2021 年 75 巻 1 号 p. 33-36
We discovered one piece of orthoquartzite gravel (Oq gravel in short) from a gravelly sand in drilled-core of the Plio-Pleistocene Osaka Group in the southern Nara basin, Southwest Japan. It is 30 mm across, 0.7 to 0.9 in sphericity, 0.9 in roundness and pale yellowish brown in color. Under microscopic observation it consists of 96% of quartz, 2% of plagioclase, 0.7% of K-feldspar and 1.3% of muscovite. On the basis of modal composition it is classified as quartz arenite. Size of sand grains included varies from 0.1 to 0.7 mm and is 0.25 mm in average (n=130). Some of quartz grains have dust rings and many show weak to moderate wavy-extinction.
The gravelly layer including Oq gravel is mostly composed of gneissose or mylonitic granites, aplite and pegmatite, with a little amount of chert, sandstone, shale and basic rocks. The granitoid gravels are supposed to be derived from the southern margin of the Ryoke Belt to the south and the sedimentary gravels are also from the Izumi Group, the Chichibu Belt or the Shimanto Belt to the further south. The Oq gravel thus seems to have derived from the same provenance of the sedimentary gravels.
The provenance of the sedimentary gravels indicates that the head of the Plio-Pleistcene northerly river has been around or beyond (cross over) the Median Tectonic Line. This result may much contribute to reconstruct the Plio-Pleistocene paleogeography around the Nara Basin.