巧家盆地黄土的粒度特征及其成因分析

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

通过对巧家盆地地区金沙江左岸阶地上的厚层黄土进行实地考察，并进行了系统采样和粒度特征分析，结果表明该区黄土的粒度组成较为一致，以10～50μm的“风成基本粒径”粗粉砂质为主，其含量在26．09％-46．06％。同时〉63μm的砂质也较大，在剖面顶部达到了28．73％，随着地层向下呈现出粗粒径减小，粘粒含量增大的趋势。巧家黄土地粒度频率曲线呈双峰态特征，与长江以南的典型红土有着相似的分布特征。其粒度组成与粒度参数特征反映了风成近源堆积特性，通过将巧家黄土的粒度参数特征与洛川黄土和河流相沉积的相对比，显示其与典型的风成黄土具有相同的成因。

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 a- chieved 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.