Research On The Characteristics Of Loess Sediments And The Provenance In Mao County-Wenchuan County

The Western Sichuan Plateau is located in the key area of environmental change,which can sensitively reflect the changes of Quaternary climate and environment.Since the early Pleistocene,loess sediments have been developed in the Western Sichuan Plateau.They are the characteristic sediments produced under the influence of the Quaternary monsoon climate in the Western Sichuan region and the important carriers of the earth environmental information records in the eastern part of the Qinghai Tibet Plateau,especially since the Pleistocene.However,the research on the coupling mechanism between the development model of loess in Western Sichuan and the rise and fall of monsoon in the region is not comprehensive.At the same time,there are many disputes about the material source of loess in Western Sichuan and whether there is homology with loess paleosol sediment in the Loess Plateau.The diversity and uncertainty of the cause,material source and sedimentary model of loess in Western Sichuan affect the Yellow River in Western Sichuan Plateau The extraction of paleoclimate information of soil sediment makes it difficult to compare with loess paleosoil sediment,deep sea sediment and polar ice core records in the Loess Plateau,which affects the regional response of climate change and global climate change in the Western Sichuan Plateau in the Quaternary period.Therefore,it is necessary to use multi index joint analysis to explain the characteristics of loess sediment and provenance in Western Sichuan.This study area is located in Wenchuan Maoxian area of the upper reaches of Minjiang River in the Western Sichuan Plateau.We will explain the provenance of the loess sediment in Wenchuan Maoxian area and its relationship with the provenance of the loess paleosol in the Loess Plateau from the aspects of sediment size composition,mineral composition,stable constant and rare earth element composition.The results of grain size analysis show that the parameters of SC,ax,lbz and BW profiles in Wenchuan Maoxian area are all less than-2.7411,indicating that the loess is formed by wind.Compared with the typical loess in the Loess Plateau,it is found that the component content of>50?m particle size in the study area is far greater than that in the Loess Plateau,and the particle size of>50?m will not be transported in a long distance,so the Loess provenance is near source.Through the quantitative analysis of the mineral composition of the whole rock,it is concluded that the crystallinity index and Ksp/Pg value of the minerals are relatively low,which shows that the chemical weathering of the Loess in Wenchuan Maoxian area is relatively weak,which has a great relationship with the relatively dry and cold climate in this area.Therefore,the stable quartz,potash feldspar,plagioclase and mica can be used as the evidence to judge the provenance.The comparison of the relative content ratio of stable minerals in Wenchuan Maoxian area,typical loess in Loess Plateau and Qinling loess shows that their mineral characteristics are obviously different.The above characteristics show that the Loess in Wenchuan Maoxian area has no homology with the Loess in the Loess Plateau and Qinling Mountains.Based on the geochemical analysis of each section in Wenchuan Maoxian area,the major element oxides in each section are Si O₂,Al₂O₃,Fe₂O₃ and Cao,which corresponds to the characteristics of quartz and clay minerals in their mineral composition.Compared with UCC,the content of Si O₂,Al₂O₃ and UCC in each profile has no difference,but Na₂O and P₂O₅are all in deficit,while Ti O₂,Fe₂O₃,Mn O and Ca O are in enrichment.As an indicator of sediment provenance discrimination,there are significant differences in the stable constant element ratio?Si O₂/Ti O₂-Ti O₂/Al₂O3,Si O₂/Al₂O₃-Ti O₂/Al₂O₃?,rare earth?Eu La_N/Yb _N,Gd _N/Yb _N La _N/Sm _N discrimination results and Lingtai section of the Loess Plateau,indicating that the Loess sediments in Wenchuan Maoxian area and the aeolian loess paleosol sediments in Northwest China have no homology.To sum up,through the study of sedimentology,mineralogy and geochemical characteristics of loess,it is concluded that the provenance of the section SC,AX,LBZ and BW in Wenchuan Maoxian area is close to that of Aeolian Loess,and there is no inheritance relationship between Aeolian Loess and paleosol.It is speculated that the potential source area of loess in the study area may be the interior of the plateau,and the dust produced by the collapse of the mountain in the study area also provides material supply for the formation of loess in the area.However,its accurate provenance needs further study.