Bristol, Mendips and Forest of Dean

Abstract

Carboniferous rocks within this region occur in a series of inliers, many occurring in the cores of periclines and anticlines. The Tournaisian and Visean strata comprise ramp carbonate successions (Avon and Pembroke Limestone groups), which show similarities with equivalent strata to the west in South Wales (Chapter 5). The main outcrops, broadly from south to north, are the Cannington Park inlier and Mendips and at Weston-super-Mare, Broadfield Down, Bristol and Avon, Cromhall and Chepstow to Monmouth (Fig. 6.1). Namurian strata are present only in the south of the region, in the Bristol and Somerset coalfields. Little information is available on the nature of these strata, though they show some similarities to the fluvial and deltaic successions of the Marros Group of South Wales (Chapter 5). Westphalian strata are present in all the coalfields, broadly from south to north, the Somerset, Bristol, Severn, Forest of Dean and Newent coalfields (Fig. 6.1). Fluvio-lacustrine deposits (South Wales Coal Measures Group) are present only in the Somerset, Bristol and the south-eastern part of the Nailsea coalfields. These coalfields are laterally contiguous beneath Mesozoic strata. Deposition was also probably laterally contiguous with the concealed Berkshire Coalfield (Chapter 7). Lateral continuity with the South Wales Coalfield (Chapter 5) is not possible to demonstrate. Strata of this facies are absent from the Newent, Forest of Dean and Severn coalfields and the Cannington Park inlier along the axis of the syn-Westphalian Usk Anticline. It is not clear if the Coal Measures were deposited and subsequently removed by erosion during late Bolsovian to early Asturian deformation or were never deposited at all. Although thinning of the succession can be demonstrated in the Nailsea Coalfield, there is no facies change to indicate passage towards a growth anticline. A Westphalian to Stephanian alluvial succession of Pennant facies (Warwickshire Group) occurs within all six coalfields, though proved Stephanian strata are restricted to the Somerset and Forest of Dean coalfields. Deposition was probably laterally contiguous with the South Wales Coalfield (Chapter 5) and the concealed Oxfordshire and Berkshire coalfields (Chapter 7).