Deltaic mass-movement trigger analysis in Lake Hallwil (Switzerland)

Subaquatic mass movements and their associated deposits are well known to serve as potential natural seismographs for paleoseismic event reconstruction in marine and lacustrine settings. However, the trigger of mass movements does not always have to be related to earthquakes solely, but can be also related to weather/climate (e.g. floods) or sediment overloading.

A crucial role in sedimentation processes can be attributed to the delta areas acting as transition zone between the subaerial and the subaquatic domain. In systems with high riverine sediment discharge, slope failures in deltas and turbidity currents can occur often. The sedimentary system of deltas is highly dynamic and their depositional regime is still poorly understood, in particular, when it comes to their role as mass-transport deposit provider. Apart from reported seismic and climatic triggers of such mass-transport deposits, also spontaneous aseismic trigger mechanisms have been reported for deltas as shown in several cases (e.g. 1687 AD Muota delta collapse in Lake Lucerne, 1996 AD Aare delta collapse in Lake Brienz, Switzerland).

New high-resolution multibeam swath bathymetric results of 10.3 km² large Lake Hallwil (Central Switzerland) indicate that multiple subaquatic mass movement/debris flow events (hereafter termed only mass movements) occurred at the Dorfbach delta. At least 5 mass-movement deposits can be identified in the proximal area of the delta, indicative for repeated slope failures at the delta front.

A series of gravity short cores, forming a transect from the deepest basin towards the immediate forefront of the mass-movement deposits, were recovered in 2021 and scanned with a multi sensor core logger (MSCL) for mass-movement related turbidites and complemented by a sedimentological description of the cores. Samples for radiocarbon and ¹³⁷Cs dating were taken for a detailed event chronology reconstruction of the partially laminated and varved sediment record.

Through the combination of morphological analyses from multibeam data and detailed core analyses, we will present first results of a mass-movment trigger analyses in Lake Hallwil, combining geochemical and geophysical investigation methods to build a mass-movement event chronlogy. Understanding this highly dynamic environment that sometimes produces catastrophic delta failures in equivalent larger settings, can help to improve natural hazard assessment and implementing safety measures for lake shores as well as coastal communities.