Giant landslides; transgressions; late Quaternary; Kazakhstan; Caspian Sea.

The history of Quaternary sea-level changes in the Caspian Sea, the world?s largest lake, is partly enigmatic, and so is the geomorphic response of its coasts. Late Pleistocene transgressions during the Early Khvalynian (ca. 40?25 ka) inundated extensive portions of the flat, low-lying semi-desert of western Kazakhstan. Cliffs cut during these highstands form a prominent escarpment tens of kilometers to several hundred kilometers from the present coast of the Caspian Sea. Satellite images, digital terrain analysis, and field mapping reveal that >300 giant landslides intersect with this escarpment. More than 100 of these slope failures mobilized volumes >108 m3 along basal failure planes with gradients as low as ~5°. All landslides share characteristics of lateral rock spreads involving competent limestones overlying weak and plastic claystones. From relative stratigraphy and new 14C data, we infer that catastrophic slope failure of over 41 km3 occurred mostly during Pleistocene Caspian sea-level highstands, while several landslides may have been reactivated or entirely originated during the Holocene. This largest cluster of terrestrial mass wasting in a tectonically quiescent setting offers an opportunity to understand how landslides erode low-relief landscapes subject to oscillating sea levels.