NEW OBSERVATIONS ON GEOMORPHOLOGY AND LOESS SEDIMENTS IN THE BAVARIAN MOLASSE BASIN – THE TERTIARY HILLS BETWEEN LANDSHUT AND REGENSBURG

For more than 50 years numerous geologists have worked on the observation and interpretation of the youngest part of Tertiary Hills in the Bavarian Molasse basin. This part is mostly covered with Pleistocene loess and loess loam. A widely accepted standard model explaining the shape of the landscape represents the results of this work. This landscape is dominated by NW-SE striking asymmetric hills and valleys. The model, based on climatic processes and particular on the aeolian loess deposition, can explain these hills. But an alternative mechanism to produce these structures is also possible, namely active faulting. In this study, we present new observations from field mapping, DEM analyses, resistivity tomography profiles and coring to underline this hypothesis. Observations are:

(a) Individual Tertiary hills extend over a length scale from 0.5 to > 6 km, dominantly striking NW-SE and show steep slopes (6 to 24°) on their WSW flanks. The thickest loess deposits of up to 20 m are adjacent to these windward SW slopes. The NE slope is usually flatter and wider with only a thin loess cover of up to 4 m. The described hills are separated by narrow valleys (50 to 150 m wide). On the valley floor pediment wedges and fluvial deposits are often lacking. (b) Small rivers in some of the valleys exhibit sharp left-stepping bends with most prominent examples near the village of Neufahrn/NB. (c) Several linear, NW-SE striking, 2 to 9 m high morphologic steps with a length of 0.5 to >10 km are observed 10 km south of the Danube river. (d) Further a Quarry log close to the Danube river flood plain near Schafhöfen reveal disturbed layers of Pleistocene fluvial sediments containing offset sand lenses and soft sediment deformation features.

These observations can challenge the classic interpretation of aeolian deposition as the dominant process forming the Tertiary hills. Instead we suggest that tectonic activity during the Quaternary may have contributed to the geomorphology of this region.