抄録
The bearing of orthoquartzitic clasts in the development of the Japanese Islands was first discussed by TOKUOKA (1967) and their precise description was given in 1970 by the same author. Orthoquartzitic clasts from the Muro group have similar characteritics with typical cratonic sandstones such as Sinian quartzites in China or other Precambrian sedimentary quartzites, and it is generally accepted that these were derived from the Precambrian cratonic basement underlying the Japanese Islands. However, the examination on orthoquartzite itself has not been fully done, especially of their occurrence and textural properties. These problems are reexamined here in detail. Eighty-nine orthoquartzitic clasts which were randomly collected by the KISHU SHIMANTO RESEARCH GROUP (1970) were observed precisely under the microscope. It is very interesting that there are many kinds of color in orthoquartzitic clasts, which are shown as colored photographs in Plate I, and that these are quite unexceptionally constituted from rounded or well-rounded quartz grains, and furthermore, that there are found several pitted grains. These facts indicate apparently that orthoquartzitic rocks were originally formed under dune environments. Textural properties of orthoquartzitic rocks were reexamined in detail after Friedman's method (FRIEDMAN, 1957 and 1962b). Grain-size distribution was obtained in twenty-five specimens of the KISHU SHIMANTO RESEARCH GROUP (1970) and in thirty-nine ones of TOKUOKA (1970). On the basis of grain-size parameters, orthoquartzites, which occur as clasts in conglomerates of the Paleogene Muro group, are safely assigned to have been formed originally under beach and/or dune environments.
