Analysis of subseismic-scale folds in complex tectonosedimentary settings: A case study from the Austrian Molasse Basin

Summary This case study from the Austrian Molasse Basin shows the results of an exploration well that targeted a deep marine axial channel belt in the foreland of the North Alpine Thrust Wedge. Borehole image logs were analysed complementary to 3D seismic data, revealing the complex internal architecture at the margin of the channel belt. Extensive folding was detected in the borehole, that was not expected from seismic analysis. Due to the close vicinity to the Alpine fold-and-thrust belt, the origin of these folds was ambiguous and related either to tectonic processes or soft-sediment deformation. Detailed geometric investigations revealed the presence of slump folds that were clearly distinguished from alpine tectonic fold trends. A full characterization of slump geometries was performed and discussed in the context of basin evolution and channel architecture. Fold hinges show significant scatter, suggesting a fairly high complexity of fold geometries, such as higher order folds or refolds. Coherent slump folds were separated from folds with higher complexity and used for subsequent paleoslope analysis. The results show that slumping was not related to gravity flows along the axial channel, but was rather triggered by tectonic movements from the south slope of the Molasse Basin.