Dan Hammarlund

The Siljan-Orsasjön Basin in south-central Sweden hosts two prominent lakes. A 100 m deep channel extends approximately 52 km across the basin but does not form a continuous connection between the lakes today. Geophysical mapping, including multibeam bathymetry and sub-bottom profiling, revealed several landslide areas associated with the steep slopes of this channel and redistributed mass-wasted glaciolacustrine varved sediments as well as later-deposited laminated lake sediments. To assess the timing and processes of sediment displacement within these subaqueous landslide areas, sediment cores were retrieved and radiocarbon dated. The results uncovered a complex stratigraphy and ruled out the possibility of a single landslide event. Instead, several events have occurred over the past 10,000 years, beginning shortly after the deglaciation. Stratigraphic analysis of the sediment cores retrieved from the mass-wasted deposits revealed various types of mass transport, including coherent slide blocks, slumps, debris flows and turbidites, intercalated with undisturbed lacustrine sediments. The oldest landslides may have been triggered by seismic activity already during the early postglacial period. Subsequent enlargements of the landslide areas were driven by lateral widening and retrogressive erosion, with major sediment slides occurring as recently as less than 500 years ago. These results have important implications for understanding geohazards, particularly risks posed to underwater infrastructure, such as cables and pipelines. The study presents a comprehensive methodology that combines high-resolution geophysical techniques with sedimentological analysis to assess the long-term of subaqueous landslides.