|dc.description.abstract||Hawke's Bay province lies within an extensive forearc basin in eastern North Island, New Zealand, that developed during the Late Miocene to present. An area of about 5 700 kmsup2; in central Hawke's Bay has been geologically mapped at 1:50 000 scale as part of an analysis of the Late Miocene-Early Pleistocene basin fill.
A substantially revised lithostratigraphic nomenclature is proposed for the Neogene succession, particularly for the latest Miocene-Early Pleistocene part (Mangaheia Group). The Tolaga Group is extended into central Hawke's Bay from the north, and incorporates the Early Miocene-Early Pliocene succession. It is proposed that the Hawke's Bay, Petane, Napier, and Poporangi Groups be abolished, and incorporated into a geographically expanded Mangaheia Group. The Petane Group is demoted to Petane Formation. Most formations within the group are now redefined as members, with the exceptions of the Esk Mudstone and Kaiwaka Formations, which are retained as separate formations.
An age model has been developed for the basin fill, chiefly using molluscan biostratigraphy. The Tongaporutuan-Kapitean boundary occurs in the Waitere Formation, and the Kapitean-Opoitian boundary occurs within the Mokonui Sandstone. The Opoitian-Waipipian boundary probably occurs in the Titiokura Formation, and the Waipipian-Mangapanian boundary in the Te Waka and Pohue Formations. The Mangapanian-Nukumaruan boundary has been identified at many localities, and occurs in several stratigraphic units, including the Papakiri Member (Matahorua Formation) and Sentry Box Formation. Geochemical analysis of glass shards in the Hikuroa Pumice Member (Petane Formation) suggests a correlation to tephra in the ODP 1124 record, and an inferred age of c. 2.15 Ma is suggested for this unit. The Plio-Pleistocene boundary is located in the Waipatiki Limestone Member (Petane Formation), and the top of the Olduvai paleomagnetic subchron occurs in the overlying Devils Elbow Mudstone Member.
The geological structure of the basin is classified into four structural domains. The axial range domain involves the eastern parts of the North Island axial ranges and there is marked oblique-slip displacement on major faults within it. Some oblique-slip is accommodated in the adjacent range front contractional domain, although dip-slip displacement is more significant. The more easterly central forearc basin domain is comparatively undeformed with only minor reverse faulting and associated folding. The eastern contractional domain comprises the inboard margin of the accretionary wedge, and is characterised by imbricate reverse and thrust faults, and associated folding. The uppermost parts of the accretionary wedge are currently undergoing gravitationally-induced collapse, expressed as deep-seated landslides and normal faulting.
While significant unconformities in the Neogene succession possibly reflect early phases in the development of the major faults in the North Island Shear Belt, most deformation of the basin fill is relatively young (post-lowermost Nukumaruan, c. 2.4 Ma), and much of this has occurred since the Early Pleistocene (c. 1.8 Ma), when deformation apparently intensified. This intensification coincides with the initiation of volcanism and rifting in the Taupo Volcanic Zone. The amount of Pliocene-Recent dextral-slip on the Ruahine Fault is likely to be less than 10 km, and there is probably less than 1 km of dextral-slip on the Mohaka Fault. New dextral-slip faults are developing in the basin fill east of the main oblique-slip faults, possibly due to dextral rotation of eastern North Island and the Hikurangi margin.
Forty-one lithofacies within the Late Miocene-Early Pleistocene sedimentary succession have been identified and grouped into six lithofacies assemblages. Each assemblage generally corresponds to a broad depositional environment. Siltstone lithofacies (inner to outer shelf) dominate the succession, followed by sandstone (shoreface-inner shelf), bioclastic (inner to middle shelf), mixed siliciclastic-bioclastic (nearshore to middle shelf), conglomerate (non-marine to shoreface), and volcaniclastic (non-marine to outer shelf) facies in decreasing abundance. In addition, thirty molluscan biofacies associations and sub-associations have been identified, representing both in situ and transported assemblages, and paleoenvironments ranging from estuarine to outer shelf settings.
Vail-type sequences, typically 20-80 m thick, are best developed in quote;middlequote; Pliocene to Early Pleistocene strata. These sequences are dominated by coarsening-upward packages of siliciclastic-dominated sediments, although bioclastic facies increase in prominence in Upper Nukumaruan cycles. Sequences are typically stacked in a strongly aggradational pattern, and although some periods of accelerated subsidence are recorded in the stratigraphic record, the aggradational nature of the succession shows that basin subsidence mostly kept pace with sediment flux during the Mangapanian to Upper Nukumaruan. Transgressive systems tracts (TSTs) typically comprise a combination of bioclastic and siliciclastic lithofacies. Highstand systems tracts (HSTs) are dominated by fine-grained siliciclastic-dominated facies. Regressive systems tracts (RSTs) may be either siliciclastic or bioclastic-dominated, although siliciclastic-dominated RSTs are most common. Lowstand systems tracts (LSTs) are mostly characterised by non-marine greywacke conglomerate beds. They sharply overlie shallow-marine rocks, and were deposited when high-bedload river systems prograded across a low-gradient coastal plain and exposed continental shelf.
Eight sequence motifs have been developed, each representing different positions across a paleoshelf. While these motifs share some similarities, unique combinations of subsidence, sediment flux, and sediment provenance have combined to differentiate them. An idealised quote;shoreline to slopequote; two-dimensional sequence model has been produced for the Nukumaruan part of the basin succession using the motifs. The model sequence illustrates the idealised distribution across a paleoshelf of the various lithofacies, macrofaunal associations, and sequence stratigraphic surfaces.
The Neogene geological history of central Hawke's Bay can be usefully subdivided into three major phases, each represented by one of the three lithostratigraphic groups documented. The Early Miocene-Early Pliocene (Otaian-Lower Opoitian) phase is represented by the Tolaga Group. This group comprises four deepening-upward bathyal-dominated packages with shelfal beds at their base. This succession is overlain by a thick sandstone (Mokonui Sandstone). Mokonui Sandstone is unconformably overlain by the Mangaheia Group (Upper Kapitean-Upper Nukumaruan), characterised by shelfal deposits with some cyclothemic intervals. Occasional upper bathyal beds occur, but appear to represent short-lived depositional phases. The uppermost Neogene phase is represented by the Middle Pleistocene (Castlecliffian) Kidnappers Group, characterised by thick non- to marginal-marine greywacke conglomerates.
The basin has been substantially inverted along its western side, involving movement on faults of the North Island Shear Belt. The stratigraphic record along the eastern margin of the forearc basin records the development of faulting and folding associated with the growth of the inboard part of the accretionary wedge, such that parts of the forearc basin succession have become involved in the accretionary wedge. The outcrop pattern of westward-younging Pliocene limestones demonstrates that the inboard margin of the accretionary wedge has migrated toward the centre of the basin over time. Younger limestone beds (e.g. Mason Ridge Formation) presently crop out close to the forearc basin axis and at lower elevations compared with older Pliocene limestone beds (e.g. Kairakau Limestone) located along the eastern margins of the basin.
Uplift of the accretionary wedge resulted in the development of a Pliocene interior seaway, which was most extensive during the Mangapanian and characterised by the development of prominent limestone formations (Te Aute lithofacies) along both margins. Although large volumes of siliciclastic sediment were entering the basin, strong tidal currents periodically swept the sea floor of siliciclastic sediment, allowing extensive continent-attached carbonate banks to develop along the western side of the basin. Thick continent-detached limestone beds developed along the eastern side of the basin due to the elevated positions of accretionary ridges. Breaches in the seaway were present at times in the areas of Kuripapango and the Manawatu Gorge. By the Lower Nukumaruan, the interior seaway became permanently closed to the south with uplift of the Mount Bruce block in northern Wairarapa.
The most important influence on the stratigraphic architecture of the Neogene succession has been the tectonic factor. In comparison with the well developed cyclothemic record in the adjacent Wanganui Basin, the cyclothemic record in the Hawke's Bay area is incomplete and poorly developed. In most parts of the succession, tectonic influences have overprinted glacio-eustatic sea-level fluctuations. Only a limited correlation can be achieved between rocks in the Mangaheia Group and the elta;sup18;O record contained in deep-sea records, as the sequences are not well enough developed in general to allow for cycle-by-cycle correlations.||en_NZ