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THE PUKETOKA FORMATION AND THE AGEOF THE HAURAKI GRABEN.Leaders: Bruce Hayward, Hugh GrenfellIntroductionThe Hauraki Graben is a major geomorphiccomponent of northern New Zealand, butwhat do we really understand about its timeand mode of formation? One much debatedquestion is - “When was it formed?” TheCoromandel-derived conglomerates of theenigmatic Puketoka Formation appear to offersome important clues (Battey 1949, Kear2000, 2004).This field trip explores the seldom-visitedtype locality of Puketoka Formation at thescenic Puketoka trig, near Maramarua(S12/046401). A second important outcrop atKidds Beach (R12/720560) on the southernshores of the Manukau Harbour will also beexamined in the context of pathways for theCoromandel-derived sediment and ageconstraints on the subsidence of the HaurakiGraben.Fig 1: View from Monument Road - hills in thebackground are the type locality of the PuketokaFormation near Maramarua.Fig. 2 Outcrop of the Puketoka Formation at KiddsBeach, southern Manukau Harbour.Age of Hauraki Graben founderingEdbrooke (2001, p.52) summarises mostcurrent thinking on the formation of thisfeature: “The Hauraki Plains and Firth ofThames are interpreted as being part of anactive continental rift structure (Hauraki Rift),extending more than 250 km from aconcealed junction with the TVZ to thenorthern Hauraki Gulf (Hochstein andBallance, 1993). The Hauraki Rift is boundedby Firth of Thames Fault and Hauraki Fault tothe west and east respectively. It consists ofup to 3 half-grabens adjacent to west-dippingmaster faults, and contains up to 2.5 km ofsediment. The rift is thought to haveoriginated during Late Miocene uplift in theback-arc region, parallel to the Coromandelvolcanic arc, and was probablyaccommodating sediment by 5-7 Ma (LateMiocene).”In recent years David Kear (e.g. Kear, 2000,2004) had made us well aware of HughBattey’s (1949) report of Coromandel-derivedcobbles in a Pliocene conglomerate atPuketoka trig, and the implications for ayoung age of formation of the HaurakiGraben that now lies between the Coromandeland Hunua Ranges.Edbrooke (2001) was aware of thisconglomerate and suggested the following toaccommodate it: “Periods of sedimentaccumulation [in the Hauraki Rift] probablyalternated with periods of sediment bypass,allowing pyroclastic flows and sediment toreach areas west of the rift (Hochstein andBallance, 1993).” By the end of today’s tripyou can make your own judgement on howplausible this explanation might be.The clasts within the type PuketokaFormation include rhyolite, vein quartz andsilicified wood, inferred to be derived fromthe northern or central Coromandel Rangevolcanic sequence (essentially of Middle andLate Miocene age, ~18-5 Ma). The oldestrhyolitic volcanism is dated at ~11 Ma(Adams et al., 1994) giving a lower ageconstraint on the conglomerate.In the southern part of the Hauraki Graben,the Kerepehi Fault has a mappable fault scarpand is considered to be active (e.g. Houghtonand Cuthbertson, 1989), suggesting that theGraben may still be slowly foundering, at

least at the southern end. In the vicinity ofMatamata, in the southern part of the Graben,the Hauraki Fault has a 400 m vertical offset(west side down) of the Waiteariki Ignimbrite(2.09 Ma, Briggs et al., 2005) and no offset ofthe Mamaku Ignimbrite (0.22 Ma) suggestinglargely Pleistocene foundering in that vicinity(Houghton and Cuthbertson, 1989, p.29).Field trip itineraryFrom Auckland City we will drive south onHwys 1 and 2 around the east and southernsides of the uplifted Hunua greywacke horst(Figure 4, 6).STOP 1. Puketoka (near Maramarua)We will drive onto private farmland and visitbluffs comprised of the type PuketokaFormation to examine the sequence andcomposition of the conglomerate clasts. Wemay also stop and look at some of the largercobbles and boulders that have been collectedoff the paddocks and used in retaining wallsby the farm house.Puketoka FormationThe Puketoka Formation is included in thelatest Miocene to Holocene Tauranga Group(Kear and Schofield 1978, Edbrooke et al.2001).At the type locality, near Maramarua, Battey(1949) described the following sequence:Top30 m poorly cemented boulder bedcontaining well rounded boulders,often more that 0.3m in diameter,of mainly andesite, with somequartz vein-stone, cryptocrystallinesilica, and bandedrhyolite.6 m conglomerate with cobbles ofgreywacke and andesite up to0.1m in diameter.45-60 m cream coloured pumice siltBase not seen.Fig. 3 Fallen block from the type localityProvenance of Puketoka sedimentGreywacke clasts are likely to be eroded fromnearby surrounding hills of the southernHunua and Hapuakohe Ranges. The andesiteclasts may be derived from the Middle-LateMiocene Kiwitahi Volcanic Group, some ofwhich outcrops today 3-10 km to the east.Some andesite might also come from theCoromandel Group of similar age on the otherside of the Hauraki Graben. The only knownpossible source for the rhyolite and quartzclasts is from the Late Miocene-PlioceneWhitianga Group that outcrops today 25-30km to the east of Puketoka on the oppositeside of the Hauraki Graben.Battey (1949, p.441) was the first to speculatethat the Puketoka Formation providesevidence for the age of the Hauraki Grabenand describes the Formation as including“pumice silts, various sands andconglomerates; these last are commonlygreywacke boulders, but in places, especiallyin the higher parts of the sequence, containboulders of vein quartz and banded rhyolitewhich, as is shown later, imply a region ofprovenance in Cape Colville Range or insome westward extension of this range.”.Puketoka Formation – its significance anduseageLater in his publication, Battey (1949) alsoincludes a “Central Volcanic Plateau” (CVZ)source for “Puketoka Formation” pumice silts(including chalazoidites) in railway cuttingsof the middle Mangatangi Valley and the eastend of Koheroa ridge (Battey 1949, p.439).

Schofield (1976) clearly considered thesedeposits (mapped as tz and pz) to be youngersediment (terrace deposits) within theTauranga Group and not part of the PuketokaFormation. He preferred a local and southern(CVZ) source for the Puketoka Formation(Schofield 1976, p.19). Kear and Schofield(1978) mapped the Formation as occurringthroughout the Waikato and of Late Plioceneto Early Pleistocene age. They appeared toprefer a Coromandel source for the sediments.Kermode (1992) mapped the Formation aspart of the Tauranga Group (tp) and having aCVZ source. Edbrooke (2001) mapped theFormation as being widespread but did notdiscuss it’s provenance. Puketoka Formationas mapped by some (e.g. Kermode 1992,Edbrooke 2001) contains a wide range oflithologies (e.g. lignites, rhyolitic ignimbritesand tephras, peat, polymict conglomerates,carbonaceous mudstone) from at least twosources. The type Puketoka Formation is ofPliocene age and contains Coromandelderivedconglomerate and pumiceoussediment. Much of the remainder of the rocksmapped elsewhere as Puketoka Formation areof Pleistocene age, contain no Coromandelderivedconglomerate clasts and thesignificant clastic rhyolitic component isderived from the CVZ. So why map them asthe same unit?Provenance of the pumiceous sediment andignimbrite of the Puketoka Formation s.l.(e.g. Edbooke 2001)These could be sourced from the CoromandelRange, Kaimai Range volcanics or CentralVolcanic Zone (CVZ) between Taupo and theBay of Plenty. Over the last 11 myrs there hasbeen a southward migration of volcanismthrough this area. Coromandel ignimbritesand pumiceous sediment comes fromWhitianga Group volcanism which north ofWaihi (i.e. the area west of Puketoka) is ofLate Miocene age (11-5 Ma, Adams et al.,1994). Around Waihi and south into theKaimai Range, rhyolitic volcanism producedextensive ignimbrite and pumiceous sedimentand rhyolite domes during the Middle andLate Pliocene (~4-2 Ma; Edbrooke 2001,Briggs et al., 2005). All rhyolitic volcanism inthe CVZ occurred during the Quaternary (post~2 Ma; Briggs et al., 2005).There is no apparent overlap in time betweenvolcanism in the NNW-trending Coromandel-Kaimai Arc and the NE-trending CVZ(Briggs et al., 2005). Thus pumiceoussediment or ignimbrite in Puketoka Formationof Pliocene age (type locality) must bederived from the southern Coromandel-Kaimai volcanism.Much of the pumiceous sediment andignimbrite mapped as Puketoka Formationaround the Manukau and Waitemata Harbours(e.g. Edbrooke, 2001) is of Early Pleistocene(1.3-1 Ma) age (Alloway et al., 2004) andtherefore likely derived from the CVZ.We will drive back to Papakura and turn westalong the southern shore of the ManukauHarbour.STOP 2. Kidds Beach, southern shoresManukau Harbour (Angelsey 2007, Berry1986, Hayward et al. 2006, 2007, Prickett2007). [Low tide 5PM, 0.7 m]The South Auckland Rock and Mineral Clubhad long noted that the gravel at Kidds Beachcontained quartzose pebbles not seenanywhere else locally and most similar towhat they collect in the Coromandel Ranges.Hearing of this, your trip leaders’ curiositywas aroused and we visited the site on severaloccasions in 2006.At Kidds Beach the normally shelly shorelineof the Manukau is replaced by a gravel beachof red, grey and white pebbles for at least a 2km stretch. These pebbles are eroded out ofthe adjacent intertidal shore platform formedof weakly bedded conglomerate with somewest and north-facing cross- bedding. Theconglomerate overlies limonitic sandstoneand includes some lenses of log-bearing,carbonaceous sandstone and mudstone.Age of the Puketoka Formation, southManukau HarbourThe geology of this area was mapped by MScstudent Keith Berry (1986), whos maps showthat this is the largest outcrop of a Pliocene“red chert conglomerate” unit that has a

number of other smaller exposures along theharbour coast between Karaka and ClarksBeach (Fig. 1). Berry also studied a numberof local drillhole sequences and showed thatthis conglomerate is underlain by earlyPliocene shallow marine Kaawa Shellbed(Opoitian-Waipipian age; Berry, 1986) and isoverlain by widespread rhyolitic pumiceoussands. These latter sediments presumablywere transported down the Waikato River tothis area from the CVZ following the start ofvolcanism in that region sometime after 2 Maago (Briggs et al., 2005). Thus the age of theconglomerate must be somewhere within thelate Pliocene (c. 3-2 Ma ago). This wasconfirmed by the discovery of the leaf of a“brassi” group (large-leaved) beech in one ofthe carbonaceous lenses (R12/f80), a groupthat had largely disappeared from NZ by 1.5-2 Ma ago (Cooper, 2004).Composition and provenance of clasts inconglomerate at Kidds BeachThe conglomerate (and modern beachgravels) is dominated by subangular pebblesof red-brown cherts and greyargillite/greywacke clearly derived from theWaipapa Group (Table 1). The nearest sourceis the uplifted Hunua Ranges, but red chertsare rare in the southern and western partsclosest to Kidds Beach (Schofield, 1976,1979). The greatest concentration of chertunits outcrop in the north-east Hunuas toWaiheke Island area (also the source ofMcCallum’s red chip used widely onAuckland footpaths). The next most commonclasts are more rounded pebbles and evencobbles of cream-white crystalline “vein”quartz. Some of this has inclusions of chertthat link its source to the Waipapa rocks, butother more massive and more coarselycrystalline quartz is more reminiscent of veinquartz typically encountered on CoromandelPeninsula. Other rarer pebble lithologies thatare most strongly linked to a probableCoromandel source are of silicified wood,chert, ?sinter, and silicified flow-bandedrhyolite. Rounded andesite pebbles also couldbe sourced from the Coromandel volcanics,but an equally possible source is the Kiwitahiandesites that outcrop along the eastern sideof the Hunuas (Black et al., 1992). As thecrow flies the nearest andesite source is theWaitakere Ranges to the north-west, but thisseems improbable as it is contrary to thedirection indicated by all the other lithologies.The shape of the pebbles reflects acombination of their hardness, jointing, anddistance transported. The softest and largestclasts are well-rounded but were locallyderived Waitemata Sandstone. The smallestand most angular pebbles are the red cherts -sourced from the hardest and most closelyjointedrocks but transported c. 30 km. Claststransported the furthest (c. 50 km) are thoseinferred to be derived from the Coromandelvolcanic region. These are all the hardestlithologies in the Coromandel Range (freshandesite, quartz, silicified rhyolite and wood)but even so most have been rounded duringtheir long journey. None of the more commonsofter lithologies found on CoromandelPeninsula (e.g. rhyolites, ignimbrites, alteredandesites) are present in the conglomerate andpresumably were not hard enough to survivethe journey. The most convincingCoromandel-sourced pebble has vein quartzand andesite together – a combination notseen in the Waitakere Ranges nor Kiwitahiandesites.Table 1. Character of the cobbles and pebblesin the Pliocene ConglomerateLithology % Size Shape ProvenanceSandstone 30

Fig.4. Simplified geological map of the Auckland region, showing location of the conglomerates at Puketoka and KiddsBeach and possible sources of their different pebble lithologies. The inferred N Hunuas-Coromandel provenance of theconglomerate points to the existence of a west-flowing “Clevedon River” prior to the major subsidence of the HaurakiGraben.West-flowing Pliocene Clevedon RiverThe probable provenance of the pebbles in theKidds Beach conglomerate require a transportroute from the North Hunuas-Waiheke area.Geological maps (e.g. Edbrooke, 2001) showthe existence of a narrow elongate “halfgraben” directly linking the two areas andnow occupied by the Clevedon Valley andArdmore-Papakura lowlands. This valley liesbetween two uplifted NW-tilted blocks ofWaipapa “greywacke” with the HunuaRanges to the south and the Whitford –Maraetai block to the north uplifted along theNW-trending Papakura Fault. Coalexploration drillhole 8427 in this “halfgraben”intersected Eocene and Miocenesedimentary rocks sitting atop greywackebasement (Edbrooke et al., 1994) and indicatethat it is clearly downfaulted and not purelyerosional through the greywackes. Thus itwould seem that a west-flowing river passedthrough this valley in the mid or late Pliocene.The present arrangement of islands andflooded valleys at the eastern end of theTamaki Strait (between Waiheke and theHunuas) suggests that several west-flowingtributaries fed this ancient “Clevedon River”,and one or more of these could haveoriginated from the vicinity of Coromandeltownship, prior to the subsidence of theHauraki Graben.A substantial west-flowing river through theAuckland region may seem counter-intuitivegiven the present geography, but further northmuch of northern Auckland and Northland ispredominatly drained by west-flowing rivers(e.g. Hoteo, Manganui, Wairoa, Hokianga)and reflect the general western tilt of thegeology of the Northland-Auckland peninsula(Figure 5). Prior to the foundering of theHauraki Graben this western tilt seems tohave extended south-eastwards to the Hunua-Coromandel region. Much of the centralCoromandel Range is tilted eastwards todayand suggests that maybe the Hauraki Grabenmay have been an area of thermal up-domingprior to the graben’s formation.

Fig. 5. Present day west-flowing Northland rivers dueto regional tiltingSummaryPuketoka Formation useThe Puketoka Formation has been used toinclude a wide variety of sediments from atleast two different sources and directions inthe Waikato and Auckland regions (e.g.Waitemata and Manukau Harbours (Kermode1992, Edbrooke, 2001) and the Waikato Basin(Kear and Schofield 1978). It seems to havebeen applied too broadly. Certainly theTamaki Estuary (e.g. St Kentigens section)and other Manukau Harbour (e.g. southeast ofIhumatao) sediments have quite a differentprovenance (i.e. CVZ derived fluvial,pumiceous sediments, palaeosols, ignimbritesand tephras). This contrasts with Kidds Beachand indeed Battey's type locality at Puketokatrig (greywacke, cherts, Coromandelrhyolites, andesites). Arguably the youngerCVZ-derived deposits should bedifferentiated from the Coromandel /greywacke-sourced sediment typified at thetype locality and Kidds Beach.Age of the Hauraki GrabenThe presence of Coromandel-derived clasts inthe Pliocene Puketoka Formation at Puketokaand Kidds Beach suggest that this middlesection of the Hauraki Graben had notfoundered far, if at all, prior to the MiddlePliocene (~3 Ma). We therefore imply thatmost foundering of this major geomorphicfeature has occurred in the last 3 myrs and isstill on-going, at least at the south end.Outstanding questions and future work1. Can the rhyolitic sediment of Pliocenesouthern Coromandel-Kaimai age bedistinguished from similar sediment ofPleistocene age derived from the CVZ –geochemistry of fresh glass?2. Can a more precise age be obtained forPuketoka Formation s.s. at the type localityand Kidds Beach? And are they of similarage?3. Has the Hauraki Graben rifted apart andwas the Coromandel and Hunua Rangesoriginally closer together?4. Are there rhyolitic sources for the Puketokaclasts buried within the Hauraki Graben?5. What is the age of the oldest sedimentwithin the Hauraki Graben (needs adrillhole)?References:Adams, C.J., Graham, I.J., Seward, D. andSkinner, D.N.B., 1995. Geochronologicaland geochemical evolution of lateCenozoic volcanism in the CoromandelPeninsula, New Zealand. New ZealandJournal of Geology and Geophysics 37:359-379.Alloway, B., Westgate, J., Pillans, B.,Pearce, N., Newnham, R., Byrami, M.,and Aarburg, S., 2004. Stratigraphy, ageand correlation of middle Pleistocenesilicic tephras in the Auckland region,New Zealand: a profilic distal record ofTaupo Volcanic Zone volcanism. NewZealand Journal of Geology &Geophysics 47: 447-480.Angelsey, K. 2007. The Kidds Beach trip -June 2006. Geocene 2: 8-9.Battey, H. M., 1949. The geology of theTuakau-Mercer area, Auckland.Transactions of the Royal Society of NZ77(3): 29-55.Berry, K. A., 1986. Stratigraphic, structuraland geophysical studies of Neogenesediments of the Manukau lowlands.Unpublished MSc thesis, University ofAuckland.Black, P. M., Briggs, R. M., Itaya, T., Dewes,E. R., Dunbar, H. M., Kawasaki, K.,Kuschel, E., and Smith, I. E. M., 1992. K-Ar age data and geochemistry of theKiwitahi Volcanics, western Hauraki Rift,

North Island, NZ. NZ Journal of Geologyand Geophysics 35: 403-413.Briggs, R. M., Houghton, B. F., McWilliams,M., and Wilson, C. J. N., 2005. 40 Ar/ 39 Arages of silicic volcanic rocks in theTauranga-kaimai area, NZ: dating thetransition between volcanism in theCoromandel Arc and the Taupo VolcanicZone. NZ Journal of Geology andGeophysics 48: 459-469.Cooper, R.A. (ed.) 2004. The New Zealandgeological timescale. Inst Geological andNuclear Sciences Monograph 22, 284 pp.Edbrooke, S. W., 2001. Geology of theAuckland Area, 1: 250,000 geologicalmap 3. Inst. Geological and NuclearSciences.Edbrooke, S. W., Sykes, R., and Pocknall, D.T., 1994. Geology of the Waikato coalmeasures, Waikato Coal Region, NZ. Inst.Geological and Nuclear SciencesMonograph 6, 2 vols.Hayward, B.W., Grenfell, H.R., Mauk, J.L.,Moore, P.R. and Mildenhall, D. 2006. Thewest-flowing “Clevedon River”,Auckland. Geological Society of NewZealand Newsletter No. 141: 24-29.Hayward, B.W., Grenfell, H.R., Mauk, J.L.,Moore, P.R. and Mildenhall, D. 2007. Thewest-flowing “Clevedon River”,Auckland. Geocene 2: 9-11.Hochstein, M. P., and Ballance, P. F., 1993.Hauraki Rift: A young, active, intracontinentalrift in a back-arc setting, inBallance, P. F., ed., Sedimentary Basinsof the World - chapter 17: South PacificSedimentary Basins, Amsterdam, ElsevierScience Publishers, p. 295-305.Houghton, B.F., and Cuthbertson, A.S. 1989.Geological Map of New Zealand 1:50,000Sheet T14BD Kaimai, New ZealandGeological Survey.Kear, D., 2000. Armchair geology, and theAlpine Fault in the North Island.Whakatane, David Kear Publisher, 18 pp.Kear, D., 2004. Reassessment of Neogenetectonism and volcanism in North Island,NZ. NZ Journal of Geology andGeophysics 47: 361-374.Kear, D. and Schofield, J.C. 1978. Geology ofthe Ngaruawahia Subdivision. NewZealand Geological Survey Bulletin 88.Kermode L.O. 1992. Geology of theAuckland urban area. Scale 1:50000.Institute of Geological and NuclearSciences geological map 2.Prickett, K. 2007. Pliocene conglomerates - aspanner in the works for archaeology.Geocene 2: 12.Schofield, J. C., 1976. Geological map of NZ,1:63 360. Sheet N48. Mangatawhiri. NZGeological Survey.Schofield, J. C., 1979. Geological map of NZ,1:63 360. Part sheets N38, N39, N42 andN43. Waiheke. NZ Geological Survey.

Fig. 6 General road and locality map.