西南北海道渡島半島第三系の地質構造発達史  [in Japanese] The geologic development in the Oshima Peninsula, southwestern Hokkaido, Japan  [in Japanese]

In Oshima Peninsula, Tertiary sedimentary rocks and volcanic rocks are distributed widely on the basements which are consist of Paleozoic and Mesozoic sedimentary rocks and Cretaceous granitic rocks. The geologic age, constituation and tectonic feature of sedimentary basements have not been clarified as yet owing to the insufficiency of investigation. Triassic to early Cretaceous type radio-laria and conodont fossils were discovered from many places within the rock units that had been considered previously to be of Paleozoic age. The Neogene rocks of the area are characterized by Green Tuff which is a thick complex of greenish altered felsic to mafic volcanic rocks mainly of the earlier half of the Miocene. The Neogene strata in the area are generally thick and are moderately folded. The folding began mainly in the pliocene time. Tertiary system is divided into seven stages, i.e. Matahachizawa, Fukuyama, Yoshioka, Kunnui, Yakumo, Kuromatsunai and Setana stages in ascending order. Matahachizawa Formation and the lower part of Fukuyama Formation are considered as late Oligocene from radiometric datings. This paper outlines the geology and geologic developments of depositional areas in Oshima Peninsula.