Stratigraphy and geochronology of the Miocene marine sequence in the Ota area, Gunma Prefecture, central Japan

Abstract

Stratigraphy of the Miocene marine sequence in the Ota area, Gunma Prefecture, central Japan is established. Based on the lithological defference, as well as on the stratigraphic relationship, the volcano-sedimentary sequence in the Ota area is divided into lower non-marine volcaniclastics of the Yabuzuka Formation and overlying marine sediments of the Midorimachi Formation (newly defined). Pumice flow deposit unconformably covers the Midorimachi Formation is suggested to be correlated with the lowest Middle Miocene felsic products such as the Oya Formation, widely distributed along the eastern margin of the Ashio Mountains. Stratigraphic correlation of the volcano-sedimentary successions around the Ashio and Yamizo Mountains suggests that the Early to Middle Miocene felsic products are divided into two stages, that is before and after the Niwaya Unconformity at about 15 Ma. Twelve samples of the Midorimachi Formation yield age diagnostic diatom fossils, which indicate that the sequence falls between the diatom biohorizons of D33 (16.7 Ma) and D35 (16.4 Ma) of the upper Crucidenticula kanayae Zone (NPD 3A).