Field trip guide to the Onland Oligocene-Miocene Sedimentary Record, Eastern Taranaki Basin Margin

This field guide affords a north to south transect through examples of the Mesozoic to Quaternary sedimentary succession exposed in the Waikato, King Country and coastal strip of the eastern Taranaki basins, with particular focus on the Oligocene and Miocene deposits and how these link into the offshore parts of Taranaki Basin. The trip starts in Hamilton and ends at Tongaporutu on the north Taranaki coast, with overnight accommodation available at either Awakino or Mokau. Primarily under both local and more distant tectonic control, the stops provide examples of the various carbonate and terrigenous (locally volcaniclastic)-dominated facies associated with marginal marine, shoreline, shelf and slope-to-basin depositional settings, and their stratigraphic architecture and wider sequence stratigraphic context. Along the way, visits are recorded to basement greywacke, serpentinite and limestone quarries

Field trip guide to Oligocene Limestones and Caves in the Waitomo District

The field guide runs from Hamilton to Waitomo to Te Anga and return in limestone-dominated country developed in transgressive sedimentary deposits of the Oligocene Te Kuiti Group – a world class example of a temperate shelf carbonate depositional system. Attention focuses on the nature, distribution and paleoenvironmental controls of the main limestone facies and some of the mixed terrigenous-carbonate facies in the Group. Along the way features of the Waitomo karst landscape are noted and the trip concludes by going underground in the Ruakuri Cave to discuss cave origins and the evidence for paleoenvironmental changes locked up in speleothems

Note on paramoudra-like carbonate concretions in the Urenui Formation, North Taranaki: possible plumbing system for a Late Miocene methane seep field

A reconnaissance study of calcitic and dolomitic tubular concretions in upper slope mudstone of the Late Miocene Urenui Formation exposed along the north Taranaki coastline indicates that they have a complex diagenetic history involving different phases of carbonate cementation and likely hydrofracturing associated with build up of fluid/gas pressures. The concretions resemble classical paramoudra in the European chalk, but are not siliceous and do not have a trace fossil origin. Stable oxygen and carbon isotope data suggest that the micritic carbonate cements in the Urenui paramoudra were probably sourced primarily from ascending methane fluid/gases, and that they precipitated entirely within the host mudstone below the seafloor. We suggest the paramoudra may mark the subsurface plumbing networks of a Late Miocene cold seep system, in which case they have relevance to the evolution and migration of hydrocarbons in Taranaki Basin, at this site perhaps focussed along the Taranaki Fault. The presence of dislodged and mass-emplaced paramoudra in the axial conglomerate of channels within the Urenui mudstone suggests there could be a connection between the loci of seep field development and slope failure and canyon cutting on the Late Miocene Taranaki margin

Stratigraphy and development of the Late Miocene-Early Pleistocene Hawke’s Bay forearc basin

A Late Miocene-Early Pleistocene mixed carbonate-siliciclastic sedimentary succession about 2 500 m thick in the Hawke’s Bay forearc basin is the focus of a basin analysis. The area under investigation covers 3 500 km2 of western and central Hawke’s Bay. The stratigraphy of Hawke’s Bay Basin is characterised by dramatic vertical and lateral facies changes and significant fluxes of siliciclastic sediment through the Late Miocene and Pliocene. This project aims to better understand the character and origin of the sedimentary succession in the basin. Geological mapping has been undertaken at a scale of 1:25000, with data managed in an ARCINFO geodatabase, following the database model employed in the IGNS QMap programme. Along the western margin of the basin there is progressive southward onlap of late Cenozoic strata on to basement. The oldest units are of Late Miocene (Tongaporutuan) age and the youngest onlap units are of latest Pliocene (Nukumaruan) age. Geological mapping of the basin fill places constraints on the magnitude (about 10 km) and timing (Pleistocene) of most of the offset on the North Island Shear Belt. Lithofacies have been described and interpreted representing fluvial, estuarine, shoreface and inner- to outer-shelf environments. Conglomerate facies are representative of sediment-saturated prograding fluvial braidplains and river deltas. These units are dominated by greywacke gravels and record the erosion of the Kaweka-Ahimanawa Ranges. Sandstone facies typically comprise very well sorted, clean non-cemented units of 10-50 m thickness that accumulated in innershelf environments. Siltstone facies probably accumulated in relatively quiet, middle- to outer-shelf water depths, and comprise well-sorted, firm non-cemented units with occasional tephra interbeds. Limestone facies represent examples of continent-attached cool-water carbonate systems that developed in response to strong tidal currents and a high nutrient flux during the Pliocene. These facies are examples of mixed siliciclastic-bioclastic sedimentary systems. Of these facies the widespread distribution and thickness of sandstone and limestone units present the most potential for hydrocarbon reservoirs. Similarly, the distribution of siltstone and mudstone beds provides adequate seal rocks. Mangapanian limestone facies have already been targeted as potential petroleum reservoirs (e.g. Kereru-1). Geological mapping suggests that potential hydrocarbon reservoir and seal rocks occur extensively in the subsurface

Rapid progradation of the Pliocene-Pleistocene continental margin, northern Taranaki Basin, New Zealand, and implications

Progradation and aggradation of the modern continental margin in northern Taranaki Basin has resulted in the deposition of a thick and rapidly accumulated Pliocene-Pleistocene sedimentary succession. It includes the predominantly muddy Giant Foresets Formation, and the underlying sandy Mangaa Formation. Investigation of the internal attributes and depositional systems associated with the Giant Foresets Formation suggests that it would provide very little effective reservoir for hydrocarbon accumulations, although it does provide essential seal and overburden properties. While the sand-dominated Mangaa Formation could be a hydrocarbon reservoir, drilling so far has yet to reveal any significant hydrocarbon shows. Undoubtedly the most significant contribution that the Giant Foresets and Mangaa Formations have had on petroleum systems in northern Taranaki Basin is the cumulative effect that rapid and substantial accumulation has had on maturation and migration of hydrocarbons in the underlying formations. Palinspastic restoration of a seismic reflection profile across the Northern Graben, together with isopach mapping of stratigraphic section for biostratigraphic stages, indicates that the thickest part of the Pliocene-Pleistocene succession is along the central axis of the Northern Graben. Deposition of this succession contributed substantially to subsidence within the graben, providing further accommodation for sediment accumulation. Isopach and structure contour maps also reveal the extent to which submarine volcanic massifs were exposed along the axis of the graben and the timing of movement on major faults

Geological structure of the forearc basin in central Hawke’s Bay, eastern North Island

Central Hawke’s Bay lies within an extensive forearc basin in eastern North Island that developed during the Late Miocene to Pleistocene. The onshore structural elements of Hawke’s Bay can be classified into four structural domains, each reflecting differing styles and scales of deformation. These domains are from west to east, the axial range domain, the range front con¬tractional domain, the central forearc basin domain, and the eastern contractional domain. Some degree of the oblique-interaction of the Australia and Pacific plates on the subduction thrust is inferred to be partitioned across the four structural domains and to be expressed dominantly as oblique-(dextral) slip on faults bordering the axial ranges, and as short¬ening on reverse faults and folds in more eastern parts of the forearc. The axial range domain involves the eastern parts of the North Island axial ranges where there is marked oblique-slip displacement on major faults. Some dextral offest is accommodated in the range front contractional domain, although dip-slip displacement is more significant. The central forearc basin domain is comparatively undeformed with only minor reverse faulting and (fault-force driven) folding. By comparison, the ad¬jacent eastern contractional domain, which comprises an accretionary wedge, is characterised by imbricate reverse and thrust faulting and associated folding. A small degree of dextral-slip is also accommodated in this domain. The uppermost parts of the inboard margin of the accretionary wedge, particularly the part onshore, is currently undergoing gravitationally-driven collapse expressed as deep-seated landslides and normal faulting. Many folds in the basin are fault-cored, several of which have been targeted in recent years by petroleum exploration companies (e.g. Hukarere-, Whakatu-and Kereru-). Most deformation of the forearc basin fill in central Hawke’s Bay is post early Nukumaruan (2.4 Ma) and much of this has occurred since the early Pleistocene (.8 Ma). Dextral-slip on Mohaka and Ruahine Faults since the Early Pliocene is likely to be less than 0 km. Significant unconformities in the basin fill reflect early phases of development of oblique-slip faults in the axial ranges. New dextral oblique-slip faults are developing in the basin fill to the east of the main oblique-slip faults bordering the ranges

Constraints on the evolution of Taranaki Fault from thermochronology and basin analysis: Implications for the Taranaki Fault play

Taranaki Fault is the major structure defining the eastern margin of Taranaki Basin and marks the juxtaposition of basement with the Late Cretaceous-Paleogene succession in the basin. Although the timing of the basement over-thrusting on Taranaki Fault and subsequent marine onlap on to the basement block are well constrained as having occurred during the Early Miocene, the age of formation of this major structure, its character, displacement history and associated regional vertical movement during the Late Cretaceous- Recent are otherwise poorly known. Here we have applied (i) apatite fission track thermochronology to Mesozoic basement encountered in exploration holes and in outcrop to constrain the amount and timing of Late Cretaceous-Eocene exhumation of the eastern side of the fault, (ii) basin analysis of the Oligocene and Miocene succession east of the fault to establish the late-Early Miocene - Early Pliocene subsidence history, and (iii), regional porosity-bulk density trends in Neogene mudstone to establish the late uplift and tilting of eastern Taranaki Basin margin, which may have been associated with the main period of charge of the underlying Taranaki Fault play. We make the following conclusions that may be useful in assessing the viability of the Taranaki Fault play. (1) Mid-Cretaceous Taniwha Formation, intersected in Te Ranga-1 was formerly extensive across the western half of the Kawhia Syncline between Port Waikato and Awakino. (2) Taranaki Fault first formed as a normalfault during the Late Cretaceous around 85±10 Ma, and formed the eastern boundary of the Taranaki Rift-Transform basin. (3) Manganui Fault, located onshore north of Awakino, formed as a steeply east dipping reverse fault and accommodated about four km of displacement during the mid-Cretaceous. (4) Uplift and erosion, involving inversion of Early Oligocene deposits, occurred along the Herangi High during the Late Oligocene. This may have been associated with initial reverse movement on Taranaki Fault. (5) During the Early Miocene (Otaian Stage) the Taranaki and Manganui Faults accommodated the westward transport of Murihiku basement into the eastern margin of Taranaki Basin, but the amount of topography generated over the Herangi High can only have been a few hundred metres in elevation. (6) The Altonian (19-16 Ma) marked the start of the collapse of the eastern margin of Taranaki Basin that lead during the Middle Miocene to the eastward retrogradation of the continental margin wedge into the King Country region. During the Late Miocene, from about 11 Ma, a thick shelf-slope continental margin wedge prograded northward into the King Country region and infilled it (Mt Messenger, Urenui, Kiore and Matemateaonga Formations). (7) During the Pliocene and Pleistocene the whole of central New Zealand, including the eastern margin of Taranaki Basin, became involved in long wavelength up-doming with 1-2 km erosion of much of the Neogene succession in the King Country region. This regionally elevated the Taranaki Fault play into which hydrocarbons may have migrated from the Northern Graben region

Petrologic evidence for earliest Miocene tectonic mobility on eastern Taranaki Basin margin

At Gibsons Beach on the west coast of central North Island, the earliest Miocene (Waitakian) Otorohanga Limestone, the top-most formation in the Te Kuiti Group, is unconformably overlain on an undulating, locally channelised erosion surface by the Early Miocene (Otaian) Papakura Limestone at the base of the Waitemata Group. The basal facies of the Papakura Limestone is a conglomerate composed exclusively of tightly packed pebble- to cobble-sized clasts of skeletal limestone sourced from the underlying Otorohanga Limestone. This petrographic and geochemical study demonstrates that the Otorohanga Limestone was partially lithified during marine and shallow-burial cementation at subsurface depths down to a few tens of metres prior to uplift, erosion and cannibalisation of the limestone clasts into the Papakura Limestone. Strontium isotope dating of fossils from both the Otorohanga and Papakura Limestones at Gibsons Beach yield comparable ages of about 22 Ma, close to the Waitakian/Otaian boundary, indicating very rapid tectonic inversion and erosion of the section occurred in the earliest Miocene. We envisage the clasts of Otorohanga Limestone were sourced from a proximal shoreline position and redeposited westwards by episodic debris flows onto a shallow-shelf accumulating mixed siliciclastic-skeletal carbonate deposits of the Papakura Limestone. Subsequent burial of both limestones by rapidly accumulating Waitemata Group sandstone and flysch instigated precipitation of widespread burial cements from pressure dissolution of carbonate material at subsurface depths from about 100 m to 1.0 km. The vertical tectonic movements registered at Gibsons Beach can be related to the oblique compression associated with the development of the Australian-Pacific plate boundary through New Zealand at about this time and coincide with overthrusting of basement into Taranaki Basin between mid-Waitakian (earliest Miocene) and Altonian (late Early Miocene) times

Sequence stratigraphy and architectural elements of the Giant Foresets Formation, northern Taranaki Basin, New Zealand

The modern continental margin in northern Taranaki Basin is underlain by a thick, mud-dominated, Pliocene and Pleistocene succession (Giant Foresets Formation, GFF) clearly imaged in seismic reflection datasets. A study focusing on the geometry and internal reflection character of the GFF has revealed structural, sedimentological, and eustatic controls on its accumulation. Isopach maps prepared for northern Taranaki Basin show shifts through time in the main loci of sediment accumulation of the Mangaa Formation and Giant Foresets Formation. During the Early Pliocene (Opoitian Stage) deposition was focused in the southern part of the Northern Graben. The prograda¬tional front moved into the vicinity of Arawa-1 and Taimana-on the Western Platform during the early-Late Pliocene (Waipipian and Mangapanian Stages), forming large mounded slope fans. Through the latest Pliocene (Mangapanian - lower Nukumaruan Stages) the progradational front moved rapidly to the north and west through and across the Northern Graben to form a distinct shelf-slope depositional front. During the Pleistocene (upper Nukumaruan Stage – Recent), the progradational front straightened out, reaching the present position of the shelf-slope break. Even during the Pleistocene, broad subsidence persisted in the Northern Graben, trapping a proportion of the sediment flux being delivered to this part of the basin. The Late Pliocene part of the GFF, particularly where it prograded on to the Western Platform, displays classic clinoform profiles, with over steepening having resulted in mass-failure of paleoslopes. Major degradation of the shelf edge and slope occurred during the Early Pleistocene, reflecting a change in the calibre and flux of sediment sourced to the continental margin. Detailed examination of part of the GFF not significantly affected by mass-failure indicates that small-scale channel levee and overbank deposits dominate slope deposition, while basin floor deposits are characterised by slope-disconnected muddy and silty basin floor fans, with little lateral continuity between systems. In a sequence stratigraphic context, many of the dominant components of each seismic unit (slumps, fans, and channel-levee complexes) were deposited during the falling (RST) and low (LST) sea level parts of a relative sea level cycle, resulting in highly asymmetric sequences. While the GFF is considered to have minor reservoir potential in terms of containing sandstone-dominated stratigraphic traps, it does afford the opportunity to study in detail how deep-water clastic systems evolved in response to the various factors that control depositional architectures, particularly in a rapidly prograding muddy continen¬tal margin system

Tubular carbonate concretions as hydrocarbon migration pathways? Examples from North Island, New Zealand

Cold seep carbonate deposits are associated with the development on the sea floor of distinctive chemosyn¬thetic animal communities and carbonate minerali¬sation as a consequence of microbially mediated anaerobic oxidation of methane. Several possible sources of the methane exist, identifiable from the carbon isotope values of the carbonate precipitates. In the modern, seep carbonates can occur on the sea floor above petroleum reservoirs where an important origin can be from ascending thermogenic hydrocar¬bons. The character of geological structures marking the ascent pathways from deep in the subsurface to shallow subsurface levels are poorly understood, but one such structure resulting from focused fluid flow may be tubular carbonate concretions. Several mudrock-dominated Cenozoic (especially Miocene) sedimentary formations in the North Island of New Zealand include carbonate concretions having a wide range of tubular morphologies. The concretions are typically oriented at high angles to bedding, and often have a central conduit that is either empty or filled with late stage cements. Stable isotope analyses (δ13C, δ18O) suggest that the carbonate cements in the concretions precipitated mainly from ascending methane, likely sourced from a mixture of deep thermogenic and shallow biogenic sources. A clear link between the tubular concretions and overlying paleo-sea floor seep-carbonate deposits exists at some sites. We suggest that the tubular carbonate concretions mark the subsurface plumbing network of cold seep systems. When exposed and accessible in outcrop, they afford an opportunity to investigate the geochemical evolution of cold seeps, and possibly also the nature of linkages between subsurface and surface portions of such a system. Seep field development has implications for the characterisation of fluid flow in sedimentary basins, for the global carbon cycle, for exerting a biogeochemical influence on the development of marine communities, and for the evaluation of future hydrocarbon resources, recovery, and drilling and production hazards. These matters remain to be fully assessed within a petroleum systems framework for New Zealand’s Cenozoic sedimentary basins