Grain-size distribution and palaeoenvironmental significance for the Neogene red clay in the middle reaches of the Huanghe River: An example from the Laogaochuan section in Fugu, Shaanxi

The Neogene red clay represented by the Laogaochuan section in the middle reaches of the Huanghe River exhibits a trimodal grain-size distribution, with dominant silt-sized particles (5 to 50 μm) and rare (particles) greater than 250 μm. The contents of particles less than 1 μm and 2 μm are relatively stable. The other grain sizes display a pattern of cyclic relief. The curves for the particles greater than 63 μm, 30 μm and median sizes are highly variable, implying that the Neogene climates were once cold and dry, and the winter monsoons were once highly variable in strength and prevailed over the study area. The stable variations in the curves for the particles less than 2 μm suggest that the summer monsoons were then very weak and less variable in strength. The observations of the grain-size curves for the Neogene red clay from the study area are generally supported by the litholigic and palaeontologic evidences.