Comparison of the Sedimentary Features of Loess between the Westerly and Monsoon Regions in China

The Yili region is located in west Xinjiang,China,and is an intermontane basin in the Tianshan Mountains.It is affected by westerly circulation all the year round.The study on loess distribution,lithology,grain size,mineral assemblages,surface texture of quartz,magnetic susceptibility and geochemical characteristics shows that the loess in the Yili region is a typical aeolian deposit.Howevert,he loess in the Yili region obviously differs from that in the Loess Plateau in some aspects.Firstly,the contents of fine silt and clay in loess in the Yili region are high,and the content of sand is low.Median size ranges from 15 μm to 20 μm,and the average proportion of the particles less than 15μm reaches 54%.Secondly,unstable minerals,such as hornblende and biotite,are more abundant than stable minerals,which are generally rich in the Loess Plateau.Thirdly,the magnetic susceptibility of loess in the Yili region is higher than that in the Loess Plateau and is related to the content of the carbonate.Fourthly,loess in the Yili region contains plentiful carbonate,occupying 12.0% ～ 33.6% of the total weight.The calcium carbonate appears mainly as hypha,pellicle and fleck in the Yili region,a westerly region,while the calcium concretion occurs widely in loess deposits in the Loess Plateau,a monsoon region.Lastly,although the paleosols can be distinguished from the loess by higher contents of Al2O3,TiO2 and K2O,the change of SiO2 content is not so significant like that in the Loess Plateau.The overall abundances of Rb and Sr in the loess and paleosol in the Yili region are higher than those in the Loess Plateau.On the contrary,The 87Sr/86Sr and Rb/Sr ratios of loess(0.7115～0.7135,0.16～0.37) and paleosol(0.7133～ 0.7137,0.34 ～ 0.87) are lower than those in the Loess Plateau(loess= 0.7153,0.55;paleosol=0.0.7171,0.73).