Holocene extreme palaeofloods recorded by slackwater deposits along the Jiacha Gorge of the Yarlung Tsangpo River valley, southern Tibetan Plateau

Natural archives of past larger floods provide insights for assessment of future extreme flood events and climate variability in High Asian rivers. However, the sedimentary characteristics and ages of palaeoflood slackwater deposits (SWDs) still remain unclear in the Yarlung Tsangpo River (YTR), the largest river around the Tibetan Plateau. In this study, three palaeoflood SWD benches along the Jiacha Gorge were systemically analyzed. The SWDs are characterized by very fine and fine sand with parallel-bedding climbing ripples. High-resolution grainsize variation from bottom to end-point part of SWD layers, as well as the C-M technique and hydrodynamic model indicates a past suspension sand deposition (as graded and uniform beds) in shallow water and at lower velocity during high flood discharges. Geochemical data and heavy-mineral analysis suggest the SWDs to have experienced only a weak weathering process, and identify the assemblage to be derived from local felsic bedrocks in the Lhasa Terrane. At least two periods of palaeoflood events, 6.3-4.7 and 3.7-3.0 ka, were identified by using the optically stimulated luminescence (OSL) dating. The minimum palaeoflood discharges were estimated to be 31,600 m3/s in the one-dimensional model, varying from 28,500 m3/s to 45,000 m3/s within a 25% variation in roughness values. The palaeoflood chronology and magnitude documented herein are obviously different from earlier work in the area, do not supporting to establish a direct linkage with palaeo-lakes and outburst floods along the middle YTR valley. These extreme palaeoflood records exhibit good consistency with strengthened monsoon and glacier meltwater during the Middle and Late Holocene in the southern Tibetan Plateau.