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 de termined for a long 1545 m continuous sedimentary sequence of marine, fluvi al, and lacustrine deposits from the Osaka Basin, southwestern Japan. Addit ionally two short geomagnetic reversal events were identified: a short even t at about 0.69 Ma lasting for about 7 kyr in the early Brunhes chron aroun d 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 shor t Matuyama event (1.60-1.58 Ma) correlated with Stage 54 event, and the ear ly stage of the Sangiran Excursion. Accumulation rate was 0.3-0.4 mm/yr fro m 3 to 2 Ma, and then increased by about 50% with a rate of 0.51 mm/yr duri ng the Olduvai subchron. High values of >0.59 mm/yr were maintained from th e Upper Olduvai boundary to the Brunhes-Matuyama boundary (from 1.77 to 0.7 8 Ma), with a peak at 0.74 mm/yr during the Jaramillo subchron. These chang es in accumulation rate probably reflect the history of tectonic subsidence of the drilling site. The magnetic polarity stratigraphy provides a chrono logical framework for sedimentological, paleoenvironmental, tephrochronolog ical, and tectonic studies of the Osaka Basin. (C) 1999 Elsevier Science B. V. All rights reserved.