ANALYSIS ON LOESS'S GRAINSIZE FEATURES AND CAUSE OF FORMATION IN QIAOJIA BASIN

There has been heavy film loess in the left bank step ground of Jinsha River in Qiaojia Basin.We have reviewed the very loess on the spot,sampled the system and analyzed the grainsize feature.The result shows the consistency of grainsize of it,with the main material coarse silt.The diameter of the coarse silt is within 10～50 μm,with content between 26.09%～46.06%.Simultaneously the 63 μm sand content is also big,has achieved 28.73% on the top of the section,with the tendency of reducing in coarse particle and increasing in clay content downward along with the stratum.What's more,the Qiaojia loess granularity frequency curve assumes the double kurtosis characteristic,has the similar distributed characteristic with Yangtze River's south of typical red soil.The average diameter of the Qiaojia loess is 37.185 μm.And the standard deviation indicates 2.38;while the asymmetry coefficient was 1.49;the kurtosis points to 2.95.And its size composition and the granularity parameter characteristic have both reflected the characteristic of near-source and piles-up.The paper discusses the similar causes in Qiaojia loess and the typical Aeolian loess,through comparing the grainsize parameter feature in Qiaojia Basin and that in fluvial sediment,and in typical loess.