和歌山県竜神村南部の日高川層群

The stratigraphy, geologic structure and sedimentology of the Hidakagawa Group in the southern part of the Ryujin village were described in the present paper. The Hidakagawa group, which is assigned as the Cretaceous (mainly the upper Cretaceous) in age, is separated from the Eocene Otonashigawa Group in the south by the Gobo-Hagi Tectonic Line, and is divided into the Nyunokawa and the Ryujin Formations in ascending order. The former, distributed in the southern part of the area, is composed mainly of flysch-like alternations of sandstone and shale, thick-bedded massive sandstones and conglomerates. It attains to 2,200m in thickness. The latter, distributed in the northern part, is characterized by predominant shale and shaly flysch, occasionally accompanied with sandy flysch. Basaltic greenstones yielded by submarine volcanisms and rhyolitic and/or rhyodacitic tuffs of subaerial origin are sometimes intercalated in it. It attains to 2,800m in thickness. The Hidakagawa group has the general strike of E-W trend, dipping to the north with high angles. There are found two anticlines and one syncline of E-W trend in the Nyunokawa formation. These structures, together with the Gobo-Hagi Tectonic Line, are obliquely cut and dislocated in some degree by the faults of NW-SE trend. There are found not a few minor folds and faults in the Ryujin formation, however, it reveals a monoclinal structure as a whole. There exist thick conglomerates called "The Nyunokawa Conglomerate" at the top of the Nyunokawa formation. Its maximum thickness attains to 400m. The clasts are mostly pebble and cobble in size, and boulders are also contained frequently. These are constituted of acidic volcanic rocks (mostly rhyolites), granitic rocks, sandstone, shale, chert and limestone, among which acidic volcanic rocks are most abundant, occupying roughly more than one-third of the total composition. Sandstones (mainly massive sandstones) were examined under the microscope. These are fine sandstones, and most of which belong to the feldspathic wacke and a part of which to the lithic wacke and the feldspathic arenite. Among constituting minerals, rock fragments and plagioclase are abundant, while quartz and K-feldspar are relatively poor in amounts. Not a few current markings were found in the Hidakagawa group. The sediments of the Nyunokawa formation were mostly transported by lateral currents from SE to NW, while those of the Ryujin formation were transported and deposited by longitudinal currents from E to W. Judging from the occurrence of the Nyunokawa Conglomerate, it might have been supplied from the southern hinterland, although no sole-marking was discovered in it. It is likely that "The Kuroshio Paleoland," situated to the south of the Shimanto Geosyncline, might have existed not only in the Paleogene but also in the Cretaceous.