Mesozoic thickness pattern in the Mid-Polish Trough

Abstract

The Mid-Polish Trough (MPT) is well recorded in the distribution of thickness of the Mesozoic sediments. Its shape was most distinctly delineated in the Early Triassic, and Early to Middle Jurassic, when thickness gradients attaining 100 m/km were reached. However, because the regional faults bordering the MPT were not active throughout its history, the existence of Mid-Polish Rift has not been confirmed. The strongest thickness gradients may have been caused by the periodical activity of the sub-Zechstein faults, which did not penetrate the Mesozoic strata due to the damping effect of plastic Zechstein salts. On the contrary, local faults, forming (mainly during the Late Triassic) syn-sedimentary grabens, are a common feature in the MPT and its surroundings. Transversal subdivision of the MPT and its slopes into at least two segments (Pomeranian and Kuiavian) is clearly visible in the thickness pattern. It is expressed by the presence of separate depocentres, reversal of asymmetry, differences in stratigraphical sequences observed on the palaeomorphological terraces south-west of the MPT, and by the structural variations after the inversion. The scale of inversion, which transformed the MPT into the Mid-Polish Swell (MPS), is unclear and needs further investigations. Estimation of the thickness of the Upper Cretaceous sediments removed by erosion is a key problem in this respect. It should take into account both, the effects of the regional inversion and the local changes resulting from the last stage of strong salt displacements.