Formation Of Vermiculated Mottles In The Laterite At Guantangcun Of Xuancheng

The accumulation Quaternary red profiles widespread in south of the Yangtze River, in these profiles red matrix often developed with unique gray vermiculated mottles, although many scholars had a lot of attention about the vermiculated red clay, but about the vermiculated red clay formation, there are still lots of disputes, such as the formation mechanism and environmental conditions. Xuancheng, Guantangcun site is one of Paleolithic sites in Shuiyangjiang Basin, the profile of the site with thickness of14m, was exposed to brown-yellowsoil, homogeneous red soil, weak vermiculated layer and strong vermiculated layer, which constitute a typical laterite profile, the vermiculated layer consists of white vermiculated mottles and red matrix, showing regular changes in the profile, it’s an ideal location to study the genesis of the vermiculated mottles.The paper focuses on three issues:(1) Dating:Accoding to magnetostratigraphy and OSL geochronological investigations, the vermiculated red clay was deposited during131-1134ka, the top of the strong vermiculated layer’s deposition age is528ka, if the strong vermiculation developmental age corresponds to the northern loess S4, S5accumulation age, the vermiculated last formation age is about398ka. Apparently, vermiculated mottles development age was significantly later than the accumulation of vermiculated layer.(2) lithological characterisitics:a. Grain size:The average content of the red matrix clay component (<2μm) is about22.2%more than the mottles clay component, an average content of18.7%. On the whole, the average particle size of the red matrix is lower than mottels. the average particle size of the weak vermiculated layer is less than the strong vermiculated layer. b. Magnetic susceptibility:Be compared to Homogeneous laterite, the magnetic susceptibility values of vermiaculated layer that can be isolated mottles depth (～3m), reduce by70%, the matrix turn pale in12depth (～12m), magnetic susceptibility values reduce by90%. Magnetic susceptibility of the trend of the matrix is roughly opposite to the mottles, on the whole, the magnetic susceptibility of the mottles is very low, Showing that ferromagnetic minerals’s decrease in vermiculated layer, and a further decrease in the mottles. c. the elemental geochemistry:the CIA value of the vermiculated layer is higher than homogeneous red clay, the CIA values of the red matrix and mottels are similar, synchronous changes in the profile, and show an increase trend from top to bottom. The mottles develop under strong chemical weathering conditions. In addition to the obvious loss of iron, other geochemical characteristics between the mottles and matrix are similar, indicating mottles formed after iron removal effect.(3) Morphological character of the mottles:get a bulk sample in strong reticulate layer (depth～10m), reconstruct3D digital shape model. the model shows that the seemingly independent mottels, are connected in space. the extension of the mottles is mainly lateral, and up forked. these features with the morphological characteristics are different from the plant roots.Integrated the evidence of dating, lithological characteristics (geochemistry, grain size, magnetic susceptibility) and morphological character, the autor infers that in Guantangcun red earth profile, vertimuclated mottles formed significantly later than the eolian deposits, affected by underground water, and removed iron. While the mottles’s characteristics of pattern, strike and branch are different from the plant roots, then the mottles may evolution from the crack, generated by surface static pressure.