Geology and detrital zircon U-Pb age of the Cretaceous Mugitani Formation in the Shimanto Belt, central Kii Peninsula, Southwest Japan

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  • Shimura Yusuke
    Department of Geology, Graduate School of Science and Technology, Shinshu University
  • Tokiwa Tetsuya
    Faculty of Science, Shinshu University
  • Takeuchi Makoto
    Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University
  • Yamamoto Koshi
    Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, Nagoya University

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  • 紀伊半島中央部に分布する四万十帯白亜系麦谷層の地質と砕屑性ジルコンU-Pb年代
  • キイ ハントウ チュウオウブ ニ ブンプ スル ヨンマンジュウタイ ハクアケイムギダニソウ ノ チシツ ト サイクズセイ ジルコン U-Pb ネンダイ

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Abstract

<p>We have investigated the Mugitani Formation located in the structurally upper part of the Cretaceous accretionary complex in the Shimanto Belt, central Kii Peninsula. The Mugitani Formation is newly divided into two units: the Izuo Unit and the Sebato Unit. The Izuo Unit is composed mainly of mudstone, broken beds of sandstone and mudstone, and mixed rock with minor amounts of sandstone and chert. The Sebato Unit is composed mainly of mudstone with minor amounts of broken beds of sandstone and mudstone, basalt, and mixed rock. The bedding planes generally strike WNW-ESE and dip moderately northward. In addition, we also carried out U-Pb dating of detrital zircons separated from three sandstones to clarify the depositional age of the Mugitani Formation. The youngest zircon grain ages and/or the youngest cluster ages indicate ca. 100-110 Ma, presenting latest Early Cretaceous (Albian). Furthermore, we found no Late Cretaceous zircon grains that are known to have been produced by the intensive igneous activity in Southwest Japan. Based on the youngest zircon grain ages, the youngest cluster ages, and age compositions, the depositional age of the Mugitani Formation corresponds to that of the Yukawa Formation, ranging from Albian to Cenomanian.</p>

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