Sedimentology and geomorphology of a relict lacustrine system in Tingri, Tibet, China

A patchy sequence of near horizontal sedimentary deposits composed of consolidated carbonate-rich laminated gravelly sand and silt are exposed on the hill slopes near Tingri, Tibet, China. They are classified as mid- to late-Pleistocene and Holocene lacustrine deposits in Chinese geological maps, but the style and nature of lacustrine deposition and the facies associations in the sequence have never been reported. This study presents a number of sections in the locality showing the varied sedimentology of the deposits. The deposits are interpreted as relict sandy or gravelly dune ridges and plains, salt pans, prograded deltas, and lacustrine sediments. Interpretation was primarily based on a comparison with modern inland lakes in the Tibetan Plateau as analogues of processes in arid mountain lake systems. Relict dune ridges appear on the margins of many Tibetan inland lakes where the highly erosive regime allowed an abundant supply of gravels and sand to deposit along lake shorelines. The strong winds in Tibet have also allowed high to medium energy shoreline systems to develop in exposed areas. The high gravel and carbonate content in the gravelly ridges provided ample resistance to erosion and they often remain a topographically pronounced feature after lake drainage. Salt pans and gravelly or sandy ridge fields are also left after lake regression along with rare lacustrine deltas represented by the lakeward prograding beds. Laminated shelly sand or silt preserved in pockets near the valley floor characterise deposition at deeper lacustrine regimes. At Tingri the exposures of the palaeoshoreline deposits can reach 20 m in thickness and are identified at an elevation ranging from 4286 to 4404 masl. Although patchy in occurrence the sequence suggests a relatively long lived relict lake system. The timing of lake phases is poorly constrained by optically stimulated luminescence mid-to-late Pleistocene and Holocene. A number of incised channels are observed in various deposits in Tingri, indicating high energy drainage during lake regression. The final drainage history leading to the disappearance of this large compound lake system is uncertain, but this rapid, high volume drainage of lake water downstream may provide a partial explanation for the rapid incision of the Arun Gorge located at the southeastern boundary of the reconstructed lake of Tingri.