GEOMORPHIC AND SEDIMENTOLOGICAL EVIDENCE OF A DRAINED LAKE SYSTEM AND ASSOCIATED ENVIRONMENTAL CHANGE TINGRI, TIBET, CHINA

A sequence of lacustrine and lake shoreline deposits in Tingri County, Tibet, China provide evidence of former higher lake levels and the subsequent regression of prehistoric lakes on the Tibetan Plateau. The evolution of this former lake system sheds light on the environmental changes on the Plateau brought about by tectonic and surficial processes in the late-Quaternary. Lake and river terraces found downstream of Phung Chu indicate a series of progressively lower drained palaeolakes along the course of Phung Chu valley near the confluence of Gama Tsangpo. Most of the sequences comprise distinct lacustrine strata. They are commonly incised by a fluvial facies composed of cross-bedded sands and cobbles making preservation of the lacustrine strata rare. On the valley margins former shorelines are marked by nearshore and shoreline facies usually represented by oscillatory beds and locally bouldery lenses. Aeolian deposits, both unconsolidated and consolidated, commonly overlie the facies identified above. Preliminary OSL dating shows that the high-energy fluvial phases at Shegar occurred between ~70ka and ~110ka. The ages were derived from sediments obtained from the highest fluvial beds associated with the highest palaeoshorelines. It is likely that these beds record the first phase of drainage and hence record the maximum possible age of the breach of the dams downstream. Moraine deposits identified at two valley constrictions along the Phung Chu valley present a likely mechanism for lake generation. This evidence strongly suggests that the late Pleistocene saw a series of glacially dammed lakes. When the natural dams were breached, the resultant torrential flood provided high energy, which powered rapid incision of the river channels and formed the present drainage pattern in southernmost Tibet. Most of the silty and clayey deposits would be quickly scoured and carried downstream as suspended load. In places the modern system is dominated by several large dunefields of barchan dunes, which are likely to be products of aeolian processes reworking the river sands left after deflation of the lakebed surface.