Radiolarian and diatom biostratigraphy of the Upper Miocene Yobito Formation near Lake Notoro, Hokkaido, Japan

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  • Motoyama Isao
    Department of Earth and Environmental Sciences, Yamagata University
  • Maruyama Toshiaki
    Department of Earth and Environmental Sciences, Yamagata University
  • Igarashi Mio
    Department of Earth and Environmental Sciences, Yamagata University
  • Eguchi Mizuki
    Department of Earth and Environmental Sciences, Yamagata University
  • Suto Megumi
    Department of Earth and Environmental Sciences, Yamagata University
  • Suzuki Masafumi
    Department of Earth and Environmental Sciences, Yamagata University

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  • 北海道能取湖付近の上部中新統呼人層の放散虫・珪藻化石層序
  • ホッカイドウ ノトロコ フキン ノ ジョウブ チュウ シントウコジンソウ ノ ホウサンチュウ ・ ケイソウ カセキソウ ジョ

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Abstract

<p>Radiolarian and diatom fossils recovered from diatomaceous sediments of the Yobito Formation in the northern region of Lake Notoro, eastern Hokkaido, enable the correlation of two stratigraphic sections, the West and East sections. Integrated radiolarian and diatom data suggest ages of 11.4-6.5 and 7.0-3.5 Ma for the West and East sections, respectively. This finding indicates that the diatomaceous sequence of the West Section is stratigraphically equivalent to the siliceous hard mudstone facies (Notoro Formation) and pyroclastic rocks (Abashiri Formation) that underlie the Yobito Formation of the East Section. This facies correlation between the Yobito and Notoro Formations probably reflects regional differences in the diagenetic history associated with silica phase transformations. Diatom data also reveal an unconformity within a sandstone layer in the diatomaceous sequence of the Yobito Formation of the West Section. This unconformity is represented by the absence of Neogene North Pacific Diatom zones (NPD) 5D and 6A. The occurrence in the interval above the unconformity of reworked radiolarian and diatom specimens of older (Miocene) age and gravel clasts of indurated mudstone suggest that the hinterland became exposed to an erosive environment at ~10 Ma.</p>

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