Research On Sedimentary Conversion Properties Of Banxi Period And Nanhua Glaciation In Hunan-Guangxi-Guizhou Neighbouring Region

Banxi Period and Nanhua glaciation are the much significant geological periods of Neoproterozoic,and these two corresponding deposit units are widely spread in South China.But since the term “Nanhua System” proposed,there has been controversial about its bottom selection.Based on the former researches,this article by means of sedimentology and sequence stratigraphy,as well as petrology,chronology and tectonic,in order to ascertain the differences of sedimentary sequences,volcanical stages,chronostratigraphy and lithofacies paleogeography between the Banxi Period and Nanhua Glaciation.The evolution of the Neoproterozoic basin is divided into four stages: the basin rifting period,the initial transgression period,the maximum ingression period and the rapid depositing period.While the former periods of evolution were determined by super-mantle plume,the rapid depositing stage was influenced by extensional effects of thermal descending.To the Chang'an glaciation,the basin evolution was under the control of sea level changes and of the climate cycling.The synchronous large-scale bimodal volcanic provinces,as well as the episodic volcanisms which correlated with the basin evolution are demonstrates that the staged basin evolvement is the sedimentary response of relative mantle plume activity.The sediment model can be deduced as “Tillites-iron formation-cap carbonates” pattern,and each unit can be found on the main continents.Besides,the new LA-ICP-MS zircon U-Pb ages verified the onset time of Chang'an glaciation is no early than 716.8±6.8 Ma,which is consists with the international cognition.Chang'an glaciation(the earliest episode of Nanhua glaciation)deposits the oldest stratum unit of the Nanhua Group,ie.the Chang'an Formation.The sedimentary differences between the Chang'an Formation and the underlying Gongdong Formation reflected in lithological characters,structures,and sedimentary facies.More over,the discrepancy of zircon U-Pb age groups of these two formations proves that the sediment conversion between Banxi Period and Nanhua Glaciation is exitence.In addition,the detrital zircon 238Pb/206 U ages show that the Chang'an Formation would have been deposited is no early than 726 Ma,and the Gongdong Formation would deposit at ca.715 Ma.Therefore,we conclude that the time of sedimentary conversion from Banxi Period to Nanhua Glaciation at ca.726Ma?715Ma.