Discovery of Early Paleocene (Danian) tuff from the Yezo Group in the Yubari area, Hokkaido, Northeast Japan

抄録

The Hakobuchi Formation in Hokkaido, Northeast Japan, consists of marine and terrestrial clastic rocks of the uppermost part of the Yezo Group. The formation was previously interpreted to range from the early Campanian to the late Paleocene, with a hiatus near the K/Pg boundary. To date, no definitive evidence for the presence of the Lower Paleocene (Danian) has been identified in the formation. In this study, we present the LA-ICPMS zircon U-Pb dates of a tuff layer in the Hakobuchi Formation found in the Yubari area. Although the tuff contains Late Cretaceous to Paleocene zircon grains with ²⁰⁶Pb/²³⁸U concordant dates of 80.8-61.7 Ma, the weighted average age of the youngest date cluster was 64.1±1.1 Ma (2σ), suggesting that the depositional age of the tuff falls within the Danian. The results provide the first radiometric evidence indicating that the Hakobuchi Formation is at least partly correlated with the Danian, and are essential for discussing the depositional age of the Hakobuchi Formation and the geochronology of the Yezo Group.