Sand sheets interaction with aeolian dune, alluvial and marginal playa beds in Late Permian Upper Rotliegend setting (western part of the Poznań Basin, Poland)

Abstract

The Upper Rotliegend deposits of the western part of the Poznan Basin, Poland, represent three main sedimentary environments: aeolian desert (dunes and sand sheets), alluvial plain (alluvial covers with wadi deposits) and marginal playa. In arid periods, dunes prograded into areas dominated by alluvial sedimentation. During wet periods, alluvial deposits eroded the aeolian deposits and prograded onto dune fields. Playa sediments were deposited among aeolian or alluvial sediments during periodic expansions of the playa. These aeolian, alluvial and playa deposits are arranged in depositional sequences. Boundaries between sequences are marked by rapid changes in depositional facies; and by erosional bounding surfaces. Maximum Wetting Surfaces have been distinguished within some parts of the depositional sequences, which enable regional correlations. The sections studied are composed of thick sequences of sand and sand-gravel sheets. The sand sheets represent periods of stratigraphic condensation. Systems of sand sheets are typical of marginal areas of ergs. We distinguish nine depositional complexes (units) numbered 1–9 from base to top, respectively. These subdivisions are based on the interpretation of the sedimentological record in palaeoclimatic terms, and in terms of major erosional boundaries, which are probably of regional extent. In the aeolian deposits, major boundaries are associated with deflation surfaces related to the stabilization of groundwater levels. In the marginal playa deposits, the most important boundaries are correlated to transgressive surfaces that developed during playa expansion. Time gaps represented by the boundaries and the scale of erosion below them, are difficult to estimate. The depositional system of the Upper Rotliegend was dominated by sand and sand-gravel sheets, and is either a unique feature in the entire Southern Permian Basin or, as we assume, sand sheets occur more commonly in the Rotliegend than has been previously thought.