The lithostratigraphy of the upper Oxfordian and lower Kimmeridgian Beds of South Dorset, England

The lithostratigraphy of the upper Oxfordian and lower Kimmeridgian Beds of South Dorset, England M. E. BROOKFIELD

BROOKFIELD, M. E., 1978. The lithostratigraphy of the upper Oxfordian and lower Kimmeridgian Beds of South Dorset, England. Proc. Geol. Ass., 89 (1), 1-32. Studies of detailed sections have validated Blake's (1875) concept of Passage Beds between the Corallian Beds (Upper Oxfordian) and Kimmeridge Clay (Kimmeridgian-Tithonian) in South Dorset. Blake's (1875) term 'Passage Beds' is applied to a coherent group of dominantly arenaceous sediments straddling the Corallian Beds-Kimmeridge Clay boundary and these beds are included in the Upper Calcareous Grit, whose upper boundary is revised to include the arenaceous facies of the lower part of the Kimmeridge Clay (in Dorset). The lower junction of the Kimmeridge Clay is raised to include only the clays of the Aulacostephanus mutablis to Zaraiskites albani Zones in the lithostratigraphic unit-Kimmeridge Clay. The faunas of the Passage Beds are akin to those of the Corallian Beds and bear only slight resemblance to those of the Kimmeridge Clay.

Department of Land Resource Science, University of Guelph, Guelph, Ontario, Canada.

CONTENTS page

INTRODUCTION .., HISTORY OF RESEARCH ZONAL SCHEME .., FOSSIL IDENTIFICATION STRATIGRAPHIC CLASSIFICATION DETAILS OF UNITS VERTICAL CHANGES IN FAUNAS ... LATERAL CHANGES IN THE CORALLIAN BEDS-LOWER KIMMERIDGE CLAY... 9. OXFORDIAN-KIMMERIDGIAN BOUNDARY ACKNOWLEDGMENTS REFERENCES 1. 2. 3. 4. 5. 6. 7. 8.

1 2 4 5 5 7 27 28 30 30 30

1. INTRODUCTION

The only continuous sections across the Corallian Beds-Kimmeridge Clay boundary in Britain occur on the Dorset coast around Weymouth, and in northern Skye, Scotland. In northern Skye, the sequence consists predominantly of shales and thin limestones, disrupted by faulting and igneous intrusions (Wright, 1973). In Dorset, a complete sequence from the Corallian Beds into the Kimmeridge Clay is exposed in the coastal cliffs east of Weymouth (Fig. 1). Here, the classic type sections of the Corallian Beds are magnificently exposed. Ziegler (1962) designated the section at Black Head (Fig. 2) as the type section of the Lower Kimmeridgian. Yet, although the 1

2

M. E . BROOKFIELD

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Fig. 1. Geological sketch maps and location of sections studied. Grid references of localities: A (SY 661765-663764) ; 0 (SY 671770); V (SY 722819); U (SY 723819) ; T (SY 725819) ; S (SY 727819); R (SY 728819) ; L (SY 733818); M (SY 733817); C (SY 743814); J (SY 746814); I (SY 749813); G-D (SY 756813-752813); P (SY 758814); Q (SY 760814) .

Corallian Beds have been studied in some detail, no detailed section through the lower Kimmeridge Clay has ever been published. The main purpose of this paper is to describe the lower Kimmeridge Clay sections of the Dorset coast, and revise the stratigraphy of the topmost Corallian Beds and lower Kimmeridge Clay in south Dorset. 2. HISTORY OF RESEARCH

The Dorset exposures of the Corallian Beds and Kimmeridge Clay were early made classic areas by Buckland & De La Beche (1835). Later, Blake (1875) and Blake & Hudleston (1877) made comprehensive studies of all the Corallian Beds and Kimmeridge Clay in England, with particular emphasis on the Dorset coast sections. Arkell (1929-37; 1935-48; 1936; 1947) revised and re-interpreted the Corallian Beds in a series of classic works; and they have recently been the subject of detailed sedimentological work, particularly of the carbonate units (Wilson, 1967; 1968; Talbot, 1973; 1974). No detailed sections of the lower Kimmeridge Clay have been measured since Blake (1875), who nevertheless described only part of the sequence. This neglect is probably due to the state of

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the exposures, which usually consist of slumped and disrupted masses of clay, including the famous mud glaciers west of Osmington Mills (Arkell, 1951). Even Arkell (1933, 385) misunderstood the sequence, since he confused Blake's (1875) Passage Beds with the upper Corallian Beds; yet Blake's (1875) Passage Beds are readily recognisable as part of the lower Kimmeridge Clay section (Fig. 3). The upper part of the sequence has never been fully described. 3. ZONAL SCHEME

A zonal scheme based on ammonites was constructed for the Oxfordian and Kimmeridgian by Salfeld (1913; 1914; 1915; 1917), Arkell (1933; 1935-48; 1945; 1947), Casey (1967), Cope (1967; 1974) and Ziegler (1962). The Upper Oxfordian zones have, however, required emendation in the light of recent work (Callomon, 1960; Wright, 1973). The Decipia decipiens Zone of Arkell (1935-48) was established in eastern England, but is not represented by ammonites in the Dorset sections. Wright (1973) has shown that Decipia sensu stricto was most abundant during deposition of strata belonging to the Perisphinctes cautisnigrae Zone. In France, Decipia-like forms are distinct from the English Decipia decipiens and occur with Ringsteadia and Microbiplices of the Ringsteadia pseudocordata Zone (Wright, 1973). Suppression of the decipiens Zone fits the Dorset sequence, where strata bearing the cautisnigrae Zone fauna ('Trigonia' clavellata Red Beds) are separated from strata bearing the pseudocordata Zone fauna (Sandsfoot Grit), by the Sandsfoot Clay without ammonites. BLAKE'S

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Unfortunately, ammonites are common only at certain horizons in the Dorset sequence. Most of the zonal boundaries cannot be placed exactly. The only ones precisely located are: the Ringsteadia pseudocordata-Pictonia baylei zonal boundary, which occurs within a highly condensed sequence; and the Rasenia cymodoce-Aulacostephanus mutabilis zonal boundary which is vaguer than it appears, due to the uncertainty in the definition of the genera Rasenia and Aulacostephanus and lack of agreement on criteria to distinguish them (Arkell & Callomon, 1963; Ziegler, 1962). 4. FOSSIL IDENTIFICATION

In the faunal lists, the ammonites are identified only to generic level. This is partly due to a lack of recent work on the relevant genera. Also, distinction of ammonite species on subtle morphological grounds has been taken to such lengths, that generic designation (which probably corresponds to distinct biological entities) is usually sufficient for zonal subdivision (d. Hallam, 1971). My large collections of ammonites, especially those from the Abbotsbury Ironstone, would certainly repay more detailed study than I was able to give them. All fossil collections are deposited in the Reading University Geology Department Museum (Museum numbers 14614-14797). Apart from the ammonites, the taxonomy of the other macrofossil groups is very uneven. The Oxfordian bivalves were revised by Arkell (1929-37). Hudleston (1880-81) studied the gastropods. No monograph of British Kimmeridgian forms is available. Identifications have hence been made mostly from the French monographs of De Loriol (1872; 1874-5; 1894) and Contejean (1895), with the names updated where possible. Occasional descriptions of species have been made in the predominantly stratigraphic works of Blake (1875) and Blake & Hudleston (1877) among others. Some of the names given in the faunal lists have been deliberately left in their original form (quotation marks), since their taxonomic status and validity is uncertain. Of the microfossils, only the foraminifera were identified, and then only to generic level, since the foraminifera genera consist of highly variable species which need statistical tests to distinguish them (Gordon, 1966). 5. STRATIGRAPHIC CLASSIFICATION

The informal stratigraphic terminology of the Dorset sections has been in use for many years, is entrenched in the literature and familiar to most workers (d. Arkell, 1936; Talbot, 1973). It seems unnecessary and pointless to propose new formal names for units. However, the lower Kimmeridge Clay sequence needs names for some of the new units described here. Despite a strong urge to formalise the nomenclature, I feel it is more consistent to continue with informal names, especially as most of these will be of only local interest. Arkell (1933; 1936) divided up the upper Corallian Beds and lower Kimmeridge Clay as shown in Table I. Talbot (1973) noted that it was impossible to separate the Glos Oolite 'Series' and the Upper Calcareous Grit. He grouped them together as the Upper Calcareous Grit, suppressing the Glos Oolite 'Series'. This modification I accept here. I also propose an upward revision of the Upper Calcareous Grit boundary. The natural sedimentary breaks within the beds studied here do not correspond with the division between the Upper Calcareous Grit and Kimmeridge Clay show in the Arkell column of Table I. The major breaks occur at the base of the Ringstead Waxy Clay and at the top of the Grey Clays of

TABLE I. Classification of the CoraUian Beds and Lower Kimmeridge Clay 0\

ARKELL 1936

THIS PAPER

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the uppercymodoce Zone and these breaks were recognised by Blake (1875) who called the lower half of the beds between these two levels the Kimmeridge Passage Beds. The Passage Beds contain similar fine-grained arenaceous facies, have a uniform clay mineralogy throughout distinct from the beds above and below, and contain a macrofauna transitional between the Oxfordian and Kimmeridgian. The junction between the Upper Calcareous Grit and Kimmeridge Clay was formerly placed at the top of the Ringstead Coral Bed (Table I), between the pseudocordata and baylei Zones. However, this makes the junction lie within the Passage Beds sequence, and it has little lithological or faunal significance. Three alternatives are possible. The first is to entirely dispense with the term Upper Calcareous Grit. The second is to revise its boundary downwards to the top of the Sandsfoot Grit, raise the lower boundary of the Kimmeridge Clay, and insert a new name-the Passage Beds between. The third alternative, adopted here, is to place the top of the Upper Calcareous Grit at the top of Blake's (1875) Passage Beds. This third alternative has the advantage that it includes in one group all the arenaceous sediments between the Osmington Oolite Group and the black clays and shales of the Kimmeridge Clay on the Dorset coast. The revised classification is shown on Table I and summarised below. For the formally minded, the suggested lithostratigraphic rank of the units is noted in parentheses.

Upper Calcareous Grit (Group) Sandy limestones and shelly limestones overlain by sandy clays and sandstones with interbedded ironstones, siltstones and minor carbonates. Subdivisions from base to top are: 1. 'Trigonia' clavellata Beds (Formation). Sandy limestones and shelly limestones with interbedded calcareous clays. Type section at Black Head. Age: cautisnigrae Zone. 2. Sandsfoot Beds (Formation): subdivided into the Sandsfoot Clay (below) and the Sandsfoot Grit (above). Type section at Sandsfoot. Age: pseudocordata Zone; possibly cautisnigrae Zone at base. 3. Passage Beds (Formation). Type section at Black Head. Age: upper pseudocordata Zone to upper cymodoce Zone. Further subdivision is discussed below. Each of these three subdivisions of the Upper Calcareous Grit show not only differing facies, but also have different clay mineral assemblages (Table I). Kimmeridge Clay (Group) Grey clays with occasional thin argillaceous limestones and concretions, passing up into sandy clays. Type sections at Black Head (lower) and Kimmeridge Bay (upper). Age: mutabilis to albani Zones (Upper Kimmeridgian to Middle Tithonian). Further subdivision of the Kimmeridge Clay is discussed in Arkell (1947) and Cope (1967; 1974). 6. DETAILS OF UNITS 1. 'Trigonia' c1avellata Beds Arkell (1936) divided these beds into three main units (Table I) which are only briefly noted here: A-The Sandy Block; B-The Chief Shell Beds (including the Clay Band); C-The Red Beds.

A. The Sandy Block consists of sandy oomicrites, oosparites and intramicrites. These show well-sorted ooliths, intraclasts and comminuted shell fragments in a dominantly micrite or

8

M. E. BROOKFIELD

microspar matrix. The Sandy Block becomes sandier to the west at Abbotsbury. In North Dorset, it apparently passes into calcareous sandstone (Gutman, 1970). B. The ChiefShell Beds consist of coarser intramicrites, oomicrites and intrasparites. The unit's name is derived from the layers of biosparite and biomicrite containing abundant bivalve shells, dominantly of Myophorella clavellata. The Chief Shell Beds show only slight variation throughout South Dorset (Arkell, 1936). C. The Red Beds consist of intraclast-bearing clays, intramicrites, intrasparites, intrabiomicrites and intrabiosparites; frequently partly cemented by siderite and ankerite (cf. Talbot, 1971; Wilson, 1967; Brookfield, 1973c). The beds are arranged in cyclical repetition and are highly condensed compared to equivalent strata in France (Cariou, Enay & Tintant, 1971). Abundant ammonites in some beds indicate thecautisnigrae Zone; though the total fauna is less diversified in terms of species than the underlying Chief Shell Beds (Arkell, 1936). The Red Beds apparently die out, or change facies westwards, since they are not recognisable at Abbotsbury (Arkell, 1936). In north Dorset they become sandier (Gutman, 1970). On the Dorset Coast, the Red Beds pass gradually upwards into the lower beds of the Sandsfoot Clay, with a gradual increase in clastic material and reduction in carbonate (Table I Fig. 2). The 'Trigpnia' clavellata Beds are typical tidal flat, lagoonal and shallow shelf carbonates (Wilson, 1968; Talbot, 1973). 2. Sandsfoot Beds A. Sandsfoot Clay. At the type section, at Sandsfoot, the clay is now hardly exposed at all, but where visible consists of sandy, silty clay, with belemnites. At Black Head, however, the clay is well-exposed and consists of sandy, silty, bioturbated clays. The lower boundary is transitional with the 'Trigonia' clavellata beds. In the basal metre a regular alternation of sandy carbonatecemented clays and sandy clays contain Deltoideum delta. The upper boundary is transitional into the Sandsfoot Grit, and the units at Black Head are really only distinguishable by their sand content. The fauna of the Sandsfoot Clay is an impoverished one, consisting mainly of belemnites and the oyster Deltoideum delta and Nanogyra nana (Table II). No ammonite has been recorded from the Sandsfoot Clay since Blake & Hudleston (1877) who noted 'Ammonites plicatilis'. The absence of ammonites and other stenohaline forms suggests a low-salinity environment. Coarse fraction studies indicate that the Sandsfoot Clay can be compared with the marginal low-salinity bay and open lagoonal environments of the Gulf of Mexico (Shepard, Phleger & Van Andel, 1960; Brookfield, 1973c). This is consistent with a change from the offshore shelf environment of the 'Trigonia' clavellata Red Beds (Talbot, 1973) into the barrier bar sands of the Sandsfoot Grit. The Sandsfoot Clay varies in thickness from 18 m at Sandsfoot to 5 m at Black Head. It lies between strata bearing cautisnigrae Zone and pseudocordata Zone ammonites and since it forms a gradation between these units, the zone boundary possibly lies somewhere within the unit. B. Sandsfoot Grit. The Sandsfoot Grit consists of a variable series of heavily bioturbated ferruginous clayey sandstones and sandy clays, and is exposed at Sandsfoot, Black Head and Osmington Mills (Fig. 4). Small exposures of the upper beds occur at Ringstead. The East Fleet section measured by Arkell (1936) is now largely obscured. Both Blake & Hudleston (1877) and Arkell (1936) commented on the lateral and vertical variability of the beds.

9

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At the Sandsfoot type section, the Sandsfoot Grit consists of a complex series of 6·5 m of ferruginous, fine-grained sandstones, completely oxidised at outcrop and in units of about 1 m thickness. Coastal excavations in 1968 showed that the fresh sediments are cemented by siderite and chamosite, and occasionally contain chamosite and limonite ooliths. Towards Black Head, the Sandsfoot Grit increases in thickness to 7·5 m, but decreases again at Osmington Mills to 6·0 m. At Black Head, the middle unit of the Sandsfoot Grit becomes more argillaceous and farther east passes into sandy, silty clays. The topmost unit in all exposures is a clayey fine-grained bioturbated sandstone sharply overlain by the Ringstead Waxy Clay. At Black Head, the basal unit contains rolled, bored and phosphatised pellets, but the junction with the Sandsfoot Clay is otherwise gradational. The Sandsfoot Grit at Sandsfoot can be divided up into three units, also recognisable at Black Head and Osmington Mills (Fig. 4). 1. The basal unit consists of medium-bedded, fine-grained clayey calcareous sandstone and sand. Quartz sand forms an average of 50 per cent of the sediment in a clayey ankerite spar matrix. Shell fragments form less than 1 per cent. Lenses of green clay are distributed throughout. Coarser lenses, about 1 em thick and up to 30 em long are composed of phosphate pellets, limonite ooliths, shell fragments and granules of metamorphic schist and quartzite (60 per cent) in a matrix of medium-grained sand and siderite microspar. These occur only at Sandsfoot and

10

M. E. BROOKFIELD

East Fleet. The unit is heavily bioturbated and contains recognisable small Thalassino ides. The macrofauna (Table II) is dominated by Chlam ys midas and belemnites, but rare Ringsteadia is recorded (Arkell, 1936 ). At Black Head, the basal unit is represented by black clayey sand (horizons U 4-7, Fig. 2), which contains oyster fragments, abundant fish fragments and phosphatic pellets suggesting a low depo sitional rate.

TABLE II. Faunas of the Sandsfoot Beds. a - abundant, c - common, Unit

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A. Sand sfoot Clay

Locality and bed numbers BIVAL VIA Mytil us un gulatus (Young and Bird) Pinna sands foo tensis Arkell Isognomon fiambarti Dollfuss Oxytom a expa nsum (Phillips) Entolium corneol um (Young and Bird) Cam pton ectes sandsfootensis Arke ll Chlamys m idas (Damon) Placun op sis radiata (Phillips) Ctenostreon probiscideum Sower by Plagiostoma rigidum (Sowerby) Trigonia sp. Mactromya aceste (d' O rbigny) Myoconcha texta (Buvignier) Astarte sub depressa (Blake and Hud leston) ?Q uenstedtia elongata Hudl eston ' Corbicella' cf. ebo racensis Ark ell A nisocardia globosa (Ro emer) Zlsocyprina tancredifo rmis (Black and Hudl eston) Pholadomya protei (Brongniart) Gon iomy a literata (J . Sowerby) Pleuromya uniform is (J. Sowerby ) Pleuromya alduini (Brongniart) Thra cia depressa (1, de C. Sowerby) Delto ideum delta (Wm. Smith) Nanogyra nana (J. Sowerby) 'Ostrea' dubiensis Contejean GASTROPODA Cerithium spp. CEPHALOPODA Rin gsteadia spp. Microbiplices sp. Paraceno ceras calloviense (Oppel) Belemnites spp. ANNELID A Serpula spp. A RTH RO PO DA Paraglyphea rostrata (Phillips) FORAMINIFERA Cribro stomoide s spp. ' Com usp ira' sp. Ammobaculites spp. Trocham m ina spp. Lenticulina

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UPPER OXFORDIAN AND LOWER KIMMERIDGIAN OF DORSET

11

The depositional environment of the basal unit closely resembles the open lagoon environment of the Gulf of Mexico (Shepard & others, 1960). 2. The middle unit, at Sandsfoot, consists of fine-grained highly ferruginous clayey sand, with some horizons cemented by ankerite spar. Most of the sediments are heavily oxidised and decalcified, and rare corroded limonite ooliths are present. Occasional granule lenses occur, as in the basal unit. Rare carbonate nodules contain chamosite ooliths in a sideritic matrix, and this may have formed a large part of the unit prior to oxidation. The unit is heavily bioturbated, with small Thalassinoides the only recognisable form. The macrofauna (Table II) is impoverished, but slightly more diverse than that of the basal unit. Pinna is occasionally recorded in life position, indicating rapid sedimentation between periods of stability of the sand surface. At Black Head, the middle unit passes into grey-green sandy clays (horizons U8a-e) and is practically identical in fauna, lithology and microfaunal content to the upper part of the Sandsfoot Clay. However, two units (U8b and U8d) contain a more diverse fauna (Table II). The depositional environment is similar to that of the basal unit, but is possibly a slightly deeper, more offshore environment. 3. The upper unit at Sandsfoot consists in fresh specimens, of sideritic chamositic clayey fine-grained quartz sandstone and sandy clay and is heavily bioturbated, though with only small Thalassinoides recognisable. The macrofauna (Table II) closely resembles that of the basal unit, but with the addition of Goniomya and Astarte. At Black Head, limonite ooliths are partially replaced by carbonate, and the fauna appears to be less diverse; though this could be due to the difficulty of collecting from the relatively poorly exposed beds. The most interesting part of the upper unit is, however, the top bed(Os, T 14 , Ms, C3 , Fig. 4). In all exposures, this consists of medium-grained sand (70 per cent) in a ferroan calciteankerite-siderite spar matrix. Carbonate-replaced limonite ooliths form up to 20 per cent of the sediment. In contrast to the underlying beds the top bed varies little in lithology from Sandsfoot to Ringstead. Large Thalassinoides are present, and the fauna is dominated by Chlamys midas. Its upper surface shows a succession of fossil assemblages marking a transition from shallow to deeper water. At Sandsfoot, a fauna of deep-burrowing suspension feeders, shallow infaunal suspension feeders, and free-living suspension feeders are disturbed by large Thalassinoides burrows. These indicate a higher energy environment than other units of the Sandsfoot Grit (Ager & Wallace, 1970). The Thalassinoides burrows were then partially cemented and exhumed, and colonised by the trace fossils Chondrites, Rhizocorallium, and the semi-infaunal bivalve Pinna, shell fragments, predominantly of Chalmys midas, were washed into depressions between the exhumed Thalassinoides burrows. The bed is sharply overlain by the Ringstead Waxy Clay. At Black Head and Osmington, the top bed is as at Sandsfoot, but is surmounted by a thin sandy clay layer. At Ringstead, the Thalassinoides are absent, but Rhizocorallium and Chlamys midas are common. This top bed appears to represent a relict sediment, as it records a succession of faunas of differing environmental character, indicating a change from relatively turbulent to quieter water conditions through time (Swift, Stanley & Curray, 1971). This culminated in the deposition of the Ringstead Waxy Clay. The overall environment of the Sandsfoot Grit is one of shallow water, dominantly above wave base. In many of its parameters, the Sandsfoot Grit closely resembles the subtidal environments of barrier bars (Davies, Ethridge & Berg, 1971; Davies, 1969) in having heavy bioturbation, slow intermittent deposition, variability of environmental energy, fine-sand-clay mixtures, well-sorted quartz sand fractions, coarser particles concentrated in lag accumulations. On this model the topmost bed would represent the drowning of the barrier bar by a relative

12

M. E. BROOKFIELD

rise in sea-level. The overlapping of relict sediments, often barrier bar or transgressive sands by shelf clays has been recorded from both the Mississippi and Niger delta areas (Allen, 1965). The Sandsfoot Beds are confined to the Dorset coast. They can not be definitely identified in the Poxwell and Broad Bench boreholes (Fig. 12); but can be correlated with the Argiles de VilIerville and Glos Sands in Normandy (Arkell, 1930; Bigot, 1930) and the Argiles noires a Deltoideum delta and Gres de Brunembert of the Boulonnais (Pruvost & Pringle, 1924). 3. The Passage Beds The unity of the Passage Beds is shown by their uniform clay mineral assemblages, the repetition of similar facies through the sequence and their similar nearshore, shallow shelf environments. Their facies and fauna is more akin to the Sandsfoot Group and 'Trigonia' clavellata Beds below, than to the Kimmeridge Clay above. In south Dorset, different facies are developed at Abbotsbury and on the coast east of Weymouth. WEYMOUTH AREA

A. Ringstead Waxy Clay

This unit was named from Ringstead, where it is now very poorly exposed. Better sections occur at Osmington and Black Head (Fig. 2). The Ringstead Waxy Clay consists of thinly bedded blue silty clay, interbedded towards its top with thinly bedded, orange-weathering, ferruginous calcareous silty clays. A line of carbonate concretions is commonly developed at the top. The Ringstead Waxy Clay sharply overlies the Sandsfoot Grit at Sandsfoot, but at Black Head, Osmington, and Ringstead contains sandy lenses and layers in the basal 30 em. It is sharply overlain, possibly disconformably by the Ringstead Coral Bed and associated strata. Some poorly preserved Microbiplices indicate the pseudocordata Zone. The Ringstead Waxy Clay decreases in thickness eastwards from 5·0 m at Sandsfoot to 3'5 mat Ringstead. This decrease may be due either to post-depositional erosion or to greater original thickness to the west. The clay consists dominantly of illite and kaolinite, with less than one per cent of quartz sand. It contains an impoverished macrofauna (Table III) of small individuals, dominated by Pseudodiadema sp. Palaeonucula menkii and Corbula prora but an abundant microfauna of foraminifera and ostracods. Slight bioturbation is shown by pyritised small burrow systems. The small size of all the specimens (less than 2 em diameter) indicates that large individuals were incapable of living on or within the sediment, probably because of a tendency to sink into the soft thixotropic mud (cf. Sanders, 1960). The rarity of deposit-feeding infauna and presence of pyritised burrow systems indicates the prevalence of reducing conditions below the sediment surface. Most of the epifauna seems to have lived during periods when the mud surface was relatively firm and layers of articulated Pseudodiadema, with small oysters attached to their surfaces are common. The presence of Pseudodiadema and ammonites, together with the diverse foraminiferal fauna indicates normal marine salinities. The sand fraction and macrofaunas most closely resemble the deeper areas of open highsalinity bays of the Gulf of Mexico (Shepard & others, 1960). It seems unlikely that the Ringstead Waxy Clay represents a lagoonal environment (Talbot, 1973), especially as there is no sign in the boreholes to the east of any barrier bar environment. B. The Nodular Beds. (new name) This informal name is proposed for the Ringstead Coral Bed, its lateral equivalents, and the overlying 'Rhactorhynchia' inconstans Bed and 'Exogyra' nana Bed (Arkell, 1936). The con-

UPPER OXFORDIAN AND LOWER KIMMERIDGIAN OF DORSET

TABLE III

13

Fauna of the Ringstead Waxy Clay. X - indicates dominant forms

BIVALVIA

"Palaeonucula menkii Roemer Pinna sp. "Corbula prora Sauvage Goniomya ? literata (J. Sow.) GASTROPODA Dicroloma sp. BRACHIOPODA Lingula sp.

ANNELIDA

Serpula sp. CEPHALOPODA Amoeboceras sp. Microbiplices sp. ?Ringsteadia sp. ECHINODERMATA xPseudodiadema sp. FORAMINIFERA Ammobaculites spp. Trochammina sp. Lenticulina spp. Dentalina sp. Lagena sp. Nodosania sp. Vaginulina sp. Brotzenia sp.

densation of this unit is apparent when its thickness is compared with equivalent sediments in Normandy where, though broken by bone bed horizons, the sediments are seven metres thick (Rioult, 1961). The Nodular Beds form a sedimentary unit of extreme complexity between the more uniform beds above and below. Sections of the Nodular Beds are shown in Fig. 5. Correlation within the beds is mainly lithostratigraphic though the brachiopod Torquirhynchia inconstans is confined to the lower half of the 'Rhactorhynchia' inconstans Bed and corals are virtually confined to the Ringstead Coral Bed. Ammonites can be used to distinguish the Coral Bed and lateral equivalents from the units above, since the junction between the pseudocordata and baylei Zones lies at the top of the Coral Bed (Arkell, 1936). The macrofaunas of the Nodular Beds are diverse and comparable with some of the faunas in the carbonate units of the underlying Corallian Beds (Tables IV, V; cf. Arkell, 1929-37). The following units may be distinguishable (Fig. 5). 1. Grey, non-oolitic sandy calcareous clay. This forms a thin clay bed below the Ringstead Coral Bed. It sharply overlies the Ringstead Waxy Clay and passes gradually upwards into the Coral Bed and laterally into the Osmington Mills Ironstone of unit 3, though lithification of the Coral Bed makes the junction often appear sharp. The bed consists of grey sandy clay distributed as small lenses and patches throughout. Bulk samples show that the sand-sized fraction amounts to 25 per cent; of which 80 per cent is sub-angular to sub-rounded quartz sand; 20 per cent of variable proportions of calcite-oolitic sand and sand-silt aggregates cemented by ferroan calcite and ankerite (0,5-2,0 mm diameter) with calcite shell fragments and small phosphatic pellets (0'5 mm and less in diameter). The macrofauna consists of only a few species, but as these are of forms which secreted calcite shells and as aragonitic forms are absent this impoverishment may be a preservation feature, especially as no ferroan calcite replacements of the aragonitic shells occur in the sand fractions. Many of the shells are heavily bored and encrusted by Serpula, Placunopsis and Nanogyra, and the borers account for most of the shell breakage in this unit. 2. The Ringstead Coral Bed. Although giving its name to this horizon, the Ringstead Coral Bed

14

M. E. BROOKFIELD

l ocali t ies

u

A ,

.... -

.-. •

V

-.;0

---.I~v

IV

•

• oJ. -

3

•• . - lit

2

R

V

•Exogyra' nana Bed

,, "

v_ v

'1 .

,.v

,

. .' e-

/

7

-.

II I

~

I

~

S' • .J.. t

s

,. • •

! ..

IV

e-

-e

.:.~

7

L

.• • IV

.L" " •

.J..

Shells and shell fragments

".

Corals

•

Limonite oolith s and carbonate-reotacsd ooliths

·v

IV

. .J- -_ 7 ;-~ III

..

e-

Ringstead Waxy Clay

V

..

• •

.:.

G

J

3

e.J,.

.-i!'-

). -;-..:..

.L

Carbonate lithificalion

o

Carbonate concretions

II

3 . -;- I

I

...,..., Eroded. phosphate - pi astered surface

Sand Clay

Fig. 5. Lithological variation within the Nodular Beds. Localitie s on Fig. 1.

is present in only two areas, at Ringstead (localities G, J , I) and at Black Head (localities Sand R), and in both areas it passes very repidly laterally into the more characteristic lithology of this horizon-limonite oolitic clay (Fig. 5). The coral bed is 5-20 em thick and varies from a hard coral limestone to nodular masses of 00 biomicrite in clay. The composition is variable; 30-80 per cent micrite or spar, 10-20 per cent quartz sand and silt, 5-40 per cent shells and shell fragments, and 5-10 per cent of carbonate-replaced limonite ooliths. The quartz sand and silt and limonite ooliths are the least variable components, and appear to form the background sediment to the other components. At its base the coral bed grades up from unit 1, although lithification makes the junction appear sharp in many places. The top is an erosion surface, with a layer of phosphatised shell fragments plastered over its surface. In one place (loc. I, Ringstead), the coral bed is overlain by a five centimetre thick haematitic sideritic limonite oolite. The fauna of the coral bed is diverse (Table IV) and comparable to the fauna of the overlying 'Rhachto rhynchia' inconstans bed: the major difference is the presence of corals, and other species associated with coral s in other Oxfordian reefs (Arkell, 1929-37). Lateral variation of faunas is pronounced, and the coral bed represents a more or less in situ deposit, modified by biological and mechanical erosion. The coral bed passes laterally into limonite oolitic clay and limonite oolite. At Black Head, the transition takes place over three metres. Features of this transition are the absence of channelling of one unit by another and the abruptness of the faunal change (few corals or other characteristic fossils of the coral bed occur in the limonite oolite, which is developed within five metres laterally from the coral bed) . Occasionally, as at Ringstead, blocks of coral with oobiomicrite envelopes lie in the limonite oolite: bioturbation has partly obscured and lithification enlarged, the original size of the blocks (Fig. 6). 3. The Osmington Mills Ironstone (new name) . This takes its name from the hamlet of Osmington Mills where half a metre of limonite-oolitic clay and limonite oolite occupy the horizon of the Coral Bed and clays below (units 1 and 2). The bed consists of heavily bioturbated grey sandy clay, with lenses and patches of limonite oolitic clayey sand or sandy clay. Irregular

15

UPPER OXFORDIAN AND LOWER KIMMERIDGIAN OF DORSET TAB LE IV

Fauna of the Ringstead Coral Bed and Osmington Mills Ironstone. (Localities from Fig. 1. Units from Fig. 5. t - denotes transitional facies . 0 - occurs, c - common? - doubtful identification. NR - not recorded) Loc ality Un it

BI VA LVIA Palaeonucula menkii Roeme r Paral/elodon cf reticula/us (Blake) Grammatodon keys erlingi (d'O rbigny) Mytilus ungulatus (Young and Bird) Li thophaga inclusa (Phillips) Modiolus durnov ariae Ar ke ll Pinna sp. Pteroperna polyodon Buvignier Gervillia sp. Isognom on subplanus {Etalion) Isognom on flambarti Dollfuss Entolium corneolum (Young and Bird) Campto nectes auritus (Schlotheim) Camptonectes sandsfo otensis Ar kell Chlamys nattheim ensis (de Lo riol) Chlamys splendens Dollfuss Chlamys midas (D amon) Eope cten anglica Arkell Aceste alternicostata Buvignier Ctenostreon prob iscideum Sowerby Limatula elliptica (Whiteaves) Plagiostoma rigidum (Sowerby) Trigonia (Trigonia) reticulata (A gassiz) Myophorel/a (Vaugonia) clavel/ata (Parkinson) Mactromy a aceste (d'Orbigny) Myo concha texta Buvignie r Myo concha saem anni Dollfuss A starte subdepressa (Blake and Hudlesto n) Astarte ovata Wm. Smith Prorokia problematica (Buvignie r) Quenstedtia cf. laevigata (Phillips) cf Sowerbya triangularis (Phillips) Anisocardia globosa (Roe mer) ?Isocyprina constantini (Dollfuss) Pholadomya aequalis J . de C. Sowe rby Pholadomya protei (Brongniart) Goniomya sp, Pleuromya uniformis (J. Sowerby) Pleuromya alduini (Brongiart ) Deltoideum delta (Wm. Smith ) Nanogyra nana (J . Sower by) GASTROPODA Bathrotomaria reticulata (J . Sowerby) Dicroloma sp. Amberleya sp. Pseudomelan ia (Oonia) sp. Nerita cf pel/ati (de Lo riol) CEPHALOPODA Ringsteadia spp. Paracenoceras cal/oviense (O ppel) ANTHOZOA Thamnast eria concinna (Milne-Edwards and H aime) Thamnasteria arachnoides (Milne-Edwards & Ha ime)

A 3

T 3

R 2

S t

0 2

K 2

M 3

G 2

P t

0 0 0

0

0

?

0

0

0

c

0

0

0

c

0

c

0 0

0

0

0

0

0

?

0

0

c

0

0

0

c

0

0

0 0

0 0

0

c

0

0

0

0

0

0

?

0

0

c

0

c

0

c

c

c

c

0

c c

0

0

0

0

0 0

c

0 0

c

0

0

c

c

0

c

0

0

0

c

0

0 0 0

0

0

0

c 0 0 0

0 0

0 0

0

0

0 0

0

0

c c

0

c c

c c

0

c

c c

c c

0

c

0

0 0 0

c c

0 0

0

0

0

c 0

0

0

0

0

0

0

0

c

c

0

0

0

0

0

0

0

16

M. E. BROOKFIELD

TABLE IV cont.

Locality Unit

A 3

T

S

t

3

Thecosmilia annularis Milne-Edwards and Haime Proto seris waltoni Milne-Edw ard s and Haime

c

ANNELIDA Serpula spp.

0

ECHINOIDEA Paracidaris jlorigemma (Phi llips)

2

M 3

c

c

c

c

0

c

c

NR

FORAMINIFERA Cribro stomoides sp. Lenticulina spp. Brotzenia spp. Citharina sp. Tristix sp. Dentalina sp.

R

NR

0

0 2

c

c

0

0

0

0

G 2

P t 0

c

0

NR 0

c

K 2

NR

NR

NR 0

c

c

0

0 0

0 0

0

carbonate concretions are common, especially immediately above the Ringstead Waxy Clay, and at the Coral Bed horizon (Fig. 7) and bored phosphatic pellets are common throughout. Bulk samples gave: 35-40 per cent of sand fraction , of which 55-60 per cent is sub-angular to sub-rounded quartz sand and silt, 35-40 per cent limonite ooliths and 5 per cent of pyrite , chamosite and rare calcite shell fragments plus lithified aggregates of limonite ooliths, sand and silt. The macrofauna is dominated by bivalves (Table IV). Despite the rapid transition into the coral bed , none of the characteristic forms of the coral bed are found in any but the most closely adjacent localities . In concretions, where lack of compaction has preserved primary sedimentary structures, flaser and lenticular bedding is common, and aggregates can be demonstrated to have been at least semi-consolidated during transpiration. Ammonites are relatively common, and frequently encrusted by epifauna. They are Ringsteadia spp . and Microbiplices sp. of the pseudocordata Zone. 4. The 'Rhactorhynchia' inconstans Bed. This lies above both the Ringstead Coral Bed and Osmington Mills Ironstone. It sharply overlies the former, but has a gradational contact with the

----

RED 'PLASTER ' OF SHELL FRAGMENTs AND PHOSPHATIC SANDT CLAY

---

.... \ .... 0 .... L 1 _'1~)""Jv ---=----,...., "" v' v - - . '. ~ ,-.

..,/

15 em

.

THECOSMllIA IN SANDY CALCITE OOLITIC 810M leRuDITE

Fig. 6. Block of Coral Bed in limonite-oolitic clay. Dotted line shows junction between biomicrudite transported with coral and addition due to lithification of the limonite-oolitic clay. Ringstead Coral Bed, loco 1, Ringstead.

17

UPPER OXFORDIAN AND LOWER KIMMERIDGIAN OF DORSET PHOSPHATIZEO

MICRITE SHELL FRAGMENTS AND

PHOSPHATIZEO LIMONITE OOLITIC

./ r

~

'--

MICRITE

r ~ ..._ ""'\

~

-

_

'::7

/

LIMONITE OOLITIC MICRITE WITH CORAL FRAGMENTS

~~~~{~~~:2~~~~=OO~T~ ;A:;~ALLY -~ ~~~-.--

LIMONITE

--

20 em

MICRITE

I

····.........

LlTHIFIEO -----". LIMONITE OOLITIC PATCHILY OOLITIC

I

CALg~:;OUS

BL AC~l:~NOY

---

BLACK CLAY

LI;;~:~~~ ~~~~RE

Fig. 7. Lithification in the transitional zone between the Ringstead Coral Bed and Osmington Ironstone; Loc, S, Black Head.

latter. It is characterised by the presence of Torquirhynchia inconstans, and consists of grey sandy bioturbated clay, with sparsely limonitic oolitic clayey sand distributed as lenses and patches throughout. Bulk samples gave 25 per cent of sand fraction of which 80 per cent is quartz sand: the other 20 per cent consists of variable proportions of calcite-oolitic sand and sand-silt aggregates, cemented with ankerite and ferroan calcite, plus small phosphatic pellets and shell fragments showing micrite envelopes due to boring algae. Limonite ooliths, usually highly corroded, form up to 2 per cent of the sand fraction. The macrofauna is relatively diverse, and comparable to that of the coral bed minus corals (Table V) though the majority of the shells are concentrated in current winnowed lag accumulations of shell fragments. Large concretions show the pre-compactional form of the bedding, and may, at least in part, represent isolated largescale asymmetric ripples. Several large concretions at Black Head contain large ammonites (Pictonia spp. of the baylei Zone) at all angles in the bed, and their origin is obscure (Fig. 8). TABLE V Faunas of the 'Rhactorhynchia' inconstans Bed and 'Exogyra' nana Bed at Black Head. A - abundant, c - common, 0 - occurs 'Rhactorhynchia' inconstans Bed (T 7 ) BIVALVIA Palaeonucula menkii Roemer Eonavicula cf quadrisulcata (J. de C. Sowerby) Parallelodon longipuncta Blake Parallelodon ? reticulatus (Blake) Grammatodon aemulus (Phillips) Grammatodon keyserlingi (d'Orbigny) Grammatodon cruciatus (Contejean) Grammatodon aft. rhomboidalis (Contejean) Mytilus ungulatus (Young and Bird) Modiolus dumovariae Arkell Modiolus bipartita (J. Sowerby) Trichites granulatus (J. Sowerby) Pteroperna polyodon Buvignier Isognomon subplanus (Etallon) Isognomon bouchardi (Oppel) Oxytoma expansus (Phillips) Entolium corneolum (Young and Bird) Camptonectes auritus (Schlotheim) Camptonectes giganteus Arkell Chlamys splendens Dollfuss Chlamys at! midas (Damon) Chlamys aft. nisus (d'Orbigny) Eopecten anglica Arkell Placunopsis radiata (Phillips)

o o o o o c o o o o o o

o o o o o o o

'Exogyra' nana Bed (T s) o

o o

o o

o

o c o o

18

M. E. BROOKFIELD

'Rhactorhynchia' inconstans Bed (T,) o o o o o o o o o o

Aceste mutabilis Arkell Ctenostreon probiscideum Sowerby Trigonia (Trigonia) reticulata (Agassiz) Myophorella (Vaugonia) clavellata (Parkinson) Mactromya aceste (d'Orbigny) Myoconcha texta (Buvignier) Myoconcha saemanni Dollfuss Astarte ovata Wm. Smith Astarte mysis de Loriol Opis (Trigonopis) corallina Damon Opis (Trigonopis) curvirostra (Wm. Smith) ? Quenstedtia gibbosa Hudleston Sowerbya triangularis (Phillips) 'Corbicella' cf eboracensis Arkell 'Corbicella' cf laevis (J. de C. Sowerby) Anisocardia elegans Dollfuss Anisocardia globosa (Roemer) Isocyprina cyreniformis (Buvignier) Isocyprina constantini (Dollfuss) Isocyprina tancrediformis (Blake and Hudleston) Corbula ? deshayesea de Loriol Pholadomya aequalis J. de C. Sowerby Pholadomya protei (Brongniart) Goniomya literata (1. Sowerby) Pleuromya uniformis (J. Sowerby) Pleuromya alduini (Brongniart) Thracia depressa (J. de C. Sowerby) Deltoideum delta (Wm. Smith) Nanogyra nana (J. Sowerby) 'Ostrea' dubiensis Contejean Actinostreon sp.

o

BRACHIOPODA Torquirhynchia inconstans (J. Sowerby)

c

ANTHOZOA Thecosmilia annularis Milne-Edwards and Haime

o

ANNELIDA Serpula spp.

c

ECHINOIDEA Paracidaris florigemma (Phillips)

o

GASTROPODA Bathrotomaria reticulata (J. Sowerby) Dicroloma sp. Cerithium sp. 'Natica' sp. Bourgetia saemanni (J. Sowerby) Pseudomelania sp. Nerinea sp. Amberleya sp. Nerita cf pellati (de Loriol)

c o o o o o o o o

CEPHALOPODA Pictonia spp. Prorasenia spp. Paracenoceras calloviense (Oppel)

c o o

'Exogyra' nana Bed (T s) o

o

o o

o o

o o

o

o o o o o o c o c c

o o o

o o c A o

c

o o

19

UPPER OXFORDIAN AND LOWER KIMMERIDGIAN OF DORSET

TABLE V cont.

'Exogyra'

'Rhactorhynchia' inconstans Bed (T 7 )

ARTHROPODA Paraglyphea cf rostrata (Phillips)

o

BRYOZOA unidentified, encrusting Torquirhynchia

o

FORAMINIFERA Ammobaculites sp. Cribrostomoides sp. Lenticulina spp. Brotzenia sp. Vaginulina sp. Citharina sp. Tristix sp. Falsopalmula sp. Lagena sp. Dentalina sp.

i

TOP

nana Bed (T s)

o o c o o o o o o

o c o o o o

o

~

/'

~F

®

<:~l

! '.. . .... ~

.

.......

'

A ..

~

)C@A

40 em

Fig. 8. Drawing of fossil distribution in carbonate concretion; loc. V, Black Head, 'Rhactorhynchia' inconstans Bed. A-ammonites, B---Ctenostreon probiscideum, C-Yalves of Deltoideum delta, D-Valve of Mytilus sp., EPleuromya uniformis in life position, F-Amberleya sp.

20

M. E. BROOKFIELD

5. The 'Exogyra' nana Bed. This consists of a thin sandy clay, with numerous small oysters-Nanogyra nana. Frequently the oysters form almost the entire bed and the accumulations are sometimes lithified, forming large lenses which appear to represent shell banks or colonies. The fauna is comparatively diverse (Table V), and the bed shows signs of condensation and erosion, with exhumed phosphatic casts of shallow and deep-burrowing bivalves, phosphatic pellets and extensive shell boring. One phosphatised Pictonia sp. indicates the baylei Zone, but this may have been derived. The bed is continuous along the Dorset coast from Weymouth to Ringstead. All the Condensed Beds show abundant signs of slow deposition, including reworked partially lithified aggregates, truncated and reworked phosphatic casts of deep-burrowing bivalves, abundant shell boring, encrustation and breakage. The flaser and lenticular lamination and large asymmetric ripples indicate a possible tidal environment and the relatively diverse faunas, with common stenohaline forms, suggests an open shelf environment. The sand fractions and faunas most closely resemble those recorded from the inner shelf and old shelf environments of the Gulf of Mexico (Shepard & others, 1960; Brookfield, 1973c). C. The 'Ostrea delta' Bed This consists of heavily bioturbated silty clays, with thin layers of large Deltoideum delta. It represents a quieter water environment than the 'Exogyra' nana bed, and contains a more diversified infauna (Table VI). It varies little in character from Sandsfoot to Ringstead. Ammonites (Rasenia spp., Prorasenia spp.) indicate the cymodoce Zone: though these are relatively rare. The bed resembles the open, high salinity bay or inner shelf environments of the Gulf of Mexico. It sharply overlies the 'Exogyra' nana bed, but grades upwards into the Blue Clay (see below). TABLE VI Fauna of the 'Ostrea delta' Bed. + - dominants BIVALVIA Palaeonucula menkii Roemer Parallelodon longipuncta Blake Parallelodon reticulatus (Blake) Grammatodon alsaticus (Roemer) Mytilus ungulatus (Young and Bird) Modiolus sp. Oxytoma expansus (Phillips) Camptonectus auritus (Schlotheim) Chlamys aff. midas (Damon) Placunopsis radiata (Phillips) Aceste sp. Myophorella (Vaugonia) clavellata (Parkinson) 'Lucina' substriata (Blake) Astarte ovata Wm. Smith Anisocardia globosa (Roemer) +Corbula prora Sauvage. Plectomya minuta (Blake) Thracia depressa (J. de C. Sowerby) +Deltoideum delta (Wm. Smith) Nanogyra nana (1. Sowerby) GASTROPODA Bathrotomaria reticulata (1. Sowerby) Dicroloma sp.

CEPHALOPODA Rasenia spp. + Belemnites spp. ANNELIDA Serpula spp. ECHINOIDEA +'Pseudodiadema' sp. FORAMINIFERA Lenticulina spp. Vaginulina sp. Brotzenia spp.

UPPER OXFORDIAN AND LOWER KIMMERIDGIAN OF DORSET

21

D. The Blue Clay (new name) This repeats the characteristics of the Ringstead Waxy Clay. It consists of grey-blue silty clay, with abundant debris of the echiniod Pseudodiadema sp. and rare bivalves. Its fauna is, however, slightly less varied (Table VII). The Blue Clay is sharply overlain by the Sandy Clays. The Blue Clay is exposed at Ringstead, Osmington and Black Head, and Sandsfoot, and persists with no lateral variation along the coast. Two thin siderite limestones contain the trace fossil, Laevicyclus, and relatively abundant ammonites Prorasenia spp. and Amoeboceras sp. indicate the cymodoce Zone. E. The Sandy Clays (new name) This name is given to the beds above the Blue Clay, and below the Grey Clays of the upper cymodoce Zone. The unit consists of three main facies arranged in cyclical repetitions. Two main cycles occur with two subsid iary cycles, which are sometimes cut out by downward channelling at the base of the main cycles (Fig. 9). Each cycle shows a typical sequence of the following. 1. A basal erosion surface on top of blue clay. 2. A lag horizon, in which rolled phosphatic pellets, shell fragments and coarse sand are concentrated. 3. Fine-grained clayey sand , fining upwards, in which an abundant and diverse macrofauna and large tree branches up to one metre long and ten centimetres in diameter are concentrated at the base and in which the macrofauna decreases in abundance and diversity upwards. The main and subsidiary cycles contain slightly different faunas in this unit (Table VIII). 4. Unit 3 gradually passes up into blue clays, identical in lithology and fauna to the Blue Clay (unit D). The blue clay, where not eroded, is overlain by unit 1 with small Thalassinoides burrows piping overlying material downwards. Frequently erosion at the base of unit 1 has cut out members or all of an underlying cycle. In unit 3, each main cycle is characterised by a different species ofTrigoniid bivalve. The subsidiary cycles tend to lack the basal lag deposit of the main cycles (Fig. 9). Ammonites recorded from both units 3 and 4 are Rasenia spp . and Amoeboceras spp . of the cymodoce Zone. These fining upwards cycles indicate regular repetitions in environmental energy. The fauna is marine (Table IX) and they may represent storm deposits of a shelf (cf. Brenner & Davies, 1973). An alternative interpretation is that they represent distal barrier bar inlet deposits, related to the Abbotsbury Ironstone barrier bar of the same age developed further west (cf. Brookfield, 1973a; 1973b). Coarse fractions of the sediments, and the composition and palaeoecology of the macrofauna indicate a close resemblance to the inner shelf near a barrier bar inlet environment of the Gulf of Mexico (Shepard & others, 1960; Brookfield, 1973c). TABLE VII

Fauna of the Blue Clay (T 10)

BIVALVIA Palaeonucula menkii Roemer Pinna sp. Anisocardia isocardioides (Blake and Hudleston) Corbula prora Sauvage Thracia depressa (J. de C. Sowerby) Nanog yra nana (J . Sowerby) BRACHIOPODA Discina sp.

ECHINOIDA 'Pseudodiadema ' sp. CEPHALOPODA Amoeboceras sp. Prorasenia sp. FORAMINIFERA Lenticulina spp. Vaginulina sp. Brotzen ia sp.

tv tv

BLACK HEAD loco U

v

SANDS FOOT

Z4

loc.O

Vttt iul selle

I~='

V

....

23 22

V

v

Subsidiary Cycle

V

& 5

1.

2. °U 3. 4.

v v

20

19

v

.. '-'-'\J._.

-

·, , .. . ~

4

r-,,

Subs. Cy<:le

. ....., .. -. · . · ,. " ., .-.I~ ··-'.-........ ......'.. .." "

BlueClay

3b •

....,.

-.:.,

v

Lee, T

v 19 v

v v

....

V

v

-. ;.,'-'V . · .. 18

'-'"

~ .. 1&

15

v

·

,

21-,' - .... 1

13 12 11

•

• v

d

.....

c

'-'"

«,

:

13 • 12

·

v. _

. ...

••• '

•

1- .... ,.- ..

....

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V

.

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0"

.... v . .,~~;-~

11' 10

b

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,

Loc.D

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•

:tt"':J;'.:... •• ~-.v"",,

"

1

----

.-

~

a b S C u red

Cycle

2 S..... eycl 1

Cycle

·L----""

Loc. O. Lee, E

.'

I'

•

0 __ ,,:._,":

Fig. 9. Correlation of the Sandy Clays on the Dorset Coast. Localities on Fig.!. I-shells, 2-sandy clay and sand, 3-burrows, ~rosion surface, Blank-day.

I:
8 ~

3

..

.-

t:I:l

~

•

'. "v-';'. . ....,. . .

3 ",_, -.":'..... '

,'...., -'

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11

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.

..,

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14

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.. .

v

ttl

RINGSTEAD

loco II

""',_",..... . -----r--- __________

V

15-18

a::

OSMINGTON

....

v

'v, 'v v. V,'v

Lee. A

:ycle I

V

14

EAST FLEET

.---..... .

v

21

V

Cycle II

.

v

v v

v

....

v

v

v v

-------

v

v

v 7

v

'"'

Subsidilry eyel

rt::l

23

UPPER OXFORDIAN AND LOWER KIMMERIDGIAN OF DORSET TABLE VIII

Fauna of the Sandy Clays Locality Bed

BIVALVIA Palaeonucula menkii Roemer Cucullaea contracta (Phillips) Parallelodon reticulatus (Blake) Grammatodon cruciatus (Contejean) Grammatodon cf. aemulus (Phillips) Grammatodon ? alsaticus (Roemer) Grammatodon aff. rhomboidalis (Contejean) Grammatodon schourovskii (Roullier) Mytilus ungulatus (Young and Bird) Modiolus durnovariae Arkell Gervillia sp. 'Inoceramus' cf. expansus Blake Camptonectes auritus (Schlotheim) Chlamys aff. midas (Damon) Placunopsis sp. Limatula elliptica (Whiteaves) Trigonia (Trigonia) reticulata (Agassiz) Myophorella (Vaugonia) clavellata (Parkinson) 'Lucina' minuscula Blake Astarte ovata Wm. Smith Astarte extensa (Phillips) Opis (Trigonopis} corallina Damon ? Sowerbya triangularis (Phillips) 'Corbicella' eboracensis Arkell Anisocardia elegans Dollfuss Isocyprina pellucida (Blake) Isocyprina cyreniformis (Buvignier) Isocyprina tancrediformis (Blake and Hudleston) Corbula prora Sauvage Corbula deshayesea de Loriol Pholadidea cf. compressa Blake Pholadomya protei (Brongniart) Pholadomya aequalis J. de C. Sowerby Goniomya literata (J. Sowerby) Pleuromya alduini (Brongniart) Pleuromya uniformis (J. Sowerby) ? Cercomya undulata (J. de C. Sowerby) Thracia depressa (J. de C. Sowerby) Deltoideum delta (Wm. Smith) cf. 'Ostrea' dubiensis Contejean Nanogyra nana (J. Sowerby) Nanogyra virgula (Defrance) Actinostreon sp. GASTROPODA Bathrotomaria reticulata (J. Sowerby) Dicroloma sp. Cerithium sp. Pseudomelania sp. ANNELIDA Serpula spp. CEPHALOPODA Amoeboceras spp. Prorasenia sp. Rasenia spp.

T ll

T I•

0

Black Head TI 3 TI •

0

TI•

0

0 0 0 0 0 0 0 0

0 0 0 0 0

0 0

0

0

0

0

0

c

c 0 0

0

0

c

0 0

0 0

0

0

0

0 0

0 0 0

c

0 0

0

c

0 0 0 0 0

0

0

0 0 0 0 0

0

0

0 0 0

0 0 0

0

0

0

0

0

0

0 0 0

TI •

24

M. E. BROOKFmLD

Localit y Bed ECHINOIDEA 'Pseudodiadema' sp. FORAMINIFERA Cribrostomoides sp. Ammobaculites sp. Trochammina sp. Lenticulina spp , Nodosaria sp. Tristix sp. Citharina sp. Vaginulina sp. Brotzenia spp, Falsopalmula sp.

Til

T I2

c

0

Black Head TI3 TI• 0

TIs

T I6

0

0 0 0

c

c

0

c

c

0

0

c

c

c

0

0

0

0 0 0

0

c c

0 0

0

F. The Grey Clays (new name) The Grey Clays (beds T 18-24' Fig . 2) consist of laminated grey-black clays with less than O· 3 per cent of fine-grained quartz sand and low amounts of silt, arranged in thinly bedded units between 30 and 70 em thick. There is little variation in this sequence in the coastal exposures. The macrofauna is abundant, but composed of only a few species (Table IX), the commonest species being Thracia depressa, Palaeonucula menkii, Corbula prora, andRasenia spp. which are represented by whole shells and abundant broken fragments . Fragments and palates of the shell-crushing fish Chimaera sp. and Microdon sp. are common and may have caused most of the shell breakage. The microfauna is relatively diverse and dominated by foraminifera and ostracods (Table IX). The ammonites indicate the cymodoce Zone. These beds can be closely compared with the prodelta environment of the Gulf of Mexico (Parker, 1956; Shepard & others, 1960) both in lithology, macrofaunal diversity and ecology and microfaunal patter. However, there was obviously no Lower Kimmeridgian delta in the area, since the beds are so thin, and a more offshore environment relative to the underlying units is likely.

ABBOTSBURY AREA Upper Calcareous Grit is not exposed west of Weymouth until near Abbotsbury, where it is in a different facies from the exposures around Weymouth. At Abbotsbury, a series of ferruginous sandstones lying above the 'Trigonia' clavellata beds passes upwards into a sandy limonite oolitic ironstone (Abbotsbury Ironstone) of cymodoce Zone age (Brookfield, 1973b; c). These were mapped together as 'passage beds' (Wilson, Welch, Robbie & Green, 1958) but it is convenient to distinguish two units of differing litho- and biofacies.

A. The Abbotsbury Sandstone (new name) This name replaces the name ' Sandstone Grits', which were formerly applied to these sandstones (Blake & Hudleston, 1877 ; Arkell, 1936). They consist of medium-thick bedded ferruginous quartz sandstones, and are best exposed at Linton Hill (SY 586 847). Blake & Hudleston (1877) and Arkell (1936) described these beds as 'Sandsfoot Grit' and correlated them with the Sandsfoot Grits at Weymouth. But the Abbotsbury Sandstone passes upwards

25

UPPER OXFORDIAN AND LOWER KIMMERIDGIAN OF DORSET

TABLE IX

Fauna of the Grey Clays Locality Bed

BIVALVIA Palaeonucula menkii Roemer Grammatodon alsaticus (Roemer) 'Inoceramus' expansus Blake 'Lucina' minuscula Blake Astarte extensa (Phillips) Protocardia pesolina (Contejean) Corbula prora Sauvage Thracia depressa (J. de C. Sowerby) Nanogyra virgula Defrance

TI7

T I"

TI •

c

c

c

T 21

T 22

T 23

c

0

c

0

T2 •

0 0

c

0 0

0

0

0

c c

0

c c

0

GASTROPODA Dicroloma sp. Cerithium sp.

c c

0

0

c 0

0

0

0

c

c

0

0

0 0

0

CEPHALOPODA Rasenia spp.

c

c

c

ECHINOIDEA 'Pseudodiadema'sp.

0

0

0

0

0

0

0

c c

c c

FORAMINIFERA Ammobaculites sp. Trochammina sp. Lenticulina spp. Brotzenia spp. Vaginulina spp. Falsopalmula sp. Citharina sp. Nodosaria sp.

T 20

0 0

0

c

0

0 0

c c c

c c

c

c 0

0

c

c c c

c 0

0 0

0

without a break into the Abbotsbury Ironstone of cymodoce Zone age and must be at least in part of baylei Zone age. They can not be considered a part of the depositional episode of the Sandsfoot Grits on the coast, which extend up no further than the middle pseudocordata Zone, since the Abbotsbury Sandstones die out as a morphological feature east of Rodden (SY 611842),3 km east of Abbotsbury and are not even lithostratigraphically connected with the Sandsfoot Grits on the coast. The type section of the Abbotsbury Sandstones is designated as the section on Linton Hill, where 8 m of medium bedded, fine-medium grained ferruginous quartz sandstones are exposed though, neither the base nor top is seen. The sandstones are very sparsely fossiliferous: only Chlamys midas is common. The top of the Abbotsbury Sandstones can be seen in Red Lane (SY 577850), where they are of lower cymodoce Zone age (Brookfield, 1973b). To the east, at Rodden, slabs of ferruginous sandstone with Chlamys midas and Deltoideum delta could at one time be seen (Arkell, 1936). The Abbotsbury Sandstones may be tentatively correlated with the lower part of the Passage Beds of the coastal sections (Ringstead Waxy Clay-'Ostrea delta' Bed). B. The Abbotsbury Ironstone The Abbotsbury Ironstone consists of up to 8 m of limonite oolitic quartz sandstones with thin interbeds of chamosite siderite mudstone, which have been described by Blake & Hudleston

26

M. E. BROOKFIELD

(1877), Arkell (1936) and Brookfield (1973b). The Ironstone was sharply overlain by Kimmeridge Clay in a temporary exposure (SY 586851) east of Abbotsbury in 1966 though the j unction is never exposed except in such excavations. The basal boundary is gradational with the Abbotsbury Sandstone. The Ironstone, though virtually restricted to the area around Abbotsbury, has also been detected to the north at Litton Cheney (SY 552908) where temporary exposures showed typical Abbotsbury Ironstone (Cope, 1971). The Abbotsbury Ironstone contains relatively abundant ammonites, dominantly Rasenia spp, of the cymodoce Zone. The Abbotsbury Ironstone may tentatively be correlated with the Blue Clay, Sandy Clays and Grey Clays of the Dorset coast. The Ironstone contains a similar fauna to unit 2 of the Sandy Clays on the coasts, except that the Ironstone has in addition several species of articulate brachiopods (Table X). The presence of limonite ooliths throughout the Sandy Clays indicate some transportation of sediment from the Abbotsbury area towards the Dorset coast and the Ironstone is considered to represent a subtidal barrier bar environment (Brookfield, 1973b). Correlation of the Abbotsbury and coastal section is illustrated in Fig. 10.

The Kimmeridge Clay This is now redefined to include the grey-black clays and shales above the Passage Beds on the Dorset Coast. Only the mutabilis Zone was examined in my study, since the higher beds of the TABLE X

Fauna of the Abbotsbury Ironstone

BIVALVIA Modiolus sp. Gervillia ? aviculoides (1. Sowerby) Entolium corneolum (Young and Bird) Camptonectes auritus (Schlotheim) Chlamys aft. midas Damon Eopecten sp. Trigonia (Trigonia) reticulata (Agassiz) Astarte ovata (Wm. Smith) Opis (Trigonopis) coral/ina Damon Sowerbya triangularis (Phillips) Anisocardia globosa (Roemer) Pholadidea abbreviata (Blake and Hudleston) Pholadomya canaliculata Roemer Goniomya literata (1. Sowerby) Pleuromya uniformis (1. Sowerby) Nanogyra nana (1. Sowerby) 'Ostrea grypheata' de Loriol GASTROPODA Bathrotomaria sp. Dicroloma sp. Chemnitzia sp. 'Natica' sp. Amberleya sp. CEPHALOPODA Rasenia spp. Paracenoceras cal/oviense (Oppel) ANNELIDA Serpula sp.

BRACHIOPODA ?Septaliphoria hudlestoni (Roilier) 'Terebratula' subsel/a (Leymerie) Ornithel/a lamp as (1. Sowerby) Aulacothyris dorsetensis (Davidson) Lingula sp. ECHINOIDEA Nucleolites scutatus (Lamarck) ARTHROPODA Eryma sp.

27

UPPER OXFORDIAN AND LOWER KIMMERIDGIAN OF DORSET AMMONITE ZONE

LOCALITIES

ABBOTSBURY

:i

BLACK HEAD

WEYMOUTH

MUTABILIS ZONE

KIMMERIDGE CLAY

8f-----+

iii

Ii2

Ili

9 I--BA-Y-LEI-Z-ON-1E••• :t=~~~:::~~~=====1!,=========:::tJ

.. z

~ S

PSEUOOCORDATA

ZONE

CAUn5NIGRAE ZONE

~' DATUM: Top of Baylei Zone

~ Oolitic Ironstone

~ Sandstone

ti53 limestone

GJ Sandy Clay

ErE

Sandy limestone

~iMetres 2 Kilometres

DClay

Fig. 10. Correlation of Abbotsbury and Coast sections, showing sand bodies.

Kimmeridge Clay have been studied by Arkell (1947), Cope (1967) and Ziegler (1962) further east at Kimmeridge. At Black Head, the junction between the Passage Beds and Kimmeridge Clay is marked by an impoverishment in macrofauna and microfauna. Only Nanogyra virgula, Thracia depressa and ammonites are common in the Kimmeridge Clay. The clay mineral assemblages change from the illite-kaolinite assemblages of the Passage Beds into the illitechlorite assemblages of the Kimmeridge Clay (Table I). The junction with the underlying Grey Clays is gradational but is conveniently taken above a horizon (T24 , Fig. 2) with large septarian concretions up to one metre in diameter. At about this horizon, the Rasenia spp. of the cymodoce Zone are replaced by Aulacostephanus spp. of the mutabilis Zone (Ziegler, 1962). 7. VERTICAL CHANGES IN FAUNAS The invertebrate faunas of the upper Corallian Beds and Passage Beds are very similar as is the microfauna of ostracods (Kilenyi, 1969), foraminifera, and microplankton (Gitmez, 1970). Different species are basically controlled by facies and are distributed sporadically through the various sections. For example, the two characteristic 'Trigonias' of the cautisnigrae Zone at Weymouth, Myophorella clavellata and Trigonia reticulata, also occur higher up in the cymodoce Zone at Weymouth. The baylei andcymodoce Zones contain species identical or allied to those

28

M. E . BROOKFIELD

of similar facies in the underlying Upper Oxfordian. A few new forms such as the brachiopod, Torquirhynchia inconstans, come in during the baylei and cymodoce Zones, and indicate spread of forms from the European continent during a minor Kimmeridgian transgression in the baylei Zone (Brookfield, 1973a). Apart from the ammonites, the vertical changes in faunas are predominantly at the species level. Thus Nanogyra nana is gradually replaced by Nanogyra virgula in the Passage Beds . The similarity of the Passage Beds faunas to the Upper Oxfordian are shown in Table XI. The faunas are dominated by euryhaline bivalves and stenohaline groups (e.g. echinoids, corals, ammonites, brachiopods and bryozoa) are common only in certain beds. This suggests that most of the sediments were formed in shallow nearshore environments of varying ecological conditions (Brookfield, 1973a). A marked change in the invertebrate faunas at outcrop occurs at the base of the mutabilis Zone, and is apparently ecologically controlled by the ubiquitous development of a black shale environment at this time in all British basins (cf. Hallam, 1967; Brookfield, 1973a). However, nektonic forms show little differentiation and microplankton remain abundant. A continuous evolutionary series of the ammonites Ringsteadia, Pictonia, Rasenia, and Aulacostephanus is traceable from the pseudocordata Zone to the Aulacostephanus eudoxus Zone. A slight decline in the micro faunas occurs in the mutabilis Zone. This reaches catastrophic proportions in the eudoxus Zone, where ostracods are practically absent (Kilenyi, 1969), arenaceous foraminifera decline from 21 species in the mutabilis Zone to 6 in the eudoxus Zone (Lloyd, 1959). Downie (1956) found very low numbers of microplankton in the zone above the Aulacostephanus autissiodorensis Zone (Pectinatites elegans Zone) (he did not study the lower zones), which contrast with the diverse microplankton recorded from the Upper Oxfordian clays of Yorkshire (Sarjeant, 1962). These faunal changes correspond with a change from mixed clastic-carbonate sediments of the cautisnigrae-cymodoce Zones, into the uniform laminated clays of the Aulacostephanus mutabilis-Pavlovia rotunda Zones. 8. LATERAL CHANGES IN THE UPPER CORALLIAN BEDS-LOWER KIMMERIDGE CLAY Comparison of the Abbotsbury and coastal exposures shows pronounced facies changes (Fig. 10) and borehole data show further changes eastwards (Fig. 11). The generally sandy sediments of the outcrop pass into much thicker sequences eastwards as shown in the Poxwell and Kimmeridge boreholes. The Dorset outcrops thus appear to represent the western edge of a basin. Unfortunately, the faunal evidence from the boreholes is meagre, and the correlation shown on Fig. 11 is based on lithostratigraphic comparison, supplemented by the small amount of fossil evidence available. The sediments in N. W. France are closely similar to those of the outcrops in Dorset, and appear to represent similar basin margin deposits. TABLE XI

Ranges of invertebrate species

'Trigonia' clavellata Beds Sandsfoot Beds Passage Beds

Confined to unit

Ranging downwards

Ranging upwards

Long ranging forms

10 3

6 6 59

2

4

8 10

5 8

23

29

UPPER OXFORDIAN AND LOWER KIMMERIDGIAN OF DORSET

BROAD BENCH =. =

fRANCE

150 Km

,

, ,

I

T

POXWELL' I LOCALITIES 1 2. 3. 4. 5. 6.

Abbotsbury Sandsfoot Black Head Poxwell (borehole) Broad Bench (borehole) Normandy (Le Havre)

MUTABILIS ZONE

VERTICAL SCALE 1cm=3m

SANDSFOOT ABBOTSBURY

-

\

\

BAYLEI ZONES

I

.,-

r

I I

:r,"

2:-

:r

I I

~ NORMANDY \

\ To:·:G';" .. T

...----1- -

e··

·· .....T·

..~.:.·v '!" ~

.0- .

-r.lo...Je , D'

n.°

\

-

, ,,I

,

\

AND

"':' T"

T"

BLACK HEAD I \

CYMODOCE

PSEUDO-

,

,,

r:.

Q T

\

\

CORDATA ZONE

"- ....

CAUTISNIGRAE ZONE

-

-

"

:r

:r -

Fig. 11. Correlation of sections in south Dorset and north-west France. Poxwell from Taitt and Kent (1939) and personal observations. Broad Bench (no. 2 borehole) from Lees and Taitt (1946), unpub. British Petroleum borehole log. and personal observations on core samples. Normandy from Rioult (1961), Arkell (1930), Bigot (1930). Lithological symbols: Shell International Standard Legend (1958).

30

M. E. BROOKFIELD

9. THE OXFORDIAN-KIMMERIDGIAN BOUNDARY The boundary is defined on the evolution of the ammonite genus Ringsteadia into the genus Pictonia, and in the type sections on the Dorset coast lies between the Ringstead Coral Bed and the 'Rhactorhynchia' inconstans bed. However, these two units are variable laterally, and the Oxfordian-Kimmeridgian boundary has little environmental significance. Apart from corals, the faunas of the Coral Bed and 'R.' inconstans bed are very similar. At Abbotsbury, a continuous sedimentary sequence spans the boundary, with little change in environment indicated. The facies faunas from the Ringstead Waxy Clay to the top of the Grey Clays are alike. The major environmental changes do not take place at the Oxfordian-Kimmeridgian boundary. They occur; at the base of the Ringstead Waxy Clay, where ferruginous sandstones (Sandsfoot Grit) are abruptly overlain by blue clays; and at the base of the mutabilis Zone, where a decrease in clastic content and marked decrease in benthic macrofauna occurs. Systematic changes in clay minerals at both horizons have been attributed to changes in source climate (Brookfield, 1973c). All this is not purely academic, as it has led to a difficulty in detecting the junction in borehole cores. The taking of the stage junction at a relatively insignificant level has also caused difficulty in correlation with other western European areas (see Arkell, 1933; 1946, for stages and discussion) . ACKNOWLEDGMENTS I gratefully acknowledge the receipt of an N.E.R.C. Research Studentship at the University of Reading, where most of this work was carried out. British Petroleum gave permission to reproduce the log of the Broad Bench borehole. Mr. L. S. O. Morris materially assisted in the examination of the cores of the Poxwell borehole in the Borings Department of the Institute of Geological Sciences. Miss E. M. Samuel gave permission to examine fossil collections in the Dorset Natural History and Archaeological Society Museum. REFERENCES AGER, D. V. & P. WALLACE, 1970. The distribution and significance of trace fossils in the uppermost Jurassic rocks of the Boulonnais, Northern France. In T. P. Crimes & J. C. Harper (eds) Trace Fossils, See I House Press, Liverpool, 1-18. ALLEN, J. R. L., 1965. Late Quaternary Niger Delta and adjacent areas: sedimentary environments and lithofacies. Amer. Assoc. Petrol. Geol. Bull., 49, 547-600. ARKELL, W. J., 1929-1937. British Corallian Lamellibranchia. Palaeont. Soc. London Mono. 392 pp. ARKELL, W. J., 1930. A comparison between the Jurassic rocks of the Calvados coast and those of southern England. Proc. Geol. Ass., 41, 396-411. ARKELL, W. J., 1933. The Jurassic System in Great Britain. Clarendon Press, Oxford. 681 pp. ARKELL W. J., 1935-48. Monograph on the ammonites of the English Corallian Beds. Palaeont. Soc. London Mono., 420 pp. ARKELL, W. J., 1936. The Corallian beds of Dorset. Part 1. The Coast. Dorset nat. hist. Arch. Soc. Proc., 57,59-93.

ARKELL, W. J., 1945. The zonesofthe UpperJurassic of Yorkshire. Yorks. Geol. Soc. Proc., 25, 339-58. ARKELL, W. J., 1946. Standard of the European Jurassic. Geol. Soc. Amer. Bull., 57, 1-34. ARKELL, W. J., 1947. The geology of the country around Weymouth, Swanage, Corge and Lulworth. Mem. geol. Surv. U.K., 386 pp. ARKELL, W. J., 1951. The structure of Spring Bottom Ridge and the origin of the mud-slides, Osmington, Dorset. Proc. Geol, Ass., 62, 21-30. ARKELL, W. J. & J. H. CALLOMON, 1963. Lower Kimmeridgian ammonites from the drift of Lincolnshire. Palaeontology, 6,219-45. BIGOT, A., 1930. Sketch of the geology of lower Normandy. Proc. Geol. Ass., 41, 363-95. BLAKE, J. F., 1875. On the Kimmeridge Clay of England. Q. Jl geol. Soc. London, 31, 196-233. BLAKE, J. F. & W. H. HUDLESTON, 1877. On the Corallian Rocks of England. Q. Jl geol. Soc. London 33, 260-405. BRENNER, R. L. & D. K. DAVIES, 1973. Stormgenerated Coquinoid Sandstone: Genesis of high-

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Jurassic) of England. Br. Mus. (Nat . Hist.) Bull. Geol., 18,231-331. GORDON, W. A., 1966. Variation and its significance in classification of some English Middle and Upper Jurassic nodosariid foraminifera. Micropalaeontology, 12, 325-33. GUTMAN, K., 1970. The Corallian beds at Todber and Whiteway hill in North Dorset. Proc. Dorset nat. Hist. Arch . Soc ., 91, 123-9. HALLAM, A ., 1967. An environmental study of the Upper Domerian and Lower Toarcian in Great Britain . Phil. Trans. R. Soc. Lond., Ser. B, 252,393-445. HALLAM, A., 1971. Facies analysis of the Lias in West Central Portugal. N. Jb. Geol. Palaeont. Abh., 139, 226-65. HUDLESTON, W. H., 1880-1. Contributions to the palaeontology of the Yorkshire Oolites. Geol. Mag., 7, 241-8, 289-98, 391-404, 481-8, 529-38; 8, 4959, 119-31. KILENYI, T . I., 1969. The Ostracoda of the Dorset Kimmeridge Clay . Palaeontology, 12, 112--60 . LEES. G . M. & A. H. TAITT, 1946. The geological results of the search for oilfields in Great Britain. Q. Jl geol. Soc. London, 101, 255-317. LLOYD, A. J ., 1959. Arenaceous foraminifera from the type Kimmeridgian . Palaeontology, 1, 298-320. PARKER, R. H. , 1956 . Macro-invertebrate assemblages as ind icators of sedimentary environments in East Mississippi Delta Region. Am. Ass. Petrol. Geol . Bull., 40, 295-376. PRUVOST, P. & J. PRINGLE, 1924. A synopsis of the geology of the Boulonnais, including a correlation of the Mesozoic rocks with those of England, with report of excursion. Proc. Geol . Ass ., 35, 29-56. RIOULT, M., 1961. Problernes de Geologie Havraise. Soc. Geol. Normandie, B 51, 32-48. SALFELD, H ., 1913 . Certain Upper Jurassic strata of England. Q. Jl geol. Soc. London, 69, 423-32. SALFELD, H ., 1914 . Die Gliederung des oberen Juras im Nordenesteuropa. N. Jb . Mineral., Beil. Bd ., 37, 125-246. SALFELD, H., 1915. Monographie der Gattung Cardioceras. Z. dtsch. Geol. Gesell., 67, 149-204. SALFELD, H ., 1917. Monographie der Gattung Ringsteadia. Palaeontographica, 62, 69-84. SANDERS, H. L., 1960. Benthic studies in Buzzards Bay III. The structure of the soft bottom community. Limnol. Oceanogr. , 5, 138-53. SARJEANT, W. A. S., 1962. Microplankton from the Ampthill Clay of Melton, S. Yorkshire. Palaeontology, 5,478-97. SHEPARD, F. P., F. B. PHLEGER & Tj . H. VAN ANDEL, 1960. Recent sediments, N. W. Gulf of Mexico. Amer. Assoc, Petrol. Geol., Tul sa, Oklahoma . SWIFT, D. J . P., D . J. STANLEY & J. R. CURRAY, 197 1. Relict sediments on continental shelves : a reconsideration. Jour . Geol., 79, 322-46. TAITT, A . H. & P. E. KENT, 1939. Note on an examination of the Poxwell anticline, Dorset. Geol. Mag., 76, 173-81. TALBOT, M. R., 1971. Calcite cements in the Corallian

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land . Palaeogeogr., Palaeoclimatol ., Palaeoecol., 4, 89-123 . WILSON, V., F. B. A. WELCH, J . A. ROBBIE & G. W. GREEN., 1958. Geology of the country around Bridport and YeoviJ. Mem. geo/. Surv. U.K. WRIGHT, J. K., 1973 . The Middle and Upper Oxfor dian and Kimmeridgian Staffin Shales at Staffin, Isle of Skye. Proc. Geol. A ss., 84, 447-57. ZIEGLER, B., 1962 . Die Ammonitengattung Aulacostephanus im Oberjura (Taxionomie, Stratigraphie, Biologie) . Pa/aeontographica, Abt. A, 119, 1-172. Rece ived 26 June 1976 Revised version recei ved 31 January 1977