The Tokibadyama Formation of Late Oligocene to Early Miocene in the central part of Oki Dogo Island mainly consists of felsic to andesitic volcanic rocks and volcaniclastic rocks．It is divided into three members ； the Nakadani Andesite，Choshigawa Rhyolite and Chikaishigawa Andesite Members，in ascending order．The upper part of Choshigawa Rhyolite Member contains intercalated lacustrine shale beds．The Chikaishigawa Andesite Member mainly consists of two pyroxene andesite and clinopyroxene andesite lavas with autobrecciation，partly of andesitic volcaniclastic rocks and olivine basalt lavas．
Most of faults and fold axes strike NE-SW to ENE-WSW．These structural trends may represent the regional stress field during Oligocene．