudstone, non-fissile; sharp base 0.50 27 Meleagrinella shell-bed, much shell hash Peterborough Member 26 Shale, deposit-feeder shale facies. Nuculaceans 0.80 55 Shale, fissile, brown; abundant Meleagrinella, 25 Shell-bed as bed 23 nuculaceans, Bositra, Dicroloma, Parainoceramus, 24 Shale, grey-green, weathers dark, burrowed. Depositfish scale feeder shale facies. Much wood, Thracia, coarse0.10 54 Shell-bed, Meleagrinella dominant, also nUCU~aceans ribbed Kosmoceras 0.23 53 Shale, fissile, brown; fauna as Bed 55, wood 23 Shell-bed, rather sparse shells, pyritous. Belemnites, associated with Parainoceramus wood, Kosmoceras, Hypsocormus tooth, winnowed 0.22 52 Clay, blocky, green; Meleagrinella, nuculaceans, shell shell hash debris 22 Shale, less fissile than bed below, gradational base 0.07 51 Shell-bed, gradational boundaries; Meleagrinella 21Shale, fissile, shelly. Abundant nuculaceans. dominant, nuculaceans, Procerithium Gradational base 0.65 50Shale,shelly; Bositra, Meleagrinella, Procerithium, 20 Shale, tough, greenish. Deposit-feeder shale facies. Dicroloma, fish scales, wood Coprolite. Gradational base 0.45 49 Alternations (5-15 cm) of 3 beds of blocky clay and 3 19 Shell-bed with clay matrix, pyritic in central 3 cm beds of shelly shale with Procerithium etc. 18 Dark Bed. Shale, not very fissile, wispy lamination 0.08 48 Shell-bed, sharp base, gradational top; nuCUlaCeanS (burrows?). Weathers conspicuously dark grey dominant, Procerithium, Meleagrinella, Kosmoceras 17 Nuculaceanshell-bed,pyritic 47Clay,blocky; Meleagrinella, Procerithium, 0.27 16 Shale as bed 14 nuculaceans 15 Shale, fissile, white-weathering; 2cm nuculacean 46 Shale, fissile; shell debris and nuculaceans. Middle 0.67 shell-bed at top 15 cm less shelly, more blocky 14 Shale, not very fissile, rather inconspicuous large 45 Shale, fissile at base passing into hard, blocky, 0.34 burrows, not very fossiliferous and rather calcareous clay; nuculaceans, Bositra, Procerithium, homogeneous.Deposit-feeder shale facies. Sporadic Kosmoceras, Belemnotheufis,burrows calcareous and pyritic septarian concretions upto 44 Clay, bluish, calcareous; gradational boundaries 0.15 30 cm diameter with well-preserved juvenile 43 Shale, less fissile than bed below, shelly, gradational 0.30 Meleagrinella, 1 .S m below top base 42 Shale, fissile, brown, pyritized shell hash. 0.67 Beds 1-13, 'Gryphaea and Reptile Beds' Kosmoceras, Hecticoceras, nuculaceans, fish scales 13 Shell-bed, rounded clasts of wood 41 Clay, greenish, becoming bluish upwards. Nuculace0.3.5 12 Shale, organic-rich, fissile in lower part, ammonites ans, articulated and disarticulated in all orientations, 11 Shell-bed. Gryphaea,Grammatodon Procerithium, Dicroloma 10 Shale, fissile, organic-rich, burrows in upper part; 40 Shale, slightly fissile becoming more so upwards, 0.36 large calcareous septarian concretions. Ammonites brown. Deposit-feeder shale facies; Procerithium, abundant Kosmoceras fragments, shell hash. Topof bed slightly 9Shell-bed,pyritous. Gryphaea burrowed 8 Shale, fissile, dark, organic-rich. Ammonites 39 Clay, greenish. Large perisphinctids form shell-layer 0.10 abundant, Gryphaea at base. Kosmoceras, Genicularia, coprolites 7 Shell-bed, pyritous at top. Gryphaea, Pinna, 38 Acutistriatum Bed. Shale, brownish, fissile especially 0.33 Kosmoceras towards top. Large burrows at top with fill of 6 Shale, fissile, grey, abundant Kosmoceras, greenish clay, also Chondrites. Kosmoceras phaeinum Meleagrinella, Pinna, spat microconchs [ K . acutistriatum J abundant, coarse5 Gryphaea shell-bed. Gryphaea shells in life ribbed Kosmoceras, nuculaceans, Parainoceramus, orientation and inverted, belemnites, ammonites, perisphinctids wood 37 Comptoni Bed. Nuculacean shell-bed, gradational 0.12 4 Shaly clay, brown. Gryphaea inverted base. Binatisphinctes, Cylindroteuthis, Procerithium 3 Gryphaea shell-bed, pyritous. Gryphaea inverted, 36 Shale, brownish, becoming less shelly upwards. 0.56 belemnites Deposit- feeder shale facies. Erymnoceras near base 2 Shaly clay, olive-brown. Transitional base 35 Nuculacean shell-bed, sharp base, becomes gradation0.09 1 Clay,silty.Sharpbase ally less shelly upwards. Meleagrinella, Procerithium
m 0.05
I"
0.22
0.20 0.10 0.12 0.08 0.02
0.68
0.01
0.25
0.28 0.54 0.15 1.17
0.10 0.20 0.16 3.80
0.02 0.46 0.04 0.17
0.01 0.09 0.05 0.14
0.03
0.03 0.04
0.06 0.06
C L A Y FM
P E T E R BO MOXERFMO OBU REG DRH,
Table 1. (Continued) Bed
m
119
photographs; indeed, photographs taken in colour versus monochrome, under different lighting conditions, or of parts of the face differing in their state of weathering. all emphasize somewhat different features.Therefore Fig. 3 and our section description should be regarded as approximations. The alternations invite speculationconcerning Milankovitch cyclicity, but thisshould await more rigorous data. In Beds 20 to 24 Duffs Foraminiferal Shale biofacies is dominant. Above this, alternations of Meleagrinella Shell Bed and Calcareous Clay biofacies are of Deposit Feeder Shale characteristic, with somebeds of facies sequence characterizes the facies. This type Grossouvrei Subzone throughout the Midlands and beyond (Duff 1975). Beds 37 (the Comptoni Bed) and 38 (the Acutistriatum Bed) are distinctive lithologically and faunally, and provide valuable regional markers. The Comptoni Bed is a nuculacean shell-bed characterized by the abundance of the perisphinctacean ammonite Binatisphinctes comptoni (Pratt). Itmarks thetop of the Coronatum Zone and probably period a of mild winnowing. The overlying Acutistriatum Bed is a dark fissile shale, rich in Kosmoceras including K. foraminifera, with (Lobokosmoceras) acutistriatum (Buckman), now regarded (Page 1991) as the micromorph of K . ( L . ) phaeinum (Buckman), the subzonal index of the lowest subzone of the Athleta Zone. Calcareous concretions occur at this horizon elsewhere in the Midlands, but are not known at Peterborough. The beds can however be readily recognized in those pits that extend high enough. The highest part of the Peterborough Member (Beds 39 to 55) was little subdivided by Callomon (1968) and was not studied by Duff (1975, 1978). In the face at the type section, similar alternations of weatheringcharacteristics to those below the Acutistriatum Bed are well developed. The beds include facies similar tothe Deposit-feederShaleand Calcareous Clay biofacies of Duff (1975). I
Kellaways Formation (no bed numbers assigned) Shell-bed inmatrix of finesand, in part cemented to calcareous sandstone. Gryphaea, some overturned, belemnites, ammonites, coprolites, wood Clay, Shell-bed,lagdeposit of belemnites and ammonites and Silt fines clay, upwards Shell-bed, abundant belemnites, wood, overturned Gryphaea, irregularly developed in ‘pockets’; up to 0.1 m Silt, clayey, etc., seen to about 1.5 m
0.14
0.06 0.02 0.15 0.10
1S0
Meleagrinella and Grammatodon. Wood is present as rounded clasts; unusual blade-like forms of pyrite 2-8cm long are common. Bed 13 marks thetop of the Jason Subzone, whichis strongly condensed in the Peterborough district (Callomon 1968; Hudson & Martill 1991). Beds 14 to 17 arethe most uniform part of the slightly succession, consisting of moderatelyorganic-rich, fissile to blocky mudstones, much less fossiliferous than the beds beneath, and belonging tothe deposit-feedershale biofacies of Duff (1975). These beds provide a high proportion of the clay worked in the Dogsthorpe and Orton pits. They represent the Obductum Subzone of the Coronatum Zone. Concretions recorded from Bed 14 in the type section are apparentlyexceptional.A large log from towards the base of Bed 14 at the type section was encrusted with gryphaeateoysters on its uppersurface, and had numerous large Parainoceramus on its lower surface. The nuculacean shell-beds towards thetop werederived by winnowing from the same facies (Duff 1975). Bed 15 shows up in the King’s Dyke face. It probably includes Bed 15 of Callomon. It was not recognized by Duff (1978) at Norman Cross nor by us at Dogsthorpe. A shell-bed (Bed 17) marks the top of the subzone; it is Bed 15 of Duff (1978) and Bed 16b of Callomon (1968), and was recognized as marking a break in sedimentation by Brinkmann (1929). Bed 18 (the Dark Bed) forms a conspicuous marker in the pits. Its base marks the base of the Grossouvrei Subzone (Callomon 1968), and the appearance in the benthic fauna of Grammatodon minimus (Leckenby).It is not otherwise distinctive faunally, being anormaldeposit-feedershale (Duff 1975). It is Bed 17 of Callomon (1968) and Bed 16 of Duff (1978). Our Bed 19 is a distinct, partly pyritic shell-bed with nuculaceans and Grammatodon, recognized also by Callomon (Bed18a) and Duff (Bed 17), andoccurring throughout the Peterborough area. AboveBed19, thenature of the succession changes from the alternation between thick shale or mudstone units and thin. distinct shell-beds. described above. to an alternation of decimetre-scale beds of mudstone o f subtly different type; shell-beds occur but are less distinct and less widely traceable. The face at the type section (Fig.2) shows this well. However it is not as easy as might besupposed tothe subdivide this part of the succession consistently. Callornon (1968) and Duff(1978) differ in detail,and our version (Fig. 3) differs from both. We have found itdifficult tocorrelate observations made on the lithologies at the face with
The Stewartby Member. The base of the Stewartby Member is marked by a sharp lithological upward change to clays that are more massively-bedded, more calcareous and generally less fossiliferous than those below. The incoming of the Calcareous Clay biofacies of Duff (1975) in the upper partof the Peterborough Member provides a partial foretaste, but the basal Stewartby Member is much thicker-bedded than is thePeterborough Member, with its characteristicalternations of lithology. A brief return to a facies similar to the Deposit-feeder Shale of Duff (1975) occurs in Beds 4 and 5 (Table l ) , 3.4 mabove the base of theMember, where fissile dark shales weatherprominently white (Fig. 2), before a return to massive calcareous clays which continue to the top of the section. Ammonite evidence (Callomon 1968) shows that the base of the Stewartby Member in the Peterof the borough pits lies within the PhaeinumSubzone Athleta Zone.
The correlation of the PeterboroughMemberthroughout EastMidlands, particularly asregards the groups of brick pits around Stewartby,Bedfordshire,and Bletchley andCalvert,Buckinghamshire, was established by Callomon (1968). Refinements toparts of Callomon’s sections have been made by Horton et al. (1974), Duff (1975, 1978), and
J . D . H U D S O N & D . M. M A R T I L L
120
Munsell Samples Metre Scale
W-
Biofacies
Colour 5Y -
13 312 411
53 52 12
51 50-
49 4a -
11
4:
311 411
4'
411
3s
Z
0
DFS CC
a z
47 46 10
DFS
42*
9
a
41 40-
DFS
39 313-
cc cc
4 12
3736-
DFS
3 12 412
3231 30-
6
5
4
3
1
2625 24 2322 21 2019 -
MSB FRS
>
a -l
31
411
0
a
2f
U
312 2:
M
312
X
211 412
M 1s
Gr
311
W
m
DFS
z z
DFS
li
N
212 DFS
312
I c3
412
2
0
0
oc
m
-
14 13 12 11 10 9-
a-
34-
"Dark bed
W
U
312
W
-l
DFS
W
a 312
DFS DFS
ssc
412 311
15 11 l-1c
-t'
G-Gr
-:-:-:l G G
? Core
-1
1-
ssc
-2
WK: 7 6-
ssc
-
"Gryphaea Reptile bed
KELLAWAYS FORMATION
missing
CAYTON CLAY FORMATION
5-
4-
-4
U
17 16 -
2-
-3
LL
0
DFS
!g:
0
la -
4 0
FRS
LL
Gr S 0
28%
15
2
311
cc
34%
7
U - AISBed 0 -- Comploni Bed
3i
44
3-
ssc
2-
ssc
Pm ABBOTSBURY CORNBRASH FORMATION
P E TF EMR B CM OO LX EA RM FY OOB UREGD RH,
Page (unpublished). Duff (1975) published a summary of the distribution of biofacies, and up to including the Acutistriatum Bed, that is reproduced with minor alterations by Martill & Hudson (1991). These sections continue to be the basis for collecting for sedimentological, palaeoecological and geochemical studies,althoughBletchley andCalvert pits have recently been closed. A new section in Wiltshire is described by Hollingworth & Wignall (1992), but elsewhere local detail depends mainly on boreholeinformation (e.g. Penn et al. 1986). Correlation away from thePeterborougharea is to alarge degree ammonite-based; individual lithologically-defined beds, with the exception of the Comptoni and Acutistriatum Beds, have not, as yet, generally been recognized. Correlatives of the ‘Gryphaea and Reptile Beds’, belonging to the Enodatum, Medea, and Jasonsubzones, are muchless stratigraphically condensedsouth-westfrom Peterborough. Gryphaeadilobotes is still a common fossil, andreptiles dooccur, butneither the lithology northe abundance of vertebrate remains characteristic of Bed 10 at Peterborough recurs (but see below for recent discoveries at Calvert). The Peterborough Member thins north and east from the Peterborough-south Lincolnshire area. The Jason Zone is overlapped northwards on to the Kellaways Formation by the Coronatum Zone and that by the Athleta Zone, which rests on the Kellaways Formation in Humberside (Penn et al. 1986). Here, the Enodatum Subzone occurs in asandyfacies,forming part of the Kellaways Formation, as at SouthCave(Callomon 1968; Page 1989). Somenotes on sections that have been importantfor recent collecting are given below.
Stewartby. The currently active pit is Quest Pit [TL 030 4201, for which the sections of Callomon (1968) and Duff (1978) areadequate, althoughthey differ in detail. Until recently the nearby Rookery Pit was in work.Afaulted portion of the latter [TL 017 414 to TL 021 412, approx.] exposes the Stewartby and basal Weymouth Members, unfortunately in abadly-weathered state. Details of these sections are recorded by K . N. Page in unpublished reports deposited with the Bedford Museum. Bletchley. The pit at Bletchley [SP 862 3261 closed in 1990. The sections of Horton et al. (1974a, b ) and Duff (1978) have more detail than that of Callomon (1968) but do not extend in detail above the Acutistriatum Bed. The section was extended by the authors, with K. L. Duff and K. N. Page, in 1990 and this upper part is detailed in Table 2. Calvert. This group of pits [SP 7022321, which for many years provided the best collecting in the southMidlands,
121
closed in 1991. The sections by Callomon (1968) and Duff (1978) are similar. Between 1986 and 1991, excavations extended beneath the basal shell bed (Bed 2cof Callomon and of Duff), exposing a further 2.2m of shale with shellbeds, with the prominent layer of concretions referred to by Callomon near the top. Recently (1989-92) N. Hollingworth has collected a number of reptiles and fish skeletons from the Jason Zone atCalvert; see Martill & Hollingworth (1992). Table 2. The Bletchley section Bed
Oxford Clay Formation, Stewartby Member 1 Mudstone, pale; exposed on shallow-angled face and
Peterborough Member 24b Shale, dark, pyrite-rich; weathered ‘rusty’ with gypsum 24a Shaly clay, dark olive-green, in part shelly. Typical Peterborough Member facies. Abundant Isocyprina,
Meleagrinella, Mesosaccella, Palaeonucula, Procerithium; no ammonites seen 23b Clay, greenish-grey, plastic; fewer fossils than bed above. Similar to bed below bench. Gap in section less than 0.5 m Position of quarry ~23a Clay, bluish-grey, plastic; sparsely fossiliferous. Cuspidaria?, Procerithium, aptychus? coprolites 22 Clay, shaly, greenish; deposit-feeder shale biofacies: g with Meleagrinella, Procerithium f Meleagrinella shell plaster e with Byssentolium d Layer of pyritic concretions over ammonites c with Binatisphincres b Ammonite plaster with large Kosmoceras phaeinum (M) Binatisphinctes, Thracia, Parainoceramus, Dicroloma, Cylindroreurhis, Lepidotes scales a with abundant Mesosaccella, Procerithium,
21
20 19
Fig. 5. Graphic log of the core from a borehole near Whittlesey [TL 251 9601, by the authors and K. L. Duff. Zero on the scale is at the base of the Oxford Clay and is approximately 27 m below ground surface at 1 m OD. Above the logged portion the coreis in weathered Oxford Clay overlain by Pleistocene-Holocene deposits. Bed numbers are assigned by comparison with the nearby type section at King’s Dyke pit (Fig. 3). Biofacies and shell-bed type as in Fig. 3; SSC: silts and silty clays (Duff 1975).
20.00
weathered at surface. Colour variations on face indicate decimetre to metre scale bedding. Bedsof concretionary limestone, onec . 15 m above base. Pyritic ammonite nuclei weather out, alsoGryphaea liruola, Genicularia, Oxytoma, belemnites etc. about 20.0 m
18
Palaeonucula, Parainoceramus, Corbulomima, Bositra, Meleagrinella, Dicroloma, Spinikosmoceras, Mecochirus, coprolites, fish scales. Some bedding surfaces nearly barren, others with shell-hash and secondary calcite Shelly shale or shell-bed, Meleagrinella-rich. Shell-hash with secondary calcite. Procerithium, Mesosaccella, Parainoceramus, Corbulomima, Genicularia Shaly clay, olive green, not fissile, hard. Typical fauna; also Hecticoceras lonsdalei Shell-bed or shelly clay; Discomiltha, Pinna, Belemnotheutis, Genicularia up to 3.5 cm,typical fauna Acutistriatum Bed. Shale, dark, fissile, sparse benthos. 2 cm wide burrows bring pale clay into dark shale for 1-2 cm at top. Goniocheirus. (Bed 17 of Callomon, Bed 18 of Duff)
0.03 0.40
0.10
1S O 2.00
0.40
0.70
-
0.40
0.20
0.20 0.05
0.30
Section continues below; see Callomon (1968) and Duff (1978)
122
H U D JS. ODNM . A R T IM L L.
& D.
Cleveland Farm Pit, AshtonKeynes. This large gravel pit [SP 070 9431 also affords fresh temporarysectionsduring excavations, and some were specially made during a visit by the Oxford Clay Working Group in 1990. The section recorded on that occasion by R. G. Clements and the authors closely resembles that published by Hollingworth & Wignall (1992). Howeverammonites determined for us by K. N. Page,anddeposited in BedfordMuseum, show thatthe alternations between organic-rich fissile shale and calcareous mudstone that characterize theupper part of thePeterborough Member (‘Lower Oxford Clay’: Hollingworth & Wignall 1992, fig. 6) belong to the Phaeinum Subzone of the AthletaZone, not theCoronatumZone.ThusthePeterborough Member-Stewartby Member contact occurs within the Phaeinum Subzone, as at Peterborough. This re-dating means thatthe fissile shales, which contain upto 16.6% TOC, even morethanBed 10 at Peterborough, belong to the same subzone as the famous ‘soft-bodied’ fauna formerly obtainedfromChristianMalford, only 20km away (e.g., Donovan & Crane 1992).
Palaeogeogaphy The general situation of the Oxford Clay outcrop within the NW European epeiric sea,and between the Borealand Tethyan ammonite fauna1 provinces, is shown in Fig. 6, and more a detailed British palaeogeography in Fig. 7. Unfortunately, little is known about the shorelines of the Oxford Clay sea, because the relevant areasareeither concealed or removed by later erosion. There is little hint of
Fig. 6. Generalized palaeogeographic map of the NW European epeiric sea in Callovian times, showing palaeolatitudes. Present-day coastlines of the British Isles and Greenland shown for guidance. The heavy line is the Alpine Front, approximately the northern margin of the Tethys ocean. Note the position of central England (cross) in a shallow sea with several islands, midway between the Tethys and the BorealSea, which had distinct ammonite faunas. Simplified and modified from Smith & Briden (1977) and Callomon (1985).
Fig. 7. Palaeogeography of southern and central England showing approximate distribution of landmasses. The Oxford Clay sea was largely surrounded by islands or larger landmasses. Modified from Bradshaw & Cripps (1992).
passage of the Oxford Clay into a marginal facies despite the abundance of driftwood and the rare presence of partially-associated dinosaurbones within it; only tothe in HumbersideandYorkshire, do north of ourarea, dominantly sandy facies appear, and these are marine. The hiatuses found in the same area (see above) could also have resulted from submarineerosion.However an indication that land was not far from Peterboroughcomesfroma clastic dyke intruded into the lower part of the Peterborough Member (Jason Zone) at Dogsthorpe (Martill & Hudson 1989). The sand in the dyke was cemented, prior to compaction, by calcite precipitated from meteoric-derived water; the immediatesource was postulated to be the Kellaways Sand beneath the Peterborough Member. Perhaps Kellaways Sand was exposed on land in the Lincolnshire-Humberside area, where the Jason Zone is absent (see above). To the south and east of our areais the London-Brabant Massif, a long-lived structural high. However there is no positive evidence for its emergence during the midCallovian, and the successions north of it at Peterborough and south of it in the Warlingham borehole,Surrey, are remarkably similar (Callomon & Cope 1971, fig. 2, table 5). Old borehole records in Sussex and Kent (e.g. Owen 1971) have also proved Oxford Clay,and do not suggest a significant facies change towards the postulated London Brabant landmass. Middle Callovian clays are present in the Bristol Channel (Warrington & Owens 1977). In the North Sea Callovian clays are reported in the Anglo-Dutch Basin, but in the Central Graben the Callovian is represented by sands (Brown 1990). The major sources of sediment to the depositional basin were probably far distant, and the general homogeneity of mineralogy of the Peterborough Member (Norry et al. this volume) supports this. Nevertheless, the Peterborough Member is not uniformly argillaceous: Macquaker(this
P E T E R B O R O U G H M E M B E R , O X F OFRMD C L A Y
volume) reports sand-grade quartz in somebeds from Stewartby. The low net accumulation rate of the sediments suggests that considerable winnowing, followed by sediment by-pass, removed much fine sediment out of ourarea, presumably intodeeper parts of the basin (Hudson & Martill 1991; Macquaker this volume). The same considerations,together with aspects of the facies discussed by Hudson & Martill (1991), strongly suggest that the sea was no morethana few tens of metres deep. The inferred palaeogeography makes river-derived a source forthe nutrients needed to sustain the productivity of the Oxford Clay sea much more likely thanocean-margin upwelling. The shallowness of the basin, and the possibility of strong seasonal variations in stratification, favours the possibility of nutrient recycling, which may also be affected by the nature of the bottom and pore waters (Ingall et al. 1993). Physical and oceanographicaspects of the palaeoenvironment are further discussed in thisvolume by Anderson et al. and palaeoecological matters by Martill et al.
Burial depth The immaturity of the Oxford Clay in the East Midlands, and the excellent preservation of aragonite, suggest shallow burial. Estimates made by extending the younger Mesozoic cover that occurs a few tens of kilometres to the east, and geotechnical data, both suggest a maximum burial depth of approximately 500 metres at Stewartby, in the late Cretaceous(Hudson 1978). ThePeterborougharea may have seen even less burial (Emery et al. 1988). Bulk organic geochemical datareported by Kenig et al. (thisvolume) show that the Oxford Clay organic matter is immature in terms of petroleum generation potential. Unpublished molecular analyses of Oxford Clay from several outcrops in south-centralEngland(Kenig, Popp & Summons, pers. comm.) show that these sediments are too immature to plot on the burial curves used to estimate source-rock maturity (Mackenzie et al. 1980), and may neverhaveexceeded 40°C. These data are of interest in view of recent publications (Green 1989; Bray et al. 1992) suggesting regional burial of theEast Midlands Shelf of 1-1.5km during the Campanian to early Palaeocene (after deposition of the currently-existing Chalk in Lincolnshire), followed by erosion starting about60Ma ago. Thoseinterpretations were based on apatite fission-track and vitrinite reflectance data from the Lincolnshire-Nottinghamshire-Leicestershire area, approximately 60-100km north and east of Peterborough, and contrast with earlier estimates of only a few hundredmetres burial in central Lincolnshire (references cited by Bray et al. 1992; Emery et al. 1988; see also discussion by Holliday 1993). Ourdata show that this proposed rapid burial cannot have affected the Oxford Clay now cropping out between Peterborough and Ashton Keynes (Fig. 1). For helpinthe field and discussions we are indebted to, among others, R. Clements, K . Page, B. Popp, T. Anderson, J . Hayes, K. Duff, A. Dawn and N. Hollingworth. For permission to workin theirpitswethanktheLondonBrickCompanyplcandEnglish China Clays. W e a r e especially grateful to T. Trinder (ECC) for his helpinprovidingdiggingmachinery a t ClevelandFarmPit.We thank B. Cox for a constructive review. This work was funded in part by NERC grant GR9/ 177.
123
References ANDERSON, T. F., POPP,B. N., WILLIAMS, A. C., Ho, L-Z. & HUDSON, J. D. 1994. The stableisotopicrecords of fossilsfrom the Peterborough Member, Oxford clay Formation (Middle Jurassic): palaeoenvironmental implications. Journal of the Geological Sociery, London, 151, 125-138. ANDREWS, C.W. 1910-13. A descriptive catalogue of the marine reptiles of the Oxford Clay. Vol. I , 205 pp.. 10 pls. (IYIO), Vol. 2, 206 pp., 13pls. (1913). London, British Muscum (Natural History). BRADSHAW, M. J. & CRIPPS,D. W. 1992. Mid Callovian. I n : COPE,J. C. W., INGHAM, J.K. & RAWSON, P.F. (eds) Atlas of Palaeogeographyand Lithofacies. Geological Society, London, Memoirs, U,120. BRAY,R. J., GREEN,P.F. & DUDDY,I . R. 1992. Thermalreconstruction using apatite fission track analysis and vitrinite reflectance: a casc study from the UK East Midlands and Southern North Sea. In: HARDMAN, R. F.P. (ed.) ExplorationBritain,Geologicalinsights for thenext decade. Geological Society of London, Special Publications, 67, 3-25. BRINKMANN, R.1929. Statistisch-biostratigraphischc Untersuchungcn an Mittcl-JurassischenAmmoniteniiberArtbegriffundStamniescntwicklung. AbhandlungenderGesellschaftderWissemchaften LU Gottingen, Mathematisch-Physikalische Klasse, Ncue Folge, Bd 13, Teil 4, 1-124 BROWN,S. 1990. Jurassic. In: GLENNIE, K. W. (ed.) Introduction to the Geology of the North Sea, 3rd edition. Blackwell, 219-254. the OxfordClay. In: CALLOMON, J. H. 1968. The KellawaysBedsand SYLVESTER BRADLEY, P.C. & FORD,T.D. (eds) The Geology of the East Midlands. Leiccstcr University Prcss, 264-290. - 1984. The mcasurcmcnt of geologicaltime. Proceedings oftheRoyal Institution of Great Britain. 56, 65-99. - 1985. The cvolution of the Jurassic ammonite family Cardioccratidae. Special Papers in Palaeontology, 33, 49-90. -& COPE,J. C. W.1971. Thc stratigraphy and ammonitesuccession of thc Oxford and Kimmeridge Clays in the Warlingham Borehole. Bulletin of the Geological Survey of Great Britain, 36, 147- 176 COX,B. M,, HUDSON,J. D. & MARI'ILL, D. M.1993.Lithostratigraphic nomenclature of the Oxford Clay (Jurassic). Proceedings of the Geologists' Association, 103 [for 19921, 343-345. DONOVAN, D. T. & CRANE, M. D. 1YY2. The typematcrial of the Jurassic cephalopod Belemnotheutis. Palaeontology. 35, 273-296. DUFF,K.L.1975.Palaeoecology of abituminousshale-thcLowerOxford Clay of central England. Palaeontology, 18, 443-482. - 1978. Biualuia from the English Lower Oxford Clay (Middle Jurassic). Monograph of the Palaeontographical Society, London.
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Received 10 June 1993; revised typescript acccpted 6 August 1993.
