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Depositional architecture of the Zabrze Beds (Namurian B) within the Main Anticline of the Upper Silesia Coal Basin, Poland

Kędzior A.

Studia Geologica Polonica

2008

Vol. 129

131-156

The Zabrze Beds (Namurian B) constitute a part of the flexural Variscan foredeep Upper Silesia Coal Basin (USCB) infill. On the Main Anticline area this unit starts the continental stage of the basin development. The Zabrze Beds were deposited on an extensive alluvial plain probably constructed by meandering river systems. The fluvial sediments were accumulated both within channels and on overbank areas. The thick (<5 m) sandstone bodies are considered as the channel fills and usually form extensive sheets whose width/thickness ratio exceeds 15. The thinner sandstone bodies, fine-grained sediments and phytogenic material were deposited on the overbank areas. The thickness of the Zabrze Beds on the Main Anticline area decreases eastward from ~250 m down to 3 m over a distance of 35 km. The thickness and number of the sandstone bodies diminish eastward with the thickness of the whole unit. The thickest bodies occur in the western part of the studied area, attaining 60 m, whereas in the eastern part they rarely exceed 30 m. As a rule, the thick sandstone bodies are laterally extensive approaching up to 10 km. Subordinately appear isolated bodies that are usually thinner. In those areas where the coarse-grained sediments are in the minority the inter- connectedness ratio is relatively low. The deposition of the thick sandstone bodies is related to stabilization of the river banks with accompanied balanced aggradations of the overbank areas, or is result of superposition of several generations of channel fills. The coal seams are numerous in the Zabrze Beds despite the small thickness of this unit. The growth of successive generations with extended eastward areas reflects increasing positive accommodation space created mainly by flexural bending of the basin substratum. The thirteen coal seams that have been identified within the Zabrze Beds, occur in different classes of thickness. Except thick (<3 m) and very thick (up to 24 m) seams, the laterally extensive and continuous coal seams are present thin (>0.3 m) and often lenticular in shape. The zones where the coal seam increase in thickness have been observed and they were the result of coalescence or the superposition of successively younger generation peat bogs. The characteristic feature of the succession of coal seams is the eastward increase of their lateral extend. The va- rious types of coal seam splitting have been observed. These phenomena were caused by lateral migration of the channel belts and growth of the crevasse splays, or by action of synsedimentary faults.