Unravelling provenance and diagenetic evolution of an Upper Rotliegend conglomerate: A single clast perspective from the Schadewalde 2/75 borehole, Saxony-Anhalt, Germany

The polymict Upper Rotliegend conglomerate of the Schadewalde 2/75 borehole, Saxony-Anhalt (Germany) represents a high-resolution archive of erosion and sediment supply along the northeastern Permian Saale Basin. The clasts are embedded in a matrix-rich, poorly sorted conglomerate, representing proximal alluvial fan deposits of the Upper Rotliegend (Havel Subgroup). Microscopic investigations of core samples from the 1407–1440 m depth interval, reveals a broad spectrum of clast lithologies, including milky quartz, various plutonic, volcanic, and metamorphic rock fragments. This lithological diversity suggests a heterogeneous provenance and reflects a complex depositional history. Our study focusses on characterising the range of clast composition and textures to unravel the provenance.

Although the dataset is still in progress, a coarse graphically textured pegmatitic biotite quartz-monzonite to monzodiorite clast already illustrates the informative potential of individual grains. Microscopic investigation reveals graphic intergrowth of quartz and K-feldspar, perthitic K-feldspar, altered plagioclase, chloritised biotite, and late diagenetic chlorite-filled micro-fractures. These microstructures suggest crystallisation from a volatile-rich melt upon slow cooling, followed by potential greenschist-facies deformation, and/or hydrothermal alteration, predating erosion, transport, and deposition of the clast. Subsequent diagenetic fluid overpressures may have triggered micro-hydrofractures during basin subsidence, facilitating chlorite precipitation. Although representing only a single clast, its characteristics suggest a nearby Permian exposure of highly evolved granitic plutons, providing insights into the source area, basin-margin transport, and alluvial-fan processes. However, its provenance assignment remains tentative, as it cannot yet be considered representative for the entire conglomerate.