Southern Region Chalk Group Lithostratigraphy: Traditional Classification - Grey Chalk

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The Grey Chalk consists of massively bedded, mostly grey or creamish-coloured, flintless chalk, with thin (centimetre scale) marl seams. It is much less argillaceous than the underlying Chalk Marl, and there is loss of the conspicuous marl / limestone rhythms, although this is gradational. In the North Downs, the Grey Chalk passes upwards into very soft, smooth, white chalk, named the White Bed (Wood, 1996). Locally, pyritic nodules are common. The Grey Chalk includes a unit of gritty, silty chalk, usually with a conspicuous lenticular structure, named Jukes-Browne Bed 7. The base of the Grey Chalk is gradational into the Chalk Marl, but the top is usually conspicuous, being marked by the erosive base of the clay-rich Plenus Marls. In Sussex, Lake et al. (1987) recorded 35 m of Grey Chalk, and Rawson et al. (1978) defining the base of this unit at a horizon rich in the brachiopod Orbirhynchia mantelliana, recorded that it was 30-38 m thick in Hampshire, 36-37 m thick in Sussex and Kent.

For the most part, the Grey Chalk is very poorly fossiliferous, exceptions being Jukes-Browne Bed 7 in which the ammonite Acantoceras jukesbrownei is locally common, and local oyster and brachiopod-rich horizons.

Macrofossil Biozonation: A. rhotomagense Zone (pars), A. jukesbrownei Zone, C. guerangeri Zone

Correlation: see Correlation with other Southern Region Chalk Group classifications

see Correlation with other UK Chalk Group successions

References

LAKE, R D, YOUNG, B, WOOD, C J & MORTIMORE, R N. 1987. Geology of the country around Lewes. Memoir of the British Geological Survey.

RAWSON, P F, CURRY, D, DILLEY, F C, HANCOCK, J M, KENNEDY, W J, NEALE, J W, WOOD, C J & WORSSAM, B C. 1978. A correlation of the Cretaceous rocks in the British Isles. Geological Society of London, Special Report No. 9, 70 pp..

WOOD, C. J. 1996. Upper Cretaceous: the Chalk Group. In SUMBLER, M. G., British Regional Geology: London and the Thames Valley. Fourth Edition. (London: HMSO for the British Geological Survey).

See: marl, Chalk Marl, Plenus Marls

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