Paleomagnetism of the upper part of the Shinzato Formation (Shimajiri Group) and the Chinen Formation, southern Okinawa-jima, southwestern Japan

Abstract

We conducted a paleomagnetic study to establish magnetostratigraphy of the upper Pliocene Shinzato Formation of the Shimajiri Group and the upper Pliocene to lower Pleistocene Chinen Formation at Urizun S, Urizun N, and Golf Links sections in Chinen Village, southern Okinawa-jima. The Chinen Formation, consisting of calcareous sandstone and sandy limestone, represents a transitional facies from the mud-dominant Shimajiri Group to the Pleistocene reef complex deposits of the Ryukyu Group. We collected samples using orientated plastic cubes and then demagnetized them progressively with alternating field to extract primary components. The samples from 5 horizons in unit B of the Urizun N section show reversed polarity. Considering previous calcareous nannofossil and planktonic foraminiferal biostratigraphic investigations in the same sections, we correlated these horizons to the subchron C2r1r. Remanents from other horizons were quite unstable and/or completely remagnetized by the present magnetic field. The Olduvai subchron was not detected in these sections.