Late Miocene and Pliocene Ostracoda from the Shimajiri Group, Kume-jima Island, Japan: Biogeographical significance of the timing of the formation of back-arc basin (Okinawa Trough)

The Shimajiri Group, sporadically distributed in Kume-jima Island, Okinawa Prefecture, Japan, is mainly formed from clastic sediments deposited during the Late Miocene (7.8–7.2 Ma, the Maja Formation) and the Middle Pliocene (about 3.2 Ma, the Aka Formation). Twenty-one samples of these sediments were taken from the Maja and the Aka Formations. Of the 102 ostracod species recorded, most of them are well known on the Japanese mainland today. Three ostracod biofacies were delineated using Q-mode cluster analysis. Modern analog technique (MAT) was used to infer the environment during the deposition of the Shimajiri Group. The Maja Formation was deposited in a littoral temperate environment near the land mass situated to the east of the continent of Asia. The Aka Formation was deposited in the temperate shallow sea in front of the delta which protrudes from the vast continental delta-faced deep sea. The depositional environment of the Aka and the Maja Formations is very closely correlated with the first stage of the formation of the Okinawa Trough. This did not increase in depth until the Middle Pliocene.