Chalk Group Lithostratigraphy: Marlborough Downs/Berkshire Downs/Chilterns - Grey Chalk

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The Grey Chalk is similar to that of the Southern Region, comprising massive bedded, yellowish-grey, marly chalk, lacking the conspicuous rhythmic sedimentation of the Chalk Marl. It is typically 20-30 m thick (Rawson et al., 1978; Shephard-Thorn et al., 1994), and contains occasional thin marl seams. In the lower part of the succession, a bed of calcarentic chalk with pelletal phosphate, small fish teeth and glauconitised and phosphatised pebbles, locally known as the 'Buckinghamshire Rag', is possibly the equivalent of Jukes-Browne Bed 7 of the Southern Region (Wood, 1996). At Totternhoe and in the Hitchin district, the latter bed equates with an erosion surface, or locally hardground characterised by the terebratulid brachiopod Ornatothyris sulcifera, overlain by a marl rich in the oyster Pycnodonte. The higher part of the Grey Chalk, equivalent to the White Bed in the North Downs, is massive, off-white chalk with curvilinear jointing (Wood, 1996). The Grey Chalk is typically sparsely fossiliferous, although terebratulid brachiopods are relatively common near the base (Wood, 1996).

Macrofossil Biozonation: A. rhotomagense Zone, A. jukesbrownei Zone, C. guerangeri Zone (Wood, 1996)

Correlation: see Correlation with other parts of the UK

References[edit]

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..

SHEPHARD-THORN. E. R., MOORLOCK, B. S. P., COX, B. M., ALLSOP, J. M. & WOOD, C. J. 1994. Geology of the country around Leighton Buzzard. Memoir of the British Geological Survey, Sheet 220 (England & Wales).

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: Grey Chalk (Southern Region), Chalk Marl, Jukes-Browne Bed 7, hardground, marl