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)
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