| Based on the regional geological evolution and the measured profiles, three tectonic sequences can be recognized from Nanhua System(Nh, Cryogenian) to Cretaceous in Western Hu’nan. The Caledonian tectonic sequence (Nh-S) reflects the evolution of South China Ocean on the southeastern margin of Yangtze Plate including its expansion, contraction and closure, which formed a complete Wilson cycle. This tectonic sequence on the nearly passive continental margin can be subdivided into 4 stages such as the clastic stage in continental extension, the clastic stage in early passive continental margin, the carbonate platform stage in mature passive continental margin, and the clastic stage in foreland basin. The Hercynian-Indosinian tectonic sequence(D-J2) reflects the evolution of Mianlue Ocean on the northern margin of Yangtze Plate, including its expansion, closure and dying processes, which formed another nearly complete Wilson cycle. This tectonic sequence on the far passive continental margin can be subdivided into the littoral clastic stage, the epicontinental carbonate platform-basin stage, the epicontinental carbonate tidal flat stage, and the clastic stage in foreland basin. The Yanshanian tectonic sequence(J3-K) reflects the formation, development, and demise processes of fault-depression basin in the continent, which formed an incomplete Wilson cycle. This continental tectonic sequence can be subdivided into the clastic stage in intracontinental extension and the clastic stage in intracontinental closure. In the Caledonian tectonic sequence, the clastic rocks(Upper Sinian Doushantuo Fm. and Lower Cambrian Niutitang Fm.) in passive continental margin expansion period, and the clastic rocks(Lower Silurian Longmaxi Fm.) in the initial stage of foreland basin are the favorable strata for shale gas. |
