Acta Musei Moraviae

The Ostrava Formation in the Czech part of the Upper Silesian Coal Basin (Serpukhovian, Upper Mississippian, Lower Namurian) – Definition of strata members, thickness and correlation horizons

The article summarizes current knowledge about the Ostrava Formation in the Czech part of the Upper Silesian Basin. The Ostrava Formation (Serpukhovian to Lower Namurian), which reaches a thickness of up to 3,430 meters, is composed, from its base, of the Petřkovice, Lower Hrušov, Upper Hrušov, Jaklovec, and Poruba members. Sedimentation within the Ostrava Formation occurred intensively in the foredeep along the Brunovistulicum block in a paralic molasse setting, influenced by episodic tectonic compression associated with the Variscan orogeny. In the southeast, within the platform area, there was a reversal marked by reduced subsidence of the basin floor and localized uplift. This is reflected in the thinning and discontinuity of sedimentation at the boundary between the Lower and Upper Hrušov Members and at the base of the Poruba Member. Sedimentation progressed gradually from above-water deltaic environments with continental sediments and coal seams, through delta-marine transitions, to shallow marine settings. The Ostrava Formation exhibits a highly diverse range of paralic lithofacies, reflecting significant lateral variability in depositional environments-ranging from shallow marine, through transitional zones between sea and land, to fully terrestrial settings. The complex coastal plain environment included swamps, wetlands, and coal-forming peatlands with lakes and a network of meandering rivers with associated floodplains. Based on the well-defined boundaries of its constituent members, thickness maps were constructed for each member of the Ostrava Formation. The impact of Variscan tectonics, progressing from west to east toward the Brunovistulicum, is evident in the variations in member thickness and sedimentation rates. Overall, member thicknesses decrease from northwest to southeast. In the foredeep, the sedimentation rate during the formation of Ostrava Formation members ranged from 385 m/Ma to 3,460 m/Ma, while on the platform, sedimentation proceeded at a much slower pace, and in some intervals, it ceased altogether.