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.
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