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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,
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