THE GANZI LOESS ORIGIN IN THE WEST SICHUAN PLATEAU

The Ganzi Loess origin in the West Sichuan Plateau has been discussed based on synthetic study in grain composition of the loess,surface texture of quartz sand,as well as other geological phenomena.On the cumulative curve of grain size distribution in Ganzi Loess of logarithmically probability graph paper,saltation mass and suspension are obvious,and the later is principal.In the suspension,the grain size is not more than 2~3φ in diameter,and the silt grains (4~8φ) are principal.The grain size parameters of Ganzi Loess were counted by the matrix method.Its mean grain size is 5.057~5.883φ, and bigger than one of fossil soil in the loess.The 1.513~1.973 of standard deviation (σφ) of fossil soil is bigger than the loess (1.205~1.778). The skewnesses (Sk) of the loess and fossil soil are -0.088~0.248.The kurtosis (Kφ) of the loess is 1.242~1.702,and the one of the fossil soil is 0.023~0.059.The quartz sand surface textures of Ganzi Loess have been observed with Scanning Electron Microscope.Most of quartz sands are blocky,and a few of grains are fusiform.On the surfaces of the quartz sands,there are a number of aeolian features,such as concentrated upturned plates meandering ridges,pocks,and others.These ones are characteristics of aeolian origin.On the other hand,a few of the quartz sands are characterized by glacial surface textures,such as all sizes conchoidal fractures,and parallel striations.In the West Ganzi,and the South Dege,by the East bank of Jinsha River,three ice wedges are discovered in the loess strata on the top of third order terrace.Furthermore,the sporo pollens are scarce in the loess strata.As above,it appears that Ganzi Loess is aeolian sediment in the periglacial environment.