Middle Pliocene magnetostratigraphy , Wanganui Basin 197 A coherent middle Pliocene magnetostratigraphy , Wanganui Basin , New Zealand

We document magnetostratigraphies for three river sections (Turakina, Rangitikei, Wanganui) in Wanganui Basin and interpret them as corresponding to the Upper Gilbert, the Gauss and lower Matuyama Chrons of the Geomagnetic Polarity Timescale, in agreement with foraminiferal biostratigraphic datums. The Gauss-Gilbert transition (3.58 Ma) is located in both the Turakina and Wanganui River sections, while the GaussMatuyama transition (2.58 Ma) is located in all three sections, as are the lower and upper boundaries of the Mammoth (3.33–3.22 Ma) and Kaena (3.11–3.04 Ma) Subchrons. Our interpretations are based in part on the re-analysis of existing datasets and in part on the acquisition and analysis of new data, particularly for the Wanganui River section. The palaeomagnetic dates of these six horizons provide the only numerical age control for a thick (up to 2000 m) mudstone succession (Tangahoe Mudstone) that accumulated chiefly in upper bathyal and outer neritic palaeoenvironments. In the Wanganui River section the mean sediment accumulation rate is estimated to have been about 1.8 m/k.y. , in the Turakina section it was about 1.5 m/k.y. , and in the Rangitikei section, the mean rate from the beginning of the Mammoth Subchron to the Hautawa Shellbed was about 1.1 m/k.y. The high rates may be associated with the progradation of slope clinoforms northward through the basin. This new palaeomagnetic timescale allows revised correlations to be made between cyclothems in the Rangitikei River section and the global Oxygen Isotope Stages (OIS) as represented in Ocean Drilling Program (ODP) Site 846. The 16 depositional sequences between the end of the Mammoth Subchron and the Gauss-Matuyama Boundary are correlated with OIS MG2 to 100. The cyclothems average 39 k.y. in duration in our age model, which is close to the 41 k.y. duration of the orbital obliquity cycles. We support the arguments advanced recently in defence of the need for local New Zealand stages as a means of classifying New Zealand sedimentary successions, and strongly oppose the proposal to move stage boundaries to selected geomagnetic polarity transitions. The primary magnetisation of New Zealand mudstone 1School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. 2Department of Earth Sciences, The University of Waikato, Private Bag 3105, Hamilton 2001, New Zealand. 3Geoscience Australia, G.P.O. Box 378, Canberra, ACT 2601, Australia. 4CRL Energy Ltd, 77 Clyde Road, P.O. Box 29 415, Christchurch, New Zealand. 5School of Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. 6Department of Geology, University of Otago, P.O. Box 56, Dunedin, New Zealand. *Author for correspondence. R04018; Received 28 September 2004; accepted 5 May 2005; Online publication date 27 July 2005 Journal of the Royal Society of New Zealand Volume 35, Numbers 1 & 2, March/June, 2005, pp 197–227 Journal of the Royal Society of New Zealand, Volume 35, 2005 198 is frequently overprinted with secondary components of diagenetic origin, and hence it is often difficult to obtain reliable magnetostratigraphic records. We suggest specific approaches, analytical methods, and criteria to help ensure robustness and coherency in the palaeomagnetic identification of chron boundaries in typical New Zealand Cenozoic mudstone successions.