Rock Avalanches in a Changing Landscape Following the Melt Down of the Scandinavian Ice Sheet, Norway

Abstract

Rock avalanches can form complex deposits for which the interpretation can be challenging, especially if they occur in valleys affected by other ‘fast’ geological processes, such as, glaciations or isostatic rebound. The mountains of western Norway enable to study rock avalanches in such a complex geological setting. Within the two valleys of Innerdalen and Innfjorddalen (~70 km afar), several rock avalanches occurred since the Late Pleistocene. The rock avalanches in Innerdalen have volumes of 31 × 106 and 23 × 106 m3 and yielded terrestrial cosmogenic nuclide 10Be ages of 14.1 ± 0.4 and 7.97 ± 0.94 ka, while those in Innfjorddalen have volumes of 15.1 × 106, 5.4 × 106 and 0.3 × 106 m3 and yielded ages of 14.3 ± 1.4, 8.79 ± 0.92 ka and 1611–12 CE, respectively. Although being of nearly similar age, the rock avalanches in both valleys occurred under different environmental settings associated with the decay of the Scandinavian ice sheet. One of which fell onto a retreating valley glacier, partly depositing as supraglacial debris (Innerdalen), while the contemporaneous one fell into a fjord, partly forming a subaqueous deposit which is today exposed due to post-glacial isostatic uplift (Innfjorddalen). The younger rock avalanches fell into the ice-free valleys onto the older rock-avalanche deposits. All of the observed rock avalanches are preserved in rock-boulder deposits distributed on the valley floor and its slopes showing a variety of geomorphological features and landforms, which are diagnostic for their paleodynamics. Numerical runout modeling using DAN3D supports the landform interpretations, which are further confirmed by 10Be ages of the rock-avalanche deposits. The presented description of rock-avalanche deposits can enable a better identification and interpretation of similar deposits in other mountain areas and contributes to the knowledge of Quaternary landscape evolution in western Norway, such as, ice-sheet thickness and post-glacial isostatic rebound.