Stratigraphic correlation of the Neogene in the San-in district, southwest Japan, in the light of radiometric dating

The Neogene of the San-in district distributed in several isolated areas is reviewed and correlated on the basis of biostratigraphic and radiometric data. The Neogene is not so abundant in reliable index fossils that doubt has recurringly been cast on in any attempt to the correlation. For the present attempt, radiometric ages of eight volcanic rocks are newly determined by K-Ar (whole rock) and fission-track (zircon) methods. Now it becomes evident that cumulative radiometric data made on Neogene volcanic rocks provide key roles in the present summary and revision of whole stratigraphic succession of late Oligocene to Pleistocene age. The following three points are conclusive. (1) some of the lowest formations, which have been regarded as the lower Miocene, are of Oligocene age. (2) the Japan Sea started to transgress and invade the San-in district about 22 Ma ago, and regressed about 10 Ma ago leaving land behind. (3) alkali rocks have intermittently erupted since 12 Ma ago. (author)