Abstract: Loess in middle Yarlung Zangbo River valley area originates in distance of near field.It formed approximately at the same epoch as Wuduo Loess did,and the Wuduo Loess is located at the eastern rim of Qinghai-Xizang plateau.The heavy mineral composition of loess in the studied area inherits from the Quaternary clastic sediments in the upper Yarlung Zangbo River vally area.The grain size of loess are predominated by 100~50 m and 50~10 m particles,which belongs to sandy loess,and whose geochemical elements are similar to that of Yulin profile at northern Loess Plateau.The forming processes of loess in the studied area can be summarized as following:A large quantities of clastic materials produced by weathering of plateau surface,i.e.physical,chemical and biological,are transported and concentrated to the valleys and basins of Yarlung Zangbo River in the function of wind,gravity,glacial and water flow.These clastic materials whose grain sizes are from sand to fine sand grade and even to silt grade lay bare on the ground surface in winter or cold period whenever precipitation and ice-snow melting water decrease,water surfaces in rivers and lakes go down relatively.At the same time,the plateau surface is controlled by westerly current.The strong wind provides force for sand shifting and dust floating.The geomorphological features of wide and narrow valleys alternating make wind speed up at narrow valley due to canalization at the one hand,slow down at wide valley due to diffusion at the other hand.Sand and dust in weak sand carring flow descend and deposite.These fine particles prevented by vegetation are evoluted to loess in warm period when precipitation increases and functions of biological,chemical and soil argillification are strengthened relatively.

Key words: Yarlung Zangbo River, loess, deposition features, causes