抄録
The Oga Peninsula is famous for the standard locality of the Green Tuff succession in Japan. The stratigraphy of the lower succession, however, remains in debate because of the complicated associations of volcanic rocks and many poor or inaccessible exposures especially in the inland area. This paper proposes a new stratigraphic unit, the Nomuragawa Formation, extensively distributed in the inland area of the west Oga Peninsula where the late Eocene to early Oligocene shallow-water to terrestrial volcanic rocks of the Monzen Formation has been believed to occupy.
The Nomuragawa Formation is unconformably underlain by the Monzen Formation and is onlapped by the late early Miocene to early middle Miocene marine Nishikurosawa Formation. This formation is composed mainly of non- to intensely welded dacite pyroclastic flow deposits and basalt to basaltic andesite lava flows and volcaniclastic deposits. Three radiometric ages determined for this formation range from 19 to 21 Ma, correlative to the Hokakejima Dacite and the Sugoroku and Daijima Basalts, both of the Daijima Formation. Our finding of the Nomuragawa Formation in the west Oga Peninsula raises a doubt on the belief of the well-established lithostratigraphic division of the Daijima Formation. Volcanic products correlative in lithology and age to the Nomuragawa Formation sparsely occur along the Japan Sea coast. They are commonly onlapped by the succeeding marine sediments, and perhaps mark the beginning of rapid opening of the Japan Sea.
