Radiolarian and diatom fossils recovered from diatomaceous sediments of the Yobito Formation in the northern region of Lake Notoro, eastern Hokkaido, enable the correlation of two stratigraphic sections, the West and East sections. Integrated radiolarian and diatom data suggest ages of 11.4-6.5 and 7.0-3.5 Ma for the West and East sections, respectively. This finding indicates that the diatomaceous sequence of the West Section is stratigraphically equivalent to the siliceous hard mudstone facies (Notoro Formation) and pyroclastic rocks (Abashiri Formation) that underlie the Yobito Formation of the East Section. This facies correlation between the Yobito and Notoro Formations probably reflects regional differences in the diagenetic history associated with silica phase transformations. Diatom data also reveal an unconformity within a sandstone layer in the diatomaceous sequence of the Yobito Formation of the West Section. This unconformity is represented by the absence of Neogene North Pacific Diatom zones (NPD) 5D and 6A. The occurrence in the interval above the unconformity of reworked radiolarian and diatom specimens of older (Miocene) age and gravel clasts of indurated mudstone suggest that the hinterland became exposed to an erosive environment at ~10 Ma.
The Hachiya Formation of the Kani Group in southern Gifu Prefecture is the oldest sediment in the eastern part of the Setouchi Geologic Province, and is composed mainly of volcaniclastic rocks deposited in a nonmarine basin. We present the results of new laser ablation-inductively coupled plasma-mass spectrometry U-Pb analyses of zircons separated from a sample of the Tochibora Welded Tuff Member in the lowest horizon of the Hachiya Formation. The analyses yielded a weighted mean 238U-206Pb age of 22.38±0.17 Ma (2σ), which constrains the time of initial sedimentation of the Hachiya Formation. The whole-rock major- and trace-element compositions of the welded tuff sample, analyzed by X-ray fluorescence, are also reported.
We conducted U-Pb dating of detrital zircons from three sandstones of the central section of the Akaishi Group, which is part of the Shimanto Belt in the Akaishi Mountains. The youngest single-grain ages are 113.6±2.3 Ma (1σ), 114.1±1.6 Ma (1σ), and 100.8±1.2 Ma (1σ) from west to east. The weighted mean ages of the youngest clusters are 115.4±1.4 Ma (1σ), 125.8±1.6 Ma (1σ), and 107.5±1.5 Ma (1σ). These results indicate that the central part of the Akaishi Group was deposited after the Aptian or Albian (Early Cretaceous). The zircon ages are slightly older than the radiolarian ages (Albian to Cenomanian), but the two sets of ages are consistent.