The Middle Pleistocene Fukuchiyama Formation is extensively distributed along the Yura River and its tributaries in the Fukuchiyama Basin, in northern Kinki District, western Japan. The Upper Member of the Fukuchiyama Formation intercalates a volcanic ash bed mainly originated from the Aso-1 tephra layer that erupted between 0.25 and 0.27 Ma. A pollen analytical study was conducted about clayey and peat beds immediately below the ash, with the identification of insect fossils from the peat as Plateumaris constricticollis (Jacoby), to reconstruct the paleoenvironment during the deposition of the member. Paleoenvironment at the studied site changed with the upward decrease in fluvial influence, from riverside and lowland Cryptomeria japonica and Alnus forest into a mosaic landscape of the Alnus and Picea lowland forest, wet grassland, and marshy vegetation. This vegetation change also indicates the upward cooling before the deposition of the volcanic ash (Aso-1) corresponding to the cold age in the Marine Isotope Stage 8 (MIS8) and suggests the occurrence of the warm period correlative to the MIS 9 within the Upper Member of the Fukuchiyama Formation.
