This paper describes occurrence of marine sediments and associated trace fossils at Shiose-no-Misaki (Cape), Oga Peninsula, NE Japan. The marine sediments constitute an upward-fining succession from sandstone to mudstone, and contain many trace fossils characteristic of the Skolithos ichnofacies. They are sorted well, and the sandstone bed at the middle of the upward-fining succession shows wavy stratification indicative of a high wave-energy. Though the stratigraphic position remains ambiguous, the Shiose-no-Misaki sediments are intruded by a dolerite of 20 Ma and most likely correlated with the late Eocene to Oligocene Monzen Formation. They represent the Oligocene to early Miocene marine sediments that occur along the eastern margin of the Japan Sea from southern Sakhalin Island to Kyushu Island, and likely support the presence of an incipient back-arc rift basin produced precedent to the early to middle Miocene rapid spreading of the Japan Sea.
