Cornbrash, Middle Jurassic, Bath—Cotswolds Province
|Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)|
Overlying the Forest Marble is the highly distinctive formation known as the Cornbrash, a term originally applied in Wiltshire to certain stony or brashy soils that are well suited to the growth of cereals. The typical lithology is a brown, fossiliferous non-oolitic, rubbly limestone with abundant shell debris and a marly matrix. Irregular marly partings give the rock a bedded appearance. The Corn- brash, which is often water bearing, frequently overlies a clay formation, and its base is marked by a good feature and by a line of villages associated with springs.
The name Cornbrash was first used in a geological sense by William Smith who acutely observed that the upper part of the formation was different from the lower in the fossils that it contained. This discovery led him to establish the principle of the orderly superposition of strata, distinguished by their fossil content. S S Buckman subsequently published an elaborate faunal analysis of the Cornbrash but this was critically modified in two classic papers by Douglas and Arkell whose account remains the basis for modern stratigraphical descriptions. Unlike the underlying formations, ammonites are sufficiently common to provide a sound basis for zonal subdivision. The Lower Cornbrash is Bathonian in age and the Upper Cornbrash is Callovian; these divisions correspond respectively to the Clydoniceras (C.) discus Subzone of the zone of the same name (P948998) and the Macrocephalites (M.) macrocephalus Zone. Four brachiopod faunas can also be recognised, the divisions being as follows:
|Upper Cornbrash||Ornithella [Micrithyridina] lagenalis|
|Lower Cornbrash||Obovothyris obovata|
According to Douglas and Arkell (1928) the Intermedia Beds are almost invariably present where the junction of the Cornbrash and Forest Marble is seen. The Obovata Beds are probably the most widespread and fossiliferous of the Lower Cornbrash and at the same time show the greatest lithological variation. At some localities Neocrassina hilpertonensis, Trigonia crucis and other bivalves occur in such profusion in the upper part of the Obovata Beds as to form a veritable ‘Astarte’–Trigonia Bed. It has been recognised by Arkell (1933, p.334), however, that the distribution of the two index brachiopods in the Lower Cornbrash may be ecologically determined and may therefore lack the stratigraphical precision suggested by Douglas and Arkell (see Cope et al., 1980, p.7). The thickness of the Cornbrash ranges from less than a metre to as much as 10 m, but is usually within the range of 1.5 to 6 m. It is about equally divided between the upper and lower divisions. At Frome and Trowbridge, however, on the north-easterly prolongation of the Mendip ‘Axis’, the Upper Cornbrash is either absent or represented only by a thin shell-lag marl which overlies a karstic surface in the top surface of the Lower Cornbrash.
- Arkell, W J. 1933. The Jurassic System in Great Britain. (Clarendon Press: Oxford.)
- Cope, J C W, Duff, K L, Parsons, C F, Torrens, H S, Wimbledon, W A, and Wright, J K. 1980. A correlation of Jurassic rocks in the British Isles. Part 2: Middle and Upper Jurassic. Special Report of the Geological Society of London, No. 15.