Late Carboniferous differential subsidence in the Campine Basin as part of the Variscan foreland system

Abstract

During the Westphalian C (middle Pennsylvanian), the southeastern Campine Basin was part of the foreland of the Variscan Orogen. Deposition was dominated by rhythmic successions of coal-mudstone-sandstone sequences. Correlations between marker horizons within these sequences indicate a consistent thinning of the intercalated units towards the northwest. On seismic data, this thinning is expressed by subtly northwestwards converging intra-Westphalian C seismic reflectors. Since sedimentation kept up with subsidence, the northwestern thinning reflects a decrease in subsidence in the same direction. We interpret this asymmetric, flexural subsidence as the result of flexural downwarping of the underlying continental crust due to an increase in tectonic load generated by the Variscan Orogen that approached from the southeast. The southeastern Campine Basin was thereby situated in a Variscan foreland system, more specifically along the forebulge flank of a foredeep. The crest of the Variscan forebulge was located further northwest in the Campine Basin. The different Westphalian C units in the study area show varying percentages (between 32 and 50%) of thickness decreases in the direction of the forebulge. Our model fits with existing thickness and burial data from the surrounding areas in the Variscan foreland, and is compatible with the published forebulge migration model in the Ruhr Coal Basin.