研究業績

We established a high-resolution chronostratigraphy in order to reconstruct the depositional paleoenvironment of the Miocene Onnagawa Formation in Akita Prefecture, NE Japan, based on a laser ablation-ICP-mass spectrometry (LA–ICP–MS) zircon U–Pb dating of two tuff beds that occur within the formation in the Yashima Area. The lower and middle parts of the Onnagawa Formation yielded ages of 12.7±0.2 Ma and 10.8±0.3 Ma (2σ), respectively. Accordingly, the stratigraphic interval that was previously defined as the Onnagawa Formation in this area was re-classified into the Sugota and Onnagawa formations in ascending stratigraphic order. The boundary between these formations has been dated to 14.2–13.4 Ma. The updated chronology indicates a high sedimentation rate at 69–378 m/m.y. during the period 11.5–10.8 Ma.