677kb pdf - Geoscience Society of New Zealand

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.