Petrology and genesis of Upper Carboniferous seams from the Ruhr region, West Germany

Abstract

The petrology of 21 seam profiles of Upper Carboniferous age (Westphalian C and B) has been studied in order to determine their depositional environments and diagenetic history. The youngest profile was drilled at a depth of 790 m and is mainly overlain by Cretaceous sediments. The oldest seam was reached at a depth of 1470 m. The seam thicknesses vary from 0.4 to 2.5 m. The main petrographic compounds are vitrinite, intertinite, liptinite, and minerals. The last group occurs as clay bands with illite, kaolinite, minor chlorite, and minor quartz contents, or as crack- or pore-filling calcite, Fe-dolomite, siderite, pyrite, or marcasite, or as syngenetic siderite concretions and pyrite crystals. The percentages of the different macerals and minerals vary mainly because of different depositional environments. Diagenetic loss or genesis of compounds is a less important factor in their distribution. Three types of profiles are distinguished by their different petrologies. Type I is most abundant, and contains much vitrinite, many clay bands, and syngenetic iron sulfides, whereas type III is rich in inertinite and certain characteristic spores. Type II is intermediate but generally contains only low percentages of minerals. Generally, this type is vitrinite-rich in the lower, and inertinite-rich in the central and upper parts of the profiles. Spores and other liptinites are much better preserved in all the seams than in clay partings or in siltstones and sandstones above and below the seams. The seams are interpreted to be former autochthonous peats. Type I profiles are probably derived from swamps which were sometimes inundated and covered by overbank deposits. Type II and III seams represent former peats which were not inundated by rivers, and partly grew under the influence of more oxidizing conditions. Therefore, they contain more inertinite and less sulfide and clay bands. They can be interpreted as former raised bogs. Diagenetic changes are expressed as increases of vitrinite reflectances (from 0.65% to 1.0%), and of liptinite reflectances; a red shift of fluorescence of liptinites was found; increasing amounts of exudatinite (and micrinite) and decreasing amounts of fluorinite and resinite were found. Minerals seem to be less affected by diagenesis; illite crystallinity, for example, remains poor.