Hiroyuki Hoshi

Early Miocene sediments of the Morozaki Group in central Japan contain deep‐sea fossils that have been dated using biostratigraphic and radiometric data. In this study, we utilize magnetostratigraphy to provide a more precise age for mudstones from just below the layer containing the fossils. Rock magnetic experiments suggest that both magnetic iron sulfide and Ti‐poor titanomagnetite carry the remanent magnetization of the mudstones. Two different stratigraphic sites have normal polarity directions with a northeastern declination, which can be correlated with Chronozone C5Dn. Given their magnetostratigraphic position near the C5Dn/C5Dr chronozone boundary (17.466 Ma) and a high sedimentation rate, the estimated age for both the sites and the deep‐sea fossils is ~17.4 Ma. The northeasterly‐directed site‐mean directions suggest clockwise tectonic rotation, most likely due to the Early Miocene clockwise rotation of Southwest Japan associated with the back‐arc opening of the Japan Sea. The deep‐sea fossils, dated at ~17.4 Ma, represent organisms deposited within a submarine structural depression formed by crustal extension during the back‐arc opening stage.