Transformation from Devonian Givetian Carbonate Platform to Famennian Bacteria-Algae Ecosystem in Litang Isolated Platform, Guangxi, and Its Significance

The carbonate platforms and the intraplatform troughs resulted from intracontinental rifting were simultaneously developed in Guangxi since the Emsian stage of Devonian. An isolated platform was found to occur at Liujing and Litang areas in central Guangxi since the Givetian stage. A fore-reef slope ecosystem characterized by tentaculites was identifiable in Mintang Fm. in Liujing during the Givetian stage. The occurrence of the reef ecosystem was inferred by the reef congregates in Mintang Fm. rather than the reef outcrop. The carbonate platform ecosystem in the Givetian Tangjiawan Fm. in Litang is composed of corals, stromatopoidea and brachiopoda. An ecosystem of a restricted and partly-open platform developed in Frasinan Gubi Fm., characterized by a small proportion of corals, brachiopoda and stromatopoidea. Frasnian Guilin Fm. in Litang is shown to be an ecosystem of a carbonate platform composed of corals, brachiopoda and stromatopoidea. An abrupt ecosystem change was observed to occur at the boundary between Frasnian and Famennian. Ecosystems of Rongxian Fm. in Liujing and Dongcun Fm. in Litang of Famennian are composed of algae reef and algae sheet. The ecosystem variation was proposed to result from the well known F-F faunal mass extinction. These ecosystem features are closely related to the forming and the evolution of the source rocks in this region.