White clam community discovered from fractured claystone of the Miocene Hayama Group, Miura Peninsula, south-central Japan

Kanie, Y.; Asami, S.; Okada, H.; Watanabe, M.

Science Report of the Yokosuka City Museum 1992(40): 31-35


Accession: 009734669

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Colony of the giant white clam, Calyptogena, is the most prominent feature of a benthic biotic community inhabiting in a unique environment of plate boundaries. A community of Calyptogena was discovered from the upper part of the Hayama Group at Ikegami of Yokosuka City in central Miura Peninsula which is correlatable to CN4 nannofossil zone (16.2-14.4 Ma). The community occurs in fractured limestones and surrounding black claystones distributing along a fault zone which extends along NW-SSE-trend. Almost all fossil shells are fragments made of aragonite. Main components of the community are Solemya tokunagai, Calyptogena spp. and tube worms. S. tokunagai is the most abundant and occurs as clusters in the claystone, whereas isolated specimens of Calyptogena spp. occurs exclusively in the brecciated limestones. Clusters of tube-worms are observed between these two tithofacies. Because of the abundant occurrences of S. tokunagai, paleodepth of the community is likely greater than 1000-1500 m where the extant colony of C. soyoae is observed in Sagami Bay. The fault zone observed here is regarded as a shared zone associated with a plate boundary. The community had thrived by taking nutrients from methane-bearing seepage and hydrosulphide ion produced by methane-reducing bacteria. Biochemical bacteria is responsible for the calcification of some part of the fine bottom sediment and breccia.