ABSTRACT
On the basis of previously established sedimentary facies analysis, the fossil composition and taphonomic features of the benthic fossil assemblages of the late Pleistocene Paleo-Tokyo Bay deposits (Kioroshi Formation) were analyzed. The bay deposits consist of various distinct sedimentary systems, including ocean beach open to the Pacific, beach in the bay, bay lagoon, flood-tidal delta and tidal inlet. The last three were found only in Paleo-Tokyo Bay, and not in modern Tokyo Bay and nearby coastal sea. These environments were formed associated with the inferred barrier islands in the eastern rim of the bay. The tidal inlet fill deposit contains dense shellbeds consisting mainly of the venerid bivalve Ruditapes philippinarum. Some of the individuals show evidence for upward escape during rapid sedimentation. In the west of the tidal inlet occurred a flood-tidal delta built into the bay. An inshore to offshore arrangement of molluscan fossil assemblages are preserved as a progradational sequence of the delta. Chion kiusiuensis and Gomphina neastartoides are common in the upper delta front, and higher-diversity molluscan assemblages occurred in the lower delta front environment. Particularly, Mactra chinensis were abundant on the physically unstable middle delta front environment. Also, a small colony of the deep-burrowing mactrid Raeta pellicula was found in the same environment. Mactrids, such as Pseudocardium sachalinensis and Tresus keenae, were also common in the bay shoreface environment in older Paleo-Tokyo Bay deposits. Such abundance of the mactrid bivalve is characteristic of the benthic associations in the Paleo- Tokyo Bay.
