宮崎県五ケ瀬川中流域の四万十川層群の地質構造と層序

The Morotsuka Group (probably Cretaceous) that widely occupies the northern part of the Shimanto terrain in the middle reaches of the Gokase is divided into two formations, the Makimine Formation, below, and the Hinokage Formation, above. The two formations are considerably different from each other in lithologic facies and geologic structure. The Makimine Formation (about 600 m thick) consists mostly of black mudstone with some intercalations of greenstones and chert. The greenstones (50-200 m) belong to a suite of oceanic basalt in composition and include massive lava, pillow lava, pillow breccia and aquagene tuff or hyaloclastite. They are conformable with underlying mudstone and constitute a characteristic features of the products of submarine volcanism. Besides, there are some intrusive sills and dykes of basalt, dolerite and gabbro. The radiolarian chert is always associated with aquagene tuff that overlies lava flow. The Hinokage Formation (about 1, 300 m) is made up of thin-to thick-bedded alternation of sandstone and shale with some beds of conglomerate and a thin layer of acid tuff in the upper part. It is divided lithologically into three members, the lower, the middle and the upper (Table 1). Sandstones are very predominant in the upper part. The two formations are in fault contact with each other at many places. The mudstones of the Makimine Formation are pervasively cleaved and sheared, being converted into phyllite or slate. They are folded in a small scale as having a wavelength of 0.5 to 1 m. The Hinokage Formation is, on the other hand, less deformed and has no such minor folds as in the Makimine Formation. However, asymmetrical, open foldings with a wavelength of 100 m or so are recognized in association with thrust faults. Their pattern and magnitude vary stratigraphically from a lens fold in the lower part to a flexure fold in the upper part. The direction and inclination of the axial planes of the folds in the Hinokage Formation are concordant with those of the thrust faults mentioned below. The Morotsuka Group appears repeatedly in three zones that are separated by low-to moderate-angled thrust faults running in a NE-SW direction and dipping to NW at intervals of 2-5 km. Thrusts of the same type but of various orders are also commonly found. Thus, the geologic structure of the group is characterized by thrust faulting which gave rise to the imbrication with a polarity facing to the north and the remarkable shortening of the sedimentary bodies. Regarding the stratigraphic division and sequence, the geologic structure and the thickness of the Morotsuka Group, this study has secured the results very different from those of previous authors (IMAI et al., 1971 and others)