The AD 1881 earthquake-triggered slump and late Holocene flood-induced turbidites from proglacial Lake Bramant, western French ALPS

Abstract

High-resolution seismic analyses on the sedimentary subsurface of the deep basin of proglacial Lake Bramant (Grandes Rousses Massif, Western French Alps) allowed the detection of a large lens-shaped body with chaotic internal reflections corresponding to a mass wasting deposit (MWD) triggered by the nearby AD 1881 Allemond earthquake (MSK intensity VU). This MWD was only retrieved at the base of a short gravity core and the top of a piston core. Sediments associated with this MWD are remoulded and laminated. Locally, blocks of sediment have preserved the original stratification. This earthquake-induced mass movement is an example of a slide that evolved into a slump. In addition, several Late Holocene turbidite and hyperpycnal deposits related to exceptional flood events were identified using highresolution sedimentological, physical and geochemical analyses. However, the identification of hyperpycnites is sometimes complicated as erosion of the basal sequence can occur during the rising limb of the flood. While the precise dating of the oldest flood event is still ongoing, two flood events aie coeval with the St. Sorlin glacier retreat following the end of the "Little Ice Age", suggesting outbursts of temporary ice contact lakes or subglacial lakes during warmer periods.