Strontium isotopic age of the Torinosu Limestone in Niyodo Village, Kochi Prefecture, SW Japan.

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  • Shiraishi Fumito
    Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University Present address: Department of Geobiology, Geoscience Center, University Göttingen
  • Hayasaka Yasutaka
    Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University
  • Takahashi Yoshio
    Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University
  • Tanimizu Masaharu
    Institute for Frontier Research on Earth Evolution, Japan Marine-Earth Science and Technology Center
  • Ishikawa Tsuyoshi
    Institute of Geosciences, Shizuoka University
  • Matsuoka Jun
    Center for Advanced Marine Core Research, Kochi University
  • Murayama Masafumi
    Center for Advanced Marine Core Research, Kochi University
  • Kano Akihiro
    Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University

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Other Title
  • 高知県仁淀村に分布する鳥巣石灰岩のストロンチウム同位体年代
  • コウチケン ニヨドムラ ニ ブンプ スル トリノス セッカイガン ノ ストロンチウム ドウイタイ ネンダイ

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Abstract

Strontium isotopic ratio provides high-resolution ages of the limestone of Yatsuji Formation (Torinosu Group) in Niyodo Village (Kochi Prefecture), on the basis of considering diagenetic effects. Results of cathodoluminescence and trace elements analysis revealed that brachiopod shell is retaining the original marine strontium isotopic value. Stromatoporoids and a calcified sponge Chatetopsis sp. tend to record higher values than the brachiopod shells collected from the same horizon. Their differences are attain to +0.00005, and which corresponds to 2.8 m.y. during late Jurassic to early Cretaceous periods. Therefore, we concluded that stromatoporoids and Chatetopsis sp. are not suitable for the precise age determination. Obtained strontium isotope data of brachiopod shells are converted to depositional age by fitting to recently proposed strontium isotopic curve (LOWESS Look-up Table Version 4). The depositional age of the limestone body of the lower Yatsuji Formation is 146.1-148.4 Ma (middle Tithonian). This age is consistent with microfossil ages, and has higher time resolution.<br>

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