Ammonites and belemnites from the Early Cretaceous Claxby Ironstone formation of Nettleton Hill

John P Green (UK)

The Early Cretaceous succession in Lincolnshire consists of a series of shallow water marine sandstones, ironstones, clays and limestones, not unlike those deposited elsewhere in the UK during early Jurassic times. In the north of the county, at Nettleton Hill, near the village of Nettleton, minor exposures of the Claxby Ironstone Formation are present.

This deposit, approximately 5.7m thick, rests unconformably on the eroded Late Jurassic Spilsby Sandstone Formation of Volgian age. The age of the ironstone ranges from the Lower Valanginian to the Lower Hauterivian stage, and is of particular interest due to the ammonite and belemnite faunas it contains. My studies over a number of years have brought to light a series of cephalopod faunas that are also prevalent in Speeton, East Yorkshire, as well as northern and southern Europe. Prominent contributors to the study of the cephalopod faunas of this formation include Lamplugh (1918), Swinnerton (1935), Casey (1973), Wright (1975) and Kemper et al (1981).

The ironstone is divided into two members: the Lower and Upper Claxby Ironstone Formations. Both these formations are characterised by a brown to purple clay matrix, rich in prominent iron ooliths, and which is highly fossiliferous. Excellent exposures were formerly present in opencast and deep mines around Nettleton (TF 1140 9868, TF 1164 9870). However, these sections are now unfortunately filled in. The current exposures at Nettleton Hill, while minor, afford the best opportunity for study at the moment.

The base of the ironstone sharply overlies the eroded Spilsby Sandstone Formation, and, at its base, is a conglomerate containing derived fossils from this bed. The Lower Claxby Ironstone Formation is characterised by a cephalopod fauna consisting of abundant belemnites, together with less common ammonites.

The characteristic Boreal (Northern European) belemnite genus, Acroteuthis subquadratus,and its allies dominate the belemnite assemblage, and rare specimens can attain a length of 20cm. Of particular interest is my observation, in the basal 20cm of the ironstone, of an apparent Hibolites sp. belemnite. This is an extremely rare element of the cephalopod fauna in Lincolnshire, until the deposition of the Lower Hauterivian Tealby Clay Formation, and this extremely rare occurrence likely represents a ‘stray’ southern European (Tethyan) immigrant. This is supported by the occurrence of extremely rare Neocomitid (Tethyan) ammonites, also found at this level at Nettleton, which invite a partial correlation with the Platylenticeras ammonite beds in Germany; this genus is recorded both there and also in the Tethys region (Rawson, personal communication 2013)

A faunal horizon of small ammonites is also present, preserved in a clay ironstone matrix, some 20cm above the base of the member. These specimens, mainly preserved as small nuclei (inner whorls), allow correlation with comparative sections in Germany and on the Russian Platform in the Boreal realm (Rawson, personal communication 2013). Among the genera recognised are apparent derivative species of Peregrinoceras sp., possible early Polyptychites, and Tollia sp.

A high degree of differentiation occurs in the degree of compression, inflation and density, and pattern of ribbing; many of these ammonites show signs of gradation into one another, and it is likely some of these specimens represent transitional forms of the genera discussed in this article. The size of the majority of the specimens (no larger than a penny) further serves to hinder identification. However, specimens from correlative sections in Germany and Russia have yielded larger specimens, which often provide firmer identification (Rawson, personal communication 2013).

The ironstone continues upwards as a very fossiliferous deposit, especially rich in bivalves, as well as belemnites of the Acroteuthis subquadratus lineage. Ammonites are very rare in these higher beds. The poor preservation of the only two specimens collected unfortunately hindered any firm identification. The top of the Lower Claxby Ironstone Member is marked by a conspicuous erosion surface, and, of interest, yields common Acroteuthis sp. that show signs of major erosion and /or possible bio- erosion. An in depth analysis of possible biological activity on these belemnite guards, after the death of the host animal, could well repay further study, as shown in a recent analysis of corroded belemnites found in the Speeton Clay Formation of East Yorkshire (Taylor et al, 2013).

Above this prominent erosion surface lies the Upper Claxby Ironstone Member, which is characterised by a brown or purple clay matrix, containing horizons of pink and grey, sometimes sugary calcite, and which has been named the ‘calcareous beds’ (Rawson, 1983). This horizon is especially notable for abundant and well-preserved brachiopods and, both lithologically and palaeontologically, is the most distinctive section of the ironstone. This bed, around one metre thick, is a condensed horizon, representing both the Upper Valanginian and Lower Hauterivian stages. Evidence of these stages is present in the form of derived fragments of Upper Valanginian ammonites, in addition to well-preserved Hauterivian species.

Particularly common among the latter are Endemoceras amblygonium and Endemoceras noricum, as well as an uncoiled genus, Distoloceras sp. While these ammonites are usually preserved as small specimens, larger fragments indicate certain individuals may have reached almost a metre in diameter.

Of particular interest is the evolutionary variation exhibited in Endemoceras. Some of the specimens exhibit weak, or sometimes, more prominent spines, mainly on the umbilical and ventro – lateral shoulders.

This spinose development is particularly well developed in Distoloceras; and indeed, this ammonite is believed to be a probable descendent of Endemoceras, although the condensed nature of the beds at the present locality excludes any biostratigraphical study. Belemnites appear to be less common in these higher beds and are attributed to several species of Acroteuthis sp.

An additional observation worth discussing is the colonisation of ammonite shells after the death of the host animal, as shown by the discovery of an Aeotostrean sp. oyster shell that bears the distinct imprint of a partial large Endemoceras sp. However, there was no sign of the ammonite itself. No doubt, this demonstrates the preservation potential of the respective groups – the strong calcitic shell of an oyster being more suited to fossilisation than the more chemically unstable ammonite shell, which was originally aragonitic. A specimen of Distoloceras sp. has also been collected that exhibits oysters attached to both sides of the shell, which could be attributed to the ammonite being subjected to repeated movement by submarine action (for example, currents), after the death of the host animal.

The ‘calcareous beds’ are overlain by an inconspicuous erosion surface, which is itself overlain by a darker, iron shot clay that has yielded the belemnite, Hibolites jaculoides, and which is taken to represent the Lower Tealby Clay Formation, of Lower Hauterivian age.

This is present at Nettleton Hill as only a feather edge, some 20cm thick in the available exposures, and is therefore not discussed further here. These succeeding Early Cretaceous beds that overlie the Claxby Ironstone in Lincolnshire are, nonetheless, of great litho logical and palaeontological interest, and these will be discussed in further detail in a future article in this magazine.

References

Casey, R. 1973. The ammonite succession at the Jurassic – Cretaceous boundary in eastern England. 193 – 266 in The Boreal Lower Cretaceous. Casey, R. and Rawson, P.F. (editors). Spec. issue geol. J. No. 5.

Kelly, S R A, and Rawson, P.F., 1983. Some late Jurassic – mid Cretaceous sections on the East Midlands Shelf, as demonstrated on a field meeting, 18 – 20 May 1979. Proc. Geol Assoc, Vol 94, 65 –– 73.

Kemper, E, Rawson, P.F., and Thieuloy, J.P. 1981. Ammonites of Tethyan ancestry in the early Cretaceous of north west Europe. Palaeontology, Vol. 24, 251 – 311.

Lamplugh, G. W. 1896. On the Speeton Series in Yorkshire and Lincolnshire. Q. J. Geol. Soc. London, Vol. 52, 179 – 220.

Taylor, P.D., Barnbrook, J.A., and Sendino, C. 2013. Endolithic biota of belemnites from the Early Cretaceous Speeton Clay Formation of North Yorkshire, UK.

Proc. York. Geol. Soc, Volume 59, p 4, 227 – 245.

Swinnerton, H.H., 1935. The rocks below the Red Chalk of Lincolnshire, and their cephalopod faunas. Q. J. Geol. Soc. London., Vol. 91, 1 – 46.

Wright, C.W. , 1975. The Hauterivian ammonite genus Lyticoceras Hyatt, 1900 and its Synonym Endemoceras Thiermann, 1963. Palaeontology, 18, 607 – 611.