Abstract
This paper details the stratigraphic framework of the Miocene Daijima Formation, southwest Oga Peninsula, Akita Prefecture, NE Japan, and proposes a new stratigraphic division of the Daijima Formation into five members: the Sugoroku-sawa Conglomerate, Sugoroku-sawa Dacite, Sugoroku Basalt, Tateyamazaki Dacite, and Tsubaki Sandstone and Conglomerate. The Tateyamazaki Dacite has previously been regarded as the representative rock of the Daijima Formation, but is now interpreted as caldera infill distributed between Cape Tateyamazaki and Cape Shiosenomisaki. Volcanic rocks of the Daijima Formation are similar in lithology, stratigraphy, and isotopic age to those of the Nomuragawa Formation that occur immediately north of the Daijima Formation. The Daijima and Nomuragawa Formations are stratigraphic equivalents being underlain by the Late Eocene to Early Oligocene Monzen Formation and overlain by the Middle Miocene Nishikurosawa Formation. Isotopic age data indicate that volcanic rocks of the Daijima and Nomuragawa formations erupted at about 21 Ma. These rocks are voluminous and bimodal in composition, and have a northwesterly paleomagnetic declination, in contrast with the present-day northerly declination. The volcanism represented by these formations is likely to have marked the beginning of the rapid opening of the Japan Sea.
