Magnetic properties of the gravel layer in Yichang area and indication for the linking of the three gorges of Yangtze River

摘要

Yichang gravel layers distributed in Yichang-Yidu-Yaqueling-ZhiJiang area are the thickest Quaternary sediments at the edge of the Jianghan Plain hilly regions, which are of great significance to the research of stratigraphic division, environment evolution and formation of the Yangtze River. By analysis of the magnetic properties of Shanxiyao section (upper), Yunchi section (middle) and Lijiayuan section (lower), which represent the intact sedimentary sequence, we found that values of magnetic parameters are obvious difference between the Yunchi bottom 9m lower parts and the upper parts. 1) Magnetic parameters such as x, SIRM and xARM are correlated with the content of magnetic minerals, whose values are obviously higher in the upper gravel layers than the lower part, which indicates that the content of magnetic minerals in the upper gravel layers is higher than the lower part. 2) Magnetic characteristics at the upper gravel layers are decided more by ferrimagnetism minerals with low coercivity, while lower parts have more magnetic minerals with high coercivity such as hematite; 3) Ferrimagnetism minerals particles in the upper gravel layers are coarser than the lower parts. Preliminary analysis shows that the differences may be caused by source change. Based on that magnetic characteristics of the upper gravel layers are close to Jinsha River sediments and the lower part are close to Qingjiang sediments, we infer that the main gravel layers source below 9m in Yunchi profile lower part are mainly from the western Hubei mountains area while in the upper part materials from the West Sichuan Province inflow into Jianghan Plain. It indicates that the three gorges had already run through. Based on dating data, it is suggested that the period of the linking of the three gorges is about 1.0-1.12MaB.P..