Thermal Remanent Magnetization and Mode of Emplacement of the Pyroclastic Deposits in the Middle Miocene Upper Donzurubo Formation, Nijo Group, Japan

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  • 二上層群上部ドンズルボー層の火砕物の残留磁化と堆積様式
  • ニジョウソウグン ジョウブ ドンズルボーソウ ノ カサイブツ ノ ザンリュウ ジカ ト タイセキ ヨウシキ

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The Upper Donzurubo Formation consists of pyroclastic-flow deposits, pyroclastic-surge deposits, hot reworked deposits, and epiclastic deposits. The lower zone of the Upper Donzurubo Formation consists mainly of epiclastic deposits, intercalated ash-fall deposits, wet surge deposits and pyroclastic-flow deposits. The middle zone consists of wet surge deposits and pyroclastic-flow deposits. The upper zone comprises epiclastic deposits and several units of wet surge and pyroclastic-flow deposits. At the beginning of deposition of the Upper Donzurubo Formation, epiclastic deposits derived from subaerial pyroclastic flows and surges were emplaced in a shallow water area. Afterwards, at least 20 pyroclastic flows and surges were emplaced on the epiclastic deposits which filled the shallow water area. Secondary vesicles and accretionary lapilli are common in the ash matrix of pyroclastic surge deposits, implying the surges were wet, generated by phreatomagmatic eruptions. Essential fragments of the pyroclasticflow deposits retain internally consistent one magnetization vector stable even at temperatures higher than about 500℃. The pyroclastic flow deposits have been supposed to have been emplaced in a subaqueous setting, but their emplacement at high temperatures in association with pyroclastic surge deposits is to be subaerial.

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