Magnetostratigraphy of Plio-Pleistocene sediments in a 1700-m core from Osaka Bay, southwestern Japan and short geomagnetic events in the middle Matuyama and early Brunhes chrons

A magnetic polarity stratigraphy spanning more than the past 3.2 Myr was determined for a long 1545 m continuous sedimentary sequence of marine, fluvial, and lacustrine deposits from the Osaka Basin, southwestern Japan. Additionally two short geomagnetic reversal events were identified: a short event at about 0.69 Ma lasting for about 7 kyr in the early Brunhes chron around the lower boundary of a marine clay deposited during a period of eustatic high sea level during the marine oxygen isotope stage 17; and another short Matuyama event (1.60–1.58 Ma) correlated with Stage 54 event, and the early stage of the Sangiran Excursion. Accumulation rate was 0.3–0.4 mm/yr from 3 to 2 Ma, and then increased by about 50% with a rate of 0.51 mm/yr during the Olduvai subchron. High values of >0.59 mm/yr were maintained from the Upper Olduvai boundary to the Brunhes–Matuyama boundary (from 1.77 to 0.78 Ma), with a peak at 0.74 mm/yr during the Jaramillo subchron. These changes in accumulation rate probably reflect the history of tectonic subsidence of the drilling site. The magnetic polarity stratigraphy provides a chronological framework for sedimentological, paleoenvironmental, tephrochronological, and tectonic studies of the Osaka Basin.