雅鲁藏布江米林宽谷段爬升沙丘粒度分异特征研究

Variation of grain sizes on a mountain climbing dune in Mainling wide valley,Yarlung Zangbo River

雅鲁藏布江米林宽谷段爬升沙丘分布广泛。选择卧龙镇西侧雅鲁藏布江右岸典型爬升沙丘,根据沙丘爬升高度和叠置沙丘地貌部位对其表面沉积物进行了系统采样。作为河谷风沙地貌的重要组成部分,相应采集了河漫滩、阶地沙丘沙样品。粒度分析表明,爬升沙丘对河漫滩沙丘、阶地沙丘在物源和形成过程方面具有继承性,其表面沉积物以细沙和极细沙为主(2.07～3.71Φ),分选中等或较好(0.20～1.41Φ),细偏,多峰。由坡脚向丘顶的沉积物粒径变细,分选变好,偏度和峰度降低。爬升沙丘表面叠置沙丘的粒度向丘顶变粗、分选变好。细沙、极细沙等粒级组分与高度相关,反映颗粒爬升能力不同。叠置条带状沙丘脊部和槽部的粒度存在分异。

Mountain climbing dunes are widely distributed in the Mainling wide valley of Yarlung Zangbo River, southern Qinghai-Tibet Plateau. The climbing dunes studied are located on the right bank of Yarlung Zangbo River at an S-shaped meander, west of Wolong town. Samples are collected according to dune climbing height and sites of super- imposed dunes along prevailing wind （NE）. As parts of the aeolian sand dune system in the valley, sediments are also sampled on dunes developing on the alluvial flat and terrace. Grain size analysis shows that sediments of the mountain climbing dune are generally char- acterized by fine and very fine sands （2.07 to 3.71φ）, moderately well sorting （0.20 to 1.41φ）, fine skewness （-0.33 to 0.61） and multi-mode grain size distribution with kurto- sis ranging from 0.66 to 3.09, exhibiting some features of both succession and variation from those of sand dunes on the upwind marginal alluvial flat and river terrace. Grain size parameters vary with height in NE-SW wind direction from the toe to the top of the climb- ing dune with mean grain size getting finer, sorting better, skewness coarser and kurtosis lower. Also, grains on super-imposed dunes grow coarser and sorting poorer from toe to dune top. There are some correlations between grain size and height, which shows a posi tive correlation for very fine sands with height and a negative relation for fine sands and height. The variation of grain sizes with height reflects different climbing ability for grains with different sizes. There exist differences in grain size characteristics between ridges and inter-ridge grooves of strip-shaped dunes.