Jurassic pebbles in the Cretaceous sandstones of the Bohemian Basin as a possible tool for reconstruction of the Late Jurassic and Late Cretaceous palaeogeography

A new find of pebbles of Jurassic silicites in the Coniacian sandstones of the Bohemian Cretaceous Basin in N Bohemia has permitted analysis of the stratigraphic extent of Jurassic sediments in the NW part of the Bohemian Massif. The studied silicites are dominated by the rhax microfacies, while bioclastic and oolitic microfacies are less common. The thickest section of Jurassic sediments in the NW part of the Bohemian Massif has been obtained from the Doubice borehole. It is represented by basal clastics overlain by a 70 m thick succession of silicite-free carbonate rocks which range in age from Callovian to Lower Kimmeridgian. These deposits are dominated by the bioclastic microfacies whereas the rhax and oolitic microfacies are missing. The studied silicite-bearing sequence is younger than the carbonate rocks in Doubice borehole and was deposited mostly in a deeper zone probably during the Late Kimmeridgian transgression, much like in the Upper Frankenalb in SE Germany. The extensive Jurassic basin in the Bohemian Massif was connected with the S part of the Polish and German basins and – via the Hessian (Saxonian) Seaway – with the Jurassic basin in SE Germany. The Jurassic sediments were mostly eroded from the Bohemian Massif during the Early Cretaceous with the exception of small relics. The remnant of the Jurassic deposits preserved in the area of the West Sudetic Island supplied coarse debris during Late Cretaceous to the Bohemian Cretaceous Basin.