Abstract
A whole rock K-Ar dating is made for the Penkenai Lava at eastern Utanobori area, northern Hokkaido. This analysis gives the age of 5.2±0.3 Ma, older than the age assignment of Late Pliocene by the previous workers. The present result also gives the upper limit of the Penke Formation, which is the uppermost formation of the Neogene marine sequence in this area. This formation is directly overlain by the Penkenai Lava and has been regarded as the Upper Pliocene, but now it is assigned to the Upper Miocene.
