Facies analysis and petrography of Permian sandstones – key for stratigraphy and depositional environment of a new tetrapod track site (Southern Permian Basin, Germany)

The Upper Rotliegend II sandstones of the Mammendorf Quarry in the Flechtingen High near Magdeburg in northern Germany yield an assemblage of continental trace fossils (reptilian and therapsid tracks, scratch and burrow traces). This assemblage is unique in the late middle Permian deposits of Europe and plays a crucial role in the correlation of pre-Zechstein continental deposits and therefore for the global biostratigraphic framework of the middle to late Permian (Schneider et al., 2020). The exact stratigraphic position of these sandstones was unclear since biostratigraphic relevant fossils and regional marker horizons were missing. To address this problem, the exposed c. 23 m thick Upper Rotliegend II section in the Mammendorf Quarry was documented at cm scale together with lithofacies patterns. 34 rock samples and 46 thin sections were studied along with the descriptions of seven drilling cores of sandstone exploration wells of up to 107 m depth as well as descriptions of two former gas exploration wells. Based on facies architectures and sandstone petrography we come to the following conclusions: (1) the Upper Rotliegend II section in the Mammendorf Quarry contains the boundary of the Dethlingen and Hannover formations (Elbe-Subgroup). (2) Therefore, the trace fossil horizon is located in the lower to middle part of the Dethlingen-Formation and consequently of late Capitanian age. (3) The transition of the formations is not characterised by a significant change in sandstone composition. Instead, it is marked by a change of facies architectures: (4) during the Dethlingen Formation, sedimentation was dominated by a braided-river-system and associated braid-plain sediments. (5) The Hannover Formation part of the profile is dominated by homogenous damp sand flat and wet sand flat as fluvial deposits recede. Sedimentation is controlled by the lake level of the perennial lake in the centre of the Southern Permian basin. (6) A 50 cm thick lacustrine Horizon found in two sandstone exploration wells at Mammendorf is correlated with a lacustrine horizon from the middle Dambeck Member, Hannover Formation, of the Uthmöden 14/78 gas exploration well. (8) The depositional environment is interpreted as a NW-NNW flowing tributary wadi-system, second to the Bebergraben-Wadi to the west. This system is characterised by strongly varying seasonal flow of water. A year-round high groundwater level can be assumed for the trace fossil bearing horizon. References: Schneider, J.W., et al. 2020, Late Paleozoic-early Mesozoic continental biostratigraphy – Links to the Standard Global Chronostratigraphic Scale: Paleoworld, v. 531, p. 186-238. doi.org/10.1016/j.palwor.2019.09.001