島根県浜田市北部の中部中新統唐鐘累層畳ヶ浦砂岩部層に見られる特異な貝殻密集産状の古生物学・生痕学的解釈  [in Japanese] Paleontologic and ichnologic interpretation for a peculiar mode of occurrence of shell concentrations in the lower part of the Tatamigaura Sandstone Member of the Middle Miocene Tougane Formation, Hamada, western part of Shimane, Japan  [in Japanese]

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Abstract

島根県浜田市の中部中新統唐鐘累層畳ヶ浦砂岩部層下部で見られた皿状およびレンズ状の貝殻密集部の形成メカニズムと形成プロセスを, 生痕学的ならびに古生物学的観点から検討した. 明瞭なポットホール状の形態が識別できないことを別にすれば, これらの貝殻密集部の形態と内部構造の特徴は共産する生痕化石<i>Piscichnus waitemata</i> の充填物として見られる化石密集部のそれに酷似する. <i>Piscichnus waitemata</i> は一部エイ類の摂食活動によって形成された構造物と考えられており, 皿状およびレンズ状の貝殻密集部もエイ類の摂食活動による生物攪拌起源である可能性が極めて高い. 皿状およびレンズ状の貝殻密集部が母岩中に孤立して産出する理由として, (1) <i>P. waitemata</i> の充填物と母岩の質的および粒度の差異が極めて少ないこと, (2) 後から形成された<i>P. waitemata</i> による破壊や上書き, そして内在型底生動物による生物攪拌作用などによって, <i>P. waitemata</i> の上部の形態情報が消去されてしまったこと, などが考えられる.

A production mechanism and process of the dish- and lens-shaped shell concentrations observed in the lower part of Tatamigaura Sandstone Member of the Middle Miocene Tougane Formation distributed in the northeastern part of Hamada City, Shimane, Japan were examined from the ichnologic and paleontologic viewpoints. The dish- and lens-shaped shell concentrations treated herein are visible on the bedding plane as the circular patches having 7 cm to 30 cm in diameter. In cross section, on the other hand, they are observed as an isolated, concave-up crescent or lens, of which thickness ranges from 1 cm to 5 cm. These shell concentrations consist mainly of graded, disarticulated bivalves, most of which show convex-down orientation, with less number of gastropods.<br>In addition to these shell-concentrations, well-preserved specimens of pothole-like trace fossil <i>Piscichnus waitemata</i>, which is interpreted to be the products of feeding behavior of rays, occur below the shell concentrations-bearing interval. The Tatamigaura <i>P. waitemata</i> occurring in fine-grained sandstone can be seen an isolated structure filled with graded, fragmented barnacles derived from the overlying coquina bed and less number of shells of host sediment origin. Morphology, size, and internal fabric of lower part of the <i>Piscichnus</i> are closely similar to those of the dish- and lens-shaped shell concentrations. These lines of evidence strongly suggest that the dish- and lens-shaped shell concentrations appear to have formed by biogenic currents by the feeding behavior of rays. Invisibility of the whole morphology of <i>P. waitemata</i> associated with dish- or lens-shaped shell concentrations is due to (1) extremely low contrast in grain size between the fill and the host sediment and (2) selective obliteration of the morphologic record at the middle to upper parts of <i>P. waitemata</i> in terms of overprinting by the younger one and/or subsequently occurred.

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 110(12), 733-745, 2004-12-15

    The Geological Society of Japan

References:  64

Cited by:  1

Codes

  • NII Article ID (NAID)
    110003024622
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • Article Type
    Journal Article
  • ISSN
    00167630
  • NDL Article ID
    7195347
  • NDL Source Classification
    ZM49(科学技術--地球科学--地質)
  • NDL Call No.
    Z15-174
  • Data Source
    CJP  CJPref  NDL  NII-ELS  J-STAGE 
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