克里雅河流域风成物质的粒度分析与讨论

本文分析研究了昆仑山北坡克里雅河流域风成物质的粒度特征。该流域四大地貌景观带的地表沉积物表层样品的机械组成差别明显,黄土的粒度组成以粉砂为主,比我国黄河中游地区的黄土粗得多,亚砂土的主要成分为微砂,沙漠地区以细砂为主。除戈壁带以"非地"形式出现以外,由沙漠中心向南至昆仑山,在西北风和东北风的作用下,风成物质的粒度由粗变细。流域出露的黄土主要是在晚更新世和全新世沉积的。

THE GRAIN SIZE OF THE WINDBORNE SEDIMENTS IN THE AREAS OF KERIYA RIVER (NORTHWESTERN CHINA)AND ITS PALEOENVIRONMENTAL INTERPRETATION

Grain size parameters of windborne sediments in the areas of the Keriya Riverwere analyzed to determine the depositional environment and to understand theclimatic conditions in the late Quaternary. The Kenya River originates from theKunlunshan Mountains and terminates in the center of the Tarim Basin. From north tosouth along the river, the deposits are clearly divided into four different belts whichare sand desert below 1 400m a.s. 1., gavel desert between 1 400-2 000m a.s. 1., loesslike sediments in the elevation 2 000--2 500m and loess between 2 500 and 4 500 ma.s.1. The loess is mainly distributed in the elevation between 2 500 and 3 400m a.s.1.Its thickness may be over ten meters. Above 3 400m the loess layer becomes thinabruptly. Above 4 500m frost weathering is a dominant process in the landscape, andthe present snow line lies at the elevation of about 5 500m.35 samples of windborne deposits collected from the surface layer from the southto north in the area of Keriya Reriya give a general picture of grain-size distributions.The loess in the research area consists mainly （over 50%） of silt with the diameterbetween 7.64-4.32 （0.005-0.050mm）. The content of clay （>7.64 , <0.005mm）is about 3%, and the percentage of fine sand （3.32- 2, 0.100-0.250mm） variesbetween 31% and 41% （Fig.1）. Compared with the loess in Luochuan of the LoessPlateau, there are much more sand particles and quite less clay in the Keriya Riverarea. The high percentage of coarser materials is generated from the short aeoliantransport distance. The loess-like sediments are made up by 60%-94% of very finesand （4.32 3.32, 0.050-0.100 mm）. The sand dunes in the desert belt contain highpercentage of fine sand and very fine sand. Only in the center of the desert, thecontent of medium-sized sand （2-1, 0.250-0.500mm） may reach 30%. Under thefunction of wind, the spatial differentiation of the grain size takes Place not onlybetween the diffelent deposit belts, but also within a single zone （Fig.2）. The strikeof the dunes indicates that the prevailing winds in the areas of Kenya hiver are NEand NW. In the upper reaches of the river, the influence of the daily upslope anddownslope winds is important. With rising temperatures during the daytime, theupslope wind becomes effective and brings large amount of dust and sand from the Takelamgan to the Kunlunshan Mountains. The dust is bought even up to the glaciers.Most scientists agree that the loess in the arid areas of central Asia, like in theSouthwest of Tarim Basin, often has an aeolian origin. The pH-value of the loessmay be as high as 9 in the north flank of Kunlunshan Mountains. Because the loesshas light color and loose structure and covers moraine and glacialfluvial sediments inPulu （Fig.3）, most part of the loess in the areas of Keriya River should be consideredas Holocene sediments. The TL-dating confirms that there was only sandaccumulation during the early and middle periods of the last glaciation.Loess appearedin the late phase of the glaciation. possibly there was loess accumulation in earlierbines. But it was not preserved because of glacial erosion during the ice ages. Underthe SEM the particles show both aeolian mechanical strtuctures and traces of frostweathering.Not only a few researcher considered the loess deposition in the north flank ofKunlunshan Mountains as a product of cold and dry climate of glaciations. But duringdrier and colder times in which the loess was sedimented in the Loess Plateau, thepresent loess region of Keriya Reriya underwent stronger aeolian and glacial process.The landscape would be barren and only sand fraction could be deposited at that bine.In the foreland of the Kunlunshan Mountains, a large belt of alluvial and glacialfluvialfans were formed. Even moraines are present in the elevatio of 1 500m a. s. 1. Thethickness of the gravel layer is 500-600m with a maximum of 900m. Owing to theuplifting of the mountains, the alluvial and glacialfluvial fans were developed atdifferent elevations. Because of limited precipitation, desert varnish appears on thegravels. The sl