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

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.

Anatomy of a Catastrophe: Reconstructing the 1936 Rock Fall and Tsunami Event in Lake Lovatnet, Western Norway

“…Similarly, facies IIIb might be analogous to the organic detritus found in deposits from the Storegga tsunami, termed facies 7 by Bondevik et al (1997a), which also represent material washed into the lake from onshore areas. Facies IIIc (light gray mud capping the sequence) has been attributed to suspension fallout following subaqueous density flows (Mulder and Alexander, 2001), but have also been correlated with seiche effects following tsunami events (e.g., Waldmann et al, 2011;Vasskog et al, 2013;Kempf et al, 2015;Nigg et al, 2021). Based on the above points, we suggest that the facies III successions represent major tsunami events that affected the whole lake basin.…”

Shallow‐Water Tsunami Deposits: Evidence From Sediment Cores and Numerical Wave Propagation of the 1601 CE Lake Lucerne Event

“…Roberts et al (2013) document sedimentary signatures of the 2007 CE subaerial landslide-generated tsunami in the onshore setting of Lake Chehalis. Nigg, Wohlwend, et al (2021) found sedimentary evidence of a prehistoric tsunami in the shallow-water and coastal area of Lake Sils, Switzerland, that are comparable to marine tsunami deposits.…”

“…Of the few shallow-water tsunami deposits studied worldwide, several authors have described sharp lower and/or upper sedimentary contacts (Abrantes et al, 2008;Goodman-Tchernov et al, 2009;Sakuna et al, 2012;Smedile et al, 2020;Van den Bergh et al, 2003), as observed at the basal contact of Unit 3. In Lake Sils, Nigg, Wohlwend, et al (2021) observed thick, normally graded sand deposits that were likely formed by the backwash currents of a prehistoric lake tsunami.…”

“…Where most tsunami evidence stems from the marine domain, tsunamis in lakes have recently been recognized as a considerable natural hazard with high magnitudes and low recurrence rates (Hilbe & Anselmetti, 2015; Kremer et al., 2015, 2021; Nigg, Wohlwend, et al., 2021; Strupler, Danciu, et al., 2018; Strupler, Hilbe, et al., 2018). The most common mechanism that generate tsunamis in freshwater environments are large subaqueous and subaerial mass movements (Hilbe & Anselmetti, 2015; Kremer et al., 2021; Mountjoy et al., 2019; Nigg, Wohlwend, et al., 2021; Roberts et al., 2013; Strupler, Bacigaluppi, et al., 2020).…”

“…Where most tsunami evidence stems from the marine domain, tsunamis in lakes have recently been recognized as a considerable natural hazard with high magnitudes and low recurrence rates (Hilbe & Anselmetti, 2015; Kremer et al., 2015, 2021; Nigg, Wohlwend, et al., 2021; Strupler, Danciu, et al., 2018; Strupler, Hilbe, et al., 2018). The most common mechanism that generate tsunamis in freshwater environments are large subaqueous and subaerial mass movements (Hilbe & Anselmetti, 2015; Kremer et al., 2021; Mountjoy et al., 2019; Nigg, Wohlwend, et al., 2021; Roberts et al., 2013; Strupler, Bacigaluppi, et al., 2020). Historic chronicles document that lacustrine tsunamis have caused fatalities, severe lake‐shore erosion, and inundation in various lakes around the world (e.g., Lake Geneva, Switzerland [563 CE Tauredunum rockfall event (Favrod, 1991; Montandon, 1925]), Lake Lucerne, Switzerland (1601 CE Unterwalden earthquake (Cysat, 1969) and 1687 CE Muota Delta collapse (Bünti, 1973; Billeter, 1923; Dietrich, 1689)), Lake Baikal, Russia (1861 CE Tsagan earthquake (Klyuchevskii et al., 2012)).…”