MAGNETOSTRATIGRAPHY OF THE UPLIFTED FORMER ATOLL OF NIUE, SOUTH-PACIFIC - IMPLICATIONS FOR ACCRETION HISTORY AND CARBONATE DIAGENESIS

Cenozoic carbonate platforms are a potential rich source of informatio n concerning eustasy but their prerequisite dating is fraught with dif ficulties. Paleomagnetism has long been established as a highly succes sful tool for dating deep-sea sediments but its applicability to shall ow-water reef carbonates has only recently been explored. Here we deri ve a detailed magnetostratigraphy of the carbonate platform at Niue on the basis of cores drilled into lagoonal facies to a maximum depth of 303 m below surface. The carbonates consist of limestone which has be en partially dolomitized and contain 16 zones of meteoric diagenesis c aused by a succession of sea-level falls. The paleomagnetic record, sp anning Chron C4r (Tortonian) to Chron C2n (late Piacenzian) and coveri ng about 7.1 m.y. of deposition and erosion, was tied to the Geomagnet ic Polarity Time Scale (GPTS) using verified control points. Coinciden ce of high accretion rates (up to 62.1 m/m.y.) and high sea levels cau sed the Tortonian-early Messinian section to mirror the contemporaneou s GPTS polarity reversal intervals. In contrast, decelerated subsidenc e coupled with frequent eustatic fluctuations yielded compressed and t runcated polarity intervals in the younger section. During late Messin ian, the accretion rate was anomalously high (114.1 m/m.y.) probably d ue to a short-lived endogenic heating event. Isothermal remanent magne tization data suggest that magnetite is the dominant magnetic carrier in the Niue carbonates. NRM intensities of limestones (mean 0.126 +/- 0.95 mA/m for n = 76) and dolomites (mean 0.121 +/- 0.097 mA/m for n = 73) are indistinguishable and their NRM/SLRM ratios are similar (clos e to 10(-2)) suggesting they underwent the same process of remanence a cquisition. The NRM at Niue are comparable in their intensities to tho se reported from San Salvador and Mururoa but much weaker than those o f deep-sea sediments. In general, meteoric diagenesis and dolomitizati on did not overprint the primary magnetization record at Niue. One exc eptional case of magnetic resetting, depicted within a well-developed meteoric diagenesis zone, suggests that magnetic overprinting cannot b e ruled out in carbonate platforms possessing weak primary magnetizati ons and therefore their polarity records must be evaluated on a case b y case basis.