Stratigraphy and Chronology of the Lower-Middle Pleistocene in the Ozu and Uchiko Basins

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  • 愛媛県大洲・内子盆地に分布する下部‐中部更新統の層序と編年
  • 愛媛県大洲・内子盆地に分布する下部-中部更新統の層序と編年
  • エヒメケン オオズ ウチコ ボンチ ニ ブンプ スル カブ チュウブ コウシントウ ノ ソウジョ ト ヘンネン

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Abstract

Fluvial-fan sediments including volcanic ashes and plant fossils are thickly accumulated in the Ozu and Uchiko Basins along the Hiji River, Ehime Prefecture, Northwest Shikoku.<br>These sediments, distributed inside the basins, can be divided into three formations Tomisuyama, Uchiko, and Ozu Formations of the Lower-Middle Pleistocene, and terrace deposits (Furuta, Sugeta, and Hatsukaichi Formations) unconformably underlain by these formations.<br>In this study, the authors discovered newly volcanic ashes and plant fossils from these strata that are usable for chronological study. According to stratigraphical observations and features of volcanic ashes, the Tomisuyama 12 ash is correlated to the Pink ash (1.0 Ma) and the Kurochibo 3 ash is correlated to the Aso-1 ash (0.25 Ma). The Tomisuyama Formation and the lower part of the Uchiko Formation contain plant fossils such as Metasequoia glyptostroboides, and Picea cf. maximowiczii. In addition, it is clear that the Shobu ash was deposited at 0.6 Ma. These results indicate that the deposits of the Lower-Middle Pleistocene were formed at least at 1.4 Ma and continued till about 0.5 Ma. Those of the Middle Pleistocene were formed about 0.3-0.2 Ma.<br>The Lower-Middle Pleistocene in the Ozu and Uchiko basins is correlated to the strata that are distributed along the Median Tectonic Line, such as the Lower-Middle part of the Shobudani Formation, Upper-Uppermost part of the Gunchu Formation, Middle-Uppermost part of the Dochu Formation, and Lower-Upper part of the Osaka Group.

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