Sedimentary sequences, seismofacies and evolution of depositional systems of the Oligo/Miocene Lower Freshwater Molasse Group, Switzerland

ABSTRACT Magnetostratigraphic chronologies, together with sedimentological, petrological, seismic and borehole data derived from the Oligo/Miocene Lower Freshwater Molasse Group of the North Alpine foreland basin enable a detailed reconstruction of alluvial architecture in relation to Alpine orogenic events. Six depositional systems are recorded in the Lower Freshwater Molasse Group. The bajada depositional system comprises 200–400-m-thick successions of ribbon channel conglomerates and overbank fines including mud- and debris-flows which were derived from the Alpine border chain. The alluvial megafan depositional system is made up of massive pebble-to-cobble conglomerates up to 3 km thick which reveal a fan-shaped geometry. This depositional environment grades downcurrent into the conglomerate channel belt depositional system, which comprises an ≈2-km-thick alternation of channel conglomerates and overbank fines. The sandstone channel belt depositional system is bordered by the 100–400-m-thick overbank fines assigned to the floodplain depositional system. At the feather edge of the basin, 50–400-m-thick lacustrine sediments in both clastic and carbonate facies represent the lacustrine depositional system. The spatial and temporal arrangement of these depositional systems was controlled by the geometrical evolution of the Molasse Basin. During periods of enhanced sediment supply and during phases of stable sliding of the entire wedge, >2000-m-thick coarsening-and thickening-upward megasequences comprising the conglomerate channel belt, alluvial megafan and bajada depositional systems were deposited in a narrow wedge-shaped basin. In the distal reaches of the basin, however, no sedimentary trend developed, and the basin fill comprises a <500-m-thick series of sandstone meander belt, floodplain and lacustrine depositional systems. During phases of accretion at the toe of the wedge, the basin widened, and prograding systems of multistorey channel sandstones extended from the thrust front to the distal reaches of the basin. The rearrangement of the depositional systems as a function of changing orogenic conditions created discordances, which are expressed seismically by onlap and erosion of beds delimiting sedimentary sequences. Whereas stable sliding of the wedge succeeded by accretion at the toe of the wedge is recorded in the proximal Lower Freshwater Molasse by a coarsening-and thickening-upward megasequence followed by erosion, the opposite trend developed in the distal reaches of the Molasse. Here, fine-grained sandstones and mudstones were deposited during periods of stable sliding, whereas phases of accretion caused a coarsening- and thickening-up megasequence to form.