A Study on the Pollen Stratigraphy of the Osaka Group, Pliocene-Pleistocene Deposits in the Osaka Basin

Abstract

From the view point of the pollen stratigraphy, the Osaka group, the typical Pliocene-Pleistocene deposits distributed in the Kinki district, Central Japan, is divided into Metasequoia Zone below and Fagus Zone above. Those divisions correspond with the stratigraphical units, that is, the Lower formation and the Upper formation of the Osaka group respectively. The former zone is characterized by the decrease of the Tertiary type tree pollen and the increase of the coniferous tree pollen which are present in Japanese Islands, and further it is subdivided into A-D subzones ascendingly. The marine beds of the latter are characterized by the high percentage of Fagus pollen, and a spectrum obtained from the fresh water bed shows the pollen composition consisting of cool-temperate to cold climate. Basing upon the compositional oscillation of the pollen diagram in regard to lithofacies changes, Fagus zone is subdivided into E-H subzones. As to the climatic changes, the first distinct cool phase is supposed to be at the boundary between B and C subzones. The climatic oscillations are assumed to occur once or twice in each subzone of which oscillation of F subzone is most distinct. During the advance from A to H subzones, the shift of the warm forest seems to have began from the mixed-forest composed of the Tertiary type coniferous trees and variegated broad-leaved trees (especially represented by high percentage of Quercus pollen) to the single tree forest such as Cryptomeria or Fagus forest. At the time of H subzone, the forest which is composed of evergreen Quercus accompanied with warm-temperate trees (Podocarpus, Paliurus, etc.) appeared in some parts, probably as the secondary forest. As to Fagus, it may be probable that the differentiation of warm-temperate and cool-temperate types took place at least at the time of G and H subzones.