Abstract

Prior to regression of the epeiric sea, the Bohemian Cretaceous Basin was affected by tectonic uplift that caused rejuvenation and erosion in the source area and a large influx of sand onto the muddy shelf. The Cretaceous sand belts of the Intrasudetic Depression (northeastern Bohemian Basin) were deposited along tectonically controlled breaks in slope on the inner shelf floor. These sand belt sequences of Turanian age, underlain, separated by, and interdigitated with shelf mudstone, have been studied along the strike of the sandstone bodies for a distance of about 50 km. The geometry of the sandstone bodies and their internal structure indicate that the initial stage of their formation was due to the infilling of large-scale scours in the shelf floor. Gigantic (exceeding 20 m), steeply cross-bedded units of sandstone, underlain by massive, coarse sandstone with coquina lag deposits were laid down on concave erosional surfaces. As the bodies developed upward, sand became finer and better sorted. Bed forms changed upward in response to a less diversified sea floor topography and low energy regime. Consequently, the gigantic cross-bedded units gave way to sand waves and dunes at the top of the sand belt sequences. Consistent offshore orientation of the gigantic foresets is believed to be related to tectonic control of sea floor topography. The sand waves and dunes were driven both offshore and parallel to the hypothetical shoreline. Numerous examples of sea floor erosion and reversal of paleocurrents (expressed by herrringbone cross-bedding) have been observed. Periodic storm currents superimposed on a tidal current system are the most likely depositional mechanisms in this tectonically controlled basin.