Introduction

The INQUA Subcommission 1-d on the Pliocene–Pleistocene boundary organized the working group on the Neogene–Quaternary boundary as the action body for Project 41 of the International Geological Correlation Program (IGCP) in 1974. As a research and contact body corresponding to the INQUA subcommission, the Japanese National Committee for Quaternary Research in the Science Council of Japan organized the Japanese National Subcommission on the Pliocene–Pleistocene Boundary, chaired by M. Itihara. The Japanese national subcommission has functioned simultaneously as the Japanese national working group for IGCP-41 for investigations of the boundary problem in Japan.

In August 1982, the Japanese national working group for IGCP-41 presented its third report at the XI INQUA Congress in Moscow (Itihara and Kuwano, 1982). That report, however, covered too many areas (Figure 24.1) to be included in this final volume. Therefore, for this purpose, it was decided to select two representative studies: this chapter, on the Pliocene–Pleistocene boundary in the Osaka Group, as a typical section of lacustrine and fluvial sediments with marine intercalations, and Chapter 25, on the Plio–Pleistocene stratigraphy in the Boso Peninsula, as a typical section of marine sediments.

Background and stratigraphic framework

Deposits of Pliocene and Pleistocene age are well exposed in Japan (Figure 24.1). In the Osaka Group in the Kinki district of central southern Honshu, strata that correlate well to the interval containing the Pliocene–Pleistocene boundary have been studied or discussed by Itihara (1961), Ishida et al. (1969), Itihara and Kamei (1970, 1982), Itihara et al. (1973, 1975, 1984), and Maenaka et al. (1977). This chapter describes the present status of the problem of the Pliocene–Pleistocene boundary in this well-described sequence.