"巫山黄土"粒度特征及其对成因的指示

为了探讨"巫山黄土"的成因, 对剖面样品进行了粒度分析.结果表明: "巫山黄土"主要由 < 50 μm的颗粒组成, ＞50 μm的砂级以上颗粒平均含量为16.9%;作为风尘"基本粒组"的10~50 μm的粗粉砂平均含量达43.1%, 为众数粒组; 5~10 μm的细粉砂颗粒含量平均为14.5%;＜5 μm的粘土颗粒含量平均为25.5%, 为次众数粒组.粒度参数、频率曲线与典型风尘沉积物相似, 显示为典型的风积成因; 粒度参数的C-M、L-M、A-M图等粒度象分析及结构参数散点图与现代河流沉积物差别明显, 而与长江中下游分布的下蜀土却基本一致.判别分析结果绝大部分为负值, 显示了风积成因为主、个别层位受到水流作用改造的成因特征. 

Grain Size Characteristics and Origin of the "Wushan Loess" at Wushan Area

Based on the grain size analyses of "Wushan Loess" at Wushan area, the causes of "Wushan Loess" are discussed in this paper. The results show as follows. The component particles of the "Wushan Loess" are predominantly less than 50 μm with the average content of > 50 μm about 16.9%; the average content of coarse silt (10-50 μm) is 43.1% as the "dominant group" of the "Wushan Loess"; the average content of 5-10 μm group is 14.5% and < 5 μm group makes up 25.5%. The characteristics of grain size parameters and frequency and probability cumulative distribution curves are the same as those of typical atmospheric dust sediment, which shows that the "Wushan Loess" is of aeolian origin. C-M, L-M, A-M chart and statistical indices of grain size distribution are obviously different from those of fluvial sediment and are the same as those of Xiashu Loess distributed in the middle and lower reaches of the Yangtze River. The most discriminant analysis result is of negative value, which displays that the aeolian origin is prevailing and individual layer is affected by water flow action.