Petrogenesis of Pliocene limestones in southern Hawke’s Bay, New Zealand: a contribution to unravelling the sequence stratigraphy and diagenetic pathways of cool-water shelf carbonate facies
Among carbonate sedimentologists, it is now appreciated that shallow-marine carbonates are not restricted to warm-water tropical settings, but can also be produced on modern cool-water shelves outside tropical latitudes. In the last two decades, many publications have not only advanced our knowledge about modern non-tropical carbonates, but have documented ancient skeletal limestones that are fossil analogues of these modern deposits. While the broad environmental and sedimentary characteristics of non-tropical deposits are becoming well established, the nature, timing, and relative importance of the various diagenetic processes affecting their evolution into limestones are yet to be fully appreciated, a topic addressed in the present study. In New Zealand, the Plio-Pleistocene (5-1.5 Ma) Te Aute limestones in southern Hawke’s Bay (eastern North Island) are non-tropical in origin, and formed in shoal areas about the margins of a forearc basin seaway adjacent to the convergent Pacific-Australian Plate boundary. Because of the dynamic tectonic setting, which led to narrowing and shallowing of the host basin over time, the studied carbonates exhibit a variety of sedimentary facies (i.e., from pure coquina to mixed carbonate-siliciclastic) and mineralogical signatures (i.e., calcite- to, more rarely, aragonite-dominated), and have undergone varying degrees of burial before being uplifted to their present position. As such, the Te Aute deposits provide a unique opportunity to: (1) decipher the suite of processes that have altered their texture and composition through time; (2) address the questions of what likely regional and more global parameters, namely tectonics and eustasy, may have driven diagenetic pathways, and how this occurred; and (3) more generally advance fundamental knowledge about models of porosity evolution in non-tropical carbonates. The present study integrates the combined results of detailed field investigations with a variety of analytical techniques, including standard, stained, and cathodoluminescence (CL) microscopic examinations of rock thin-sections, X-ray diffraction (XRD), and elemental (ICP-OES and microprobe) and stable isotope (δ¹⁸O and δ¹³C) geochemistry. Sedimentologic analysis has identified 14 different facies types that correspond to a spectrum of depositional settings associated with both continent-attached shelves and isolated upthrust-cored antiforms within the forearc seaway. Facies are representative of environments ranging from below storm-wave base, in outer shelf-type locations, to predominantly shallow-water shoreface-type conditions under the influence of strong tidal current flows and storm waves. A new petrographic approach for studying cool-water carbonate diagenesis has been developed. The method entails recording the high-resolution chronological occurrence of all diagenetic features within samples, and noting their vertical and lateral distribution within sedimentary formations. It is demonstrated that the attributes of pre-compaction diagenetic suites between consecutive sedimentary discontinuities change vertically in relation to the stacking of depositional facies. This original approach, by also revealing marine-cemented condensed horizons and key surfaces otherwise unrecognized in the field, has helped unlock the cryptic sedimentary architecture within the limestones. Thus, the Te Aute deposits appear to be constructed of metre-scale (1-15m) sequences that could be integrated into a sequence stratigraphic scheme, emphasizing that not only the production and deposition, but also the diagenesis of these carbonate deposits, were forced by glacio-eustatically and tectonically-driven processes. In order to better explain the probable progression of diagenetic environments that moved through Te Aute limestones, concepts that classically underpin the sequence stratigraphic analysis of sedimentary successions have been used to group cement zones, revealed under cathodoluminescent (CL) light, into diagenetic discontinuity-bounded CL sequences. In most Pliocene limestones, the procedure, in combination with geochemical data, has: (1) identified recurring and correlative motifs of pre- and post-compaction CL sequence stratigraphies; (2) enabled regional mapping of CL sequences; (3) highlighted the spatial geometry and timing of emplacement of cementing aquifers; and (4) led to the construction of paleohydrologic models, invoking location of recharge areas and fluid-flow directions. Pre-compaction CL sequences formed locally from marine and more widely from oxidized meteoric (or mixed marine-meteoric) waters after exposure of limestone caps due to relative drops in sea level. Post-compaction burial CL sequences precipitated from reduced fluids recharged from areas of basin margin exposure, while the next CL sequences in the chronological suite are related to the ingress of oxidized meteoric waters as a result of uplift.
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