Middle to Late Pleistocene Fossil Foraminiferal Assemblages and Paleoenvironmental Changes in the Kioroshi Formation, Shimosa Group, Omiya Upland, Central Japan

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vWe identified three fossil foraminiferal assemblages from the cored sediments (GS-KG-1) of the Middle to Upper Pleistocene Kioroshi Formation, Shimosa Group, at Kawaguchi, Saitama Prefecture, central Japan. The Kioroshi Formation in the study area is composed of lower muddy incised-valley fills and upper widely distributed, alternating beds of sand and mud. On the basis of the fossil foraminiferal assemblages, the formation is subdivided into three zones: Zones I and II in the lower part and Zone III in the upper part, in ascending order. Zone I is characterized by the Ammonia beccarii forma 1 assemblage, strongly indicating an inner bay environment. The Buccella frigida assemblage occurs in Zone II, and represents an inner to middle bay environment. The high abundance of B. frigida in Zone II suggests an influence of cold water. Zone III is characterized by the Miliolidae-Rosalina australis-Elphidium kusiroense assemblage, and seems to represent a middle to outer bay environment. The faunal succession in the cored sediments evidences development of a bay environment related to the late period of transgression from the Marine Isotope Stage 6 to Stage 5. In addition, our results show that the high abundance of B. frigida is a critical tool to distinguish the incised-valley fills of the Kioroshi Formation from the uppermost latest Pleistocene-Holocene incised-valley fills in Arakawa, Nakagawa, and Tokyo Lowlands.