The Sequence and structure of the southern portion of the East Crop of the South Wales Coalfield

Abstract

Summary The paper describes the sequence and structure of the Upper Carboniferous rocks along that part of the East Crop of the South Wales Coalfield between Pontypool in the north and Risca in the south. The Coal Measures of this region have hitherto been regarded as representing a thin but nevertheless complete succession. Nowhere, however, along this portion of the East Crop is there a complete sequence of Coal Measures, as that formation is developed in more westerly parts of the coalfield. Indeed, there is no outcrop of the Lower Coal Series over some considerable part of this tract; that series is represented in other parts only by its basal strata. In certain transverse valleys a fuller (but still incomplete) sequence of the Lower Coal Series is represented at outcrop by measures including the Similis-Pulchra Zone. Likewise the Pennant Series, which in this paper is subdivided into Upper, Middle, and Lower groups, is incomplete; only the Upper and Middle groups are represented along most of the East Crop. The Upper Pennant Group is that portion of the Pennant Series which has hitherto been regarded as alone constituting the “Pennant Sandstone” of Monmouthshire. The Middle Pennant Group, which contains massive conglomerates and carries coloured measures previously known as the Deri Group, has in the past been erroneously included in the Lower Coal Series. Thus the relationship of the Coal Measures along the East Crop is conditioned by major unconformity of Morganian age, which was accompanied by overstep and overlap, so that high horizons within the Pennant Series rest unconformably upon the Lower Coal Series and sharply overstep on to the Millstone Grit. The region has suffered from movements of two definite ages :— (a) An earlier period, of Morganian age, resulting in the elevation of a land-mass east of the coalfield from which was derived sediment incorporated in the Middle Pennant Group. (b) A later, Hercynian (Variscan) period. These combined movements have resulted in monoclinal folding along north-north-east to south-south-west lines, the older rocks being more steeply inclined than those of the Pennant Series. The dominant north-north-east to south-south-west folding of the eastern end of the South Wales Coalfield is described, and the considerable structural similarity between the east crops of the South Wales and Forest of Dean coalfields is indicated. The effects of Morganian uplift are considered to have been widespread and may have affected an area including the Forest of Wyre, Forest of Dean, and Somerset and Gloucestershire coalfields. The paper includes a comparison of the sequence in the area under description with that in other more westerly parts of the South Wales Coalfield and presents a suggested correlation of the Pennant Series of these areas. The Forest of Dean sequence is correlated with that of South Wales.