The depositional environment of the singular Middle Miocene strata from southwestern Tomioka City, Gunma Prefecture, central Japan, from the view point of fossil ostracod assemblage

Abstract

We present a micropaleontological re-examination of the depositional environment of the ‘Kojyakui-sho’ sediments of the Middle Miocene Obata Formation, part of the Tomioka Group, located along the Kabura-gawa River near Tajima, Tomioka City, western Gunma Prefecture, central Japan. These sediments contain sublittoral and bathyal ostracods, and were deposited in the bathyal zone on a quartz-porphyry bedrock seafloor during the late Early to early Middle Miocene. Planktonic foraminifera and ostracods within these samples indicate that the ‘Kojyakui-sho’ sediments formed during the Miocene Climatic Optimum; this is the second reported occurrence of these Miocene Climatic Optimum bathyal ostracod species in Japan.