ABSTRACT

During the Middle-to-Late Miocene, diatomaceous sediments were deposited in the North Pacific margin and marginal basins. The Onnagawa Formation is one of such deposits, which shows cyclic sedimentary rhythms reflecting oscillations of the marine environment in the Japan Sea. However, the age of the Onnagawa Formation is still poorly constrained due to the poor preservation of siliceous microfossils. To better constrain its age, we performed U-Pb dating of zircon grains from a tuff layer from the middle part of the Onnagawa Formation, and obtained an age of 11.18 ± 0.37 Ma. By combining our data with previously reported radiolarian biostratigraphy from the same section, we improved the age model of the diagenetically altered and lithified sediments of the Onnagawa Formation.