LANDSLIDE-INDUCED LIQUEFACTION DEPOSITS IN THE VORDERRHEIN RIVER VALLEY, EASTERN SWISS ALPS

The Flims landslide, located in the eastern Swiss Alps, is the largest postglacial landslide in Europe. About 9400 years ago, 10-12 km³ of Cretaceous limestone detached from the northern wall of the Vorderrhein river valley. The rock mass rapidly fragmented and, upon impact with the valley floor, liquefied approximately 1 km³ of late-glacial and postglacial sediments. A slurry of liquefied sediment traveled down the Vorderrhein and 16 km up the valley of the Hinterrhein, its largest tributary. Huge fragments of rockslide material (tomas), up to 100 m across and 50 m high, were rafted up to 11 km on the liquefied slurry. The sheet of liquefied sediments, referred to as the “Bonaduz gravel”, is locally >60 m thick and fines upward from cobble gravel at the base to sand at the top. At several sites within and at the margins of the Flims landslide, we documented vertical and near-vertical tabular bodies of fining-upward matrix-supported gravel containing silt rip-up clasts and ‘Pavoni pipes’, all of which are typical of Bonaduz gravel. A coarse, poorly sorted cobble facies, which we informally refer to as the “proto-Bonaduz” facies, interfingers with and underlies Flims rockslide debris. We hypothesize that the proto-Bonaduz sediments were liquefied by the Flims rockslide and are an immature, coarse Bonaduz gravel facies. The older, smaller Tamins landslide was a barrier to the downvalley flow of the Bonaduz gravel, splitting the flowing mass into two lobes. One of the two lobes burst through the Tamins landslide barrier, rafting masses of Tamins landslide debris hundreds of metres to many kilometres down the Vorderrhein valley. A tongue of proto-Bonaduz gravel rode up the Tamins barrier, probably as the basal carpet and leading edge of the mass flow. Up to several metres of thinly bedded sand and fine gravel locally forms a conformable cap on the Bonaduz gravel and probably was deposited by downvalley flow of water escaping from the Bonaduz sediment sequence. The impact of the Flims rockslide on the Vorderrhein valley floor likely disarticulated the outer portions of the Tamins landslide barrier, creating subparallel elongate tomas that moved 150 m to 1.4 km away from the barrier. Existing mass flow classifications do not adequately capture the characteristics of the Bonaduz gravel or explain its transport and depositional mechanisms.