Abstract
SUMMARY Palaeomagnetic and rock magnetic studies were performed on a seaward transect of continental shelf sediments from sites 1071 and 1072 of Ocean Drilling Programme Leg 174A on the New Jersey margin. The sediments recorded a polarity change from reversed to normal, which was tentatively interpreted as the Matuyama–Brunhes boundary. The polarity boundary is closely correlated with the sequence boundary pp3(s), which is considered to have formed as an erosional unconformity during regression and subsequent transgression related to global eustacy. Rock magnetic analyses indicate that the sediment contains comparable amounts of magnetite and greigite (Fe3S4) with possible trace amounts of haematite. A marked change in rock magnetic properties is recorded at Site 1072 with an increase in greigite concentration at the polarity boundary (62.4 metres below seafloor), where the grain size does not change. At Site 1071, greigite is dominant from 2 m below pp3(s) to 0.8 m above it. These intervals are characterized by higher peak coercivities of remanence and subdued remanence in both the magnetite and greigite components. Palaeomagnetic analysis indicates that the intervals just below pp3(s) have dual-component magnetizations of reversed polarity and are considered to have been deposited during the Matuyama Chron (C1r.1r) and were possibly remagnetized during the Brunhes Chron through the formation of the pp3(s) surface. Detrital magnetite and early diagenetic greigite might have carried the reversed polarity magnetization. During the formation of pp3(s) in the Brunhes Chron, greigite might have formed at the oxidation front with ongoing downward formation due to oxidation of pyrite by percolating fresh water, which might be a cause of the remagnetization. Our study indicates that careful rock magnetic investigation is necessary for magnetostratigraphic studies of continental shelf deposits in order to recognize remagnetizations induced by sea-level changes.
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