和歌山県中東部の日高川層群湯川累層・美山累層 : 紀伊半島四万十累帯の研究(その12)
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概要
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In the surveyed area, the Yukawa Formation occupies the northern part and the Miyama Formation outcrops in the southern part. Being separated by the Yanase fault in the north from the Hanazono Formation, and by the Yukawa fault in the south from the Miyama Formation, the Yukawa Formation consists of massive and bedded sandstone, flysch-type alternation of sandstone and mudstone, and is accompanied with black shale. It is lithologically divided into the Y 1, Y 2 and Y 3 Members in ascending order, and its total thickness attains to 3400 m. Based on radiolarian fossils, the formation is assigned to be late Albian to Turonian in age. The Yukawa Formation was deformed into a large synclinorium, of which fold axis is trending in east-west direction. The Miyama Formation is mainly composed of massive sandstone, flysch-type alternation of sandstone and shale, and black shale, being accompanied with lenticular exotic blocks of greenstones, chert and red/green shale. The formation is divided into the M 1, M 2 and M 3 Members, being separated by faults from each other, and the geologic age of each member becomes younger toward south. Whole strata are severely deformed and repeated by numerous reverse faults. Abundant radiolarian fossils found from black shale suggest the geologic age of the Miyama Formation to be Turonian to early Campanian. The Yukawa Formation is supposed to deposited in a sedimentary basin situated on an outer continental shelf or on an upper continental slope, while the Miyama Formation is an accretionary complex formed around trench floor during Late Cretaceous subduction process.
- 1991-01-25