Quaternary sediment in the Yichang area: Implications for the formation of the Three Gorges of the Yangtze River

Sediment data from the Yichang area in the Jianghan Basin of Hubei Province in China suggest deposition in a lacustrine environment prior to 0.75 Ma, B.P., followed by incision of the Yangtze River. The earliest Quaternary Yunchi Formation accumulated in an alluvial fan to fan-delta environment. The subsequent Shanxiyao Formation was deposited in an environment that changed from fan-delta to lacustrine. The distribution of sedimentary facies suggests the presence of a lake in the Yichang area prior to 0.75 Ma, B.P. The lack of sediments contemporaneous with the Yunchi and Shanxiyao Formations in other areas of the Jianghan Basin, suggests that this ancient lake was limited to the Yichang area. This lake predates the present Yangtze River in the Yichang area and the Jianghan basin. Provenance studies of gravels in the Yunchi and Shanxiyao Formations, as well as gravels in terraces and the channel of the Yangtze River indicate a variety of sediment sources, but suggest that no material from the area west of the Three Gorges had been carried into the Yichang area prior to 0.75 Ma, B.P. The Yangtze River cut through Three Gorges area only after 0.75 Ma, B.P.