K–Ar age and petrography of Tomaruyama Lava and Isoya Lava, Southwest Hokkaido, Japan: Identification of Quaternary volcano in the vicinity of Suttsu and Kamoenai districts

Abstract

Since 2020, a four-year study of the potential geological disposal of nuclear waste has been conducted for the Kamoenai and Suttsu areas of Hokkaido. In the determination of candidate sites for nuclear waste disposal, Quaternary volcanoes must be at least 15 km from such repositories. Potassium–Ar ages of late Cenozoic volcanic rocks in the Kamoenai and Suttsu region were determined to evaluate potential sites for the geological disposal of nuclear waste. We obtained ages of 4.96 ± 0.15 Ma for the Tomaruyama Lava (early Pliocene) and 2.70 ± 0.60 Ma (late Pliocene to early Pleistocene) for the Isoya Lava. This suggests the Suttsu area is an unsuitable site for a nuclear waste repository. The Isoya Lava consists of andesite to dacite with icelandite-like characteristics, high FeO*/MgO ratios, similar to those of the nearby early Pleistocene Raidenmisaki Volcanic Breccia Formation.