Pattern And Evolution Of The Devonian-Middle Triassic Tectono-palaeogeography In Hunan And Guangxi Provinces

This paper is devoted to the basic geologic research on the marine oil and gas exploration in the regions of Hunan and Guangxi province. On the premise of the full research on the results of previous studies, using plenty of outcrop section as the main foundation data, based on the theory of sedimentology and tectonics, combining of field geologic survey, microscope section analysis, geochemical detection, and lithofacies palaeogeographic map, this paper does systematically sedimentology and tectonics study on the Devonian - Middle Triassic strata in Hunan and Guangxi Provinces, and defines the pattern and evolution of the Devonian-Middle Triassic Tectono-palaeogeography in Hunan and Guangxi Provinces. On this basis of dependent evidence, a variety of deposit types of a special platform-basin-dome-depression in the research area on the tectonic activity, relative sea level changes in the sedimentary filling response are detailedly profiled. Besides, the filling material feature of superposition and the reconstructed residual basin and its Tectono-palaeogeography significance are discussed in depth.Three major facies groups and 14 types of the sedimentary facies are recognized in the strata of Late Paleozoic to Early-Middle Triassic in Hunan and Guangxi province, including weathering residual, alluvial fan and fluvial in the continental sedimentary facies group; delta, tidal flats and lagoon in the sea-land transition facies group ; terrigenous clastic non-barrier shore, ramp carbonate platform, rimmed carbonate platform, isolated carbonate platform, clastic - carbonate mixed platform, basin between platforms, undercompensation basin and compensation turbidite basin in the marine sedimentary facies group. Evolution of the Devonian-Middle Triassic Tectono-palaeogeography in Hunan and Guangxi Provinces can be divided into two major stages, including Devonian - Early Permian extensional chasmic transgressive basin development stage and Late Permian - Middle Triassic Back-arc chasmic marine facies basin development stage. In the period of Late Devonian - Late Devonian, the regions of Hunan, Guangxi province took place extension rift. The palaeotopotraphy declined gradually from north to south, depression-platform White pattern has been formed and the area of isolated platform is larger. The Qinzhou-Fangcheng depression's continuous deposition was not interrupted after the Caledonian period. Shallow-water Carbonate platform during the Carboniferous period was extended, and the distribution of deep-water basin between platforms was restricted. The shoal developed, and the differentiation of the depression-platform was not obvious. The Early Permian took place transgression. The water flooded the whole region. As a result, the differentiation of the depression-platform was significant and the scope of isolated platform was shrinking. In the Late Permian, the Qinzhou-Fangcheng depression closure and the fold inversion caused by the Dongwu Movement. Uplift occurred in the Yunkai Area. The deep-water basin was contraction, and the differentiation of the depression-platform was significant. In the Triassic Period, carbonate platform transferred to carbonate ramp characterized by a large area oolitic shoal. This period was until the Triassic Period, when the independently depressions in the Yunnan,Guizhou and Guangxi regions merged into the Nanpanjiang depression characterized by the thick turbidite sedimentary basin and the isolated platforms were submerged .The regions of Hunan and Guangxi, which were in the composite position intersected by a variety of Tectonic system and experienced a long-term evolution of plate tectonics and the intracontinental orogeny process, formed a complex basin-mountain system and developed a unique platform-basin-dome-depression sedimentary system. Through the research on the sedimentary filling feature and the distribution of basin unconformity of the residual basin in the regions of Hunan and Guangxi, the filling response characteristics of tectonic movements and changes in relative sea level caused by different sediment types are discussed. Comprehensive study shows that, this area in the Devonian - Middle Triassic experienced 5 tectonic movements and 3 transgression events, which controlled 3 Coal-forming periods, 4 reef-forming stages, 3 creature mass extinction events, 2 period when the depression-platform's differentiation was obviously and 1 period when the differentiation was not significant.