会津柳津(やないず)地方における後期中新世陥没盆地の形成について
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概要
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The middle to the upper Miocene series in the Yanaizu region is divided into five formations, namaly the Ogino, the Urushikubo, the Shiotsubo, the Sunagohara, and the Fujitoge formations in ascending order, all of which are in conformable relation except that the unconformities at the base of the Sunagohara formation and partly at that of the Fujitoge formation. The Urushikubo formation may unconformably overlie the Ogino formation in part. The Ogino formation and the lower part of the Urushikubo formation consist of pyroclastic rocks. The upper part of the Urushikubo formation, the Shiotsubo formation, and the lower part of the Fujitoge formation consist of shale, sandstone, and siltstone respectively. The Sunagohara formation and the upper part of the Fujitoge formation consist of pyroclastic rocks. Basaltic, dacitic, and rhyolitic dykes intrude into the Urushikubo and the Shiotsubo formations. The fundamental geologic structure is nearly flat, except for some flexure, synclines and anticlines which have N-S trending axes. The horizontally bedded Sunagohara formation is distributed in a restricted area, about 5 km in diameter, and abuts on the steedly dipping surface of the unconformity at the margin of the basin. Basal conglomerate, which is very poorly sorted and angular talus-like breccia including boulder gravel with 3m in maximum diameter, is distributed along the margin. It would be deposited just in front of the abrupt cliff which has been preserved as the present steep surface of the unconformity. Normal faults striking parallel to the surface of the unconformity are found in the basement and by them such a cliff would formed. It is concluded that the collapse basin with 5 km in diameter was formed by the faulting which had thrown down the basin side abut 400 m before deposition of the Sunagohara formation. Before the formation of this collapse basin, the domal uplift which is 25 km in diameter and 500 m in height took place. Judging from that the collapse basin was filled with pyroclastic rocks which was deposited soon after the collapse, the domal uplift is ascribed to the magmatic activity. The other uplift with the axis of NW-SE trend, which was no relation to volcanism had took place in this region before the domal uplift.
- 1978-03-25