Nam Tso Lacustrine Sediments and the Ancient Big Lake in Northern Tibet Plateau
纳木错湖相沉积与藏北高原古大湖

The ancient lakeshore is distributed extensively in northern Tibet plateau, and the lake sediments as well as the lake landform are well developed. At present, 4～6 grades of lake terraces around the Nam Tso can be distinctly recognized, which exceed the modern surface of the Nam Tso in altitude by 3～12 m, 15～22 m, 25～30 m, 35～45 m and 60～150 m, respectively. The first, second and third grades of terraces are all built terraces, and the fourth to sixth grades of terraces are also built terrace in plain area, but are base seat terraces at the sloping foot of the bedrock hill. These terraces are all composed of well-sorted sand, silt, clay and sub-clay with horizontal bedding and sometimes distinct tiny bedding. The highest lake sediments are 150 m higher than the modern surface of the lake. On the smooth lake terrace 40 m below the morden lake surface are tens of (more than 50) lakeshore mounds composed of lakeside gravels. Along the bank of the Nam Tso and on the islands of the lake, various kinds of lake-erosion topographies can be seen, probably related to the change of the surface of the lake. The EW-trending Xiongqu-Naqu valley, 30 km in length and 1.5～3.5 km in width, connects with the Namucuo basin and the Rencuo-Jiurucuo basin in the west. In the eastern part of the valley, it connects with a new NWW-trending valley. Nearby the watershed of the valley and on its east and west sides, Quaternary lake sediments comprising the top of the second lake terrace of the Nam Tso becomes the bottom of the wide valley, indicating that the Nam Tso and the Rencuo-Jiurucuo on its west seem to have been a connected big lake when the third and second grade lake terrace of the Nam Tso were formed. Besides, the watershed between the Rencuo and the Mujiucuo on its west is as wide as 8 km with the elevation lower than 4 700 m, and its height above the Rencuogongma does not exceed 50 m, which suggests that the Nam Tso and the Selincuo also constistuted a connectied big lake rather than being two lakes linked together by rivers when the third and second grade lake terrace of the Nam Tso were formed. These two periods and the period during which the fourth grade of lake terrace and higher lake sediments of the Nam Tso were piled up belong to the several high lake surface stages in the growing history of ancient big lake in northern Tibet, and the highest lake surface stage of the Nam Tso, the Shencuo and the Bamucuo and most ancient lakes in the southeast part of northern Tibet plateau were all connected, which can be called ancient Qiangtang East Lake. The Zharinanmucuo and the Xurucuo and other ancient lakes in the southwest of northern Tibetan plateau were also connected, which can be called ancient Qiangtang West Lake. Furthermore, the two ancient lakes were probably linked with each other in a particular period, and became genuine ancient Qiangtang Lake. According to the distribution of the highest lakeshore and the marks of the lake sediments, it is suggested