Macrofossil succession of the Burnham Formation
(Upper Cretaceous, Upper Turonian Stage) of North Ormsby,
Lincolnshire.
By J.P. Green
The large disused quarry at North Ormsby ( TF2893 ) north of Louth, Lincolnshire, displays an important sequence of beds of the Burnham Chalk Formation (Upper Cretaceous, Upper Turonian Stage) and at present constitutes the best exposure of the beds in the county. Similar beds exposed at Ulceby Vale Quarry (TA104133) in North Lincolnshire were described in terms of both stratigraphy and palaeontology, by Wood (1992) and by Hildreth (1999 and 2012).
The North Ormsby section was measured and described in stratigraphical terms by Wood and Smith [1978] although little information on the macrofauna was published. Hill (1902) was the first to identify the Sternotaxis plana biozone in this area, and Rowe (1929) provided an admirable macrofaunal list in his account. The aim of the present work is, therefore, to augment and build upon the work of previous authors, and place, where possible, the recorded macrofossils in a stratigraphical framework. Attention will be drawn, in particular, to affinities and differences in terms of distribution of the fauna in comparison to related sections. Wood and Smith (1978) established important flint and marl marker horizons for the Chalk of Lincolnshire and Yorkshire, and these will be referred to within this account.
The Burnham Chalk exposed at this locality consists in general terms of thick bedded chalk, interbedded with marls and beds of predominantly tabular and semi tabular flint. About halfway up the sequence, above the level of the Wootton Marls, the chalk gives way to thinner bedding, and beds of largely semi tabular carious flints. It is noteworthy that the chalk at this locality is relatively soft, and appears to yield a relatively more diverse macrofauna than successions further north. However, it is uncertain as to what extent this is attributable to improved collecting conditions in these soft chalks, rather than to a more favourable ecological environment (Wood 1992).
The lowest marker horizon present in the quarry face is a prominent marly bedding plane, located 0.5 m below the first major marker bed, the Ravendale Flint. The beds beneath this bedding plane are largely obscured by talus. This marked bedding plane is taken as the Welton/Burnham Chalk Formation boundary. The beds beneath the Ravendale Flint have yielded Sternotaxis plana (Mantell) and fragmented inoceramids. In a correlative section at West Ravendale road cutting (TA2206 ) cidarid plates and spines have been recorded, along with Infulaster excentricus (Woodward). At the present locality, a smooth, poorly preserved ammonite has been recorded, just beneath the Ravendale Flint, tentatively referred to Puzosia sp. by Whitham (personal communication 1997). This genus is characteristic of the Chalk Rock hardground complex, located near the base of the plana zone in southern England (Wright 1979).
The beds between the Ravendale Flint and the lowermost Triple Tabular Flint are around 2.5 m thick, and are on the whole relatively thick bedded and have yielded S. plana, together with Orbirhynchia sp. and fragmented large inoceramids. The earliest Echinocorys sp. Recorded by the author has been found in flint preservation, just above the middle Triple Tabular Flint, and the beds within the Triple Tabular Flint sequence, up to the horizon of the overlying North Ormsby Marl (the type locality of Wood and Smith 1978) are around 2.4 m thick, and have yielded S. plana and I. excentricus in relative abundance, together with fragmented inoceramids, possibly Inoceramus lamarcki Parkinson and related forms. These bivalves are especially common within the Triple Tabular Flint series at this locality.
Several specimens of thick tested large Echinocorys sp. have been recorded just beneath the North Ormsby Marl, which at this locality is around 7 cm thick. This echinoid, as usually recorded from the plana zone, displays a round, domed test form, and of interest is a specimen that displays a low test form, and slightly elongated length. This evolutionary variation within Echinocorys is displayed throughout the Chalk sequence, and their real biostratigraphical value in the Chalk of the Northern Province has yet to be fully realised (Whitham 1991).
A loosely coiled ammonite has also been collected from just beneath the North Ormsby Marl with fine ribbing punctuated by raised collars, indicative of Hyphantoceras reussianum (Orbigny). The occurrence of this species is of particular interest, in that it lies stratigraphically below the occurrences noted in the Northern Province Chalk, by both Whitham (1991) and Wood (1992). This ammonite has also been recorded by the author at the Ulceby Vale Quarry, albeit in loose material.
The thin bedded Chalk overlying the North Ormsby Marl has yielded the brachiopods Orbirhynchia sp., Gibbithyris sp. and Kingena elegans Owen. The occurrence of this brachiopod fauna at this level corresponds with that at the same horizon at Kilnwick Percy, East Yorkshire (SE843504) noted by Whitham (1991). A relatively barren Chalk bed extends about 1 m upwards to the next major marker horizon, the 25 cm thick Ludborough Flint (Wood and Smith 1978) this being the so-called White Flint of Rowe (1929).
The succession from the Ludborough Flint up to the next marker bed, the Thornton Curtis Marl, is around 2 m thick, and yields rare Echinocorys, usually large, thick tested distorted examples, which upon examination display a rounded, domed test form. Other fossils from this sequence are rare, but include Orbirhynchia sp, a large Gibbithyris sp. and fragmented large inoceramids. A poorly preserved ammonite tentatively referred to Lewesiceras sp. has also been recorded from this bed.
A distinct lithological change takes place above the level of the overlying paired Wootton Marl sequence, which lies around 1.3 m above the Thornton Curtis Marl; the bed immediately below the Wootton Marls yields rare distorted Echinocorys sp. The predominantly thick bedded chalks with tabular flints give way to thinner bedded chalks with semi tabular, carious flint bands. This also appears to coincide with a more abundant macrofauna. The next major marker horizon in the Northern Province Chalk, the Ulceby Marl, lies some metres above the Wootton Marls; at the present locality it is absent, due to its presence in the unexposed sequence of beds that lie above the quarry. Hence, the top of the North Ormsby section falls somewhere between the Wootton Marl sequence and the Ulceby Marl.
Finely weathered sections in the carious flint sequence have yielded abundant Orbirhynchia sp. and an undescribed fauna of tiny bivalves, as well as small Porosphaera sponges, and crinoid and bryozoan debris. The beds just above the Wootton Marl sequence have produced a partially articulated asteroid, assigned to Metopaster? sp. At a horizon recorded as 7 m above the Ludborough Flint, the large ammonite Lewesiceras mantelli Wright and Wright also occurs, along with well preserved S. plana and Parasmilia sp.
Inoceramid bivalves in the carious flint sequence consist in the main of large fragmented forms probably referable to I. lamarcki stuemckei Heinz, and small thin shelled forms, assigned to Mytiloides sp. It is noteworthy that the echinoid Micraster corbovis Forbes has not been observed by the author in situ at this locality, but it occurs abundantly, and in excellent preservation, in scree material. There is an undoubted horizon of these echinoids towards the top of the exposure, but difficulties of access currently preclude clarification of this. A band of M. corbovis has been recorded by the author some 3 m below the Ulceby Marl at the Ulceby Vale Pit; it has yet to be found at North Ormsby.
Other species have been recorded in scree material by the author, derived from levels above the Wootton Marls. These include three dimensional sponges, including Doryderma? sp, Ventriculites? sp, and other unidentified forms. Echinoids include a partial test of Tylocidaris? sp, and an Echinocorys that displays an inflated test and a pointed top, again demonstrating the evolutionary variation within this genus. A tooth of the squatiniform shark Scapanorhynchus sp. has also been recorded, along with fragments of large pachydiscid ammonites. Of particular note is a poorly preserved partial specimen of an unknown heteromorphic ammonite, possibly referable to the inner whorls of H. reussianum.
The total thickness of the North Ormsby section, from the bedding plane beneath the Ravendale Flint to around 4 m above the Wootton Marls, is around 15 m. The beds persist several metres further, but difficulties of access preclude a final and true figure. Wood (1992) has stated that the Burnham Chalk of the Louth area is, on the whole, relatively thicker than successions nearer the Humber.
This overview of the macrofauna of the North Ormsby Quarry section is of particular importance, in view of the fact that it affords the best exposure of the Burnham Chalk in this area. Further investigation, particularly of the occurrence and stratigraphical placing of the rare ammonite faunas and the level of the Micraster corbovisbeds, are topics that could repay further study.
References.
Hildreth, P.N. 1999. Field meeting in North Lincolnshire. Humberside Geologist 13, 25-26.
Hildreth, P.N. 2012.A section of Burnham Chalk at Ulceby Vale Quarry, North Lincolnshire. Humberside Geologist 15.
Hill, W. 1902. Note on the Upper chalk of Lincolnshire. Geol. Mag, Vol. 9, 404 – 406.
Rowe, A.W. 1929. The zones of the White Chalk of Lincolnshire. Naturalist 875, 411-439.
Whitham, F, 1991. The stratigraphy of the Upper Cretaceous Ferriby, Welton and Burnham formations north of the Humber, north east England.Proceedings of the Yorkshire Geological Society 48, 227-254.
Wood, C.J., in Gaunt, G. D., and Fletcher, T P, 1992. Geology of the country around Kingston upon Hull and Brigg. Memoir for 1:50 000 geological sheets 80 and 89 (England and Wales), HMSO, London, pp 77-101.
Wood, C.J, and Smith, E. G. 1978. Lithostratigraphical classification of the chalk of North Yorkshire, Humberside and Lincolnshire. Proceedings of the Yorkshire Geological Society, 42, 263-87.
Wright, C.W. 1979. The ammonites of the English Chalk Rock (Upper Turonian). Bulletin British Museum (Natural History) Geology Series 31, No. 4
Author's contact details :-
John P Green, B.A., 225, Yarborough road, Grimsby, North East Lincolnshire, DN34 4DW.
E – mail - jpgreen73@hotmail.co.uk
[final revised typescript received 7th August 2012]