The distribution of Late Kimmeridgian and Portlandian ostracoda in southern England

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Revue de micropaléontologie 51 (2008) 221–238

Original article

The distribution of Late Kimmeridgian and Portlandian ostracoda in southern England La r´epartition des ostracodes du Kimm´eridgien sup´erieur au Portlandien en Angleterre m´eridionale Ian P. Wilkinson British Geological Survey, Keyworth, Nottingham NG12 5GG, United Kingdom

Abstract The vertical distribution of ostracods in the Upper Kimmeridgian and Portlandian (sensu anglico) succession in three cored boreholes at Hartwell, Tisbury and Fairlight, are compared to other successions in southern England. The Upper Kimmeridge Clay Formation yields rich, but low diversity ostracod faunas, characterised by stratigraphically restricted species of Aaleniella, Galliaecytheridea, Klentnicella, Macrodentina, Mandelstamia, Micrommatocythere, Paralesleya and Prohutsonia. Several continue into the Portlandian, but some species, belonging to genera such as Cytherelloidea, Paracypris, Fabanella, Galliaecytheridea, Klieana, Paraschuleridea, Eocytheridea, Paranotacythere, Procytheropteron, Rectocythere and Macrodentina, appear for the first time. Biostratigraphical subdivision is made difficult by provincialism caused by decreasing salinities and facies change during the Late Portlandian. Marine taxa such as Protocythere, Macrocypris, Paraschuleridea, Paranotacythere, Procytheropteron and Rectocythere were replaced by euryhaline forms, such as species of Fabanella and Mantelliana, and fresh-oligohaline species of the genera Cypridea, Scabriculocypris, Alicenula and Rhinocypris. © 2007 Ian P. Wilkinson. Published by Elsevier Masson SAS. All rights reserved. R´esum´e La r´epartition verticale des ostracodes dans le Kimm´eridgien Sup´erieur et le Portlandien (sensu anglico) dans trois forages a` Hartwell, Tisbury et Fairlight est compar´ee a` celles d’autres coupes d’Angleterre m´eridionale. La partie sup´erieure de la formation de Kimmeridge Clay a livr´e de riches faunes d’ostracodes, peu diversifi´ees, caract´eris´ees par des esp`eces stratigraphiquement restreintes des genres Aaleniella, Galliaecytheridea, Klentnicella, Macrodentina, Mandelstamia, Micrommatocythere, Paralesleya et Prohutsonia. Plusieurs esp`eces connues d`es le Kimm´eridgien persistent dans le Portlandien, d’autres, appartenant aux genres Cytherelloidea, Paracypris, Fabanella, Galliaecytheridea, Klieana, Paraschuleridea, Eocytheridea, Paranotacythere, Procytheropteron, Rectocythere et Macrodentina font leur apparition. La subdivision de biostratigraphique est rendue difficile par le provincialisme provoqu´e par la baisse de la salinit´e et le changement des faci`es pendant le Portlandien sup´erieur. Des genres marins tels que Protocythere, Macrocypris, Paraschuleridea, Paranotacythere, Procytheropteron et Rectocythere ont e´ t´e remplac´es par des formes euryhaline, telles que les esp`eces de Fabanella et Mantelliana et les esp`eces dulc¸aquicoles-oligohalines des genres Cypridea, Scabriculocypris, Alicenula et Rhinocypris. © 2007 Ian P. Wilkinson. Published by Elsevier Masson SAS. All rights reserved. Keywords: Kimmeridgian; Portlandian; Ostracods; Biostratigraphy; Palaeoecology; Great-Britain Mots cl´es : Kimm´eridgien ; Portlandien ; Ostracodes ; Biostratigraphie ; Pal´eo´ecologie ; Grande-Bretagne

1. Introduction There was a major change in the environment of the southern England at the close of the Jurassic and the succession pre-

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serves evidence of the passage from fully marine through to the fresh-oligohaline conditions. The change from a biostratigraphy based on marine and brackish marine faunas through to the fresh-oligohaline, Cypridea-rich assemblages of the Weald Clay presents difficulties in dating the transitional phase. Three cored boreholes at Hartwell (near the classic Bugle Pit locality, National Grid Reference SP7926 1223), Tisbury (at the eastern

0035-1598/$ – see front matter © 2007 Ian P. Wilkinson. Published by Elsevier Masson SAS. All rights reserved. doi:10.1016/j.revmic.2007.08.005

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Fig. 1. Sketch map of southern England showing the localities mentioned in the text. Fig. 1. Carte sch´ematique du Sud de l’Angleterre montrant les localit´es mentionn´ees dans le texte.

end of the Vale of Wardour, Dorset, National grid reference 9359 2907) and Fairlight (a short distance from Hastings, National Grid Reference TQ8592 1173) (Fig. 1) have been studied to determine the distribution of ostracods through the latest Kimmeridgian and Portlandian of southern England. These three successions are used to subdivide the succession on the basis of their ostracod content and results are compared with other key published localities at Portesham, the Dorset Coast, Bugle Pit and Swindon. The top of the Kimmeridgian succession in southern England is more complete than that of eastern England, where erosion has removed the succession above the pectinatus Zone. However, the higher ostracod zones in East Anglia and Lincolnshire (Wilkinson, 1983a, 1983b) can be recognised in the lower part of the succession examined for the present study. 1.1. Previous works 1.1.1. Upper Kimmeridgian It was not until the late 1950s and 1960s that detailed investigations on British Kimmeridgian ostracods began. Taxa from the stratotype area were systematically described by Malz (1958) who described several new species, principally members of the genus Macrodentina. Neale and Kilenyi (1961) described species of Mandelstamia, and Kilenyi (1965) erected Oertliana (a junior synonym of Dicrorygma Poag, 1962). However, the most important studies on British Kimmeridgian ostracod was published by Kilenyi (1969) and Christensen and Kilenyi (1970). Kilenyi (1969), showed that the upper part of the Kimmeridge Clay Formation yielded abundant,

but low diversity, ostracod faunas characterised by Galliaecytheridea spinosa (Kilenyi, 1969), Galliaecytheridea polita (Kilenyi, 1969), Dicrorygma (O.) brotzeni Christensen, 1965, Prohutsonia pustulata (Kilenyi, 1969) and, at some horizons in the upper part of the sequence, Hechticythere serpentina (Anderson, 1941) and Klentnicella nealei (Kilenyi, 1969). Christensen and Kilenyi (1970) and Christensen (1974) developed a biostratigraphical framework and attempted to correlate across north-western Europe, recognising four ostracod zones in the Upper Kimmeridgian: Galliaecytheridea compressa Zone, G. polita Zone, G. spinosa Zone and Mandelstamia maculata Zone. They encountered difficulties due to the low diversity and patchy distribution, which prevented the accurate location of zonal boundaries. 1.1.2. Portlandian Whilst describing the fresh-water ostracods from the “Purbeckian” of southern England, Jones (1885) described (by original designation) Cythere retirugata, C. retirugata var. rugulata, and C. transiens (Malz, 1958, corrected the generic assignment in his classic work on the genus Macrodentina). The stage was not examined further until Anderson (1941) recognised a number of species at Swindon, including by original designation, Cythere retirugata var. rugulata, C. retirugata var. textilis and Cytheridea politula, as well as several new species and varieties: Cytheridea retirugata var. decorata, Cytheridea visceralis, Cythereis serpentina, C. euscarca and Cytherella?decipiens. The taxonomy of these species was subsequently modified by Barker (1966a, 1966b) and Anderson (1985).

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By far the most important work on Portlandian ostracods is the one of Christensen and Kilenyi (1970). In Britain, it is that by Barker (1966a, 1966b), who described faunas from Dorset and Aylesbury. He noted that in Dorset, the first occurrence of typical Portlandian species was in the uppermost part of the Kimmeridge Clay, 0.3 m below the Massive Bed at Hounstout Cliff and Black Nore Sandstone at West Wear Cliff. This fauna included Macrodentina (M.) transiens (Jones, 1885), Macrodentina (D.) retirugata (Jones, 1885), Macrodentina (P.) rudis Malz, 1958, G. compressa Christensen and Kilenyi, 1970, Paranotacythere (U.) rimosa (Martin, 1940), P. (U.) levis (Barker, 1966a), Prohutsonia elongata (Barker, 1966a), Galliaecytheridea postrotunda Oertli, 1957 (sensu Barker, 1966a), H. serpentina and Eocytheridea eusarca (Anderson, 1941). The Portland Stone has six species in common with the Portland Sand, but Macrodentina (M.) rugulata (Jones, 1885) and Procytheropteron bicostata Barker, 1966a, are peculiar to it and, significantly, G. compressa, M. (P.) rudis and P. elongata were not present (Barker, 1966a). The influence of the decreasing salinities on the ostracods during the Late Portlandian (“Purbeckian”) was recorded by Barker (1966b). Fully marine forms (e.g. species of Protocythere, Macrocypris, Paraschuleridea, Paranotacythere and Procytheropteron) were gradually replaced by euryhaline forms, such as species of Fabanella and Mantelliana, and finally fresh-oligohaline species of the genera Cypridea, Klieana, Scabriculocypris, Alicenula and Rhinocypris. Anderson developed the concept of salinity control in his ‘faunicycles’ in the non-marine Cretaceous of southern England (Anderson, 1985, and references therein), a concept further discussed by Horne (1995, 2002) who concluded that they were unusable. 1.2. Stratigraphy A number of different definitions for the Kimmeridgian and Portlandian have been proposed, resulting in confusion where the epithets sensu anglico and sensu gallico are not used. Throughout this paper, the terms are used sensu anglico. The modern concept of the Kimmeridge Clay Formation is based on the virtually unbroken sequence seen on the Dorset coast between Black Head and Ringstead Bay and between Brandy Bay and Chapman’s Pool. Webster (1816) whilst mapping the Isle of Wight and Purbeck, first referred to these deposits as “Kimmeridge Strata”. The earliest use of the Kimmeridgian as a stage was by d’Orbigny (1842–1851: p. 610) who coined the term “l’´etage kimm´eridgien” for that period of time during which the Kimmeridge Clay was deposited. In Britain, this is generally considered to be between incoming of Pictonia baylei and the extinction of Virgatopavlovia fittoni. The Upper Kimmeridge Clay is divided into 27 beds on the basis of lithology, palaeontological characteristics and geophysical log signatures (Gallois, 2000; Gallois and Etches, 2001). D’Orbigny (1842–1851) defined the Portlandian as the highest stage of the Jurassic with its type section in Dorset. His concept of “l’´etage portlandien” included the Portland Sand and Portland Stone and he listed the diagnostic macrofauna as, by original designation, Ammonites giganteus, Ammonites

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irius and Trigonia gibbosa. Salfeld (1913) defined the base of the Portlandian stage on the incoming of Gravesia gravesiana and, as a result of his work, it became apparent that d’Orbigny had erroneously correlated Kimmeridgian limestones in France (containing Gravesia) with the limestones of the Portland Stone of Dorset (Arkell, 1946). Cope (1978) and Wimbledon and Cope (1978) have fixed the base of the Portlandian (sensu anglico) at the base of the Progalbanites albani Zone at Hounstout Cliff, Dorset. The top of the stage is more difficult to locate in southern England, as the Portlandian/Berriasian boundary is within the non-marine lower part of the Purbeck facies, although it has been defined using charophytes and ostracods (Feist et al., 1995). 2. Stratigraphical distribution of ostracods 2.1. Tisbury borehole 2.1.1. Stratigraphy Although the Tisbury borehole shows some similarities to the Dorset coast, there are several notable differences, the most noticeable being the absence of oil shales and the absence of the Massive Bed (Fig. 2). The stratigraphy is interpreted as follows (after Wimbledon, 1976). Portland Sand Formation

Kimmeridge Clay Formation

Tisbury Member (top not seen) Chicksgrove Member Wardour Member Base not seen

0–9.0 m 9.0–13.9 m 13.9–28.87 m 28.87–126.70 m

The top of the hudlestoni Zone (Late Kimmeridgian) is placed at 125.82 m where interbedded mudstones and oil shales (Bed KC45) changes to paler grey mudstones with only thin oil shales (Bed KC46). A thin cement stone at this horizon is taken to be the lateral equivalent of the White Stone Band. Gallois and Etches (2001) correlated the base of the silty interval, at 91.50 m, with the Chapman’s Pool Pebble Bed, situated at the base of Kimmeridge Clay Bed KC55 of the coastal section, at the base of the rotunda Zone. Deposits of rotunda age are traced through to a depth of 50.83 m, herein, although the Kimmeridge Clay beds of Dorset cannot be recognised with confidence. The fittoni Zone is inferred to be situated between 48.20 and 38.75 m, on the basis of the ostracod assemblage, although on lithological evidence the top of the Kimmeridge Clay Formation is slightly higher. Between 38.75 and 36.7 m, the silty Kimmeridge Clay becomes more arenaceous and passes up through siltstone, sandy siltstone and silty sandstone. Although barren of microfaunas, this sandy interval (at least through to 31.7 m) contains dinoflagellate cysts indicative of a fittoni age (Riding, 1993). The base of the Portland Sand Formation is placed at 28.87 m where the Upper Lydite Bed can be recognised (Gallois and Etches, 2001) and representing a break in sedimentation (sequence boundary K10 of Wignall, 1991). Much of the Wardour Member comprises sandstone with occasional thin limestones, but the upper part is a bioturbated siltstone. Much of this interval is believed to be of albani age, although the highest part may be of glaucolithus age (Wimbledon, 1976; Coe,

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Fig. 2. Distribution of ostracods through the Upper Kimmeridgian and Portlandian of Tisbury borehole. M: Tisbury Member; CL: Chicksgrove Limestone Member; WSB: White Stone Band; WM: Wardour Member; ULB: Upper Lydite Bed; CPPB: Chapman’s Pool Pebble Bed. Fig. 2. R´epartition des ostracodes dans le Kimm´eridgien Sup´erieur et le Portlandien du sondage de Tisbury.

1996). The 4.0 m of micrite that overlies the Wardour Member is equivalent to the Chicksgrove Limestone Member of the Vale of Wardour, where it is of glaucolithus age (Wimbledon, 1976). The Tisbury Member at the top of the borehole comprises calcareous sandstone, with occasional bioturbated siltstones. 2.1.2. Ostracod distribution (Fig. 2) Ostracods have a patchy distribution throughout the Tisbury borehole, very rare or absent in some intervals and common in others. It is assumed that this is a function of the environment of deposition and low oxygenated bottom water conditions (Wilkinson, 1983b). Although the lowest samples proved barren, or yielded very rare specimens, an ostracod assemblage of ten species was found at a depth of 122.65 m. Dicrorygma, including D. brotzeni, and Micrommatocythere reticulata nov. sp. dominated the fauna, but rare specimens of Mandelstamia tumida (Christensen and Kilenyi, 1994) are present and very rare specimens of Paralesleya perforata (Witte and Lissenberg, 1991) were also recorded. Comparison with the coastal sequence and boreholes in eastern England indicates that the inception of D. brotzeni is in the late pectinatus Zone and M. tumida is restricted to the same Ammonite Zone. P. perforata has been recorded in the rotunda/fittoni zonal boundary interval of Dorset, in the highest Argiles de Wimereux and basal Assises de Croi of the Boulonnais (?latest pectinatus to pallasioides/rotunda zones) and in the Upper Kimmeridgian deposits off Denmark and the Netherlands

(Witte and Lissenberg, 1994). Its presence at 122.65 m and again at 48.2 and 45.4 m, implies a late pectinatus to early fittoni age for this interval. It is surprising that the ostracod zonal index for the late hudlestoni to early fittoni zones, G. spinosa, does not occur between the samples at 122.65 and 76.30 m in the borehole, but this may be due to palaeoenvironmental considerations. The 119.9 m to 87.20 m interval yielded very sparse assemblages. Rare specimens of Pseudohutsonia pustulata (Kilenyi, 1969) were found in a number of samples above 115.85 m, Paranotacythere (U.) caputmortuum (Martin, 1957) was present at 96.4 m and Schuleridea moderata Christensen and Kilenyi, 1970, was recorded in a number of samples above 96.4 m. It is not possible to place an age on this interval with certainty, although, P. pustulata ranges from the pectinatus through to the late rotunda zones in Dorset (its inception is within the hudlestoni Zone in eastern England). P. (U.) caputmortuum extends throughout the Late Kimmeridgian; its inception is in the autissiodorensis Zone in Germany (Schudack, 1993, 1994) and it ranges up into the earliest Portlandian in the southern North Sea Basin (Witte and Lissenberg, 1994), but it has only been recorded in the hudlestoni Zone of England (Wilkinson, 1983b). A sudden influx of species occurs between 84.25 and 81.00 m. S. moderata occurs in abundance, H. serpentina is common and A. inornata (Kilenyi, 1969), Aaleniella gracilis (Christensen and Kilenyi, 1970), Macrodentina (M.) sp. cf. transiens and K. nealei are also present. This is clearly an important biostratigraphical event and the fauna resembles that within the rotunda Zone of

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Dorset (Christensen and Kilenyi, 1970). G. spinosa is consistently present between 76.30 and 52.3 m, dominating the faunas at several horizons. This assemblage is similar to that described from the late rotunda Zone of the Dorset Coast (Kilenyi, 1969; Christensen and Kilenyi, 1970) and the younger part of the Late Kimmeridgian in the offshore boreholes of the southern North Sea Basin (Witte and Lissenberg, 1994). G. spinosa continues to dominate faunas between 48.20 and 38.19 m, with common H. serpentina. Several other species appear for the first time in the borehole: Procytheropteron brodei (Jones, 1894), Procytheropteron cf. prolongatum (Sharapova, 1939) and Macrodentina sp. appear at 48.20 m, P. bicostata at 45.40 m (in the same sample as the last specimens of Dicrorygma brotzeni), and Paranotacythere cf. rimosa at 40.50 m. These species are characteristic of the latest Kimmeridgian (late fittoni Zone) and Early Portlandian on the Dorset coast. Barker (1966a) recorded them from the uppermost 0.3 m of the Kimmeridge Clay, just below the Massive Bed and Black Nore Sandstone. G. polita and G. compressa (characteristic of the ‘mid’ and ‘late’ fittoni Zone of the coastal sequence) were not encountered in the Tisbury borehole. The reason for this is unclear at the present time, but is probably due to removal of the upper part of the fittoni Zone by erosion. The occurrence of E. eusarca at 38.19 m is interpreted as indicating a late Fittoni age, by comparison with other areas of southern Britain. It is unfortunate that the highest Kimmeridge Clay and Lower Portland Sand Formation (Wardour Member) proves to be barren of ostracods, although wood chips, moulds of molluscs, echinoid spines and fish debris together with very rare agglutinating foraminifera were observed. Most of the Chicksgrove Member is similarly barren of ostracods, although a small fauna was found at 10.35 m, dominated by Macrodentina retirugata and E. eusarca, but also including Procytheropteron barkeri, ?Paranotacythere (U.) levis (a single partly decalcified carapace) and a number of indeterminate fragments. This fauna is considered to be of Portlandian age and consistent with the glaucolithus age of the type area. The presence of E. eusarca is considered significant biostratigraphically. 2.2. Fairlight borehole 2.2.1. Stratigraphy (Fig. 3) Deposits across the Kimmeridgian/Portlandian boundary interval do not crop out in south-eastern England. The stratigraphy can be summarized as follows. Lulworth Formation Portland Formation Kimmeridge Clay Formation

Gypsiferous Member (top not seen) Portland Stone Member equivalent Portland Sand Member equivalent Upper part, base not seen

320.0–339.9 m 339.9–352.4 m 352.4–373.6 m 373.6–396.8 m

Unlike Dorset, where the Kimmeridge Clay Formation/Portlandian Formation boundary is placed at the base of the arenaceous Massive Bed, in south-eastern England coeval sediments are predominantly argillaceous throughout. As the sequence is dominated by clays, silty clays and siltstones, the position of the Kimmeridgian/Portlandian boundary is difficult

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to locate lithologically and it can best be described as transitional. The Rotunda Nodule Bed is recognised at 391.9 m depth and the base of the Purbeck Formation can be placed at an erosion surface at 339.91 m. The lack of ammonite control in the Gypsiferous Member prevents an accurate assessment of its age, but Anderson argued that part of the sequence is missing here, so that the Warren Faunicycle (sensu Anderson, 1985 and references therein) rests on either the okusensis or kerberus zones. Thus, the anguiformis and opressus zones, as well as the Quainton Faunicycle, are missing. The macrofauna and stratigraphy of the Wealden and Purbeck beds of the Fairlight borehole has been discussed in detail by Morter (1981). On macrofaunal grounds, the base of the Portlandian is placed at 373.5 m depth, immediately below a more sandier interval. The macrofauna of the borehole is dominated by bivalves, but other groups are also represented (e.g. gastropods, brachiopods, serpulids, crustacea and fish). Ammonites are rare and fragmentary due to the diameter of the core. Those below 394.7 m can be placed within Pavlovia; those between 367.6 and 368.5 m depth are assigned to Epivirgatites and Glaucolithites has been recorded from the 358.1 to 368.5 m interval (Cox, 1976). 2.2.2. Ostracod distribution Ostracod diversity through the late Kimmeridgian and Portlandian of the borehole is generally low (Fig. 3). However, the Fairlight borehole, although imperfect in terms of distribution of ostracods, brings the British range of several important species more in line with those of mainland Europe. The three samples taken below the presumed Rotunda Nodule Bed yielded at best seven species of ostracod, with H. serpentina and Paracypris problematica (Kilenyi, 1969), dominating the faunas. Other biostratigraphically useful species include D. brotzeni, P. pustulata, K. nealei and G. spinosa. The occurrence of H. serpentina is surprising here because elsewhere in southern England its first up sequence appearance is closer to the top of the Kimmeridge Clay (Barker, 1966a; Christensen and Kilenyi, 1970). However, the remainder of the fauna is as expected for the top of the G. spinosa ostracod Zone (top of the pallasioides Zone). Immediately overlying the “Rotunda nodule Bed” (at a depth of 389.92 m) the fauna is much reduced in diversity and richness. Single specimens of P. pustulata and G. spinosa were encountered and the stratigraphically lowest specimen of M. (D.) retirugata was also found in the sample. Within an interval generally barren of ostracods, rare specimens of G. polita were encountered (at 382.83 and 379.78 m). This is an important record for it is the index of the eponymous ostracod zone which equates with the mid-fittoni Zone of the Dorset coastal sequence (Kilenyi, 1969, 1978; Christensen and Kilenyi, 1970). It has also been recorded in the Dutch Sector of the Southern North Sea Basin (Witte and Lissenberg, 1994) and in the Danish Embayment (Christensen and Kilenyi, 1970; Christensen, 1988). Thus, although rare, it is biostratigraphically important. The albani Zone (Portlandian) (between 373.53 and 367.51 m) is, unfortunately, generally devoid of ostracods. However, the highest sample examined (at 367.59 m) yielded a small

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Fig. 3. Distribution of ostracods through the Upper Kimmeridgian and Portlandian of Fairlight borehole. The T. fittoni Zone (C. dunkeri Subzone) was placed in the Warren Faunicycle by Anderson (1985). RNB: Rotunda Nodule Bed. For lithological ornament, see Fig. 2. Fig. 3. R´epartition des ostracodes dans le Kimm´eridgien Sup´erieur et le Portlandien dans le sondage de Fairlight. La Zone a` T. fittoni (Sous-zone a` C. dunkeri) a e´ t´e plac´ee dans Faunicycle Warren par Anderson (1985). RNB : Rotunda Nodule Bed. Pour la l´egende lithologique, voir Fig. 2.

fauna of H. serpentina and M. (D.) retirugata, a crushed carapace of Cytherelloidea sp. cf. paraweberi (Oertli, 1957) (sensu Barker, 1966a) and G. compressa. This last named species was used as a zonal index by Christensen (1974) equating with the latest Fittoni and early Portlandian. The glaucolithus Zone (between 367.51 and 356.35 m) yielded abundant ostracods, particularly H. serpentina, M. (D.) retirugata, G. compressa, and less common P. (U.) caputmortuum and P. elongata. The first three listed continue from below and the G. compressa Ostracod Zone is indicated. P. (U.) caputmortuum has been widely reported from the Upper Kimmeridgian and Lower Portlandian (sensu anglico) deposits in Germany, northern France and the Dutch Sector of the Southern North Sea. In Britain, it has previously been recorded only as rare fragments to date, but it is common in the sample from 364.54 m of the Fairlight borehole. P. elongata

has a broad geographical range, being present in the “Upper Kimmeridgian or Lower Portlandian” of the Dutch Sector of the Southern North Sea Basin (Witte and Lissenberg, 1994). The stratigraphically highest British occurrence of D. (O.) brotzeni is in the Fairlight borehole, at 364.54 m, for although it has been recorded from the Portlandian in the North Sea Basin and Danish Embayment, in southern England it has not been noted above the Rotunda Zone. Ammonite control is wanting in the Portland Stone (Cox, 1976) and ostracods were found in only two samples from this part of the sequence. A single specimen of M. (M.) transiens was recovered at 343.2 m, but a moderate fauna was found at 340.16 m. This, the highest sample to yield a marine fauna, contained common M. (M.) transiens, P. (U.) rimosa, P. (U.) caputmortuum and H. serpentina. Euryhaline taxa, such as Fabanella, were absent and fully marine shallow water condi-

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tions are interpreted. Species recorded are typical of the Upper Portlandian okusensis–kerberus zones elsewhere in southern England and show several similarities to contemporaneous assemblages in the southern North Sea and Germany. The base of the Purbeck Beds is placed at 339.91 m. The incoming of Damonella ellipsoidea (Wolburg, 1962) with Fabanella boloniensis (Jones, 1882), Mantelliana purbeckensis (Forbes, 1855) and Cypridea dunkeri (Jones, 1855), at 338.36–338.44 m is considered to indicate the Warren Faunicycle (Anderson, 1985). If this interpretation is correct, then the basal (Quainton) faunicycle is either exceedingly thin (and thus unexamined) or missing. The Theriosynoecum forbesii Zone (sensu Horne, 1995) was not recognised. Overlying samples failed to yield ostracods until 22 m higher, where taxa are characteristic of the Berriasian Upway faunicle, so that the position of the Portlandian / Berriasian boundary is unclear in this borehole.

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2.3. Hartwell borehole and Bugle Pit 2.3.1. Stratigraphy The borehole was cored, below 2.20 m, with excellent recovery below 2.85 m, and was terminated in the Oxfordian Ampthill Clay Formation. The upper part of the borehole was examined for the present study (Fig. 4).

‘Purbeck Formation’ Portland Formation Kimmeridge Clay Formation

Portland Stone Member Portland Sand Member Hartwell Silt Member Swindon Member Elmhurst Silt Member Watermead Clay Member

2.85–5.16 m 5.16–12.19 m 12.19–17.95 m 17.95–29.45 m 29.45–35.57 m 35.57–43.65 m 43.65–47.20 m

Fig. 4. Distribution of ostracods through the Upper Kimmeridgian and Portlandian of Hartwell borehole. Note: As the succession here is conformable, this interval may be of anguiformis age. The ostracod fauna here and in similar beds in the Bugle Pit appear to be related to the Quainton Faunicycle of Anderson (1985), but older than in Dorset, where the Quainton Faunicycle rests on the oppressus Zone. WCM: Watermead Clay Member; pen: “Pendle”; usm: “Upper Shelly Marl”; cl: Creamy Limestone Bed; cs: Crendon Sand Bed; arl: Aylesbury Rubbly Limestone Bed; gl: Glauconitic Bed; ul: Upper Lydite Bed; ll: Lower Lydite Bed. Fig. 4. R´epartition des ostracodes dans le Kimm´eridgien Sup´erieur et le Portlandien du sondage d’Hartwell. Note : cet intervalle pourrait appartenir a` la Zone a` anguiformis. La faune d’ostracodes ainsi que celle de Bugle Pit apparaˆıt comme appartenant au Faunicycle Quainton d’Anderson (1985), mais plus ancienne que celle du Dorset o`u le Faunicycle Quainton repose sur la Zone a` oppresus. WCM : Watermead Clay Member ; pen : « Pendle » ; usm : « Upper Shelly Marl » ; cl : Creamy Limestone Bed ; cs : Crendon Sand Bed ; arl : Aylesbury Rubbly Limestone Bed ; gl : Glauconitic Bed ; ul : Upper Lydite Bed ; ll : Lower Lydite Bed.

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The Upper Kimmeridge Clay in the Hartwell borehole differs from that of the Dorset Coast in that it is in part much siltier. It can be subdivided on a combination of lithology and macrofaunal content into members (Oates, 1991). The Hartwell Silt Member and Emhurst Silt Member are separated by the argillaceous Swindon Clay Member. The Hartwell Silt and Swindon Clay members are of pallasioides age and the Elmhurst Silt falls within the pectinatus Zone (Cox et al., 1994). The periods of erosion associated with the Upper Lydite Bed at 17.95 m and the Lower Lydite Bed at 35.57 m coincide with K9 (pectinatus Zone) and P1 (glaucolithus Zone) sequence boundaries of Wignall (1991) and Coe (1996). The lowest part of the formation discussed herein is the Watermead Clay Member (Kimmeridge Clay Beds 44–45 of Gallois and Cox, 1974; Cox and Gallois, 1979, 1981), between 47.20 and 43.65 m, which is assigned to the hudlestoni Zone (Cox et al., 1994). The ‘Purbeck Formation’ comprises an alternation of thin limestones and marls, becoming siltier and sandier towards the base. The definition and nomenclature of this formation is under examination and modifications will be proposed by Hopson et al. (in press), but in the meantime the traditional name is retained. A laminated marl crowded with ostracods (known as “Pendle” by quarrymen) is situated between 4.61 and 4.99 m. This has generally been taken to be the base of the ‘Purbeck Formation’ (e.g. Radley, 1991), but it is underlain by a 0.17 m thick mudstone (the Upper Shelly Marl of Radley, 1991), which is considered to form the base of the ‘Purbeck Formation’ by some (Cox et al., 1994; Horton et al., 1995). The ‘non-marine’ ‘Purbeck Formation’ cannot be related to the marine macrofaunal zonation directly, but the continuous sedimentation across the Portland / Purbeck formational boundary and the fact that the stratigraphical highest Portlandian deposits are of kerberus age, may indicate an anguiformis age for the Pendle. The ‘non-marine’ sequence is older here compared to the Dorset coastal sequence (Wimbledon, 1980; Cox et al., 1994). It is not possible from the small diameter core to recognise the Portlandian ammonite zones per se, but the lithological characteristics of the borehole permit extrapolation with nearby quarries where calibration is possible. Thus, the Creamy Limestones, Crendon Sands and upper part of the Aylesbury Limestone (sometimes called Rubbly Limestone) is considered to be of kerberus age. Much of the Aylesbury Limestone can be placed into the okusensis Zone and although the Glauconitic Beds of the Portland Sand Member are predominantly of glaucolithus age, the uppermost part may fall within the earliest okusensis Zone. 2.3.2. Ostracod distribution (Fig. 4) The Holman’s Bridge Shale Member (Wheatleyensis Zone; Upper Kimmeridgian) with its flood proportions of M. (X.) maculata (Kilenyi, 1961) (a characteristic species throughout southern and eastern England), is overlain by the Watermead Clay Member. Its lower bed (KC44) (at 46.25–46.38 m) yielded a fauna dominated by S. moderata (56.48%) and P. pustulata (31.61%), but included smaller numbers of D. brotzeni (3.63%), M. reticulata (1.55%) as well as the zonal index Eocytheropteron

aquitanum (6.74%). This fauna compares closely with that of the E. aquitanum Ostracod zone of eastern England (Wilkinson, 1983a, 1983b). Unfortunately, the succeeding sample (44.0 m) from Bed KC45 proved much less fossiliferous, although a single specimen of P. pustulata indicates a relationship with the association in Bed KC44 elsewhere in southern and eastern England. The Elmhurst Silt Member (pectinatus Zone) lacked ostracods. However, sample of the Swindon Clay Member contained a small fauna; P. pustulata and D. brotzeni continued up from below, but rare specimens of M. rudis, M. foveolata and G. spinosa were also present. The last named species, which was recorded at 30.0 m, proves the presence of the eponymous ostracod zone, which is seen to extend up into the Hartwell Silt Member where the species was again found (at 24.0 m). The larger part of the Glauconitic Beds of the Portland Sand Member (between 17.50 and 15.0 m depth) was devoid of ostracods. At the very top of the member (12.90 m depth), where the proportion of glauconite was greatly reduced, a sparse fauna was recorded. Here, at a level that probably falls within the earliest part of the okusensis Zone, G. compressa makes its first appearance in the borehole. This ostracod zonal index is accompanied by H. serpentina, M. (M.) transiens, Paraschuleridea buglensis Barker, 1966b, Rectocythere (L.) visceralis (Anderson, 1941) and P. barkeri (Anderson, 1971), species that extend through the Portlandian. A number of these species (H. serpentina, M. (M.) transiens and G. compressa) have their origins in the fittoni Zone, on the Dorset coast, but that part of the sequence is missing here due to erosion. Others (P. buglensis, R. (L.) visceralis and P. barkeri) are more restricted in their geographical distribution, but have been recorded in the Portland Stone Member. At the base of the Portland Stone Member, within the Aylesbury (or ‘Rubbly’) Limestone (11.90 m depth), H. serpentina, R. (L.) visceralis, M. (M.) transiens and ?P. buglensis were again recovered, together with the first representative of P. bicostata in the borehole. The last named species has been recorded from the Upper Kimmeridge Clay (fittoni Zone) on the south coast of England and ranges up through much of the Portlandian. Essentially similar faunas were also recorded in the overlying Crendon Sand (at 9.80 m) and Creamy Limestones (6.0 m) (of the Portland Stone Member), both of which are considered to be of kerberus age. There is a notable increase in the numbers of specimens up-sequence in the Portland Stone Member, but the faunas are essentially marine or brackish marine in composition, euryhaline species were not seen. The Pendle, near the base of the Purbeck Formation contained abundant ostracods, which, for the first time in the borehole (at 4.99 m), indicate reducing salinities. A few species (but almost 50% of the specimens) range up from below, e.g. Paracypris sp. (0.3%), P. buglensis (4.6%), H. serpentina (5%) and P. barkeri (37.5%). However, the largest number of taxa (and a little over 50% of the specimens) appear for the first time: M. (M.) rugulata (15%), M. (D.) retirugata (17%), P. (U.) rimosa (10%), Macrocypris alexanderi nov. sp. (3.6%). Also appear-

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Fig. 5. Distribution of ostracods through the Portlandian of Bugle Pit, Hartwell, near Aylesbury (after Barker, 1966b). Vertical scale in metres. Pe: Pendle; usm: Upper Shelly Marl. ´ Fig. 5. R´epartition des ostracodes dans le Portlandien de Bugle Pit, Hartwell, pr`es d’Aylesbury (d’apr`es Barker, 1966). Echelle verticale en m`etres. Pe : Pendle ; usm : Upper Shelly Marl.

ing for the first time in the borehole is F. boloniensis (7%), a species that is euryhaline in nature and the harbinger of reducing salinities. The fauna is interpreted as brackish marine, true fresh water taxa such as Cypridea and Alicenula are not present even in the highest sample (at 3.0 m) where the limestones yielded a small fauna comprising Klieana alata (Martin, 1940). Although species of Cypridea, Mantelliana Scabriculocypris, Damonella and Rhinocypris had not entered the region at this time (?Anguiformis Zone), the faunas show a resemblance to the Quainton Faunicycle of Anderson (1985) and references. Hartwell borehole was sited adjacent to the old, and now infilled, Bugle Pit, from where Barker (1966b) recovered an important fauna (Fig. 5). The assemblage from the Crendon Sand and Creamy Limestone beds are essentially similar to those in the Hartwell borehole. One noticeable difference, however, is that F. boloniensis appears at the base of the Creamy Limestone, lower than its first record in the borehole, and K. alata also has a stratigraphically lower first occurrence. Significantly, immediately above the “Pendle”, M. purbeckensis and Cypridea tumescens praecursor (Oertli, 1963), characteristic of the Quainton Faunicycle (of Anderson, 1985) appear for the first time. A little higher in the pit, C. dunkeri, Alicenula leguminella (Forbes, 1855), Rhinocypris jurassica (Martin,

1940), Stenostroemia fragilis (Martin, 1940) and S. decipiens (Anderson, 1941) occur, characteristic of the Warren Faunicycle. T. forbesii (Jones, 1855), the index for the eponymous zone of Horne (1995), had not entered the record at this point. The stratigraphically lowest faunicycle sensu Anderson (1985), the Quainton Faunicycle, overlies the oppressus Zone in Dorset. However, in the Hartwell borehole and in the Bugle Pit, these faunas appear in deposits immediately and conformably overlying the kerberus Zone. This appears to be a conundrum, but it should be remembered that Anderson’s faunicycles were always defined in terms of the palaeoenvironmental conditions and that any biostratigraphical value was secondary to this. Clearly Quainton- and Warren-environments were established at an earlier date in Buckinghamshire compared to Dorset, but it is not surprising that similar ecological niches contain similar associations. The diachronous nature of the earliest Purbeck facies has been pointed out several times (Allen and Wimbledon, 1991; Cox et al., 1994; Horne, 2002). 3. Biostratigraphy Although late Kimmeridgian and Portlandian ostracods from southern England have been examined a number of times from

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Fig. 6. Distribution of ostracods along the Dorset coastal section (composite) after Kilenyi (1969), Christensen and Kilenyi (1970), Barker (1966a), Anderson (1985) and unpublished data. O–P–P: oppressus–primitivus–preplicomphalus zones; Tf: Theriosynoecum fittoni Zone. Fig. 6. R´epartition des ostracodes le long de la cˆote du Dorset (composite), d’apr`es Kilenyi (1969), Christensen and Kilenyi (1970), Barker (1966a), Anderson (1985), et donn´ees non publi´ees. O–P–P : zones a` oppressus–primitivus–preplicomphalus zones ; Tf : Zone a` Theriosynoecum fittoni.

a taxonomic point of view, biostratigraphical application has been more limited. The stratigraphical distribution of ostracods in the Fairlight, Tisbury and Hartwell boreholes can be related to Dorset (Barker, 1966a; Christensen and Kilenyi, 1970; Christensen, 1974; Barker et al., 1975) (Figs. 6 and 7), Swindon (Sylvester-Bradley, 1941; Anderson, 1941) (Fig. 8) and Ayles-

bury (Barker, 1966b) (Fig. 9) and placed into a biostratigraphical framework. This biostratigraphy builds on and updates that used by Christensen and Kilenyi (1970) and Kilenyi (1978) as a result of the inclusion of further data and can be related, in part, to the ostracod distribution and biostratigraphy recognised in eastern England (Wilkinson, 1983a, 1983b).

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Fig. 7. Distribution of ostracods through the Portlandian (basal Purbeck) succession at Portesham, southern Dorset (after Barker et al., 1975). Fig. 7. R´epartition des ostracodes dans le Portlandien (Purbeckien basal) de Portesham, Sud du Dorset (d’apr`es Barker et al., 1975).

Fig. 8. Distribution of ostracods through the Portlandian and Berriasian succession at Swindon (lithostratigraphy and ostracod data after Sylvester-Bradley, 1941; Anderson, 1941). Fig. 8. R´epartition des ostracodes dans le Portlandien et le Berriasien de Swindon (lithostratigraphie et ostracodes d’apr`es Sylvester-Bradley, 1941 ; Anderson, 1941).

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Definition: In Britain, the zone is defined by the total range of E. aquitanum. Remarks: The sparse faunas that Kilenyi (1969) found in Dorset led Christensen and Kilenyi (1970) to extend the upper boundary of the M. (X.) maculata Zone up to the first occurrence of G. spinosa, although M. (X.) maculata is not present above Kimmeridge Clay Bed 42. Wilkinson (1983a, 1983b) erected the E. aquitanum Zone to close this stratigraphical gap in eastern England. In Dorset, the index species is quite rare, although Kilenyi (1969) shows it to be present just above the Basalt Stone Band (i.e. in Kimmeridge Clay Bed 44 sensu Gallois and Cox, 1974). The species was recorded in the Hartwell borehole in the Watermead Clay Member of the Kimmeridge Clay Formation. Other species that appear for the first time in the ostracod zone include M. reticulata and P. pustulata. M. reticulata is synonymous with Monoceratina sp. 1 of Kilenyi (1969). The zone can be recognised in southern and eastern England and the southern North Sea. 3.2. G. spinosa Zone (Fig. 10)

Fig. 9. Relationship between ostracod and macrofaunal zonal schemes and lithostratigraphy. O–P–P: oppressus–primitivus–preplicomphalus ammonite zones. Fig. 9. Corr´elations entre les zonations d’ostracodes et de macrofaunes avec la lithostratigraphie. O–P–P : zones a` oppressus–primitivus–preplicomphalus.

3.1. E. aquitanum Zone Stratigraphical range: Early hudlestoni Zone. Typical species: E. aquitanum, P. pustulata, M. reticulata, S. moderata and, in eastern England, Macrodentina (P.) woottonensis.

Stratigraphical range: Kimmeridge Clay Bed KC48 to the upper part of the Kimmeridge Clay (late fittoni Zone) in Dorset. Kimmeridge Clay Beds 45–47 (pectinatus Zone) in Eastern England (higher beds were removed by erosion). Typical species: G. spinosa, D. (O.) brotzeni, M. reticulata, P. pustulata, S. moderata and M. (X.) tumida. In southern England P. problematica, H. serpentina, K. nealei, Aaleniella inornata and A. gracilis occur. M. (P.) woottonensis is confined to eastern England. Definition: The base of the zone is defined by the inception of G. spinosa and its upper boundary is recognised by the appearance of the succeeding zonal index, G. polita. The zone is divided into subzones based on the appearance of M. (X.) tumida, the co appearance of H. serpentina and M. (P.) rudis, and the total range of the genus Aaleniella (A. inornata and A. gracilis). Remarks: The LADs of S. moderata, M. (X.) tumida and M. (P.) woottonensis are in the lower part of the ostracod zone (probably within the Pectinatus Zone), the genus Aaleniella is restricted to the middle part of the rotunda Zone, and the FADs of P. problematica, K. nealei and H. serpentina are near the top

Fig. 10. Ostracods from the G. spinosa Zone. C: carapace; LV: left valve; RV: right valve. Scale bars: 100 ␮ except Fig. 10(7) and (8): 30 ␮. 1, 2. G. spinosa Kilenyi, 1969. Tisbury borehole (−55.7 m), Late Kimmeridgian, rotunda Zone. 1, C, right view, female, MPK10063. 2, C, left view, male. MPK10064. 3. Mandelstamia (Xeromandelstamia) tumida Christensen and Kilenyi, 1970. North Wootton borehole. RV, female, MPK10145. 4. H. serpentina (Anderson, 1941). C, left view, Fairlight borehole (−364.54 m), Portlandian, glaucolithus Zone, MPK10072. 5. P. pustulata (Kilenyi, 1969). C, left view, Fairlight borehole (−391.97 m), Late Kimmeridgian, Pallasioides Zone, MPK10070. 6. Dicrorygma (Orthorygma) brotzeni Christensen, 1965. C, left view, Tisbury borehole (−70.65 m), Upper Kimmeridgian, rotunda Zone, MPK10615. 7. M. reticulata nov. sp. C, right view, holotype, Hartwell borehole (−46.25−46.38 m), Upper Kimmeridgian, hudlestoni Zone, MPK10629. 8. A. (Danocythere) gracilis Christensen and Kilenyi, 1970. RV, Tisbury borehole (−81.0 m), Late Kimmeridgian, rotunda Zone, MPK10610. 9. M. (Polydentina) rudis Malz, 1958. LV, female, Hartwell borehole (−33.00 m), Late Kimmeridgian, pallasioides Zone, MPK10081. 10. A. (Danocythere) inornata (Kilenyi, 1969). C, right view, Tisbury borehole (−70.65 m), Late Kimmeridgian, rotunda Zone, MPK10085. Fig. 10. Ostracodes de la Zone a` G. spinosa. C : carapace ; VD : valve droite ; VG : valve gauche. Barres d’´echelle : 100 ␮ sauf Fig. 10(7) et (8) : 30 ␮. 1, 2. G. spinosa Kilenyi, 1969. Sondage de Tisbury (−55,7 m), Kimm´eridgien Sup., Zone a` rotunda. 1, C, vue droite, femelle, MPK10063. 2, C, vue gauche, mˆale. MPK10064. 3. Mandelstamia (Xeromandelstamia) tumida Christensen et Kilenyi. 1970. Sondage de North Wootton, VD, femelle, MPK10145. 4. H. serpentina (Anderson, 1941). C, vue gauche, sondage de Fairlight (−364,54 m), Portlandien, Zone a` glaucolithus, MPK10072. 5. P. pustulata (Kilenyi, 1969). C, vue gauche, sondage de Fairlight (−391,97 m), Kimm´eridgien Sup., Zone a` pallasioides, MPK10070. 6. Dicrorygma (Orthorygma) brotzeni Christensen, 1965. C, vue gauche, sondage de Tisbury (−70,65 m), Kimm´eridgien Sup., Zone a` rotunda, MPK10615. 7. M. reticulata nov. sp. C, vue droite, holotype, sondage d’Hartwell (−46,25 to −46,38 m), Kimm´eridgien Sup., Zone a` hudlestoni, MPK10629. 8. A. (Danocythere) gracilis (Christensen et Kilenyi, 1970). VD, sondage de Tisbury (−81,0 m), Kimm´eridgien Sup., Zone a` rotunda, MPK10610. 9. M. (Polydentina) rudis Malz, 1958. VG, femelle, sondage d’Hartwell (−33,00 m), Kimm´eridgien Sup., Zone a` pallasioides, MPK10081. 10. A. (Danocythere) inornata (Kilenyi, 1969). C, vue droite, sondage de Tisbury (−70,65 m), Kimm´eridgien Sup., Zone a` rotunda, MPK10085.

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of the ostracod zone (in the upper part of the rotunda Zone). The zone can be recognised in southern and eastern England and the southern North Sea. 3.3. M. (X.) tumida Subzone Stratigraphical range: Late hudlestoni and late pallasioides zones. Typical species: P. pustulata, D. (O.) brotzeni. Definition: The first upward appearance of M. (X.) tumida to the appearance of the succeeding subzonal indices. Remarks: The subzone can be recognised in southern and eastern England and the southern North Sea. 3.4. Hechticythere serpentine / M. (P.) rudis Subzone Stratigraphical range: Late pallasioides and early rotunda zones. Typical species: P. problematica, K. nealei, H. serpentina. Definition: The first upward appearance of H. serpentina and M. (P.) rudis to the appearance of the succeeding subzonal indices. 3.5. A. inornata / A. gracilis Subzone Stratigraphical range: Late rotunda Zone. Typical species: P. problematica, K. nealei, H. serpentina, M. (P.) rudis. Definition: The total range of Aaleniella in the British Kimmeridgian. Remarks: The subzone has been recognised only in southern England. 3.6. G. polita Zone (Fig. 11) Stratigraphical range: Mid to late fittoni Zone. Typical species: This is situated in a sparsely populated part of the Kimmeridge Clay in southern England. Definition: The total range of G. polita. Remarks: Christensen and Kilenyi (1970) used this species as a zonal indicator. The upper and lower zonal boundaries are, however poorly defined due to the scarcity of ostracods. The zone has been recognised in southern England and the Southern North Sea basin. 3.7. G. compressa Zone (Fig. 12) Stratigraphical range: Late fittoni to okusensis zones. Typical species: Cytherelloidea sp (=C. paraweberi sensu Barker, 1966b), E. eusarca, G. compressa, Galliaecytheridea sp. (=G. postrotunda sensu Barker, 1966a), H. serpentina, M. (D.) retirugata, M. (P.) rudis, M. (M.) transiens, P. (U.) rimosa, P. buglensis, P. barkeri and P. elongata. Definition: The base of the zone is defined by the inception of G. compressa and the upper boundary is defined by the inception of the superadjacent zonal index. Remarks: This zone was recognised by Christensen (1974) who quoted the range given by Barker (1966a), from just below

Fig. 11. Ostracods from the G. polita Zone. C: carapace; LV: left valve; RV: right valve. Scale bars: 100 ␮. 1–3. G. polita (Kilenyi, 1969). Fairlight borehole, Upper Kimmeridgian, fittoni Zone. 1, LV, female (−397.78 m), MPK10619. 2, C, right view, female (−382.83 m), MPK10620. 3, RV, male (−382.83 m), MPK10621. Fig. 11. Ostracodes de la Zone a` G. polita. C : carapace ; VD : valve droite ; VG : valve gauche. Barres d’´echelle : 100 ␮. 1–3. G. polita (Kilenyi, 1969). Sondage de Fairlight, Kimm´eridgien Sup., Zone a` fittoni. 1, VG, femelle (−397,78 m), MPK10619. 2, C, vue droite, femelle (−382,83 m), MPK10620. 3, VD, droite, mˆale (−382,83 m), MPK10621.

the Massive Bed on the Dorset coast, through to the top of the Portland Sand Formation. The zone can be recognised in southern England, North Sea and Danish embayment. 3.8. E. eusarca Subzone Stratigraphical range: Late fittoni to late albani zones. Typical species: As for the zone, except P. buglensis. Definition: The base is defined by the FAD E. eusarca and its upper boundary is defined by the incoming of the succeeding index. Remarks: The subzone has been recognised only in southern England.

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Fig. 12. Ostracods from the G. compressa Zone. C: carapace; LV: left valve; RV: right valve. Scale bars: 100 ␮. 1, 2. G. compressa Christensen and Kilenyi, 1970. Fairlight borehole (−364.54 m), Portlandian, glaucolithus Zone. 1, RV, female, MPK10616. 2, RV, male, MPK10054. 3. E. eusarca (Anderson, 1941). C, right view, Tisbury borehole (−10.35 m), Portlandian, glaucolithus Zone, MPK10627. 4. P. buglensis Barker, 1966b. LV, Hartwell borehole (−4.99 m), Portlandian (the “Pendle” of the Purbeck Fm.), anguiformis Zone, MPK10626. Fig. 12. Ostracodes de la Zone a` G. compressa. C : carapace ; VD : valve droite ; VG : valve gauche. Barres d’´echelle ; 100 ␮. 1, 2. G. compressa (Christensen et Kilenyi, 1970). Sondage de Fairlight (−364,54 m), Portlandien, Zone a` glaucolithus. 1, VD, femelle, MPK10616. 2, VD, mˆale, MPK10054. 3. E. eusarca (Anderson, 1941). C, vue droite, sondage de Tisbury (−10,35 m), Portlandien, Zone a` glaucolithus, MPK10627. 4. P. buglensis Barker, 1966b. VG, sondage d’Hartwell (−4,99 m), Portlandien (le « Pendle » de la Fm. de Purbeck), Zone a` anguiformis, MPK10626.

3.9. P. buglensis Subzone Stratigraphical range: Late albani and okusensis zones. Typical species: As for the zone. Definition: The FAD of P. buglensis to the appearance of R. (L.) visceralis. Remarks: The zonal index is frequently found in the Swindon and Aylesbury districts, as well as in the Hartwell borehole and Bugle Pit. Although the subzone can be recognised as far south as Portesham (Barker et al., 1975), it has not yet been recognised in the Dorset coastal section. 3.10. R. (L.) visceralis Zone (Fig. 13) Stratigraphical range: okusensis–anguiformis zones. Typical species: R. (L.) visceralis, F. boloniensis, P. bicosta, Paracypris weedonensis. Definition: The lower boundary of the zone is defined by the inception of R. (L.) visceralis. The fauna extends into in the basal Purbeck, where it is replaced by assemblages characterised by T. forbesii Zone (sensu Horne, 1995). At some localities where T. forbesii is missing from the assemblage, R. (L.) visceralis is found with species of Cypridea, Damonella etc., typical of Anderson’s Quainton and Warren faunicycles (1985, and references therein). Remarks: The distribution of the assemblage is controlled by palaeoenvironment and facies so that the upper boundary is

difficult to define. However, the index species appears to be euryhaline. In the lower part of its range, such as in Dorset (Barker, 1966a), it occurs with taxa such as Galliaecytheridea, Hechticythere, Paracypris, Macrocypris and Procytheropteron. In the Bugle Pit (Barker, 1966b), it is found with M. purbeckensis, but disappears from the record before the first appearance of Cypridea, Rhinocypris, etc. At Portesham, however, Barker et al. (1975) shows R. (L.) visceralis to occur with Cypridea tumescens (Anderson, 1939), C. dunkeri papulata (Anderson, 1941) and Scabriculocypris acanthoides Anderson, 1941, as well as T. forbesii. At Swindon (Sylvester-Bradley, 1941), the index is similarly found with Cypridea, Scabriculocypris, Damonella, Stenestroemia and Fabanella. R. (L.) visceralis is, therefore, a useful species correlating the marine and nonmarine environments. The zonal index has been recognised in the Swindon, Aylesbury and Hartwell districts and as far south as Portesham, but it has not been recognised in the Dorset coastal section. 4. Systematic palaeontology All material is deposited in the Palaeontological collections of the British Geological Survey, Keyworth, Nottinghamshire, NG12 5GG. Family MACROCYPRIDIDAE M¨uller, 1912 Genus Macrocypris Brady, 1867

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Fig. 13. Ostracods from the R. (L.) visceralis Zone. C: carapace; LV: left valve; RV: right valve. Scale bars: 100 ␮. 1. Rectocythere (Lydicythere) visceralis (Anderson, 1941). LV, Hartwell borehole (−11.90 m); Portlandian, okusensis Zone, MPK10641. 2. F. boloniensis (Jones, 1882). RV, male, Hartwell borehole (−4.99 m), Portlandian (the “Pendle” of the Purbeck Formation), anguiformis Zone, MPK10609. 3. P. bicostata Barker, 1966a. LV, Tisbury borehole (−40.50 m), Upper Kimmeridgian, fittoni Zone, MPK10065. 4, 5. M. alexanderi nov. sp. Hartwell borehole (−4.99 m), Portlandian (the “Pendle” of the Purbeck Formation), anguiformis Zone. 4, C, right view, MPK10603, (holotype). 5, LV, MPK10604 (paratype). Fig. 13. Ostracodes de la Zone a` R. (L.) visceralis Zone. C : carapace ; VD : valve droite ; VG : valve gauche. 1. Rectocythere (Lydicythere) visceralis (Anderson, 1941). VG, sondage d’Hartwell (−11,90 m), Portlandien, Zone a` okusensis, MPK10641. 2. F. boloniensis (Jones, 1882). VD, mˆale, sondage d’Hartwell (−4,99 m), Portlandien (le « Pendle » de la formation de Purbeck), Zone a` anguiformis, MPK10609. 3. P. bicostata Barker, 1966a. VG, sondage de Tisbury (−40,50 m), Kimm´eridgien Sup., Zone a` fittoni, MPK10065. 4, 5. M. alexanderi nov. sp. Sondage d’Hartwell (−4,99 m), Portlandien (le « Pendle » de la formation de Purbeck), Zone a` anguiformis. 4, C, vue droite MPK10603, holotype. 5, VG, MPK10604 (paratype).

Macrocypris alexanderi nov. sp. (Figs. 13[4, 5]) 1941. Macrocypris horatiana Jones and Sherborn-Anderson, p. 380, Pl. 19, Fig. 16. 1966b. ¿‘Macrocypris” sp. - Barker, p. 484, Pl. 9, Figs. 3, 4. Derivation of name: After the author’s son. Holotype: Carapace MPK10603 from Hartwell borehole (−4.99 m); Portlandian (the “Pendle” of the Purbeck Formation); anguiformis Zone. Paratype: Left valve MPK10604 from the same locality as the holotype. Diagnosis: A species of Macrocypris with high evenly arched dorsal margin and a marked mid-ventral concavity. Dimensions (mm)

MPK10603 (holotype) MPK10604 (paratype)

Length

Height

0.80 0.71

0.39 0.38

Description: Left valve slightly larger than the right. Dorsal margin high and evenly arched, cardinal angles not con-

spicuous. Ventral margin sinuous with a marked mid-ventral concavity. Anterior margin broadly rounded. Posterior margin acutely rounded. Posterior and ventral margins appear to be obliquely down-turned due to the curvature of the carapace. Surface smooth. Highest and widest at mid length, maximum length immediately above the ventral margin. Internally, inner margin broad, anterior and posterior vestibulae wide, marginal pore canals numerous both anteriorly and posteriorly. Hinge apparently as for the genus: that of the left valve comprises a smooth median ridge becoming denticulate each end, but the terminal elements were poorly preserved and it is not known whether they are locellate. Muscle scars not seen. Remarks: M. horatiana Jones and Sherborn (1888) from the Bathonian is considerably smaller than the present species and has a more tapered shape and its highest point is anterior of mid-length. The present species appears to be closely related to M. horatiana in the sense of Witte and Lissenberg (1994), but their figured specimens differ in being acutely pointed posteriorly and in having weak cardinal angles that give a slightly

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more angular appearance. The present species is rarely found as individual valves so that the internal details are not fully known. Distribution and range: Hartwell borehole: Basal Purbeck Limestone (“Pendle”), Portlandian (anguiformis Zone). Bugle Pit, Hartwell, and Warren House Farm, Stewkley: Portland Formation (Portland Stone Member, Creamy Limestone Bed) and basal Purbeck Limestone Formation, Portlandian (kerberus and anguiformis zones). Swindon: Town Gardens Quarry, Purbeck Formation (“Middle Pebbly Bed”). Family CYTHERURIDAE M¨uller, 1894 Subfamily CYTHERURINAE M¨uller, 1894 Genus Micrommatocythere Wilkinson, 1983a Micrommatocythere reticulata nov. sp.(Fig. 10(7)) 1969. Monoceratina sp. 1 Kilenyi - p. 145, Pl. 29, Figs. 22–24. 1983a. Micrommatocythere sp. Wilkinson-Wilkinson, p. 23. 1983b. Micrommatocythere sp. Wilkinson-Wilkinson, p. 171, 173, 175. Derivation of name: After the characteristic reticulate ornamentation. Holotype: MPK10629. Carapace, from Hartwell borehole (−46.25 to −46.38 m), Upper Kimmeridgian, hudlestoni Zone. Material: 48 valves and carapaces. Diagnosis: A reticulate species of Micrommatocythere with a postero-ventral swelling forming a weak ala-like projection. Dimensions (mm)

MPK10626

Length

Height

0.37

0.19

Description: Carapace small, caudate. Dorsal margin straight to slightly sinuous, ventral margin slightly concave, anterior margin broadly rounded, posterior drawn out into a dorsally positioned caudal process. Anterior margin flattened to form a narrow flange. Anterior cardinal angle well developed. Lateral surface reticulate Eye tubercle small but prominent with, immediately below and posterior of it a short, thick, vertically disposed “riblet” formed by the coalesced muri of the reticulation that form a ‘rosette’ around it. Lateral surface swollen postero-ventrally to form a weak ala-like projection. Internal details as for the genus. Remarks: Kilenyi (1969) placed this species into Monoceratina, but it is here placed into Micrommatocythere. This species differs from “Monoceratina sp. 2” of Kilenyi (1969) (which is also considered to be a species of Micrommatocythere) in the alate appearance, in the concave ventral margin and in the reticulate rather than coarsely punctate ornament. Distribution and range: Eastern England- North Wootton borehole (National Grid Reference TF 6439 2457): Upper Kimmeridge Clay beds KC44-KC45 (hudlestoni Zone). Hunstanton borehole (National Grid Reference: TF 6857 4078): Upper Kimmeridge Clay Beds KC44-KC47 (hudlestoni and pectinatus zones). Skegness borehole (National Grid Reference TF 5711 6398): Upper Kimmeridge Clay beds KC44-47 (hudlestoni and pectinatus zones). Southern England- Hartwell borehole: Kimmeridge Clay Formation (Watermead Clay

237

Member), KC44, and Portland Formation (Portland Sand Member) (hudlestoni and okusensis zones). Fairlight borehole: Upper Kimmeridge Clay (pallasioides Zone). Tisbury borehole: Upper Kimmeridge Clay (pectinatus to rotunda zones). Micrommatocythere sp. MPA368 Remarks: This species has a small, coarsely punctate carapace with a broadly rounded anterior margin, caudate posterior margin, straight dorsal margin and concave ventral margin. It differs from M. reticulata in lacking the ala-like projection, in being punctate and in having a convex ventral margin. It is rare in the latest pallasioides Zone of the Fairlight borehole. Genus Paranotacythere Bassiouni, 1974 Paranotacythere (Unicosta) sp. MPA358 Remarks: A moderately small, weakly punctate and weakly sulcate species with an almost continuous marginal rib, broken only behind the eye tubercle. Dorsally the marginal rib is arched, partially interrupting the outline; anteriorly and ventrally it is evenly curved; the dorsal and ventral parts of the rib unite at an acute angle near the weak posterior cardinal angle. Paranotacythere (U.) sp. MPA358 is rare in the Portland Sand equivalent (glaucolithus Zone) of the Fairlight borehole. 5. Conclusion Ostracods in the Upper Kimmeridgian and Portlandian (sensu anglico) succession in southern England are rich, but low in diversity. The assemblages comprise species of Aaleniella, Galliaecytheridea, Klentnicella, Macrodentina, Mandelstamia, Micrommatocythere, Paralesleya and Prohutsonia. Ostracods in the Portlandian are influenced by rapidly changing environments so that marine taxa such as Protocythere, Macrocypris, Paraschuleridea, Paranotacythere, Procytheropteron and Rectocythere were replaced by euryhaline forms, such as species of Fabanella and Mantelliana, and fresh to oligohaline species of the genera Cypridea, Scabriculocypris, Alicenula and Rhinocypris. Rapidly changing environmental conditions make biostratigraphical subdivision difficult, but it is possible to recognise the G. compressa Zone (youngest) (including the E. eusarca and P. buglensis subzones); G. polita Zone; G. spinosa Zone (subdivided into the M. (X.) tumida, H. serpentina / M. (P.) rudis and A. inornata / A. gracilis subzones) and E. aquitanum Zone (oldest). The G. compressa Zone is succeeded by Wealden assemblages characterised by Cypridea, Theriosynoecum, etc., described by Anderson (1985) and Horne (1995, 2002). Acknowledgements The author publishes with permission of the Executive Director of the British Geological Survey (N.E.R.C.). The author also acknowledges the reviewers, D. Horne and J.-P. Colin, for their constructive comments.

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