The Uchimura Mountains lies in the southern part of the Takai-Utsukusigahara Belt of the northern Fossa Maguna region. The consolidaded sedimentary rocks, which has been called the Uchimura Formation in this range, are of early Miocene age and exceeded 5000 meters in total thickness. They are gently folded and faulted, and intruded by some plutonic or dike rocks of late Miocene to Pliocene age. The exposed lower Miocene section in this mountains has been consists mainly of volcanic and normal clastic sediments. The former crops out in the eastern area, the latter in the western area of this mountains. The volcanic sediments, which are conformably succeeded by basaltic to dacitic sediments, are separated into five conformable members: the Ayashizawa greenish pyroclastics, 0-700meters thick; the Minamikadozawa tuff, 150-1000 meters thick; the Hirogawara greenish pyroclastics, 300 meters thick; the Kokuzou basalt, 400 meters thick; and the Hosoonotaki Volcanics, 300-1400 meters thick. These volcanic members has sometimes interfingering lenticular normal sediments, that is the Iriyama sandstone member (700 meters thick) andthe Shirayazawa sandstone shale member (100-800 meters thick). Normal clastic sediments are separated into two conformable members : the Takeshitouge sandstone, 250 meters thick and the Ichinose sandstone conglomerate member, 500 meters thick. The Misayamatouge basalt is a interfingering lenticular member in these normal sediments. These volcanic and normal clastic members have interfingering relationships vastly at the center part of the Uchimura Mountains. Volcanic sediments of a eastern part have a regional dip to the north. Normal sediments, on the contrarily, folded into some gently plunging anticlines and synclines. Two prominent sets of faults cut the formation at the center part of this region ; the one trending northwestward and the other about eastwestward. Stratigraphycal and tectonic features in the Uchimura Mountains can be assigned to the reflection of a basement structure of the Neogene formation, especially the bending structure of the Median Tectonic Lines (MTL) just at this point.
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