The vent breccia including amoeboid volcanic clasts along the Cape Kakuda shoreline, Niigata City

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  • 新潟市角田岬海岸のアメーバ状火山岩塊を含む火道角礫岩
  • ニイガタシ カクダミサキ カイガン ノ アメーバジョウ カザン ガンカイ オ フクム カドウカクレキガン

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Basaltic andesite hyaloclastites of the Miocene Teradomari stage are distributed along the Cape Kakuda shoreline, southwest of Niigata City. Polymictic volcanic breccia consisting of andesitic basalt clasts and various kinds of andesite clasts form two perpendicular columns about 30m in diameter. As the pyroclastic layer around Cape Kakuda dips 20°-30°, the shape of the polymictic volcanic breccia suggests a volcanic vent. The polymictic volcanic breccia is clast-supported, mainly composed of boulders over 30cm in diameter. This includes 20-50cm rounded or subrounded gravel, basaltic amoeboid clasts, and 5×2m lapilli tuff blocks containing red scoria. The majority the basaltic amoeboid clasts exceed 30cm, with some of them reaching 1m in size. They are surrounded by a glassy chilled margin, but the inside of individual clasts is brecciated into angular fragments about < 5 cm in size. This suggests quenching fragmentation in water, in which amoeboid clasts were crushed into angular fragments while keeping their external forms, from which it can be interpreted that the vent filled with water. Many dykes intrude the polymictic volcanic breccia. These dykes have the same lithologic character as the amoeboid clasts, therefore it is certain the amoeboid clasts are from these dykes. This rock, which is formed by the mingling of wet sediment and fragments of the host magma, is known as peperite. The presence of rounded gravels suggests that the crater formed in shallow water near a beach. Furthermore, the presence of the red scoria within the lapilli tuff block suggests that the blocks were in an aerial environment and that a portion of the crater was on land. Speculating on the eruption style in such an environment, it can be surmised that this was a Surtseyan eruption, as indicated by Kokelaar (1983).

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