@article{oai:soar-ir.repo.nii.ac.jp:00012093, author = {美ケ原団体研究グループ}, issue = {2}, journal = {信州大学理学部紀要}, month = {Mar}, note = {Utsukushi-ga-hara Collaborative Research Group found upper Miocene Green tuff sequence in the south western part of Utsukushi-ga-hara Plateau, central Nagano Prefecture, central Japan. The sequence has been denned as the Kotaki-yama Group by the research group. Neogene successions of the area can be divided three major groups, which are the Uchimura Formation, the Kotaki-yama Group and the Enrei Formation. Lower Miocene Uchimura Formation mainly consists of altered pyroclastic rocks and was penetrated by quartz diorite in the middle to late Miocene. The Kotaki-yama Group unconformably covers their basement rocks with a basal conglomerate. Gravels of the conglomerate are commonly angular and same rock immediately under the unconformity. The Group can be divided into four formations on the basis of its lithology. They are the Daimon-zawa, the Konako-zawa, the Hon-zawa and the Minami-zawa Formations in ascending order. The Daimon-zawa Formation is composed of andestic lava and tuff breccia with intercalated layers of normal sediments. The Konako-zawa Formation unconformably overlies the Daimon-zawa Formation and consists mostly of andesitic pyroclastic rocks. The Hon-zawa Formation rests unconformably upon the Konako-zawa Formation in the upstream of the Konako-zawa Gorge and is composed mainly of andesitic to dacitic volcanic rocks. These Formations mentioned above are unconformably overlain by the Minami-zawa Formation, which is composed of conglomerate, sandstone, mudstone, lava and pyroclastic rocks in the order of from the base to upward. The lowermost part of Plio-Pleistocene Enrei Formation is the Sanjiro Tuff Member, which rests unconformably upon the Kotaki-yama Group. The Enrei Formation consists of andesitic pyroclastic rocks with interbeds of nomal sediments and so called flat lava in the upper part. Beginning of the sedimentary basin of the Kotaki-yama Group must be Collaps. Lithofacies of the basal conglomarate and restored form of the unconformity are supporting the idea. The basin has 5 kilometers width and 12 kilometers length elongating north to south. Following elevation of the Kotaki-yama Group toward the end of the Miocene, this area was subjected to stresses reviving earlier faults and leading to the formation of the Enrei Collapsed Basin trending in north-west to south-east direction., Article, 信州大学理学部紀要 25(2): 79-99(1991)}, pages = {79--99}, title = {長野県中央部美ケ原高原南西部の地質}, volume = {25}, year = {1991} }