Abstract
Upper Miocene tephra beds considered likely to represent a widespread tephra were found in the Tsugawa (Aga Town), Nanatani (Kamo City), and Sumon (Uonuma City) areas of central Niigata Prefecture, northeast Japan. The Otbt Tephra Bed in the Nomura Formation at Tsugawa, the Dtbt Tephra Bed in the Minamiimogawa Formation at Kamo, and the Isbt Tephra Bed in the Tsunagi Formation at Sumon are correlated with one another based on petrographic characteristics, the chemical composition of volcanic glass shards, and diatom biostratigraphy. These tephra beds are characterized by extremely low MgO (<0.01 wt.%) and CaO (<0.40 wt.%) contents in volcanic glass shards. The three tephra beds are assigned to the lowest part of the Neogene North Pacific diatom zone NPD 5D, between diatom biohorizons D56 (10.0 Ma) and D57 (9.4 Ma), with an estimated age of 9.9 Ma.
