SEDIMENTARY SEQUENCES, SEISMOFACIES AND EVOLUTION OF DEPOSITIONAL SYSTEMS OF THE OLIGO MIOCENE LOWER FRESH-WATER MOLASSE GROUP, SWITZERLAND/

Magnetostratigraphic chronologies, together with sedimentological, pet rological, seismic and borehole data derived from the Oligo/Miocene Lo wer Freshwater Molasse Group of the North Alpine foreland basin enable a detailed reconstruction of alluvial architecture in relation to Alp ine orogenic events. Six depositional systems are recorded in the Lowe r Freshwater Molasse Group. The bajada depositional system comprises 2 00-400-m-thick successions of ribbon channel conglomerates and overban k fines including mud- and debris-flows which were derived from the Al pine border chain. The alluvial megafan depositional system is made up of massive pebble-to-cobble conglomerates up to 3 km thick which reve al a fan-shaped geometry. This depositional environment grades downcur rent into the conglomerate channel belt depositional system, which com prises an approximate to 2-km-thick alternation of channel conglomerat es and overbank fines. The sandstone channel belt depositional system is bordered by the 100-400-m-thick overbank fines assigned to the floo dplain depositional system. At the feather edge of the basin, 50-400-m -thick lacustrine sediments in both elastic and carbonate facies repre sent the lacustrine depositional system. The spatial and temporal arra ngement of these depositional systems was controlled by the geometrica l 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 s ystems were deposited in a narrow wedge-shaped basin. In the distal re aches of the basin, however, no sedimentary trend developed, and the b asin fill comprises a < 500-m-thick series of sandstone meander belt, floodplain and lacustrine depositional systems. During phases of accre tion at the toe of the wedge, the basin widened, and prograding system s 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 discord ances, which are expressed seismically by onlap and erosion of beds de limiting sedimentary sequences. Whereas stable sliding of the wedge su cceeded by accretion at the toe of the wedge is recorded in the proxim al Lower Freshwater Molasse by a coarsening- and thickening-upward meg asequence followed by erosion, the opposite trend developed in the dis tal reaches of the Molasse. Here, fine-grained sandstones and mudstone s were deposited during periods of stable sliding, whereas phases of a ccretion caused a coarsening- and thickening-up megasequence to form.