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

Abstract

We measured the uranium-lead (U-Pb) and fission-track (FT) ages of single zircon grains from three samples (TKB , TKB , and TKB ) 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 .. Ma ( σ) and .. Ma ( σ) for TKB , and .. Ma ( σ) and .. Ma ( σ) for TKB , respectively. Their U-Pb and FT ages overlap within their respective σ uncertainties, which suggests that their ages correspond to the eruption ages. Specifically, the TKB 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 TKB are reset to their U-Pb ages or younger. The pooled FT age for all of the analyzed zircons ( .. Ma; σ) and weighted mean U-Pb age for zircon grains within the youngest age range ( .. Ma; σ) are coincident within their respective σ uncertainties. Therefore, their ages could also show the eruption age. TKB and TKB are from the Quaternary Sarabetsu Formation, and their U-Pb and FT ages correspond to the stratigraphy. Although TKB 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 TKB , which is located in the easternmost part of the study area, is consistent with this stratigraphic trend.

U–Pb and fission‐track dating of Miocene hydrocarbon source rocks in the Akita Basin, Northeast Japan, and implications for the timing of paleoceanographic changes in the sea of Japan

“…Of the 28 concordant grains, 17 grains with ages of 10.56-8.87 Ma were selected as the youngest cluster particles, resulting in a weighted mean age of 9.59 ± 0.16 Ma (2σ). (Iwano et al, 2012;Nakajima et al, 2019;Niwa et al, 2020). We obtained a U-Pb age of 11.55 ± 0.22 Ma (2σ) and an FT age of 12.3 ± 1.4 Ma (2σ) for sample UG-1.…”

“…250°C; Iwano et al, 2012), thus representing more reliable eruption ages. If fission‐track and U–Pb ages coincide well with each other, dated ages may reliably indicate the eruption ages of volcanic and volcaniclastic rocks (Iwano et al, 2012;Nakajima et al, 2019; Niwa et al, 2020). We obtained a U–Pb age of 11.55 ± 0.22 Ma (2 σ ) and an FT age of 12.3 ± 1.4 Ma (2 σ ) for sample UG‐1.…”

“…The fission tracks of the zircon grains of samples UG‐1 and UG‐3 were interpreted to be totally thermally reset. For these reasons, both the U–Pb and FT ages of samples UG‐1 and UG‐3 are reliable ages representing eruption ages of tephra (Nakajima et al, 2019; Niwa et al, 2020).…”

Imaging of a serpentinite complex in the Kamuikotan Zone, northern Japan, from magnetotelluric soundings

“…Previous geological studies have identified faults at the boundaries between the Shirikomadake serpentinite complex and surrounding sedimentary rocks (Igi 1959;Katoh et al 1979;Oka 1985). The early Pleistocene Sarabetsu Formation near the center of the study area (Oka and Igarashi 1993;Niwa et al 2020) contacts with the serpentinite complex by fault and is vertically inclined near the contact, indicating that uplift of the complex occurred during the Pleistocene or later. In addition, tectonic fault blocks within the serpentinite complex contain Cretaceous sedimentary rocks (Katoh et al 1979), suggesting that Cretaceous rocks from the surrounding area were dragged into the serpentinite complex during the Quaternary (or earlier) uplift.…”