Field meeting on the Lincolnshire Limestone of Southern Lincolnshire and Leicestershire 19 September 1976

Field Meeting on the Lincolnshire Limestone of Southern Lincolnshire and Leicestershire 19 September 1976 Report by the Director: Michael Ashton ASHTON, M. 1978. Field Meeting in the Lincolnshire Limestone of Southern Lincolnshire and Leicestershire. Proc. Geol. Ass. 90 (1),15-20. The report records the itinerary of a one day excursion to four localities (Greetham, Th istleton , Stainby, and Leadenham) where the Lincolnsh ire Limestone is exposed . The ob jecti ves of the meeting were to examine the sections in the light of recent research on the litho- and biostratigraphy of the formation and to demonstrate the contrasts between the Lower Lincolnshire Limestone sequences of south and central Lincolnshire. The proposed correlations are illustrated on detailed, measured sections.

Department of Geology, University of Newcastle upon Tyne, Newcastle, NEl 7RU.

Unites A-E are approximately equivalent to the Lower Lincolnshire Limestone of earlier workers (Woodward, 1894; Richardson, 1939a; 1940; Hollingworth & Taylor, 1951 ; Taylor, 1963 ; Kent , 1966 ; Sylvester-Bradley, 1968). Wherever appropriate, the new stratigraphical units have been related to pre viously recogn ised subdivisions.

1. INTRODUCTION

The purpose of the meeting was to examine the stratigraph y of the Lincolnshire Limestone (Middle Jurassic) in the light of current studies (Ashton, 1976 ; and Ashton & Parsons , in preparation). Except for the occasional record ing of ammonite finds (Barker & Torrens, 1971; Senior & Earland-Bennet, 1973) and review articles (Kent , 1966; Sylvester-Bradley, 1968) little has been added to our knowledge of the Lincolnshire Limestone of southern Lincolnshire and Leicestershire since the work of Richardson (1939a; 1939b; 1940) , Kent (1940) , Hollingworth & Taylor (1951) , and Evans (1952) . Few detailed sections have been published to show the exact relationships of the proposed stratigraphical subdi visions of the formation (see especially Kent, 1966) and only a handful of the collected ammonites have been figured (Barker & Torrens, 1971; Ashton , 1976). During this excursion, four sections in the Lower Lincolnshire Limestone (Figs. 1 and 3) were examined and their lithostratigraphical relationships discussed. In addition, the age of the formation and its biostratigraphical subdivision were outlined in respect of fresh ammonite discoveries and re-examination of all earlier finds.

3. WCALITIES VISITED Greetham Quarry (Grid Ref. SK 933146)

The thirty-three members of the party assembled at Greetham Quarry (Fig. 1), where the director briefly outlined the programme for the day. At Greetham, many of the characteristics that have hindered the understanding of the lithostratigraphical relationships within the Lincolnshire Limestone, are well displayed; in particular, drastic vert ical and lateral changes of facies and thickness can be demonstrated by contrasting the successions developed at the northeastern and south-western ends of the quarry (Fig. 2). The party examined the north-eastern end first where attention was focused on the highest unit (Unit D, see Figs 2. and 3). Considerable time was spent examining its base, which here is typified by 'rolled' Theocosmilia sp., Lucina bellona d'Orbigny, tiny gastropods (all of which are decalcified) and large Thalassinoides sp. networks. This is a distinctive horizon, which can be traced across the whole of south Lincolnsh ire and Leicestershire northwards to Lincoln, where it occurs in a slightly modified form (generally lacking the faunal element) . Consequently it is considered to be of fundamental importance in the correlation of the Lincolnshire Limestone (Fig. 3). Underlying Unit D is a facies complex of ooid- and peloid-calcarenites (Figs 2. and 3) within which perhaps the most striking beds of the succession occur: the

2. A NOTE ON THE LITHOSTRATIGRAPHICAL TERMINOLOGY As full deta ils of the lithostratigraphical revision, discussed on this excursion , have not yet been published, a new terminology for the Lincolnshire Limestone will not be used here . However, to overcome this problem each of the newly recognised lithostratigraphic units (member status) is here designated by a capital letter (A , B, C, etc.). Essentially A is the oldest, B the next oldest and so on, although lateral variations complicate this simple picture . 15

16

MICHAEL ASHTON

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'Greetham Fossil beds'. These richly fossiliferous calcilutites overlie an oyster and Chomatoseris sp. encrusted hardground and contain a fauna largely adapted to living in soft substrates. Common amongst the forms present are Pholadomya lirata (1. Sowerby), Pinna cuneata Phillips, Modiolus imbricatus (J. Sowerby), Osteomya dilata (Phillips), Astarte minima Phillips, Linsdallia quadrata (J. de C. Sowerby), Protocardia sp., abundant "Nerinea" spp. and ?Montlivaltia ?sp. nov. Occasional Isastrea sp. colonies, bored by Lithophaga sp., also occur, while both bioturbation and discrete burrows are also very evident. The party then proceeded to the south-western end of the quarry, where the sequence was seen to differ in a number of respects (Fig. 2). Most noticeably the 'Greetham Fossil beds' appear to have been practically cut out by the erosive downcutting of the overlying unit. In addition, facies (horizon y: see Fig. 2) and thickness variations (horizon z: see Fig. 2), over only a few metres, were demonstrated.

Lunch was taken before travelling northwards to Thistleton. Thistleton Quarry (SK 903180) The abandoned British Steel pit near Thistleton (Fig. 1) provides one of the few remaining exposures of the Grantham Formation (formerly Lower Estuarine Series, see Kent, 1975) and the lowest part of the Lincolnshire Limestone Formation. It is readily correlated with Greetham Quarry by means of the hardground and capping fossiliferous beds discussed previously (Fig. 3), and together therefore, these sections provide an almost complete composite sequence through the Lower Lincolnshire Limestone of this area; only the uppermost unit (E) is missing. Of particular note in this sequence are the fine exposures in Unit A (the local equivalents of the Blue Beds of Richardson, 1939b, 466) and the apparent rhythmic make-up of the sediments in Unit B. Indeed Unit B in

17

FIELD MEETING ON THE UNCS UMESTONE

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south Lincolnshire and Leicestershire appea rs to be compo sed of a number of shallowing-upward rhythms (ooid-calcarenite s passing up into laminated peloidal calcarenites), probably resulting from the progradation of a tidal flat . Each rhythm appears to be term inated by a transg ression resulting from increased subsidence that suppr esses terrigenous sediment influx and generally heralds a time of non-deposition. In the absence of preserve d sediment, either a hardground or erosive surface records the 'transgressive event' , while during the ensuing per iod of relative sea level stability, ren ewed progradation of the tidal flat initiates another cycle. The deta ils of these rhythms will be discussed elsewhe re by the director. After examining some of the evidence for these rhythms the party continued northwards to Leadenham, briefly stopping off at Stainby on route. Stainby Quarry (SK 910233)

The main purpose of this brief stop was to illustrate the 2

lateral pers istence of Unit D (Fig. 3) and in particular its distinctive base (Plate lA & B), which is superbly exposed here. The overlying Unit E , in which the celebratedAcanthothyris crossi (J. F. Walker) occurs, is also present, completing the Lower Lincolnshire Limestone sequence. At the very top of the section the lowest few metres of the Upper Lincolnshire Limestone are barely accessible and were examined with difficulty. Leadenharn Quarry (SK 962523)

On arrival at Leadenham (formerly Dove's Quarry) the party was met by Mr. Marsh of Lincoln , who showed the members a large collection of Lincolnsh ire Limeston e fossils. Most notable amongst these was a very large Sonninia (Fissilobiceras) sp. After admiring the many well-preserved specimens , the group proceeded into the quarry, wher e a succession, strikingl y different from those seen at the previous localitie s, is expo sed. The Lower Lincolnsh ire Limestone at Lead enh am is more typical of the central Lincolnsh ire region (E vans, 1952)

18

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Fig. 3. Correlation chart for the Lower Lincolnshire Limestone sections at Greetham, Thistleton, Stainby and Leadenham. than south Lincolnshire (Fig. 3). In particular, the chalky white calcilutites of Unit C, with the ir prolific molluscan fauna , are quite different from any of the facies seen further south. The fauna appears to have suffered little or no post-mortem transportation, as practically all of the burrowing bivalves (Pholadomya lirata (1. Sowerby), and Pleuromya uniformis (1. Sowerby) are the most common) occur in their position of life as do the serni-infaunal, byssally-attached Pinna cuneata Phillips. Furthermore, few other forms show any sign of wear or

significant breakage; only the epifaunal bivalves are disart iculated and even the se single valves are whole. This faun a probably reflects very closely the preservable fauna of the living assemblages, that inhabited the quiet, nutrient-rich, 'soft-bo tto med', lagoonal environment, represented by th is facies. While many members continued to discuss the faunal relat ionships seen in Unit C, the rest of the party ascended another 'quarry-platform' to examine higher levels, including the thick ooid-calcarenite, that forms

19

FIELD MEETING ON THE UNCS UMESTONE

Plate 1B. Close-up view of the base of Unit D (arrowed) at Stainby Quarry. (SK 910233).

Plate 1A. View of the upper part of the west face of Stainby Quarry (SK 910233), with Unit D marked .

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Plate l C. View of the south-east face at Leadenham Quarry (SK 962523). The bed , from which the Sonninia (Fissilobiceras) eii.fissilobata (Waagen) was recovered, is arrowed. The boundaries between Units D, C and Bare marked. Unit E is not clear seen on this view.

20

MICHAEL ASHTON

the base (here lacking the fauna seen in the south) of Unit D. During this examination, one of the members (M. Ashton, no relation to the Director) uncovered a large Sonninia (Fissilobiceras) aft. fissilobata (Waagen) from bed Ib of Unit D (see Plate IC and Fig. 3). As ammonites are so rare in the Lincolnshire Limestone, Mr. Ashton kindly agreed to donate the specimen to the British Museum (No. C 80406). The Director explained that recent work (Ashton & Parsons, in preparation) has shown that such specimens, which are typical of the ovalis Subzone ofthe laeviuscula Zone (Parsons, 1974), have only been collected from horizons above the base of Unit D; all previous discoveries from lower stratigraphic levels are indicative of the discites Zone. Therefore the base of Unit D appears to be a very important

stratigraphical divide, which seems to coincide with the discites-laeviuscula zonal boundary. After this fitting end to the excursion the party dispersed.

ACKNOWLEDGMENTS The co-operation of Peterborough Quarries Ltd. and Howard's Ltd. is gratefully acknowledged. The Director would also like to thank Miss M. Pugh and Mr. M. Rosenbaum for help in organising the excursion and Dr. W. Wimbledon for skilfully extracting the ammonite found at Leadenham. Mrs. C. Cochrane prepared the ill ustrations.

References ASHTON, M. 1976. New Evidence for the age of the Lincolnshire Limestone Formation (Bajocian) of Eastern England. Trans Leic. lit. phil. Soc., 70, 21-34. BARKER, M. J. & H. S. TORRENS. 1971. A new ammonite from the southernmost outcrop of the Lower Lincolnshire Limestone (Middle Jurassic). Trans Leic. lit. phil. Soc., 65, 49-56. EVANS, W. D. 1952. The Jurassic Rocks ofthe Lincoln District. Proc. Geol. Ass., 63, 316-35. HOLLINGWORTH, S. E. & J. H. TAYLOR. 1951. The Northampton Sand Ironstone, Stratigraphy, Structure and Reserves. Mem. Geol. Surv. U.K., 211 pp. KENT, P. E. 1940. A short outline of the stratigraphy of the Lincolnshire Limestone. Lines nats. Union Trans., 10, 48-58. KENT, P. E. 1966. A review of the correlation of the Lincolnshire Limestone (Inferior Oolite). Trans Leic. lit. phil. Soc., 60,57-69. KENT, P. E. 1975. The Grantham Formation in the East Midlands: Revision of the Middle Jurassic, Lower Estuarine Deposits. Mercian Geol., 5, 305-27. PARSONS, C. F. 1974. The sauzei and 'so-called' sowerbyi Zones of the Lower Bajocian. Newsl. Stratigr., 3, 153-80.

RICHARDSON, L. 1939a. A weekend field meeting in the Stamford district. Proc. Geol. Ass., 50,29-45. RICHARDSON, L. 1939b. A weekend field meeting in the Grantham district. Proc Geol. Ass., 50, 463-75. RICHARDSON, L. 1940. Field meeting at Lincoln. Proc. Geol. Ass., 51, 246-56. SENIOR, J. R. & P. M. EARLAND-BENNETT. 1973. The Bajocian ammonite Hyperlioceras rudidiscites S. Buckman in eastern England and its significance. Proc. Yorks geol. Soc., 39,319-26. SYLVESTER-BRADLEY, P. C. 1968. The Inferior Oolite Series. Pp. 211-16 in The Geology of the East Midlands. Editors: P. C. Sylvester-Bradley and T. D. Ford. 400 pp. University Press, Leicester. TAYLOR, J. H. 1963. Geology of the Country around Kettering, Corby and Oundle. Mem. Geol. Surv. U.K. 149 pp. WOODWARD, H. B. 1894. The Jurassic rocks of Britain. Vol. IV, Lower Oolitic rocks of England. Mem. Geol. Surv. U.K., 628 pp. Received 18 January 1978