RECONSTRUCTION OF AN EARLY PENNSYLVANIAN FLUVIAL SYSTEM BASED ON GEOMETRY OF SANDSTONE BODIES AND COAL SEAMS: THE ZABRZE BEDS OF THE UPPER SILESIA COAL BASIN, POLAND

摘要

. The Zabrze Beds (Kinderscoutian), together with the underlying Jejkowice Beds (late Chokerian– Alportian) of local extent, are the oldest deposits of the fully terrestrial phase of sedimentation in the Upper Silesia Coal Basin. The maximum thickness zone of Zabrze Beds is trending SSW–NNE and reflects the migrating narrow zone of basin-floor main subsidence. The westward erosional termination of Zabrze Beds is due to the tectonic thrusting of the Devonian to Mississippian rocks of the Variscan Moravo-Silesian orogen, whereas their eastward thinning and pinch-out towards the craton of Ma?opolska Block is due to the related forebulge growth. The present-day tectonic structure of the basin, with the east-trending Main Anticline near Zabrze, is due to the Cenozoic northward thrusting of the Carpathian orogenic front. The deposition of Zabrze Beds occurred in a north-trending alluvial valley formed and filled in by a bedload-dominated, sandy river system. In the southern part of the basin, the alluvial deposits are characterized by numerous thick, multistorey, sheet-like channel-belt sandstone bodies with sparse overbank and phytogenic deposits, indicating a braided river system. This alluvial architecture passes northwards – in the basin’s area of Main Anticline – into an architecture composed of smaller, isolated sandstone bodies of single-storey channel belts with a much higher relative proportion of overbank and phytogenic deposits, indicating a meandering river system. The down-valley transformation of the fluvial system from braided into meandering is attributed to such factors as a spatially differential sediment supply to the system and a non-uniform axial valley gradient.