New Zealand Journal of Geology and Geophysics abstracts
Discriminating cool-water from warm-water carbonates and their
diagenetic environments using element geochemistry: the Oligocene
Tikorangi Formation (Taranaki Basin) and the dolomite effect
Steven D. Hood
Campbell S. Nelson
Peter J. J. Kamp
Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
Abstract Fields portrayed within bivariate element
plots have been used to distinguish between carbonates formed in warm-
(tropical) water and cool- (temperate) water depositional settings.
Here, element concentrations (Ca, Mg, Sr, Na, Fe, and Mn) have been
determined for the carbonate fraction of bulk samples from the late
Oligocene Tikorangi Formation, a subsurface, mixed dolomite-calcite,
cool-water limestone sequence in Taranaki Basin, New Zealand. While the
occurrence of dolomite is rare in New Zealand Cenozoic carbonates, and
in cool-water carbonates more generally, the dolomite in the Tikorangi
carbonates is shown to have a dramatic effect on the “traditional”
positioning of cool-water limestone fields within bivariate element
plots. Rare undolomitised, wholly calcitic carbonate samples in the
Tikorangi Formation have the following average composition: Mg 2800
ppm; Ca 319 100 ppm; Na 800 ppm; Fe 6300 ppm; Sr 2400 ppm; and Mn 300
ppm. Tikorangi Formation dolomite-rich samples (>15% dolomite) have
average values of: Mg 53 400 ppm; Ca 290 400 ppm; Na 4700 ppm; Fe
28 100 ppm; Sr 5400 ppm; and Mn 500 ppm. Element-element plots for
dolomite-bearing samples show elevated Mg, Na, and Sr values compared
with most other low-Mg calcite New Zealand Cenozoic limestones. The
increased trace element contents are directly attributable to the trace
element-enriched nature of the burial-derived dolomites, termed here
the “dolomite effect”. Fe levels in the Tikorangi Formation carbonates
far exceed both modern and ancient cool-water and warm-water analogues,
while Sr values are also higher than those in modern Tasmanian
cool-water carbonates, and approach modern Bahaman warm-water carbonate
values. Trace element data used in conjunction with more traditional
petrographic data have aided in the diagenetic interpretation of the
carbonate-dominated Tikorangi sequence. The geochemical results have
been particularly useful for providing more definitive evidence for
deep burial dolomitisation of the deposits under the influence of
marine-modified pore fluids.
Keywords carbonate geochemistry; trace elements;
cool-water limestone; dolomite; Tikorangi Formation; Oligocene;
Taranaki Basin; New Zealand; reservoir
G03066; Received 22 August 2003; accepted 5 March 2004; Online
publication date 1 December 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47:
857-869
0028-8306/04/4704-0857© The Royal Society of New Zealand 2004
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