Reginald L. Hermanns, Norges geologiske undersøkelse (Norway)
Lars H. Blikra, Norges geologiske undersøkelse (Norway)
Susan Ivy-Ochs, ETH Hoenggerberg (Switzerland)
Peter Kubik, ETH Hoenggerberg (Switzerland)
Rudolf Naumann, GeoForschungsZentrum Potsdam (Germany)
Hallgeir Dahle, SINTEF, Rock and Soil Mechanics (Norway)
The chronology and evolution of large rock-slope failures are important data for the analysis of geohazards. Although geophysical and bathymetric data from fjords in western Norway indicate frequent rock avalanching in the late Holocene, onshore only the age of few deposits of large massive rock-slope failures could have been defined. To fill this gap of data we performed surface exposure dating with the cosmogenic nuclide 10Be of massive rock-slope failures of the rock-avalanche type and of a translational slide in bedrock in the fjordland of western Norway. The first results indicate that six rock avalanches occurred between 12 and 9.5 kyr. This suggests high postglacial activity of large failures clustering at the end of the Late Pleistocene/Early Holocene. However, we also obtained younger ages of rock avalanches at 8, 7 and with a maximum age of 1.5 kyr. The youngest rock avalanche occurred at the locality of Berill, close to a documented postglacial reverse fault. Another rock avalanche, lying along the prolongation of the fault, has an age of ∼8200 years, corresponding in age with the submarine Storegga slide in the Atlantic along the Norwegian shelf at the same latitude. The new cosmogenic data thus question whether there might be large seismic events at that time causing large slides in both environments. The same age was also obtained from a chaotic boulder deposit comprising clasts up to 3 m in size along the coast of the inner Tafjord. This deposit was first interpreted to be an outburst flood deposit from a larger rock-avalanche dam. However, the mineralogical composition of the landslide dam as well as fluvial deposits between the dam and the coast indicate completely different crystalline source rocks. Therefore we interpret now, that these are tsunami deposits of reworked moraine deposits. These might be results of local tsunamis from rock avalanches or even caused by a possibly 20 - 30 m high tsunami caused by the Storegga slide. Finally, we could obtain three ages of the sliding surface of a translational slide on the Otrøya island, just west of the city of Molde. These ages indicate that sliding started with the retreat of the ice sheet at the end of the last glacial maximum with a slip rate of 1.5 +/- 0.44 mm/yr and slowed down to about half of that rate in the mid Holocene. We anticipate that further rockslides will be dated, which will finally allow, when compared to offshore data, to evaluate better causes and trigger of large rockslides in western Norway.