Revision of the age of the “Upper Miocene” Mitoku Formation in Misasa Town, south-central Tottori Prefecture, western Japan

The “Upper Miocene” Mitoku Formation in the south–central part of Tottori Prefecture, southwest Honshu, Japan, is a stratigraphic type of Mitoku-type flora, which is thought to represent the origins of the modern temperate forest vegetation on the islands of Japan. In this study, zircon U–Pb dating and palynostratigraphic analyses were undertaken to determine the age of the Mitoku Formation at the Naru site, north of the famous Mitoku-san historic site and place of scenic beauty, where Mitoku-type Misasa–Naru flora have been previously reported. The ages obtained for the pumice lapilli–tuff and tuff beds, which are intercalated with plant-fossil-bearing beds at the Naru site, are ca. 4.7 Ma (mid-Early Pliocene). The preliminary palynological results reveal that the flora comprise both cool and warm temperate elements, with the common and continuous occurrence of Cathaya pollen, which can be correlated with the NP-5 Neogene palynostratigraphic zonation (ca. 5.5–3.0 Ma). Warm elements in the Misasa–Naru flora were once considered to be remnants of mid-Miocene flora. However, our results show that the flora represent a warm temperate climate during the Early Pliocene, and might necessitate revision of the sequential changes in the Mitoku-type floras in this area, and even to the regional stratigraphy.