雅鲁藏布江河谷黄土的沉积特征及成因

Deposition Features and Causes of Loess in Yarlung Zangbo River Valley Area

在野外调查和室内资料分析的基础上 ,对西藏雅鲁藏布江中部流域河谷地区黄土的形成时代、物质特征及成因进行了研究 ,认为该黄土是近源黄土 ,形成时代与青藏高原东缘武都黄土相近。黄土的重矿物组成明显受到雅鲁藏布江河谷第四纪碎屑物质的影响 ,其地球化学元素构成与黄土高原北部榆林剖面相近 ,粒度组成以 10 0～ 5 0μm和 5 0～ 10μm的颗粒为主 ,为沙黄土。该黄土是在雅鲁藏布江河谷丰富碎屑物质、宽谷与峡谷相间地貌特征的基础上 ,受冬季或寒冷时期西风急流和河谷与山地间热力差异产生的谷风搬运堆积 ,并在温暖时期经历生物、化学。

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 \%μ\%m and 50~10 \%μ\%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.