2020 Volume 71 Issue 3 Pages 215-233
Diatom biostratigraphic analysis and fission-track (FT) dating were carried out for the Upper Miocene to Pliocene successions exposed along the Tsubokawa River (Tsubokawa Section), in Shichinohe Town, Aomori Prefecture, Northeast Japan. The Upper Miocene to Pliocene are divided into the Wadagawa, Kotsubogawa and Ichinowatari formations in ascending order. The Wadagawa Formation in the Tsubokawa Section was correlated to the diatom zones NPD5C and NPD6A. The FT age of a pumice lapilli tuff layer of the Ichinowatari Fomation was dated at 4.1 ± 0.4 Ma (1σ). The age of each formation estimated from stratigraphic relations, biostratigraphy and FT age is as follows; Wadagawa Fomation: 12–8 Ma, Kotsubogawa Formation: 10–8 Ma, Ichinowatari Formation: about 4 Ma. Andesitic volcanic rocks (10–7 Ma) correlated to the Kotsubogawa Formation are widely distributed in the northernmost part of the Ou Backbone Range and the Osoreyama Mountains. This shows that vigorous andesitic submarine volcanism simultaneously occurred in these wide areas in 10–7 Ma.