Chalk Group Lithostratigraphy: Marlborough Downs/Berkshire Downs/Chilterns - Chalk Rock: Middle Hardground Suite

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The Middle Hardground Suite comprises the Fognam Farm Hardground overlain by the Leigh Hill Hardground (Bromley & Gale, 1982). The Fognam Farm Hardground, highly convolute and glauconitised, and overlain by a pebble bed of glauconitised intraclasts, occurs over most of the region, except the north-eastern Chilterns, where it fades and disappears (Bromley & Gale, 1982). It was formed by lowered sea level and associated erosion in the Mid and Late Turonian, and equates with the interval between the Southerham and Caburn Marls in the Lewes Nodular Chalk Member of the Southern Region (Gale, 1996).

The Leigh Hill Hardground overlies the Fognam Farm Hardground in the Marlborough Downs, and is typically strongly glauconised, though its morphology varies from flat to highly convoluted (Bromley & Gale, 1982). Passing norteastwards into the Chilterns, there is uncertainty concerning the correlation of the Leigh Hill Hardground, and so here, the second hardground from the top of the Chalk Rock is separately named the Blounts Farm Hardground.

Northeastwards, as the Middle Hardground Suite weakens and disappears, two marl seams make their appearance in the succession, the Latimer Marl, overlain by the Reed Marl (Bromley & Gale, 1982). Field relations suggest that the Latimer Marl (c. 10 cm thick at Latimer [TQ 005 993], its type locality) correlates with marly chalk that locally overlies one of the minor hardgrounds above the Fognam Farm Hardground (Bromley & Gale, 1982). The Latimer Marl was equated with the Sourtherham Marl 1 (in the lower Lewes Nodular Chalk Member of the Southern Region) by Mortimore & Wood (1986), but this is an error (C J Wood, pers. comm., 1997), as suggested by Gale's (1996) correlation of the Fognam Marl with Southerham Marl 1. Field relations suggest that the Reed Marl, comprising c. 2cm of plastic grey marl, is coeval with the Blounts Farm Hardground. Mortimore & Wood (1986) equated the Reed Marl with the Caburn Marl in the lower Lewes Nodular Chalk Member of the Southern Region.

Macrofossil Biozonation: upper T. lata Zone

Correlation: see Correlation with other parts of the UK

References[edit]

BROMLEY, R G & GALE, A S. 1982. The lithostratigraphy of the English Chalk Rock. Cretaceous Research, Vol. 3, 273 - 306.

GALE, A S. 1996. Turonian correlation and sequence stratigraphy of the Chalk in southern England. In HESSELBO, S P & PARKINSON, D N (eds), Sequence Stratigraphy in British Geology, Geological Society Special Publication, No. 103, pp. 177-195.

MORTIMORE, R N & WOOD, C J.1986. The distribution of flint in the English Chalk, with particular reference to the 'Brandon Flint Series' and the high Turonian flint maximum. In SIEVEKING, G de C & HART, M B. The scientific study of flint and chert. Cambridge University Press, Cambridge.

See: marl, hardground, Lewes Nodular Chalk Member (Southern Region), Southerham Marl 1, Caburn Marl, Turonian