Controls on evaporite facies distribution during the early phases of the Zechstein Basin development (Zechstein 1, Upper Permian): Implications from the marginal part of the basin in SW Poland

Abstract

Amongst the least understood issues in the giant evaporite basin evolution is their early development. Here, we use a unique set of archival data from the Fore-Sudetic Homocline in SW Poland to investigate the controls on evaporite facies distribution in the marginal part of the Zechstein Basin after the Zechstein Sea transgression (Upper Permian, Z1 cycle). Based on integrated data from boreholes, subsurface mining survey and seismic profiling we constructed a 3D structural model, which showed that the carbonate, sulphate and chloride facies boundaries originally extended W–E, irrespective of the fault patterns in the basement. The thickness distribution for the Zechstein Limestone (Ca1), Lower Anhydrite (A1d) and Upper Anhydrite (A1g) evidences carbonate and sulphate platform development in respective evaporitic phases. Although the sea bathymetry and progressive seawater evaporation were the main factors responsible for the northward progradation of the evaporite facies, the relief over the sulphate platforms implies a significant influence of sea currents on the sulphate deposition. The channelised circulation of variably saturated seawater decreased the overall accumulation rate in the channels and promoted the high-and-trough topography development over the platforms. The Upper Anhydrite (A1d) top relief was evened by the Oldest Halite (Na1) rocksalt deposition. Prior to the Zechstein Limestone (Ca1) deposition, the distribution of clastic facies was mostly restrained by the dune relief inherited after the aeolian sedimentation in the Rotliegend. This old relief partly influenced also the Zechstein Limestone (Ca1) carbonate thickness and subfacies distribution and was eventually buried by the Lower Anhydrite (A1d) sulphates. There is no evidence on tectonic controls on evaporite depositional patterns throughout the Z1 cycle in the studied area.