MULTI-SCALE SUBSIDENCE VARIABILITY AND BASEMENT HERITAGE IN EPICONTINENTAL BASINS – CASE STUDIES FROM THE MESOZOIC OF SOUTH-WESTERN GERMANY

Isopach maps of large epicontinental basins typically show complex patterns of dispersed local to regional depocentres and intrabasinal swells. Differential subsidence within the basin is closely coupled with facies distribution of marine or nonmarine deposits, the geometry of regional unconformities and drainage patterns of syndepositional fluvial systems. It has direct impact on the regional distribution of today’s geopotentials of these deposits, e. g. for hydrocarbon exploration, mineral water resources, CO₂ sequestration, or industrial minerals. During the last years, the Geological Survey of Baden-Wuerttemberg (Germany) has improved the resolution of 3D modeling on a regional scale. From this modeling a consistent set of isopach maps and cross sections has been derived.

The lateral scale of the thickness variations in Late Permian to late Jurassic epicontinental sediments ranges from local sinks and uplifts only a few kilometres across to intrabasinal zones of retarded or increased subsidence 10 - 100 km wide. Their position and orientation has remained stable over most of the studied time span, i. e., for more than 100 Ma, but variations in subsidence rates were not in phase at different locations. Temporal variations of differential subsidence between neighbouring locations range in scale from relatively short local subsidence pulses within single 100 ka or 400 ka depositional cycles to long-term spatial trends of thickness gradients over several biostratigraphic zones and even stages. Yet, temporal variability and spatial differentiation of these isopach anomalies and larger subsidence structures are not in accordance with simple thermal subsidence after an initial basin-forming tectonic event. We interpret these isopach structures as near-surface reflection of slow but ongoing shear along older, large-scale basement shear zones. The relevance of this long intracontinental deformation to paleogeographic reconstruction of plate boundaries and outlines has yet to be investigated.