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Structural controls on Upper Palaeozoic sedimentation in south-east Wales

British Geological Survey, Bryn Eithyn Hall, Llanfarian, Aberystwyth, Dyfed SY23 4BY, UK

Marked thickness changes in early Dinantian (Courceyan-Chadian) sediments occur across the Cardiff-Cowbridge Anticline, a major east-west Variscan fold in the Vale of Glamorgan. They resulted from differential subsidence over an active concealed basement fault zone, the Vale of Glamorgan Axis, which is coincident with the hinge of the Anticline; the latter formed in response to inversion on this basement fault. A Caledonian ancestry for the Vale of Glamorgan Axis is indicated by pronounced Upper Old Red Sandstone overstep across it.

Across the Severn Estuary, between South Wales and the Bristol-Mendips region, there is a significant offset of isopachs and facies belts in both the Courceyan-Chadian and Arundian sequences. It is suggested that these result from intra-Carboniferous dextral strike-slip along a major fault zone underlying the Severn Estuary (the Severn Estuary Fault Zone), and that the Vale of Glamorgan Axis was a synthetic structure, developed in conjunction with this strike-slip movement. By inference, the Severn Estuary Fault Zone shares a common history with the Vale of Glamorgan Axis, extending back to the Devonian and possibly earlier.

Late Dinantian (Holkerian) facies distribution reflects uplift and emergence on the north-south Usk Anticline and Malvern Lineament. Continued uplift is recorded in Silesian sequences in south-east Wales and the Bristol-Mendips area. The orientation of structures bordering the Severn Estuary is consistent with their development in a right-lateral strike-slip zone.

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