Magnetostratigraphy of the uplifted former atoll of Niue, South Pacific: implications for accretion history and carbonate diagenesis

Abstract

Cenozoic carbonate platforms are a potential rich source of information concerning eustasy but their prerequisite dating is fraught with difficulties. Paleomagnetism has long been established as a highly successful tool for dating deep-sea sediments but its applicability to shallow-water reef carbonates has only recently been explored. Here we derive 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 been partially dolomitized and contain 16 zones of meteoric diagenesis caused by a succession of sea-level falls. The paleomagnetic record, spanning Chron C4r (Tortonian) to Chron C2n (late Piacenzian) and covering about 7.1 m.y. of deposition and erosion, was tied to the Geomagnetic Polarity Time Scale (GPTS) using verified control points. Coincidence of high accretion rates (up to 62.1 m/m.y.) and high sea levels caused the Tortonian-early Messinian section to mirror the contemporaneous GPTS polarity reversal intervals. In contrast, decelerated subsidence coupled with frequent eustatic fluctuations yielded compressed and truncated polarity intervals in the younger section. During late Messinian, the accretion rate was anomalously high (114.1 m/m.y.) probably due to a short-lived endogenic heating event. Isothermal remanent magnetization 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/SIRM ratios are similar (close to 10 −2) suggesting they underwent the same process of remanence acquisition. The NRM at Niue are comparable in their intensities to those reported from San Salvador and Mururoa but much weaker than those of deep-sea sediments. In general, meteoric diagenesis and dolomitization did not overprint the primary magnetization record at Niue. One exceptional case of magnetic resetting, depicted within a well-developed meteoric diagenesis zone, suggests that magnetic overprinting cannot be ruled out in carbonate platforms possessing weak primary magnetizations and therefore their polarity records must be evaluated on a case by case basis.