The effect of paleotopography, tectonics and sediment supply on quality of coal seams in continental basins of central and western Bohemia (Westphalian), Czech Republic

Abstract

The Radnice Member, the most important coal-bearing unit of the continental basins in central and western Bohemia (Czech Republic), is used here to demonstrate the interplay of controls on mire formation and distribution in continental settings. Its Middle Westphalian strata are interpreted as tectonically active or erosively incised river valley infills principally controlled by tectonic subsidence and clastic input. Another important controlling factor was the paleotopography, with some hills up to 200 m above the valley floor, which was responsible for the distribution of siliciclastics within the dendritic depocenter. The interplay of these factors is described here in terms of base-level changes expressed in the form of A/S ratio (accommodation/sediment supply ratio, [Martinsen, O.J., Ryseth, A., Helland-Hansen, W., Flesche, H., Torkildsen, G., Idil, S., 1999. Stratigraphic base level and fluvial architecture: Ericson Sandstone (Campanian), Rock Springs Uplift, SW Wyoming, USA. Sedimentology, 46, 235–259]). Vertical changes in A/S ratio were related mostly to fluctuation of tectonic subsidence whereas lateral variations of this ratio between valleys or within them were connected with uneven clastic supply of individual valleys from their source areas. In valleys where clastic input overreached the accommodation space production (A/S < 1), clastics filled all the available accommodation and no or small mires disturbed by fluvial deposition developed. Valleys, where clastic input was smaller than increment of accommodation space (A/S > 1) due to high subsidence rate or low clastic input, sediments transported by axial drainage were confined to the upper valley parts. The lower and middle parts of the valleys which starved of clastic sedimentation provided ideal conditions for long-lasting peat accretion. As a result, two economically important groups of coals developed in the Radnice Member in the Kladno-Rakovník Basin: the older Radnice group and the younger Lubná group. A high rate of base-level rise was typical of the deposition of the Radnice group and led to the formation of extensive mires in lower parts of the valleys that had a restricted clastic influx. The mires of the Lubná group formed during a time when the base-level rise was still high but lower than during the deposition of the Radnice group. Mostly small but long-lasting mires developed in sedimentary shadows located along the valley margins or in smaller valleys away from the main clastic transport and fluvial dynamics.