ALBERT

Description: Earth is heading towards a climate that was last experienced more than 3 Myr during the “mid-Pliocene warm period”1. Atmospheric carbon dioxide (pCO2) concentrations were ~400 ppm, global sea level oscillated in response to orbital forcing2,3 and peak global mean sea level (GMSL) may have reached ~20 m above present4,5. For sea-level rise of this magnitude extensive retreat or collapse of the Greenland, West Antarctic and marine based sectors of the East Antarctic ice sheets are required. Yet the relative amplitude of sea-level variations within glacial-interglacial cycles remains poorly-constrained. Here, we show sea-level varied on average by 13 ± 5 m over glacial-interglacial cycles during the mid- to late Pliocene, ~3.3 - 2.5 Myrs. We calibrated a theoretical relationship between modern sediment transport by waves and water depth and then applied the technique to Pliocene grain size in shallow-marine sediments from Whanganui Basin, New Zealand, thereby estimating past sea level variation. The resulting PlioSeaNZ record is independent of the deep ocean oxygen isotope (δ18O) record for global ice volume3, and in phase with ~20 kyr duration cycles of insolation over Antarctica, paced by eccentricity-modulated orbital precession between 3.3 and 2.7 Ma6. Thereafter, sea-level fluctuations are paced by ~41 kyr cycles in Earth's axial tilt as ice sheets stabilise on Antarctica and intensify in the northern hemisphere3,6. Sensu stricto, we provide the amplitude of relative sea-level (RSL) change, rather than absolute GMSL change. However, glacio-isostatic adjustment (GIA) simulations of RSL change, show that the PlioSeaNZ record approximates eustatic sea level (ESL), defined here as GMSL unregistered to the centre of the Earth. Nonetheless, under conservative assumptions, our estimates limit maximum Pliocene sea level to less than +25 m and provide new constraints on polar ice-volume variability under climate conditions Earth is on track to experience this century.

Description: The diagenetic evolution of thick, cool-water Pliocene limestones that formed within a forearc basin to accretionary wedge setting in eastern North Island, New Zealand, can be usefully tracked by applying at the thin-section scale the concepts of stratal patterns (onlap, offlap, discontinuity surfaces) in sedimentary sequences. The petrographic approach, supported by geochemical data, involves recognizing genetically related packages of zoned cements under cathodoluminescent (CL) light, named cement suites, which are bounded in thin section by (correlative) diagenetic discontinuities, including dissolution surfaces, renucleation surfaces and/or fractures. Based initially on detailed petrographic study of the early Pliocene Kairakau Limestone formation, a bryozoan-epifaunal bivalve-barnacle grainstone to rudstone, this procedure identifies five distinctive cement suites labelled K1-K5 separated, respectively, by discontinuity surfaces d1 to d4, and referred to collectively as the Kairakau diagenetic motif. Suites K1 and K2 have a pre-compaction origin and are inferred to have formed in a sedimentary system paced by high-frequency glacio-eustasy cycles, and reflecting deposition in transgressive (TST), highstand (HST) and regressive (RST) systems tracts, followed by initial shallow burial. Cement suite K1 is developed only locally, typically immediately above (early TST) and sometimes below (late RST) sequence boundaries. It consists of neomorphosed marine turbid cements growing upon the abraded surface of skeletons, and is bounded above by a dissolution surface (d1). Pre-compaction cement suite K2 has this dissolution surface at its base and a fracture surface (d2) at its top. K2 cements formed from oxidizing waters, either under shallow marine burial or, more likely, mixed marine-meteoric influences; they are inferred to relate mainly to the HST-RST portion of a depositional cycle. Post-compaction cement suites K3-K5 comprise pore-filling and fracture-hosted cements that formed during the burial of depositional sequences by overlying sequences, and during subsequent uplift. Suite K3 comprises ferroan calcite cements precipitated from compaction-driven reduced fluids that are terminated against fracture event d3, whereas suites K4 and K5 are interpreted as telogenetic cement phases that formed from meteoric, dominantly oxidizing, waters during uplift and exhumation of the whole succession and are separated by fracture and dissolution surface d4. Significantly, the same Kairakau diagenetic motif is developed in all the other Pliocene limestone occurrences in the study area. In an attempt to explain the emplacement of the successive cementing aquifers within limestones of different ages and separated by thick siliciclastic deposits, a cement stratigraphic model for the Pliocene succession concludes the paper, utilizing the concept of onlap and downlap cementational trends within the pre- and post-compaction cement suites of the eastern North Island carbonates. Ideal pre-compaction onlap-downlap diagenetic suites K1 and K2 mimic the evolution of the depositional environment from marine to subaerial forced mainly by short-term high-frequency (104 - 105 a) relative sea-level changes, whereas their post-compaction counterparts (suites K3-K5) record burial followed by exhumation of the sediment pile forced by subsidence and tectonic mechanisms of longer duration (105-107 a).

Description: An investigation of the taphonomy, palaeoecology and stratigraphy of cool-water skeletal concentrations (shell beds) of the Matemateaonga Formation (Late Miocene-Early Pliocene) of Wanganui Basin, New Zealand, has provided the basis for the classification of taphofacies presented here. Two taphofacies described from transgressive systems tracts include the amalgamated shell bed and sediment starved shell bed taphofacies, representing skeletal concentration dominated by wave and current agitation, and sediment starvation, respectively. A further five taphofacies described from highstand and regressive systems tracts exhibit a gradient of sedimentological, taphonomic and palaeoecological properties that result from variation in storm event and fair-weather wave processes across the palaeoshelf bathymetric gradient. A principal components analysis of semi-quantitative data (53 observations) from sequences in Manutahi-1 well core demonstrates that taphonomic properties may be limited to particular systems tracts in some cases, but can also be repeated in different system tracts where the depositional environments are similar. Taphofacies, which are contained within siliciclastic-dominated portions of sequences (highstand and regressive systems tracts) possess little direct relevance to sequence stratigraphic analyses, but do provide valuable information on environmental conditions, in particular, depth relative to storm and fair-weather wave base, and proximity to shoreline.

Description: The nontropical Oligocene carbonate-rich Tikorangi Formation is an important oil producer in the Taranaki Basin, New Zealand. Hydrocarbons are hosted and produced from mineralized, natural fracture systems. Petrographic, trace-element, stable-isotope (δ18O and δ13C), and fluid-inclusion data have enabled a complex sequence of eight paragenetic events to be determined. The Tikorangi Formation host rock was cemented by low-Mg calcite (event 1) during burial diagenesis, from temperatures of 27°C, corresponding to 0.5 km burial, and continued until 37°C, 1-km burial depth, producing tight, pressure-dissolved fabrics with essentially no porosity and permeability. The host rock was partially dolomitized (5–50%) (event 2) by Ca- and Fe-rich dolomite rhombohedra at burial depths and temperatures of 1.0–1.5 km and 35–50°C without secondary porosity development. Subsequent brittle fracturing formed by Neogene compression (event 3) is constrained to a period following lithification and dolomitization, but before precipitation of first-generation vein calcite (event 4). This initial ferroan low-Mg vein calcite formed after a period of burial from Fe-rich, meteorically modified fluids at temperatures of about 50–60°C and 1.4–1.9 km burial depth. Baroque dolomite formed (event 5), following a period of Mg-enriched basinal fluid input precursory to hydrocarbon emplacement per se. The dolomite formed mainly as a primary cement but also as a calcite replacement at temperatures following further burial to 2–2.5 km and temperatures of 65–80°C. Formation of celestite and quartzine phases (event 6) coincided with or marginally postdated dolomite at similar depths and temperatures to event 6 and formed as both replacements and cements. Second-generation ferroan vein calcite formed (event 7) at cooler temperatures (53–65°C), perhaps resulting from the introduction of cooler meteoric fluids from upsection. The presence of petroleum-fluid inclusions in the second-generation calcite suggests precursory hydrocarbon-bearing fluids have migrated, along with aqueous fluids from about 10 Ma, with hydrocarbon emplacement (event 8) occurring in the last 6 m.y. following a period of rapid late Miocene burial. An improved understanding of the paragenesis of the Tikorangi Formation may assist in hydrocarbon production from its reservoirs. Steven Hood is a postdoctoral research fellow in the Department of Earth Sciences at the University of Waikato in Hamilton, New Zealand. While recipient of a University of Waikato doctoral scholarship, he completed a Ph.D. on the subsurface stratigraphy and petrology of the middle Tertiary Tikorangi Formation fracture reservoir in Taranaki Basin in 2000. His research interests are currently focused on the petrology, paragenesis, and petroleum geology of cool-water carbonates, especially in New Zealand.Cam Nelson received B.Sc. and B.Sc. (honors) degrees in geology at Victoria University, Wellington. He lectured in the Department of Geology at the University of Auckland, where he received his Ph.D. before joining the Department of Earth Sciences at the University of Waikato in Hamilton in 1971 as its founding geological staff member. He was department chairperson from 1988 to 1996 and has been a professor since 1991. His research interests are in sedimentary and marine geology and stratigraphy and Cenozoic paleooceanography and paleoclimatology of the southwest Pacific region. He is past president and office holder of the Geological Society of New Zealand and was elected a fellow of the Royal Society of New Zealand in 1994. Peter Kamp is professor of Earth Sciences at the University of Waikato in Hamilton. He received his M.Sc. degree and his Ph.D. from the University of Waikato. His research interests are in the analysis of sedimentary basins, particularly those of Late Cretaceous–Cenozoic age in New Zealand. Another major research interest involves the techniques of fission-track analysis and (U–Th)/He thermochronometry. His research applications involve the thermal history of sedimentary basins and the exhumation history of basement provinces–mountain belts.