Mineralogy and geochemistry of carbonate mineralization and their role for environmental analyses of lower permian clastic and volcaniclastic sediments (Stockheim Trough, Federal Republic of Germany)

摘要

In the Permian of the Stockheim Trough, a volcaniclastic to elastic sedimentary series was mineralogically and geochemically studied for its types of carbonate mineralization. Mineralogy permits a division into two lithofacies types (Type I: stratabound/stratiform, Type II: faciesbound) and helps constraining the T interval of postdepositional alteration (100 °C to 135 °C). Four processes have been responsible for carbonate precipitation (evapotranspiration, pedogenesis, hydrothermal replacement, and cementation). This classification is refined by use of chemical parameters like the CaO/MgO ratio, Sr, Mn contents and O- and C-isotopes, in specific. During early basin subsidence, volcanites, locally suffering intensive Fe-carbonatization, were extruded. After that, bimodal volcaniclastic sediments were spread out in the Stockheim Basin. Their good permeability and the preponderance of labile volcanic components were favourable for hydrothermal carbonization. The major elements for carbonates were delivered by decomposition of organic matter (C), replacement of plagioclase (Ca) and originated from circulating meteoric waters (O). Overlying evaporitic lacustrine limestones were built-up by ground-water discharge in distal fan positions and outpouring of hot brines. In a playa environment dolomite was concentrated in calcretes. Subsequent to erosion, increased evapotranspiration and brackish-marine conditions substituted for these continental conditions. This change is well documented by a shift in the C- and O-isotope patterns of carbonate