Abstract
Marine diatoms occur in the Miocene marine sequence of the Arakawa and Ichinokawa Formations distributed along the Tokigawa River in the southern part of the Hiki Hills, central Japan. Twenty-two samples yield age diagnostic diatom fossils, which indicates the sequence belongs to the lower Denticulopsis lauta Zone (NPD 4A). Combining with lithologic characters with diatom geochronology, the lithostratigraphic boundary between the underlying siltstone of the Arakawa Formation and overlying sandy siltstone with poorly sorted breccia of the Ichinokawa Formation is dated as 15.4 Ma. Fission track age of thick felsic tuff within the Arakawa Formation is also determined. Litho- and chrono-stratigraphies of the Hiki and Iwadono Hills are well correlated with those of the Miocene marine sequences in and around the Kanto Mountains.
