Revision of bio- and carbon isotope stratigraphy across the Turonian/Coniacian boundary in the northwest Pacific: A potential auxiliary section of the Coniacian Stage in the North Pacific Province

The recognition of the Turonian/Coniacian boundary in the western North Pacific Province has remained ambiguous owing to the limited occurrence of age-diagnostic species and the low resolution of existing carbon isotope records. The Yezo Group, exposed in Hokkaido, northern Japan, represents a key Cretaceous marine succession in the northwestern Pacific. Here, we present a new high-resolution carbon isotope curve of bulk organic carbon (δ 13 C org ) across the Turonian–Coniacian boundary interval of the Yezo Group, based on exposures along the upper reaches of the Kotanbetsu River, in northwestern Hokkaido, and constrained by newly obtained macrofossil data. The resulting integrated stratigraphy revises previous stratigraphic correlations and allows precise identification of the stratigraphic position of the Turonian/Coniacian and lower/middle Coniacian boundaries, as well as several key carbon isotope events. The Kotanbetsu River section of the Yezo Group thus constitutes a suitable auxiliary reference section for defining the base of the Coniacian Stage in the North Pacific Province. The abundant tuff beds at or near the Turonian/Coniacian boundary further make the Kotanbetsu River section a promising succession for refining the numerical ages of the Turonian and Coniacian stages.