Comment on esurf-2022-20

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 (Paleo-) Lake Bonaduz caused intensely fluidized rock material, which formed the so-called Bonaduz Gravel 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 dill logs. Our study provides new insights into the distribution, thickness, and internal structure of the Bonaduz Gravel, the Toma and Cresta 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 new field evidence that the Bonaduz Gravel formed an onlap onto the Ils Aults and is thus the stratigraphically younger unit. The Toma/Cresta hills consist of blocky cores with an agglomeration of smaller mixed sediments, which drift and override the Toma/Cresta core, causing their smoothly shaped top. We consider simultaneous transport of the Cresta within the Bonaduz Gravel, yet 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 outburst-flood deposits, in particular of Toma hills.