Description, correlation and depositional history of Miocene sediments outcropping along North Taranaki Coast

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King, P.R.; Scott, G.H.; Robinson, P.H. 1993 Description, correlation and depositional history of Miocene sediments outcropping along North Taranaki Coast. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences monograph 5 199 p.

Abstract: A section of Neogene sedimentary rocks is almost continuously ''posed in coastal cliffs southwards from Awakino to just northeast of Motunui, north Taranaki. The sequence ranges in age from Altonian to Tonga Porutuan, and includes the stratotype for the Tongaporutuan Stage. The stratigraphic interval representing the Altonian to early Waiauan stages is very thin or missing. However, much of the latter interval is represented north of Awaidno, near Tirua Point, and its biostratigraphy is reviewed. The coastal region is underlain by an overthrust block of Mesozoic basement, bounded to the west by the Taranaki Fault. The outcropping Miocene sediments form part of a sequence that thins eastwards and progressively onlaps and overtops this basement block. North of Awakino, and a few kilometres inland, basement (Triassic Murihiku Supergroup) crop out. At its southern outcrop limit, basement is unconformably overlain by ?Otaian- to Altonian-aged shelf sandstones of the Mokau Formation.In turn, these sandstones are overlain by transgressive, deeper water mudstones (Altonian-aged) that are lateral equivalents of the Manganui Formation (Wai-iti Group), widely distributed within the Taranaki Basin. Manganui Formation sediments are the lowermost exposed along the coast (at Awakino river mouth). These comprise several metres of weakly-bedded, silty mudstones (Altonian), overlain by a thin, condensed, unconformity-bound, glauconitic and calcareous gritty sandstone (early Clifdenian). Overlying Neogene sediments exposed along the coast range in age from late Waiauan to late Tongaporutuan; they belong to the Mohakatino, Mount Messenger and Urenui Formations of the Wai-iti Group. These formations are restricted to the northeast and east of the Taranaki Basin, and to the neighbouring North Wanganui Basin. The north Taranaki coastal sequence provides a useful window for examining subsidence history and depositional patterns during the mid-late Miocene, along a part of the Taranaki Basin's eastern margin. This exposed sequence can be broadly correlated with subsurface sequences in nearby drillholes in the Taranaki Peninsula. The correlation is based on wireline log character and is facilitated by biostratigraphic information, particularly the dextral and sinistral coiling signatures of Globorotalia miotumida. The upper Mohakatino to Urenui measured section is a representative transect through a basin floor to uppermost slope progradational system. An unconformity at the top of the outcropping Manganui Formation probably relates to mid Neogene tilting and sea-floor erosion. In the late mid-Neogene, following a period of renewed subsidence, Mohakatino sediments were deposited in a deep marginal trough. At first detritus derived from coeval submarine volcanism to the northwest was re-deposited in well-stratified, highly volcaniclastic sandstone beds (Purupuru Tuff Member). Thereafter, upper Mohakatino clastic sediments, comprising thick-bedded sandstones and interbed sil tstones in varying proportions (Perry Sandstone and Tawariki Siltstone members), were deposited on the basin floor by high concentration gravity flows. Penecontemporaneous tectonic movements of the sea floor produced large-scale intraformational deformation of some Tawariki beds. Along the coast, total Mohakatino Formation thickness is around 270 m. The Mount Messenger Formation is around 700 m thick. It was deposited rapidly, and records an increased supply of clastic detritus into the deep water basin in the study area. The formation consists of upper and lower sandstone units and an intervening sandstone and siltstone interbedded unit. Thin- and thick-bedded sands were deposited by liquefied flows and high density turbidity currents, initially within submarine fan environments on the basin floor. Sediment influx exceeded subsidence rates, resulting in basin floor aggradation and slope progradation, with some accompanying episodes of slope failure. Ultimately, upper Mount Messenger turbidites were deposited in coalescing channel/levee or fan lobe complexes at the base of slope. Mount Messenger sediments are generally well-stratified and intensely bioturbated. Various other features include: massive and graded bedding, minor cross-stratification, Bouma sequences, broad scouring, rip-up clast horizons, local channeling, and occasional syn-sedimentary deformation structures. The Urenui Formation is around 850 m thick in the coastal section. It typifies deposition on a gradually shallowing and outbuilding slope. Relatively thick, massive or weakly-bedded, muddy siltstone sequences predominate, and are characteristic of slope areas by-passed by coarse sediment In addition, three isolated channel or incised canyon complexes are exposed at different stratigraphic levels within the Urenui Formation. Channel axial facies include residual debris flow conglomerates, shell hash horizons, and thick-bedded sandstones; scoured sandstones occur in laterally adjacent levee/overbank complexes or broadly confined channels. Thinly interbedded siltstones and sandstones (with some cross-lamination and ripple bedding) characterise late phases of channel infill. Using inferred paleoenvironments and correlative seismic reflection signatures in a sequence stratigraphic framework, the upper Mohakatino-Urenui succession theoretically occupies various positions within a lowstand systems tract. Fine-grained sequences of the Urenui Formation could arguably relate to a late highstand systems tract. Several possible oscillations in relative base-levels are identified in the Mount Messenger and Urenui Formations. However, no clear enstatic signature is recorded; the interacting factors of basin subsidence and sediment supply primarily controlled depositional patterns. (auths)

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