Magnetostratigraphy And Origin Of The Red Earth Formation In Jiujiang City Of Jiangxi Province China

In this study, magnetostratigraphy, grain size, magnetic susceptibility and geochemistry were analyzed for the samples taken from the Jiujiang city of Jiangxi province and the grain size and geochemistry results were compared with the typical eolian deposit , as well as the lacustrine and fluvial sediments.Magnetostratigraphic study indicates that the B/M boundary is found to be located within the middle-lower part of the vermiculated red soil(VRS) at a depth of 15.5m and the Jaramillo normal polarity subzone of the Matuyama negative polarity zone doesn't appear in this section. The results show that the Jiijiang section deposited from later period of early Pleistocene.The grain-size analytical results reveal that the Jiujiang Xiashu loess and VRS sample above 20m consist of fine uniform particles. The distributions are bimodal, with the dominant grain-size fraction of 5～50μm, and the >63μm fraction occupies only a small amount. These are essentially similar to the grain-size distributions of the typical Quaternary aeolian loess-paleosol deposit and the Chuanxi loess, but differ from the multimodal patterns of the lacustrine and fluvial sediments, while the distributions of the Jiujiang VRS below 20m are multimodal, which similar to the fluvial sediments. The grain-size parameters for the Jiujiang Xiashu loess and VRS above 20m are also similar to those of Quaternary loess-paleosol and the Chuanxi loess samples. The main grain size of the Jiujiang Xiashu loess and VRS are about 6.4-7.0φand 7.1-7.3φrespectively, close to those of the Quaternary loess-paleosol and the Chuanxi loess samples. The mean square deviation values of the Jiujiang Xiashu loess and VRS samples above 20m and the Quaternary loess-paleosol samples are similar, which are smaller than the Jiujiang VRS samples below 20m and the lacustrine and fluvial sediments, indicating a much better sorting and relatively stable depositional dynamics, characteristic of eolian dust deposits. Both the Jiujiang Xiashu loess and VRS samples above 20m are positively skewed, which are similar to the Quaternary loess-paleosol and the Chuanxi loess samples, however the skewness values of the VRS samples below 20m are smaller than the Quaternary loess-paleosol and some of them are negatively skewed. The kurtosis values of the Jiujiang Xiashu loess and VRS samples above 20m are uniform and similar to the Quaternary loess-paleosol and the Chuanxi loess samples, however the kurtosis values of the VRS samples below 20m vary in a wide range.The results above show that the parent material of the Jiujiang Xiashu loess and VRS samples above 20m are aeolian origin and the VRS samples below 20m are different from the Quaternary loess-paleosol samples.The Jiujiang Xiashu loess and VRS underwent much stronger chemical weathering than the Quaternary loess, and it finished the primary process of chemical weathering characterized by leaching of Ca and Na, and almost reached the secondary process characterized by leaching of K. The chemical weathering intensity of the Jiujiang Xiashu loess, VRS and the Quaternary loess-paleosol rank in the following order: the Jiujiang VRS>the Jiujiang Xiashu loess>the Quaternary paleosol >the Quaternary loess, it shows that the paleoclimate in southchina was warmer and wetter than northchina and it became drier and cooler during the Quaternary. The REE distribution patterns of the Jiujiang Xiashu loess , VRS, Quaternary loess and Chuanxi loess samples also show extreme homogeneity, suggesting the same origin of these deposits.