Astronomical calibration of a southern hemisphere Plio-Pleistocene reference section, Wanganui Basin, New Zealand

Abstract

Wanganui Basin, New Zealand, contains one of the most complete late Neogene stratigraphic records in the world. The ca. 3km thick basin-fill for the last 3.6Ma comprises 58 superposed fifth and sixth-order shallow marine sedimentary cycles which correspond to individual 100 and 41ka sea-level cycles since oxygen isotope stage MG6. Stages MG6 to 5 are represented by marine cyclothems, whereas stages 17 to 4 are represented by a suite of coeval and younger uplifted marine terrace sequences. Additionally, a predominantly glacial loess stratigraphy exists for isotope stages 12-2. The Milankovitch-frequency, shallow marine, cyclostratigraphy of Wanganui Basin is here correlated with the astronomically-calibrated Plio-Pleistocene timescale of Lourens et al. (1996, Paleoceanography, 11, 391–413). An integrated chronology is presented for Wanganui Basin based on radiometric ages on interbedded rhyolitic tephra, on biostatigraphic data, on paleomagnetic polarity measurements, and on cycle correlations with the oxygen isotope timescale. Numeric ages on the tephra are consistent with the interpreted magnetostratigraphy and cyclostratigraphy, and fit well with the astronomically-calibrated timescale. Our cyclostratigraphic correlations provide age estimates for 116 stratigraphic horizons in Wanganui Basin that are not otherwise able to be dated, and thereby establish an astronomical chronostratigraphy for the New Zealand Plio-Pleistocene. The historic subdivision of the New Zealand marine Plio-Pleistocene is based on the biostratigraphy of shallow marine strata in both Wanganui and East Coast basins, North Island. Based on the new cyclostratigraphic correlations we re-evaluate the age of all New Zealand PlioPleistocene stage and substage boundaries. After making these revisions, the current age estimates for the base of the Opoitian, Waipipian, Mangapanian, Nukumaruan, Castlecliffian, and Haweran Stages are 5.25, 3.60, 3.03, 2.46, 1.07, and 0.34Ma, respectively, and the ages of the Hautawan-Marahauan (intra-Nukumaruan) and Okehuan-Putikian (intra-Castlecliffian) Substage boundaries are 2.15 and 0.78Ma, respectively.