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On both sides of the Jinsha River valley near Benzilan, Deqen, northwest of Yunnan, China, a series of varied lacustrine deposits have been discovered under the fourth terrace (Q(3)(2)). Landslide debris has been identified underlying the lacustrine deposits, indicating that the lacustrine deposits were caused by a landslide damming the Jinsha River. U-series dating shows that the age of the upper and middle portions of the lacustrine deposits on the right bank were 55.4 +/- 3.5 and 82.1 +/- 6.6 ka respectively. Lacustrine deposits on the left bank have been dated to 122.0 +/- 12.4 ka at the bottom, which indicates that the large-scale landslide was formed before 122.0 +/- 12.4 ka and the paleo-lake was developed within the early stage to the mid-stage of the Late Pleistocene. The characteristic of spore-pollen and high content of CaCO3 in the lacustrine clays reveal that the paleoclimate was semi-arid with less heavy rainfall. Considering the regional neotectonic background, the authors prefer to interpret that the Benzilan ancient landslide-dammed lake was caused by an ancient earthquake. The discovery of the ancient landslide-dammed lake provides significance to the understanding of the formation and evolvement of calamitous geological hazards along large rivers. (C) 2010 Elsevier Ltd and INQUA. All rights reserved.