Evidence for Storegga tsunami run-up at the head of Nordfjord, western Norway
Résumé
Numerical simulations show that the Storegga tsunami (∼8 150 cal a BP) penetrated into fjords and sounds along the Norwegian coast because of its large wavelengths of about 600-800 km. Despite this, deposits from the tsunami have not previously been reported from the innermost fjord district, probably because suitable sedimentary archives are lacking in the steep fjord landscape. Here we describe a persistent and well-defined depositional unit, up to 3 m thick, found in a lake sequence at the head of the 106-km-long Nordfjord, which we interpret as a deposit from the Storegga tsunami. The unit is identified and mapped in seismic profiles as a continuous semi-transparent unit which in places cut deeply into the underlying strata. In sediment cores the unit consists of grey silt with sub-units containing shell or plant fragments, gravel and sand clasts, and redeposited lake mud. Radiocarbon ages date the deposit to the time of the Storegga tsunami (∼8 150 cal a BP). We estimate a minimum run-up of 1-7.5 m for the tsunami based on the inferred elevation of the lake outlet relative to sea level at that time.