Abstract
Marine diatoms occur in the Miocene marine sequence formally assigned to the lowest part of the Tsuchishio Formation distributed along the Arakawa River in the northern part of the Hiki Hills area, central Japan. Nineteen samples yield age diagnostic diatom fossils, which indicate the sequence belonging to the lower Denticulopsis lauta Zone (NPD 4A). The absence of both Denticulopsis praelauta and Cavitatus lanceolatus surely represents that the age of the investigated sequence is precisely placed between the last occurrence (LO) of D. praelauta (D41 : 15.7Ma) and the first occurrence (FO) of C. lanceolatus (D41.5 : 15.6Ma). This age-constraint requires the revision of the previously established stratigraphy of the extremely deformed Miocene series in this area, because the reported fission track ages are apparently inconsistent with the diatom chronology of this study.
