O01_P02

New evidence for the persistence of the Ilanzersee (Flims rockslide)

Grischott R^1,2, Wacker L², von Poschinger A³, Gilli A⁴

¹Büro Für Technische Geologie, Sargans, Switzerland, ²Laboratory for Ion Beam Physics, Zurich, Switzerland, ³Private adress, Kempfenhausen, Germany, ⁴Department of Earth Sciences, Zurich, Switzerland

The Flims rockslide is one of the largest known rockslide in the Alps and had a strong influence on the landscape evolution in the Vorderrhein-Valley. The Flims rockslide (volume 9–12 km³) has been dated to 9400 cal yr BP with the radiocarbon method [2]. The Vorderrhein was completely blocked by a more than 600 m-thick landslide dam and a lake, Ilanzersee, formed upstream [1]. Its maximum level obviously did not reach higher up than 930 m a.s.l. After the breach of the dam, an important sediment transport down the Rhine valley had occurred. Nevertheless, a relict lake existed for a longer time, probably for centuries. A level of about 820 m a.s.l that was held for fairly long time. The duration of this second lake level, until the lake was finally emptied, has not been clear so far. Drill cores retrieved along a transect of 4 km on top of the former delta plain of Ilanzersee revealed the continuous presence of typical fine-grained delta sediments overlain by recent fluvial sediments. A wood fragment embedded in delta sediment was dated to 8900-9000 cal yr BP and supports geological evidence found elsewhere, that the lake persisted during quite a long time. More samples from other drillings will be analysed to underline this first evidence.

[1]: Von Poschinger 2005

[2]: Deplazes et al., 2007