THE SOUTH WALES COALFIELD - LOW-GRADE METAMORPHISM IN A FORELAND BASIN SETTING

The South Wales Coalfield comprises Namurian, Westphalian and Stephani an sequences which developed in the external zone of the Variscan Orog enic Belt. Sedimentation is thought to have been concentrated in a par alic basin related to crustal flexure in a foreland basin setting, lin ked to Variscan tectonic loading to the south. It has long been recogn ized that coal rank varies across the Coalfield, from high-volatile bi tuminous rank in the east and southeast, to anthracite in the northwes t. This is confirmed here on the basis of comprehensive volatile matte r and vitrinite reflectance data sets, computer contoured to generate isovol and isoreflectance maps. A steady increase in coalification wit h depth is also recorded. Clay mineral investigations show the presenc e of illite and kaolinite across the entire coalfield area, while neit her pyrophyllite nor mixed clays were detected. Illite crystallinity v alues from mudstones are in the range 1.20-0.44 Delta degrees 2 theta. Vitrinite reflectance data indicate maturation temperatures in the ra nge 123-290 degrees C; clay mineral and IC values suggest temperatures up to 230 degrees C, while previous descriptions of pyrophyllite from the Coalfield would allow locally for even higher temperatures. Coali fication appears to have occurred rapidly (maximum 5 Ma) following dep osition, implying high heat flux during Westphalian times. This is als o indicated by fluid inclusion evidence from quartz crystals in clay i ronstone nodules. The inference is of a high geothermal gradient (c. 5 0 degrees C km(-1)) during Westphalian times, contrasting with an earl ier gradient estimate of 20-37 degrees C km(-1). Relatively high gradi ents have also been determined previously for the Ruhr Coalfield, whic h occupies a similar tectonic setting in the external zone of the Vari scan Orogenic Belt. Such a gradient is at odds with the proposed forel and basin tectonic setting for the South Wales Coalfield, in which low geothermal gradients would be anticipated.