<国立科博専報>北海道広尾郡大樹町付近の第三系
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Tertiary system developed in the eastern side of the Hidaka massif is divided into two groups of the Biroo and Rekifunegawa (in ascending order), of which the latter overlies the former with conformity. Biroo Group consists of porphyrite and green tuff, Hikatagawa Sandstone, Nakanogawa Formation, Nubinaigawa Mudstone, and Odamura Shale (in ascendingorder); Rekifunegawa Group is subdivided into Toyonigawa Conglomerate and Taiki Formation (in ascending order). The green tuff and porphyrite represent the lowest part of the Biroo Group, and both originate from the same igneous activity. The green tuff bed (less than 35 m in thickness) is overlain by the Hikatagawa Sandstone with conformity. The Hikatagawa consists of medium-grained, gray, tufaceous sandstone which includes fossiliferous concretions. Nakanogawa Formation is subdivided into two parts of dark gray mudstone in the lower part and the thin rhythmic alternation of sandy siltstone and mudstone in the upper. The Nakanogawa is measured about 500 meters in thickness. Nupinaigawa Mudstone consists of gray, massive mudstone which measured about 750 meters in thickness. The Nupinai yields rather abundant mulluscan fossils. Odamura Shale is characterized by the platy hard shale of about 130 meters in thickness. Toyonigawa Conglomerate is overlying the Odamura Shale with conformity and includes pebbles of black shale, granitic rocks, and mudstone derived from the underlying strata. The Toyonigawa is very variable in its thickness from north (about 200 m) to south (about 2100 m). Molluscan fossils are collected from the gray sandstone in the lowest part of this formation. Taiki Formation consists mainly of gray, massive mudstone, but it is interfingering to the conglomerate or conglomeratic sandstone in the lowest part of this formation. The thickness of the Taiki is more than 500 meters. The Biroo and Rekifunegawa Groups are cut by the Biroo fault in the western border and are folding together with the axes of north to south direction. Being judged from the molluscan fauna, the Hikatagawa and the Nupinaigawa are of middle Miocene and Toyonigawa Conglomerate and Taiki Formation are of late Miocene. Then, the Biroo Group, Toyonigawa Conglomerate, and Taiki Formation are correlated respectively with Takinoue Formation, Kawabata Formation, and Oiwake Formation in the central part of Hokkaido.
- 国立科学博物館の論文
- 1972-11-30
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