米沢盆地南東部における後期中新世板谷火山活動

概要

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Along the Ohu backbone area on the prefectural boundary, the alternation of pyroclastic layers were developed thickly and broadly. Among them, the Itaya formation is distributed in the southeastern part of Yonezawa Basin and northwest part of Fukushima Basin. The relation, between the Itaya formation and underlying formations of the middle Miocene and granitic basement is found to be in the remarkable unconformity. The Itaya formation is divided into six members by lithofacies, namely, Itaya-1(I-1), -2, -3, -4, -5 and -6, in the ascending order. These are composed almost as follows, I-1:fine tuff (250 m), I-2:lapilli tuff (200m), I-3:alternation of sandstone and mudstone (300m), I-4:tuff (80m), I-5:lapilli tuff (300m), and I-6:welded tuff (600m). The activity of Itaya eruption is divided two stages. The early one had formed I-1 and I-2 members, and the late one I-4, I-5 and I-6 members. In the early stage, the activity had gone into eruption at the depression basin, 10 kilometers in diameter, about 400 meters in depth. The talus deposits and low gravity anomalies in this area gives an evidence of the collapsed basin. The latter eruption had broken out mainly on land. As a result, the well vesicated pumice and welded tuff are found in those pyroclastic deposits as the sufficient proof of this type of eruption.
日本地質学会の論文
1979-03-30