ORGANIC MATTER AS INDICATOR OF THE DEGREE OF METAMORPHISM OF THE CARBONIFEROUS ROCKS IN THE SOUTH WALES COALFIELDS

Abstract

Isoreflectance maps of vitrinites in coals and phytoclasts (coaly inclusions) in the Coal Measures of S. Wales show a progressive increase in vitrinite reflectance with increase in coal rank and grade of metamorphism towards the anthracite area in the west, without significant variation in reflectances of coals with present depth. Differences between values of vitrinite reflectance in coals and phytoclasts in the same profile are recorded and attributed to variations in depositional environments, and to susceptibility to alteration of the host rocks. Spores in the Coal Measures are progressively carbonized (from golden brown to black) towards the west, and this is not a function of present depth of burial. Reflectance of vitrinites in coals, degree of spore carbonization and coal rank are used in definition of three different diagenetic zones in the coalfield. The Coal Measures could have acted as a source for accumulations of gas in the Permo‐Triassic sequence of the south Irish Sea, if necessary geologic conditions are present. Burial metamorphism (application of Hilt's law) could have caused the coalification of S. Wales coals up to the high and medium volatile bituminous stages (up to the late diagenetic stage). A relationship is established between the lateral variation in coal rank in the coalfield and the regional variation in coal rank in the southern British Isles, where two regions of markedly different geothermal history are recognised. The Diagenetic Zone in the E and S parts of the coalfields marks the final stage in the thermal history of the cold region (the W margin of London Stable Massif), whilst the Anchimetamorphic Zone in the NW part (the anthracite area) represents the final stage in the thermal history of the hot region (the S margin of St. George's Land).Phytoclasts and spores are recorded for the first time from the Dinantian massive limestones of S. Wales.