Characteristics and evolution of a dynamic prograding continental margin: the late Neogene Giant Foresets Formation, northern Taranaki Basin, New Zealand
The Pliocene-Pleistocene Giant Foresets Formation represents the progradation and development of the modem continental margin in northern Taranaki Basin. The dynamic evolution of this formation is fundamentally linked to a Pliocene-Pleistocene increase in sediment flux sourced from South Island, to extensional graben formation in northern Taranaki Basin, to flexural subsidence in Wanganui Basin, and to uplift and erosion in King Country Basin. The development of the Giant Foresets Formation is summarised in this thesis by a series of paleogeographic maps based on the interpretation of wireline data from hydrocarbon exploration holes, industry-acquired seismic reflection data, and the paleoenvironmental interpretation of foraminiferal data derived from well cuttings samples. Outbuilding of the modem continental shelf-slope margin began during the Early Miocene as the regressive phase of a 1ˢᵗ-order megacycle. Four 2ⁿᵈ-order cycles superimposed on this megacycle have been identified in Taranaki, Wanganui and King Country Basins. The evolution of the Giant Foresets Formation is closely linked to the youngest two megacycles: the Mid Miocene-earliest Pliocene Whangamomona Group/megacycle, and the mid Pliocene-Pleistocene Rangitikei Supergroup/megacycle. The Whangamomona Group includes the Otunui, Mount Messenger, Urenui and Kiore Formations. The continental margin associated with the Whangamomona Group had only limited extent in southern and eastern parts of Taranaki Basin. The Rangitikei Supergroup in Wanganui Basin includes the Tangahoe Mudstone and younger Plio-Pleistocene sediments. The Giant Foresets Formation represents the Rangitikei Supergroup in northern Taranaki Basin. Revision of the biostratigraphy of four well sections, and integration with biostratigraphic data from other well sections, has highlighted the development in northern Taranaki Basin of a region-wide unconformity between Miocene (Late Tongaporutuan to Late Kapitean) and Pliocene (Early to mid Opoitian) strata, prior to deposition of the Giant Foresets Formation. This is expressed as a paraconformity across much of the northern part of the Western Stable Platform (Ariki Formation), thinner condensed intervals in the Northern Graben, and an erosional unconformity across the Turi Fault Zone. The Ariki Formation is related to at least three important events: (i) the limited extent of progradation of the Whangamomona Group foreset front; (ii) initiation of extension of the Northern Graben; and (iii), the timing of the Tangahoe pulldown in Wanganui Basin. Palinspastic restoration of a complete seismic reflection profile, and backstripping and decompaction of eleven well sections, suggests that paleobathymetric relief may have been locally important in extending the duration of terrigenous sediment starvation. Six wireline facies have been identified in the Giant Foresets Formation and underlying units. These are: (i) hemipelagic facies; (ii) basin floor fan facies; (iii) slope fan facies; (iv) shelf facies; (v) slumped facies; and (vi) condensed facies. A number of subfacies have also been identified. Construction of a fence diagram using the facies motifs identified in this study has highlighted the distribution of volcaniclastic versus siliciclastic and hemipelagic sedimentation patterns in both time and space in northern Taranaki Basin. The Miocene was dominated by hemipelagic facies interfingering with coarser lithologies sourced predominantly from reworked volcaniclastic material, which travelled some distance into the northwestern region of northern Taranaki Basin. The Plio-Pleistocene Giant Foresets Formation is characterised by a hemipelagic motif, reflecting the dominantly fine-grained nature of the unit, although the formation displays an overall coarsening-upward trend, reflecting increasing proximity to the advancing foreset front and the development of shelf deposits. Localised Pliocene basin floor fans in the Northern Graben (Mangaa Formation) emphasize the role of the graben as a sink for sedimentation during the Opoitian and Waipipian. Identification and mapping of more than 70 seismic units within the Giant Foresets Formation in northern Taranaki Basin has allowed the post-Miocene sediment distribution patterns to be characterised. Isopach maps and structure contour maps demonstrate the translation of depocentres through time, and illustrate how the sediment distribution pattern associated with the advancing continental margin gradually changed from one of focused sedimentation in the south, and along the axis of the Northern Graben (Mangaa Formation), to a more linear front. Numerous channels and channel systems have been mapped in the Mangapanian to Nukumaruan shelf succession, often preferentially oriented along the axis of the Northern Graben. The muddy texture of the Giant Foresets Formation is also shown by the seismic characteristics of the formation, with architectural elements and depositional styles reflecting mud- and mixed mud/sand-rich depositional systems. Basin floor (bottomset) facies are characterised by low- relief mounds and/or gull-wing geometries of channel-levee systems, or sub-parallel reflectors, while slope (foreset) facies are dominated by chaotic reflection configurations consistent with slumped deposits or numerous stacked small-scale channel-levee systems on the middle and lower slope. Much of the shallowing of water depth in northern Taranaki basin occurred during deposition of bottomset facies. Depositional environments changed from mid/lower bathyal during the Late Pliocene, to upper bathyal during the Waipipian and Mangapanian. However, the most rapid aggradation and progradation of the continental margin occurred during the Mangapanian to Late Nukumaruan, with progradational rates as high as 35 km/m.y. recorded. While the coarsest sediment textures are associated with shelf (topset) facies, and the youngest (degradational) foreset units, examination of benthic foraminiferal samples indicate that there was effective transport from the shelf to the base of slope throughout the depositional history of the Giant Foresets Formation. A dramatic (uphole) decrease in planktic foraminiferal percentage between the Ariki/ Manganui Formations and the Giant Foresets Formation, with a corresponding change in watermass conditions but no dramatic change in water depth, suggests that the progradation of the continental margin during the latest Miocene and Early Pliocene acted to deflect coastal upwelling cells away from northern Taranaki Basin, decreasing available surface water nutrients, and thus reducing the abundance of planktic foraminifera during this time. Cyclicity within the Giant Foresets Formation is evident on seismic reflection profiles, in wireline motifs, and from paleontological evidence. Sequences may be defined by a particular wireline motif, or set of motifs, the use of seismic reflection configurations to identify components of depositional environments interpreted to represent regressive (RST) and lowstand (LST) sea level conditions, and through the identification of mixed deep water and shallow water benthic foraminiferal assemblages. Basinward of the shelf margin, the highstand systems tract (HST) is often only represented by a bright, high amplitude and laterally continuous reflector that rolls over into shelf deposits. Transgressive (TST) and HST components of a relative sea level cycle cannot be separated from each other on seismic reflection profiles of the shelf deposits of the Giant Foresets Formation. Sequence boundaries often correspond to a bold seismic reflector horizon. A number of discrete high order sequences have been identified, but many of these cannot be consistently correlated between well sections. However, three levels of cyclicity can be established: 2ⁿᵈ-order cycles related to progradation of the Whangamomona Group and Rangitikei Supergroup, 4ᵗʰ-order (400 ka) cycles, and 5ᵗʰ - order (100 ka) cycles (possibly related to glacio-eustatic controls). Sequences are highly asymmetrical, with most of the deposition occurring during lowering and low sea level conditions. Sediment flux is the predominant factor controlling the seismic character of well sections. However, three levels of cyclicity can be established: 2ⁿᵈ-order cycles related to progradation of the Whangamomona Group and Rangitikei Supergroup, 4ᵗʰ-order (400 ka) cycles, and 5ᵗʰ-order (100 ka) cycles (possibly related to glacio-eustatic controls). Sequences are highly asymmetrical, with most of the deposition occurring during lowering and low sea level conditions. Sediment flux is the predominant factor controlling the seismic character of the Late Opoitian to Mangapanian parts of the Giant Foresets Formation, while changes in relative sea level during the Nukumaruan to Castlecliffian increasingly influenced stratal architecture. The shoreline is not considered to have moved basinward of the shelf break during any of the sea level cycles represented in the Giant Foresets Formation, except possibly for a few cycles during the Late Nukumaruan and Castlecliffian.
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