U-Pb and fission-track ages from the Hokutan Group, northern Hyogo Prefecture, SW Japan

Although the Hokutan Group is considered to provide evidence for the Miocene opening of the Japan Sea, the ages of the formations within the group are poorly constrained. We dated eight tuff beds from the Hokutan Group in the Tajima-Myokensan and Sobudake areas, where the type localities of the formations are located, using U-Pb and fission-track (FT) methods. Uranium contents were measured using laser ablation-inductively coupled plasma-mass spectrometry. A tuff bed in the upper Takayagani Formation and two in the basal part of the Yoka Formation yielded a mean U-Pb age of ~21.5 Ma, which is within the range of FT ages (26-19 Ma) of the samples. Accordingly, the boundary between these two formations is dated at ~21 Ma. U-Pb ages of ca. 16-15 Ma and FT ages of ca. 16-11 Ma were obtained from a tuff in the Toyooka Formation and from four tuffs in the Muraoka Formation, consistent with previously reported fossil ages. Considering the radiometric and fossil ages, the depositional age of the upper Toyooka Formation is taken as 17.0-16.5 Ma and that of the lower Muraoka Formation is taken as 16.5 Ma. The FT ages imply that the middle-upper Muraoka Formation is younger than 15-14 Ma.