K-Ar ages of four acidic tuff beds intercalated in the Cretaceous Yezo Group n the Tappu ara, northwest Hokkaido were determined. Biotite from the lower Coniacian in the Ua Member is dated at 89.7±2.0 Ma, one from the Lower Turonian horizon in the Mj-k Member at 92.8±2.0 Ma, one from the lowermost Turonian horizon in the Mj-k member at 98.0±2.1 Ma, and one from the uppermost Cenomanian horizon in the Mf-h Memver at 93.1±2.2 Ma. Considering relatively large error in K-Ar age, four ages bracketing the Cenomanian-Turonian boundary area nearly the same with a weighted average of 93.1±1.2 (1 σ) Ma. This result suggests 93 Ma for the Cenomanian-Turonian boundary. Although the age of Cenomanian-Turonian boundary has been placed at about 91 Ma, new data indicate that the boundary age should be revised.
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