Castle Conglomerate Unit of the Upper Silesian Basin (Czech Republic and Poland): a record of the onset of Late Mississippian C2 glaciation?

Abstract

The Castle Conglomerate Unit in the Poruba Member (Early Namurian, Mississippian) is a lithosome composed of coarse-grained sandstones and conglomerates. Its thickness, lithology and extensive area make it unique in the paralic sequence of the Ostrava Formation of the Upper Silesian Basin. The unit was studied on the basis of information from exploratory boreholes, mines and its outcrop at the type locality in the Lučina Valley in Silesian Ostrava. Locally, the unit attains a thickness of up to 115 m (average 24.4 m). Its present-day area is approximately 992 km2. The axis of the conglomerate facies (with character of interbeds) in the unit has a NNE-SSW alignment and coincides with the axis of the basin’s maximum subsidence during the sedimentation of the Poruba Member. The maximum aggregate thickness of the conglomerate facies is 60 m; their present-day area of occurrence is approximately 480 km2. The Castle Conglomerate Unit represents deposits of a low-sinuosity river system. The upper part of the multistorey channel fill is characterised by channels filled with sand bedforms and occasional bars. Palaeocurrents show a low spread of vectors, primarily to the NNW. The lower part does not crop out; it is known, therefore, only from boreholes. The presence of the unit’s coarse-grained clastics inside the basin filling, which is dominated by siltstone and medium-grained sandstone, indicates a major drop in the base level, which is interpreted here as a drop in the sea level. We correlate this event with the major glacio-eustatic sea level drop at 323 Ma that can be connected with the onset of glaciation interval C2 of the Late Paleozoic Ice Age. Because the resolution of available stratigraphic and geochronologic data is not currently sufficient, an alternative explanation of the tectonic origin cannot be ruled out of the discussion and may also be valid in regard to the active Variscan foreland.