New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 223-236 223 0028-8306/97/4002-0223 $7.00/0 © The Royal Society of New Zealand 1997 Basement geology of Taranaki and Wanganui Basins, New Zealand N. MORTIMER underlying and adjacent pre-Late Cretaceous crystalline A. J. TULLOCH basement, units of which are exposed in the North and South Islands. Institute of Geological & Nuclear Sciences The distribution of geological units beneath and adjacent Private Bag 1930 to these two basins has important implications for the Dunedin, New Zealand Paleozoic and Mesozoic tectonic evolution of New Zealand. The area straddles the North and South Islands, regions T. R. IRELAND which are commonly treated separately in tectonic analyses Research School of Earth Sciences (e.g., see comments by Black 1994). It is valuable to know Australian National University if recognised terranes and igneous suites continue between Canberra ACT 0200, Australia the two islands and to the north and west of New Zealand. A knowledge of sub-basin geology also helps with studies of the provenance and paleogeography of basin strata and Abstract We present a revised interpretation of the in the reconstruction of reservoir sandstone depositional basement geology beneath Late Cretaceous to Cenozoic systems. Taranaki and Wanganui Basins of central New Zealand, The onland basement rocks of New Zealand are well based on new petrographic, geochemical, and geo- characterised on a regional (i.e. 1:1 000 000) scale and can chronological data from 30 oil exploration wells. Recently be most simply divided into Eastern and Western Provinces published structural and magnetic interpretations of the area that are separated by the Median Tectonic Zone (MTZ). The assist in the interpolation and extrapolation of geological Eastern Province is dominated by Late Paleozoic—Mesozoic boundaries. Torlesse and Waipapa Terranes have been indurated sandstone and mudstone with subordinate mafic identified in Wanganui Basin, and Murihiku Terrane in volcanics and chert, in part overprinted by the Haast Schist. eastern Taranaki Basin, but Maitai and Brook Street Terrane The Western Province consists of early Paleozoic siliciclastic rocks have not been recognised. Separation Point Suite, and carbonate rock, intruded and metamorphosed by mid- Karamea Suite, and Median Tectonic Zone igneous rocks Paleozoic and Cretaceous granitoids. The MTZ is character- are all identified on the basis of characteristic petrography, ised by a zone of Carboniferous and Early Triassic to Early geochemistry, and/or age. SHRIMP U-Pb zircon measure- Cretaceous volcanic, plutonic, and sedimentary rocks, whose ments on igneous samples from western Taranaki wells do not give precise ages but do provide useful constraints: Motueka-1 granite is latest Devonian - earliest Carbon- iferous; Tangaroa-1 and Toropuihi-1 are Carboniferous; and Surville-1 is Cretaceous (cf. Separation Point Suite). Our c long 170°E 180 interpretation of sub-basin geology is compatible with previously observed onland relationships in the North and 200km lat 35°S ~ South Islands. Keywords North Island; Taranaki; Wanganui; Eastern North Island Pacific Province; Western Province; Median Tectonic Zone; Ocean terranes; granitoids; petrology; petrography; geochemistry; U-Pb dating; zircon STUDY AREA INTRODUCTION Wellington Taranaki and Wanganui Basins (Fig. 1) are two major, Late Cretaceous—Cenozoic sedimentary basins in central New Chatham Islands Zealand, that are, respectively, hydrocarbon-producing and V hydrocarbon-prospective. The clastic sedimentary rocks of South Island the basins were all ultimately derived from erosion of the 45°S- Stewart Island I G95075 Fig. 1 Location of the study area in the New Zealand region. T, Received 20 December 1995; accepted 12 September 1996 Taranaki Basin; W, Wanganui Basin. 224 New Zealand Journal of Geology and Geophysics, 1997, Vol. 41 * 38 tangaroa-1 ,- KH EASTERN PROVINCE Ariki-t» ••' Te Ranga-1» & MEDIAN TECTONIC: ZONE Wainui-1» . | . r 1 Torlesse Terrane # Moa 1B L, ,_J Rakaia (r), Pahau (p) Pukearuhe-1 Caples Terrane (c) Waipapa Terrane (w) McKee-1 & \ ToeToe-1 Maitai Terrane Tane-1» lnglewood-1 Taranga-1 i Murihiku Terrane Witiora-1 Mt. TaranakW ; ™"=-' « pUniwhakau-1 ;' Rotokare-1. C X X j Rotoroa Complex tl (MTZ) WESTERN PROVINCE ,*+*+*' Mainly Separation •*^^ Point Suite r>+ Mainly Karamea r + + I Suite |S;7] Buller Terrane (b) f.;./*'/*j Takaka Terrane (t) STRUCTURES Major Cenozoic —""" faults Esk Head Melange Haast Schist SAMPLE SITES • well penetrates basement o well penetrates cover only • other subsurface sampling POSITIVE MAGNETIC ANOMALIES •-- >+100 gamma 42°S 172°E 174° 176° Fig. 2 Geology in the vicinity of Taranaki and Wanganui Basins. Location of oil exploration wells referred to in the text, and selected magnetic anomalies (from Hunt 1978) are also shown. KH, Kawhia Harbour; PP, Pio Pio; FR, Fishermans Rock; MT, Mt Tongariro: RT, Rangipo hydro tunnel; WT, Whakapapa—Tawhitikuri hydro tunnel; LT, Lake Taupo; KR, Kaimanawa Range; PU, Port Underwood Unshaded areas are water and Late Cretaceous to Quaternary cover. Geology from Sporli (1978), Cooper & Tulloch (1992), and Mortimer (1993, 1995). nature and contacts with the flanking Eastern and Western PREVIOUS WORK AND SCOPE OF STUDY Provinces are the topic of ongoing research (Kimbrough et al. 1994). The rocks of the Eastern and Western Provinces Cope & Reed (1967) examined material from 10 onshore have been divided into a number of petrographically and North Island oil exploration wells and proposed correlations geochemically distinct tectonostratigraphic terranes and of the indurated sandstone and schist in the wells with igneous suites (Fig. 2). Details of these divisions are beyond various facies of the New Zealand geosyncline (Eastern the scope of this paper, but recent summaries have been Province). Wodzicki (1974) examined basement material provided by Roser & Korsch (1988), Tulloch (1988), from four offshore oil exploration wells in the area of Fig. 2 Bradshaw (1989), Cooper & Tulloch (1992), Mortimer and showed that various Western Province igneous and (1993, 1995), Black (1994), Kimbrough et al. (1994), and metamorphic rocks were represented. The provenance of Muiretal. (1994). Cretaceous—Cenozoic sandstones in western and eastern Mortimer et al.—Taranaki & Wanganui Basin basement 225 Taranaki Basin broadly reflects derivation from Western and The Kaitieke-1 sandstone is a volcanic litharenite and Eastern Province sources, respectively (e.g., Smale 1992). contrasts with the other sandstones which are feldspathic In this paper we present a revised and updated litharenites and lithic feldsarenites (Folk et al. 1970) (Fig. 3). interpretation of the distribution of basement geological units The well is located only 8 km along-strike from surface beneath Taranaki and Wanganui Basins (area of Fig. 2). Since outcrops of the volcanic litharenite-dominated Waipapa the above studies were completed, much new information Terrane (Sporli 1978; Beetham & Watters 1985; Black 1994). has become available. In particular, our interpretations are On the basis of detrital modes and geographic location, we based on: therefore correlate Kaitieke-1 basement with the Waipapa Terrane. The closest basement outcrops to Stantiall-1, (1) petrographic and/or geochemical analyses of basement Young-1, Santoft-1 A, and Parikino-1 wells are Rakaia material from a total of 30 offshore and onshore oil Torlesse sandstones exposed in the North Island axial ranges exploration wells, including re-examination of material (Sporli 1978; Beetham & Watters 1985). The detrital modes from the earlier studies (Tables 1, 2); and the Ti/Zr and La/Sc ratios of these samples fall within (2) U-Pb SHRIMP dates on zircons from igneous rocks in the range of Rakaia Torlesse sandstones (Fig. 3,4A; see also four wells (Table 3); Mortimer 1995 for other chemical similarities), and outside (3) interpretations of offshore magnetic anomalies (Hunt the range of the more volcaniclastic Eastern Province 1978; Davy 1992); terranes which lie west of the Torlesse. The lower La/Sc ratio of the Parikino-1 pelite as compared to the psammite (4) recent maps of basin and sub-basin structure from seismic (Fig. 4A) is highly distinctive of Torlesse rather than Caples reflection studies (Anderton 1981; Thrasher & Cahill and Waipapa volcaniclastic terranes (Roser et al. 1993; 1990); Mortimer 1993 and references therein). (5) contemporary subdivisions of onland geology into All Wanganui Basin basement samples contain authi- terranes, metamorphic facies, and igneous suites with genic pumpellyite, prehnite, or epidote, but zeolites are which to correlate the exploration well material (e.g., conspicuously absent (Table 1); the rocks have thus Tulloch 1988; Mortimer 1993, 1995; Black 1994; experienced at least prehnite-pumpellyite facies meta- Kimbrough et al. 1994; Muir et al. 1994, 1996); and morphism. Although regional metamorphic gradients are (6) supplementary subsurface information from xenoliths, present in all New Zealand terranes (e.g., Bishop 1972; Boles dredge hauls, and tunnels (Beetham & Watters 1985; 1974; Black et al. 1993; Mortimer 1993), grade of Graham 1985, 1987; Carter et al. 1988; Gamble et al. metamorphism generally varies within known limits and, 1994). we believe, can be used to supplement terrane correlations made on the basis of detrital petrographic and geochemical Onland geological units are, of course, defined not just criteria. The prehnite-pumpellyite to greenschist facies