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Discriminating cool‐water from warm‐water carbonates and their diagenetic environments using element geochemistry: The Oligocene Tikorangi Formation (Taranaki Basin) and the dolomite effect

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 53400 ppm; Ca 290400 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.