The Werra cyclotheme (Upper Permian,Fore-Sudetic Monocline,Poland): Insights into fluctuations of the sedimentary environment from organic geochemical studies

A study of the extracts of samples recording a transgressive–regressive succession of the Werra cyclothem, Zechstein (Upper Permian), from the southern margin of the European Permian Basin (the Fore-Sudetic Monocline, Poland) provides an insight into major sources of organic matter and diagenetic processes. The studied cross-section comprises all lithologies representative for the European basin, including transgressive sandstones (Weissliegend) and organic-rich shales (Kupferschiefer) followed by carbonates as well as regressive anhydrites with intercalations of rock salt. Due to the variable influence of overlapping diagenetic processes that affected the organic matter, i.e. maturation and late diagenetic oxidation related to base metal mineralisation, a reliable comparison of the biomarker results is only possible for the major upper part of the section (ca. 38 m, including carbonates and evaporites) but not for the oxidised first 2–3 m from the base of the Kupferschiefer.The transition from carbonate to evaporate sedimentation is associated with a shift to predominant even C-numbered n-alkanes, increased abundance of carotanes and high homohopane index reflecting enhanced reducing conditions. The presence of the C₂₅ regular isoprenoid, squalane, biphytane and the rapid decrease in the pristane/phytane ratio in the evaporites are mainly controlled by the important contribution from both halophilic and methanogenic archaea. The occurrence of gammacerane in the anhydrites suggests development of water column stratification at some stages of the sea regression. This is associated with appearance of specific aryl isoprenoids with a lycopane carbon skeleton most likely related to Botrycoccus braunii race L algae. Lycopane derivative occurrence suggests that such an algal race could have occurred since Permian (currently known Recent-Eocene). The presence of the abundant freshwater/brackish algal biomarkers in evaporititic deposits can be envisaged in the costal part of the Zechstein basin with temporary salt wedge estuary water stratification. The algae bloomed in the top fertile fresh/brackish water layer fed by rivers, and the algal biomass was deposited on the sea floor covered with evaporitic brine. The stratification periodically broke down during precipitation of the rock salt, presumably due to a decrease in riverine water input, as revealed by characteristic disappearance of gammacerane and hopane distributions similar to those observed for the carbonate rocks.The methylphenanthrenes/phenanthrene ratio was used together with methyldibenzothiophenes/ dibenzothiophene ratio to assess maturity. These maturity estimations indicate that the peak of oil window has been reached, which is confirmed by other biomarker maturity parameters based on sterane and hopane distributions. The maturity stage of oil generation and expulsion was further confirmed by the presence of solid bitumen as cements in the sandstone underlying Kupferschiefer and bitumen veins and lenses in the carbonates.