Pattern And Evolution Of Paleozoic Tectono-palaeogeography In The Middle Yangtze Region

This paper is devoted to the basic geologic research on the marine oil and gas exploration in south China.In this paper, on the basis of the summary of previous research, using lots of outcrop section as the foundation data, based on the theory of sedimentology and tectonics, combining field geologic survey, microscope section analysis, geochemical detection, and lithofacies palaeogeographic map, pattern and evolution of tectono-palaeogeography is studied systematically. Besides, the two questions tectonic attribute of South China Basin in the Caledonian period and Xuefeng Orogen are discussed on basis of dependent evidence.3 facies groups and 15 kinds of sedimentary facies are recognized in the Palaeozoic strata in Middle Yangtze Region, including weathering residual facies, alluvial fan facies and fluvial facies in continental facies group; delta facies and estuarine facies in transitional facies group; clastic tidal flat facies, clastic non-barrier shore facies, ramp carbonate platform facies, rimmed carbonate platform facies, carbonate platform-basin facies, mixed platform facies, mixed continental shelf facies, offshore closed basin facies, undercompensation basin facies, turbidite basin facies in marine facies group. Evolution of Paleozoic tectono-palaeogeography is divided into three stages, including stable subsidence of intracontinental sea basin in Early Cambrian-Early Ordovician, compressional uplifting in Middle Ordovician-Silurian, transgressive basin after Caledonian orogeny in Neopaleozolc. The palaeotopography declined gradually form northwest to southeast in Early Cambrian-Early Ordovician, large scale rimmed carbonate platform sedimentary system hold leading position. The paleogeograhpical framework experienced great transformation in Middle Ordovician-Silurian. Qianzhong Uplift, Jiangnan-Xuefeng Uplift and Cathaysia uplifted successively. Blackshale of offshore closed basin facies of Wufeng formation in late Ordovician and clastic tidal flat deposition are typical facies during this stage, the Middle and Upper Yangtze Region uplifted in Middle-late Silurian. Transgression is major controlling mechanism of distribution of facies in Devonian-Permian, clastic offshore facies, clastic tidal flat facies changed each other with carbonate platform and carbonate shelf.In view of tectonic attribution of South China Basin in Caledonian stage, dependent evidence suggested that Yangtze Block and Cathaysia Block had matched in Caledonian stage; South China Basin is not ocean basin which segregated two blocks, but intracontinental sea basin. This evidence chiefly include two sides, firtly Caledonian granites in Hunan, Guangxi and Jiangxi assume planar distribution, and their element of geochemical detection suggested that they are compatible with the characteristics of crustal derived S-type granites, they are significantly different compared with granites forming in collision zone of oceancrust to continental crust. Secondly the sedimentary characteristics of Caledonian strata in Hunan, Guangxi, Jiangxi suggested that there is not discontinuity of facies on both sides of Jiangshan-Shaoxing Fault Zone, which is considered to be Caledonian suture zone of Yangtze Block and Cathaysia Block, but the interdigitation, continuity of facies or assuming the united paleogeographic unit.In view of tectonic attribution of Xuefeng ancient land, distribution of Caledonian, Indo-China and Yanshanian uncomformable surface suggested that intensity of orogenesis in Xuefeng area descended gradually form southeast to northwest, influence region spread gradually towards northwest from Caledonian to Indo-Chinese epoch. The sedimentary characteristics in Xuefeng area suggested that Xuefeng experienced complex vicissitudes of uplift-depression from Sinian to Cretaceous, it can be divided into five stages, including carbonate slope-shelf basin in Early Sinian-Middle Ordovician, uplifting and denuding in Late Ordovician-Late Carboniferous, carbonate platform in Permian, carbonate shelf in Early Triassic, uplift and intermountain basin in Middle Triassic-Cretaceous. Combining the above conclusion that there was not collision event of oceancrust to continental crust in Caledonian, we can draw a conclusion that Xuefeng ancient land which is marked by outcropped land of Proterozoic group is not an ancient land as a matter of fact, but a multipled, superimposed intracontinental orogen which experienced multiphasic intracontinental orogenic event.