Deglaciation history and subsequent lake dynamics in the Siljan region, south-central Sweden, based on new LiDAR evidence and sediment records

The Siljan region hosts Europe's largest impact structure. The high-relief landscape, with a central granite dome bordered by lake basins, contains an array of glacial and shore-level landforms. We investigated its deglaciation history by mapping and analysing landforms on high resolution LiDAR (light detection and ranging)-based digital surface models coupled with well-dated sediment successions from peat and lake sediment cores. The granite dome and bordering areas are characterized by streamlined terrain and ribbed moraine with a streamlined overprint. These suggest an ice-flow direction from north-northwest (NNW) with wet-based thermal conditions prior to deglaciation. During its retreat, the ice sheet was split into thinner plateau ice and thicker basin ice. Sets of low-gradient glaciofluvial erosion channels suggest intense ice-lateral meltwater drainage across gradually ice-freed slopes, while 'down-the-slope' erosion channels and eskers show meltwater drainage from stagnated plateau ice. Thick basin ice receded with a subaqueous margin across the deep Siljan-Orsasjon Basin c. 10,700-10,500 cal. bp. During ice recession the ingression of the Baltic Ancylus Lake led to diachronous formation of highest shoreline marks, from similar to 207 m in the south to similar to 220 m above sea level (a.s.l.) in the north. Differential uplift resulted in shallowing of the water body, which led to the isolation of the Siljan-Orsasjon Basin from the Baltic Basin at c. 9800 cal. bp. The post-isolation water body - the 'Ancient Lake Siljan' - was drained through the ancient angstrom kero Channel with a water level at 168-169 m a.s.l. during c. 1000 years. A later rerouting of the outlet to the present course was initiated at c. 8800 cal. bp, which led to a lake-level lowering of 6-7 m to today's level of Lake Siljan (similar to 162 m a.s.l.). This study shows the strength of an integrated methodological approach for deciphering the evolution of a complex landscape, combining highly resolved geomorphological analysis with well-dated sediment successions.

Automated summary

The Siljan region in Europe hosts the largest impact structure, and a study on its deglaciation history reveals the details of ice sheet retreat and the formation process of changes in the ancient lake's water level.