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 rocks on the basis of their petrological content but also by using of Young-1, Stantiall-1, Santoft-IA, Parikino-1, and fossil, stratigraphic, lithofacies, and structural data. Despite Kaitieke-1 are similar to the observed metamorphic grade the fact that the oil exploration well core and cuttings do of the Rakaia Torlesse and Waipapa Terranes with which not provide this extra information, we are confident that our we correlate them, and distinctly different from the zeolite petrological-geochronological approach yields valid facies Murihiku Terrane and Pahau Torlesse Terranes (cf. interpretations. Boles 1974; Black et al. 1993; Black 1994). Subsurface basement rocks in and near Wanganui Basin have also been sampled by means other than oil exploration PETROLOGICAL RESULTS wells. Fishermans Rock in Cook Strait (P51016) is a As expected from the studies of Cope & Reed (1967) and weathered tzIIA psammitic schist (Mortimer pers. obs.) of Wodzicki (1974), samples from wells in the eastern half of Rakaia Torlesse affinity (B. P. Roser in Carter et al. 1988, Fig. 2 (east of long. 174°E) have the features of Eastern see their table 1 for chemical analysis). Graham (1985) and Province metasedimentary rocks, and those in the western Beetham & Watters (1985) noted Waipapa Terrane volcanic half of Fig. 1 (west of long. 174°E) have the features of litharenites in the Whakapapa-Tawhitikuri tunnel but Western Province and/or Median Tectonic Zone plutonic and Torlesse Terrane feldsarenites in the Rangipo tunnel of the metamorphic rocks. For convenience, we describe the well Tongariro Power Development project. Graham (1987) samples in these eastern and western groups. A summary of described contact-metamorphosed Torlesse sandstone the principal petrographic features of all well samples used xenoliths from Mt Tongariro, thus narrowing the position in this study is given in Table 1. of the Torlesse—Waipapa boundary south of Lake Taupo to within c. 10 km. Eastern wells Taranaki Basin Wanganui Basin The detrital modes of sandstones from onshore Taranaki Whole rock geochemical analyses of samples from Basin wells Kiore-1, Pukearuhe-1, Rotokare-1, Tatu-1, and Wanganui Basin wells are given in Table 2 and Palmer et Uruti-1 are distinctly less quartz and lithic rich than the al. (1995). As noted by Cope & Reed (1967), Kaitieke-1, Wanganui Basin sandstones mentioned above, and are Young-1, Stantiall-1, and Santoft-1A penetrated indurated similar to compositions reported from the Murihiku Terrane clastic sedimentary rocks, and Parikino-1 penetrated schist. (Fig. 3). The abundance of tuffaceous and calcareous All sandstones are unfoliated except in Santoft-1 A (too fine material in the sandstones is also typical of Murihiku grained to be classified using Bishop's (1972) textural zone sandstones (e.g., Boles 1974) and atypical of other Eastern scheme—it might be IIA) and in Parikino-1 (textural zone Province terranes. The geochemical composition of IIA-B). sandstones from the above five wells and from Table 1 Summary of petrographic data and preferred correlation of basement samples from Taranaki and Wanganui oil exploration wells.
Well Depth (m) Lat. (S) Long. (E) Type P no. Lithology Notes Correlation Eastern wells (Wanganui, onshore Taranaki, and Te Ranga-1) Kaitieke-1 393.2-393.8 39°03.20' 175°17.83' Cuttings 33377 Medium ss Q5, F20, L75. Quartz—prehnite veins, intermediate volcanic clasts Wai papa Kiore-1 534.3 39°13.43' 174°33.73' Core 30651 Fine-medium ss Ql 1, F41, L48. Volcanic lithics, chloritised glass fragments. Calcic plagioclase Murihiku Kiore-1 534.9 39°13.43' 174°33.73' Core 51395 Fine ss-siltstone Quartz rich, heulandite patches, detrital muscovite Murihiku Manutahi-1 1389.5 39°41.12' 174°25.02' Core 51396 Fine ss Quartz rich, detrital biotite, zeolitised glass shards, much sericite Murihiku Parikino-1 2312.2 39°48.07' 175°08.83' Core 30501 Schist tzIIB-IHA, quartz, ab., muse, chlorite, titanite, epidote. Strain-slip cleavage Torlesse Pukearuhe-1 3132-3138 38°53.65' 174°30.58' Cuttings 51393 Medium ss Q9, F37, L54. Acid volcanic and plutonic lithics. Heulandite and laumontite Murihiku Puniwhakau-1 2146.1 39°19.10' 174°42.42' Core 30850 Fine ss Quartz rich, detrital biotite Murihiku Puniwhakau-1 2146.1 39°19.10' 174°30.58' Core 51394 Fine ss Quartz rich. Pink zeolitised patches Murihiku Rotokare-1 3232.7 39°24.85' 174°24.17' Core 51390 Medium ss Q7, F37, L56. Volcanic lithic, zeolites (including heulandite). Albitised feldspar Murihiku Santoft-IA 2627.4-2630.5 40°12.32' 175° 12.41' Core 29822 Fine ss-siltstone Microfaulted and incipient pressure solution cleavage. Pumpellyite Torlesse Stantiall-1 2085.5-2087.0 40°05.02' 175°20.03' Core 16573 Medium ss Q30, F32, L38. Detrital muscovite, biotite, epidote. Pumpellyitised feldspar Torlesse Tatu-1 857.1 38°55.12' 174°55.02' Core 30608 Medium ss Q15, F38, L47. Volcanic lithics. Heulanditised glass shards. Calcic plagioclase Murihiku Tatu-1 857.7 38°55.12' 174°55.02' Core 51391 Fine ss Quartz and feldspar rich, volcanic lithics, authigenic zeolites. Calcic plagioclase Murihiku Te Ranga-1 3877-3882.5 38°12.15' 174°37.90' Cuttings 51397 Calc. siltstone Calcareous matrix, calcite veins, detrital quartz, biotite prominent Murihiku Uruti-1 341.3-342.9 38°56.37' 174°34.77' Core 16570 Calc. medium ss Q10, F50, L40. Detrital biotite. Zeolite (phillipsite?) veins. Albitised feldspar Murihiku Uruti-2 1546.3-1549.3 38°57.67' 174°30.75' Core 16572 Calc. siltstone Quartz rich, detrital epidote, biotite, muscovite, authigenic chlorite, zeolite Murihiku Young-1 1025.7-1028.4 40° 17.63' 175°30.00' Core 30502 Medium ss Q29, F33, L38. Detrital biotite, muscovite. Incipient pressure solution cleavage Torlesse Young-1 1031.5-1034.8 40° 17.63' 175°30.00' Core 30503 Medium ss Q44, F27, L29. Detrital biotite. Matrix sericite prominent. Pumpellyite. Torlesse z; 3< Western wells (offshore Taranaki and offshore South Island) ea l Ariki-1 4814 38°12.09' 173°41.85' Cuttings 11826 Various Basalt-andesite, and volcaniclastic ss. Secondary ab, chlorite, calcite, hematite MTZ N Kiwa-1 3850-3853 39°48.65' 172°41.88' Cuttings 50886 Granitoid Fine-grained kaolinised ?biotite granite. Biotite now pale brown micaceous clay Separation Point 3 Kongahu-1 2015-2016 41°14.85' 171°52.47' Core 44741 Granitoid Coarse-grained biotite granite; brown biotite altered to chlorite and muscovite Karamea Suite & Maui-2 3469-3563 39°36.77' 173°26.97' Cuttings 39790 Granitoid Medium-grained hornblende diorite. Brown/green hornblende, access magnetite MTZ s Maui-4 3850-3905 40°02.40' 173°14.45' Cuttings 39792 Granitoid Sodic leucogranite (Wodzicki 1974) Separation Point Moa-IB 3523-3546 38°29.72' 173°21.18' Cuttings 39787 Schist Biotite, hornblende, pyroxene-bearing schist (Wodzicki 1974) Takaka Terrane E. Motueka-1 1564 40°31.43' 173c29.02' Cuttings 51535 Granitoid Fine-medium grained biotite granite; c. 5% green biotite; magnetite, titanite Pz I-type granite o North Tasman- 1 2722-2725 40° 12.01' 173° 16.33' Cuttings 11828 Granitoid Fine-grained chloritised biotite granite. Pink K-feldspar. Contaminated with Cz ss Karamea Suite O Ruby Bay-1 268-281 41°14.15' 173°05.28' Core 39887 Granitoid Medium-grained biotite hornblende diorite with 2-3% quartz and K-feldspar MTZ a Surville-1 2199 40°43.33' 173°26.83' Cuttings 50890 Granitoid Fine-medium grained biotite granite; c. 2% green biotite, trace opaque Separation Point Qo* Tane-1 4471 38°56.33' 172°38.33' Cuttings 50888 Granitoid Medium-grained biotite granodiorite; c. 3% green biotite, trace titanite Separation Point (TO Tane-1 4471 38°56.33' 172°38.33' Cuttings 50889 Granitoid Medium-grained biotite granodiorite; c. 3% green biotite, trace titanite Separation Point » Tangaroa-1 3984.5-3984.7 38°10.78' 173°52.32' Core 51291 Silicified rhyolite Feldspar phenocrysts, spherulitic groundmass. Sec. quartz, ep, chlorite, sericite MTZ basement Tangaroa-1 3985.8 38°10.78' 173°52.32' Core 51295 Basalt Basalt. Rare clinopyroxene phenocrysts to 1 mm. Secondary epidote and chlorite MTZ basement o Taranga-1 4197 38°58' 173°15' Core 54827 Granitoid Medium-grained biotite quartz-monzodiorite. Trace magnetite, titanite Separation Point o Toropuihi-1 2192 40°51.25' 171°56.73' Cuttings 50892 Granitoid Fine-grained biotite granite; trace olive-green biotite, trace fluorite Pz A-type granite *& Wainui-I 3892-3894 38°27.87' 173°18.51' Cuttings 11827 Matrix-rich ss Single cutting (c. I % of sample) of recryt. siliceous metasandstone Takaka Terrane >- 4-. 2 o a p Table 2 X-ray fluorescence (XRF) analyses of basement core and cuttings material from Taranaki and Wanganui wells. 3 P Well Pno. Rock type SiO2 TiO2 AI2O3 Fe2O3T MnO MgO CaO Na2O K2O P2O5 LOI TOTAL Ba Ce Cr Cu Ga La Nb Ni Pb Rb Sc Sr Th U V Y Zn Zr Eastern iwells K.iore-1 51395 fine ss 59.52 0.86 16.82 6.73 0.07 2.46 2.05 3.83 2.87 0.21 4.55 99.97 464 52 76 36 18 20 7 25 19 95 15 210 8 1.9 160 28 90 171 p crq Manutahi-1 51396 fine ss 64.24 0.70 16.12 5.11 0.07 1.98 1.51 3.73 3.59 0.16 2.99 100.20 421 54 35 20 22 25 8 15 17 139 14 160 11 3.5 119 33 86 207 p 3 Puni-1 51394 fine ss 59.44 0.89 16.65 6.36 0.09 2.84 3.05 3.76 2.74 0.20 4.03 100.05 498 48 54 37 20 18 9 24 19 115 16 595 11 2.8 151 31 104 194 C Rotokare-1 51390 med ss 64.60 0.70 16.01 4.75 0.06 1.91 1.53 3.66 3.56 0.16 3.14 100.08 418 51 34 20 21 23 9 13 19 139 13 161 12 2.7 113 32 80 204 03 Tatu-1 51391 fine ss 62.72 0.72 16.19 5.95 0.08 1.95 2.41 3.01 3.41 0.12 3.49 100.05 477 44 32 24 23 19 10 12 18 142 13 430 8 2.0 124 29 97 204 Uruti-1 16570 calc ss 53.68 0.74 16.69 8.23 0.10 2.17 4.76 4.03 1.79 0.18 7.00 99.37 520 55 35 24 20 23 8 15 18 52 15 647 8 1.9 149 25 99 160 Western wells K.iwa-1 50886 granite* 76.25 0.10 11.99 0.47 0.05 0.23 1.51 2.87 4.24 0.01 1.55 99.27 1055 15 8 1 15 11 3 <1 20 87 <1 348 4 1.0 8 2 63 60 Kongahu-1 44741 granite 73.29 0.24 14.00 2.08 0.04 0.47 0.29 2.95 5.56 0.16 1.06 100.14 360 47 5 4 19 21 9 5 33 297 3 61 15 5.0 22 50 30 96 Maui-2 39790 diorite* 46.51 1.51 17.46 11.60 0.20 6.17 8.28 3.18 1.00 0.38 3.23 99.52 458 40 29 71 23 12 4 20 7 21 33 947 2 <1.0 257 25 134 76 Motueka-1 51535i granite* 70.29 0.23 15.84 2.28 0.05 0.47 1.03 4.82 4.14 0.07 0.70 99.92 985 53 2 5 22 25 13 3 15 160 3 164 16 6.0 12 40 36 227 Motueka-1 51535ii granite* 72.01 0.14 15.29 1.34 0.02 0.17 0.82 4.73 4.37 0.03 0.79 98.92 922 41 3 6 18 18 9 4 14 151 2 155 13 4.8 11 22 21 151 N Tasman-1 11828 granite* 72.87 0.22 14.50 1.40 <0.01 <0.01 0.81 3.55 4.95 0.17 0.73 99.20 760 42 16 26 <1 25 21 35 <1 183 25 83 14 9.0 20 19 156 118 Ruby Bay 39887 diorite 57.38 0.94 17.23 7.49 0.13 3.20 6.22 3.81 2.11 0.31 0.82 99.64 566 57 23 32 20 24 9 14 15 70 23 609 10 2.0 140 37 84 234 Surville-1 50890i granite* 70.34 0.16 15.81 1.12 0.03 0.49 1.68 4.64 4.24 0.06 0.80 99.37 735 14 <2 6 19 11 4 2 20 163 1 671 7 1.0 15 3 36 64 Surville-1 50890M granite* 70.53 0.21 15.54 1.25 0.03 0.39 1.72 4.77 4.06 0.07 1.33 99.90 689 25 5 8 19 11 5 3 23 150 2 634 12 1.7 17 1 34 78 Tane-1 50888 granite* 67.90 0.25 16.94 1.31 0.02 0.32 2.63 5.70 2.36 0.08 1.87 99.38 1171 35 3 5 18 19 5 1 19 52 1 1038 4 1.0 20 3 50 136 Tane-1 50889 granite* 67.67 0.20 16.94 0.94 0.02 0.21 2.13 6.19 2.45 0.08 2.58 99.41 1193 43 3 6 15 24 4 1 9 54 2 772 4 1.0 19 2 30 127 Tangaroa-1 51291 rhyolite 73.33 0.35 14.75 1.30 0.05 1.07 4.25 3.39 0.44 0.08 1.04 100.05 137 21 2 22 14 9 3 1 19 11 6 440 7 2.0 39 13 52 189 Tangaroa-1 51295 basalt 49.15 1.23 16.26 11.61 0.18 5.55 7.99 3.38 0.73 0.17 4.03 100.28 201 16 46 108 18 4 <2 17 12 17 40 288 2 1.0 373 21 97 61 Taranga-1 54827 granite 68.15 0.38 15.66 3.01 0.03 1.22 1.17 5.33 2.60 0.18 1.81 99.54 741 37 2 4 21 17 4 2 14 62 4 577 7 1.1 47 4 49 120 Toropuihi-1 50892 granite* 75.61 0.04 12.73 1.04 0.01 0.05 0.55 3.82 4.70 0.01 0.65 99.21 204 47 <2 2 35 20 50 10 47 512 1 15 37 13.0 5 129 92 121 Witiora-1 50895 granite* 77 58 0.06 12.11 0.37 0.01 0.08 0.54 3.82 4.12 0.01 0.60 99.30 907 14 <2 1 13 8 3 0 7 79 0 73 7 1.0 7 3 9 53 Major elements wt%, trace elements ppm. * indicates cuttings, not core, analysed; , total iron as Fe2C>3; LOI, loss on ignition. Analytical methods, along with analyses of Stantiall-1, Santo ft-1 A, Young-1, Kaitieke-1, and Parikino-1 well samples are given in Palmer et al. (1995). Analyst Ken Palmer, Victoria University, except for North Tasman-•1 granite PI 1828 which was analysed by John Hunt, Spectrachem Analytical. to 228 New Zealand Journal of Geology and Geophysics, 1997, Vol. 40 Table 3 U-Pb analyses of zircon from Toropuihi-1, Tangaroa-1, Motueka-1, and Surville-1 rocks. 238Tj/206p Analysis U Th Th/U 204pb/206pb 207pb/206pb b Age (Ma) Toropuihi-1 1.1 2674 711 0.27 0.00004 ± 0.00003 0.05386 ±0.00105 19.91 ±0.45 315.6 ± 7.0 1.2 7189 3579 0.50 0.00015 ±0.00003 0.05608 + 0.00044 20.09 + 0.41 312.1 ± 6.2 2.1 3680 903 0.25 0.00015 ±0.00004 0.05380 + 0.00040 19.99 ±0.42 314.4 ± 6.5 2.2 4157 1382 0.33 0.00044 ± 0.00007 0.05737 + 0.00059 20.43 + 0.43 306.5 ± 6.4 2.3 7073 2875 0.41 0.00102 ± 0.00008 0.06614 + 0.00053 22.29 + 0.46 278.9 ± 5.6 3.1 7662 2758 0.36 0.00116 ±0.00007 0.06761 ± 0.00048 21.51 ±0.47 288.3 ± 6.1 4.1 337 609 1.81 0.00018 ±0.00041 0.05584 ±0.00173 28.18 ±0.73 224.1 + 5.7 5.1 2424 1225 0.51 0.00010 + 0.00006 0.05393 + 0.00060 20.28 + 0.44 310.0 ± 6.6 5.2 1211 426 0.35 0.00018 ±0.00012 0.05331 ±0.00111 20.10 ±0.46 312.9 ± 7.0 6.1 4339 2112 0.49 0.00331 ±0.00017 0.10286 ±0.00063 22.19 ±0.48 268.9+ 5.7 7.1 4473 1489 0.33 0.00202 + 0.00014 0.08356 ± 0.00069 20.44 ± 0.42 297.8 ± 6.0 8.1 4779 2284 0.48 0.00153 ±0.00013 0.07445 ± 0.00087 27.58 ±0.59 224.2 ± 4.7 Tangaroa-i 1869 0.44 0.00286 ± 0.00306 0.08744 + 0.00218 25.46 + 0.77 239.0 ± 7.2 1.12 42541590 1481 0.36 0.00302 + 0.00324 0.09235 ± 0.00047 25.58 ± 0.69 235.4 ± 6.3 2.1 2528 990 0.39 0.00543 ± 0.00565 0.11915 ±0.00124 19.40 ± 0.49 299.4 ± 7.6 2.2 2317 916 0.40 0.00278 ±0.00310 0.08932 ±0.00177 21.68 ±0.55 277.8+ 7.0 3.1 988 450 0.46 0.00066 ±0.00101 0.05382 ± 0.00096 19.08 ± 0.48 327.3 ± 8.2 3.2 983 443 0.45 0.00065 ±0.00114 0.05548 + 0.00053 19.15 ±0.48 325.5 ± 8.2 3.3 625 270 0.43 0.00066 ±0.00107 0.05496 ±0.00154 21.33 ±0.99 293.1 ± 13.6 3.4 744 327 0.44 0.00063 ±0.00170 0.05557 ± 0.00099 20.54 ± 0.53 303.9 ± 7.8 4.1 397 299 0.75 0.00412 ±0.00504 0.08480 + 0.00192 22.00 ± 0.85 275.2 ± 10.6 4.2 1080 786 0.73 0.00757 ± 0.00807 0.15702 ±0.02552 21.78 ±0.55 255.4 ± 6.4 5.1 7495 3420 0.46 0.00078 + 0.00081 0.06072 + 0.00203 19.56 ±0.94 317.1 ± 15.3 5.2 6490 2277 0.35 0.00098 ±0.00103 0.06250 + 0.00142 22.96 + 0.89 270.5 ± 10.5 6.1 480 252 0.53 0.00158 ±0.00254 0.05613 ±0.00095 19.68 ±0.51 316.8 ± 8.2 6.2 537 285 0.53 0.00241 ±0.00355 0.05913 ±0.00123 19.68 + 0.52 315.7 + 8.4 7.1 574 182 0.32 0.00243 ± 0.00320 0.06660 + 0.00042 21.01 +0.53 293.7 ± 7.4 7.2 530 169 0.32 0.00133 + 0.00141 0.06243 ± 0.00087 21.88 + 0.56 283.6 ± 7.2 8.1 3654 1090 0.30 0.00044 ± 0.00053 0.05643 ± 0.00040 21.85 ±0.55 285.8 ± 7.2 8.2 3588 1070 0.30 0.00050 ± 0.00066 0.05605 + 0.00038 21.74 + 0.54 287.4 ± 7.2 9.1 90 55 0.62 0.01794 + 0.02519 0.11567 + 0.00163 16.09 + 0.43 360.6 ± 9.7 9.2 74 45 0.61 0.01321 ±0.01839 0.11906 ±0.00432 18.19 ±0.55 318.7+ 9.6 9.3 120 70 0.59 0.00940 + 0.01094 0.08728 + 0.00239 19.64 ±0.59 306.6 ± 9.3 Motueka-1 1.1 3057 2320 0.76 0.00012 ±0.00004 0.05401 ±0.00042 17.02 ±0.19 370.1 ± 4.0 2.1 538 370 0.69 0.00115 ±0.00031 0.05945 ±0.00156 17.72 + 0.28 353.8 ± 5.5 3.1 2314 1018 0.44 0.00017 + 0.00006 0.05248 ±0.00071 17.71 ±0.20 356.7 ± 3.9 4.1 335 172 0.51 0.00091 ±0.00063 0.06749 + 0.00147 19.23 + 0.31 323.8 ± 5.2 5.1 393 220 0.56 0.00162 ±0.00047 0.06636 ±0.00198 18.88 + 0.36 330.2 ± 6.1 6.1 754 555 0.74 0.00056 ± 0.00036 0.05872 ±0.00172 17.78 + 0.30 352.8 ± 5.7 7.1 515 318 0.62 0.00106 + 0.00024 0.06135 ±0.00185 17.68 ±0.26 353.8 ± 5.0 8.1 290 142 0.49 0.00042 + 0.00037 0.06864 ±0.00159 18.37 ±0.40 338.2 ± 7.2 9.1 563 370 0.66 0.00095 + 0.00038 0.05742 ±0.00136 19.01 ±0.30 331.1 ± 5.1 10.1 292 243 0.83 0.00175 + 0.00044 0.07096 ±0.00213 18.29 ±0.32 338.8 ± 5.8 11.1 495 313 0.63 0.00098 ± 0.00052 0.06127 + 0.00154 19.41 ±0.47 320.9 ± 7.5 12.1 696 106 0.15 0.00043 ± 0.00071 0.05590 + 0.00110 18.01 ±0.30 347.5+ 5.7 13.1 933 576 0.62 0.00001 ±0.00001 0.05591 ± 0.00078 17.55 ±0.27 356.5+ 5.3 14.1 686 323 0.47 0.00090 ± 0.00086 0.05795 ± 0.00086 16.81 +0.31 370.9+ 6.6 Surville-1 1.1 1292 864 0.67 0.00104 ±0.00059 0.05444 ±0.00217 53.96 ± 0.96 119.0 ± 2.1 2.2 121 72 0.59 0.00787 ± 0.00260 0.07712 ±0.00421 49.90+ 1.44 125.4+ 3.6 3.1 182 73 0.40 0.00387 ± 0.00340 0.08580 ± 0.00701 54.35 ±1.89 114.1 ± 4.0 4.1 1012 716 0.71 0.00180 + 0.00055 0.06271 ±0.00155 55.74 + 0.79 114.2 ± 1.6 5.1 637 472 0.74 0.00162 ±0.00083 0.07817 ±0.00220 56.18 ± 1.21 111.4 ± 2.4 6.1 303 262 0.86 0.00623 ±0.00159 0.08800 ± 0.00337 55.24 ± 1.36 112.0 ± 2.8 7.1 1078 733 0.68 0.00163 + 0.00045 0.06121 ±0.00152 55.78 ±0.81 114.3 ± 1.7 8.1 359 281 0.78 0.00811 ±0.00148 0.10042 ±0.00533 51.81 + 1.07 117.7+ 2.5 9.1 837 542 0.65 0.00094 ± 0.00058 0.07353 ± 0.00192 57.62 ± 1.34 109.2 ± 2.5 10.1 181 172 0.95 0.00473 ± 0.00207 0.09043 + 0.00451 55.44 ± 1.73 111.3 ± 3.5 11.1 101 99 0.97 0.08576 + 0.03168 0.44765 + 0.02837 28.84 ± 1.94 124.6 ± 10.7 12.1 41 35 0.86 0.02463 ±0.01981 0.22505 ±0.01868 43.83 ± 3.03 117.6 ± 8.6 13.1 270 148 0.55 0.00221 ±0.00271 0.07851 ±0.00313 58.21 ± 1.71 106.2 ± 3.1 14.1 756 512 0.68 0.00596 + 0.00276 0.06824 ± 0.00242 48.54 ± 0.93 128.7 ± 2.5 15.1 149 108 0.73 0.00001 ±0.00001 0.08070 ±0.01019 49.49 ± 2.49 124.4 ± 6.4 Analyst T. R. Ireland. Analytical methods and operating conditions are similar to those described in Muir et al. (1994). Mortimer et al.—Taranaki & Wanganui Basin basement 229 El Kaitieke-1 D Taranaki Basin Kiore-1 Kaitieke-1 • Wanganui Basin o Kiore-1 Uruti-1 O Pukearuhe-1 60- Rotokare-1 Parikino-1 • Rotokare-1 SantofMA Tatu-1 Stantiall-1 40- Young-1 Uruti-1 2fJ.Puni-1 Stantiall-1 Tatu-' • Manutahi-f + Young-1 La/Sc 1200 i Maui-'2-. a. 3 Ruby Bay-1 Fig. 3 Petrographic plot of visually estimated modes of medium- and coarse-grained sandstones from eastern wells of Taranaki and MEDIAN TECTONIC Wanganui Basins, and of point-counted populations from selected 400 ZONE ROCKS Kiwa-1 New Zealand terranes. Q, F, L, total quartz, feldspar, lithic grains, respectively. North Island Torlesse (« = 19) and southern Waipapa O = 25) data from Finlow-Bates (1970), Beetham & Watters (1985), and Mortimer (1995); Murihiku data (n = 21) from MacKinnon (1983). Hexagons indicate one standard deviation 50 Toropuihi-1 80 SiO (wt%) about the mean. 2 Puniwhakau-1 and Manutahi-1 are permissibly Murihiku, 100O but cannot be uniquely distinguished from Caples-Waipapa Tangaroa-1 sandstones using chemical criteria alone (Fig. 4A). o rhyolite The presence of zeolites in Uruti-1 and 2, Tatu-1, Kiore-1, • basalt 100 Puniwhakau-1, Rotokare-1, Pukearuhe-1, and Manutahi-1 further suggests a correlation with the Murihiku Terrane (cf. descriptions by Boles 1974; Black et al. 1993). The lack of BJ prehnite-, pumpellyite-, or epidote-bearing assemblages rules « 10 out a correlation with Maitai, Caples, Waipapa, and Rakaia •5 Torlesse Terranes. S, Lord Howe Rise Western wells • rhyolites o Lake Roxburgh Wells for which only petrographic data could be used for tonalites interpretation include Ariki-1, Moa-IB, Tasman-1, and s 0.1 Wainui-1. Ariki-1 cuttings were of altered basalt, andesite, Rb Ba Th U Nb K La Ce Sr P Zr Ti Y ?dacite, and volcaniclastic sandstone, all with abundant metamorphic chlorite, hematite, and sericite. The igneous rocks in Ariki-1 are not dissimilar to those in nearby Fig. 4 Geochemical plots of well samples. A, Sandstones and Tangaroa-1, for which core was available and which we schists from eastern wells plotted on La/Sc versus Ti/Zr binary studied in greater detail (see below). We concur with diagram. Murihiku, Caples-Waipapa, and Torlesse fields from Wodzicki's (1974) correlation of quartzofeldspathic and Roser et al. (1993). Arrow for Parikino-1 links analyses of calcareous schist in Moa-IB with the Onekaka Schist psammitic (psam) and pelitic (pel) portions of core (pelitic at arrow tip). High metamorphic grade of Wanganui Basin samples suggests (Takaka Terrane of Cooper & Tulioch 1992); such rock types Caples-Waipapa and Torlesse correlations; low metamorphic grade are essentially absent from the Buller Terrane and are, as of Taranaki samples suggests a Murihiku correlation (see text). yet, unknown from the MTZ. The presence of a single cutting B, Granitoids from western wells plotted on SiC>2 versus Sr of recrystallised siliceous sandstone in the nearby Wainui-1 diagram, comparing compositions of granitoid samples with is also compatible with a Takaka Terrane correlation. respect to onland reference suites. Fields from Tulioch (unpubl. data). C, Basalt and rhyolite from Tangaroa-1 plotted on We can add little to Wodzicki's (1974) detailed primitive mantle-normalised multi-element diagram (of Sun & description of the indurated ignimbrite-rich breccia- McDonough 1989). Both samples have calc-alkaline and conglomerate in Tasman-1 (Fig. 2). Wodzicki (1974) subduction-related character. Nb concentration of basalt is plotted tentatively favoured a correlation with the late Early at practical detection limit (2 ppm). Other data from McDougall Cretaceous Hawks Crag Breccia but noted that the Tasman-1 & van der Lingen (1974) and Tulioch (unpubl. data). 230 New Zealand Journal of Geology and Geophysics, 1997, Vol. 40 clasts were dominated by ignimbrite and soda granite instead were probably derived from Rotoroa Complex-like of potassium-rich granite. We note that the early Late correlatives (R. B. Stewart pers. comm. 1992; Gamble et al Cretaceous Beebys Conglomerate near Nelson is also a 1994). reddish colour, and contains a clast assemblage more like the Tasman-1 breccia (e.g., P43602; Johnston 1990). Thus, there may be closer onland analogues of the Tasman-1 ION PROBE ZIRCON U-Pb ANALYSES breccia than realised by Wodzicki (1974). Methods Geochemical data are presented for 12 wells which penetrate igneous basement of the MTZ or Western Province Zircon was separated and analysed on the ion probe (Table 2, Fig. 4B-C). Similar results for replicate samples (SHRIMP) at the Australian National University in an of two wells, in which cuttings were hand picked by different attempt to better constrain the affinities of rocks from four operators, supports the validity of whole rock analysis using of the wells. Isotopic data are presented in Table 3 and Fig. 5 relatively small (c. 15 g) samples of cuttings. Only 1.5 g of Except for Tangaroa-1, for which core of rhyolite was cuttings were available for XRF analysis of the North available, all zircon samples were separated from the same Tasman-1 sample. hand-picked cuttings of granite used for geochemical Basement in Tane-1, Surville-1, Kiwa-1, Witiora-1, and analysis. The clearest, most prismatic grains were selected Taranga-1 can be confidently assigned to the Cretaceous for analysis, but there is a possibility that cuttings and Separation Point Suite (SPS); in addition to high Sr (Fig. 4B), subsequent zircon separates were derived from more than all except Witiora-1 have high Sr/Y ratios (144-634) one basement component. 206 238 characteristic of the SPS (Tulloch & Rabone 1993; Muir et The zircon data are normalised to Pb/ U of 0.0928 al. 1995). Witiora-1 rock has low Y and Rb and is still for the 572 Ma SL13 standard. The data plotted in Fig. 5 considered to belong to the SPS despite K20>Na20 and have not been corrected for common Pb contributions. For depleted Sr because these values probably result from Phanerozoic zircons, the estimation of common Pb by wav 204 206 extreme fractionation. Surville-1 has unusually high K O of the Pb/ Pb results in large uncertainties of the 2 207 and Rb for SPS, but low Y, Nb, and apparent lack of zircon radiogenic Pb abundance. For these zircons, common Pb inheritance render correlation with Rahu Suite (Tulloch is determined by a lever rule between common Pb on the 1983) unlikely. abscissa and radiogenic Pb on the concordia curve, and the 2 238 Toropuihi-1 bottomed in biotite granite with high Ga, Y, ages are derived from the °6pb*/ U ratios. All zircon and Nb indicative of an A-type affinity, similar to granites analyses should lie on a line connecting the radiogenic age immediately onshore at Whakapoi Point and at Cape to common Pb (dashed lines in Fig. 5) if they are consisten* Foulwind c. 100 km to the southwest (Cooper & Tulloch with a single magmatic age and variable contributions of a 1992). Such granites are rare elsewhere in New Zealand. common Pb component. Large common Pb contributions are apparent by discordant analyses in Fig. 5, and those with Kongahu-1 and, less confidently, North Tasman-1 and large common Pb contributions often appear to have lost Pb Motueka-1 are both correlated with the S-type granites of as well, when compared with more concordant zircons. the Karamea Suite, most abundantly exposed onshore in the Discordant zircons are apparent in all four rocks, and Karamea Batholith (Fig. 2) (Tulloch 1988). Motueka-1 (and determining a crystallisation age for such zircon populations North Tasman-1) has somewhat high Na for typical Karamea can be subjective. For this work, we adopt the simple Suite, but the zircon data discussed below confirm a mid- approach that a concordant population that has the highest Paleozoic age. Although the Motueka-1 rock contains a trace U-Pb age defines the magmatic age. A spread of data away of magnetite, Na and Sr are not high enough to suggest from the best-fit line indicates either: (1) the presence of correlation with I-type granites of the Paleozoic Paringa both magmatic and inherited grains; (2) beam spots Suite (Cooper & Tulloch 1992); correlation with MTZ- overlapping magmatic rim and inherited core; (3) Pb-loss; associated Carboniferous granites is a possibility. or (4) some combination of 1, 2 and 3. Wodzicki (1974), using only petrographic criteria, correlated basement in Maui-2 and Ruby Bay-1 with the Rotoroa Complex (part of the Median Tectonic Zone; Results Kimbrough et al. 1993, 1994). The chemical data presented Approximately half of the Toropuihi-1 zircons (Fig. 5 A) in this study (e.g., moderate Sr values, high Y compared to form a tight cluster around concordia at 312 + 7 Ma, with SPS; Fig. 4B) confirm these earlier correlations. the remaining zircons being younger, with higher common Tangaroa-1 intersected interbedded rhyolite and basalt. Pb, and scattered. No older inherited grains are apparent and Both are considerably altered and the rhyolite is strongly the scatter is probably due to Pb loss. The tight cluster silicified. The rhyolite analysis is characterised by very low indicates the Carboniferous age is almost certainly K and Rb. A mantle-normalised multi-element plot of the magmatic. Strontium isotope analysis of the whole rock analyses (Fig. 4C) exhibits enrichments in Rb, Ba, and Sr yields Sr = 15.39 ± 0.01 ppm and 87Sr/86Sr = and Nb-depletion, characteristic of calc-alkaline, subduction- 1.1619 ±0.0001. Rb concentration from duplicate XRF related suites. We know of no obvious onshore correlatives analysis is 512 ± 2 ppm. Model ages based on assumed Sr- of this basalt-rhyolite basement, although a possible plutonic initial ratios of 0.705 and 0.715 range from 314 to 318 Ma. correlative of the rhyolite is the Lake Roxburgh Tonalite on These ages overlap with, and thus are consistent with, the the western edge of the MTZ in Fiordland (Kimbrough et zircon age of 312 ± 7 Ma. Conversely, the high 87Rb/86Sr al. 1994), which has similar chemistry (Fig. 4C). Tangaroa- ratio (100-101), together with the error on the zircon age, 1 rhyolite is clearly chemically dissimilar from Cretaceous do not allow a precise calculation of the Sr initial ratio. rhyolites on the Lord Howe Rise (McDougall & van der Tangaroa-1 zircons (Fig. 5B), separated from rhyolite, Lingen 1974) (Fig. 4C). At least some foliated amphibolite are scattered over the concordia plot, making a unique and diorite xenoliths from Mt Taranaki are not cognate and interpretation very subjective. The oldest, most concordant Mortimer et al.—Taranaki & Wanganui Basin basement 231 0.15 0.15 20 25 20 25 238(J/206pb 238(J/206pb 0.15 0.15 CM 0.00. 15 20 25 30 30 40 50 60 70 238(J/206pb Fig. 5 Tera-Wasserburg U-Pb zircon concordia diagrams for Tangaroa-1 rhyolite and Toropuihi-1, Motueka-1, and Surville-1 granites. A single magmatic population with different contributions from common Pb would lie on a line (dashed) between the inferred radiogenic composition and the common Pb composition on the abscissa. However, all these samples appear to have been affected by Pb loss as indicated by the scattered analyses with younger ages and/or high common Pb. Toropuihi-1 gives a relatively well constrained Carboniferous age, and although Tangaroa-1 shows more scatter, it too is likely Carboniferous. Motueka-1 is distinctly older (possibly Devonian), similar to the Karamea Suite (Muir et al. 1994, 1996). Surville-1 is Early Cretaceous, but there is considerable scatter between 110 and 125 Ma. cluster of six grains gives a mean 238U/206Pb age of 321 Ma, data alone, although in either case the magmatic age is Early although one grain with high common Pb is significantly Cretaceous, and the data are consistent with the assignment older than this at c. 361 Ma. Cretaceous zircons are of Surville-1 to the Separation Point Suite on the basis of conspicuously absent; because of the large scatter, we can chemical affinities and age (Fig. 4B) (Kimbrough et al. only assign a Carboniferous magmatic age to the rock. 1994). Motueka-1 zircons (Fig. 5C) cluster quite closely in an Although interpretation of the U-Pb data from these four age range of 330-370 Ma, although the scatter exceeds rocks is problematical when it comes to assigning a analytical error for a single statistical population. A good magmatic age and associated error, it is apparent that the cluster at c. 350 Ma is suggestive of an igneous crystal- basement in Toropuihi-1, Tangaroa-1, and Motueka-1 is lisation age, in which case two 370 Ma zircons are inherited. Carboniferous-Devonian in age, whereas Surville-1 Alternatively, if 370 Ma is a magmatic age, most of the basement is Cretaceous. The former three samples therefore zircons have lost Pb. The low degree of scatter covering all have affinities with Paleozoic Western Province or MTZ the zircons suggests a latest Devonian or earliest Carbon- granitoids, whereas Surville-1 is likely to correlate with the iferous magmatic age. Separation Point Suite. Surville-1 zircons (Fig. 5D) cluster in a restricted age range c. 110-125 Ma but with excessive scatter for this to MAGNETIC ANOMALIES be a single population. The data could be interpreted either as representing a 125 Ma crystallisation age with Pb-loss The short wavelength and linear nature of the Junction affecting many grains, or as a 110 Ma granite containing Magnetic Anomaly (JMA—between Pio Pio and near Tahi- inherited 125 Ma grains. Either interpretation would be 1 in Fig. 2) and its correspondence with the Dun Mountain difficult to justify on the basis of the Surville-1 zircon U-Pb Ophiolite Belt of the Maitai Terrane throughout New Zealand 232 New Zealand Journal of Geology and Geophysics, 1997, Vol. 40 has been summarised by Hunt (1978). A prominent long sparse and some major issues remain unresolved. Onshore, wavelength, high amplitude anomaly, west of the JMA, also Eastern Province units (terranes) occur as simple structural extends south from Mt Taranaki to near Tahi-1 (Fig. 2; belts of varying width and are thus, to a large extent, equivalent to anomaly "R" of Davy 1992). From near Tahi- amenable to simple interpolation and extrapolation beneath 1 to D'Urville Island, magnetic anomalies become indistinct, the basins. However, the Western Province contains plutons but between D'Urville Island and the Rotoroa Complex, the and batholiths that are commonly smaller than the spacing JMA and the western anomaly merge into a single, broad, between the oil exploration wells, so our attempts to draw very strong magnetic anomaly (Fig. 2). This single strong geological boundaries around them are highly schematic. anomaly is caused by the close and parallel disposition of Below we discuss our assumptions, methods, and problems the highly magnetic rocks of the Maitai Terrane, Brook Street in sequentially assembling Fig. 6 on a unit-by-unit basis. Terrane, and Median Tectonic Zone; at a local scale, individual anomalies are still resolvable (Hunt 1978). East Maitai Terrane of the JMA, Davy (1992) has emphasised a magnetically The starting point for the sub-basin geological interpretation quiet region corresponding to the sandstone and schist of of Fig. 6 is the correlation of the JMA with the Dun Mountain the Caples, Torlesse, and Waipapa Terranes. Ophiolite Belt of the Maitai Terrane (Hunt 1978). Although Non-basement magnetic signatures in the area of Fig. 2 ophiolitic or metasedimentary rocks of the Maitai Terrane include those generated by: (1) the Quaternary andesitic have not been penetrated by any wells, we confidently draw stratovolcano of Mt Taranaki, which has a separate, short Maitai Terrane as a narrow, discontinuous belt between its wavelength anomaly, slightly offset from the broad western surface exposures at Pio Pio and D'Urville Island (Fig. 2) anomaly (Davy 1992); (2) buried Miocene andesite The near-constant width of the Maitai Terrane shown ir volcanoes in offshore Taranaki, north of about latitude 39°S Fig. 6 is schematic. The absence of the JMA south of Tahi- (Herzer 1995); and (3) a Late Cretaceous igneous complex 1 and north of D'Urville Island (Fig. 2) coincides with ar. in the Torlesse Terrane at latitude 42°S. area of considerable tectonic complexity in Taranaki Basir West of about longitude 174°E, Davy (1992, fig. 7) (P. R. King pers. comm. 1996) and may reflect local excisiori showed an area of high magnetic contrast, which he of the ophiolite belt (Fig. 6). interpreted in terms of linear magnetic anomalies (this contrast is not as obvious on Fig. 2, which is simplified from Torlesse Terrane Hunt 1978). Although we do not necessarily accept that these The western boundary of the Torlesse Terrane is well defined anomalies are all as linear as Davy claims (cf. Hunt 1978 in the northern and southern parts of Fig. 6 (Beetham & and Davy 1992), we do regard the high contrast as typical Walters 1985; Graham 1985, 1987; Mortimer 1993, 1995) of magnetic signatures of the Western Province, which in between, it must lie to the west (we prefer just west) o: contains a variety of highly magnetic plutons (Tulloch 1989). the Wanganui Basin wells and Fishermans Rock. With such This variable magnetic character of Western Province and a boundary, Parikino-1 schist is sensibly SSW along-strike MTZ units means that magnetic anomalies are of limited from the Torlesse schist in the Kaimanawa Range, but is use in distinguishing these units from Brook Street and c. 50 km farther northwest across-strike than a simple 1\TNE Maitai Terranes. The JMA, however, is an invaluable extrapolation of Torlesse schist from Port Underwood to near magnetic datum in interpreting basement geology beneath Kapiti Island would suggest (Fig. 2, 6). Taranaki and Wanganui Basins (see below). The many NNE- and northeast-striking faults in Wanganui Basin outlined by Anderton (1981) may be BASIN AND SUB-BASIN STRUCTURE continuations of the Wairau and/or Picton Faults (Fig. 6) However, the absence of north-, NNW-, or northwest- Interpretation of seismic reflection lines in the area by striking faults in seismic sections (Anderton 1981; Carter et Anderton (1981), Thrasher & Cahill (1990), and Lewis et al. 1988; Thrasher & Cahill 1990; Lewis et al. 1994) does al. (1994) has identified a number of major faults that offset not support a northwest-striking sinistral fault with 50 km the basement-cover unconformity by up to 6 km. The offset in Cook Strait to account for the apparent displacement Taranaki and Manaia Faults (Fig. 6) are two of the most of the Torlesse-Waipapa boundary (cf. Cope & Reed 1967. important in the area and have had a long and complex fig. 1). Lack of basement sampling in northern Cook Strait history (e.g., King & Thrasher 1992). Mortimer (1993) has presently precludes solution of this geometrical problem. drawn attention to the fact that the Picton Fault (Fig. 6) is a In Fig. 6 we have, for simplicity, drawn the western edge of major metamorphic and structural boundary within the the Torlesse Terrane with a north—south strike under much Marlborough Schist. of Wanganui Basin. The position of the Esk Head Melange From our experience in onshore South Island, boundaries in Fig. 2 and 6 is from Mortimer (1995) and reference:- of Mesozoic terranes often coincide with Cenozoic faults. therein. Given the lack of precision in locating contacts between basement geological units beneath Taranaki and Wanganui Caples and Waipapa Terranes Basins, we have deliberately chosen to position them along These litharenite-dominated terranes lie in identical these major basement-cover faults where geometry permits. structural positions between the Rakaia Torlesse and Maitai Terranes, with Caples Terrane exposed in the South Island DISCUSSION and Waipapa Terrane mainly in the North Island. Nomen- clature of the Waipapa Terrane is in a state of flux; it is Our preferred basement map of Taranaki and Wanganui probably a composite terrane, and reliable discrimination Basins is shown in Fig. 6. Although an improvement on of its various parts requires lithofacies, paleontological, and earlier versions, it is still essentially an outline, as data are structural data (Black 1994). Neither Caples nor Waipapa Mortimer et al.—Taranaki & Wanganui Basin basement 233 38°l Ixxxx'xxxxv EASTERN PROVINCE 'j< x y x x x x xi & MEDIAN TECTONIC Taranaki Fault xxxxxx; xxxxx: ZONE xxx: Torlesse Terrane Rakaia (r), Pahau (p) xxx Caples Terrane (c) xxxxxxx Waipapa Terrane (w) fXXXXXXX/' txxxxxxx Maitai Terrane fcxxxxxx xxxxxx xxxxxx Murihiku Terrane xxxxx +\xx *'xx Brook St Terrane X xx; Plutonic rocks of xx; Median Tectonic Zone XXX xxxx WESTERN PROVINCE xxxxx \xxxxxxx Undifferentiated xxxxxxx \ Esk Head Melange Haast Schist SAMPLE SITES • well penetrates basement O well penetrates cover only • other subsurface sampling 172°E 174° Fig. 6 Interpretation of pre-Late Cretaceous basement geology beneath the Taranaki and Wanganui Basins based on interpretations made in this paper and data of Anderton (1981), Thrasher & Cahill (1990), Mortimer (1993, 1995), and Lewis et al. (1994). All contacts are speculative. Highly speculative units and contacts are shown by question marks. See Fig. 2 for names of wells. Open circles correspond to location of wells in which Late Cretaceous — Early Cenozoic sandstones may have an MTZ and/or combined Eastern and Western Province provenance (e.g., Smale 1992). Terrane has been intersected in any exploration well in the genetic belt as schist in the Kaimanawa Range. Exactly how 250 km between Kaitieke-1 and the South Island; con- far under Wanganui Basin the schist west of Picton Fault sequently, we show only undifferentiated Caples and extends is unknown. At present there are simply not enough Waipapa Terranes beneath western Wanganui Basin (Fig. 6). data to reconcile the mutual geometry of the Caples, Waipapa, and Torlesse Terranes (see above), and different Haast Schist parts of the Haast Schist between the North and South The occurrence of schist in Parikino-1 and Santoft-IA, and Islands, except as broadly continuous belts (Fig. 6). at Fishermans Rock and Kapiti Island, supports previous interpretations that Haast Schist is continuous from the South Murihiku Terrane Island to the Kaimanawa Range (Fig. 6) (Cope & Reed 1967; Murihiku Terrane lies west of Maitai Terrane and is Mortimer 1993). Mortimer (1993) emphasised major penetrated by nine holes. The position of the western edge differences in the mesoscopic and macroscopic style of schist of the Murihiku Terrane in Fig. 6 is obtained as follows: the deformation east and west of Picton Fault, and specifically oldest strata on the western limb of the synclinorium in assigned schist near Port Underwood (Fig. 2) to the same Murihiku rocks near Kawhia Harbour are Oretian (Late 234 New Zealand Journal of Geology and Geophysics, 1997, Vol. 40 Carnian — Early Norian; Kear 1960), which, by comparison correlatives (including the relatively sodic Echinus Granite • with the Murihiku Terrane of Southland, suggests that only are obvious. Correlation of the Motueka-1 granite with the c. 2 km stratigraphic thickness of earlier Triassic strata are Karamea Suite suggests a considerable narrowing of the missing at Kawhia Harbour. So, unless an otherwise Takaka Terrane because, onshore, S-type granites of the unobserved anticlinorium. repeats the strata, the Murihiku Karamea Suite appear to be restricted to the Buller Terrane Terrane probably does not extend much farther west than (Cooper & Tulloch 1992). However, complete northward Te Ranga-1 (Fig. 2). Use of the Taranaki Fault as a excision of Takaka Terrane is unlikely given its occurrence convenient western edge to the Murihiku Terrane requires in Moa-IB. An alternative interpretation is a previously it to taper southwards against the Maitai Terrane (Fig. 6). unrecorded intrusion of Karamea Suite granites into the This wedge shape accords with the occurrence of Murihiku Takaka Terrane. Terrane near Nelson only as attenuated narrow fault slivers As discussed by Cooper & Tulloch (1992), the granite (Johnston 1981) that are too small to be shown at the scale of Toropuihi-1 is similar in its A-type chemistry to the of Fig. 2 and 6. Foulwind Granite, although a distinctly older age ol 327 ± 6 Ma was reported by Muir et al. (1994) for the same Brook Street Terrane and edge of Eastern Province unit. In Nelson, the Brook Street Terrane also occurs only as narrow fault slivers (Johnston 1981, 1990), considerably Median Tectonic Zone narrower than its maximum outcrop width of some 16 km Correlations with the MTZ can be made for the plutonic in the Takitimu Mountains of Southland. Because distinctive rocks in Ruby Bay-1 and Maui-2, and some xenoliths erupted pyroxene-rich lavas and sandstones of the Brook Street from Mt Taranaki (Gamble et al. 1994) are consistent with Terrane were not penetrated by any of the wells, and there derivation from subjacent MTZ crust. No obvious is no clearly definable linear magnetic anomaly to denote correlatives for the basalt and rhyolite encountered in its presence, the depiction of the Brook Street Terrane Tangaroa-1 occur onshore, but a possible plutonic equivalent beneath Taranaki Basin on Fig. 6 is highly speculative. which overlaps in age within error, is the Echinus Granite However, our reason for assuming its continuity north from (310 ± 5 Ma; Kimbrough et al. 1993), which appears to forrr D'Urville Island is that Brook Street-like rocks have been basement to the MTZ on Pepin Island (Fig. 2, 6) (Beresforc dredged on the West Norfolk Ridge, 800 km to the northwest et al. 1996). Tangaroa-1 rhyolite is chemically similar to the of Taranaki Basin (Mortimer & Herzer 1995), and possible slightly older (c. 340 Ma; Kimbrough et al. 1994) (Fig. 4C) broad correlatives may also occur in New Caledonia Lake Roxburgh Tonalite in eastern Fiordland, which may (Campbell 1984). also form older basement to the MTZ. Wide compositional In Fig. 6, we give Brook Street Terrane an arbitrary range and subduction-related calc-alkaline chemistry of the (Takitimu) outcrop width of c. 15 km, and south of Mt Tangaroa-1 rocks are thus consistent with a provisional Taranaki we have selected the Manaia Fault as its obvious Median Tectonic Zone correlation. Given the broad likely western edge. An alternative interpretation entirely similarity between Ariki-1 cuttings and Tangaroa-1 core, we omitting Brook Street Terrane from Fig. 6 would result in have included Ariki-1 in the MTZ (Fig. 6). The fact that MTZ the western edge of the Eastern Province coinciding with igneous rocks have been recovered in dredges on the West the Taranaki Fault, as shown by Cope & Reed (1967) and Norfolk Ridge (Mortimer & Herzer 1995) gives us Wodzicki (1974). confidence in extending the MTZ north throughout the whole area of Fig. 6. Although we agree with Wodzicki's (1974) Western Province tentative correlation of the red acid igneous breccia in Tasman-1 with Late Cretaceous cover units, because the Buller or Takaka Terrane-like metasedimentary rocks occur stratigraphic age of the unit is unknown, we cannot entirely in only two of the offshore western wells shown in Fig. 6. rule out still older correlation (e.g., with the Rainy River The high proportion of Separation Point Granite in six of Conglomerate of the Median Tectonic Zone; Johnston 1990). the offshore wells, suggests a greater areal extent of this unit offshore, compared to its onshore distribution in South Island (Fig. 6). Because granites of the Separation Point Suite intrude rocks of both the Western Province and the Median CONCLUSIONS Tectonic Zone (Kimbrough et al. 1994), they do not constrain On the basis of petrography, geochemistry, and U-Pb age, the position of the Western Province/Median Tectonic Zone we correlate core and cuttings from 30 oil exploration wells boundary. in Taranaki and Wanganui Basins with onshore geological North Tasman-1 and Motueka-1 lie well to the east of units (Tables 1,2,3). We have identified distinctive Torlesse most Paleozoic granitoid rocks, the exception being the Terrane, Murihiku Terrane, Median Tectonic Zone, and Echinus Granite of Pepin Island (Kimbrough et al. 1993; Separation Point Suite lithologies in a number of wells. Beresford et al. 1996). The (oxidised) green biotite and Maitai and Brook Street Terrane rocks have apparently not presence of magnetite and titanite in the Motueka-1 sample been intersected by wells, though the presence of the former suggest an I-type affinity. The chemistry of this rock does is indicated by the prominent Junction Magnetic Anomaly not suggest a clear association with any of the recognised (Hunt 1978). We have used the well samples, along with Paleozoic suites, and isotopic study is required to further interpretations of magnetic and reflection seismic data, to characterise it. The chemistry of the rock is comparable to construct a new geological map of the pre-Late Cretaceous the S-type Karamea Suite rather than the I-type Paringa Suite basement beneath west-central New Zealand (Fig. 6). (Cooper & Tulloch 1992), except in its relatively high Na2O Due to low sampling density, the locations of boundaries content; correlation with I-type granitoid rocks associated between major rock units are imprecisely located and their with the MTZ is another possibility, although no clear nature is indeterminate. Major questions left unanswered by Mortimer et al.—Taranaki & Wanganui Basin basement 235 the study include: (1) Caples-Waipapa-Torlesse-Haast Schist Campbell, H. J. 1984: Petrography and metamorphism of the geometry between the North and South Islands; (2) Teremba Group (Permian—Lower Triassic) and Baie de continuity of the Maitai Terrane near Tahi-1; (3) extent of St.-Vincent Group (Upper Triassic - Lower Jurassic), New the Brook Street Terrane north of D'Urville Island; (4) Caledonia. Journal of The Royal Society of New Zealand correlation of Motueka-1 granite; and (5) regional geometry 14: 335-348. of MTZ and Western Province units. However, Fig. 6 is a Carter, L.; Lewis, K. B.; Davey, F. 1988: Faults in Cook Strait and significant improvement on earlier efforts: it shows overall their bearing on the structure of central New Zealand. New continuity of offshore and onshore geology and demonstrates Zealand journal of geology and geophysics 31: 431—446. a greater regional extent of New Zealand basement terranes Cooper, R. A.; Tulloch, A. J. 1992: Early Palaeozoic terranes in and igneous suites than previously has been established. New Zealand and their relationship to the Lachlan Fold Belt. Tectonophysics 214: 129-144. Cope, R. N.; Reed, J. J. 1967: The Cretaceous paleogeology of the Taranaki—Cook Strait area. Proceedings of the ACKNOWLEDGMENTS Australasian Institute of Mining and Metallurgy 222: We acknowledge the following oil companies for drilling to 63-72. basement, thereby supplying material for this study: Amoco N.Z., Davy, B. 1992: Magnetic anomalies of the New Zealand basement. Anzpac Petroleum, Champlin Oil & Refining, Esso Exploration In: Proceedings of the 1991 New Zealand Oil Exploration & Production, Home Energy, Marathon Petroleum, N.Z. Acquitane Conference. Wellington, Ministry of Commerce, Crown Petroleum, N.Z. Oil & Gas, N.Z. Petroleum, Shell BP & Todd Oil Operations Group. Services, Superior Oil, and Tasman Petroleum. We also thank Neville Orr for thin sections, Stewart Bush for rock powders, Ken Finlow-Bates, T. 1970: Quantitative examination of immature Palmer and John Hunt for X-ray fluorescence analyses, Bob sandstones by point-count and X-ray modal analysis. Tane Stewart, John Gamble, and Joe McKee for information on Mt 16: 163-173. Taranaki xenoliths, Ian Graham for Sr isotopic analysis of Folk, R. L.; Andrews, P. B.; Lewis, D. W. 1970: Detrital Toropuihi-1 cuttings, and Glenn Thrasher, Peter King, Mac Beggs, sedimentary rock classification and nomenclature for use Alva Challis, Bill Watters, and Bryan Davy for helpful discussions. in New Zealand. New Zealand journal of geology and Earlier versions of the manuscript were improved by pre- geophysics 13: 937-968. submission reviews from Hamish Campbell, Peter King, and Mac Gamble, J.; McKee, J.; Grapes, R.; Bennett, D. 1994: The crust Beggs, and journal reviews from Tim Little and Russell Korsch. Institute of Geological & Nuclear Sciences contribution number beneath Taranaki Volcano imaged by xenoliths from 886. andesites in the Stratford lahars. Geological Society of New Zealand miscellaneous publication 80A. 70. Graham, I. J. 1985: Rb-Sr geochronology and geochemistry of Torlesse metasediments from the central North Island, New REFERENCES Zealand. Chemical geology (isotope geoscience section) Anderton, P. 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