The succession and structure of the north-eastern area of the South Wales Coalfield

Abstract

Summary The paper deals with that portion of the South Wales Coalfield lying between Pontypool, Blaenavon and Tredegar : that is, the northern part of the East Crop and the eastern part of the North Crop. Although of much smaller thickness than farther west, the Coal Measures in the area have previously been regarded as constituting a full succession. It is now found that nowhere in the area is there a complete succession, the topmost beds of the Lower Coal Series and the lowermost beds of the Pennant Series being absent as a result of a major unconformity of Morganian age. The overlap and overstep developed by this unconformity increases in magnitude eastwards where the gradual disappearance of successive strata have been mapped; the major marine horizons of the Lower Coal Series have also been located and mapped across the area. The palaeontological evidence shows that the highly coloured beds below the Tillery Vein coal seam belong, not to the Lower Coal Series as previously thought, but to the Middle Pennant Group. The palaeontological and mapping evidence further shows that within the present area the Lower Pennant Group is completely absent along the East Crop, and in the south the Middle Pennant Group rests on the basal part of the upper similis-pulchra Zone of the Lower Coal Series. The Coal Measures in the area have been affected by at least two periods of earth-movements, an earlier period of Morganian age and a later Hercynian (Armorican) period. Nevertheless, the folding due to these movements is very gentle, the strong monoclinal folding found in the south-eastern region of the coalfield being absent in the present area. The faults trend mainly in a north-westerly direction and are all tensional structures. They are regarded as accommodation fractures initiated and developed during Coal Measures times.