山形県吉野地域における中期中新世の陥没構造

概要

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It is generally recognized that the sedimentary basins formed by the Green Tuff orogeny in the period of its earliest stage collapsed after local upheaval, and were followed by the volcanic activities. In these basins, the talus-like conglomerates deposited at first and normal sediments did succesively. Such collapsed basins formed at the Nishioga, Daijima and Nishikurosawa stage are found in the Green Tuff region. The genetic and developing process of the basins confirmed at the Nishioga and Daijima stages. In the Yoshino district, the lower to the middle Miocene systems are divided into four formations, that is the Taro formation, the Yoshino formation, the Motozawa formation and the Hasedo formation. The Motozawa formation deposited in the collapsed basin formed after the deposition of the Yoshino formation. The cliff of the collapsed basin was about 700 meters high at that time. The Motozawa formation abutted against the cliff. The lowest part of this formation is marine sediments. The upper Yoshino formation is marine sediments, too. The Yoshino formation is conformably overlain by the Motozawa formation, except the Motoyashiki area where locates at the margin of the basin. The facts above mentioned tell that the collapsed basin accumulated the Motozawa formation was formed under the sea. The writer discusses about this basin in this paper.
日本地質学会の論文
1979-03-30