ジルコンのU–Pb法と（U–Th）/He法から推定された北海道南西部，長万部地域（黒松内低地帯）の約1 Maのテフラ

抄録

Zircon double-dating (U–Pb and (U–Th)/He) was performed on a tephra in the Pleistocene Chiraigawa Formation, Oshamambe district (Kuromatsunai lowland), southwestern Hokkaido. The obtained ages were 1.035 ± 0.048 Ma and 1.030 ± 0.054 Ma, respectively, which are mutually consistent. These results indicate that the zircons from this tephra crystallized in a magma at ca. 1.0 Ma and an eruption producing the tephra happened instantly. The applied methodology appears to be robust for dating Quaternary volcanic activity, similarly to another zircon double-dating (U–Pb and fission-track) approach. Furthermore, U–Pb dating of a tephra in the Pleistocene Setana Formation yielded an age of 1.063 ± 0.037 Ma. These results indicate that the depositional age of the Setana Formation is in close agreement with previous estimates, but that the Chiraigawa Formation, which was previously assumed to be Middle Pleistocene, is Early Pleistocene. The ages also indicate that the Setana and Chiraigawa formations were deposited in a very short time interval.