| Summary: | Lake Orsa is situated in the county of Dalarna in central Sweden. The lake is part of the Siljan Ring, which formed approximately 380 Ma by the largest known impact in Europe. The area is of high interest due to its location during the Weichselian deglaciation. The deglacial history in the area is complex, and the behavior of the receding ice sheet is to some extent not yet fully understood. Submarine landslides were revealed during a geophysical survey, aimed as a site study for a drilling project with the purpose to retrieve an undisturbed sedimentary sequence for studying late- to postglacial evolution in the area. The largest landslide in Lake Orsa mobilized more than 620 000 m3 of sediment. It has a length of over 630 m and is over 400 m wide. The slides in Lake Orsa are characterized as both confined and emergent submarine landslides. Units with different sedimentological properties have been identified, with a plausible weak horizon in between. The upper unit is believed to prevent up-ward movement of water or gas, possibly leading to overpressure in the lower sediments. The landslides are thought to have occurred during several occasions and seem not to be related to one single event. Plausible causes may be a combination of steep slopes, overpressure zones, and/or low strength horizons in the sub-bottom. The landslides in Lake Orsa have several similarities with the Finneidfjord slide which occurred in northern Norway 1996. The slide was responsible for the loss of four human lives and destroyed nearby houses and a road. Characterizing and understanding submarine landslides are thus an important task to be able to protect citizens as well as infrastructure. Dating the landslides will be possible once the retrieved sediment core during the drilling campaign in one of the slide deposits has been analysed. Relating the slides to the postglacial evolution of the area has proven to be difficult without an age.
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