Subsidence and inversion in the western part of Polish Basin - data from seismic velocities

Abstract

Seismic interval velocities in the Baltic Formation (Triassic - Lower Bunter) have been used to calculate the scale of Mesozoic subsidence, latest Cretaceous/Palaeogene inversion and erosion in the western part of the Polish Basin. The study has been concentrated on the reconstruction of thickness of the Upper Cretaceous sediments which have been removed from vast areas after their inversion. The data enable a rough estimation only and a qualitative comparison of these processes in different areas. Their value is also limited in the territories affected by salt tectonics. It is suggested that the axial zone of the northwestern (Pomeranian) zone of the Mid-Polish Trough (MPT) started to rise at the end of Turonian. As a result, a thin cover of the Upper Cretaceous sediments was deposited here and removed later in the last stage of regional inversion of the MPT - its transformation into the Mid-Polish Swell. Farther to the south-east - but still within the Pomeranian segment - the local Zechstein salt displacements interfered with the regional uplift and caused the differentiation of the Upper Cretaceous thickness. Northwestern branch of the Mid-Polish Swell (Kołobrzeg Anticline) inverted rather in a single pulse, at the end of Cretaceous. Along the southern slope of the Polish Basin (Fore-Sudetic Monocline) the primary Upper Cretaceous thickness increased to the north and west reaching the maximum of more than 1000 m at the Polish-German boundary. In the area of the present pre-Cretaceous subcrops the Upper Cretaceous cover was thinner (300 m and less). The investigations should be continued including more reliable data from sonic logs which are scarce so far.