Abstract
Diatom biostratigraphy is first established for the Miocene marine sequence of the Tsuchishio Formation distributed along the Arakawa River, Aketo stratigraphic section, in the northern part of the Hiki Hills area, central Japan. Thirty-two samples yield age diagnostic diatom fossils, which indicate the sequence to be restricted between two diatom biohorizons, the acme/last common occurrence of Denticulopsis hustedtii (D 55.8 : 10.1 Ma) and the first occurrence of Denticulopsis dimorpha (D56 : 10.0 Ma) in the uppermost part of the Thalassiosira yabei Zone (NPD 5 C). The age-determination based on the high resolution diatom biostratigraphy allows precise correlation to the other several Miocene marine sequences in central Japan. The present correlation model, especially, shows that regression facies from the hemipelagic Tsuchishio Formation to the deltaic Yagii Formation delayed ca. 1.5 m.y. than the similar facies change from the Haraichi Formation to the Itahana Formation in the Tomioka area, Gunma Prefecture.
