Epicontinental basins in Poland: Devonian to Cretaceous - relationships between the crystalline basement and sedimentary infill

Abstract

Correlation between the structure of crystalline crust and the geometry and evolution of the epicontinental basins in Poland has been attempted. Using the data of the DSS and gravity measurements, several crustal blocks and their separating fractures have been distinguished. Northern and eastern parts of the country are characterized by typical three-layered cratonic crust of the Svecofennian age (unit A) while the southwestern part - by two-layered Variscan crust (unit C). The former was a basement of northeastern slopes of the Devonian through Cretaceous epicontinental basins whereas the latter underlies the southwestern part of the Permian-Mesozoic basins. Between the both there is - in the northwestern and central parts of the country - a belt of anomalous crust, 150-200 km wide (unit B). It is characterized by a thick upper layer of extremely low velocities (5.8-5.9 km/s) and in its northeastern zone (B’) - by a transitional crust-mantle layer with intermediate velocities 7.8-7.9 km/s. The latter coincides with the axial zone of the Permian-Mesozoic basin called Mid-Polish Trough (MPT) which was later inverted to form the Mid-Polish Swell (MPS). Earlier it was an external zone of the Devonian-Carboniferous epicontinental basins. The second zone of this unit (B”) adjoining from the south-west is perhaps equivalent to the external belts of the Variscan orogen, including its foredeep. At the end of the Early Palaeozoic the entire unit B was probably an assemblage composed of small crustal blocks of unknown origin: either of modified cratonic crust (proximal terranes?) or newly formed pre-Variscan crust or both. Transversal fractures of the NE-SW trend subdivide the area into individual segments: Baltic, Pomeranian, Kujavian and Małopolska ones. In particular, the last of them (southeastern part of the country) shows a different pattern of crustal units. Correlation with crustal fractures in the central areas of Poland is here difficult because of very sparse data. In the whole area studied some of the presumed fractures in the crystalline basement coincide with shallower fault zones recorded by geological investigations but some others do not.