The South Wales Coalfield: Low grade metamorphism in a foreland basin setting?

The South Wales Coalfield comprises Namurian, Westphalian and Stephanian sequences which developed in the external zone of the Variscan Orogenic Belt. Sedimentation is thought to have been concentrated in a paralic basin related to crustal flexure in a foreland basin setting, linked to Variscan tectonic loading to the south. It has long been recognized that coal rank varies across the Coalfield, from high-volatile bituminous rank in the east and southeast, to anthracite in the northwest. This is confirmed here on the basis of comprehensive volatile matter and vitrinite reflectance data sets, computer contoured to generate isovol and isoreflectance maps. A steady increase in coalification with depth is also recorded. Clay mineral investigations show the presence of illite and kaolinite across the entire coalfield area, while neither pyrophyllite nor mixed clays were detected. Illite crystallinity values from mudstones are in the range 1.20-0.44 Delta degrees 2 theta. Vitrinite reflectance data indicate maturation temperatures in the range 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. Coalification appears to have occurred rapidly (maximum 5 Ma) following deposition, implying high heat flux during Westphalian times. This is also indicated by fluid inclusion evidence from quartz crystals in clay ironstone nodules. The inference is of a high geothermal gradient (c. 50 degrees C km(-1)) during Westphalian times, contrasting with an earlier gradient estimate of 20-37 degrees C km(-1). Relatively high gradients have also been determined previously for the Ruhr Coalfield, which occupies a similar tectonic setting in the external zone of the Variscan Orogenic Belt. Such a gradient is at odds with the proposed foreland basin tectonic setting for the South Wales Coalfield, in which low geothermal gradients would be anticipated.