鮮新統池子層のシロウリガイ類群集の種組成, 産状とタフォノミー

抄録

Little-disturbed fossil colonies of a deep-sea giant white clam, Calyptogena sp. 1 are found in a gigantic displaced block consisting of cemented shell beds, from the lower Pliocene Ikego Formation, on the northeast coast of Sagami Bay. In the dense bed of Calyptogena more than half of the individuals are conjoined, and most of them are embedded parallel to the bedding. This mode of occurrence, along with oxygen-isotope data, is interpreted as nearly in-situ, or only slightly dislocated from the habitat. Calyptogena is occasionally associated with Lucinoma spectabile, Conchocele disjuncta and Acharax sp. Also, bathyal gastropods, such as Neptunea and Buccinum species are present, suggesting a similar bathymetric and climatic regime to the modern Calyptogena colonies in Sagami Bay. Individual shell beds are very similar to the planar concretions widely found just beneath the modern Calyptogena colonies. Judging from the tectonic setting of the area where earthquake occurs repeatedly, the thick succession of such concretionary layers may be interpreted as representing recurrent rapid burial events triggered by earthquakes. Stratigraphic change in the areal extent of cemented layer and density of fossil occurrence, show a stepwise increase upwards. This may be an indication of cyclic activity of seepage in the area.