高知県佐川地域の七良谷層から最上部ジュラ系アンモノイドの産出  [in Japanese] Occurrence of the uppermost Jurassic ammonoids from the Naradani Formation in Sakawa area, Kochi, Southwest Japan  [in Japanese]

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Abstract

高知県佐川地域に分布する七良谷層の模式層序周辺の泥質砂岩中から,最上部ジュラ系を示す2種類のアンモノイド化石を発見した.そのうち<i>Aspidoceras</i>属は,テチス海地域の最上部ジュラ系から多産し,<i>Hybonoticeras</i>属は同地域のキンメリッジアン−チトニアン階境界付近を示準するタクサである.これらの化石の産出により,七良谷層は最上部ジュラ系(キンメリッジアン−チトニアン階)に対比される可能性が高い.この結論は放散虫化石層序とおおむね調和的である.これまで七良谷層は,上部ジュラ系−下部白亜系鳥巣層群の層序的下位にあたる地層と考えられてきた.しかし七良谷層から産出したアンモノイドの示す時代は,鳥巣層群産アンモノイドのレンジと明らかに重複し,アンモノイド化石からは両岩相層序ユニットの時代差は識別できない.したがって,今後,七良谷層と鳥巣層群の層序関係を再検討する必要がある.

The Naradani Formation is located to the south of the Torinosu Group in the Sakawa district of Kochi, southwest Japan, and consists of a linear, narrowly distributed series of marine clastic deposits that contain allochthonous limestone blocks. This formation was previously correlated to the Bajocian–Bathonian Middle Jurassic using brachiopods extracted from the limestone blocks, with later radiolarian correlations suggesting an Oxfordian–Kimmeridgian Upper Jurassic age. The formation has also been interpreted to underlie the Upper Jurassic–Lower Cretaceous Torinosu Group. However, the discovery of two stage-diagnostic aspidoceratid ammonoids, <i>Aspidoceras</i> sp. and <i>Hybonoticeras</i> sp., from muddy sandstones in the stratotype area of the Naradani Formation casts doubt on these previous interpretations. <i>Aspidoceras</i> was widespread during the latest Jurassic, and the presence of <i>Hybonoticeras</i> limits this short stratigraphic interval to the Kimmeridgian–Tithonian boundary in various Tethyan sections. This clearly suggests a latest Jurassic age for the Naradani Formation and generally supports the radiolarian biostratigraphy age-assignment. However, the ranges of ammonoids from the Naradani Formation overlap those from the Torinosu Group, meaning that it is not possible to determine an age-difference between these two lithostratigraphic units. Further research is needed to test other hypotheses, for example to determine whether heterotopic facies relationships exist between the Naradani Formation and the Torinosu Group.

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 118(11), 741-747, 2012

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130003363881
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • ISSN
    0016-7630
  • NDL Article ID
    024131291
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  J-STAGE 
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