Journal of the Royal Society of New Zealand abstracts
A coherent middle Pliocene magnetostratigraphy, Wanganui Basin, New
Zealand
Gillian M. Turner1, Peter J. J. Kamp2*, Avon
P. McIntyre2,3, Shaun Hayton2,4, Donald M. McGuire5,
and Gary S. Wilson6
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.
Abstract 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 Gauss-Matuyama 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 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.
Keywords palaeomagnetism; magnetostratigraphy;
geomagnetic polarity timescale; Wanganui Basin; middle Pliocene;
sequence stratigraphy; oxygen isotope stages
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
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