High-resolution magnetostratigraphy of mid-Pliocene (3.3-3.0 Ma) shallow-marine sediments, Whanganui Basin, New Zealand

Resumen:
Orbital timescale, well-dated, shallow-marine records of the mid-Pliocene warm period (3.3-3.0 Ma) from tectonically uncomplicated continental margins are rare. They are, however, of interest for understanding global sea level changes in a climate characterised by atmospheric CO2 concentrations of similar to 400 ppm and temperatures 2-3 degrees C warmer than pre-industrial times. We present here a new high-resolution magnetostratigraphy for two laterally adjacent drill cores (Tiriraukawa-1 and Siberia-1) from the Whanganui Basin of New Zealand, that continuously recovered (98 per cent) a similar to 350-m-thick succession of cyclical shallow-marine (inner to outer shelf) sediments, spanning the mid- (3.3-3.0 Ma) to late Pliocene (3.0-2.6 Ma). Palaeomagnetic analysis was carried out on 1282 specimens, taken at intervals of 1 m (corresponding to approximately 1000 yr) throughout both cores. The sediments show very weak natural remanent magnetization (NRM) and magnetic susceptibility, in the ranges of 10(-4)-10(-3) A m(-1) and 10(-4)-10(-3) SI, respectively. NRMs generally comprise three components, a variable low blocking temperature (T-b) component interpreted to be thermoviscous; an intermediate (150-290 degrees C) T-b component, which is thought to be the survivor of the primary detrital magnetization; and a high (>290 degrees C) T-b component, considered to be secondary in origin and often antipodal to the characteristic intermediate T-b component. Despite thermal alteration hindering the separation of these components, progressive demagnetization enables recognition of the polarity of the characteristic remanent magnetization in most cases. Magnetic polarity zonations of the two cores are consistent and show a bottom to top sequence of polarity intervals: R1-N1-R2-N2-R3. Independent age constraints and the previously defined basin chronostratigraphic framework enable a clear correlation to the Gauss Chron of the Geomagnetic Polarity Timescale (GPTS; Ogg 2012) including subchrons C2An.2r (Mammoth), C2An.2n, C2An.1r (Kaena) and C2An.1n. The uppermost reversed polarity interval, R3, which is estimated to have occurred between 3.02 and 2.99 Ma with a duration of about 30 000 yr, is currently not recognized in the GPTS (Ogg 2012).

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