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Depositional Transformation from Banxi Period to Nanhua Glacial Period in Southeast Margin of Yangtze Block and Its Implications to Stratigraphic Correlation
详细信息   
摘要
The depositional-structural transformation and the regional correlation of relevant important strata units from Banxi period to Nanhua glacial period in southeast margin of Yangtze block have been a major focus of debate for a long time. Predecessors had been regarded that they wear conformity without obvious boundary surface between the Danzhou Group and Nanhua glacial deposition in North Guangxi and East Guizhou.However,the authors’ survey of some important sections in Guizhou-Guangxi neighboring region,such as Jinping Longli section,Liping Zhaoxing section,Congjiang lijiapo section,Sanjiang Silikou section and Luocheng Huangjin section,indicate that the transforming surface from Banxi period into Nanhua glacial period was clearly and sharply,for the upper part and down part of the surface were different completely in rock component,contexture,construction,and depositional facies types. The regional change of the transformation token on that the stratigraphic missing of the Banxi Group and its equivalences with a regressive progradational sequences become more and more obvious from the margin to the interior of Yangtze Block,and in reverse,the sedimentary sequences of the Nanhua glacial period with a transgressing process gradually become more and more integrated from the interior to the margin.We think that this was the depositional records of "Xuefeng Movement" brought forward by predecessor.Therefore,it was more probability and feasibility in the Hunan-Guizhou-Guangxi neighboring region of the southeast margin than the Gorges region in the interior of Yangtze Block to find a relatively integrated sedimentary sequence of Nanhua glacial period,and so the region should be the most favorable area to select the Global Standard Section and Point(GSSP) for "Nanhua System". At the same time,the available geo-chronological data suggest that the top boundary of Banxi period should be towards 720Ma,which is close to the onset age of Sturtian Glacial stage,and should not be limited at 780Ma subjectively.The most important is that the depositional sequence of Banxi period was a tact sedimentary cycle standing for the filling of mid-Neoproterozoic rift basin,and also there was not observable and identifiable hiatus in the sequence.so this sequence should not be dismembered or fragmented,otherwise we will be going against the principle of International Commission on Stratigraphy(ICS) for the classification of Precambrian strata.Therefore,at present,we consider that 720Ma is reasonable for the top boundary age of Banxi Group. And now,the first thing is to get more distinct and clear concept and implication of "Nanhua System".So it is necessary for its foundation to research and discuss deeply the tectonic settings of the Banxi Group and its equivalences and the depositional transformation from Banxi period to the Nanhua glacial period.

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