Holocene Pyroclastic Fall Deposits along the Pacific Coastal Region of Eastern Hokkaido

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  • FURUKAWA Ryuta
    Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology
  • NANAYAMA Futoshi
    Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology

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  • 北海道東部太平洋沿岸域における完新世の降下火砕堆積物
  • ホッカイドウ トウブ タイヘイヨウ エンガンイキ ニ オケル カンシン セイ ノ コウカ カサイ タイセキブツ

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Volcanic eruptions and tsunami events in Hokkaido are mainly attributed by the subducting Pacific plate. Marsh and lagoon deposits along the Pacific coast of eastern Hokkaido contain depositional records of the Holocene large-scale tsunamis and explosive volcanic eruptions. Coring and sampling using the Geoslicer technique at 182 sampling sites from 10 areas revealed thickness of widespread pyroclastic fall deposits and extent of the area covered by tsunami deposits. The pyroclastic fall deposits constitute mostly fine volcanic ash less than 3 centimeter thick and are well-preserved indicating calm depositional environments. Source volcanoes and ages of the pyroclastic fall deposits are identified by the phenocryst assemblage, shape and major element composition of glass shards, and stratigraphy. The pyroclastic fall deposits of Tarumai-a (Ta-a), Tarumai-c (Ta-c) and Hokkaido-Komagatake-c2 (Ko-c2) from southwestern Hokkaido, and Baitoushan-Tomakomai (B-Tm) from northern Korea/China are abundant throughout the study area. The Usu-b (Us-b) and Tarumai-b (Ta-b) pyroclastic fall deposits from southwestern Hokkaido can only be found in the southern part of the study area. Most of the pyroclastic fall deposits have more extensive distributions suggesting underestimated eruptive volumes by previous researches.

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