GEOLOGY OF THE - GNS Science

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Figure 8 East of Wairoa the coastal plain is advancing seaward; former sea cliffs are visible on the inland shore of the WhakakiLagoon (left). Mahia Peninsula (far distance) is part of a rising structural high. Photo CN47895125: D.L. HomerFigure 9 The flat surfaces ofPortland Island (foreground) andMahia Peninsula are marineterraces cut into the Late Miocenerocks. The terrace on PortlandIsland is 125 000 years old andabout 110m above sea level.Terraces at southern MahiaPeninsula are tilled to the WNW;thetilling and uplift are related to activegrowth of the Lachlan Ridge, thecrest of which is offshore,immediately east (right) of thepeninsula. Photo: Stephen Jones,Ti'eble Court Photos, Gisborne9
STRATIGRAPHYThe rocks of the Raukumara area are here subdi videdinto and described as five major units. based mainly ontheir age and their structural history:Lale Jurassic to Early Cretaceous basementin-place Cretaceous to Oligocene rocksthe Easl Coasl Allochthon (Earl y Cretaceous 10Oligocene displaced rocks)Miocene and Pliocene rocks. andQuaternary sediments.The nature of the contact between the first two units isdi sputed in some areas. The Early C re taceo us toOligocene rocks are subdi vided on the basis of structuralhistory. In the south west oflhe map area basement rocksare unconfo rmabl y overla in by an in- pl ace(autoc htho no us) sedime ntary seque nce of Ea rl yCretaceous to Oligocene age. However, in the northeast(nonheasl of Matawai-Te Karaka) equivalent rocks arepresenl as a series of thrust sheels (the East CoaslAllochthon ofMoore I988a) some ofwhich are displacedat least tens of kilometres fro m their original locationsa nd , over large areas, thrust over th e equivale ntautochthonous sequence. In the southwest a regionalunconformit-y is present between the Ea rl y Cretaceous toO ligocene in-place seq uence and the overl ying MiocenePli ocene unil. The th rusl sheels o f lh e East CoastAllochthon are also overlain by Miocene-Pliocene beds.LATE JTerrane nomenclatureRASSIC TO EARLY C RETACEO USBASEMENTTorlesse composite terraneThe indurated qumtzofeldspathic and lithic sandstone andmudslone ("greywacke") of the nonhwestem Raukumaraa rea have commo nl y been ma ppe d as TorlesseSupergroup (e.g. Suggate et al. 1978). Torlesse rocksform the orth Island ax ial ranges and much of theSoulhern Alps; they range in age from Carboniferous 10Earl y Cretaceous.The basement rocks of New Zealand are also mapped interms of tectonostratigraph ic terranes (e.g. Coombs el al.1976; Bishop et al. 1985; Bradshaw 1993; Fig. 10). TheLate Jurassic to Early Cretaceous basement rocks of lheRaukumara area are a part of the Torlesse compositeterrane, one ofseveral Late Paleozoic to Early Cretaceous.fault-bounded tectonostratigraphic units of low grademetasedimentary rocks which constitute the EasternProvince of ew Zealand. The monOlOnous sequencesof indurated, interbedded sandstone and mudstone typicalof the Torlesse are inferred to have accumulated mainlyin deep marine environments, within an accreti onaryprism.toThe original definition of the Torlesse terrane was givenby Coombs el al. (1976). Later workers subdi vided lheTorlesse rocks into a number oft.erranes and subterranesbut for some areas contl icting interpretat ions have beengiven. Bradshaw el al. ( 198 1), Bradshaw (1989) andSilberling et. al. ( 1988) subdi vided the To rlesse intosubterranes, the easternmost of which (Pahau subterrane)included the basement rocks of the Rauk umara area.Others subdi vided the Pahau subterrane in lO two units.both of which they termed terranes - Pahau terrane. ofLate Jurassic to Earl y Cretaceous (?Barremian) age. anda younger (Apti an-Albian) Mata Ri ver terrane (Sporli &Ballance 1985, 1989). The boundary between Pahau andMata Ri ver terran es was originally placed near theWhakatane Fau lt, i.e. west of the area of the Raukumaramap (Sporli & BaJiance 1985, 1989). but laler movedeaslward by 40 km. to inlersecl the Bay of Plenty coastnearTorere. 18 kill eaSI ofOpoti ki (A ila & Sparli 1992).The validity of "Mata River terrane" remains unproven.If Mata Ri ver te rra ne is found to be a validtcclOnostratigraphic unit, the name is inappropriate, sincethe Mata Ri ver catchment lies entirely within middleCretaceous and you nger cover strata. Mortimer ( J995)subd ivided the To rlesse rocks of the eastern North Island,based on differences in sandstone petrographic modes.age. bulk chemical composition and the known andinferred extent of melange belts; his work suggested theremay be no basis for a distinct Mata Ri ver terrane. Hede fin ed two major basement units east of the TaupoVolcanic Zone. name ly the Pahau subterrane and apreviously unrecognjsed Waioeka subterrane, both of LateJurassic to Early Cretaceous age. The boundary betweenthe Pahau and Waioeka sllbterranes was drawn at a majorzone of melange inferred to extend south from the Bayof Plenly coast as far as a poi nl WSW of LakeWaikaremoana (the Whakatane me lange; Mortimer1995). All basement rocks of the Raukumara area areeast of lhe Whakalane melange and within his Waioekasubterrane. Two distinct petrofacies were differentiated(Waioeka petrofacies and Omaio petrofacies). Thissubdi vision into subterranes and petrofacies was acceptedby Field, Uruski et al. ( 1997).Begg & Johnslon (2000) elevated units formerly knownas slIbterranes (e.g. Pahau subterrane) to fu ll terranestatus. on the basis of their presumed discrete geologicalorigins, and applied the name "Torlesse (composite)terrane" for Torlesse rocks in general. Kamp's (1999)work on fi ssio n track lhe rmochro no logy of lh eRaukumara basement rocks found Mortimer's ( 1995)subdi vision to be appropriate; he also elevated thesubterranes to full terrane status. Following Mortimer( 1995), Begg & Johnston (2000) and Kamp ( 1999) thebasement rocks of the Rallkllmara area are here mappedas Waioeka terrane, a part of the Torlesse compositeterrane.
Page 3: GEOLOGY OF THERAUKUMARA AREAScale 1
Page 10 and 11: Wairoa--+---_.f:..Oweka StreamWaiku
Page 12 and 13: Onshore physiography and geologyEas
Page 16 and 17: SEDIMENTARY AND VOLCANIC ROCKS..~an
Page 18 and 19: Figure 12 Torlesse sandstone(pale g
Page 21 and 22: In the Konmga-Matawai area the high
Page 23 and 24: eccia (Speden 1975).The Raukumara S
Page 25 and 26: e within the East Coast Allochthon
Page 27 and 28: distal. oceanic setting. are now at
Page 29 and 30: Figure 20 Folded centimetre- to dec
Page 31 and 32: Figure 22 Chevron folds inwell bedd
Page 33 and 34: Figure 23 On the point separating H
Page 35 and 36: Moore 1957). carTe Puia Middle Mioc
Page 37 and 38: In northern Raukumara Peninsula th
Page 39 and 40: QUATERARY SEDIMENTSSediments deposi
Page 41 and 42: Shallow marine sediments near Cape
Page 43 and 44: Figure 32 Wharekia (1 106 m, Yl5158
Page 45 and 46: Figure 34 The Tiniroto Landslide(ou
Page 47 and 48: (Whakoau Fault of Mazengarb el al.
Page 49 and 50: GEOLOGICAL RESOURCESOnly the northe
Page 51 and 52: shallow wells were dug in and near
Page 53 and 54: GEOLOGICAL HAZARDSVolcanic activity
Page 55 and 56: Seismoleclonic hazard(by M. Stirlil
Page 57 and 58: AVAILABILITY OF QMAP DATAACKNOWLEDG
Page 59 and 60: volcanicity. New Zealand journal of
Page 61 and 62: Hill. P.H. 1974: Taitai Series rock
Page 63 and 64: Mortimer. . 1992: Petrological repo
Page 65 and 66:
Strong. C.P. 1976: Cretaceous foram
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