新潟県入広瀬村大白川地域の足尾帯の地質

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The Ashio Belt in the Oshirakawa area consists of four sedimentary units; (I) the Kuromatagawa chert-black shale unit (500-600 m), (II) the Kakinoki shale-sandstone alternation unit (600-800 m), (III) the Irihirose greenstone-chert unit (200-300 m) and (IV) the Oshirakawa pebbly shale-greenstone-chert unit (1500-2000 m) in ascending order of apparent sequence. The Kuromatagawa chert-black shale unit is dominantly composed of bedded and massive cherts, interbedded by black shale. The cherts are occasionally gradational to black shale. This unit is folded and thermally metamorphosed by Cretaceous granites. The Kakinoki shale-sandstone alternation unit refers to the shaly flysh accompaning basaltic greenstones. The lower half of the Irihirose greenstone-chert unit comprises basaltic greenstones, and the upper half is dominantly composed of bedded and massive cherts, characterized by the occurrence of the bedded red cherts. The Oshirakawa pebbly shale-greenstone-chert unit occupies the upper half of the surveyed area. Olistostrome facies are predominant in the unit with olistoliths of siliceous sandstones, cherts, greenstones and limestones. Middle Permian fusulinids such as Neoschwagerina and Chusenella were found in the limestone olistoliths. Besides, laterally extending strata of cherts, greenstones and sandstones are intercalated. Some greenstones accompany limestones and skarns at their top. Layered sandstones commonly contain 5-15% potash feldspar, while in sandstone olistoliths, they are scarcely found. The early Jurassic radiolarian assemblage (the Parahsuum simplum assemblage) (YAO, 1984) was detected from the layered bedded chert situated at the middle horizon of the unit, interbedded in the olistostrom.es. From the Kuromatagawa chert-black shale unit, the middle Permain to late Triassic conodonts were reported. Thus, the Ashio Belt in the Oshirakawa area is composed of the piles of the sedimentary bodies of different facies and ages, at least from middle Permian to early Jurassic. These apparent successions are almost similar to those in the Tanba and northern Mino Belts in southwest Japan, and the middle Jurassic might also be expected in the surveyed area.
1987-07-25

新潟県入広瀬村大白川地域の足尾帯の地質

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