Geophysical evidence of massive hyperconcentrated push waves with embedded toma hills caused by the Flims rockslide, Switzerland

Abstract

Rockslides and rock avalanches are amongst the most destructive natural hazards in the alpine environment. The Flims rockslide is the largest known rock-slope failure in the Alps, which provides excellent outcrops and has fascinated researchers since the early 20th century. The postulated impact of the Flims rockslide on Lake Bonaduz caused intensely fluidized rock material, which formed the Bonaduz Formation and toma hills, probably accompanied by a catastrophic impact wave. So far, this hypothesized sequence of events is based only on sedimentological and geomorphic analyses. We present electrical resistivity tomography (ERT) profiles, which we correlated with the sedimentological information obtained from outcrops and drill logs. Here, geophysical evidence on a metre and decametre scale complements prior outcrop and sample intervals with much smaller representativeness. Our study provides new insights into the distribution, thickness, and internal structure of the Bonaduz Formation and the toma hills as well as other flood deposits around the Ils Aults, where we studied the sediment to a depth of up to 160 m. There is geophysical evidence that the Bonaduz Formation formed an onlap onto the Ils Aults and is thus the stratigraphically younger unit. The toma hills consist of blocky cores with an agglomeration of smaller mixed sediments, which drift and override the toma core, causing their smoothly shaped top. We consider simultaneous transport of the hills within the Bonaduz Formation but a slightly slower movement at the front due to a bulldozing effect. This study contributes to an improved understanding of (i) the complex stratigraphical context of the Tamins and Flims deposits, (ii) water-rich entrainment in rock avalanches, and (iii) the genesis and transport of toma hills.