西南北海道定山渓地域の中新統と地質構造

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The stratigraphy and geologic structure of the Miocene Series are examined at the Jozankei area, southwestern Hokkaido. The Miocene strata is divided into the Teine, Motoyama and Hyakumatsuzawa Formations in ascending order. The quartz porphyry intruded into the central part of the area. The Motoyama Formation is composed of sandstone, siltstone, mudstone, conglomerate and dacitic volcanic breccia. The Hyakumatsuzawa Formation, covering the Motoyama Formation, consists of andestic volcanic breccia and tuff breccia, tuffaceous sandstone, siltstone and mudstone, and accompanies andesite lava. The radiolarian fossils and a K-Ar age obtained from these geologic series indicate that the Motoyama and Hyakumatsuzawa Formations accumulated during the Middle Miocene and during the period from Middle Miocene to Late Miocene, respectively. The quartz porphyry is considered to have intruded during the period ranging from the later stage of deposition of the Motoyama Formation to the earlier stage of the Hyakumatsuzawa Formation. On the other hand, three phases of structural movements are recognized in the area as follows: (1) NW-SE trending folds, (2) NNW-SSE or N-S trending fissures represented by the intrusion of the quartz porphyry, (3) NNE-SSW trending folds. Based on the stratigraphic correlation in the area, structures of the first and second phases are inferred to have been formed during 11-12 Ma and 10-llMa, respectively, while the third one since the Late Miocene.
地学団体研究会の論文
1989-01-25

西南北海道定山渓地域の中新統と地質構造

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