Steven D. Hood

A skeletal assemblage classification system for non-tropical carbonate deposits based on New Zealand Cenozoic limestones

Abstract Cenozoic limestones in New Zealand are mainly skeletal grainstones and packstones formed under non-tropical climatic conditions in open marine shelf or ramp environments. Following petrographic analysis of the nature and abundance of the skeletal components in nearly 500 samples of these limestones, a complete linkage cluster analysis identified seven major skeletal assemblages that may be regarded as subdivisions of the single foramol skeletal association defined by Lees and Buller (1972) for temperate-region carbonate deposits. The seven assemblages are given contracted names, as follows: (a) barnamol = barnacle/bivalve-dominated; (b) bimol = bivalve-dominated; (c) bryomol = bryozoan/bivalve-dominated; (d) echinofor = echinoderm/benthic foraminiferal-dominated; (e) nannofor = nannofossil/planktonic foraminiferal-dominated; (f) rhodalgal = calcareous red algal-dominated; and (g) rhodechfor = calcareous red algal/echinoderm/benthic foraminiferal-dominated. A composite triangular classification diagram has been devised for naming the skeletal assemblage of an unknown sample on the basis of its three main skeletal components. The diagram successfully characterises more than 85% of the New Zealand Cenozoic limestone samples and also appears to be applicable for the skeletal assemblage designation of many overseas examples of non-tropical carbonate deposits. Limitations relate mainly to locally common skeletal types (e.g. serpulids, brachiopods) that are presently not incorporated into the New Zealand-based scheme. The general ecological preferences of the main skeletal contributors in each of the seven skeletal assemblages form a basis for relating the assemblages to broad shelf habitats. Consequently, as well as the benefits of providing a more consistent skeletal assemblage terminology for comparative studies between different workers, the scheme can assist with the paleoenvironmental interpretation of non-tropical skeletal carbonate facies.

Pliocene Te Aute limestones, New Zealand: Expanding concepts for cool‐water shelf carbonates

Abstract Acceptance of a spectrum of warm‐ through cold‐water shallow‐marine carbonate facies has become of fundamental importance for correctly interpreting the origin and significance of all ancient platform limestones. Among other attributes, properties that have become a hallmark for characterising many Cenozoic non‐tropical occurrences include: (1) the presence of common bryozoan and epifaunal bivalve skeletons; (2) a calcite‐dominated mineralogy; (3) relatively thin deposits exhibiting low rates of sediment accumulation; (4) an overall destructive early diagenetic regime; and (5) that major porosity destruction and lithification occur mainly in response to chemical compaction of calcitic skeletons during moderate to deep burial. The Pliocene Te Aute limestones are non‐tropical skeletal carbonates formed at paleolatitudes near 40–42°S under the influence of commonly strong tidal flows along the margins of an actively deforming and differentially uplifting forearc basin seaway, immediately inboard of the convergent Pacific‐Australian plate boundary off eastern North Island, New Zealand. This dynamic depositional and tectonic setting strongly influenced both the style and subsequent diagenetic evolution of the limestones. Some of the Te Aute limestones exhibit the above kinds of “normal” non‐tropical characteristics, but others do not. For example, many are barnacle and/or bivalve dominated, and several include attributes that at least superficially resemble properties of certain tropical carbonates. In this regard, a number of the limestones are infaunal bivalve rich and dominated by an aragonite over a calcite primary mineralogy, with consequently relatively high diagenetic potential. Individual limestone units are also often rather thick (e.g., up to 50–300 m), with accumulation rates from 0.2 to 0.5 m/ka, and locally as high as 1 m/ka. Moreover, there can be a remarkable array of diagenetic features in the limestones, involving grain alteration and/or cementation to widely varying extents within any, or some combination of, the marine phreatic, burial, and meteoric diagenetic environments, including locally widespread development of meteoric cement sourced from aragonite dissolution. The message is that non‐tropical shelf carbonates include a more diverse array of geological settings, of skeletal and mineralogical facies, and of diagenetic features than current sedimentary models mainly advocate. While several attributes positively distinguish tropical from non‐tropical limestones, continued detailed documentation of the wide spectrum of shallow‐marine carbonate deposits formed outside tropical regions remains an important challenge in carbonate sedimentology.

Cementation scenarios for New Zealand Cenozoic nontropical limestones

Abstract Cenozoic limestones are widely distributed in New Zealand, especially in the Oligocene‐earliest Miocene in both islands, and the Pliocene‐Pleistocene in North Island. A spectrum of limestone types exists, but all are skeletal‐dominated (>70%), with usually <20% interparticle cement‐matrix and <10% siliciclasts, and they have facies attributes typical of nontropical carbonates. The range of diagenetic features identified within the limestones is the basis for assigning them to a small number of “end‐member” cementation classes that are inferred to be associated with four, broad, diagenetic settings. Class I limestones have very open fabrics dominated by abraded bryomol (bryozoan + bivalve mollusc) facies skeletons coated with isopachous rinds of typically nonferroan, dull to blotchy luminescent, fibrous to bladed spar, often with porosity occlusion by detrital and/or precipitated micrite. The limestones are usually thin and rare, of Oligocene — early Miocene age, and are interpreted to have formed...

Modification of fracture porosity by multiphase vein mineralization in an Oligocene nontropical carbonate reservoir, Taranaki Basin, New Zealand

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 (18O and 13C), 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 27C, corresponding to 0.5 km burial, and continued until 37C, 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 35–50C 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 50–60C 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 65–80C. 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 (53–65C), 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.

Lithostratigraphy and depositional episodes of the Oligocene carbonate-rich Tikorangi Formation, Taranaki Basin, New Zealand

Abstract The subsurface Oligocene Tikorangi Formation is a unique and important oil producer in the onshore Waihapa‐Ngaere Field, Taranaki Basin, being the only carbonate and fracture‐producing reservoir within the basin. Core sample data from seven onshore wells (foredeep megafacies) and a single offshore well (basinal megafacies) are correlated with a suite of sonic and gamma‐ray geophysical well log data to derive interpretative carbonate facies for the Tikorangi Formation. Four mixed siliciclastic ‐carbonate to carbonate facies have been defined: facies A— calcareous siliciclastite (<25% carbonate); facies B—very calcareous siliciclastite (25–50% carbonate); facies C— muddy limestone (50–75% carbonate); and facies D—coarse limestone (>75% carbonate). Single or interbedded combinations of these facies form the basis for identifying nine major lithostratigraphic units in the Tikorangi Formation that are correlatable between the eight wells in this study. The Tikorangi Formation accumulated across a shelf‐slope‐basin margin within a tectonically diversified basin setting, notably involving considerable off‐shelf redeposition of sediment into a bounding foredeep. Analysis of gamma, sonic, and resistivity well logs identifies five major episodes of sedimentary evolution. Episode I comprises re‐trogradational siliciclastic‐dominated redeposited units associated with foredeep subsidence. Episode II is a continuation of episode I retrogradation, but with increased mass‐redeposited carbonate influx during accelerated foredeep subsidence and relative sea‐level rise, the top marking the maximum flooding surface. Episode III involves a progradational sequence comprising relatively pure redeposited carbonate units associated with declining subsidence rates and minimal siliciclastic input, with movement of facies belts basinward. Episode IV consists of prograding aggradation involving essentially static facies belts dominated by often thick, periodically mass‐emplaced, carbonate‐rich units separated by thin background siliciclastic shale‐like units. Episode V is a retrogradational sequence marking the reintroduction of siliciclastic material into the basin following uplift of Mesozoic basement associated with accelerated compressional tectonics along the Australia‐Pacific plate boundary, initially diluting and ultimately extinguishing carbonate production factories and terminating deposition of the Tikorangi Formation.

Petrogenesis of diachronous mixed siliciclastic-carbonate megafacies in the cool-water Oligocene Tikorangi Formation, Taranaki Basin, New Zealand

Abstract The Oligocene (Whaingaroan‐Waitakian) Tikorangi Formation is a totally subsurface, litho‐stratigraphically complex, mixed siliciclastic‐limestone‐rich sequence forming an important fracture reservoir within Taranaki Basin, New Zealand. Petrographically the formation comprises a spectrum of interbedded rock types ranging from calcareous mudstone to wackestone to packstone to clean sparry grainstone. Skeletal and textural varieties within these rock types have aided in the identification of three environmentally distinctive megafacies for the Tikorangi Formation rocks—shelfal, foredeep, and basinal. Data from these megafacies have been used to detail previous conclusions on the petrogenesis and to further refine depositional paleoenvironmental models for the Tikorangi Formation in the central eastern Taranaki Basin margin. Shelfal Megafacies 1 rocks (reference well Hu Road‐1 A) are latest Oligocene (early Waitakian) in age and formed on or proximal to the Patea‐Tongaporutu‐Herangi basement high. They are characterised by coarse, skeletal‐rich, pure sparry grainstone comprising shallow water, high energy taxa (bryozoans, barnacles, red algae) and admixtures of coarse well‐rounded lithic sand derived from Mesozoic basement greywacke. This facies type has previously gone unrecorded in the Tikorangi Formation. Megafacies 2 is a latest Oligocene (early Waitakian) foredeep megafacies (formerly named shelfal facies) formed immediately basinward and west of the shelfal basement platform. It accumulated relatively rapidly (>20 cm/ka) from redeposition of shelfal megafacies biota that became intermixed with bathyal taxa to produce a spectrum of typically mudstone through to sparry grainstone. The resulting skeletal mix (bivalve, echinoderm, planktic and benthic foraminiferal, red algal, bryozoan, nannofossil) is unlike that in any of the age‐equivalent limestone units in neighbouring onland King Country Basin. Megafacies 3 is an Oligocene (Whaingaroan‐Waitakian) offshore basinal megafacies (formerly termed bathyal facies) of planktic foraminiferal‐nannofossil‐siliciclastic wackestone and mudstone formed away from redepositional influences. The siliciclastic input in this distal basinal setting (sedimentation rates <7 mm/ka) was probably sourced mainly from oceanic currents carrying suspended sediment from South Island provenances exposed at this time. Tikorangi Formation rocks record the Taranaki Basins only period of carbonate‐dominated sedimentation across a full range of shelfal, foredeep, and basinal settings. Depositional controls on the three contrasting megafacies were fundamentally the interplay of an evolving and complex plate tectonic setting, including development of a carbonate foredeep, changes in relative sea level within an overall transgressive regime, and changing availability, sources, and modes of deposition of both bioclastic and siliciclastic sediments. The mixed siliciclastic‐carbonate nature of the formation, and its skeletal assemblages, low‐Mg calcite mineralogy, and delayed deep burial diagenetic history, are features consistent with formation in temperate‐latitude cool waters.

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.

Mixed glauconitic-carbonate-siliciclastic surficial sediments on the north Kaipara continental margin, northwestern North Island, New Zealand

Abstract A mosaic of siliciclastic and mixed carbonate-siliciclastic sediments and authigenic minerals occurs at shelf and slope depths (30–1015 m water depth) on the open, wave-dominated north Kaipara continental margin (NKCM) off northern New Zealand. Texture and composition define five surficial sediment facies. Facies 1 (siliciclastic sand) comprises generally well-sorted fine sands that extend to outer shelf depths. Facies 2 (glauconitic sand) is composed of 30–95% authigenic glauconite grains at 150–400 m water depth in central to northern portions of NKCM. Facies 3 (mixed bryozoan-siliciclastic sand) occurs only in northernmost NKCM and involves a conspicuous (>40%) bryozoan carbonate content. Facies 4 (pelletal mud) occupies the mid shelf (100–150 m water depth) in northern NKCM and consists of muddy sediment with >30% mixed carbonate-siliciclastic pellets of probable fecal origin. Facies 5 (foraminiferal mud and sand) contains >30% foraminiferal tests at slope depths in southern NKCM and at both slope and mid-outer shelf depths towards the north. The siliciclastic mineralogy is consistent with mainly distant provenances to the south of the NKCM in central North Island and northern South Island, while much of the bryozoan material is likely reworked from the Three Kings carbonate platform to the north. No single shelf sedimentation model explains the complex facies distributions on the NKCM because the deposits record the interplay of several present and past hydrodynamic and sediment supply controls. While truly modern deposits may occur inshore, the bulk of NKCM surficial sediments are mainly palimpsest and/or relict deposits.

The enigma of intricately fitted beach boulders near Raglan, New Zealand

ABSTRACT An intertidal rocky platform tucked in behind a rocky headland on open-ocean Gibson Beach, near Raglan, supports an agglomeration of cobble- to large-boulder-sized clasts of Cenozoic sandstone and limestone. Rather than exhibiting just point contacts, many larger clasts are tightly interlocked and fitted with their neighbours and/or the underlying platform bedrock. Clast interface geometry relates to the strength contrast between adjacent rock types, linked to their calcite (cement) content. The end-product is an armoured, highly stable framework of boulder clasts resembling a giant three-dimensional jigsaw puzzle. While the direct impact of breaking waves likely plays a role in in situ jostling of boulders, we speculate that mechanical abrasion and fitting between larger clasts may also be promoted and maintained by in situ microvibration of the boulders as a consequence of wave-induced microseismic shaking within the cliff-backed rocky platform and headland, especially during major storm wave assault from the southwest.