Sedimentary Sequences, Seismic Facies, Subsidence Analysis, and Evolution of the Burdigalian Upper Marine Molasse Group, Central Switzerland

Abstract

The sedimentological interpretation of seismic and borehole data derived from the Burdigalian Upper Marine Molasse Group of the central North Alpine foreland basin enables a detailed reconstruction of the shallow-marine architecture. The different seismic facies are assigned to the shoreface/foreshore, nearshore, and offshore depositional systems. Mapping of the seismic facies on the seismic line reveals the presence of two prograding sequences (Luzern and St. Gallen formations) that are separated by an unconformity. The subsidence analysis suggests that the evolution of the Upper Marine Molasse was primarily controlled by the interaction between tectonic subsidence and sediment supply. The distal transgressions of the Luzern and St. Gallen formations and the establishment of shallow-marine environments were caused by enhanced subsidence rates in the distal reaches associated with increased sediment supply rates. The development of the sequence boundaries separating the Luzern and St. Gallen formations and the regression at the end of the St. Gallen Formation, however, were controlled by uplift in the distal part of the basin. The subsidence and sediment flux analyses of the Upper Marine Molasse allow an improved understanding of the tectonic and denudation history of the northern Alps. Out-of-sequence thickening in the rear of the northern Alps by underplating of the Aar massif and forward thrusting of the Helvetic nappes is interpreted to have controlled uplift in the distal part of the basin and the formation of the sequence boundaries. In-sequence crustal thickening at the tip of the orogenic wedge by forward underthrusting of Lower Freshwater Molasse thrust sheets beneath Molasse deposits associated with enhanced erosion of the northern Alps caused a northward shift of the location of the major orogenic load and an increase of the sediment supply to the Molasse basin. These processes are interpreted to have controlled the distal shifts of the basin depocenter and the distal transgressions of the Luzern and St. Gallen formations.