The Fulnau landslide and former Lake Seewen in the northern Swiss Jura Mountains

Investigations of the former lake at Seewen in the Canton of Solothurn, Swi tzerland, over the past 30 years have revealed a landslide history and a la ke evolution during late Pleistocene and Holocene times. As the only Late G lacial lake known in the northern Jura Mountains, its existence has been do cumented by historical chronicles up to its final drainage in the late 16th century. Geological mapping indicates that former Lake Seewen originated w hen rocks from the Homberg Mountain slid into a river gorge near Fulnau and blocked the flow of the river Seebach. Age dating of the older lake sedime nts using radiocarbon and pollen indicates that this landslide happened aro und 13,000 to 13,500 BP. The lake extended upstream in an easterly directio n leaching its maximum extension in the late sixth ol early seventh century AD. Although the lake sediments reach a thickness of more than 23 m above debris deposits, the water depth appears to have been limited to not more t han 10 m. These lake sediments are highly cohesive, rich in clay and organi c matter with silty layers. X-ray investigations revealed the existence of numerous "dropstones" in Late Glacial deposits. Holocene sedimentation took place at rates averaging 1.5 mm/a back to 7,000 BP and 2.6 mm/a between 7, 000 and 9.000 BP. whereas the Late Glacial sedimentation rates are somewhat lower at 2.1 mm/a. Thin silty-sandy layers allow a correlation of differen t drill cores and indicate short-term changes in the sedimentation conditio ns which might he triggered by exceptional flood-events, storms, subaquatic slumping or earthquakes.