Role of buckling and sub-salt basement activity on the evolution of salt pillow structure – insights from numerical models and seismic data from the Polish Basin

The presence of the thick Zechstein (Upper Permian – Wuchiapingian to Changshingian) evaporites strongly controlled deformation style within the Polish Basin that formed the eastern periphery of the epicontinental Permian-Mesozoic Central European Basin System, both during its Triassic to Early Cretaceous subsidence as well as during its Late Cretaceous to Paleogene inversion. Traditionally, formation of Zechstein evaporites has been associated with deposition of evaporitic cyclothems within large depression, with essentially no tectonic influence on evaporitic depo-systems. Furthermore, development of salt structures, in particular of salt pillows, has been commonly attributed to thin-skinned tectonics with minor role played by sub-salt fault zones. A newly developed tectono-stratigraphic model, constructed using seismic data calibrated by deep research wells from the central part of the basin (Bydgoszcz – Szubin area), suggests a significant role of Late Permian localized extension and deposition of syn-extensional Zechstein evaporites within the half-graben controlled by a deeply rooted normal fault. Consecutive basin inversion was associated with substantial uplift of the hangingwall block, formation of salt pillow built of locally overthickened evaporites deposited during active extension, and buckling of the Mesozoic supra-salt overburden. In order to test this new tectono-stratigraphic scenario of deposition and deformation of the Zechstein evaporites our own finite element-based numerical model has been used. In this study, we considered a two-dimensional plane strain model. The deformation was studied using the incompressible Stokes equations. A simplified stratigraphic sequence of the evaporitic series composed of alternated salt and anhydrite layers with a total initial thickness of 1 km has been used as it depicts general characteristics of the initial model constructed using seismic data. In several model runs, various geometrical and mechanical parameters of the salt and anhydrite layers and of the overburden have been considered. Additionally, various scenarios of deformation including the different rates of deformation and fault basement activity have been tested. We also took into account various models of sedimentation and erosion processes. Obtained results fully confirmed that indeed basement localised subsidence and later inversion might have played a much more important role during the deposition of Zechstein evaporites and then during the formation of salt pillows than previously assumed.