Planation surfaces in the Three Yangtze Gorges area: Evidences from the Cretaceous to Quaternary deposits in the Yichang region, Hubei

There are three major depositional cycles recognized in the Cretaceous to Quaternary deposits in the Yichang region, Hubei. Each cycle is composed of the gravel beds in the lower part and silt and mud beds in the upper part. These depositional cycles are interpreted to be the responses to the three phases of tectonic evolution and three phases of planation surfaces in the Three Yangtze Gorges areas west of Yichang. Referenced to the previous data, organic fossils and age determinations for the Quaternary Shanxiyao Formation, the planation surfaces cited above were dated at the Late Cretaceous-Eocene for the Exian planation surfaces, the Pliocene-beginning of the early Pleistocene for the Shanyuanian planation surfaces, and the end of the early Pleistocene-the early stage of the middle Pleistocene for the Yunmengian planation surfaces, respectively.