A review of the timing of coalification in the light of coal seam erosion, clastic dykes and coal clasts

摘要

Coal seam erosion, clastic dykes penetrating coal seams, and studies of coal clasts occurring in Late Paleozoic basins are used to demonstrate that coalification to the rank of subbituminous and bituminous coal occurred rapidly at shallow. depth in some parts of the Variscan orogen, implying a precondition of high paleotemperatures for the coal basins. The origin of clastic dykes, particularly in the (?) Bolsovian (=Westphalian C) deposits of the Plzen and Kladno-Rakovnik continental basins (Czech Republic) is shown to be a relevant factor in determining the timing of coalification. [Pesek J. (1978). Erosion and clastic dikes in coal seams of the central Bohemian basins and their significance for determination of plant substance coalification. Folia Musei Rerum Naturalium Bohemiae Occidentalis, Geologica 12, 1-34.] is of the opinion that the 50-100 cm deep fractures filled with superposed elastics originated in "fully ranked" high volatile bituminous coal, exposed during a hiatus between the (?) Early Westphalian C (Early Bolsovian) and Late Westphalian D (Late Asturian). These dykes have either irregular shape or form conic bodies with sharp boundaries. Their filling does not show any signs of postsedimentary deformation. Coal clasts occurring in clastic sediments either close to or far above the roof of coal seams are a few millimetres up to several centimetres in size, being subangular or angular. However, there also exist clasts of which one side is angular to subangular, whereas the opposite side is markedly sharp. In some basins coal clasts are embedded in units of the same age. They are mostly of somewhat lower rank (by 0.01-0.03 percent) than the nearest coal seam. Whereas [Gayer, R. A., Pesek, J., Sykorova, L, Valterova, P., 1996. Coal clasts in the upper Westphalian sequence of the South Wales coal basin: implication for the timing of maturation and fracture permeability. In: Gayer, R., Harris, J. (Eds.), Coalbed methane and coal geology, Geological Society Special Publication 109, pp. 103-130.] presumed in the South Wales foreland Basin a burial level of about 1 km with an elevated geothermal gradient (60 deg C/km), enhanced by hot fluids along thrusts, resulting in rapid coalification to the bituminous coal rank, no deep burial of organic matter could have occurred in the intramontane basins of the Bohemian Massif. Coalification of the Late Paleozoic seams must have proceeded at a very high rate. The only logical reason leading to such rapid coalification of biomass could have been high value of geothennal gradient at least in the basins of the Bohemian Massif.