Holocene megafloods? Stories of the lacustrine strata along the Nyang River, Tibet

The occurrence of lacustrine sediments along the Yarlung-Tsangpo, southern Tibet, has evoked the arguments regarding the megafloods scenario and paleo-lakes dammed by monsoon driven valley glaciers in the early Holocene. According to our field survey of the terraces constituted by lacustrine deposits on the top along the middle Yarlung-Tsangpo and its tributary, Nyang River, we are able to establish a successive litho-column that reveals the evolution of the paleo-lake. The 150 m sedimentary sequence was reconstructed by compiling the outcrops exposed among the terraces on the elevation of ca. 2900-3100 m. The strata mainly consist of alternating sand and silt units and distinctly record two remarkable varve sections. Our resulted stratigraphy shows at least two sets of coarsening- upwards sequence beginning from the varve layers and then gradually transiting to interbeded sand/silt with abundant ripple rich cross-beds, parallel lamination, syn-depositional deformation and even showing channel gravels in certain horizons. The facies identification presents the evolution of depositional environments from pro-delta to the beach ridges of the frontal floodplain and indicates at least two aggradational stages of the glacier-related outwash delta. Radiocarbon and OSL dates reveal an aging-downwards trend as the stratigraphic order, confirming the assumption that the recorded sequence was deposited under the successive deposition of the investigated terrace. They further suggest that the younger paleo-lake took place before 32 ka and the older one occurred no earlier than 50 ka, both are far older than Holocene. As a summary, from Marine Isotope Stage 4 to 2, two sedimentary sequences of delta evolution from pro-delta, delta front to floodplain are recorded along the Nyang River; this is believed as a result of glacier-damming events occurring in the middle reach of the Yarlung-Tsangpo.