Reconstructing post-mid Pleistocene glacial-dammed paleolakes in the Tsangpo Gorge, southeastern Tibetan Plateau

Abstract

The paleolakes of the Yarlung Tsangpo Gorge area of the southeastern Tibetan Plateau provide an important basis for testing the hypothesis that glacial dams have effectively prevented the Yarlung Tsangpo River from cutting into the edge of the Plateau. However, sedimentological evidence for the recurrence of these paleolakes during the Quaternary remains inadequate. During this research we conducted extensive geological surveys, and sedimentary and geochronological analyses, of the river terraces and valley sediments upstream of the Yarlung Tsangpo Gorge. Three fluvial-lacustrine terraces formed since the last glacial period were identified from analyses of 17 sedimentary sites, with the following depositional ages, from new to old: 0.6–0.4 ka (T1), 9–5 ka (T2) and ≥50–20 ka (T3), respectively. The sediments of T1 and T2 are <30 m thick with an overall pattern of reversely-graded sequences, each reflecting a dammed paleolake event (named Paleolake I and Paleolake II). The sediments in T3 are the thickest (>265 m), with the highest preserved terraces in the river valley. More than five depositional cycles of fluvial and lacustrine phases in this terrace reflect many sporadic damming events during the last glacial period (representing Paleolake III). Additionally, the earliest phase of sedimentation of the dammed lakes revealed by analyses of drill cores in the valley fill may have begun in the Mid Pleistocene. The sedimentary sequences of these paleolakes revealed that frequent glacial-damming events have occurred in response to climate change since the Mid Pleistocene, supporting the theory that glacial dams and outburst floods have played an influential role in the geomorphological evolution of the margins of the Tibetan Plateau.