Early Pleistocene Paleogeography of the Onshore Central Hawke's Bay
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Late Miocene – Early Pleistocene paleogeography of the onshore central Hawke’s Bay sector of the forearc basin, eastern North Island, New Zealand, and some implications for hydrocarbon prospectivity K.J. Bland1,2, P.J.J.Kamp1 and C.S. Nelson1 1Department of Earth and Ocean Sciences, The University cession have become involved in the accretionary of Waikato, Private Bag 3105, Hamilton, New Zealand. wedge, which has migrated westward through time. 2GNS Science, P.O. Box 30-368, Lower Hutt 5040, New Zealand. Email: [email protected] Uplift of the inboard margin of the accretionary wedge since the latest Miocene helped to cause an interior Abstract seaway to develop to the west during the Pliocene. Distinctive coarse-grained bioclastic carbonate The timing of trap formation in relation to the tim- sediments of the Te Aute lithofacies were deposited ing of source rock burial and maturation are impor- along both margins of the seaway, which was most tant considerations in evaluating the hydrocarbon extensive during the Late Pliocene (Mangapanian). prospectivity of onshore parts of the forearc basin Although significant volumes of siliciclastic sedi- in central Hawke’s Bay. We describe here aspects ment were supplied to the basin during the Pliocene, of the Late Miocene to Early Pleistocene paleoge- strong tidal currents periodically swept much of ography for the area based on detailed field map- these sediments northeastward. Tidal connections ping and lithofacies analysis, to help constrain pe- existed during the Pliocene into Wanganui Basin in troleum systems evaluations. Key conclusions are: the vicinity of Kuripapango and Manawatu Gorge. • Most deformation of the forearc basin fill ap- By the latest Pliocene (lower Nukumaruan), the in- pears to be relatively young (i.e. post-2 Ma). This terior seaway became closed in the south with uplift deformation has occurred after a major phase of of the Mount Bruce block in northern Wairarapa. Late Miocene to Pliocene sediment accumulation, and is particularly significant along the north- Potential reservoirs within the map area include both western and southeastern margins of the basin. shelf and redeposited sandstone beds in the Miocene • The axis of the forearc basin in central to Early Pliocene Tolaga Group. Thick, coarse- Hawke’s Bay appears to have undergone little grained, variably cemented Plio-Pleistocene lime- structural deformation. Gentle force and reverse stone lithofacies in the Mangaheia Group are wide- faults in the subsurface may be suitable traps. spread along the margins of the basin, and have been • The most widespread potential reser- the targets for several past exploration programmes. voir beds are Miocene sandstone beds. However, drilling has shown that the attractiveness • Potential hydrocarbon source rocks are most- of the Pliocene limestone facies as reservoir beds is ly absent from western parts of the basin due to limited because they quickly pass laterally into silici- significant Neogene uplift and erosion. They clastic mudstone away from the margins of the basin. are, however, probably still widely preserved beneath central parts of the basin where uplift Keywords: and erosion have been much less pronounced. Paleogeography; Hawke’s Bay; East Coast Basin; • Miocene structures within the axis of the basin, Hikurangi margin; Neogene; Tolaga Group; Manga- buried by the Late Miocene to Pleistocene silici- heia Group; Kidnappers Group; Te Aute lithofacies clastic succession, are likely exploration targets. Introduction The forearc basin has been substantially inverted along its western side since the latest Pliocene, Paleogeographic reconstructions of a general nature resulting in erosion of older sediments, includ- covering all of eastern North Island were published ing potential source rocks, down to basement in in Beu (1995) and Field et al. (1997). Since then ranges flanking its western side. The stratigraphy new and detailed geological mapping (1:50,000 along the eastern margin of the forearc basin, and scale) has been undertaken over most of the forearc particularly the outcrop pattern of westward-young- basin in central parts of Hawke’s Bay (Bland, ing Plio-Pleistocene limestones, records the devel- 2006), underpinned by rationalisation of the prior opment of faulting and folding associated with the lithostratigraphy, which was unnecessarily complex elevation and growth of the inboard part of the ac- (Bland et al., 2007; Kamp et al., 2007). This has cretionary wedge. Parts of the forearc basin suc- provided a sound basis for re-interpretation of the 2008 New Zealand Petroleum Conference Proceedings 2 paleogeographic development of the basin, which it is clear that the subducted oceanic Pacific slab was has also been aided by new sedimentological analy- progressively emplaced in a southwest direction be- ses available for the extensive carbonate units that neath North Island during the Neogene (Kamp & accumulated along the margins of the basin during Furlong, 2006). This implies that the Hikurangi the Pliocene (Bland et al., 2004; Caron, 2002; Ca- Trough, the structural trench offshore to the east ron et al., 2004, 2006). In this paper we present where the oceanic plate starts subducting beneath a series of new paleogeographic maps for central the overriding Australia plate, progressively de- parts of the Hawke’s Bay region and describe then veloped to the south from East Cape to Kaikoura in relation to sedimentological and structural infor- during the Neogene, and a slab has been beneath mation for corresponding formations and members. central parts of Hawke’s Bay only since 10 – 12 These maps have been depicted relative to the mod- Ma. This places constraint on the age of the start of ern coastline without allowing for the magnitude of convergent margin processes and hence of the ac- dextral displacement on faults, or the effects of crus- cretionary prism and forearc basin; Tongaporutuan tal shortening or tectonic rotation, mainly because it and younger strata could have accumulated in a con- is difficult to precisely quantify such displacements vergent margin setting, whereas older strata (Early and partition them into particular intervals. The and Middle Miocene) may be better associated paleogeographic maps, by virtue of the summaries with the prior, possibly strike-slip, tectonic setting. of facies and depositional systems they represent, may aid hydrocarbon exploration in the basin, with the caveat that they have been built up chiefly from The sedimentary fill exposed at the surface in the examination of units that crop out in the region, forearc basin in central parts of Hawke’s Bay com- together with reference to exploration hole data. prises a transgressive-regressive Late Miocene (Tongaporutuan) to Early Pleistocene (c. 10-1.6 Geological setting Ma) succession about 2,500 m thick. Progressive onlap of late Neogene sediments onto basement is Hawke’s Bay Province in eastern North Island lies evident along the western margin of the basin. The within the forearc region of the Hikurangi ocean- oldest beds overlying basement are of Tongaporu- continent convergent margin, the northern part of tuan (Late Miocene) age, and occur in the vicinity the obliquely convergent Australia-Pacific plate of the Napier-Taupo Highway (State Highway 5). boundary zone through continental New Zealand Farther to the south and in the Ohara Depression, (Fig. 1 & 2). This paper focuses on the area on the youngest beds over basement are of lower Nu- land between Raupunga in the north and the Ohara kumaruan (Late Pliocene) age. In general, the basin Depression and Mason Ridge in the south (Fig. 1). contains a mixed carbonate-siliciclastic succession, The lowlands in central Hawke’s Bay and much of with carbonates becoming more common, although the embayment offshore (Hawke Bay), lie within volumetrically limited, in the Pliocene section. the principal forearc basin of the convergent mar- Conglomerates derived from erosion of the Trias- gin, between the inboard part of the accretionary sic-Jurassic basement exposed in the axial ranges prism that involves the hill country south of Cape only become significant from the Late Pliocene. Kidnappers, and the high country of the axial rang- es in the west (Ruahine, Kaweka and Ahimanawa Lithostratigraphy and outcrop patterns ranges) (Pettinga, 1982; Kamp 1982). The part of the accretionary prism underlying the Maraetotara The Neogene lithostratigraphy and its nomenclature Plateau and the hills to the south contains a Late for central parts of Hawke’s Bay are summarised in Miocene accretionary slope basin that accumulated Fig. 3, and the distribution of many of the forma- turbidites and mass-emplaced sandstone beds in tions are illustrated in Fig. 4. At the highest level, bathyal environments, and is structurally associ- the stratigraphy comprises three groups: (i) the To- ated with bounding melange zones that formed as laga Group, incorporating Early Miocene (Otaian) imbricate thrust faults (van der Lingen & Pettinga, to Early Pliocene (lower Opoitian) formations; (ii) 1980) The forearc basin differs in being much more the Mangaheia Group, incorporating upper Opoitian structurally simple and in containing a longer-lived to Nukumaruan (Late Pliocene-Early Pleistocene) stratigraphic succession, possibly as much as 5 km formations, and (iii) the middle Pleistocene (Castle- thick (Cutten, 1994). Based on the most recent ro- cliffian) Kidnappers Group, which mostly com- tation data describing the relative movement of the prises alluvial