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Lincolnshire Borehole Proves Greater Extent of the Scarborough Formation (Jurassic : Bajocian)

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Lincolnshire Borehole Proves Greater Extent of the Scarborough Formation (Jurassic : Bajocian) J.micropalaeontol., 1: 141-147, July 1982 Lincolnshire borehole proves greater extent of the Scarborough Formation (Jurassic : Bajocian) M.J. BRADSHAW & R.H. BATE Department of Geological Sciences, University of Aston, Birmingham, B4 7ET & Department of Palaeontology, British Museum (Natural History), Cromwell Road, London, SW7 5BD ABSTRACT-The ostracod fauna of a thin unit of shelly mudstones encountered directly beneath the Great Oolite Group in a recently drilled borehole in north Lincolnshire confirms the presence of the Scarborough Formation, formerly thought of as being confined to the Cleveland Basin. Sandstones and coals separating the Scarborough Formation from typical upper Lincolnshire Limestone calcarenites were probably deposited by delta progradation and are correlated with the regressive Yons Nab Beds and lower Gristhorpe Member of the Cleveland Basin. Discovery of the Gristhorpe Member and the Scarborough Formation beneath the basal Great Oolite unconformity on the East Midlands Shelf suggests that the youngest limestone facies of the Lincolnshire Limestone can be no younger than earliest sauzei Zone, and that the erosional event which preceded deposition of the Great Oolite is of late humphriesianum Zone or younger age. A revised palaeogeography for Scarborough Formation times envisages that following initial westward marine transgression along the axis of the Cleveland Basin, the sea broadened and at its maximum extent covered much of the Market Weighton Block. INTRODUCTION Between the early Aalenian Dogger Formation and article, Zone or Subzone used without qualification the early Callovian Cornbrash Limestone in the Cleve- refers to ammonite biozones which are regarded as land Basin is a series of thick fluvio-deltaic sandstones tantamount to chronozones; Torrens, 1980). It was and mudstones separated by four thinner, shallow marine therefore with great surprise that a recently drilled bore- deposits, which together compose the Ravenscar Group. hole at Nettleton, north Lincolnshire (Grid Ref. TF Middle Jurassic strata attenuate southward toward the 12459823), some 17 km east of the Bajocian outcrop, East Midlands Shelf and are missing over much of the cored a sandy and argillaceous sequence between typical Market Weighton Block due to both syndepositional upper Lincolnshire Limestone and basal Upper Estuarine i hinning and later erosion, but they reappear on north “Series”, which includes beds whose macrofaunas re- Humberside and thicken southward through Lincoln- semble those of the Scarborough Formation. This paper shire (see Fig. 1 for locations and structural setting). presents a detailed description and interpretation of the The Bajocian of Lincolnshire consists mainly of com- upper Lincolnshire Limestone to basal Upper Estuarine positionally variable but dominantly oolitic limestones, “Series” succession at Nettleton, and important bio- and northward advances of the Lincolnshire Limestone stratigraphic confirmation of age using the ostracod Sea account for the medial two marine units of the fauna. The effect of the Scarborough Formation’s dis- Cleveland succession. At outcrop, however, the topmost covery in Lincolnshire on our understanding of Middle Lincolnshire Limestone (Inferior Oolite Group) is of Jurassic sedimentation patterns is also discussed. (early Bajocian laeviuscufa Subzone or slightly younger *age(Ashton, 1980; Ashton & Parsons, in press), and is SEDIMENTOLOGY AND PALAEONTOLOGY separated by an erosional unconformity from sand- Lincolnshire Limestone stratigraphy has been re- stones and mudstones of the basal Upper Estuarine viewed by Kent (1966) and recently revised by Ashton “Series” (Great Oolite Group), which have not been (1 975,1980). On south Humberside the formation is split biostratigraphically dated but are thought to be of late informally into a lower unit of oolitic and bioclastic, Bajocian or earliest Bathonian age (Bradshaw, 1978). often micritic limestones with varying terrigenous con- No marine representative of the Scarborough Formation, tent (Kirton Beds), and an upper, coarser, often cross- the youngest marine unit within the Ravenscar Group bedded bioclastic oolite (Hibaldstow Beds). The Kirton of North Yorkshire (Fig. 2) and of ?late sauzei to hum- Beds contain a medial argillaceous unit (Raventhorpe phriesianum Zone age (Parsons, 1977) has been recog- Member of Ashton, 1975), which reflects a phase of nised on the East Midlands Shelf (throughout this southward progradation of the Yorkshire Delta. A lower 141 Bradshaw & Bate 0- km 30 DURHAM Inferred limit of marine basin during Scarborough Fmn timas limit of beds older than the Crinoid Grit NORTH YORKSHIRE Fig. 1 limestone tongue continues into the Cleveland Basin as An account of the lower and middle Lincolnshire the marine Blowgill Beds (Bate, 1967; Hemingway, Limestone at Nettleton is contained in Bradshaw & 1974) whilst an upper limestone tongue, the Cave Oolite, Penney (1982). The Raventhorpe Member and associated lateral equivalent of the Hibaldstow Beds and also the beds are more terrigenous than in their type section at upper Kirton Beds, persists as the Cayton Bay Formation Kirton-in-I,indsey (see Ashton, 1975), suggesting a (of Parsons, 1980; Lebberston Member of Hemingway stronger deltaic influence. The Hibaldstow Beds are and Knox, 1973). themselves unexceptional, but instead of being directly 142 Scarborough Formation in Lincolnshire 143 Bradshaw & Bate and unconformably overlain by the Upper Estuarine Trigonia, Nicaniella (Pressastarte), ? Homomya, Lingula “Series”, they grade up into the sandy and argillaceous and serpulids. A low diversity ostracod fauna (sampled sequence illustrated by Fig. 2. Moderately sorted cal- between 341.47 and 341.49 m, and contained on faunal carenites containing ooids and subordinate bioclasts slide number MPA 17235 in the Institute of Geological become finer grained, very poorly sorted and strongly Sciences Collections, Leeds, consists of Malzia uni- bioturbated, and contain a high proportion of coarse silt carinata Bate, M. bicarinata Bate, Praeschuleridea to very fine sand grade peloidal material, much of which subtrigona intermedia Bate, Fuhrbergiella (Fuhrber- is probably of faecal origin. This transition is ac- giella) horrida horrida Brand & Malz and Glyptocythere companied by the appearance of large numbers of medium scitula Bate. The mudstones are strongly bioturbated and to coarse sand-sized peloids and structureless crypto- contain a relatively high proportion of articulated bi- crystalline intraclasts, reflecting an increase in ooid and valves. Bioturbation is less intense in the claystones skeletal micritization. Intraclasts of quartz-silty wacke- where shell plasters frequently display large numbers of stone also occur and skeletal debris is mainly of mollus- similarly orientated Gervillella valves and Procerithium. can origin ; gastropods (including micromorphic forms) 15 cm of thinly interlayered siltstone and carbonaceous and pectinids are especially abundant, but ostracods and claystone, lacking a calcareous macrofauna, caps this benthic foraminifera are also very common. Grainstones thin (1.17 m) marine sequence beneath a second sharp give way upward to darker argillaceous packstones and contact at 340.52 m. The beds above the contact are then variably calcareous and carbonaceous dark grey mud- easily recognisable as the basal Upper Estuarine stones; thin layers of coarser oolitic material are common. “Series”. This upward increase in mud content is accompanied by a Typically in the East Midlands, the Upper Estuarine higher proportion of whole shells; bivalves dominate, “Series” is divisible into two, a lower rootletted fining- mostly pteriaceans (Gervillella, Isognomon) and pectinids upward sequence of white, yellowish grey and olive grey (both thin-shelled Propeumussium (Parvamussium) and sandstones and siltstones and brown, black and green thicker shelled Camptonectes), but shallow burrowing clay tones, lacking a calcareous macrofauna, succeeded genera (astartids and articids) form a significant minor by a stack of vertically repetitive sequences or rhythms, element and gastropods and serpulids are common. With- each with an erosive bivalve-rich base and a rootletted top in the overlying greenish grey to light olive grey siltstones (Bradshaw, 1978). At Nettleton, only the lower member and sandstones an abundant macrofauna of pectinids appears to be present. Sandstones fine upward into (again mostly P. (Parvamussium) ) and “Astute ” minima mudstones and claystones of varying shades of green and (Phillips), accompanied by Gervillella, Meleagrinella cf. brown ; black, intensely carbonaceous layers also occur lycetti (Rollier), ? Trigonia and high spired gastropods and the sequence is rootletted throughout. Only the base is lost upward whilst macroscopic plant debris remains of the Upper Estuarine “Series” is illustrated in Fig. 2, abundant. Though intensely bioturbated in places, the and the reader is referred to Bradshaw & Penney (1982) sandstones display even lamination with rare low angle for the rest of the Great Oolite succession. erosion surfaces, and contain carbonaceous and micaceous siltstone interlaminae. Thin coal layers occur STRATI GRA PHY toward the top of the sandstones, which are followed The Hibaldstow Beds of north Lincolnshire and south by thinly interlayered very fine sandstone, dark car- Humberside pass north of the Humber into the Cave bonaceous mudstone and minor coal, with rootlets pene- Oolite, a limestone tongue laterally
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