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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
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Page 6 and 7: Fig.4. Simplified geological map of
Page 8 and 9: North Island, NZ. NZ Journal of Geo

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