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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).
Page 4 and 5: Schofield (1976) clearly considered
Page 6 and 7: Fig.4. Simplified geological map of
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