Fusuline biotic turnover across the Guadalupian-Lopingian (Middle-Upper Permian) boundary in mid-oceanic carbonate buildups: Biostratigraphy of accreted limestone in Japan

摘要

Two sections in Upper Middle to Lower Upper Permian shallow-water limestones at Kamura and Akasaka in southwest Japan were analyzed for detailed lithostratigraphy and biostratigraphy. Both sections represent ancient seamount-capping carbonate buildups developed on a basaltic basement in a mid-oceanic environment. The occurrence of abundant Tethyan fusulines allows the recognition of well-defined biostratigraphic zonation in both sections and their mutual correlation. The Upper Guadalupian (Middle Permian) Lepidolina/Yabeina Zone is overlain conformably by the Lower Lopingian (Upper Permian) Codonofusiella-Reichelina Zone with a 13 m-thick transitional interval barren of index taxa. The Guadalupian-Lopingian (G-L) boundary is marked by the First Appearance Datum (FAD) of the Lopingian Codonofusiella-Reichelina assemblage in both sections. This study recognizes for the first time the G-L boundary horizon in a mid-oceanic shallow-water environment. In addition, the shallow-water carbonates in the study sections record the extinction of the Middle Permian large-sized fusuline family Verbeekinidae at the G-L boundary in mid-Panthalassa, as well as in shallow-water Tethyan shelf areas, demonstrating positively that the G-L boundary mass extinction occurred on a global scale. The abrupt elimination of large-shelled fusulines, followed by the domination of small-shelled fusulines may indicate that environmental stress occurred at the end of Guadalupian. The dying-out of symbiotic algae may have caused the selective extinction of the large-shelled fusulines.