延安晚更新世黄土颗粒微观各向异性特征及其成因分析

Microscopic anisotropy and genesis of Late Pleistocene loess particles in Yan’an

土颗粒的形状及其空间方位的择优取向是导致宏观土体各向异性的重要原因之一。以具有明显各向异性的晚更新世马兰黄土为研究对象,借助Quanta FEG型电子显微镜扫描照片所包含的相关信息,揭示其微结构在水平向和垂直向上的差异,并对该微观结构的各向异性成因进行了分析。发现:(1)水平向和垂直向切片中颗粒的圆形度R_0具有正态分布特征,无论垂直向还是水平向土壤颗粒均以次圆形、圆形和非常圆颗粒为主。(2)水平方向切片中颗粒的方位角具有一定的WN-ES的优势方向,而垂直向切片中土颗粒的长轴有与地面平行的优势取向,且埋深越深这种趋势越明显。(3)水平切片中土颗粒的各向同性性质更为显著,而垂直向切片中颗粒的各向异性性质更为突出,且随埋深的增加,各向异性增强。(4)水平向和垂直向切片中颗粒总的概率分布基本表现为随机分布,粉粒在数量上占绝对优势是导致这种无序分布的重要原因,相对于粉粒而言细砂则可在一定程度上体现其排列具有方向性。分析表明,颗粒的方位角和各向异性率对黄土微观各向异性具有较好的指示作用。同时,颗粒的方向性对黄土高原的古气候也具有重要研究意义。

The shape and dominant spatial orientation of soil particles are the important cause of macroscopic soil anisotropy.Based on Quanta FEG scanning electron micrograph,the late Pleistocene Malan loess with obvious anisotropy was studied to reveal the difference of the microstructure in the horizontal and vertical directions and the anisotropic cause of the microstructure.The results show that the roundness R0 of the particles in the horizontal and vertical slices has normal distribution characteristics.Subcircular,circular and very circular particles are the main types of soil particles in both vertical and horizontal directions.The azimuth of the particles in the horizontal slice has a certain dominant direction of WN-ES,while the long axis of the soil particles in the vertical slice has the dominant orientation parallel to the ground,and the deeper the buried depth is,the more obvious the trend is.The isotropic nature of the soil particles in the horizontal section is more significant,while the anisotropic properties of the particles in the vertical section are more prominent,and the anisotropy increases with the buried depth.The total probability distribution of particles in horizontal and vertical slices is basically disordered.The quantitative advantage of the particles is a major reason for this disordered distribution.Compared with the fine particles,fine sand,to some extent,has the preference of a certain orientation in alignment.The analysis shows that the azimuth and anisotropy of the particles have a good indication to microscopic anisotropy of the loess.At the same time,the direction of particles also is of significance for the study of paleoclimate of the Loess Plateau.