Carboniferous rocks in this region occur in a broadly east-west trending syncline, the\ud core of which includes the South Wales and Pembrokeshire coalfields (Fig. 5.1).\ud Tournaisian and Visean strata (Avon and Pembroke Limestone groups) represent\ud deposition on a southward prograding carbonate ramp evolving into a carbonate shelf\ud (Wright 1987), in a succession which shows similarities to that of the Bristol and\ud Mendips areas (Chapter 6). The main outcrops, in south Pembrokeshire, Gower and\ud the Vale of Glamorgan, occur along the southern periphery of the coalfields and are\ud commonly affected by Variscan thrusting and folding. Thinner successions occur\ud along what is termed the East Crop and North Crop of the South Wales Coalfield,\ud where much of the Visean succession is absent due to sub-Namurian and intra-Visean\ud unconformities. Namurian fluvio-deltaic deposits (Marros Group) flank the South\ud Wales and Pembrokeshire coalfields. Much of the lower and middle Namurian\ud succession is absent across the region, except in the west of the South Wales Coalfield\ud where only small parts are absent beneath an intra-Namurian unconformity.\ud Westphalian fluvio-lacustrine deposits (South Wales Coal Measures Group) form the\ud South Wales and Pembrokeshire coalfields, located to the east and west of\ud Carmarthen Bay, respectively. Westphalian to Stephanian Pennant alluvial facies\ud (Warwickshire Group) occur in the core of the South Wales Coalfield syncline.\ud Deposition of the South Wales Coal Measures and Warwickshire groups was\ud probably laterally contiguous with those in the Bristol and Somerset coalfields\ud (Chapter 6), but the Usk-Cowbridge High controlled and restricted sedimentation for\ud much of the Carboniferous, with pre-Namurian uplift and erosion removing the\ud Tournaisian and Visean succession. Later uplift is also believed to have caused\ud attenuation of the Warwickshire Group in the east of the South Wales Coalfield. The\ud lithostratigraphical nomenclature for the region is that of Waters et al. (2007; 2009)
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