Ammonites of the Ampthill Clay, Lincolnshire

John P Green (UK)

The Ampthill Clay Formation of the UK, of Late Jurassic (Oxfordian) age, represents a series of highly fossiliferous marine mudstones that form part of the Ancholme Clay Group in North Lincolnshire (Gaunt et al, 1992); but are almost unexposed in the county other than at an excellent exposure of the Ringsteadia pseudocordata zone at South Ferriby Quarry (SE 992204). Therefore, this shortage of natural exposures means that any information, which can be obtained from other exposures in this county, is of the utmost significance.

Minor stream exposures at Kingerby Beck, North Owersby in North Lincolnshire (TF 0519 9340) have revealed a rich and well-preserved fossil fauna. These minor exposures have been placed by Gaunt et al (1992) within the Amoeboceras glosense zone, therefore lying at a differing stratigraphical horizon to the South Ferriby Quarry. Also, in contrast to the latter locality, the fossils exhibit a much higher degree of preservation and are therefore easier to collect.

Unfortunately, biostratigraphical bed-by-bed collecting is largely impractical at Kingerby Beck, due mainly to the very minor nature of the exposures; indeed, the majority of fossils have been collected from patches of clay exposed on the stream bed. The Ampthill Clay Formation, where exposed, is present as undifferentiated pale grey mudstones, with scattered calcareous concretions. It is these that are the major source of the prolific and well-preserved fossil faunas, particularly ammonites. Some of these concretions are very large – a sledgehammer is often required to break these open.

The fossils collected mainly represent a nektonic (swimming) fauna, with benthic (bottom dwelling) specimens being relatively scarce. Among the latter are well-preserved specimens of the large oyster Gryphaea dilotata, often encrusted with serpulids, as well as small gastropods (Dicroloma? sp.) preserved in concretions. Among the nekton, rare specimens of the belemnite Pachyteuthis abbreviata occur, together with a prolific ammonite fauna.

Wright (2003) discussed a temporary exposure of a clay ironstone facies of the Ampthill clay Formation at South Marston, Wiltshire, in the Ringsteadia pseudocordata zone. This also contained an abundant and well-preserved ammonite fauna, characterised by an assemblage dominated by Ringsteadia sp, together with Amoeboceras sp. and perisphinctids. Interestingly, the mode of preservation of these ammonites is somewhat similar to Kingerby Beck, despite lying at a different zonal horizon. Many of the ammonites are preserved only as sections of the body chambers or the phragmocones (innermost whorls). This is attributed to the introduction of micrite and phosphate into the shell, resulting in three-dimensional preservation being only partially achieved (Wright, 2003).

Based on recent collecting at Kingerby Beck, Amoeboceras sp. appears the most common ammonite, with subordinate Perisphinctes sp. also abundant. Amoeboceras is a Boreal (northern European) ammonite, being most abundant in Northern England and Scotland. The South Marston site in Wiltshire represents the most southerly occurrence of this ammonite present in any great numbers (Wright, 2003). Therefore, Kingerby Beck is a key site for the future potential collection and study of this ammonite in Northern England.

In terms of preservation, large, smooth body chambers (representing the adult growth stages) are common, along with densely ribbed phragmocones representing the early growth stages, and also micro conch forms. Among the perisphinctids, very large body chambers attributed to the subgenus Arisphinctes sp. have been found, as well as smaller fragments that most likely represent Perisphinctes spp.

However, in contrast to Amoeboceras, innermost whirls (phragmocones) of Perisphinctes sp. are rather scarce. Rare examples of smooth body chamber fragments of possible Ringsteadia? sp. have also been found, but no determinate inner whorl fragments are yet known. Ringsteadia sp. is also represented by common body chamber preservation with crushed inner whorls in the R. psuedocordata zone at the South Ferriby Quarry (Whitham, 1992). In contrast to Kingerby Beck, Ringsteadia is by far the most abundant ammonite at South Marston.

In terms of collecting, the Kingerby Beck site represents a prolific source of well-preserved fossils, particularly ammonites. It also represents an interesting case study for ammonite preservation that demonstrates preservation bias, which can cause drawbacks for ammonite studies as discussed by Wright (2003). Therefore, Kingerby Beck represents a very rare exposure of this highly fossiliferous deposit in Lincolnshire and, as such, future collection and further research of the ammonite faunas, in particular, is an area that may well repay study.

References

Gaunt, G. D, Fletcher, T.J, and Wood, C.J. Geology of the country around Kingston-upon-Hull and Brigg. HMSO for Geological Survey of Great Britain. P 57 – 70.

Whitham, F. 1992. The Geology of Middlegate Quarry, South Ferriby, North Lincolnshire. Humberside Geologist No. 10, p 4 -7.

Wright, J.K. 2003. New exposures of the Ampthill Clay near Swindon, Wiltshire, and their significance within the succession of the Oxfordian/Kimmeridgian boundary beds in southern England. Proceedings of the Geologists Association, Volume 114 Part 2.