In situ fossil chemosynthetic community found from the lower Pleistocene Kazusa Group, Yokohama City, central Japan

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  • Majima Ryuichi
    Geological Institute, Faculty of Education, Yokohama National University
  • Tate Yukiko
    Geological Institute, Faculty of Education, Yokohama National University
  • Shibasaki Takuji
    Geological Institute, Faculty of Education, Yokohama National University

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Other Title
  • 横浜市の上総層群から発見された現地性化学合成貝化石群集
  • ヨコハマシ ノ カズサソウグン カラ ハッケンサレタ ゲンチセイ カガク ゴウ

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Description

A fossil methane seep community is preserved in marine strata of the lower Pleistocene Kazusa Group, Yokohama City, central Japan. This community occurs in a 9m-high and 9m-width outcrop, and consists of lower (4.5m in maximum height, 9m in width) and upper (3m in maximum height and 6m in width) lenticular layers within the silty sand matrix. Each layer abundantly yields large-size (about 10cm in shell length), articulated bivalves, such as Lucinoma, Conchocele, and Acharax, all of which have been considered to have symbiotic, chemoautotrophic bacteria in their gills. Both the layers are cemented by carbonates characterized by very light carbon (δ^<13>C=-55.06‰ (PDB)). The molluscan fossils associated with the shell layers show a paleo-bathymetry ranging from 100m to 200m in depth. Above observations evidently indicate that these fossil layers are an in situ chemosynthetic community in the ancient methane cold-seep site. The mode of occurrence of the two layers probably records a fluctuated history of upwelling fluid including methane seeped from unknown subsurface sources.

Journal

  • Fossils

    Fossils 61 (0), 47-54, 1996

    Palaeontological Society of Japan

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