Geology of the Ostrava-Karviná coalfield, Upper Silesian Basin, Czech Republic, and its influence on mining

Abstract

The Ostrava-Karviná coalfield represents the Czechoslovak part of the Upper Silesian Basin, which is one of the most important European coal deposits. It is an exception in Europe, as the major part of the coal reserves are of Namurian age. The formation of the Upper Silesian Basin resulted largely from the tectonism that occurred during three periods. These were the origin of the Cadomian, Variscan and Alpine structural stages. The Variscan evolution comprises: the platform development (Middle Devonian—Early Carboniferous), the period of marine molasse (latest Viséan-Namurian A), paralic molasse (Namurian A) and terrigenous molasse (Namurian B and C-Westphalian). Two major basinal tectonic units with contrasting facies and tectonic structures originated in the latest Viséan, as a result of the Variscan mobilization: the Variscan foredeep, along the western margin of the basin (with thick coal-bearing Carboniferous and complicated fold and fault structures), and the Upper Silesian stable block, an extensive platform in the eastern part of the basin, with a reduced thickness of sediments and simple tectonic structure. The Alpine tectonic movements rejuvenated the older major faults, influenced the origin of a very sharply marked pre-Tertiary paleorelief, formed on the top of Carboniferous deposits, and led to the nappe structure of the Mesozoic and Tertiary rocks which cover southern parts of the Ostrava-Karviná Coalfield. The paralic Ostrava Formation (Namurian A) is up to 3000 m thick, with individual cycles varying from 6 to 20 m in thickness, reflecting the broad variation in depositional environments. It contains more than 170 coal seams, with an average thickness of 73 cm. About 80 marine and brackish horizons, more than 100 freshwater horizons, 16 coal tonsteins and 30 layers of “whetstone” rocks serve for correlation. The continental Karviná Formation (Namurian B, C and Westphalian A), deposited after tectonic inversion, is up to 1000 m thick. It contains up to 90 coal seams, 180 cm thick on average (maximum up to 16 m). The cycles are thicker and laterally less stable than those in the Ostrava Formation and contain frequent layers of conglomerates and coarse-grained sandstones. Only 25 freshwater horizons are known. The typical feature of both formations is the frequent occurrence of tuffogenous layers (whetstone rocks and tonsteins) which are important strategraphic key horizons. The coal seams mainly comprise bituminous coal. At the lower boundary of the limnic Karviná Formation there is an abrupt increase in the abundance of inertinite (up to 80% of the coal). The majority of coals are of a high quality and can be used for coke production. The sulphur and phosphorus content is very low (less than 0.7% and 0.02–0.11%, respectively). The geological reserves are estimated to be about 16,000 million t of coal. The variegated barren rocks, the so-called red beds, have been recorded over the entire southwestern part of the basin. The genesis of this rocks is discussed. The following geological phenomena complicate the deep mining activity: irregular development and distribution of the coal seams; complex tectonic structure; erosion wash-outs; the effect of the formation of red beds on coal beds; gas, rock and water bursts; anomalous geomechanic features; and the complicated paleorelief of the coal-bearing Carboniferous. These phenomena and their influence on the mining activity are described.