Permalink : http://hdl.handle.net/2115/36758

A re-investigation on the pre-Tertiary System that lies within the Kamikawa Basin, central Hokkaido, was carried out from a radiolarian biostratigraphic point of view. As a result, Tosshozan Formation and the Toma Formation, which have been previously included in the Sorachi Group and regarded as Triassic and Permian-Triassic sequences respectively, are now seen to consist of olistostromes of the Early Cretaceous age (Berriasian ?-Valanginian in the former formation and Barremian to Aptian in the latter formation). The Toma Formation in particular includes many exotic blocks of various geological ages: middle Permian and late Triassic limestones, late Triassic chert, late Jurassic chert and limestones, siliceous shales or acidic tuffs of Early Cretaceous, and sandstones and greenstones. The matrix of this formation consists of pebbly shale which is strongly sheared. A similar oliststrome belt of early Cretaceous age extends from the western flank of the southern Hidaka Belt to Sakhalin and seems to represent a convergent zone along the Eurasian plate margin during the early Cretaceous time. Additionally, we have found that the Takasu and the Kaimei Formations in the studied region belong to the Middle Yezo Supergroup, while the Hidaka Supergroup is here represented by the Aibetsu Formation belonging to the late Cretaceous sequence. Consequently, we proposed a revision of stratigraphy of the Pippu-Toma regions.