John P Green (UK)
The large, disused quarry at North Ormsby [O.S. grid ref. TF2893], north of Louth in 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 Pit [TA104133] in North Lincolnshire have described in terms of both stratigraphy and palaeontology, by Wood (1992) and, more recently, by Hildreth (1999, and in press).
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 S. plana biozone of the Burnham Chalk Formation in this area, and Rowe (1929) provided an admirable macrofaunal list in his account. Therefore, my aim is to build on the work of previous authors, and place the recorded macrofossils in a stratigraphical context. In addition, Wood and Smith (1978) established important flint and marl marker horizons for the chalk of Lincolnshire and Yorkshire, and these shall be referred to in this account.
The Burnham Chalk, as exposed at this locality, consists in general terms of thick bedded chalk, interbedded with marl seams and marl layers, and beds of predominantly tabular and semi-tabular flint bands. 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 flint bands.
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 the soft chalk, 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 50cm below the first major marker bed, the Ravendale Flint. The beds beneath this bedding plane are largely obscured by talus, but this is taken as the Welton/Burnham Chalk Formation boundary. The beds beneath the Ravendale Flint have yielded the echinoid Sternotaxis Plana (Mantell) and fragmented inoceramids, and in a correlative section at West Ravendale road cutting [TA2200], cidarid plates and spines have been recorded, along with the characteristic Northern Chalk Province echinoid, 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 (in conversation, 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 basal triple tabular flint have yielded S. Plana, together with Orbirhynchia sp. and large, fragmented inoceramids. The earliest recorded Echinocorys sp. 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). These are about 2.4m thick and have yielded S. plana and I. excentricus in relative abundance, together with fragmented inoceramids, possibly referable to Inoceramus lamarcki (Parkinson) and related forms. In terms of their abundance at North Ormsby, these bivalves are especially common in the triple flint series.
Several specimens of large, thick-tested Echinocorys sp. have been recorded just beneath the North Ormsby Marl, which at this locality is about 7cm thick. This echinoid, usually recorded from the plana zone, tends to display a round, domed test form and, of interest, 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 Northern Province chalk has yet to be fully realised (Whitham, 1991).
A loosely coiled ammonite has also been collected just beneath the North Ormsby Marl, the 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).
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 corresponding with that at the same horizon at Kilnwick Percy in East Yorkshire [SE843504] noted by Whitham (1991). A relatively barren chalk bed extends about one metre upwards to the next major marker horizon, the 25cm-thick Ludborough Flint, this being the so called White Flint of Rowe (1904).
The succession from the Ludborough flint up to the next marker bed, the Thornton Curtis Marl, is about two metres thick, and yields rare Echinocorys, usually large, thick-tested distorted examples, which display a rounded, domed test form. Other fossils from this sequence are rare, and include Orbirhynchia sp, a large Gibbithyris sp. and large, fragmented 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 about 1.3m 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. Therefore, the top of the North Ormsby section falls somewhere between the Wootton Marl sequence and the Ulceby Marl Band.
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 parkinsoni?. At a horizon recorded 7m 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 and small, thin-shelled forms, assigned to Mytiloides sp. It is noteworthy that I have not seen the echinoid, Micraster corbovis (Forbes), 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 prevent clarification of this.
Other species have been recorded in scree material by me, 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 squatimiform 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 about 4m above the Wootton Marls, is about 15m. The beds persist several metres further, but difficulties of access prevent 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 in the north of the county.
This overview of the macrofauna of this area is of particular importance, in view of the fact that the present locality affords the best exposure of the Burnham Chalk in this area. Further investigation, particularly the occurrence and stratigraphical placing of the rare ammonite faunas, and the level of the Micraster corbovis beds, are topics that could, in particular, repay further study.
About the author
John P Green is an amateur palaeontologist from Cleethorpes in Lincolnshire. His current research interests are focused on the biostratigraphy of the late Cretaceous chalk of northern England, and the latest Jurassic and earliest Cretaceous ammonite faunas of North Lincolnshire. He can be contacted at: firstname.lastname@example.org.
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