A tsunamigenic delta collapse and its associated tsunami deposits in and around Lake Sils, Switzerland

Valentin Nigg (), Stephan Wohlwend, Michael Hilbe, Benjamin Bellwald, Stefano C. Fabbri, Gregory F. Souza, Florian Donau, Reto Grischott, Michael Strasser and Flavio S. Anselmetti
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Valentin Nigg: University of Bern
Stephan Wohlwend: ETH Zurich
Michael Hilbe: University of Bern
Benjamin Bellwald: Volcanic Basin Petroleum Research (VBPR)
Stefano C. Fabbri: University of Bern
Gregory F. Souza: ETH Zurich
Florian Donau: ETH Zurich
Reto Grischott: ETH Zurich
Michael Strasser: University of Innsbruck
Flavio S. Anselmetti: University of Bern

Natural Hazards: Journal of the International Society for the Prevention and Mitigation of Natural Hazards, 2021, vol. 107, issue 2, No 3, 1069-1103

Abstract: Abstract Large lacustrine mass movements and delta collapses are increasingly being considered as potential tsunamigenic sources and therefore hazardous for the population and infrastructure along lakeshores. Although historical reports document tsunami events in several lakes in Switzerland, and although the propagation of lake tsunamis has been studied by numerical wave modeling, only little is known about on- and offshore lacustrine tsunami deposits. In Lake Sils, Switzerland, a large prehistoric mass-movement deposit originating from the Isola Delta with a minimum estimated volume of 6.5 × 106 m3 and a basinal thickness of > 6 m in the seismic record has been identified by previous studies and radiocarbon dated to around 700 Common Era. Here, we combine (i) comprehensive sedimentological investigation of sediment cores recovered from the on- and offshore settings, (ii) mineralogical fingerprinting of the inflows from key catchments to characterize sediment provenance, and (iii) numerical tsunami modeling, to test the hypothesis of a tsunamigenic delta collapse in Lake Sils. We observe a clastic event deposit consisting of coarse-grained, fining-upward sand overlying an organic-rich peat deposit in the shallow water. This layer thins and fines landward on the coastal plain. Toward the deeper water (20–40 m), the deposit transforms into a thicker and more heterogeneous sediment package with multiple sequences of fining-upward sand and a well-pronounced clay cap at the top. Radiocarbon dating of the peat underlying the event deposit yields a maximum age of 225–419 calibrated Common Era. The tsunami models, which indicate wave heights reaching up to 5 m, simulate areas of inundation that coincide with the location of event deposits. Based on our results, we propose that the historically undocumented Isola Delta collapse generated a basin-wide tsunami that inundated the lakeshore, transporting large amounts of unconsolidated sediment along the lakeshore toward the coastal plain and into the deeper lake basin.

Keywords: Delta collapse; Tsunami deposit; Lacustrine tsunami; Tsunami modeling; Sedimentology (search for similar items in EconPapers)
Date: 2021
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DOI: 10.1007/s11069-021-04533-y

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