Zircon U-Pb dating of a tuff layer from the Miocene Onnagawa Formation in Northern Japan

alternation rhythms have been interpreted as reflecting orbital-forced climate changes (Tada, 1991), timescales of the sedimentary cycles have not been precisely constrained. Knowledge of the mechanism controlling the sediment cycles of the Onnagawa Formation is critical to understand the evolution of the Japan Sea basin and its associated changes in the environment during the Middle-to-Late Miocene, when the global climate faced a significant cooling phase (e.g., Holbourn et al., 2013). Chronostratigraphy of the Onnagawa Formation has been developed by diatom biostratigraphy of diatomaceous sediment in the Oga Peninsula (Koizumi et al., 2009). However, many outcrops of the Onnagawa Formation have been suffered from silica diagenesis, which dissolved most diatom frustules composed of opalA and reprecipitated as opal-CT (e.g., Koizumi et al., 2009; Tada and Iijima, 1983). Consequently, preservation of diatom frustules became very poor and diatom biostratigraphy could be barely applicable. Although the Onnagawa Formation in the studied area is suffered from silica diagenesis, it is well exposed, and its continuous sequence can be obtained by splicing several sections. Zircon U-Pb dating of a tuff layer from the Miocene Onnagawa Formation in Northern Japan