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

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(4) . Holocene Pyroclastic Fall Deposits along the Pacific Coastal Region of Eastern Hokkaido Ryuta FURUKAWA῍, Futoshi NANAYAMA῍ 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 +2, sampling sites from +* 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 - 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-c, (Ko-c,) 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. Key words : pyroclastic fall deposit, Hokkaido, Holocene, Tarumai volcano, Hokkaido-Komagatake volcano. +ῌ ῐ ῎ ῑ ῏. ( ij$MKkl*-m!../nop.

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(125) 362. ῍ῐῒῑῌ῏῎ ῑ. Fig. 1. Variation diagrams for TiO,-K,O contents of glass shards by microprobe analyses. Each symbol shows mean and deviation in normalized wt.ῌ. Fields of source-known volcanic glass shards by Katsui et al. (+312), Furuta et al. (+320), Tokui (+33-) and Fukusawa et al. (+332) are shaded in upper-left plot. A-Tn is a working standard sample analyzed in each analysis..

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(301) Table A+. (continued). 371.

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