Controls on basal Zechstein (Wuchiapingian) evaporite deposition in SW Poland

Abstract

The development of basal Zechstein (Wuchiapingian) strata in SW Poland indicates the existence of a varied relief inherited after the flooding of the pre-existing depression by the transgressing Zechstein sea. The deeper parts of the basin were the place of development of thin basinal Zechstein Limestone showing sedimentary condensation manifested by bored and encrusted grains and thick evaporites (mostly halite). In shallow parts, Zechstein Limestone reefs followed by thinner evaporite sequences (dominated by anhydrite) were formed. The analysis of 2D sections extracted from 3D seismic data showed that instead of three conventionally recognized evaporitic units in the PZ1 cycle, five units occur (from the base to the top: Lower Anhydrite, Lower Oldest Halite, Middle Anhydrite, Upper Oldest Halite, Upper Anhydrite). In any particular place their number may vary from two (Lower Anhydrite at the base of the PZ1 cycle and Upper Anhydrite at the top of the PZ1 cycle) to five. Two units of Lower Anhydrite occur throughout the platform and basinal zones, showing a deepening-upwards (transgressive) trend. Halite sedimentation, in the deepest parts of salt basins, began shortly after the deposition of the upper Lower Anhydrite while on sulphate platform areas, sulphate deposition continued. The Lower Oldest Halite deposits occur in the depressions. Between the halite basins, anhydrite platforms occur, and the thickness of anhydrite platform deposits is smaller than is observed in salt basins. The Upper Oldest Halite in turn is recorded above the anhydrite platform. The two halite units represent different phases of development of halite basins. Formation of the Lower Oldest Halite basins was related to pre-Zechstein depressions, although in some cases their syndepositional subsidence was controlled by reactivation of former faults within the sub-Zechstein basement. In turn, the Upper Oldest Halite basins used the accommodation space created due to anhydritization of the Lower Anhydrite deposits, that were composed originally of selenitic gypsum, and the associated volume loss. 3D seismic data, constrained by borehole data, show that the PZ1 evaporites in SW Poland were deposited in a far more complex and dynamic system than previously assumed.