塔里木盆地边缘山地的黄土沉积

LOESS DEPOSITS IN THE SURROUNDING MOUNTAINS OF TARIM BASIN, NORTHWESTERN CHINA

依据在昆仑山北坡若羌、于田和昆仑山西缘塔什库尔干境内的野外工作 ,分析研究了新疆黄土的分布规律、形成年代、物质成分、沉积环境和沉积区的环境变化。在新疆境内 ,黄土分布的海拔高度有由南向北降低的趋势 ,黄土的年龄各地差别较大 ,在低海拔的部位还保存有“老黄土” ,但在高海拔位置只见“新黄土”、全新世和现代黄土。本区黄土虽然颗粒较粗 ,但矿物组成及化学成分和洛川黄土有相似性。在昆仑山北坡高海拔地区现存的黄土应是在当地较温暖湿润的时期沉积的。

Loess and loess like sediments are vastly spread over the marginal mountains of the Tarim Basin and Zhungaer Basin in Xinjiang of northwestern China. Around the basins there are often two different aeolian sedimentation belts, which are separately covered by loess, and loess like deposits. Generally speaking, the loess is more intensively developed in the southern margin of the basin than in the northern. On the northern flank of Kunlun Mountains,loess is mainly distributed between 2 500m and 3 400m a.s.l.(above see level), and its thickness is seldom over two dozen meters. Small loess patches with a thickness of about 1m may occur even at the elevation over 5 000 m a.s.l. But in the Tianshan Mountains, the loess is considerably restricted to 950m and 2 400 m a.s.l. on the southern flank, and 800m and 2 000m on the northern side. The samples taken from Tashikuergan to Yütian of the northern flank of Kunlun Mountains show the grain size of loess tends to increase from West to East. Compared with Loess Plateau, the loess in Xinjiang is generally coarser probably owing to its shorter transport distance. The samples from Pulu in the upper reaches of Keriya River indicate that the percentage of heavy minerals in the loess is 5.5% on average. The content of stable and very stable minerals is only some 20%. One third of the heavy minerals is hornblende. Besides, the content of epidote, iron minerals and black mica is relatively high also. From the field investigation and laboratory analysis, it concludes that the preserved loess in the surrounding mountains of the Tarim Basin has been deposited mainly in Holocene. There the loess accumulation takes place principally during the intergalcial periods. Under the more severe climatic conditions of glacial times, the vegetation in the present loess zone would deteriorate considerably, and a stronger deflation and sand accumulation would replace the process of loess sedimentation. But in the northern flank of Tianshan Mountains, several layers of paleosol are present in the loess section, and both Holocene and late Pleistocene loess is observed.