1978 年 17 巻 2 号 p. 77-85
This paper clarifies that the Late Pliocene to Middle Pleistocene formations distribute in the Hiji area, north of Beppu Bay and these formations are correlated to the Takio Formation of the Oita Group in the Oita area, south of Beppu Bay, where is a standard area for the late Cenozoic stratigraphy in Kyushu.
The Oita Group in the Hiji area is stratigraphically divided into the Hiji Formation, Oga Formation, Katano Volcanic Rocks and Kanagoe Volcanic Rocks in ascending order. Lower member of the Hiji Formation yields marine molluscan fossils living in coastal water associated with oceanic water and they indicate Late Pliocene to Early Pleistocene. The Hiji Formation consists of hornblende andesitic pyroclastic rock and marine elastic rocks, and the Oga Formation consists of augite hypersthene andesitic pyroclastic rocks and fresh water elastic rocks. The Katano and Kanagoe Volcanic Rocks are hornblende augite hypersthene andesite and augite hypersthene andesite lavas respectively.
Change of volcanic activity in the Hiji area coincides with that of the formations corresponding to the Takio Formation in surroundings of the Hiji area. Because the basal part of the Hiji Formation suffers epithermal alteration, age of the Usa Group in the marginal area of Beppu Bay which was correlated to Miocene must be reviewed.