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

New insights into the condensed nature and stratigraphic significance of the Late Neogene Ariki Formation, Taranaki Basin

The Ariki Formation is a distinctive Late Miocene – Early Pliocene marl facies rich in planktic foraminifera, reaching thicknesses in the range 70 - 109 m in most exploration holes drilled into the Western Platform northwest of Taranaki Peninsula. In Awatea-1 and Mangaa-1 in the Northern Graben, however, there are two marl units separated by the Mangaa “B” Sands. The lower unit has the same upper Tongaporutuan and Kapitean age as the lower part of the marl on the Western Platform, and the upper marl has an Upper Opoitian - Waipipian age, similar to the upper part of the Ariki Formation on the platform. In other holes located on the margins of the graben there can be one thin marly horizon, which usually correlates with the upper marl unit in Awatea-1 and Mangaa-1. The presence of two marly units in the Northern Graben, which are probably amalgamated on the western Platform, suggests two periods of late Neogene condensed sedimentation in northern Taranaki Basin arising from siliciclastic sediment starvation, separated by a period of submarine fan accumulation (Mangaa ‘B’ sands) following subsidence of the Northern Graben. We attribute the initial interval of marl accumulation mainly to a marked landward shift in the position of coastal onlap in central and southern Taranaki and in the region east of the Taranaki Fault Zone (southern King Country and northern Wanganui regions), which effectively shut-off the supply of siliciclastic sediment to northern Taranaki Basin, thereby enabling marl to accumulate. The start of accumulation of the upper part of the Ariki Formation and its marly correlatives in and around the Northern Graben, is attributed to a younger (upper Opoitian) landward shift in the position of coastal onlap, this time involving the formation of the Wanganui Basin depocentre and Toru Trough, which trapped the contemporary siliciclastic sediment being supplied from the south. A lower Opoitian phase of progradation between these two phases of retrogradation led to accumulation of the lower part of the Mangaa Formation (Mangaa ’B’ sands), which was limited in its extent to the Northern Graben because bounding normal faults had by then developed sea floor relief precluding mass-emplaced siliciclastic sediment from being deposited on the higher standing Western Platform. The accumulation of Ariki Formation marl in northern Taranaki Basin ended during the mid-Pliocene due to progradation of a thick continental margin wedge (Giant Foresets Formation) across the Northern Graben and Western Platform

Eastern Taranaki Basin field guide.

Linking the onshore and offshore parts of Eastern Taranaki Basin: Insights to stratigraphic architecture, sedimentary facies, sequence stratigraphy, paleogeography and hydrocarbon exploration from the on land record

Evolution of the Giant Foresets Formation, northern Taranaki Basin, New Zealand

Plio-Pleistocene aggradation and progradation has resulted in the rapid outbuilding of the continental shelf margin, northern Taranaki Basin. Seismic reflection profiles reveal that this outbuilding is characterised by bold clinoforms which offlap in a basinward direction. This stacked succession of clinoforms, collectively termed the Giant Foresets Formation, obtains thicknesses of over 2 km in places, and has had a significant effect on the thermal regime of the region. This integrated study was initiated to document the Late Neogene evolution of this formation, and thereby gain insights on sedimentary distribution patterns, timing of sedimentation, and controls on progradation and aggradation. Latest Miocene extension in the northern Taranaki Basin, related to rotation of the Hikurangi subduction zone, greatly influenced sedimentation patterns in the Pliocene. Palinspastic reconstruction shows that initial extension of the Northern Graben occurred before Giant Foresets Formation sedimentation began. Sediment, sourced from erosion to the east, was preferentially funneled into the newly created Northern Graben during the late Miocene and early Pliocene, while areas to the north and west underwent a period of sediment starvation. During the late Pliocene, and into the Pleistocene, sediment accumulation outpaced graben extension, and by the end of the Mangapanian, the graben was overtopped. During this period, the progradational front associated with the outbuilding of the continental shelf-slope margin advanced northwards. Throughout the Nukumaruan, continuing to the present day, shelf migration was extremely rapid. While at least seven cyclical sea level changes, with an approximate periodicity of 400 ka (fourth-order) have been identified, overall, depths shallowed from dominantly bathyal, to dominantly shelfal

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