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＜紀要論文＞
Stratigraphical Studies of the Chichibu Belt in Western Shikoku
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| 概要 | In this paper the stratigraphy of the Upper Palaeozoic strata of the Chichibu belt in western Shikoku is described and also their stratigraphical, structural and metamorphic relationships with the nor...therly adjacent Mikabu green rocks and Sambagawa metamorphic rocks are analysed and discussed. In connection with the correlation palaeontologic descriptions are given for certain selected fusuline and coral species. The Palaeozoic strata of western Shikoku, about 5300m in total thickness, are of eugeosynclinal lithologic assemblage, consisting primarily of sandstone, slate or phyllite, chert and altered basic volcanic rocks (greenstones) with a small amount of limestone. Their stratigraphic sequence and correlation with the already established sequences outside the present area are summarized in Fig.5. The Carboniferous is represented by the Beedeina schellwieni zone and the Beedeina higoensis zone of the Oyabu Formation and also the Triticites yayamadakensis zone of the middle part of the Kaifuki Formation in the Nomura area. They are predominantly slate and sandstone with some intercalations of chert and green rock. The Kume Formation, which is distributed in the north and northeastern areas, consists mostly of black slate and phyllite and can be correlated with the above entioned formations, on the basis of its stratigraphical position below the Lower Permian formation. The Lower Permian itself is represented by the Saitaro Formation in the northeastern Odamiyama and the northern Sugeta areas. It is dominantly thick green rocks with some beds of chert, and are characteristically accompanied with some dolomite lenses at the basal part. On the southern Oozu area, however, the Lower Permian strata consist of black slate and intercalated chert and sandstone. The lower part of the Uwagawa Formation is referred to the Pseudoschwagerina morikawai zone and the lower part of the Tanosuji Formation to the Pseudoschwagerina minatoi zone. The Middle Permian is represented by the middle and upper parts of the Uwagawa Formation and the main part of the Nomura Formation of the Nomura area. They are composed of slate with occasional interbeds of chert and sandstone. Locally green rock is also contained. Owing to the paucity of fossil evidence, the main part of the above mentioned formations annot be exactly dated, but the middle part of the Uwagawa Formation is referred to the Neoschwagerina simplex zone and the uppermost part of the Nomura Formation to the Yabeina globosa zone. The uppermost part of the Palaeozoic strata of this area is the Masagoya Formation which unconformably overlies the Lower Permian Nakakubo Formation. It is referable to the Yabeina globosa zone in the basal part and may range upward to Upper Permian. The formation consists of slate with some intercalates of sandstone and green rock. Sandstones are dominantly of medium-grained feldspathic wacke, having more than 15 percent matrix, but partly feldspathic arenite. Greenstones are of ophiolitic assemblage, consisting of altered basalt, dolerite, spilite, diorite, microgabbro and basaltic tuff and tuff breccia. They are products of submarine volcanism. The thickest pile of the greenstones, about 700m, is accumulated in the northern part of the area in the early Permian Saitaro Formation. The Mikabu green rocks which are distributed along the boundary area between the Chichibu belt on the south and the Sambagawa metamorphic belt on the north are continuously extended to the green rock member of the Saitaro Formaion. Thus the Mikabu green rocks in western Shikoku are nothing but a member of this formation and early Permian in age. There is an anticline along the boundary of the Sambagawa and Chichibu belts. Thus, the green schists of the southern marginal belt of the Sambagawa and the green rocks of the Saitaro Formation of the Chichibu occupy the northern and southern wings of the anticline, whereas the Kume Formation occupies the axial part. The bed of green schist of the northern wing is extended to the Minawa Formation which is widely distributed in the main part of the Sambagawa belt. The Chichibu belt in the present area is divided into two metamorphic zones, mainly on the basis of the distribution and assemblage of the metamorphic minerals in the green rocks. They are a low grade prehnite-pumpellyite zone (Zone I) and a higher grade glaucophane schist zone (Zone Ⅱ). Their mineral assemblages are shown in Table 4. The boundary between the two zones, which is represented by the amphibole isograd, runs in ENE-WSW direction, cutting obliquely the stratigraphical boundaries and a major structural form.続きを見る |
| 目次 | 1.Introduction 2.Description 3.Discussion 4.Summary of Results 5.Referencs |
本文ファイル
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19_03_p387
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13.5 MB | 573 |
詳細
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| 登録日 | 2015.10.30 |
| 更新日 | 2021.02.18 |