09:30 〜 09:45
[SCG51-09] 放射年代と層序に基づいた富山堆積盆に分布する中新統南砺・八尾層群の精密な年代層序
キーワード：層序学、年代学、富山堆積盆、中新世
Recent developments on radiometric dating for the last several decades provide new and/or robust views to Hadean to Quaternary chronostratigraphy. In contrast, stratigraphy including lithology, paleontology, and paleomagnetism gives the context of geological events. Although recent geological studies tend to focus on radiometric dates, stratigraphy is also able to constrain geological ages of rocks. Considered these situations, lithostratigraphy and zircon U-Pb dates for the Miocene Nanto and Yatsuo groups in the Toyama basin, SW Japan, were examined in this study. In this presentation, we discuss their more refined chronostratigraphy, comparing among radiometric dates, lithostratigraphy, biostratigraphy, and magnetostratigraphy.
The Lower to Middle Miocene Nanto and Yatsuo groups were deposited and formed during the main stage (ca. 23-15 Ma) of the back-arc spreading in the Japan Sea [1]. Their stratigraphic subdivisions shown in the attached figure are based on geological survey and literature review [2]. Additionally, 22.8 ± 0.2, 23.5 ± 0.7, 17.1 ± 0.4, and 17.0 ± 0.2 Ma (±2σ) of zircon 238U-206Pb weighted mean dates were obtained from the Johana (welded tuff), Nirehara (rhyolite gravel), Iwaine (pumiceous lapilli tuff), and Iozen formations (rhyolitic lava) in this study, using a quadrupole ICP-MS and a multiple collector ICP-MS coupled with laser ablation systems (LA-ICP-MS).
Obtained lithostratigraphy and zircon U-Pb dates were compared to radiometric dates (zircon U-Pb, zircon fission-track, whole-rock K-Ar, and calcite Rb-Sr dating) and chronostratigraphies (magnetostratigraphy and microfossil biostratigraphy) by previous studies (e.g., [2] - [7]). Depositional and emplacement ages of the target strata can be summarized as following stages on the basis of those results: (1) the Nanto stage (23.0-22.3 Ma), (2) the Iwaine stage (17.5-17.3 Ma) after formation of an unconformity (22.3-17.5 Ma), (3) the Iozen-Kurosedani stage (17.3-16.6 Ma), and (4) the Higashibessho-Fukuhira stage (16.6-15.2 Ma). Of course, formation age of each formation was estimated more narrowly (formation ages of several formations could be extrapolated in 0.01-0.001 Myr scales). These ages were constrained much narrowly than recent works [2, 6]. Moreover, these results propose that combination between developing radiometric dating techniques and detailed stratigraphic data enables us to discuss highly resolved chronostratigraphy and formation processes of Miocene sedimentary basins in 0.1-0.001 Myr scales.
This study was funded by the Ministry of Economy, Trade and Industry, Japan as part of its R&D supporting program titled “Establishment of Technology for Comprehensive Evaluation of the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (JPJ007597) (Fiscal Years 2023).”
References
[1] Yamada et al. (2023) 130th Ann. Meet. Geol. Soc. Japan, T15-P-19.
[2] Yamada and Takahashi (2021) Jour. Geol. Soc. Japan, 127, 507–525.
[3] Yanagisawa (1999) Bull. Geol. Surv. Japan, 50, 67–81.
[4] Tamaki et al. (2006) Bull. Geol. Surv. Japan, 57, 73–88.
[5] Takeuchi et al. (2017) Geology of the Tomari District. Quadrangle Ser, 1:50000, Geol. Surv. Japan, AIST.
[6] Nakajima et al. (2019) Jour. Geol. Soc. Japan, 125, 483–516.
[7] Yoshida et al. (2019) Sci. Rep., 9, 1003.
The Lower to Middle Miocene Nanto and Yatsuo groups were deposited and formed during the main stage (ca. 23-15 Ma) of the back-arc spreading in the Japan Sea [1]. Their stratigraphic subdivisions shown in the attached figure are based on geological survey and literature review [2]. Additionally, 22.8 ± 0.2, 23.5 ± 0.7, 17.1 ± 0.4, and 17.0 ± 0.2 Ma (±2σ) of zircon 238U-206Pb weighted mean dates were obtained from the Johana (welded tuff), Nirehara (rhyolite gravel), Iwaine (pumiceous lapilli tuff), and Iozen formations (rhyolitic lava) in this study, using a quadrupole ICP-MS and a multiple collector ICP-MS coupled with laser ablation systems (LA-ICP-MS).
Obtained lithostratigraphy and zircon U-Pb dates were compared to radiometric dates (zircon U-Pb, zircon fission-track, whole-rock K-Ar, and calcite Rb-Sr dating) and chronostratigraphies (magnetostratigraphy and microfossil biostratigraphy) by previous studies (e.g., [2] - [7]). Depositional and emplacement ages of the target strata can be summarized as following stages on the basis of those results: (1) the Nanto stage (23.0-22.3 Ma), (2) the Iwaine stage (17.5-17.3 Ma) after formation of an unconformity (22.3-17.5 Ma), (3) the Iozen-Kurosedani stage (17.3-16.6 Ma), and (4) the Higashibessho-Fukuhira stage (16.6-15.2 Ma). Of course, formation age of each formation was estimated more narrowly (formation ages of several formations could be extrapolated in 0.01-0.001 Myr scales). These ages were constrained much narrowly than recent works [2, 6]. Moreover, these results propose that combination between developing radiometric dating techniques and detailed stratigraphic data enables us to discuss highly resolved chronostratigraphy and formation processes of Miocene sedimentary basins in 0.1-0.001 Myr scales.
This study was funded by the Ministry of Economy, Trade and Industry, Japan as part of its R&D supporting program titled “Establishment of Technology for Comprehensive Evaluation of the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (JPJ007597) (Fiscal Years 2023).”
References
[1] Yamada et al. (2023) 130th Ann. Meet. Geol. Soc. Japan, T15-P-19.
[2] Yamada and Takahashi (2021) Jour. Geol. Soc. Japan, 127, 507–525.
[3] Yanagisawa (1999) Bull. Geol. Surv. Japan, 50, 67–81.
[4] Tamaki et al. (2006) Bull. Geol. Surv. Japan, 57, 73–88.
[5] Takeuchi et al. (2017) Geology of the Tomari District. Quadrangle Ser, 1:50000, Geol. Surv. Japan, AIST.
[6] Nakajima et al. (2019) Jour. Geol. Soc. Japan, 125, 483–516.
[7] Yoshida et al. (2019) Sci. Rep., 9, 1003.