Coal clasts in the Bolsovian (Westphalian C) sequence of the Kladno-Rakovník Continental Basin (Czech Republic): implications for the timing of maturation

Angular coal clasts confined to coarse-grained feldspathic sandstone deposited in the Tuchlovice Mine pit in the Kladno-Rakovník basin were studied. These fragments of various orientation consisting of banded, dull and bright coal of Bolsovian age (=Westphalian C) occur as an admixture being some tenths of a millimetre up to 40x30x20 mm large. The clasts are coalified into high volatile bituminous coal showing an average reflectance corresponding to vitrinite (Rr = 0.64%). They reveal no signs of progressing diagenetic changes after their redeposition. Some clasts exhibit cracks filled with clay minerals, which constitute common admixture in coal seams of this basin. Cracks do not extend into the adjacent sediment. Their origin is connected with coalification processes in original coal seam, which occurred prior to erosion. The investigation of miospores showed that the coal clasts come from the eroded Lubná coal seams of Bolsovian age. Since the Radnice Member is the oldest unit of the central and west Bohemian basins, the coalification of organic matter into high volatile bituminous coal must have occurred within this member, i.e., during some hundred thousand years at a depth of only a few tens, maximum first hundred metres. The achieved results argue for a high paleotemperature gradient, which must have existed during the Carboniferous in the Bohemian Massif.Similar conditions are known, e.g., from South Wales basin in UK (Gayer et al., 1996) and from the Upper Silesian basin (Kožušníková et al., 1999)