A lateglacial rock avalanche event, Tianchi Lake, Tien Shan, Xinjiang

The genesis of a diamicton deposit that dams Tianchi Lake in the eastern Tien Shan, Xinjiang, China, has been debated for many years, and it has generally been considered to have been either the result of landsliding or glacial deposition. Using geomorphic, sedimentological and geochronological techniques we re-examined this deposit to help elucidate its origin. The outer margin of the deposit can be traced up the adjacent hillside, where a series of steps within the bedrock are present and likely represent landslide scars. Within the lake the deposit has an undulating surface. The deposit is unstratified, comprising angular monomictic clasts that coarsen upward into boulders, which armor the surface. All these features suggest that the deposit was formed by landsliding. Furthermore, 10Be cosmogenic radionuclide surface exposure dating of boulders on the levee of the deposit show that the deposit probably formed during the Lateglacial, possibly at ∼12 ka and is likely coincident to the Younger Dryas Stade. The sedimentological and geomorphic evidence support our view that the diamicton deposit is not of glacial origin, but was deposited by rock avalanching during the Lateglacial.