紀伊山地中央部の秩父帯・四万十帯

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There are two lithologic units in strata in the central area of the Kii Mountains. One of them belongs to the Chichibu Belt and is subdivided into three formations : B, C and D in ascending order. The other unit of Cretaceous in age is a member of the Shimanto Belt. Besides them, there is O Formation exclusively found between the two units. The B Formation is composed of mudstone, chert and large limestone bodies accompanied with greenstones. The limestone bodies contain Late Triassic conodonts, and the mudstone rarely yields Jurassic radiolarians. The C Formation is made up of thick chert beds, intercalated with layers of mudstone, sandstone, acidic tuff, limestone and greenstones. Late Paleozoic and Mesozoic fossils are found sporadically in this formation. The D Formation is characterized by conglomerate containing abundant pebbles and cobbles of limestone. The O Formation consists largely of mudstone with subordinate sandstone. The constituent of the formation is a chaotic sediment involving blocks rich in dimensional and lithological diversity, yielding fossils of various ages. The Cretaceous strata in the Shimanto Belt comprise sand- and mud-abundant portions. In each mud-abundant portion are a few horizons involving greenstones, chert and acidic tuff. Among them, the siliceous parts yield Cretaceous radiolarians. Such juxtaposition of rocks of different ages as shown in the B and C formations is interpreted as the product of tectonic and/or sedimentary rearrangement. From the lithofacies and fossil contents, the formation of the chaotic part of the O Formation was due to mingling during and after sedimentary processes. The B, C and D formations, gently dipping north to northeast, overlie structurally the Cretaceous strata and partly the O Formation. The Cretaceous strata dipping 40°to 60°northwest show recurrence of the muddy portions. The B or C formation and the Cretaceous strata or the O Formation are in fault contact with each other. This fault having gentle dip corresponds to the Butsuzo Tectonic Line. The Cretaceous strata in the Shimanto Belt may be in fault contact directly with the Sanbagawa metamorphics in western half of the area without the B, C and D formations or the Chichibu Belt.
国立科学博物館の論文
1981-09-22

紀伊山地中央部の秩父帯・四万十帯

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