Volcanic Stratigraphy in Western Oita, Kyushu, Japan- Part 2:The Oyama -Tsuetate Area

摘要

The volcanic stratigraphy of Western Oita, Japan, is clarified through detailed geologicalfieldwork, petrographic study and age determination. The oldest volcanic activity is identified as thatresponsible for the Okubodai Andesite (4.4 Ma), followed by that for the Shakadake Andesite (3.2-3.1Ma), Yoshinomoto Andesite (2.8-2.4 Ma), Matsubara Andesite (2.4-2.1 Ma) and KaratomariyamaPyroclastic Flow Deposit (2.2 Ma). The Kamitarumizu Andesite, Fukuro Andesite, Kusu Group andOno Rhyolite made up a succession ranging from 1.7 to 1.3 Ma and were followed by the ShimotsuruPyroclastic Flow Deposit (1.1 Ma), Okubodai Pyroclastic Flow Deposit (1.05 Ma), YamakogawaRhyolite activity (1.3-1.0 Ma) and subsequent formation of the Yabakei Pyroclastic Flow Deposit(1.05-0.97 Ma). The volcanic activity of the Itsumaichi Dacite and Hanahira Pyroclastic Flow Depositoccurred at 0.9-0.8 Ma. Volcanic activity related to deposition of the Oyama Formation, TsuetateFormation, Iketsuru Pyroclastic Rock, Kan'non'iwa Andesite and Kan'non'iwa Pyroclastic FlowDeposit is inferred to have occurred at approximately 0.7 Ma, succeeded by the activity related to theHaneyama Rhyolite and Kameishiyama-Yoshitakeyama Dacite. The volcanic activities ofTakenosakotani Pyroclastic Flow Deposit and Sugihira Pyroclastic Flow Deposit were 0.62-0.58 Ma,and that of Amagase Pyroclastic Flow Deposit was 0.3 Ma.