Catalogue of mass movement deposits in Lake Geneva during the last ~ 4000 years
Presented by Katrina KREMER on 23 Sep 2013 from 17:15 to 19:15
Type: Poster presentation
Session: Poster session
Similarly to steep oceanic continental margins, lake slopes can collapse, producing large sublacustrine landslides and tsunamis. Lake sediments are excellent natural archives of such mass movements and their studies allow the reconstruction of these events on historic and prehistoric timescales. In Lake Geneva (central basin, called ‘Grand-lac’), more than 100 km of high-resolution seismic reflection profiles reveal the ~ 30 upper meters of late Holocene sedimentation history divided into two sequences: (a) The upper 5 m thick sequence, characterized by parallel, continuous and high-amplitude reflections intercalated with transparent horizons, is interpreted as hemipelagic sediments interbedded with turbiditic deposits due to floods, forming the ‘background’ lake sediments. (b) The lower 25 m thick sequence consists of large mass movement units characterized by lense-shaped, transparent to chaotic seismic facies with irregular lower boundaries. These chaotic seismic units alternate with decimeter-scale intervals of "background" seismic facies identified in the upper sequence. 14C dating of one distal 12-m-long sediment core reveals that the mass movements deposited between 3690 and 1342 cal BP. The Grand-Lac mass movement’s catalogue reveals units/deposits with varied size, distribution and facies which lead to different interpretations of deposition and trigger processes. Among them, two outstanding and contrasted deposits point to different causes: - The largest event deposit in this sequence is a 6 m-thick bed, with thinning upwards granulometry and an erosive base that covers the entire deep basin with a minimum volume of 0.25 km3. This layer can be associated with the Tauredunum rockfall event of 563 AD (1342 cal BP; Kremer et al., 2012). This historical event is known because of large human and material loss in the Rhone valley and in the old city of Geneva due to a tsunami. - The oldest mass movement imaged in our seismic sequence, is situated on the same horizon than (at least) one large mass movement offshore Thonon and is dated at 3690 cal BP. Its scar lies at >100 m water depth and thus was probably caused by an earthquake. Numeric modeling indicates that this mass movement was likely tsunamigenic and may be linked to an occupation gap of pile dwellers living on the lake’s shores (Kremer et al., in review). Overall, this catalogue shows that mass movement events occurred at least 6 times over ~ 2400 years and that at least two of them were large enough to induce high tsunami waves over the lake. This project is funded by the Swiss National Fund nr. 200021-121666 References Kremer K., Simpson G., Girardclos S. (2012), Giant Lake Geneva tsunami in AD 563, Nature Geoscience 5, 756-757. Kremer K., Marillier F., Hilbe M., Simpson G., Dupuy D., Yrro B., Rachoud-Schneider A.-M., Corboud P., Bellwald B., Wildi W., Girardclos S., Pile dwellers occupation gap in Lake Geneva (France-Switzerland) possibly explained by an earthquake – slide – tsunami event during Early Bronze Age, in review at EPSL.