Kimmeridge Clay Ostracoda of the North Wootton Borehole, Norfolk, England

Ostracoda from the Kimmeridge Clay Formation (Upper Jurassic) of the North Wootton Borehole was examined from a biostratigraphical point of view. The ostracod zonal scheme, previously erected for the stratotype section in Dorset, is modified and the faunas are related to the 47 beds which can be recognised by a combination of macrofaunal and lithological characteristics. Seven ostracod assemblage zones are recognised in the borehole, of these three are new and one is subdivided into two subzones. One new genus, Micrommatocythere, together with four new species and one new subspecies–Micrommatocythere edmundi, Paranotacythere (Unicosta) effusa, Mandelstamia (Xeromandelstamia) horrida, Macrodentina (Polydentina) woottonensis and Galliaecytheridea mandelstami kilenyii–are described.


INTIRODUCTION
The Kimmeridge Clay of Norfolk is poorly represented at outcrop due to the thick and extensive drift deposits which blanket the region. As a consequence, prior to the recent work on borehole material by Gallois & Cox (1974, 1976, Glallois (1979a, b) and Cope ( 1 974), little information had been published since 18 15 when William Smith called the formation T h e Oaktree Clay It was mentioned by Rose (1835) and in the Memoirs of the Geological Survey (Whitaker, Skertchly & Ju,kes-Browne, 1893;Woodward, 1895, andStrahan, 1920) as well as by Pringle (1923) and Kent (1947) in connection with drilling programmes for hydrocarbon exploration.
The gently folded Kimmeridge Clay of Norfolk, which disconformably overlies the Ampthill Clay (Oxfordian) and is itself overlain unconformably by the "Lower Market, before disappearing t o the south east of Mundford (Gallois & Cox, 1974, Gallois, 1979b (Fig. 1). This southward reduction in thickness is accentuate,d by the erosion of the top of the Upper Kimmeridge Clay which took place prior t o the deposition of the Sandringham Sands.
T h e most complete sequence of Kimmeridge Clay in eastern Enigland yet descriibed is the continuously cored North Wootton Borehole (Grid Ref. TF 6439 2457), situated 5 k m to the north east of Kings Lynn (Fig. I ) .
Between the depths 19.-75 and 119.45m, the entire sequence from the buylei t o the lower part o f the pecti-natus ammonite zones of the Kimmeridgian has been proved. Thirty five beds in the Lower and 12 beds in the Upper Kimmeridge Clay have been recognised on the basis of a combination of macrofaunal and lithological characteristics (Gallois & Cox, 1976;Gallois, 1979a). These beds a r e cyclic in naturewith silts o r silty mudstones (kerogen-rich at some intervals) fining upwards t o dark grey mudstones and finally, as a result of an increase in the calcium carbonate content. t o pale grey mudstones. The rhythms can be traced over large distances and used to correlate the Kimmeridge Clay in East Anglia with the stratotype of Dorset (Cox & Gallois,198 1) and boreholes in Wiltshire and Lincolnshire.
Although the u p p t r part of the pectirzutus ammonite Zone and above is absent, the excellent faunal and lithological control in the North Wootton Borehole, provides an extremely good standard for eastern England.

PREVIOUS RESEARCH
Kimmeridge Clay Ostracoda of the Dorset coast of southern England have been well documented in the last 25 years, initially by German workers (Malz, 1958a(Malz, , 1958bClashoff, 1964), but later by Kilenyi (1965Kilenyi ( , 1969Kilenyi ( , 1978 and Christensen & Kilenyi ( 1 970). Virtually all of this work has concentrated on the stratotype sections of the Kimmeridge Bay area, Dorset, and until now there has been a paucity of information about ostracod faunas from eastern England.
Ostracoda from the stratotype area have been shown to be extremely useful biostratigraphically by Christensen & Kilenyi (1970). Their zonal scheme has been used, more o r less successfully, over much of North West Europe including the North German Basin, North West Borehole.
Polish Basin, North West Danish Embayment and Paris Basin. Although much of the eudoxus t o lower part of the wheatleyensis ammonite zones are barren of Ostracoda and the wheutleyensis to pectinatus ammonite zones yield only poor assemblages, five ostracod assemblage zones were recognised. Later a sixth ostracod zone was added by Christensen (1 974), the Galliaecytherideu compressa Zone. T h e relationship between the Dorset and Norfolk ostracod zonal schemes together with the ammonite zonation is summarised in Table 1 .

OSTRACODA FROM THE NORTH WOOTTON BOREHOLE Lower Kimmeridge Clay Ostracoda
The Lower Kimmeridge Clay in the North Wootton Borehole extends from a depth of 1 19.40 to 5 1.15 m and comprises the baylei to autissiodorensis ammonite zones and Beds 1-35 of Gallois & Cox (1 976). In its upper part (Beds 29 and 3 1-35) oil shales arc common, the thickest seams being found near the top of theeudoxus ammonite Zone, Bed 32. It is at this interval that the ostracod faunas become sparse o r barren, nevertheless stratigraphically significant assemblages have been recovered throughout most of the sequence (Fig. 2). Diversity was found to be low-of the 39 samples examined the majority had a diversity of five o r less and only eight samples exceeded ten.
T h e zonal scheme of Christensen & Kilenyi (1 970) is recognisable but, in the light of new evidence from the North Wootton Borehole, it is in part modified. a. The Galliaecytheridea dissimilis ostracod Zone. The lowest ostracod zone of the Kimmeridge Clay, the Galliuecytheridea dissimilis Zone, is represented by the 11 9.40-100.39m interval of the North Wootton Borehole. It encompasses the haylei, cymodoce and lowest part of the mutabilis ammonite zones and corre-  Gallois & Cox (1976). Although G. dissimilis was not encountered in the lowest two beds, its almost consistent occurrence in large numbers in Beds 3-15 renders it an excellent marker. It is comrnonly found associated with the long ranging species C;. dorsetensis, G. postrotunda, Schuleridea trieheli (ii species which exhibits precocious sexual dimorphism), Dicrorygma reticulata and Paranotucythere extendata

R. cymodoce
In Beds 1 and 2, at the base of the baylei ammonite Zone, Macrodentina pulchra was recovered in large numbers. This species has been recorded from the Upper Oxfordian and the lowest part of the baylei ammonite zones in N.W. Germany, but only from the Upper Oxfordian of Dorset (Glashoff, 1964 Gallois and Cox, 1976). Lithology: 1. mudstone, dark and medium grey; 2. oil shale; 3. Pale grey, very calcareous mudstones; 4. very shelly mudstone; 5. silty mudstone.
(7. dissimilis occurs iin flood proportions between Beck 3 and 5 (upper part of the baylei and basal cymodoce zones) at depths of 118.80-1 17.55 m, where it is frequently found with Macrodentina intercostulatu. This latter species is diagnostic of deposits ranging from the Upper Oxfordian to1 the lowest part of the baylei arnrnonite zone of N.W. Europe (Glashoff, 1964), but the highest record in the present borehole is the basal part of the cymodoce ammonite Zone (Bed 5 ) .
The ostracod faunas of Beds 5-9 in the cymodoce ammonite Zone are sparse, but G. dissimilis is generally present and in the upper part Macrod,entina ( M . ) cica(ricos,a and Cytherella fullonica have also been noted. Rich assemblages were recovered from the upper part of the cymodoce Zone (Beds 10-14, 11.5-109.8 m depth) where Galliaecytheridea punctatu, a characteristic species of the baylei and cyrnodoce ammonite zones (Kilenyi, 1969) was recovered. Although two damaged instars tentatively assigned to Mandelstamiu rectilineu were recovered from Bed 7, from a depth of 1 17.0 m, the species occurred commonly for the first time in Bed 13 (at I10.7m). This species is restricted to the mutabilis and basal eudoxus ammoinite zones of Dorset (Christensen & Kilenyi, 1970, fig. 2) but it has also be'en recorded from the Upper Oxfordian (Glashoff, 1964;Whatley, 1965). b. The Macrodentina (Polydentina) proclivis ostracod Zone. Gai'liaecytheridea dissimilis extends into the lower mutlibilis ammonite Zone, Bed 15, where it occurs in small1 numbers. The extinction level in the borehole is taken to be at the top of this bed, at a depth of 108.20m, at approximately the same stratigraphical level as in Dormjet. Between 107.110 and 99.0m (Beds 16-18), assemblages were sparse and of very low diversity (6 in Bed 17, but less than 3 elsewhere). Specimens of the index species of the Gallitzecytheridea elongutu ostracod Zome of Christensen & Kilenyi (1970) were not recovered and this gap in the zonal scheme also occurs near the base of the mutuliilis ammonite Zone in Dorset (Kilenyi, 1969). It is here proposed that a new zone is erecited to fill this gap. 'The low diversity of the assemblages restricts the choice of the index species to Mucrodentina (Polydentina) proclivis, which first appears at the base of the mutubilis ammonite Zone in the North Wootton Borehole as well as in Ilorset (IKilenyi, 1969). The zone fossil is not common, but was recovered from Bed 15, where it occurred with G. dissimili,s, Bed 17 and again in Bed 2 1 where it was found together with G. elongatu. Within the Macrodentina (Polydentina) proclivis ostra cod Zone, numerous long ranging species become extinct -Schulrridea triebeli, Gulliuecytheridea frugilis, G. postrotunda and itlicrommatocythere edmundi as well as, at the base, G. d,issimilis. c. The Galliaecytheridia elongata ostracod Zone. The first occurrence of the ost racod zone fossil, C;. elongata, in I h r s e t is in the middle part of the mutubilis ammonite Zone, a little below Bed 22. the Supracorallina Bed (Kilenyi, 1969). In the North Wootton Borehole, the first appearance of the zone fossil is at the base of Bed 18, at a depth of 100.3 m, although it is rare until Bed 2 1 (Bed 20 was too thin to be sampled for micropalaeontological examination). The upper boundary of the ostracod Zone is a little below the top of the eudoxus ammonite Zone, in Bed 3 I , but the highest bed of the eudoxus Zone (Bed 32), being composed almost entirely of oil shale, contains a very sparse, poorly preserved fauna.
The ostracod assemblages of the C. elongutu ostracod Zone of the North Wootton Borehole, fall into two subzones. The lower is characterised by the association of G. mundelstami kilenyii subsp. nov. with G. elongata from the base of the ostracod Zone up to Bed 23, at the top of the mutubilis ammpnite Zone. The upper subzone is recognised by the first appearance of Macrodentinu (Polydentinu) steghausi steghuusi ( = M . (P.) proclivis striatu of Kilenyi, 1969) which extends from Bed 24 to Bed 30.
Within the C. mandelstumi kilenyii ostracod Subzone, G . dorsetensis, Paranotocythere (Unicosta) nealei and Macrodentina (Polydentina) proclivis become extinct. A single specimen of Nodophthalmocythere tripartita was recovered from Bed 18 (from a depth of 99.00 m) in the middle part of the mutabilis ammonite Zone and can be used to correlate this interval with Dorset (Kilenyi, 1969;Christensen & Kilenyi, 1970) and perhaps the Paris Basin (Malz, 1958a). Cytherelloideu weberi was present at the top of the subzone (Bed 23).
The M . (P.) steghausi steghausi ostracod Subzone contains several species not seen in lower levels of the North Wootton Borehole. Amongst these are Mucrodentina muculata and Cytherelloidea paruweberi which are present at a slightly lower stratigraphical level in Dorset (Kilenyi, 1969), but above the Supracorallina Bed (Bed 22 of Gallois & Cox, 1976; Cox & Gallois, I 98 1). The last occurrence of Dicrorygmu reticularu, Exophthulmocythere fuhrhergensis and Purunotacythere (Unicostu) extendata is in this ostracod subzone. d. Un-named zone. The extinction level of G. elongata is in Bed 31, at the top of the eudoxus ammonite Zone. This is followed by an interval of thick oil shales which is generally barren of Ostracoda but with very sparse faunas at some levels. Mandelstumiu rectilineu, which is extremely common throughout the mutabilis and eudoxus ammonite Zones (sometimes reaching flood proportions), is present in small numbers in Bed 32 and 33 where it is accompanied by very rare fragmentary Cytheropteron sp. The remainder of the Lower Kimmeridgian (autissiodorensis ammonite Zone, the upper part of Bed 33 to 35) of the North Wootton Borehole is barren of Ostracoda.

Upper Kimmeridge Clay Ostracoda
The Upper Kimmeridge Clay extends from a depth of 5 1.15 m to the unconformable contact with the Sandringham Sands at 19.75 m. It includes the elegans to the lower part of the pectinatus ammonite Zones, Beds 36-47 of Cox & Gallois (in Gallois, 1979a). The lower part of the sequence was deposited under similar conditions to those at the top of the Lower Kimmeridge Clay and oil shales are common throughout the elegans and lowest part of thescitulus ammonite Zones (Beds 36 and 37). Ostracoda are sparse throughout most of the Upper Kimmeridge Clay, but where oil shale seams are absent or thin, small populations have been recovered. Diversities are always low, generally between one and three, but in Bed 45, at the top of the hudlestoni ammonite Zone, they were slightly higher at seven (Fig. 3). a. Un-named zone. The elegans ammonite Zone (Bed 36) was barren of Ostracoda and in this respect identical to Dorset. Although the lower part of the scitulus ammonite Zone (Beds 37 and basal 38) was also barren of Ostracoda, the higher part of Bed 38 contained a very sparse fauna. Schuleridea moderata made its first appearance in the borehole at this level and the known range is therefore slightly extended; previously it had been recovered only from the hudlestoni and pectinatus ammonite zones of Dorset (Christensen & Kilenyi, 1970). The rare specimens of Paranotacythere (Unicosta) sp. A have close affinities with P. (Unicosta) neulei and may be a phylogenetic link between that species and P. rimosa of the Portlandian. Paranotacythere (Unicosta) effisa sp. nov. occurs rarely in this zone, but extends up into the base of the succeeding zone where it forms 75% of the ostracod population. c. The Mandelstamia (Xeromandelstamia) maculata ostracod Zone. In Dorset, Mandelstamia (X.) muculata is found in the wheatleyensis ammonite Zone, where it is restricted to the mudstones a little above the Grey Ledge Stone Band and slightly below The Blackstone (Kilenyi, 1969), Beds 41 and the base of 42 as defined by Cox & Gallois (1981). In the North Wootton Borehole the species was recovered from the upper part of Bed 40 to Bed 42. It occurred in flood proportions at 39.2m in the middle part of Bed 41 and within the thick oil shale at 35.1 m (Bed 42). With the exception of the sample from the very calcareous interval at the top of Bed 40 which has a diversity of 5 , samples from this zone were frequently almost monospecific. Very rare specimens of an angular Galliaecytheridea with a pointed, slightly spinose posterior, were recovered from Beds 40 and 42 and appear to be a form intermediate between G. mandelstami kilenyii and G . spinosa. d. Un-named Zone. Between 35.0 and 33.25m, that is the upper part of Bed 42 and 43, samples were very poorly fossiliferous or barren. Small numbers of Paranotacythere (Unicosta) sp. cf. nealei and Schuleridea moderata were recovered, but this interval could not be included in any of the named ostracod zones. e. The Eocytheropteron aquitanum ostracod Zone. The paucity of the ammonite populations in Dorset, led Christensen & Kilenyi (1970) to extend the M . (X.) maculata Zone up to the first occurrence of G. spinosa, although the ostracod zone fossil is absent above Bed 42. Eocytheropteron aquitanum was recovered from the upper part of the hudlestoni ammonite Zone, Bed 44, in Dorset (Kilenyi, 1969) and it has also been recorded from Vile d'OlCron, near St. Denis (Donze, 1960) and between Cap de la Creche and Boulogne-sur-Mer (Oertli, 1963) although the exact stratigraphical positions are not clear. In the North Wootton Borehole, E. aquitanum was recovered from Bed 44, between 33.25 and 26.90m and is used as the ostracod zone fossil corresponding with the middle and upper part of the hudlestoni ammonite Zone. It extends up into the basal part of the overlying ostracod zone where it is found together with G. spinosa. f. The Galliaecytheridea spinosa ostracod Zone. In Dorset, the G. spinosa ostracod Zone ranges from the upper part of the pectinatus to the lower part of the fittoni ammonite Zones (Kilenyi 1969;Christensen & Kilenyi, 1970). In the North Wootton Borehole, the nominate zone fossil first evolved during the latest hudlestoni ammonite Zone (Bed 45) but it was not recovered from the overlyingpectinatus ammonite Zone (Beds 46 and 47). Mandelstamia (Xeromandelstamia) tumida is common in the lower part of Bed 45 and Macrodentina (Polydentina) woottonensis dominates the assemblage in the middle part. Faunas become sparse in the top of the bed, but at 22.5m Schuleridea moderata occurs in flood proportions. Paranotacythere (Unicosta) pustulata and Mandelstamia (Xeromandelstamia) tumida occur at a lower stratigraphical level in this borehole compared to Dorset where they do not occur below the upper part of the pectinatus ammonite Zone (Bed 48) (Kilenyi, 1969;Christensen & Kilenyi, 1970 Gallois, 1979a). Whilst Bed 46 is barren of Ostracoda, very rare specimens of the long ranging species Schuleridea moderata have been recovered from Bed 47.
All specimens used in this study are housed with the Institute of Geological Sciences, Leeds, England. anterior vestibule narrow; marginal pore canals few, simple and straight. Hinge antimerodont, an ocular sinus is situated behind the anterior terminal element. A vertical, slightly arcuate row of four small, rounded adductor scars and a V-shaped frontal scar is situated slightly anterior of the mid point. Sexual dimorphism not observed. Remarks. Micrommutocythere gen. nov. bears a superficial resemblance t o some species of the genera Monocerutinu and Pseudocythere, but differs in possessing an eye tubercle, an antimerodont hinge and four adductor muscle scars. It is placed in the family Cytheruridae o n the basis of the hinge, muscle scar arrangement and the posterodorsal caudal process. Description. Carapace small and thinly shelled. Dorsal margin straight and subparallel with the sinuous ventral margin which has a marked mid-ventral concavity best seen when viewed internally. Anterior margin broadly rounded. Posterior cardinal angle prominent and posterior margin drawn out in a dorsally situated caudal process. The greatest width is posterior of mid-length and t h e highest point is at the anterior cardinal angle. Surface finely punctate. Eye tubercle small, but distinct.
T h e posteroventral area exhibits a weakly developed ala which partly obscures the marked mid-ventral concavity. An extremely weak sulcus is present. Internal margin moderately broad. Anterior vestibule narrow. Marginal pore canals few, straight and simple, 9 anteriorly and 4 posteriorly. Hinge antimerodont. A vertical, slightly arcuate row of four small, round adductor muscle scars and a V-shaped frontal scar are situated slightly anterior of mid-length. Dimensions (mm)

Length Height
Remarks. T h e fact that this species has a small, but distinct eye tubercle and an antimerodont hinge distinguishes it from several species of Monocerutinu and Pseudocythere which it superfically resembles.  differs in outline, particularly when viewed dorsally and in being weakly alate. Micrommatocythere edmundi sp. nov. has been recovered from several horizons from the P. baylei and A . mutabilis ammonite Zones (G. dissimilis and M . proclivis ostracod Zones), Lower Kimmeridgian, Beds 1-17 of Gallois & Cox (1976).
(PI. 2 , figs. 11-12) Derivation of name. Latin, effusa profuse, relating to its flood occurrence in Bed 40 of the Kimmeridge Clay of the North Wootton Borehole. Diagnosis. A robust, reticulate species of Paranotacythere (Unicosta) with smooth sulcus and caudal process. A very conspicuous posteroventral rib is joined to the main U-shaped rib system which surrounds the sulcus, at a very prominent tubercle, which may bear a spine. It extends towards the posterior a short distance, partly obscuring the ventral margin, before bifurcating. Holotype. MPK 3670, LV, from the lower part of the wheatleyensis ammonite Zone ( M . maculata ostracod Zone), Bed 40, of the Upper Kimmeridge Clay at a depth of 40.4m in the North Wootton Borehole. Description. Carapace large and robust. Dorsal margin straight; ventral margin convex, curving smoothly upward to the blunt caudal process; anterior margin broadly rounded. The lateral surface is ornamented by a system of ribs and intercostal reticulation. The sulcus is wide and smooth and the caudal process is also without ornamentation, and separated from the reticulate part of the lateral surface by a subdued, arcuate rib. The main rib system is U-shaped, about the sulcus into which an obliquely orientated dorsal rib runs. A mid-dorsal tubercle is situated at the junction between the vertical posteromedian and dorsal ribs. The posteroventral part of the U-shaped rib system bears a prominent tubercle, surmounted by a short spine, from which extends a distinct rib that partly overhangs the ventral margin before bifurcating, one rib curving upward to merge with the reticulation in the posteromedian area while the other runs parallel to the ventral margin as far as the arcuate ridge in front of the caudal process. A subdued rib runs longitudinally, above mid-height, in the posteromedian area, joining the U-shaped rib system with the arcuate rib at the caudal process. A distinct eye tubercle is present. Internally, features are as for the genus and subgenus as defined by Bassiouni (1974). Males are slightly more elongate than the females. Description. Dorsal and ventral margins straight and subparallel; posteroventral margin gently curved upwards to the rounded posterior margin; anterior margin broadly rounded. Surface ornamented by a series of small irregular tubercles and a reticulation which gives the surface a very rough appearance. A deep sulcus is present anterior of mid length in which a large, high, conical, subcentral tubercle is situated. Inner margin moderately broad, selvage well developed, anterior and posteroventral vestibules very narrow. Marginal pore canals few, straight and simple. Muscle scars not observed. Hinge, as for subgenus with crenulate terminal elements and smooth median bar/groove. Sexual dimorphism extremely pronounced, the male being much larger and more inflated than the female. Dimensions (mm) Length Height HolotypedLV, MPK 3664 0.62 0.36 Paratype 0 MPK 3663 0.55 0.3 1 Remarks. The ornament of Mandelstamia (X) horrida serves to distinguish the species from all other members of the genus. The species was found to be confined to the upper part of the P . scitulus ammonite Zone (Upper Kimmeridgian), Bed 39 of Gallois & Cox (1976) where it occured in flood proportions. Galliaecytheridea mandelstarni (Ljubimova) with ovoid, slightly elongate carapace; marked posterior cardinal angle; posterior margin extended into a rounded point at mid height; punctate ornament coarse in the median area becoming finer towards the margins. Holotype. MPK 3655, carapace, from the upper part of the nmtabilis ammonite :Zone (lower part of the G. elongata ostracod :Zone), Bed 22, of the Lower Kimmeridge Clay at a depth of 89.7m in the North Wootton Borehole. Dimensions (mm) Length Height Holotype car., MPK 3655 0.58 0.36 Remarks. Kilenyi (1 969) has described this form in details under the name Galliaecytheridea cf. mandelstami (Ljubimova, 1955). The subspecies G. mandelstumi kilenyii ssp. nov. differs from G. mandelstumi mandelstami in being smaller and more elongate. Its stratigraphical distribution is confined to the higher part of the A . mutabilis ammonite Zone (the lower part of the G. elongatu ostracod Zone), Beds 18-23 of Gallois & Cox (1976).