LAKE-LEVEL CHANGE OF NAM CO,TIBET,SINCE THE LATE PLEISTOCENE AND ENVIRONMENT INFORMATION OF CLAY MINERALS IN LACUSTRINE DEPOSITS

Field surveys of lacustrine sediments and lakeshore levees and leveling surveys of lake terraces at peripheries of Nam Co-the largest lake with the highest altitude above sea level in Tibet-indicate that there are 6 lake terraces below the lakeshore of Nam Co 48 m above the lake level and highstand la custrine sediments from 48 to 139.2 m above the lake level.Isotopic dating of the lacustrine sediments shows that the development of Nam Co and the evolution of the ancient large lake on the southeastern North Tibetan Plateau can be divided into three stages:① the ancient large lake stage at 116~37 ka BP, ② the outflow lake stage at 37~30 ka BP, and ③ the Nam Co stage since 30 ka BP.Clay minerals in lacustrine sediments of Nam Co since the late Pleistocene were analyzed by X-ray diffraction, and according to the analytic results, the paper discusses the environment information of clay minerals by using the ratio method, kaolinite method and X-ray diffraction method.The changes of the clay mineral composition show the climatic and environmental features of a dry temperate arid and semi-arid area.The study provide new data for the study of the lake evolution, climate change, paleogeographic change and the plateau uplift process.