Sedimentary environment and geomorphological significance of the gravel bed in Yichang

Abstract：The gravel bed in Yichang is more than 100m thick and composed of 22 layers. It lies over Cretaceous red bed and covered with Middle Pleistocene plinthitic red earth. The authors analyzed the sedimentary facies of the gravel bed. The upper and middle fan subfacies of alluvial fan exist in Shanxiyao and Yunchi profiles from the bottom upward. There is a hiatus between Lijiayuan profile and the above two profiles. The middle and lower fan subfacies exist in Lijiayuan profile from the bottom upward. Grain size analysis also shows that the outcropped sand body has the characteristics of braided river and alluvial fan, which is consistent with the result of sedimentary facies analysis. It is thus concluded that the very thick gravel bed in Yichang is a typical alluvial fan and seems to be mainly the upper part of the fan. Such wide development of the alluvial fan can only occur under the condition of strong hydrodynamic action. Through a study of the geomorphology and environmental significance of the formation of the fan, the authors preliminarily consider that the outcropped gravel bed in Yichang was produced by the formation of the Three Gorges. Data available also show that the gravel bed was formed in 1.08～0.73 Ma B.P., whereas the Three Gorges were formed before 1.0Ma B.P.