Orthoquartzitic Gravels in the Paleogene Muro Group, Southwest Japan

Abstract

The Shimanto Terrain facing to the Pacific Ocean was a geosynclinal area during the Mesozoic to lower Tertiary. The Muro group in the Kii Peninsula, Southwest Japan, occupies the southern part and upper horizon of the geosyncline. In the previous paper (TOKUOKA, 1967) the writer reported on the coarser clastic sediments of the Shimanto Terrain in the Kii Peninsula, and discussed its development. He concluded that the provenances of the Shimanto geosyncline were mainly in the northern geanticlinal regions, but that there must have been an old land to the south of the geosyncline, now foundered to ocean depths, from the discovery of exotic gravels. These exotic gravels are orthoquartzites, which have never been found in the present Japanese Islands. He collected and examined 391 samples of orthoquartzitic gravels from the indurated conglomerates in the Muro group at the southern extremity of the Kii Peninsula. In the present paper he will describe their occurrence, size, shape, roundness, rock color and internal sedimentary features. Eighty-six specimens observed under the microscope to clarify their textural and compositional properties will be described. Also considerations will be given to orthoquartzites themselves. Orthoquartzitic gravels in the Muro group have similar characteristics with typical cratonic sandstones such as Sinian quartzites in China or other Precambrian sedimentary quartzites which are distributed worldwide. It may be concluded that there had once been a source area, a part of which must be composed of Precambrian quartzites, to the south of the Shimanto geosyncline, and that it sank below the Pacific Ocean at the close of the geosynclinal development (probably at the end of the lower Miocene).