Lithofacies and fossil assemblages of the Upper Cretaceous Sada Limestone, Shimanto City, Kochi Prefecture, Shikoku, Japan

DOI IR Web Site 72 References
  • Nobuhara Takami
    Geological Institute, Faculty of Education, Shizuoka University
  • Onda Daigaku
    Department of Environment and Natural Sciences, Graduation School of Environment and Information Sciences, Yokohama National University
  • Kikuchi Naoki
    Department of Natural Environmental Science, Faculty of Science, Kochi University
  • Kondo Yasuo
    Department of Natural Environmental Science, Faculty of Science, Kochi University
  • Matsubara Kei
    Department of Natural Environmental Science, Faculty of Science, Kochi University
  • Amano Kazutaka
    Department of Geoscience, Joetsu University of Education
  • Jenkins Robert G.
    Faculty of Education and Human Sciences, Yokohama National University
  • Hikida Yoshinori
    Nakagawa Museum of Natural History
  • Majima Ryuichi
    Faculty of Education and Human Sciences, Yokohama National University

Bibliographic Information

Other Title
  • 高知県四万十市の上部白亜系佐田石灰岩の岩相と化石群集
  • コウチケン シマントシ ノ ジョウブ ハクアケイ サダ セッカイガン ノ ガンソウ ト カセキ グンシュウ

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Abstract

The Sada Limestone in the Upper Cretaceous Nakamura Formation, Shimanto Belt, Shikoku, Japan is known to yield abundant "Thyasira" and "Serpula" fossils. However, the whole assemblage has never been studied to an appropriate depth. We present a preliminary report on the mode of occurrence, lithology, and faunal composition of the Sada Limestone followed by discussion on its origin. The limestone crops out at more than 10 localities in an elliptical area (250m in NE-SW, 400m in NW-SE direction) surrounded by Upper Cretaceous siltstone. The boundary between the limestone and the siltstone is gradational, which suggests that the limestone was autochthonous. The Sada Limestone consists of three lithofacies: 1) banded limestone (detrital micrite with parallel calcite-sheets, showing inverted stromatactoid structure); 2) "Thyasira"-"Serpula" limestone (detrital micrite containing abundant autochthonous "Thyasira" and "Serpula"); and 3) sparsely-fossiliferous massive limestone, composed mainly of recrystallized sparite. The three lithofacies alternate with each other with intervals of several meters. The "Thyasira"-"Serpula" assemblage is associated with other chemosynthetic bivalves, such as Myrtea? sp. and Solemya sp. "Thyasira" shells are large (8cm of maximum shell length) and conjoined. The shells are densely packed in the limestone, whereas in the surrounding siltstone molluscan fosssils are rare. The taxonomic composition, mode of occurrence, and lithological characteristics such as inverted stromatactoid structure suggest that the Sada Limestone is seep-related and the fauna represents a chemosynthetic ecosystem similar to many other thyasirid-rich carbonates worldwide. However, to verify the hypothesis requires further biogeochemical approaches such as stable isotope and biomarker analyses.

Journal

  • Fossils

    Fossils 84 (0), 47-60, 2008

    Palaeontological Society of Japan

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