Abstract:
This study establishes a magnetostratigraphy for the Pliocene sediments (Opoitian, Waipipian, and Mangapanian stages) exposed in the Turakina River, near the center of the Wanganui Basin. The basin is structurally simple, and its geological history is well known from the work of the Superior Oil Company (Turner, 1944) and Fleming (1953). The sediments are very weakly magnetised massive marine mudstones and sandy mudstones which were deposited very rapidly at shelf depths. Towards the end of the Waipipian stage, the basin was cut off from open ocean access, water depths decreased, and rapid sedimentation continued. Deposition rates of 1 to 2 mm/yr kept up with the tectonic deepening of the basin, as over 2000m of sediment were deposited in about 1.2 m.y.
Along with their weak magnetisation, the sediments display magnetic viscosity with decay time constants of 50 min to months and longer. Specimen preparation and handling methods have been refined and tested to ensure there is no opportunity for the acquisition of spurious magnetisations. The bulk magnetic behavior of the sediments resembles that of much younger unconsolidated and unaltered sediments. Drying the mudstones in the Earth's field creates small very hard secondary remanences. Thermal demagnetisation has been used to determine primary magnetisation directions. Bulk magnetic properties of the sediments have been studied, and the changes in those properties with heating have been used to determine the magnetic mineralogy. Scanning electron microscopy has also been used. The primary magnetisation is carried by an iron sulfide mineral, tentatively identified as pyrrhotite. The sediments also contain hematite and a few large detrital titanomagnetites, but these seem to play no part in the preservation of the primary magnetisation, which is a chemical remanent magnetisation (CRM) resulting from reducing conditions associated with organic matter in the sediments. This CRM probably had its origin very soon after deposition, as has been found in other Pliocene marine sediments (Linssen, 1988).
Combined remagnetisation circles analysis (McFadden & McElhinny, 1988) has been used as the basis for the magnetostratigraphic analysis and interpretation. Over 90% of all specimens and 88% of all sites have been included in the final result. There is good agreement between the interpreted magnetostratigraphy and the international time scale; most of the Turakina section was deposited during the Gauss chron. Correlation with other New Zealand magnetostratigraphies is also good, and demonstrates the difficulties with Pliocene biostratigraphic datums and stage correlations between eastern and western North Island basins.
