赤石山地北部の四万十帯

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The undifferentiated Mesozoic rocks of the Shimanto belt of the northern Akaishi mountains are divided into eight lithostratigraphic units which are separated by N-S trending faults (Fig. 1): namely, the Senjo-dake (3,100m thick), Kita-dake (2,500m), Akaishi-dage (1,000 m), Notori-dake (1,500 m), Nishimata (1,500 m), Hirogochi (1,500 m), Dentsukutoge (unknown), and Narada (1,600 m) Units. The Kita-dake Unit is conformably overlain by the Akaishi-dake Unit at the Akaishi-dake area, and by the Notori-dake Unit at the Notori-dake area, respectively, the mutual relation of the latters is unknown owing to intervening fault. The lack of marker beds beyond faulted boundaries in addition to the strong deformation and extreme scarcity of fossils in all units makes it impossible to correlate units among themselves. Some fossils roughly indicating the late Jurassic to Cretaceous age have been reported (OKUBO et al., 1958, 1959; KANO, 1981) from an area to the south of this study. Among the boundary faults, the Okaramatsu-yama fault seems to be the most significant one since it separates the Shimanto belt of the Akaishi Mountains into two lithologically and stratigraphically contrasting parts. Major strucure of the western part is characterised by several isoclinal folds of which the axial planes dip steeply to the east or west. The Kita-dake Unit, which consists mainly of pelitic and psammitic rocks accompanied by minor chert, acidic to basic volcanics and rare limestone especially in its basal part, crops out as the core of major anticlinorium. It is noteworthy that the Kita-dake Unit includes pebbly and/or block-like chert, green rock and sandstone in phyllitic pelitic rocks, and it is reffered to be sedimentary melange. The axial part of this anticlinorium occurs along the backbone of the Akaishi Mountains. In the western limb of the anticlinorium, most of the Senjo-dake Unit, in which psammitic sediments are predominant, is overturned. In the eastern limb, on the other hand, also psammitic sediments are predominant, and the Nishimata Unit dips to the east while the Hirogochi Unit dips to the west, both units being right-side-up. Pelitic sediments are predominant in the eastern side of the Okaramatsu-yama fault. Although the structure of the Dentsuku-toge Unit is not yet fully understood due to its complicated nature, that of the Narada Unit is a symmetric anticline in which the axial plane dips steeply to the west. Basic volcanics including pillow lava crop out along the axial part of the anticline. In the Shimanto belt of this district, deformation is intence and transposition structure is widespread. Metamorphic recrystallization is also marked throughout the area. Pumpellyite occurs in most rocks including psammitic rocks. Actinolite and prehnite are common. The metamorphism of the Shimanto belt of this district may correspond to a transitional facies between prehnite-pumpellyite meta-graywacke and pumpellyite-actinolite facies. In the northeastern part of this district, the Shimanto rocks are intruded by the Kaikoma-Hoo and Yakejizo granites and the contact-effect is superimposed on the regional metamorphism.
地学団体研究会の論文
1983-11-25

赤石山地北部の四万十帯

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