Bolsovian (Pennsylvanian) tropical peat depositional environments: The example of the Ruhr Basin, Germany

Abstract

A set of 11 Bolsovian coal seams from the German Ruhr Coal Basin were analyzed in profiles for their organic geochemical and petrographic composition, aiming at the reconstruction of depositional environments, mainly in terms of water supply and redox conditions. For this purpose, established maceral indices and A/I ratio were employed. The seams show the same basic depositional pathways of terrestrialisation and paludification as reported for Duckmantian coal seams from the same basin. The presence of coals formed as ombrotrophic peat indicates that climate was still wet enough for the formation of domed bogs until the Upper Bolsovian, although the typical densosporite facies does not occur within the uppermost third of the analyzed succession. Very high TS values for seam Parsifal II indicate the influence of marine waters at the peat stage. A marine ingression event was not reported before for this stratigraphical level within the Ruhr Basin. Data also reveal some variability in vitrinite reflectance (0.65–0.74%) within a given coal seam which cannot be due to thermal maturation. For seams with decreased VRr values, a correlation with high liptinite contents was found. Rock-Eval HI values do not show any strong variation with depths in the recorded maturity interval. Tmax values vary between 418 °C and 444 °C showing dependence on both, thermal maturity and liptinite content. Decreasing contents of inertinite macerals throughout the Pennsylvanian in the Ruhr Basin suggest climatic changes during that time span.