CONODONT BIOSTRATIGRAPHY OF REGIONAL YANGSHUOAN STAGE(UPPER FAMENNIAN,DEVONIAN)IN CHINA
The regional Yangshuoan Stage,named after the Yangshuo County in Guilin,Guangxi,represents the penultimate Devonian stage in China and is approximately equivalent to the upper Famennian.Its base is defined by the first occurrence datum(FOD)of the index conodont Palmatolepis rugosa trachytera and its top by the FOD of the foraminifera Eoendothyra regularis.The stratotype section of the Yangshuoan Stage is near Tieshan in Guilin City,and it comprises-236 m of shallow-water intraclastic,oolitic wackestone or grainstones with minor intercalations of black,organic-rich calcareous mudstones that accumulated on an isolated carbonate platform.A total of 149 conodont samples were collected at intervals between 0.25 m and 18 m within the"Wuzhishan"and Rongxian(=Yonghsien)formations at Tieshan and adjacent Xiaolongmen sections,among which 94 samples yielded conodonts and 55 were barren.Amongst the conodonts,reworking processes cannot be ruled out finally;however,there are no differences in the way they are obtained.Representatives of the genera Palmatolepis,Neopolygnathus,Polygnathus,Pseudopolygnathus,Bispathodus,Icriodus,Rhodalepis,Mehlina,and Branmehla have been collected from these successions and the faunas enables the establishment of six conodont biozones,including the undifferentiated and regionally expanded Pa.marginifera marginifera,Pa.rugosa trachytera,Pa.perlobata postera,and Pa.gracilis expansa zones as well as the Bi.costatus and Bi.ultimus ultimus zones in ascending order,providing a high-resolution correlation with other equivalent sections in the peri-Palaeotethys region.Based on a detailed analysis of conodont biostratigraphy,two intervals-involving sudden intercalations of black,organic-rich rocks within very shallow-water shoal carbonate sediments-were recognized in the upper part of the regionally expanded Pa.rugosa trachytera Zone and at the top of the likewise regionally expanded Pa.gracilis expansa Zone,respectively.These two major lithological changes represent sedimentary responses to sea-level rises and marine dysoxic events,probably paired with unsteady eutrophic conditions in the South China marginal sea and can be correlated with the global Annulata Events and Dasberg Crisis.
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