THE 1996 AD DELTA COLLAPSE AND LARGE TURBIDITE IN LAKE BRIENZ

Abstract

In spring 1996 AD, the occurrence of a large mass-transport was detected by a series of events, which happened in Lake Brienz, Switzerland: turbidity increase and oxygen depletion in deep waters, release of an old corpse into surface waters and occurrence of a small tsunami-like wave. This mass-transport generated a large turbidite deposit, which is studied here by combining high-resolution seismic and sedimentary cores. This turbidite deposit correlates to a prominent onlapping unit in the seismic record. Attaining a maximum of 90 cm in thickness, it is longitudinally graded and thins out towards the end of the lake basin. Thickness distribution map shows that the turbidite extends over ∼ 8.5 km2 and has a total volume of 2.72 ⁎ 106 m3, which amounts to ∼ 8.7 yr of the lake's annual sediment input. It consists of normally graded sand to silt-sized sediment containing clasts of hemipelagic sediments, topped by a thin, white, clay-sized layer. The source area, the exact dating and the possible trigger of this turbidite deposit, as well as its flow mechanism and ecological impact are presented along with environmental data (river inflow, wind and lake-level measurements). The combined results indicate that no particular cause (i.e., earthquake, explosion, flood, wind storm, lake-level change, seiche or sediment dredging) triggered the 1996 AD large turbidite deposit. Instead, it was due to a delta slope failure most likely caused by normal sediment accumulation.