雅江河谷佛掌沙丘表层沉积物粒度特征

造山带河流的河谷深切,滑坡堰塞湖及其溃决洪水频发,洪水滞流沉积物经风力改造,在两岸形成大面积沙丘（以白格堰塞湖溃决洪水为例）。对雅鲁藏布江沿岸沙丘表层沉积物粒度特征的研究,可以为鉴别河谷中的古洪水沉积与风成沉积提供一定的参考意义,并为该区域防风治沙工程提供理论支持。选取雅鲁藏布江中下游河漫滩的爬坡沙丘——佛掌沙丘为研究对象,分析其与其上四个叠置的新月形沙丘表层沉积物的粒度特征和分异规律,揭示其沉积环境。粒度分析结果表明：佛掌沙丘表层沉积物以中砂为优势粒径,分选较差,极正偏,尖窄峰态。叠置新月形沙丘的粒度分布模式为坡顶粒径最粗,分选最好;就整个佛掌沙丘而言,随高度上升,沉积物的粒径变细,分选变好,这是风力分选与重力分异共同作用的结果。利用萨胡判别式判定该沙丘沉积环境为河流沉积,但将粒度数据与其他区域的河岸沙丘粒度特征进行对比发现该沙丘为典型的风成沙丘,可能是由于该区域物源较近,且受地形阻挡风力分选作用不明显,导致萨胡判别式无法很好地区分风力沉积与河流沉积。根据与河漫滩沙样的粒度特征对比发现,佛掌沙丘主要由河流中携带的物质提供沙源。

Variation of grain sizes for surface sediments of Fozhang dune in Yarlung Zangbo River Valley

Mountain rivers are generally featured by deep-cut river valley and steep side slopes, which is prone to landslide-dammed lake and outburst floods. Taking 2018.10.10 Baige landslide-dammed lake in Jinsha river as an example, wind reworked the outburst flood sediments and developed a plenty of sand dunes at the bank of Jinsha river. This studies about aeolian dunes in the Yarlung Zangbo River, to some extent, could provide some implication for the sedimentary distinction of aeolian sand and original flood deposit, and has some significance for engineering control of aeolian dune in this region. In this paper, a slope climbing dune in the floodplain of the middle and lower reaches of Yarlung Zangbo River, we analyzed the grain size characteristics and and spatial differentiation of surface sediments on four overlapping crescent-shaped dunes of a climbing dune, named Fozhang dune in the lower Yarlung Zangbo. The results of particle size analysis show that frequency distribution curves of the surface sediment of Fozhang dune, dominated by poorly sorted medium sand, is extremely positive biased and narrowly peaked. In the case of four individual superimposed crescent-shaped dune, the coarsest and best sorted particle fraction emerges at the top of the slope. On the whole, the surface sediment of Fozhang dune has become finer and better sorted towards the crest, which results from the combined sorting of wind and gravity. The grain size characteristics of Fozhang dune is comparable to other aeolian dunes of similar genesis. However, Sahu criterion indicates that the Fozhang dune is fluvial-originated. Maybe Sahu criterion cannot identify aeolian sand of very proximal material source from river floodplain. The comparison of grain size characteristics of sand samples with those of floodplain suggests that Fozhang dune is mainly provided with sand source by materials carried by rivers.