Abstract
Seven tuff samples, 5 from the Kibi Group (so-called mountain gravel) and 2 from the Katsuta Group were dated by using the fission-track technique. These samples were collected from the Kibi Plateau area in Okayama Prefecture. Fission-track ages of 34.3 ± 1.8 Ma (Yoshii Formation), 35.8 ± 1.3 Ma and 34.0 ± 2.7 Ma (Tomiyosi Formation), 27.4 ± 1.9 Ma and 27.1 ± 1.5 Ma (Tsudaka Formation) were obtained from the Kibi Group. Until now, the Kibi Group was considered to be deposited during the Quaternary period due to its unconsolidated nature. These dating results indicate that the Kibi Group is of Eocene to Oligocene in age. From the Katsuta Group, fission track ages of 17.9 ± 2.1 Ma (Yoshino Formation) and 16.2 ± 2.1 Ma (Takakura Formation) were obtained. These data agree with microfossil dating.
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