Neogene chronostratigraphy and bio-events in the Japanese Islands

Abstract

The Neogene marine sequences on the Pacific side of southern Japan yield rich planktonic foraminifers throughout the entire succession, which is directly correlative with the world's tropical region. In northern Japan and on the Sea of Japan side, 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. The recently established biostratigraphic and chronostratigraphic correlation of the typical Neogene sequences of Japan by means of planktonic microfossils is presented. The chronologic framework has been achieved by incorporating datum levels established by microplanktonic biostratigraphy, radiometric dating related to biohorizons, magnetostratigraphy and the geomagnetic polarity time scale. The resultant correlation discloses a more precise definition of the chronologic succession of benthic marine faunas. The distribution of Neogene molluscan faunas in time and space is summarized. One of the most pronounced bio-events is a fluorishing of tropical and subtropical marine molluscs and larger foraminifers throughout nearly all of the Japanese Islands during the latest early Miocene or the earliest middle Miocene, at about 16 Ma. Such a climatically controlled mid-Neogene bio-event has also been recognized in the Pacific region. The improved biochronologic framework thus provides a basis for the world-wide correlation of Neogene events in the Japanese Islands.