Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Transgressive nearshore limestone of the Korycany Member (Cenomanian) in the central part of the Bohemian Cretaceous Basin and the discussion on the origin of the basin
Published online: 7 December 2020
AbstractTwo localities of shallow-water nearshore limestone were discovered in the central part of the Bohemian Cretaceous Basin (Figs. 1 and 2). The limestone form the basal part of the transgressive Korycany Member of the Peruc-Korycany Formation (Upper Cenomanian) in two deep boreholes J 470 812 Skalka and 416 320 Ralsko, situated within a dense borehole network. Sandstone and sandy siltstone representing the remaining part of the Korycany Member were abruptly overlain by marlstone of the Bílá Hora Formation (Fig. 3). Apart from the two other drillcores with a reduced limestone layers, all other drillcores contain only sandstone and siltstone of the Korycany Member. Limestone whose extent has been estimated 0.20-0.30 km2 laterally passes into sandstone. It was deposited at the foot of morphological basement elevations in small areas with limited siliciclastic input (Figs. 4 and 5). The thickness of the Korycany Member is strongly reduced on the elevations whose uppermost part were exposed during limestone sedimentation. In the borehole J 470 812 Skalka, the limestone reaching 4.10 m in thickness include fenestral bioclastic wackestone in the lower part, whereas the upper part is formed by bioclastic floatstone (Figs. 6 to 10). The bioclasts (20-25 %) floating in micrite matrix comprise predominantly bivalves, foraminifera and red algae, while gastropods are very rare. The content of terrigenous quartz grains oscillate between 15 and 25 %. The irregular fenestral structures are filled with sparite. Limestones were deposited in the shallow protected nearshore zone (open lagoon) without washing out ofmicrite. Occasionally, the sediment was exposed in the intertidal zone and desiccation (fenestral) structures originated. In the borehole 416 320 Ralsko, the limestone thickness is 2.30 m. Limestone consists of bioclastic-intraclastic floatstone with sparite matrix (Figs. 11 to 13). It was deposited in a very shallow nearshore zone under low-tide sea level with high water energy washing out micrite. Micritic intraclast with dispersed bioclasts and quartz grains were transported from the nearby protected area (open lagoon). Intraclast formation resulted from destruction of partially lithified micritic sediment during extreme storms or, alternatively, originated during transgression phase. The new localities of nearshore limestones in the central (axial) part of the Bohemian Cretaceous Basin enlarge the number of limestones localities in an analogous position. Many elevations still existed in the axial part of the basin at the end of the Cenomanian, having been submerged as late as during the Lower to Middle Turonian. The long-term existence of elevations in the central part of the basin support the idea that the Bohemian Cretaceous Basin formation was controlled mainly by global Cenomanian transgression invading the topographically lower part of the Bohemian Massif. The idea of the strike-slipe (pull-apart) type of the basin lacks evidence because there are no documented strike-slipe systems during the formation and existence of the basin. The thickness of the Korycany Member is mainly controlled by the basement morphology, being the highest in the narrow, marginal nearshore zone off the axial part of the basin. In addition, the thickness of the Korycany Member accumulated during approx. 1-1.5 Ma is low. There is no tectonic depocenter migration and the value of „total“ subsidence about 0.1mm/year or less is characteristic of the cratonic basins or basin on the passive continental margin.
Published online: 7 December 2020