CHARACTERISTICS OF CLAY MINERAL ASSEMBLAGE OF XIASHU LOESS AND THEIR PALEOENVIRONMENTAL SIGNIFICANCE

30 samples of Xiashu loess taken from Dagang section in Zhenjiang,Jiangsu Province were analysed by the X-ray diffractemeter system.Particle-size analysis and magnetic measurements were also conducted to help explain the forming environment of the Xiashu loess.Conclusions can be drawn as follows:In Xiashu loess,the clay minerals are composed of illite(52.8%~70%),vermiculite(20.6%~38.5%),kaolinite(6.0%~11.1%) and small amount of 1.4 nm-interstratified mineral.The clay mineral assemblage,therefore,is illite-vermiculite-kaolinite pattern.The content of illite is relatively higher in paleosol than in loess,while the content of vermiculite is just the opposite.This may indicate that these two clay minerals have close relations in origin,supporting that illite can be partly formed from vermiculite during the soil development.For the whole section illite has a generally increasing trend upward,while vermiculite and kaolinite show a reverse trend.There are obvious differences in clay mineral assemblage between the Xiashu loess and the North China loess.First,the former has a higher amount of vermiculite without any of chlorite and smectite.Second,the crystallinity of illite in Xiashu loess is generally worse.They can be explained as the difference of climate between two regions,that Xiashu loess has experienced a weathering process under warmer and more humid climate than the North China loess.According to the content of kaolinite,the Xiashu loess is still in a moderate weathering in general.Furthermore,the content of illite is relatively higher in paleosol units than in loess units,while the content of vermiculite is the opposite in Dagang section.This negative relationship in content between illite and vermiculite shows that they may have close relations in origin,supporting that illite is partly formed from vermiculite during their soil development.