Palaeogeothermics in the Czechoslovak part of the Upper Silesian Basin

SUMMARY The coalification data (vitrinite reflectance, percentage R,) from 49 boreholes located in the Czechoslovak part of the Upper Silesian Basin were processed and the palaeotemperature gradient which prevailed during the Namurian and the Early Westphalian was estimated. Very high gradients (mean: 95 K km-') are calculated at least in the western part of the basin during the Namurian A. At that time, the Ostrava Formation (FM) was deposited with a high subsidence rate. The corresponding heat flow density of 200 mW m-2 may not reflect the characteristic heat flow density of the region, but may represent a subsurface value within the uppermost 2-3 km thick layer, probably sustained by the convective system inside the basin. The gradient during the Namurian B, C and Westphalian A is lower (mean: 77K km-' in the Karvina FM). It is suggested that the decrease in the gradient coincides with changes in the basin development. An intraNamurian hiatus occurred at the turn of the Namurian A and B. The lower thermal regime which governed during the sedimentation of the Karvina FM, results also from the R, data of the Ostrava FM at the southern rim and in the Karvina part of the basin (mean gradient 60-65 K km-I). Due to the relatively small thickness of the Ostrava FM and/or the big thickness of the Karvina FM in these areas, the sediments of the Ostrava FM reached the maximum temperature during the deposition of the Karvina FM. Thus, the R, data must reflect the lower gradient. By applying palaeogradients estimated in both formations together with present mean thermal conductivities within them, a heat flow density of 115-130 mW m-2 was computed. This value agrees well with the heat flow of 125mW m-' estimated for the Westphalian A in the Ruhr Basin by the same method.