Landscapes and sediments of the ‘Yangtze First Bend’ valley along the Jinsha River and development of the valley

The First Bend of the Yangtze River refers to a segment of incised valleys in the upper reaches of the Jinsha River where the river makes a 135° turn from SSE to NNE. This segment is located on the southeast margin of the Tibetan Plateau, that is, the transition zone from the Tibetan Plateau to the Yunnan-Guizhou Plateau. According to the geomorphic and sedimentary characteristics of valleys, it can be divided into three sectors: the Qizong-Xialuoyu sector, the Tiger Leaping Gorge sector, and the Daju Basin sector. Their landforms and Quaternary sedimentary characteristics are described in detail in this paper. The origins and formation ages of alluvial sediments, lacustrine sediments, and glacial and glaciofluvial sediments in these sectors, especially 150～250m thick buried sediments, are also discussed on the basis of isotopic ages and related analytical data. The development of the Jinsha River valleys and the uplift of the Tibetan Plateau since the Miocene are also discussed.