A New GSSP Candidate for the Induan-Olenekian Boundary (early Trias-Sic) from the Waili Area, Nanpanjiang Basin, South China

Despite decades of effort to define the Global Stratotype Section and Point (GSSP) for the Induan–Olenekian boundary (IOB), it has not yet been formally ratified, mainly due to controversies on the selection of appropriate markers. In this talk, we review the historical progress of the IOB GSSP candidates and present new data from the Waili area in the Nanpanjiang Basin, South China. We argue that the Waili area represents an ideal candidate for the IOB GSSP, based on its well-preserved conodont and ammonoid successions, and ash layers that provide high-precision U-Pb ages to calibrate the boundary horizon. Four conodont zones are identified across the IOB interval. Four ammonoid zones are also recognized, including the late Induan Clypites typus zone, and the early Olenekian Guangxiceltites zone, Kashmirites kapila zone and Flemingites rusiradiatus zone. Due to the complex taxonomic identification of Novispathoduswaageni, classicaly used as the primary index for defining the IOB, we suggest using instead the first occurrence (FO) of Eurygnathoduscostatus, which is widely distributed and shorter lived than Nv. waageni. Additionally, it corresponds to the Guangxiceltites ammonoid zone. High-precision CA-ID-TIMS U-Pb dating of a ash bed, located immediately above the Kashmirites kapila beds and ~0.2 m above the FO of Eu. costatus, yields an absolute age of 250.703 ± 0.048 Ma. The carbon isotope profile can also serve as a valuable correlation tool for sections lacking age diagnostic fossils. At Waili, the positive carbon isotope excursion (P2) known to occur around the IOB is documented within the Kashmirites kapila zone. The ammonoid, conodont, and carbon isotope records can be well correlated with those from other GSSP candidates: the Chaohu (South China) and Mud (Spiti, India) sections. To sum up present data, the Waili area integrates all biostratigraphic and geochemical markers of both Mud and Chaohu sections, and additionally provides high precision U-Pb age constraints. Despite ongoing debates on the selection of boundary markers for the GSSP, the Waili area, by integrating multiple stratigraphic datasets in a single locality, therefore represents a highly suitable candidate for the IOB GSSP.