A Middle-Upper Devonian Boundary Section in the Open Platform,Platform Margin Facies of Guilin,South China

The Caiziyan Middle and Upper Devonian boundary section is located approximately 30 km northeast of Guilin.It hosts relatively abundant benthic and common-rare pelagic fossils,including brachiopods,corals,tentaculites,and conodonts,which may serve as a better suitable section for pelagic and neritic stratigraphic correlation.In this section.10"standard" conodont zones are recognized across the Givetian-Frasnian boundary,including,in descending order,the Lower hassi Zone,punctata Zone,transitans Zone,the Upper falsiovalis Zone,the Lower falsiovalis Zone,disparilis Zone,the Upper hermanni-cristatus Zone,the Lower hermanni-cristatus Zone,the Upper varcus Zone,and the Middle varcus Zone,all of which are defined by the first occurrence of their defining conodont species.The Middle-Upper Devonian (Givetian-Frasnian)boundary is defined by the first occurrence of Ancyrodella pristina in accordance with the GIobal Stratotype Section and Point(GSSP),which is assigned at 6.2m above the base of bed 19 in the Caiziyan section.