Stratigraphy of the late Cenozoic strata in the Akan area: with special emphasis on significance of the lower Miocene in the eastern Hokkaido, northern Japan

A revision of the Neogene lithostratigraphy of the Akan area of eastern Hokkaido is based on a reexamination of lithofacies and diatom biostratigraphy. The Neogene strata are divided into the Chichappugawa, Tonokita, Chichappu, and Kotan formations, in ascending order. The lower Miocene Chichappugawa Formation is newly defined and named in this study. The age of the Tonokita Formation is early middle Miocene, the Chichappu Formation is middle to late Miocene and the Kotan Formation is latest Miocene to Pliocene. The relationship between the Chichappugawa and Tonokita formations is inferred to be an unconformity, which suggests tectonic movement between the early and middle Miocene. The discovery of early Miocene strata in the Akan area indicates that the early Miocene sedimentary basin in eastern Hokkaido extended toward the northeast and east, and that it was connected to the Sea of Okhotsk.