Abstract
A geological field survey and zircon U–Pb dating were conducted for the volcanic and pyroclastic rocks of the Middle Miocene Hinokigawa Formation, distributed widely in the western Shimokita Peninsula, to re-evaluate the chronostratigraphy of the formation in the Hinokigawa, Otokogawa, Takiyama, and Ushitaki basins. A U–Pb age of 13.4 Ma was obtained from obsidian in the Hinokigawa basin, which is consistent with the previously suggested Middle Miocene age. A fan-shaped depression structure was observed in the Ushitaki basin around Hotokegaura and is filled with the newly defined Hotokegaura Tuff and Fukuura Rhyolite Lava. A caldera-collapse breccia occurs at the boundary between the lava and basement rocks and is interpreted as syn-caldera-forming ejecta. The Hotokegaura Tuff and Fukuura Rhyolite Lava that infill the newly defined “Hotokegaura Caldera” yield zircon U–Pb ages of 4.5 and 4.4 Ma, respectively, and the Nuidoishiyama Intrusive Body in the east of the caldera yields an age of 4.7 Ma. The Maruyama Rhyolite Lava, which occurs near the southeastern boundary of the caldera, yields a U–Pb age of 4.0 Ma. Therefore, the main activity of the Hotokegaura Caldera occurred during the Early Pliocene. In contrast, a U–Pb age of 7.6 Ma (Late Miocene) was obtained for the Ushitaki Tuff in the south of the Hotokegaura Caldera.
