Rudistove vapence belohorskeho souvrstvi u Biliny, paleogeografie, stratigrafie, sedimentologie

The open lagoon (bay) originated on the foot of a crystalline elevation (the so-called Most-Teplice elevation) during the Lower Turonian transgression at the NNW margin of the Bohemian Cretaceous Basin (Figs 1 and 3). The carbonates up to 12 m thick, due to limited siliciclastic input, deposited in the lagoon. Microfacies analysis enabled to distinguish two microfacies, comparables with SMF 9 and SMF 12-Bs sensu Flügel (2004). Microfacies indicate two various zones in the lagoon with different energy regime. Sparitic, bioclastic grainstones and rudstones with well-rounded and sorted bioclasts providing mutual support were deposited (Figs 4 to 7) in very shallow zone near the beach, under the low tide sea level. Floatstones (micritic carbonates ) with abundant, floating, less rounded and sorted bioclasts accumulated (Figs 8 to 11) in deeper zone with low water energy, below the fair-weather wave base. Except for bioclasts only some peloids were found. The scattered terrestrial grains (quartz, muscovite, gneisses) are present in small amount (max. 3 vol.%). The bioclasts predominantly comprise the fragments of rudist shells (around 95 vol.%). Sparse red algae, echinoderms, bryozoans and molluscs were identified. The siliciclastic sands with sponge spicules admixture simultaneously with carbonates settled in the adjacent open sea. The carbonate sand was transported away from the lagoon, forming thin layers in siliciclastic sands (Fig. 3) during extremely strong storms or ebb currents. The psammitic sequence with thin layers of the rudist carbonates belongs to the uppermost part of the Bílá Hora Formation of the Lower to Middle Turonian (Fig. 2). Rudist carbonates deposited in the lagoon correspond to a simple noncyclic transgressive succession. The sedimentation of rudist carbonates was suddenly terminated due to a rapid sea-level rise during the Middle Turonian transgression. The transgression was followed by sedimentation of basinal marlstones. Marlstones covered both the rudist carbonates and the uppermost part of the crystalline elevation.