Ages Of The Recent Two Episodes Of Glacially Dammed Lakes Along The Upstream Of The Yarlung Zangbo Gorge

The Yarlung Zangbo river in southeast Tibet turns abruptly southward at the eastern Himalayan syntaxis, forming the world’s largest grand canyon. Along its entrance, there are many episodes of ancient dammed-lake deposits and multiple-level lacustrine terraces. Besides, very thick fluvio-lacustrine sediments are buried beneath the cover of the riverbed. Currently, the timing, ages and extents of these glacially dammed lakes remain controversial issues.This thesis focuses on sedimentology and chronology of valley deposits in this area. Based on various data, it makes division of sediment facies to many terrace profiles and drilling cores. In addition, this work establishes an approximate timeline of upper valley deposits and determines the timeing, ages and extents of ancient dammed lakes by using luminescence signals from different minerals (quartz and K-feldspars) in combination with radiocarbon dating. The results would help further understand the landscape evolutionary history and the relationship among tectonics, climate, and surface processes, and evaluate the role of megafloods from the failure of moraine-dammed lakes on the incision in the gorge.Analysis of outcrop profiles and sediment facies of drilling cores suggest that the valley deposits along the entrance of the Yarlung Zangbo Great Canyon consist of many cycles of fluvial-lacustrine sequences, which gradually change from clay interbedded with silty sand to fine sand beds with abundant horizontal laminations and climbing ripples, even imbricated gravels of fluvial facies from lower to upper. Lacustrine sediments appear widely along the upstream at the confluence of the Yarlung Zangbo River and the Zelunglung glacier and its tributary drainages. Meanwhile, lacustrine sediments also are buried hundreds of meters below the riverbed.In terms of optically stimulated luminescence (OSL) and radiocarbon dating, this thesis comes to the following conclusions.1) OSL:This work has made a series of condition testing to choose suitable grain sizes and measurement procedures for the study region.2) The ages of glacially dammed lakes:The results reveal at least three glacially dammed lake events (Ⅰ, Ⅱ and Ⅲ) which have deposition ages of 7-9ka(Ⅰ), 20-30ka(Ⅱ) and >40ka(Ⅲ), respectively. The formation of paleo-dammed lakes reflects that the Zelunglung Glacier in the west slope of the Mt. Namche Barwa had progressively advanced to block the Yarlung Tsangpo River during the early Last Glacial, the Last Glacial Maximum and early Holocene. The recent two episodes of glacially dammed lakes produced two steps of lacustrine terraces (T1,T2) correspondingly, which are of elevations 2906-2956m and 3100-3060m, and ages 7ka (T1) and 20ka (T2), respectively.3) The extents of glacially dammed lakes:The glacially dammed lake I has a relatively smaller extent. Its lacustrine sediments are distributed mainly from Datuoka to Mirui with maximum thickness about 5-8m. Its end is roughly at south to Milin county. The terrace T1 of this lake is 20m high above the river level, composed mostly of clay beds which are covered by aeolian sand. The glacially dammed lake II occupies a large area with the end roughly nearby Lang county. Its sediments are exposed from Datuoka to Wolong. Its terrace T2 is 200-250m above the river, composed mostly of the rhythmic beds of clay and sand. After the later fluvial erosion, the lacustrine sediments of this lake formed 1-3 levels of secondary terraces. The glacially dammed lake III is largely covered by the sediments from glacially dammed lakes I and II, with scattered exposures in the bottoms of Miruicun terraces. The drills in the river bed at Datuoka and Miling reveal that its deposit thickness is more than 100m.