Brunhes/Matuyama polarity epoch boundary in the Osaka Group of the Senriyama Hills, southwest Japan

Abstract Magnetic measurements were carried out on core samples of the Osaka Group distributed in the Senriyama Hills, north of Osaka Bay. The studied section, about 53 m thick, includes four layers of clay or silts, and intercalated sand and gravel layers. The four muddy layers, which have been known as the Ma 3, Ma 4, Ma 5 and Ma 6 “marine clay” beds (in ascending order), seem to have been deposited in shallow marine environments, possibly associated with transgression during interglacial periods. The mud layers were sampled using a new type of core sampler, designed to mark the north direction on the cores. The magnetic analysis revealed that the upper part of the section (Ma 5 and Ma 6) has normal polarity, and that the lower part (Ma 3) has reversed polarity. The Ma 4 bed, which was formerly assigned to the Brunhes Chron, showed reversed polarity in the lower part, while its upper part had downward inclinations. This result shows that the Brunhes/Matuyama polarity transition occurred in an interglacial period. The Ma 4 layer is assigned to the period of marine transgression corresponding to the oxygen isotope stages 19-18.