Conditions of accumulation and sedimentary architecture of the upper Westphalian Cracow Sandstone Series (Upper Silesia Coal Basin, Poland)

Marek Doktor

Abstract


Warunki sedymentacji i architektura krakowskiej serii piaskowcowej Górnośląskiego Zagłębia Węglowego

The Cracow Sandstone Series (upper Westphalian) forms the uppermost segment of the coal-bearing succession that makes up the bulk of the Variscan foredeep-basin fill in the Upper Silesia Coal Basin. The series, up to 1640 m in stratigraphic thickness, consists entirely of non-marine deposits. Fiveteen lithofacies have been distinguished in the sediments of this series. The Cracow Sandstone Series is subdivided here in two lithofacies associations. The sandstone association consists mainly of medium- to coarse-grained sandstones that form packages up to several tens of metres thick (max. 140 m), with surfaces of erosion at base. These bodies are separated by less voluminous packages of the mudstone association that consists mainly of mudstones and coal seams, which locally make up the predominant part in the sequences of this association. The sediments of the sandstone association are believed to have originated within wide channel tracts of distal sandy braided rivers. The sediments of the mudstone association with the interbedded coal seams are interpreted as floodplain deposits. The predominant type of peat bogs, represented now in the coals of the Cracow Sandstone Series, were wet forest swamps. The peat bogs were probably slightly domed and their margins received clastic material from adjacent channels. This resulted in the frequent lateral splitting of the coal seams. The large-scale splitting of seams is associated with lateral transition of fine-grained floodplain deposits into coarse-grained channel deposits. The vertical alternation of the channel and floodplain deposits is the result of natural processes on an alluvial plain that resulted in shifting positions of depositional environments, first of all avulsion of the whole fluvial tracts. The coal seams in the Cracow Sandstone Series do not form extensive sheets of persistent thickness, and their geometry depends on the course and evolution of the network of fluvial tracts in the alluvial system. The internal geometry of the Cracow Sandstone Series was controlled to a large extent by differential compaction of sediments, notably by rapid compaction of peat.

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