marl seams and chalk stratigraphy: note on an occurrence with nodular beds throughout kent and...

CORRESPONDENCE

Marl seams and Chalk stratigraphy: note on an occurrence withnodular beds throughout Kent and Surrey recognised in 1937 as a

basal marker of the Upper Chalk

s. C. A. Holmes

5, Hampton Grove, Ewell, Epsom, Surrey KT171LA

Although British Geological Survey practice no longeraccepts a division between Upper and Middle Chalk,definitive basal Upper Chalk beds were recorded inKent from Survey field mapping in 1936 (Holmes,1937, p. 352) and traced at localities all along theNorth Downs (Holmes, in Dines, Holmes & Robbie,1954 [1971], pp. 38, 39, 40, 41, 42, 43; in Dines,Buchan, Holmes & Bristow 1969, pp. 99, 100, 108,109, 110, 111; 1962, p. 33). The same beds are to befound also across the Canterbury & Folkestone andthe Maidstone one-inch sheets. Subsequently thisdetailed sequence was recognised throughout Surrey(Gray, 1965).

Clearly demonstrated on the field meeting ofAugust 23-25, 1980 (G.A. Circulars 819, 820), theequivalent on the South Downs, where Marl I isexposed in the Beachy Head cliff for example, isembodied in the Bridgewick Marls of Mortimore(1986, pp. 109, 111 Fig. 9, 112 Fig. 10, 113).

Consequently, it is helpful to find the same marl,

nodular beds and flint occurrence on the North Downsappearing to me in descriptions by Robinson (1986,pp. 154, 156 Fig. 10, 159 Fig. 13), part of his Fan BayMarls and Bantam Hole Hardgrounds, and toestablish that this stratigraphic element is indeedwidespread as one among other lithological units thatcorrelate between the Chalk of the North and SouthDowns.

In Geological Survey publications, a standardsequence near Chatham of the basal Upper Chalkbeds (Holmes, in Dines et al., 1954[1971], p. 39) wasdefined first. It consists of a prominent and persistentmarl seam, a metre or more below a band of largeflints, and overlying some one and a half metres ofnodular chalk with fossiliferous hardgrounds and somenodular flints; beneath this is another, persistent bandof large flints. Recalling that its first mention was inthe Proceedings of 1937 now seems appropriate to1987.

References

DINES, H. G., S. C. A. HOLMES & J. A. ROBBIE. 1954.[1971]. Geology of the country around Chatham. Mem.Geol. Suru., Gt. Brit.

-, S. BUCHAN, S. C. A. HOLMES & C. R.BRISTOW. 1969. Geology of the country aroundSevenoaks. Mem. Geol. Surv., Gt. Brit.

GRAY, D. A. 1965. The stratigraphical significance ofelectrical resistivity marker bands in the Cretaceous strataof the Leatherhead (Fetcham Mill) Borehole, Surrey. Bull.Geol. Suru., oc Brit. 23, VI.

HOLMES, S. C. A. 1937. Field Meeting at Wrotham andBorough Green, Kent. Proc. Geol. Ass., 48, 350-53.

- 1962, in Summ. Prog. Geol. Surv. Gt. Brit, for 1961,33.

MORTIMORE, R. N. 1986. Stratigraphy of the UpperCretaceus White Chalk of Sussex. Proc. Geol. Ass., 97,97-139.

ROBINSON, N. D. 1986. Stratigraphy of the Chalk Groupof the North Downs, southeast England. Proc. Geol. Ass.,97, 141-70.

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