Flood-plain Deposits and Fossil Assemblages of the Middle Pleistocene Kiyokawa Formation, Shimosa Group, Eastern Japan

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Sedimentary processes accumulating and preserving terrestrial vertebrate fossil assemblages in a flood plain were examined from the middle Pleistocene Kiyokawa Formation in Chiba Prefecture, eastern Japan.The flood-plain deposit is divided into three depositional units by lithology and fossil assemblages: A, B, and C Units. Unit A consists of massive mud with many plants remains from wash load by flooding. Unit B, which is composed of ill-sorted muddy sands or sandy muds, contains numerous wood debris, isolated terrestrial vertebrate remains, freshwater molluscan shells, and mudstone clasts. It may have been accumulated by a mudflow accompanying flooding. These fossil assemblages also indicate that these sediments come from a larger area than the stream channel. Unit C consists of massive muds and alternating layers of silts and fine-grained sands with in-situ freshwater bivalves and turtle remains. These fossils indicate that Unit C is a deposit of still water as a pond or shallow lake environment.