New Zealand Journal of Geology and Geophysics abstracts
Integrated stratigraphy of the Waitakian-Otaian Stage boundary stratotype,
Early Miocene, New Zealand
H. E. G. MORGANS1
A. R. EDWARDS2
G. H. SCOTT1
I. J. GRAHAM3
P. J. J. KAMP4
T. C. MUMME1*
G. J. WILSON1
G. S. WILSON1
1Institute of Geological & Nuclear Sciences Ltd
P.O. Box 30 368
Lower Hutt, New Zealand
2Stratigraphic Solutions Ltd
P.O. Box 295
Waikanae, New Zealand
3Institute of Geological & Nuclear Sciences Ltd
P.O. Box 31 312
Lower Hutt, New Zealand
4Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
*Present address: 47 Parkvale Road, Karori, Wellington, New Zealand.
Abstract The base of the type section of the Otaian Stage at
Bluecliffs, South Canterbury, is recognised as the stratotype for the boundary
between the Waitakian and Otaian Stages. Principal problems with the boundary
are the restriction of existing bioevent proxies to shelf and upper slope
environments and its uncertain age. These topics are addressed by a
multidisplinary study of a 125 m section about the boundary, which
examines its lithostratigraphy, depositional setting, biostratigraphy,
correlation, and geochronology.
The lower siltstone lithofacies (0-38.5 m) was deposited at upper bathyal
depths (200-600 m) in a marginal basin which was partially sheltered from
fully oceanic circulation by a submarine high and islands. The site was covered
by cool-temperate water and was probably adjacent to the Subtropical
Convergence. This unit is succeeded by the banded lithofacies (38.5-106 m)
and the upper siltstone lithofacies (basal 19 m studied). Paleodepth
probably declined up-sequence, but deposition at shelf depths is not definitely
indicated. A cyclic pattern of abundance spikes in benthic and planktonic
foraminifera commences 9 m above base and extends to 73 m in the
banded lithofacies. Oxygen isotope excursions (up to 2.08%) in Euuvigerina
miozea and Cibicides novozelandicus are greatest within the
interval containing the abundance spikes. The stage boundary occurs in the
banded lithofacies at the highest abundance spike (73 m). Although
condensed intervals might affect the completeness of the section, they are not
associated with sedimentary discontinuities, and we consider that the section
is suitable as a biostratigraphic reference.
Spores, pollens, dinoflagellates, calcareous nannofossils, foraminifera,
bryozoans, and ostracods are preserved near the boundary, but molluscs
principally occur higher, in the shallower upper siltstone lithofacies.
Siliceous microfossils are rare. There is considerable scope for further
biostratigraphic research.
The primary event marking the boundary at 73 m is the appearance of the
benthic foraminifer Ehrenbergina marwicki. This is a distinctive and
widely distributed event but is restricted to shelf and upper bathyal
environments. Supplementary events in planktonic foraminifera and calcareous
nannofossils were researched. Highest occurrences of Globigerina
brazieri and G. euapertura are recorded at 47 and 58 m. There
is a marked decline in relative abundance of Paragloborotalia spp. at
62 m. Helicosphaera carteri becomes more abundant than H.
euphratis between 56 and 87 m. These events are not exact proxies for
the boundary but they may usefully indicate proximity to it. They occur in the
interval of prominent spikes in foraminiferal abundance.
The Waitakian-Otaian boundary is dated at 21.7 Ma by strontium isotopes.
Stable primary remanence could not be determined in a pilot paleomagnetic study
of Bluecliffs specimens. However, specimens trended towards reversed polarity,
and remagnetisation great circle analysis will allow directions to be
calculated in future collections.
Keywords Oligocene; Miocene; Waitakian; Otaian; stage
boundary stratotype; lithostratigraphy; biostratigraphy; geochronology;
integrated stratigraphy; Bluecliffs Silt; Otekaike Limestone; planktonic
foraminifera; benthic foraminifera; calcareous nannofossils; Mollusca;
palynoflora; dinoflagellates; Radiolaria; oxygen isotopes; carbon isotopes;
strontium isotopes
New Zealand Journal of Geology & Geophysics, 1999, Vol. 42:
581-614
0028-8306/99/4204-0581 $7.00/0 (c) The Royal Society of New Zealand
1999
PDF file of entire paper: medium quality (7551K); (scanned from paper original: notes about this process)
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