Geology of the Ohata Formation in the Shimokita Peninsula

Abstract

During Pliocene to Pleistocene ages, a caldera swarm was formed by felcic volcanisms at Shimokita Peninsula.We examined stratigraphy of Ohata Formation, which is composed of intra-caldera pyroclastic deposit within the Yagen caldera, one of the calderas.The depositional age was determined by the K-Ar method as 2.6Ma-3.0Ma.The formation was divided by their petrographical features into four members: Yaichirousawa, Ohatagawa, Hairosawa, and Komenasawa Pumice Tuffs. The lithological features of the volcaniclastic deposits imply that they emplaced as subaqueous volcaniclastic flows within the caldera basin that had collapsed prior to the deposition of Ohata Formation.