著者

斎藤 靖二

出版者

東北大学

雑誌

東北大學理學部地質學古生物學教室研究邦文報告 (ISSN:00824658)

巻号頁・発行日

vol.62, pp.55-67, 1966-03-30

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The Paleozoic System distributed in the Setamai district, southern Kitakami massif, is classified on the ba-5is of rock facies as follows; Toyoma Formation Permian Kanokura Formation Sakamotozawa Formation ~Unconformity~ Nagaiwa Formation Onimaru Formation Carboniferous ~Unconformity~ Odaira Formation Arisu Formation Orikabe Formation The Orikabe, Arisu, and Odaira Formations are composed chiefly of "schalstein" and black slate. The acidic to andesitic pyroclastics intercalated in the Orikabe Formation indicate the former existence of volcanic activities. The Arisu Formation and the lower part of the Odaira Formation are composed mostly of andesitic pyroclastics, which decrease in thickness upwards to the middle part of the Odaira Formation. This shows that intense volcanism was periodic during deposition of those formations. The formations other than mentioned above do not show such intense volcanic activity. The Paleozoic System of this area is folded and faulted into blocks and thus their structures are complicated. The Hizume-Kesennuma tectonic line which extends in NNW-SSE direction is situated in the western part of the area and the eastern margin is defined by the grandodiorite which metamorphosed the sedimentaries in contact ther with. The most striking feature of the geology of this district is the structural differences between the Carboniferous and the Permian Systems. This unconformity was discovered by Minato (1942) at the base of the Permian Sakamotozawa Formation in the Setamai district who said that it represents the "Setamai folding" of thee Late Carboniferous. The folding axes of the Carboniferous System trend nearly N-S, whereas that of the Permian System is generally NNW-SSE in direction, being parallel with the Hizume-Kesennuma tectonic line. The Permian System is distributed in the northeastern, southeastern and central parts of the district and along the tectonic line. In the central part, in general, the Permian strata form a large-syncline, with NNW-SSE trend plunging to the south and parallel with the tectonic line. On the other hand the Carboniferous System which is restricted in distribution to the eastern side of the tectonic line, is developed on the outerside of the large-syncline. In the east of Komata it occurs in the Permian System as a block uplifted by faulting. The Orikabe Formation is distributed along the western wing of the syncline extending from Orikabe to Okuhinotsuchi in north to south direction though it is separated into several blocks by faults. The Arisu Formation is developed on the outerside of the Orikabe Formation which follows the syncline. The Odaira Formation is distributed generally farther to the outerside than the lower formations though it is directly overlain with unconformity by the Sakamotozawa Formation at the west of Mt. Odaira, and is bordered by the Onimaru Formation on its outerside. Concerning the Carboniferous System it seems that each of the formations is developed on the westward and eastward sides of the central part of the area in ascending order, and all of them are covered with unconformity by the Permian Sakamotozawa Formation. Furthermore, the Carboniferous System sometimes develops a large-anticline and gently dipping fluted homoclines, features which cannot be seen in the intensely folded Permian System. It seems that these evidences reflect not only the lithofacies but also the geologic structures of the Carboniferous System before the deposition of the Sakamotozawa Formation. The differences of the geologic structures between the Carboniferous and the Permian Systems, as mentioned above, are the results of the "Shizu folding" and the "Setamal folding" of Minato (1942). Therefore it is supposed that a large-anticline with axis of nearly N-S trend might have been formed and denuded in the Setamai district before the deposition of the Sakamotozawa Formation. The geological structures must have been fairly complicated, but concerning the details more precise investigation must be undertaken.