Surface exposure dating of the Flims landslide, Graubünden, Switzerland

Sixteen boulder and bedrock surfaces related to the Flims landslide (volume 8–12 km3) were dated with 36Cl and 10Be. Exposure ages range from 4900 ± 250 yr to 15,440 ± 1480 yr, including corrections made due to snow cover and karst erosion. Ages of 11,410 ± 590 yr and 13,340 ± 1090 yr were obtained on bedrock surfaces outside of the landslide zone. These are minima for deglaciation of Segnes valley and provide constraints on possible amounts of inherited nuclides. Based on seven boulder ages, which range from 8200 ± 260 to 9520 ± 990 yr, a mean of 8900 ± 700 years is calculated for the Flims landslide. We exclude three outliers (one significantly older and two significantly younger than the others) and the ages from the Cassons bedrock site from the mean calculation. The latter is excluded as shielding due to snow and vegetation cover is difficult to constrain there. The oldest boulder on the landslide yielded an age (15440 ± 1480 yr) more than 5000 years older than any other boulder age, suggesting that it included the surface of the pre-slide bedrock. The exposure ages are consistent with a single failure event. The erratics and patches of till lying on the landslide debris must have been carried piggy-back on top of the landslide rather than having been deposited by the late Pleistocene Vorderrhein Glacier. The Flims rockslide is about the same age as the Köfels (9800 yr) and Kandertal (9600 yr) rockslides. All three occurred during the marked transition to warmer, wetter conditions during the early Holocene.