Recent progresses in the Neogene bio- and chronostratigraphy in Japan have been achieved by the establishment of microplanktonic biostratigraphy in land-based marine sequences and recognizing of datum levels for the precise interregional correlation, within the framework of radiometric time scale. The microplanktonic time scale has been accomplished by incorporating various methods of radiometric dating, magnetostratigraphy, geomagnetic polarity time scale, and detailed biostratigraphies of deep sea cores and typical land-exposed sections.
The Neogene marine sequences in the Pacific side of southern Japan yields much tropical planktonic foraminifers throughout the entire succession, which are directly correlative with those characterizing the world's tropical region. In northern Japan and also on the coast of the Sea of Japan, however, warm water elements are restricted to a lower part of the Neogene, and cold-water inhabitants dominate younger stratigraphic horizons, where correlations can be made mainly by diatoms and additionally by radiolarians and calcareous nannoplankton. Some planktonic foraminifers characterizing middle latitude areas are also useful for the correlation of sequences in the Pacific side of southern Japan with those in the Sea of Japan side.
The resultant correlation by means of planktonic microfossils made clear its relation to the biostratigraphy of benthic marine molluscs and larger foraminifers, and some refinements in the Neogene history of Japan became necessary. To achieve precise interregional correlation of the Japanese Neogene sequences, most important problems to be solved in future are further refinements of radiometric calibrations in the microplanktonic time scale and the stratigraphic relationship between datum levels of planktonic foraminifers and those based upon other fossil groups. The bio- and chronostratigraphic correlation. within the framework of the newly accomplished microplanktonic time scale provides a basis for achieving global correlations of the Neogene events.
