関東平野北西縁の地質

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In this paper, the writers will deseribe the geology of the western region of the Gunma Prefecture, Central Japan. The area includes the Kabura, Usui and Karasu Valleys, and most eastern part of Nagano Prefecture. Eastern part of this area is underlain by Neogene marine and non-marine sediments of Early to Late Miocene. During the Late Miocene and Early Pliocene, western part of this area had become a site of intense volcanism. South margin of the area is bordered by the Shimonita Tectonic Zone which is 3 km in width and composed of Cretaceous to Paleogene sediments and intrusives, and has E-W trend. Lowest Miocene beds which consist of the Ushibuse, Shimonita and Obata Formations are distributed inparallel with the Shimonita Tectonic Zone. Lower to Middle Miocene beds are Idozawa and Fukushima Formations. They are comparatively widespread around the Myogi Mountain Mass. Middle Miocene Yoshii and Itahana Formations trend northwest and dip northeast. Lower part of the Itahana Formation is of marine deposits and upper part is of non-marine. Late Miocene and Early Pliocene volcanic rocks in this area, that is, Akima Formation and its correlatives unconformably cover the Lower and Middle Miocene formations. The sites of intense volcanism are distributed in the west end of the Gunma Prefecture, because there are many lava flows and intrusive rocks arranged in a narrow zone from north to south. These volcanisms began with rhyolitic magma activity and then were followed by andesitic one. Volcanoes in Quaternary bestride the boundary between Gunma and Nagano Prefectures.
日本地質学会の論文
1981-03-30

関東平野北西縁の地質

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