U–Pb and fission-track dating of Miocene hydrocarbon source rocks in the Akita Basin, Northeast Japan, and implications for the timing of paleoceanographic changes in the sea of Japan

Middle to Late Miocene organic-rich siliceous mudstones (Onnagawa Formation) in the Akita Basin, Northeast Japan have been an important target for both paleoceanographic studies and hydrocarbon exploration in Japan. However, the reliable age of their formation has remained poorly constrained. Here, we report new zircon U–Pb and fission-track ages of the Onnagawa Formation from a previously well-studied outcrop route in the Yashima area, central Akita Basin. The thin tuff bed in the lower Onnagawa Formation was dated at around 11.6 Ma, whereas thin tuff beds in the upper Onnagawa Formation was dated at 10.4–9.6 Ma. The new age model constrains the base of the succession as older than 15.6–13.8 Ma and the top of the succession as 8.7–8.2 Ma. The results suggest that the lowest part of the succession was deposited before the Onnagawa Stage. The new age model indicates a rapid deposition in the lower Onnagawa Stage. The new age model also clarifies a temporary decrease in the sedimentation rate during 10.9–9.4 Ma, which coincided with a hiatus or slow deposition reported from other areas along the Sea of Japan coast. The new age model also revises the timing of paleoceanographic changes, of the best hydrocarbon source horizon, and of hydrothermal activity responsible for seafloor chemoautotrophic communities in the Akita Basin. This revised timing reveals that the onset of paleoceanographic changes from oxidizing to anoxic bottom environments favorable for hydrocarbon source rock formation was closely related to the tectonic uplift of the Northeast Japan Arc at ~12 Ma, whereas the timing of hydrothermal activity was related to the following extensional tectonics at ~9 Ma. The results of this study thus shed light on hitherto unclarified relationships between tectonics, volcanism, and paleoceanographic changes in the Sea of Japan.