| Chinese Quaternary loess is accumulated sedimentary materials blown from one area to other areas by the wind.Owing to its wide distribution,the continuity of deposition and its sensitivity to global environmental changes and paleo-climate evolution,an investigation into it will be of great significance for the study of paleo-environment,as well as for the agricultural production and engineering projects,among others.So far,the study of loess in China has been fruitful,which enables China to be one of the top three media of recording evolutions of the Quaternary environments.Besides,it pioneered a new path of research in this field.However,as a unique type of Chinese quaternary sediment,the loess in the lower reaches of Yangtze River,and the dust deposition with its by-products in the Delta Region of the river and in the Three Gorges region are studied much less and less fruitful when compared with the accomplishments made in other regions.The disparity can be ascribed to the following reasons:one is that the loess in the former regions is much denser than the latter regions.Another factor is that the humid climate condition in the Southern regions has a great impact on the transformation of the primitive dust deposition.The studied region is located in the Wushan regions of the Three Gorges of Yangtze River.And the 41 samples of Quaternary Deposit as well as 1 sample of bedrock are taken from WTP1 and WS3 sites in Wushan Chongqing province.The samples are then dried naturally and grinded for Grain-size test and magnetism test. Grain-size is the fundamental physical features of sediment.And grain-size analysis is an important means to study the sediment environment,and the process and mechanism of shipping as well.The basic feature of grain-size can not only count as the quantitative basis for the classification and naming of sediments,but also as a clue to trace what the environment was like in their formation.We arrive at the following conclusion based on relevant experiment and analysis of the results:According to the grain-size analyses of 41 samples of the Quaternary Deposit from WTP1 and WS3 sites in Wushan Chongqing province,based on minute comparison of the grain-size features with typical loess,the genesis of Wushan Quaternary Deposit is discussed.The results show that:Wushan Quaternary Deposit mainly consists of 5～50um particles.The average percentage of the grain-size distribution accounts for 53.44%,65.37%respectively in WTP1 and WS3 sites.And 5～50um particle group is the "dominant group" of Wushan Quaternary Deposit.The average content of＞50μm particles and＜5um particles account for 0.79%,15.54% and 45.78%,19.09%respectively.The grain-size frequency distribution presents single-peak mode.And the particle parameters are almostly coincident.It can be concluded that Wushan Quaternary Deposit is also the atmospheric dust sediment based on grain-size elementary analyses.The quality of magnetic susceptibility(x) is a reflection of the strength of magnetism.The average magnetic susceptibility of WS3 Quaternary Deposit of the whole profile accounts for the 9.184(10-8m3·kg-1),ranging between 7.310～10.857.The magnetic susceptibility of WTP1 Quaternary Deposit of the whole profile fluctuates between 0.965～2.349, with a mean of 1.687(10-8m3·kg-1),less than one-fifth of WS3.The contrast may be caused by the two different magnetic mechanism.On the whole,the magnetic susceptibility of the two profiles is low,reflecting that the role of weathering is not obvious.Frequency magnetic susceptibility(xfd) is used to prove the existence of the super-paramagnetic minerals.According to the result,WS3 profiles frequency magnetic susceptibility is between -0.427～0.524,with an average of 0.064,and all sections xfd＜2.0%,indicates that the fundamental particles do not exist in the profile.Frequency magnetic susceptibility at WTP1 profile is fluctuated between -2.507～0.376,with an average of -0.386,the whole profile being xfd＜2.0%,indicates that there are basically no SP particles in the profiles,the discovery that the grain-size WS3 profiles are more coarse than the granularity WTP1 profile well illustrates the point."Hysteresis" phenomenon is one of the important characteristics which distinguish anti-ferromagnetic materials from magnetic materials.In view of the form of the hysteresis loop,the WS3 profile Samples are higher and thinner than the WTP1 ones,indicating that the former magnetic mineral is mainly composed of hematite,the latter magnetite.The hysteresis test of the bedrock sample through WTP1 profile(WTP1-br) indicates that "back line" has the shape of almost a straight line,reflecting that it basically does not contain magnetic substances.This result is in good consistency with the characteristics of Grain size.Consequently,the hypothesis that the Wushan Quaternary Deposit was formed by wind transport is disconfirmed.As is discussed above,the characteristics of grain-size and magnetic susceptibility of Wushan Quaternary Deposit have shown little effect of weathering,which exhibits certain similarities with the composition of the North.But it appears to be abnormal that the magnetic susceptibility of the samples taken on mountain is high while that of the samples taken below mountain is low,which is worth further study.
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