New Zealand Journal of Geology and Geophysics abstracts
Strontium isotope dating of the New Zealand Oligocene
Campbell S. Nelson1
Daphne Lee2
Phillip Maxwell3
Roland Maas4
Peter J. J. Kamp1
Steve Cooke5, 1
1 Department
of Earth Sciences
University of
Waikato
Private Bag 3105
Hamilton, New
Zealand
email:
c.nelson@waikato.ac.nz
2 Department
of Geology
University of
Otago
P.O. Box 56
Dunedin, New
Zealand
3 Bathgates
Road
RD 10
Waimate, New
Zealand
4 School
of Earth Sciences
University of
Melbourne
Victoria 3010,
Australia
5 School
of Ocean and Earth Sciences
Southampton
Oceanography Centre
Southampton
SO14 3ZH, UK
Abstract One of the least well resolved portions
of the New Zealand Cenozoic time-scale is that centred on and about the
Oligocene Epoch, internationally regarded as spanning
c. 10 m.y. from 33.7 to 23.8 Ma. We have determined the 87Sr/86Sr
ratios and derived absolute ages for 77 macrofossil samples collected
from several biostratigraphically dated mid-Tertiary sections in the
South Auckland (North Island) and North Otago/South Canterbury (South
Island) regions. While the site-specific stratigraphic significance of
our ages remains to be assessed, we present them here to foster wider
consideration and discussion in relation to evolving absolute age
schemes for the New Zealand Oligocene biostratigraphic stages. Initial
results suggest: (1) The approximate boundary ages for the mid-Tertiary
Stages are: Runangan/Whaingaroan, 34.8 Ma; early Whaingaroan/late
Whaingaroan, 31.0 Ma; Whaingaroan/Duntroonian, 28.5 Ma;
Duntroonian/Waitakian, 25.5 Ma; Waitakian/Otaian, 22.2 Ma. These values
are mainly older than ages assigned over the past decade.(2) The early
Whaingaroan Stage, traditionally held to be entirely within the
Oligocene, and to define its base, extends back across the
Eocene/Oligocene boundary at 33.7 Ma into the late Eocene by up to 1.1
m.y., as previously suspected by Morgans et al. (1996) on
biostratigraphic grounds.(3) There has been considerable uncertainty
about placement of the Waitakian Stage over the past two decades,
whether entirely in the Miocene, entirely in the Oligocene, or
straddling both epochs. Our Sr dating shows that the Oligocene/Miocene
boundary (23.8 Ma) lies about midway through the Waitakian Stage, in
agreement with Graham et al. (2000).(4) The Whaingaroan/Duntroonian
boundary approximates the international early–late Oligocene one (28.5
Ma).Comparisons with recently published Oligocene stable oxygen isotope
records suggest that ?18O maxima and attendant sea-level lowering,
with possibly significant unconformity development, may be anticipated
on three occasions in the early Whaingaroan, two or three in the late
Whaingaroan, two in the Duntroonian, and at least two in the Waitakian.
The unconformities bounding formations and members in the Oligocene
successions may relate to these sea-level changes, and so be regionally
correlatable, including the well publicised Marshall Paraconformity of
latest Whaingaroan (c. 29 Ma) age.
Keywords New Zealand; Oligocene; Whaingaroan
Stage; Duntroonian Stage; Waitakian Stage; absolute age; strontium
isotope dating; fossils
G03055; Received 29 May 2003; accepted 1 October 2003; Online
publication date 1 December 2004
New Zealand Journal of Geology &
Geophysics, 2004, Vol. 47: 719-730
0028-8306/04/4704-0719© The Royal Society of New Zealand 2004
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