北部北上山地の先宮古統堆積岩類に関する研究の総括と現状

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The studies of the Pre-Miyakoan (Lower Cretaceous) stratigraphy and geologic development of the Northern Kitakami Massif are reviewed. In the Northern Kitakami Massif, a thick sedimentary complex which attains a hundred thousand meters, consisting of shale, chert, sandstone and greenstone intercalating with lenticular limestone are extensively distributed. They were believed to be an "eugeosynclinal" deposits. The area is divided into three "tectonic belts", namely "Northern Kitakami Belt", "Iwaizumi Belt" and "Taro Belt" by the "Kuzumaki Tectonoic Line" and "Taro Tectonic Line" trending NNW-SSE from the west (Kano, 1958; Shimazu et al., 1970; Sugimoto, 1974a). The geologic age of them was believed to be Carboniferous to Permian, Triassic to Jurassic and Jurassic to Lower Cretaceous respectively, from the fossil evidence occurred from the lenticular limestones. And the above tectonic lines were regarded to have controled the eastward shift of the sedimentary basin. During the past decade, however, it has become accepted from the paleontological evidence of conodont that the many chert beds are Permian to Triassic in age (Toyohara et al., 1980; Murai et al., 1981, 1983, 1985, 1986) and of radiolalia from only one locality of "Taro Belt" that shale is Jurassic in age (Taketani and Minoura, 1984). The most of the paleontologically dated rocks of Permian and Triassic ages are now considered to be exotic blocks or sheets included in the Jurassic to Early Cretaceous clastic sediments. The strata of the Northern Kitakami Massif are presumably emplaced in a trench, where land-derived sandstone and shale and ocean-derived chert and limestone which covers the seamount constituents can encountered and be mixed together. So the sediments of the Northern Kitakami Massif can be taken as an accretionary body. The differences are recognized between the western area and eastern area devided by Iwaizumi Tectonic Line in the occurrence of the fossils and the compositions of the sandstone. The western area, Kuzumaki-Kamaishi Belt, is characterized by including the Permian to Triassic exotic blocks and by distributing of lithic-, plagioclase rich elastics, and the eastern area, Akka-Tanohata Belt, is characterized by including only Mesozoic exotic blocks and being composed of quartz and potash feldspar rich elastics which originated from granite. In the present, the significance of this "line" is obscure, but the "line" can be traced to the northward, which passes between the Kamiiso and Esashi-Katsurako districts of the Southwestern Hokkaido.
地学団体研究会の論文
1988-07-25

北部北上山地の先宮古統堆積岩類に関する研究の総括と現状

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