静岡県掛川層群の五百済テフラ層と新潟地域西山層のChw-3テフラ層との対比

概要

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The Iozumi Tephra Bed is one of the prominent tephra beds of more than 10m thick in the Kakegawa Group, Shizuoka Prefecture along the Pacific Ocean side. This Tephra Bed was correlated to the Chw-3 Tephra Bed in the Nishiyama Formation in the Niigata region of Japan Sea side on the basis of their horizon, petrography and chemical composition of volcanic glass shards. They are mainly composed of micropumice-type glass shards and biotite and hornblende are dominant as mafic minerals in common. Chemical composition of glass shards is also coincident, especially characterized as higher MgO contents around 0.3% compared with the tephras of the nearer horizon. The Iozumi Tephra Bed is composed of more than ten gravity flow units containing pumice clasts of more than 10cm in the Kakegawa region, which were possibly passed from pyroclastic flows. While, in the Niigata region, the Chw-3 Tephra Bed is thin of less than 10cm and fine-grained, interpreted as air-fall tephra. These settings imply that the Iozumi-Chw-3 Tephra was derived from pyroclastic flows accompaning co-ignimbrite ash. The age of the Iozumi-Chw-3 Tephra Bed is inferred to be around 2.2Ma.
新潟大学の論文