Petrography, inorganic and organic geochemistry of Lower Permian carbonaceous fan sequences ("Brandschiefer Series") - Federal Republic of Germany: Constraints to their paleogeography and assessment of their source rock potential

Abstract

Lower Permian clastic and volcaniclastic sediments from the western edge of the Bohemian Massif (northern Bavaria, F.R.G.) were studied petrographically and chemically to decipher their complex history of deposition, their origin of elements (C specifically) and to assess their economic potential (hydrocarbons, Ba, U). The deposit is an alluvial fan sequence, among which pyroclastic sediments are intercalated. Chemical discrimination plots allow these acid to intermediate tuffs to be assigned to the calc-alkaline clan. These volcanites and the bimodal clastic sediments plead for the assumption of an active continental margin. Zr, Ba, Th, Ce and P are delivered by volcanic activity into these basins, while the organic matter originated from plant debris. Postsedimentary redeposition led to the removal of elements from the basinal fine-grained sediments and trapping of Ba, Na and hydrocarbons into the more porous mid-fan sections. The organic matter plays a minor role in U fixation, since it had already lost its sorption capacity at the time of basin evolution. S, C and Fe plots as well as the use of the DOP (=degree of pyritization) led to the conclusion that this pyrite in the more central parts has been syngenetically precipitated under iron-limited conditions. This environment may be considered as an area of U preconcentration and Ba mineralization. Moreover, it shows a good sourcerock potential, mainly for gas. © 1988.