米山層直下の礫岩層(吉尾礫岩層)およびそれにまつわる造構運動について

The latest Miocene Yoshio conglomerates are exposed at Yoshio, Kashiwazaki City, Niigata Prefecture. The Yoshio conglomerates, 150 m in maximum thickness, are pursued for about 4 km on the east limb of an anticline, and are composed of irregular alternation of conglomerates, sandstone and mudstone. The conglomerate beds, composed of five to eight beds, are the thickest at Harai-gawa and Kosugi, and the thinnest at Hijirigahana in the northwest. The pebbles at the former localities are larger than those at the latter. The size,shape, roundness, distribution and composition of the pebbles were examined at twelve localities, and the results are shown in Fig. 3. The conglomerates are polymictic, consisting mainly of pebbles of sandstone, shale, granitic rocks, metagabbro (pre-Tertiary) and Tertiary volcanic rocks. Rarely they contain pebbles of serpentinite, epidote amphibolite and limestone. Significant variation of roundness and composition of pebbles is not recognized throughout the conglomerate beds both in vertical and horizontal directions. Descriptions mainly of the pre-Tertiary pebbles are given. Granitic rocks are biotite granite, hornblende-biotite diorite, augite-hornblende diorite and metadiorite. Megascopically most of the metagabbro are compact and black, some being characterized by parallel orientation of hornblende. Others are rich in hornblende and feldspars, and show gneissose texture. Metagabbro is composed mainly of common hornblende, clinozoisite, epidote, chlorite and plagioclase, of which the last is sometimes saussuritized. Most of the common hornblende have yellowish green to bluish green color, and rarely have a brownish core and bluish green rim. They also contain augite rarely, which seems to be a relic mineral. Epidote amphibolite shows weak schistosity. It is composed mainly of common hornblende, epidote, clinozoisite, chlorite and albite. Serpentinite pebbles are very rare, and only two pebbles were discover ed. It is composed mainly of serpentine and some opaque minerals. Heavy minerals in matrices of the conglomerate beds were examined, and the result is shown in Fig. 6. They consist mainly of pyroxene, hornblende and clinozoisite, of which the last is derived probably from metagabbro. Pebbles of epidote amphibolite, serpentinite and metagabbro, and their petrographic characteristics suggest that the hinterland was the Joetsu Metamorphic Belt. These pre-Tertiary rocks are now not exposed in and around the Mt. Yoneyama area. But the large boulders of granitic rocks and the like rocks indicate that the hinterland was not very far from the present area. Moreover, the Yoshio conglomerates are overlain by a thick series of the Pliocene pyroclastic rocks (Yoneyama formation). Namely, the pyroclastic rocks of the Yoneyama formation were piled up after the upheaval of the hinterland, which supplied the pebbles. The size of the pebbles and the thickness of the conglomerates show that the pebbles were supplied from the south of Harai-gawa and Kosugi. In the area of Mt. Yoneyama and to the south the latest Miocene formations are absent, and the Pliocene pyroclastic rocks lie unconformably on the middle Miocene mudstones. The movements of the hinterland are supported also by the facts mentioned above.