北上山地外縁地向斜地域の層位学的研究

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Detailed investigation has been made on the stratigraphy of the northern Kitakami massif, Northeast Japan. The northern Kitakami massif which occupies the northeastern side of the Hayachine tectonic belt (Yoshida and Katada, 1964), is subdivided into three belts ; (a) the North Kitakami belt, (b) the Iwaizumi belt and (c) the Taro belt from the west to the east. The latter two belts are dealt with in this article in detail. (a) The North Kitakami belt It is mainly composed of the Permian strata. (b) The Iwaizumi belt The Iwaizumi belt is separated from the North Kitakami belt by the Kuzumaki tectonic line. Stratigraphic sequence newly established in this belt is as follows. Through the present investigation, it became evident that the formations distributed in this belt are Mesozoic in age, in spite of the previous view that they were of Permian. The fossil conodonts indicating the late Triassic in age were newly found from the Kisawahata and Magidai formations. They are correlative with the Kamiiso Limestone of Hokkaido (Sakagami et al, 1969) and Natsu-domari Formation of Aomori Prefecture (Murata and Nagai, 1972). Calcareous algae which may indicate Mesozoic (Jurassic?) were collected from the limestone lens of the Sawayamagawa Formation. Stromatoporoids of the so-called "Torinosu type" indicating the late Jurassic in age were previously collected by Hase (1952 MS) from the "Iwaizumi Formation" which almost corresponds with the Takayashiki Formation in this article. Besides the formations mentioned above, the Jurassic (?) Akashika Formation is developed in the eastern margin of the Iwaizumi belt. (c) The Taro belt The Taro belt is separated from the Iwaizurni belt by the Taro tectonic line. Stratigraphic sequence developed in this belt is as shown below. Hexacorals and stromatoporoids of the so-called "Torinosu-type" were found from the Koshimeguri Formation. The Omoto Formation yielded some molluscan and plant fossils. The latter is of the so-called "Ryoseki type" indicating the early Cretaceous in age. Based on the sedimentary facies and fossils, it may be said that the Omoto Formation has been deposited under a brackish to shallow marine environment. On the other hand, andesitic pyroclastic rocks are developed in the upper part of the formation. This represents the initial phase of the igneous activities. Large quantity of volcanic rocks of the Hara-chiyama Formation represents the maximum phase of the activities in the early Cretaceous. The Hara-chiyama Formation is correlative with the Kanaigaura, Niitsuki and Yamadori formations in the southern Kitakami massif. The Rikuchu Group which shows steep and narrow folded structures and the granodiorites are unconformably overlain by the Lower Cretaceous (Aptian-Albian) Miyako Group which gently dips eastwards. The conspicuous diastrophism represented by this unconformity was named the Oshima orogenic movement by Kobayashi (1941). These three belts are commonly characterized by the steeply dipping structures with NW-SE or NNW-SSE trends. However, the North Kitakami Belt, which principally shows NW-SE trends, is separated from the Iwaizumi and Taro belts with NNW-SSE〜N-S trend by the Kuzumaki tectonic line. The Early Cretaceous granodiorites (110-120 m. y.) are mainly distributed in the northeastern part of the northern Kitakami massif. They occur as concordant intrusive masses to the folded Mesozoic strata both in the Iwaizumi and Taro belts. Thus it is-highly probable that the structures of these two belts were accomplished by the same orogenic movement. Based upon the distribution pattern of the geosynclinal sequences of the Paleozoic and Mesozoic systems in the Kitakami massif, it is generally accepted that the younger sequences tend to develop in the northeastern part rather than the southwestern part. Actually, the North Kitakami belt in which dominates the Paleozoic sequences of the Chichibu geosyncline occurs in the western part of this area, while both the Iwaizumi and Tar6 belts represented by the Mesozoic geosynclinal sequence occupy the northeastern part. From these facts it may be safely concluded that the geosynclinal basin was shifted from the west to the east during the Paleozoic to Mesozoic time, though several tectonic barriers might be existed in the course of the structural development.
東北大学の論文
1974-03-10

北上山地外縁地向斜地域の層位学的研究

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