Abstract
Sedimentological and paleontological study was carried out on the Plio-Pleistocene Tokai Group, from the lower part of the Kuragari Formation to lower part of the Oizumi Formation, at Chikarao in Tado-cho, Mie Prefecture, Central Japan. Six sedimentary facies, namely A, B, C, D, E and F, are recognized based on the sedimentary logs (scale 1/100) with 210m thickness along the main route. These sedimentary facies associations indicate the gravelly and sandy meandering (partly braided) river systems associated with backmarsh and backswamp on the ancient floodplain. The analysis of pollen fossils carried out for 11 samples through the same route. The vegetation can be interpreted as mixing forests of fallen broad-leaved trees and the coniferous trees on an floodplain. While, the cold climate indicators of fossil Menyanthes found from a peaty silt layer just above the Karegawa volcanic ash layer. The analysis of diatom fossils for 15 samples collected along the main route. The diatoms of 28 genera (101 species) were identified, and these assemblage data suggest environments of limno-philous cool water, shallow swamp and bog inhabiting aquatic plants. 183 beetle fossils consist of 6 families found in the peaty silt layer just above the Karegawa volcanic ash layer. These beetle fossils suggest mainly the marshy and shallow pond condition, including subarctic and temperate species such as Ilybius cf. poppiusi, Plateumaris constricticollis constricticollis, and Apotomopterus maacki. Therefore, the peaty silt layer just above the Karegawa volcanic ash layer was deposited during the cold period. The climate of that time was colder than that in the present. Synthesizing the analytical results of the beetle, pollen and diatom fossils, the shifting period to the colder climate was located above the Karegawa volcanic ash layer, situated near the Plio-Pleistocene boundary about 1.75 Ma.
