Was Zechstein basin in Poland just large “hole in the ground”? – new extensional model based on reinterpreted regional seismic data

Syntectonic growth strata in extensional settings are characterized by gross divergent geometries towards the master normal faults. Similar depositional patterns might be expected for evaporitic successions deposited during active extension although for salt-rich systems it might be very difficult to discern initial syn-tectonic depositional thickness variations from those caused by syn-depositional or early post-depositional halokinetic flow. Evaporites can be also passively infilling accommodation space that was created before their deposition either by tectonics or by erosion. Re-evaluation of regional coverage of seismic data from the Polish Basin provided evidences that, contrary to previous views, formation of Zechstein (Wuchiapingian – Changshingian) evaporites was significantly controlled by active extension. This sedimentary basin was located within the eastern periphery of the large epicontinental Permian-Mesozoic Central European Basin System and was filled with several kilometers of siliciclastics and carbonates, and thick Zechstein evaporites. Its axial part, the Mid-Polish Trough, characterized by the thickest Permo-Mesozoic sedimentary cover and developed partly above the Teisseyre–Tornquist Zone, underwent substantial Late Cretaceous–Paleogene inversion and currently forms a large regional anticlinal structure referred to as the Mid-Polish Swell. Some previous models of deposition of Zechstein evaporites assumed that evaporation started after catastrophic flooding of vast paleo-topographic depression (“hole in the ground” model). Regional coverage of seismic data was used to map major sub-Zechstein fault network that were responsible for crustal extension during development of the Polish Basin and were reactivated as reverse fault zones during its regional Late Cretaceous – Paleogene inversion. Those sub-Zechstein fault zones are often associated with locally increased thickness of Zechstein evaporites (salt pillows). It could be shown that such thickness increase could be in turn associated with gradual thickness increase of particular Zechstein cyclothems towards sub-Zechstein fault zones. All those observations were used to construct regional model of the Zechstein basin that was controlled by regional extension. As a consequence, Zechstein evaporites deposited within the axial part of the basin have been partly reinterpreted as growth strata related to active basement extensional tectonics.