Alluvial deposits liquefied by the Flims rock slide

The origin of the Bonaduz gravel (Switzerland) in the vicinity of the large Holocene Flims rock slide deposits has been discussed for a century. The internal structure shows many features that imply transport as an over-sized mud/debris stream. This flow of gravel, sand and fines must have been triggered by the mobilisation of the valley alluvium by the impact of the rock slide. The mobilised mud with a volume of several 100 million m3 must have had enormous transport capacities as large units of loose material were transported over kilometres without breaking apart. Accordingly, the shear forces in the flow must have been negligible. Similar features have been found at several other sites in the Alps, all in the vicinity of large rock slides. Our interpretation of these sediments as catastrophic mass movement deposits suggests that such events could happen again, also in other parts of the world.