Abstract
Chronostratigraphy of the Miocene Series in the Choshi area, central Japan, is established on the basis of diatom biostratigraphy, K-Ar dating and paleomagnetic polarity. The previously described volcano-sedimentary succession of the Metogahana Formation is divided into two units, i.e. the lower andesite lava flows and the tuffaceous sandstone of the Senninzuka Formation (newly proposed), and the overlying marine siltstone of the Metogahana Formation (re-defined). The Senninzuka Formation can be correlated to the normal polarity interval around the Chron C6A (ca.21-20 Ma), whereas the diatom fossils from the Metogahana Formation indicate a latest early Miocene age (ca.17-16 Ma). The time gap of about 4 m.y. suggests the presence of an unconformity between these two units.
