Characters of grain-size and susceptibility of the Neogene Laogaochuan red clay section, Shaanxi, and its paleoenvironment significance

Abstract：Sample analyses show that the Neogene red clay at the Laogaochuan section exhibits a trimodal grain-size distribution, suggesting that the red clay is composed of components of multiple origins and that components of different origins make different contributions to the total. The silt-sized grains (5-50μm) are predominant, of which the content of grains of 10-50μm in size is higher than that of grains of 5-10μm and grains >250μm in size are rare. The mean, >63μm and >30μm grain-size curves are highly variable, while the <2μm grain-size curve is relatively stable. The features indicate that the Neogene climate was mainly dry and cold and the winter monsoons had once changed greatly in strength and prevailed over the study area and that the summer monsoons were relatively weaker with little change. The changes of the grain-size curves are supported by the lithologic and biologic evidence, suggesting that the climate then was mainly dry-cold and sometimes alternately warm-wet and cool-dry. The lithology in the field is manifested by alternation of clay beds with calcic concretionary beds and the grain-size curves are manifested by fluctuations of peaks and valleys. The susceptibility curve shows that the climate of the Yushean Stage was warmer and wetter than that of the Baodean Stage.