Zircon U-Pb and fission-track ages for tephra interbedded in Neogene and Quaternary in Horonobe area, northern Hokkaido

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  • Niwa Masakazu
    Tono Geoscience Center, Japan Atomic Energy Agency
  • Amamiya Hiroki
    Tono Geoscience Center, Japan Atomic Energy Agency Present address; QJ Science Co., Ltd.
  • Yonaga Yusuke
    Tono Geoscience Center, Japan Atomic Energy Agency Present address; Chi-ken Sogo Consultants Co., Ltd.
  • Ogita Yasuhiro
    Tono Geoscience Center, Japan Atomic Energy Agency
  • Yasue Ken-ichi
    Tono Geoscience Center, Japan Atomic Energy Agency Department of Earth System Science, School of Sustainable Design, University of Toyama
  • Iwano Hideki
    Kyoto Fission-Track Co., Ltd.
  • Danhara Tohru
    Kyoto Fission-Track Co., Ltd.
  • Hirata Takafumi
    Geochemical Research Center, Graduate School of Science, The University of Tokyo

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Other Title
  • 北海道北部，幌延地域の新第三系～第四系に挟在するテフラのジルコンU-Pbおよびフィッション・トラック年代
  • ホッカイドウ ホクブ,ホロノベ チイキ ノ シン ダイサンケイ~ダイヨンケイ ニ キョウザイスル テフラ ノ ジルコン U-Pb オヨビ フィッション ・ トラック ネンダイ

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Abstract

<p>We measured the uranium-lead (U-Pb) and fission-track (FT) ages of single zircon grains from three samples (TKB3, TKB6, and TKB7) of tephras that are intercalated with Neogene to Quaternary deposits in the Horonobe area, Hokkaido, Japan, to understand their eruption ages. The weighted mean U-Pb ages and pooled FT ages of the youngest zircon grains are 1.43±0.18 Ma (2σ) and 1.14±0.40 Ma (1σ) for TKB3, and 7.55±0.14 Ma (2σ) and 6.62±0.43 Ma (1σ) for TKB6, respectively. Their U-Pb and FT ages overlap within their respective 2σ uncertainties, which suggests that their ages correspond to the eruption ages. Specifically, the TKB6 tephra age is consistent with the diatom fossil ages from the overlying and underlying silt deposits. The FT ages for most of the zircons in TKB7 are reset to their U-Pb ages or younger. The pooled FT age for all of the analyzed zircons (1.52±0.19 Ma; 1σ) and weighted mean U-Pb age for zircon grains within the youngest age range (1.34±0.16 Ma; 2σ) are coincident within their respective 2σ uncertainties. Therefore, their ages could also show the eruption age. TKB3 and TKB7 are from the Quaternary Sarabetsu Formation, and their U-Pb and FT ages correspond to the stratigraphy. Although TKB6 lies at the boundary between the Yuchi and Koetoi formations, its U-Pb and FT ages are significantly older than the reported depositional age of the Yuchi Formation. The Neogene to Quaternary depositional age of this horizon tends to be older from west to east in the Horonobe area. The older age of TKB6, which is located in the easternmost part of the study area, is consistent with this stratigraphic trend.</p>

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