The fol/owing three papers formed a symposium on "Vulcanicity and VegetatiOl' in the Rotorua District" at the 1962 Conference. GEOLOGY OF THE ROTORUA DISTRICT J. HEALY N.2.c'Geological Survey, D.S.1.R., Rotorua INTRODUCTION has a sharp descent into the Whakatane Graben and Reporoa lowlands, but on the The Rotorua district is 3500 square miles east it slopes gently down to the RBngitaiki in area, extending north from Maroa and River. This flows along the faul: angle Murupara to the Bay of Plenty coast between plateau and ranges, tlowing in between Matata and Tauranga. The Taupo ignimbrite gorges between the tectonic Volcanic Zone, which lies as a belt north- basins of Galatea, Waiohau and Te Mahoe. east from Ruapehu to White Island,passes through the centre of the area. In this North-west of the Taupo Volcanic Zone account the rocks have been divided into a the Patetere Plateau is gently arch, d along number of groups on a lithologic basis, and a north-south axis at an elevation of 2000 ft., their distribution and approximate age but terminates on the north against the range are shown in Figure 1. These groups do Kaimai Ranges and Whakamarama Plateau, not include late Quaternary volcanic ash, also an ignimbrite plateau. On the west it descends towards the Tokoroa Plat"au, and which mantles the eniire area superficially . and in places is more than 40 ft thick. on the east it gives way to the ;(aharoa Plateau, which north of Lake Rotorua is a little over 1500 ft. above sea level hut falls PHYSIOGRAPHY to 1200 ft. north of Rotoma and 1(00 ft. at Matata. On the north this plateau descends Broadly speaking, the district consists of gradually to sea level in Makettl Basin. two extensive ignimbrite plateaux between Tauranga Basin is a tectonic fe,lture in which the Taupo Volcanic Zone extends as which the post-gracial rise of sea-level pro- a 20-mile wide belt between Maroa and the duced a drowned valley system \vhich is Whakatane Graben. On the extreme east are now Tauranga Harbour, cut off frolll the sea the ranges reaching from Te Whaiti to the by large sand spits and bars. Make:u Basin sea at Whakatane. They reach altitudes up is occupied by swamp, and Whakatane to 3500 ft. but fall gradually towards the Graben has been filled by Holocene volcanic north. They are bounded by fault scarps of detrital material.'A remnant of the Kaharoa two intersecting systems - a north-south Plateau remains between RotolUa and set of dextral transcurrent faults, and a set Okataina, Rotorua Caldera being a basin of north-east faults of normal type, down- within the plateau. , thrown to the north-west and parallel to the Taupo Volcanic Zone. Both systems The Taupo Volcanic Zone contains varied form prominent high fault scarps along the structural and volcanic relief. Maroa Vol- eastern side of the Galatea and Waiohau canic Centre is a cluster of rhyolite domes basins. and flows, 10 miles in diameter; from which the slopes fall to north-west and south-east The Kaingaroa Plateau has a remarkably into Atiamuri and Reporoa valley!; respec- flat surface 1800 ft. above sea level at tively. Both valleys extend north-east for 15 Kaingaroa. To the north there is a 200 ft. miles at the feet of the flanking ignimbrite scarp around the erosional margin of the plateaux, and between them are a series of topmost ignimbrite sheet, and the remainder north-east ranges. These are faul t blocks of the plateau is more dissected and falls tilted away from Ngakura valley, which is gradually towards Te Teko, where it ends a central graben. against the lower Rangitaiki Plains. The youthful volcanic cone of Edgecumbe rises Farther north-east are the huge volcanic from the western side of the plateau, which - piles of Tarawera and Haroharo. 1 hese ar~ ...."" LEGEND < c: Alluvium, .sand-dun~s. swamps, t'" (') etc. -z> Alluvium, lake sediments, tuffs. -(') Rhyolitic lavas. >-i "' Pumice flows (non-welded). >z Andesitic and docitic lavas and " ignimbrites. < [!J Lake sediments and interbeded tn tuffs. ~ -o. Miscellaneous non~morine z, pumiceous sediments. Marine sandstones. Ignimbrites (welded types). Greywocke. -- - -,", FIGURE 1. Geological map of the Rotorua area HEALY: (;EOLOGY OF ~OTORUA DISTRICT 55 rhyolite lavas of the Okataina Volcanic a greater amount of pumice, metimes Centre, extruded on the floor of Haroharo completely recrystallized during t Ie cooling Caldera, within which Rotokakahi, Tikitapu, process, which weather to give t,~e rock a Okareka, Okataina, the eastern end of honey-combed appearance. The ldest one Rotoiti, Rotoehu and Rotoma are marginal is the Te Kopia Ignimbrite (1 , a dark lakes dammed by the lavas, and Tarawera glassy, crystal-rich type which orms the lake is a remaining part of the floor blocked base of the Paeroa Fault scarp. T e Paeroa off by coalescing flows from Tarawera and Ignimbrite (2), which forms t e highest Haroharo. Two miles downstream from the part of the Paeroa Range and the lock west outlet, the Tarawera River plunges over the of it, is a light grey quartz-rich type similar margin of one of the flows, and enters the to the Rangitaiki Ignimbrite bu( differing lower Tarawera Valley. in containing abundant biotite. 'J!he Rangi- taiki Ignimbrites form the bas~ of the North-east of Tarawera volcano the Taupo Kaingaroa Plateau and outcrop on both Zone narrows abruptly and enters the flanks. On top of the plateap.I are the Whakatane Graben. This is bounded by Kaingaroa Igni.mbrites (6), including an steep fault scarps south of Matata and at upper hard, fine-grained type th minor Awakeri. Seaward the graben probably plagioclase and no quartz, and a I wer dark enters White Island Trench (Fleming 1952). grey to black, glassy type contai 'ng lumps of white and grey-streaked pumi"".1 ROCKS OF THE DISTRICT The Waiotapu Ignimbrite (3) lis red to The oldest rocks, usually known as "grey- pink in colour, contains no quarjtz, and at wackes", are sandstones, argillites and con- Waiotapu contains abundant c<!>arse flat- glomerates, which form the ranges east of tened lenticles. On the Patetere Piateau and the Rangitaiki River, and two small ranges farther west it is generally pinkl and finer near Otamarakau. Jurassic fossils have been grained. The Matahina and Mam<l.ku ignim- found at Awakeri and east of Taneatua, brites (5) were spread in the spape of a though the rocks may locally be Lower huge circle centred on Rotorua qtldera and Cretaceous elsewhere. These are the base- Okataina Volcanic Centre, from 'iihich they ment rocks of the region. Beneath the were probably erupted. The UPPer parts of Kaingaroa Plateau they are believed on geo- these two ignimbrites are soft and pumice- physical evidence to lie above sea level ous and the lower parts dense arid welded, (Beck & Robertson 1955), but in the Taupo and both contain quartz. Volcanic Zone are faulted down to much greater depths (Modriniak & Studt 1959). Those described above are rpyolitic to dacitic in composition, but igni~brites of The most widely distributed rocks are the Whakamarama and Kaimail area are ignimbrites. These are believed to be the dacite and those of the Papamci>a Ranges products of large scale fissure eruptions, west of Te Puke are andesitic.1 They are probably from ring fractures (Healy 1962). older than the ignimbrites erupte<!l from the The material, of ash, lapilli and block Taupo Volcanic Zone. i grade, spread as pyroclastic pumice flows Some ignimbrites are extremely pumice- and settled by compaction after coming to ous and are not welded, though I they may rest hot enough to retain plasticity and to be well compacted, and some, eriIpted into become welded in the lower parts. One water, were interbedded with iake sedi- sheet, the Matahina Ignimbrite (5 on the ments. Rocks of this type form th~ southern eastern side), flowed at least 70 miles, and half of the Paeroa Range (oldest Iformation individual sheets had volumes which in of the Huka Group-Grindley 1959), and some cases exceeded' 50 cubic miles. older ones form the block we!!t of the The ignimbrites vary in colour from pink Ngakura Graben (Ohakuri Forination of and mauve to grey and even black. They Grindley). Still younger, loosely ~ompacted may contain abundant phenocrysts of pumice breccias formed by Ji>yroclastic plagioclase and quartz, or very few; and flows from the Maroa Volcanic <Centre (1) generally the upper and softer zones contain and from vents in and around th~ Okataina 56 VULCANICITY AND VEGETATION Volcanic Centre (2) are the youngest beds ignimbrites pass' up into 1 pumiceous tuffs of this type with any extent. These include over:' which Jies the Matahina Ignimbrite, the Waitahanui Breccias of Grange (1937) taken to be of Hawera age. Siltstones and and Hapacangi Rhyolite Pumice and Mihi sandstones with lignites ,and interbedded Breccia of Grindley (1959). pumiceous tuffs border the Tauranga Har- bour, and include the Tauranga beds of The final stage of the volcanism at the two Henderson and Bartrum (1913). Included centres was the eruption of rhyolite lavas also are sands, conglomerates and tuffs at in the forra of domes, and there are notable Maketu, Matata and Awakeri. They range in accumulations of rhyolite at Maroa and age from Castlecliffian' to Hawera. Younger Okataina. In general the rhyolites of the Hawera siltstones, sandstones and tuffs outer. rings west of both centres are older were laid down in lakes jin the Ngakuru, thall'thos(' within the centres, and are con~ Horohoro and Rotorua areas. The bulk of sidered to have been erupted from surround- the Holocene sediments is alluvial pumice.