Nelson Land District

GEOLOGY – LAND DISTRICTS OF NEW ZEALAND

by Geoffrey Conrad Shaw, B.SC., New Zealand Geological Survey, Lower Hutt and Graeme Roy Stevens, M.SC.(N.Z.), PH.D.(CANTAB.), Paleontologist, New Zealand Geological Survey, Lower Hutt.

North Auckland Land District

For millions of years North Auckland has been tectonically relatively stable and has been worn down to an area generally of low relief. Long-continued weathering on gentle slopes in a warm, moist climate has caused most of the rocks to decay deeply, with the result that the stratigraphy and structure of the region are difficult to decipher. The oldest rocks are complexly folded greywackes and argillites that were deposited in the New Zealand Geosyncline. Some are known to be Permian; others may be Triassic and Jurassic and contain small deposits of manganese ore which have been worked. These rocks, which are exposed only in eastern North Auckland, are broken into blocks by faults.

Other old rocks of North Auckland include basic volcanic and associated intrusive rocks that form the prominent steep hills of central and northern North Auckland, such as the Tangihua and Mangakahia Ranges. They may range in age from late Jurassic to Eocene. Small copper deposits occur at the contact of these rocks with the Cretaceous sedimentary rocks that surround them. At North Cape there is a large mass of ultramafic rocks, including a huge deposit of serpentine at Surville Cliffs that may be quarried on a large scale for use in fertiliser.

Soft, deeply weathered, Upper Cretaceous, Paleocene, and early Eocene sandstones and siliceous clay-stones make up most of the lowland areas of central and northern North Auckland. The details of their stratigraphy and structure are obscure. In many areas these, together with other Tertiary rocks and small masses of serpentine, are chaotically arranged and appear to have been emplaced as large slumps during the mid-Tertiary, when North Auckland experienced its latest period of diastrophism.

Mid and late Eocene and Oligocene strata form small areas of central and northern North Auckland. They include mid-Eocene coal measures, mined mainly from seams at Kamo, Kawakawa, and Hikurangi, and Oligocene limestones. These are of two types, an argillaceous (“hydraulic”) limestone which supplies Northland's cement industry, and a crystalline limestone used mainly for agricultural lime. Much of the southern half of North Auckland is made up of lower Miocene sandstones. These underlie Auckland City.

There has been little geophysical work done in Northland and it is not known how thick are the Cretaceous and Tertiary sediments of eastern Northland, nor what lies beneath them. Quaternary sands form the isthmus of the Ninety-Mile Beach peninsula, the long arms of the entrance to the Kaipara Harbour, and the south arm of Manakau Harbour.

Volcanic activity occurred intermittently in Northland during the Tertiary. Lower Miocene andesitic lavas and agglomerates form the Waitakere Ranges west of Auckland city, and similar rocks of Miocene age occur in eastern North Auckland, forming, for example, the jagged peaks of Manaia and Bream Head near Whangarei. There are small dacite plugs and eroded dacite volcanoes of Pliocene age east of Maungaturoto and near Whangarei.

During the late Pliocene and Quaternary only basalt was erupted in North Auckland. The older of these lavas forms the Tutamoe and Waipoua plateaux. Younger basalt flows, poured out in fissure eruptions in the Pleistocene, form the Kerikeri plateau adjoining the Bay of Islands. Some of these lavas have weathered to form bauxite ore. There are numerous younger flows and scoria cones in the Kaikohe, Kaeo, and Whangarei areas, and hot springs occur at Ngawha, near Kaikohe. At Auckland City basalt has been erupted in late Pleistocene and Holocene times from some 60 centres. Scoria cones, such as Mount Eden, Mount Wellington, and One Tree Hill, are prominent city landmarks, of which the youngest is Rangitoto Island, a lava cone some 850 ft high, probably less than 1,000 years old.

South Auckland Land District

For convenience of geological description the large South Auckland Land District may be considered as two regions: one composed of mainly sedimentary rocks on the western side; the other dominantly volcanic and comprising the Coromandel Peninsula, Kaimai Range, and plateau of the central volcanic area.

The Sedimentary Area

In the sedimentary area basement rocks of the New Zealand Geosyncline are exposed in many places or lie at fairly shallow depths. Towards the west, fossiliferous Triassic and Jurassic strata are warped into a major downfold — the Kawhia Syncline — which was developed during the Rangitata Orogeny (seediagram 11). Although partly hidden by younger rocks, this syncline can be traced from Port Waikato to the Awakino River and probably continues under the Tertiary rocks of Taranaki. The Triassic and Jurassic rocks of the syncline are largely fossiliferous and of the kind that were deposited on the continental shelf. They include the New Zealand type Jurassic section. The Triassic rocks (Balfour Series) comprise some 10,000 ft thickness of conglomerates, siltstones, and sandstones; the Lower Jurassic strata (Herangi Series) consist of marine siltstone and sandstone overlain by conglomerates which contain tree trunks and other plant fossils. The Herangi Series ranges from about 2,000–6,000 ft in thickness. The Middle and Upper Jurassic strata (Kawhia Series), which succeed it, total some 8,000 ft in thickness. The lowest beds are partly non-marine, with plant fossils; the upper are marine, with abundant fossils, including ammonites and belemnites. The southern shores of Kawhia Harbour afford a rich collecting ground for these and other Jurassic fossils. The uppermost Jurassic rocks (Oteke Series), totalling some 10,000 ft in thickness, comprise siltstones, sandstones, and conglomerates, with abundant plant remains in the upper part. These young Jurassic strata are best exposed at Port Waikato, where they contain excellent plant fossils and are in places crowded with belemnites.

Far to the east of the Kawhia Syncline poorly fossiliferous rocks, of a kind that were deposited in the deep water of the geosyncline, project in numerous outcrops between Tongariro National Park and the Firth of Thames, some being encircled by ignimbrite erupted from the volcanic region. Among the largest areas of these rocks are the Hauhungaroa and Rangitoto Ranges (diagram 11). The Waikato basin (Hamilton lowlands) is almost enclosed by greywacke hills, which the Waikato River cuts through at Karapiro hydro-electric station and in the Taupiri Gorge. The high Hakarimata and Taupiri greywacke ranges are continued to the north by lower Triassic and Jurassic greywacke ranges that separate the lowlands of Huntly and Mercer from the Hauraki Plains. They are succeeded by the greywackes of the Hunua Hills (some 2,000 ft high) which lie west of the Firth of Thames.

Thick Tertiary sediments may lie below Pleistocene alluvium in major structural depressions, such as the Hamilton lowlands and Hauraki Plains, that were formed by the movements of the Kaikoura orogeny. The alluvium of the Hamilton lowlands (seediagram 10) includes pumice terraces built in the late Pleistocene and Holocene when the Waikato River brought pumice from eruptions in the central volcanic region. The youngest terrace was formed after the Taupo eruptions of approximately 1,800 years ago. Fine pumice sand is quarried in many pits at Horotiu near Hamilton.

Erosion has removed the cover of Tertiary strata from much of the region; the deposits that survive, mainly in faulted depressions, in few areas average more than a thousand feet in thickness. In the northern part of the region upper Eocene coal measures, resting on deeply leached greywacke, form the base of the Tertiary succession. A few mines work a small area south-west of Ngaruawahia, but the largest mines and opencast pits are in patches of coal measures within an area of some 40 sq. miles near Huntly, and at Kimihia and Kopuku north-east of Huntly (seediagram 10). Where coal measures are absent in the northern part of the region Oligocene marine strata are the oldest Tertiary rocks; they are mainly sandstone and prominent limestone. In the south-west part of the land district, Eocene coal measures are absent and thick Oligocene limestone (in which occur many sinkholes and caves, including those at Waitomo) is the dominant formation. Succeeding this to the south are thick layers of alternating Miocene sandstone and mudstone.

Mention has already been made of the prominent extinct basaltic volcanoes of south-west Auckland. Two large forest-covered eroded volcanoes, Mount Karioi (2,840 ft) at Raglan Harbour, and Mount Pirongia (3,156 ft) are the largest in a chain of cones that extends in a straight line for approximately 35 miles. South of Auckland city there are a number of Pleistocene basalt volcanoes, of which one of the largest and best preserved is Pukekohe Hill, a gently sloping, basalt lava cone.

A major fault marked by a high and greatly eroded scarp separates the Firth of Thames and the Hauraki Plains from the Coromandel Peninsula and its continuation as the Kaimai Range: Miocene andesites and Pliocene rhyolites and ignimbrites make up most of this region. The Jurassic greywackes that form the foundation of the range are exposed only in the north, for example in the Moehau Peninsula.

Central Volcanic Area

The Volcanic Plateau which forms the eastern part of this land district is an irregular, elevated area of some 10,000 sq. miles of dominantly acid volcanic rocks. Here the Taupo Volcanic Zone, a narrow, north-east-trending axial belt within the plateau, contains active volcanoes, geysers, and boiling springs, making the region one of New Zealand's principal tourist and holiday centres. Except at the southern extremity of the Kaimai Range, no sharp structural boundary separates the western part of the volcanic plateau from the remainder of the land district; the margin is highly irregular where erosion has nibbled at the edge of ignimbrite sheets that flooded out from the interior of the plateau in Pliocene and Pleistocene times. To the east and south-east, however, the plateau ends abruptly at the foot of mountain ranges which are in a large part fault bounded.

Although an elevated region, it is by no means a simple plateau, but rather the warped, broken, and partly dissected remnants of a plateau coated with thick deposits of pumice ash. These remnants (for example, the Kaingaroa, Mamaku, and West Taupo plateaus) owe their flat surfaces to the presence of hard ignimbrite as horizontal or gently tilted sheets (seediagram 11). Ignimbrite is a broad term covering a range of acid volcanic rocks, some hard, some soft, that were poured out in huge eruptions in the Pliocene and Pleistocene to form extensive thick layers, some of which may contain tens of cubic miles of this rock. It is not clearly understood in what form ignimbrites were erupted — they may have come out as greatly foamed-up lava, or as white-hot particles and lumps carried along in “glowing avalanche” eruptions.

Gravity measurements in recent years have revealed much about the general geological structure of the Volcanic Plateau. Greywacke basement rocks are thought to lie below the volcanic rocks which are only a few hundreds of feet thick in the marginal parts of the plateau, but they are enormously thick (as much as 2 miles) in a belt of structural depressions that extends from Tongariro National Park to the Bay of Plenty. These depressions, filled with ignimbrite, pumice, lava, and old lake sediments, have been formed after the expulsion of huge quantities of molten rock. Large blocks of the outer part of the crust have foundered along faults, and volcanic debris has accumulated.

The Taupo Volcanic Zone, to which is confined the most active faulting and volcanic activity, extends for some 120 miles from Ruapehu to White Island. North-east-trending faults bound the Taupo zone in many places, and many others can be followed within the zone: many of the hot springs, rhyolite domes, and craters lie on or close to the faults. Ruapehu (9,175 ft), Ngauruhoe (7,515 ft), and Tongariro (6,517 ft) are large, active, andesite volcanoes at the southern end of the zone. Pihanga (4,352 ft) and Kakaramea (4,266 ft) are the largest of a group of extinct forest-covered volcanoes of basaltic and basaltic-andesitic composition that lie between Tongariro and Lake Taupo.

Lake Taupo occupies a huge depression formed partly by faulting, partly by eruptions. Thick pumice deposits surround the lake, and radiocarbon dating of charcoal fragments shows that there has been a succession of pumice eruptions from the eastern side of the lake during the past 10,000 years. Some of these eruptions were small bursts, others gigantic explosions that spread pumice over thousands of square miles. The latest and perhaps grandest occurred approximately 1,800 years ago and showered pumice as far as Gisborne. A prominent landmark near Taupo township is Mount Tauhara (3,566 ft), an extinct dacite volcano that stands boldly above the ignimbrite plateau.

The Taupo Volcanic Zone is broadest in its middle section, between the northern end of Lake Taupo and the Rotorua lakes area. Here is the greatest concentration of hot springs and the most diversified picturesque scenery of the volcanic plateau. Domes and flows of glassy rhyolite lava are clustered at each end of this section in two “volcanic centres” – the Maroa centre to the south, the Okataina centre to the north. Between these centres the zone has been arched and then broken by parallel faults into a series of tilted blocks and a central graben. A striking fault scarp, the Paeroa scarp, marks one of these faults.

The Maroa centre, north of Lake Taupo, is a group some 10 miles across of rhyolite domes and flows, erupted along faults. Perlitic rock is quarried from some of these lavas and processed locally to make “expanded perlite”. The Okataina centre is a cluster about 15 miles across of domes and flows, among which are picturesque lakes such as Rotoiti, Rotoehu, Okataina, and Tarawera. The largest of these rhyolite domes are Haroharo and Tarawera. The latter is an elongated group of three steep-sided, rhyolite domes that have grown above the remnants of an older dome. The great Tarawera eruption of 10 June 1886 threw out basalt from a chain of new craters along the mountain's summit, and a further 6 miles of craters were blown open at the foot of the mountain and along the Waimangu Valley. Lake Rotomahana occupies the largest of these craters. Lake Rotorua occupies a sunken caldera further to the west; lake beaches cut in the surrounding hills show that the surface was once some 300 ft higher. Ash beds erupted from the Okataina centre mantled the landscape to a depth of many feet, the youngest of these being the Kaharoa shower, which may have been blown out as little as 800 years ago, probably from the site of Mount Tarawera.

Beyond the Rotorua lakes the Taupo zone becomes narrower and enters the sediment-filled Whakatane graben (fault trench) which is continued as the submarine White Island trench. Whale Island is an extinct, eroded andesite cone about 5 miles from Whakatane; White Island, some 30 miles to sea, is an active andesite volcano.

Taranaki Land District

Of the 12 land districts of New Zealand, none has a simpler surface geology than Taranaki, the smallest. At the Awakino River, the Mesozoic strata of the Kawhia Syncline (diagram 11) disappear beneath Tertiary marine beds which extend southwards to form wide lowlands pierced in the west by a line of volcanic vents along which has grown a chain of andesite volcanoes culminating in the giant cone of Mount Egmont (8,260 ft), with its secondary cone, Fantham Peak, on its eastern flank. These Tertiary rocks, eroded to form plains, underlie Taranaki's farm land, and successive eruptions from the volcanoes have mantled them with thick deposits of fertile ash which forms the basis of its agricultural productivity.

Tertiary sedimentation began in Taranaki in the early Eocene with deposition of coal measures followed by limestone and other relatively thin sediments in the Oligocene. During early Miocene times the Taranaki area became a broad, rapidly sinking basin in which accumulated more than 15,000 ft of marine sedimentary rocks. Some of these rocks are shown to the left of section B in diagram 5.

Very late in New Zealand's geological history, these rocks began to be raised above the sea, first in the north, then successively southwards. As a result, the older beds are progressively more deeply buried towards the south, and younger beds are encountered southwards at the surface. The clearest exposures of this fairly continuous sequence of southward-dipping strata are seen south of the Awakino River estuary down the length of the western coast of Taranaki. The oldest strata encountered are Lower Miocene (Pareora Series) alternating beds of sandstone and mudstone. Further south is massive sandstone, followed by coal measures which contain seams that are worked in the Ohura Valley and the Mokau area; these are overlain by mudstones. Much andesite tuff occurs in the succeeding strata. Thousands of feet thickness of upper Miocene sandstones and mudstone are well exposed along the coastal section north of Waitara.

Oil seeps have long been known on the Taranaki coast at New Plymouth, since the early settlers first found petroleum floating on the sea near the volcanic Sugar Loaf Islands. The first drilling rigs were erected in 1865 and for many years a limited supply of petroleum has been won from pumps on the beach near New Plymouth. Many small companies investigated the region for oil, but large-scale operations did not begin until 1939, just prior to the outbreak of the Second World War, when deep exploratory boreholes were put down at Inglewood and Devon. Since 1959 a number of boreholes, reaching depths of as much as 13,000 ft at Kapuni, south of Mount Egmont, have revealed the presence of a reservoir of natural gas below the thick cover of Tertiary marine strata. Examination of fossil pollen grains from samples of this coal shows that the vegetation of these Taranaki coal measures was very similar to and contemporary with the coals of the Waikato.

The volcanic centres of Taranaki are of two ages. The northern and eastern group, comprising the Sugar Loaves, Whareorino, and Pehimatea, are of early Pleistocene age and dacitic composition. The central Taranaki volcanoes, andesitic in composition, are three vents aligned along a single north-west — south-east line, all of upper Pleistocene and Holocene age; Kaitake being the oldest, then Pouakai, followed by Egmont. That Egmont was last active less than 360 years ago is shown by radiocarbon dating of charcoal from a Maori oven covered by the latest volcanic ash from Egmont.

Thick deposits of unsorted masses of volcanic debris which surround these volcanoes, forming the Taranaki Ring Plain, are interpreted as volcanic mudflow deposits (lahars) that swept down the mountain slopes during Pleistocene times. The surfaces of some of the upper lahars are marked by clusters of hillocks composed of huge blocks; these lahar mounds are particularly numerous in the Inglewood and Opunake areas.

Wellington Land District

The Wellington Land District comprises the Rimutaka-Tararua Range (south of the Manawatu Gorge) and the sedimentary basins on either side (Palmerston North — Wanganui and Eketahuna-Wairarapa), but it extends to the west of the range north of the Manawatu Gorge to include the Rangitikei area and Tongariro National Park.

The Rimutaka-Tararua Range is composed of greywacke, which is thought to be mainly of Triassic age. The Upper Triassic shell Monotis richmondiana is known from the headwaters of the Otaki River. The entire Wellington Peninsula is also composed of similar greywacke and is evidently of about the same age, as worm tubes and ichthyosaur vertebrae thought to be of Triassic age have been found around the Wellington coast. The Triassic greywacke extends north from the Manawatu Gorge forming the Ruahine and Kaweka Ranges of Hawke's Bay Land District, but in the northern part of Wellington Land District the Kaimanawa Range projects as a fault-bounded block westwards towards Tongariro National Park. The Kaimanawa greywacke is thought to be older than that of the main range and to include the Permian as well as part of the Triassic.

The Rimutaka-Tararua-Ruahine Ranges have been pushed up along major faults (see section B of diagram 5) and so have well defined boundaries (seediagram 2). Sedimentary basins, floored by greywacke, flank the ranges to east and west, and throughout Tertiary time have received the products of erosion of the ranges — first the eastern basin and then the western basin. Filling of the eastern basin began in the Cretaceous and was completed by the Miocene; its contents have now been folded and faulted. Filling of the western basin began in the Miocene and is still continuing, with many large river systems (for example, Wanganui, Rangitikei, Manawatu) draining the ranges and actively extending the coastline seawards with their loads of mud, sand, and shingle.

The Kaikoura Orogeny in the Pliocene and Pleistocene affected both basins, but the sediments filling the eastern basin became greatly crumpled whereas those of the western were broken into blocks by faults. Therefore, as may be seen from section B of diagram 5, the sediments of the western basin are fiat lying or gently dipping, though they have been dislocated by faults to form “highs” and “lows”. In the past some of the “highs” have attracted the attention of oil companies as indicating likely oil traps; an exploratory bore at Mount Stewart, near Feilding, was drilled to basement greywacke, but no oil traces were found.

As is also shown in section B of diagram 5, the sediments filling the eastern basin have been distorted by folding as well as faulting. In many places the older Cretaceous rocks, hard argillites and sandstones, have been broken into a series of slivers which, when exposed at the surface, form prominent serrated dark-coloured hills, contrasting with the low-lying surrounding lighter coloured Tertiary rocks. These hills are features of the Wairarapa and Dannevirke areas and have been locally called taipos (“devils”). To the south of the Wairarapa, in the Cape Palliser area, the basement Triassic-Jurassic greywacke is exposed at the surface, as well as the overlying Lower Cretaceous sandstones and argillites.

Uplift of the sedimentary rocks filling both eastern and western basins has exposed many fine sequences of marine rocks, all richly fossiliferous. The coastal section extending from Wanganui to Waitotara is the type section for the Wanganui Series (Pliocene-mid Pleistocene). Progressive uplift of the Wanganui-Rangitikei area has forced major rivers, such as the Wanganui and Rangitikei, to incise deeply and so form spectacular gorges.

Faulting of the Wellington Land District in the Pliocene and Pleistocene has produced a series of north-east — south-west trending faults, cutting both basement greywacke and young sedimentary rocks alike. Section B of diagram 5 shows some of these faults in simplified form; many are active at the present time and have been the foci of frequent earth-quakes. These active faults are shown in diagram 8.

Several fault zones cut the Wanganui coast and, as may be seen from diagram 8, are the south-western continuation of the Rotorua-Taupo fault zone with its volcanoes and hot springs.

The Wellington Peninsula is traversed by a number of faults, all recently active, which are conspicuous features of the landscape. As seen in diagram 8, the Wellington faults are continuations of South Island faults. The Wairau Fault probably continues off shore along the west Wellington coast, in part accounting for the steepness of the present coastline, and in all likelihood passes between Kapiti Island and the mainland. The Owhariu and Wellington Faults, both continuations of the Awatere Fault, are well expressed in the topography, especially the latter, which for most of its length in Wellington City and the Hutt Valley is marked by a prominent scarp that forms one side of Wellington Harbour. The Wellington Fault continues through the Tararua Range, where its course is marked by scarps, saddles, and valleys, and it forms the eastern boundary of the range from the Mangatainoka River north to the Manawatu Gorge and beyond. Buckling along the southern end of the Wellington Fault has formed a series of basins of which that occupied by Wellington Harbour and the Lower Hutt Valley is the largest.

The Wairarapa Valley is bounded by two major faults, the East Wairarapa, delimiting the coastal Aorangi Mountains, and the West Wairarapa, delimiting the Rimutaka and Tararua Ranges. Both faults are continuations of faults in the Inland and Seaward Kaikoura Ranges (seediagram 8).

The West Wairarapa Fault was last active in 1855, when it moved vertically 10–12 ft and tilted the land to the west as far as the western Wellington coast, causing an uplift of 7 ft in eastern Wellington Harbour and the Hutt Valley and 5 ft in Wellington itself. Movement on the East Wairarapa Fault in 1942 caused severe damage in the Masterton area and many small scarps were formed along the fault line. Both Wairarapa faults continue north-eastwards into Hawke's Bay as a broad fault zone, containing innumerable small faults (seediagram 8).

During the Pleistocene glacial episodes, much of the Wellington Land District, especially the ranges and the Wellington Peninsula, was subjected to a frost climate with freeze and thaw as active erosion agents. Valley glaciers formed in the high parts of the Tararua Range and the entire landscape was smoothed by frost action, with the production of much coarse angular debris which accumulated in the lowlands and mantled many of the higher slopes. With the fall in sea level during the Pleistocene (as water was abstracted from the seas to form ice sheets on land), much of the near-shore part of the sea floor was exposed around the Wellington coastline. This allowed wind to erode the fine sediment of the old sea floor which in some areas was blown inland to form thick deposits of loess mantling the topography.

In the Pliocene marine straits connected the eastern and western sedimentary basins, crossing sags in the rising ranges. One such strait was formed on the site of the present Manawatu Gorge. During the Pleistocene glacial periods the sea withdrew from the Manawatu Strait, which may have been temporarily bridged by gravel deposits derived from erosion of the ranges, but the sea probably returned during some of the earlier interglacials. When the strait was not occupied by the sea the Manawatu River flowed through it from east to west. With the gradual uplift of the ranges throughout the Pleistocene, and the permanent exclusion of the sea, the Manawatu River was forced to cut a deep gorge across the rising country in order to maintain its course. The Wellington Peninsula was probably joined to the South Island in the Pliocene (seediagram 7). During the Pleistocene, however, this land bridge was progressively eroded and lowered so that when sea level rose in post-glacial times an open seaway, the present-day Cook Strait, was formed.

Spilites (altered submarine basalt flows), with their associated red shales and cherts, occur in the Wellington greywackes, though they are of scattered occurrence and do not attain a great thickness in any one locality. Igneous intrusions, probably of early Tertiary age, are found in the Brocken Range — Ngahape area, Wairarapa.

The volcanics of Tongariro National Park are included in Wellington Land District. These consist of five major andesitic volcanoes — Ruapehu, Tongariro, Ngauruhoe, Kakaramea, and Pihanga — together with several associated minor volcanoes and vents. Tongariro volcano is the oldest and dates from the lower Pleistocene. The volcano built itself to some considerable height, but in the middle Pleistocene it exploded violently to form an enormous crater on the rim of which several smaller cones were built, including the active Ngauruhoe on the south rim. Kakaramea volcano, immediately south of Tokaanu, is slightly younger than Tongariro, but became extinct earlier, and Pihanga volcano, immediately south of Turangi, had a similar short-lived history. The Ruapehu volcano was initiated in the late Pleistocene and was rapidly built to a height similar to that of Tongariro prior to its explosive phase, but it also has lost some of its height through explosions. Ruapehu is still active. Extensive ring plains, built up by mud-flows (lahars) down the flanks of the volcanoes, surround the main vents. They can be traced for some distance (for example, to Waiouru, Ohakune, etc.) and are responsible for the barren country traversed by the main highway (Desert Road) north of Waiouru. A lahar derived from Ruapehu swept down the Wangaehu River and caused the Tangiwai rail disaster of 1953.

The volcanic centres of Tongariro National Park Park are aligned along a major crustal weakness marked by faulting, volcanic activity, hot springs, etc. As may be seen from diagram 2 and diagram 8, this weakness continues north-eastwards through the Rotorua-Taupo area and into the south-west Pacific via White Island, Kermadec Islands, Tonga, and Samoa.

Hawke's Bay Land District

This land district separates readily into three regions of geological structure; the main forms of the eastern Wellington district are continued into Hawke's Bay. The western boundary is formed by high, rugged mountain ranges running south-west — north-east and composed of Triassic-Jurassic greywacke. The east is characterised by broken, hilly country formed from Cretaceous and Tertiary sediments. Here complex earth movements have upthrust small ridges or tilted blocks, exposing anticlinal cores of late Cretaceous rocks. The third region, which is a continuation of the wide Wairarapa trough from Woodville through the Takapau Plains to Mohaka and Wairoa on the shore of Hawke Bay, is filled with thick Upper Tertiary and Pleistocene sediments. The land district lies within the zone of active crustal instability.

The massive greywacke ranges of the first region cover less than a quarter of the Hawke's Bay Land District. The greywackes are here largely unfossiliferous but are probably of Triassic and Jurassic age. They were probably first pushed above the sea in the Rangitata Orogeny (late Jurassic and early Cretaceous) but the high country they then formed most likely had only a temporary existence, being periodically reduced by erosion and reconstituted by earth movements throughout Cretaceous and Tertiary times (seediagram 7). The Kaikoura Orogeny of the Pliocene and early Pleistocene gave the Ruahine Range its present development. The Kuripapango Trough, which is a sag in the range to the west of Napier, was probably formed in much the same way and at the same time as the Manawatu Gorge.

The range as a whole has been pushed up along major faults to the east and west (seediagram 5). The active Wellington Fault, continuing from further south, lies along the eastern edge of the range, while numerous other small faults, also active, are present. Earthquake tremors are frequent in this area and surface movements have occurred on many of the faults.

Greywacke similar to that of the Ruahine Range probably underlies most of the area to the east of the range, as is shown in diagram 5. Tilting movements along faults have exposed this basement greywacke in a number of places (for example, Waewaepa and Wakarara Ranges) (seediagram 12), but here it is only moderately folded and faulted and is possibly younger than the Ruahine greywacke. Further east again (for instance, in the Otane Range) (seediagram 12), the basement greywacke is not itself exposed, and the oldest rocks at the surface are Lower Cretaceous fossiliferous sandstones with local patches of grits and conglomerates, deposited in basins developed after the Rangitata Orogeny.

Spilites, with their associated red shales and jaspers, occur widely in the Hawke's Bay greywackes, and at Maharahara they contain small quantities of copper, which was worked for a short time.

The coastal series of low ranges and hilly country forming the second region run parallel to the Ruahine axis, from Mahia to Palliser Bay in the Wairarapa, bordering and projecting into the Wairarapa — Hawke Bay — Wairoa trough. Structurally the much folded and faulted late Cretaceous and Tertiary sediments of the coastal zone form a complex fold system trending north-east — south-west (diagram 12). The Akitio Syncline lies inland, running parallel to the southern Hawke's Bay coastline and ending at Hastings. Along the seaward side are complexly faulted, tilted, and folded strata, mainly of massive mudstones which form a number of parallel anticlines and synclines. The striking hogback of the Silver Range north of Elsthorpe is composed of alternating sandstones and mudstones of Miocene age. Kahuranaki is a complexly faulted mass of upper Pliocene cemented shelly limestone, and the well known Te Mata Peak is part of a prominent dip slope in the same limestone on the west flank of the Elsthorpe Anticline. The limestone forms prominent dip slopes throughout central Hawke's Bay and is often quarried for agricultural use; very occasionally some layers are pure and are used in glass manufacture (for example, Pakipaki, Waipawa Gorge, etc.). Fossils are abundant in most of the Tertiary rocks of the district; the Dannevirke Series is based on the sequences exposed near Te Uri and Waipawa. The late Cretaceous rocks do not contain many fossils, but New Zealand's largest ammonite (2 ft in diameter) was found near Porangahau.

The complex landscape of the region is thought to have resulted mainly from earth movements during the Kaikoura Orogeny, which have continued to the present day. Hence the characteristic broken ridges and non-conformity of sediments laid down during this period — shearing and faulting forming deep depressions which became filled with thousands of feet of rapidly deposited sandstones and conglomerates, grits, and claystones. Those of the Dannevirke Series are extremely fine grained and form bentonitic clays which have marked swelling properties due to the mineral montmorillonite and outcrop throughout the area where the Tertiary-Cretaceous boundary is exposed. Bentonite is quarried from coastal deposits south of Porangahau and is used as a drilling mud and as a binder, filler, and plasticiser in many industries, notably ceramics. It is also valuable as an absorbant, emulsifier, chemical water softener, and suspension medium. Due to its peculiar rheological properties, it is responsible for much local slipping in Hawke's Bay, and it has been suggested that much of the overthrusting of Upper Tertiary beds was made possible by the bentonite “greasing” the underlying Cretaceous rocks. Other sediments with economic value include some of the Tertiary sandstones, which have been found useful as moulding sands and are used in small foundries in Napier and Dannevirke. Oil companies have surveyed much of Hawke's Bay area, but so far without promising results, mainly because of the complex, small-scale faulting and folding. Pliocene lignites have been found in the Dannevirke area, but are of very poor quality and too thin to work.

The central lowlands lie between the two preceding regions. Structurally they consist of two shallow basins (the Ruataniwha Depression and the Heretaunga Plains) where the underlying Tertiary and lower Pleistocene are covered with younger alluvium which is still being deposited in the broad flood plains of the rivers. Ignimbrite and pumice pebbles abound in the upper Pleistocene and Holocene gravels of the lowlands, and ash layers are abundant in some of the Wanganui Series sediments, especially those of the middle Pleistocene (for instance, near the base of the Clifton — Cape Kidnappers coastal section).

The Ruataniwha Depression (which includes the Takapau Plains) is filled with Pleistocene and Holocene terrace deposits and is crossed by the river system of the Tukituki and its tributary, the Waipawa. Most of the present topography results from tilting and river terracing under frost-climate conditions during the Pleistocene glaciations. Towards the foot of the Ruahines the surface is tilted and terrace systems may be seen in the harder calcareous silt-stones and conglomerates of the earlier Pleistocene.

The Heretaunga Plains is a strongly alluviated area, as it lies in the lower reaches of three large rivers (Ngaruroro, Tukituki, Tutaekuri). Fossiliferous marine Pliocene rocks form the margin of the plains and these rocks also outcrop at Scinde Island, Napier city.

That Hawke Bay is probably a structural feature is evident from the inward dip of the surrounding rocks, especially the complex of anticlines and synclines of the coastal highlands which plunge into the depression from the south, apparently to reappear on the northern side.

The enormous quantities of rock carried down stream by the rivers into the lowland areas have left many thick beds of conglomerates and greywacke pebbles which are suitable for road metal. The Heretaunga Plains form a very important ground-water system, containing sufficient water to irrigate the entire fertile area of the plains and provide for ancillary industries. Drilling has shown that the water-bearing beds are thick and large reserves have been proven.

The Wairarapa Fault Zone continues into this region of Hawke's Bay District and many active faults are known (seediagram 8). Earthquakes of considerable intensity are frequent in the region and the Napier earthquake of 1931, which devastated the city of Napier, raised the sea bed at Ahuriri 7–9 ft to form dry land, now used by industry and housing. The Pahiatua earthquake of 1934 occurred in the same fault zone and the Wairoa earthquake of 1932 in the continuation of the zone on the northern side of Hawke Bay.

Gisborne Land District

The western boundary of this land district lies along a continuation of the south-west–north-east-trending North Island mountain chain, composed mainly of hard Jurassic and Cretaceous greywacke, argillites, siltstones, and sandstones. To the north in the East Cape area lies the volcanic region of Matakaoa, a complex mass thought to have been erupted from local volcanoes in Jurassic-Cretaceous times. The central and eastern part of the district is composed of Tertiary sediments forming a north-eastern continuation of the Hawke Bay–Wairoa trough.

The greywacke ranges form the main divide between the Bay of Plenty and the East Coast. The Raukumara Range reaches over 5,000 ft and the Huiarau over 4,000 ft, forming a series of massive ridges, densely forested and deeply dissected by precipitous gorges. The area contains some of the most inaccessible country in the North Island.

The greywacke ranges of the Gisborne district contain rather more fossils than those of the Wellington and Hawke's Bay districts and these give a better indication of age. Lower Jurassic fossils are found in the greywacke of the Ikawhenua Range near Taneatua and Upper Jurassic fossils from that of the Raungaehe Range near Awakeri; it is therefore likely that the greywacke on the western boundary of the Gisborne district is dominantly of Jurassic age. Toward the east Lower Cretaceous fossils occur in greywacke in the upper Waimana Valley and it is likely that this Lower Cretaceous greywacke extends still further eastwards to form the rugged hills of the Urewera country, immediately inland from the Bay of Plenty coast east of Opotiki. The Raukumara Range, forming the central axis of the district, is composed mainly of sparsely fossiliferous argillites, sandstones, and conglomerates of Cretaceous age which have been folded and faulted to form a series of prominent domes. In many of the deeply cut river valleys of the area, for example, the Motu and its tributary the Mangaotane, the rocks of these domes have been exposed; these have been mapped and used as the basis for the Taitai, Clarence, and Raukumara Series (Lower and Upper Cretaceous). All strata contain the fossil clam Inoceramus in varying amount, including some giant specimens up to 1½ ft long.

Taitai Series rocks are well exposed in the Tapuwaeroa Valley (where they extend to within 10 miles of the east coast) and in the centre of domes in the Te Puia area. They are of two types: a compacted sandstone and siltstone (Mokoiwi beds) and a hard dark sandstone, with pockets of conglomerate containing well rounded pebbles of quartz porphyry, feldspar, and greywacke (Koranga beds). The latter are very resistant to erosion and form some of the most striking peaks in the Raukumara Range, including Mount Hikurangi (5,753 ft) the highest non-volcanic mountain in the North Island, Mount Aorangi (4,091 ft) and Mount Taitai (2,012 ft), whose rugged tops tower precipitously over the surrounding ridge country.

Clarence and Raukumara Series rocks occur widely and form high-level hilly country in the Motu and Mata areas. Siltstones and mudstones are the main rock types, but hard sandstone beds may occur. Carbonaceous sandstones and mudstones, greensands, and bentonitic clays of the Mata Series are exposed in the Mata and Tapuwaeroa Valleys.

The Matakaoa volcanics take the form of a plateau broken by the east-west fault-bounded Wharekahika trench, which is filled with Tertiary sediments. The plateau stands at about 1,000 ft and extends from Matakaoa Point to Cape Runaway. The volcanics are mainly basaltic and extensive deposits of pillow lavas (submarine basalt flows) provide evidence of submarine eruptions. The rocks appear to correspond closely to the Tangihua Volcanics of Northland and are of about the same age, as Upper Jurassic or Lower Cretaceous fossils are found in a small body of limestone in the basalts near Cape Runaway. A smaller area of the same volcanics occurs south of the sedimentary trench, in the Hicks Bay–Te Araroa area.

The Matakaoa area was probably a volcanic island in the New Zealand Geosyncline during the Rangitata Orogeny but had a varied history throughout the Tertiary (seediagram 7) and was the source of the volcanic pebbles found in conglomerates at various levels in the Gisborne Cretaceous and Tertiary sediments. In the Pliocene the sea invaded the Wharekahika trench to form a strait between the main land mass and Matakaoa. The Matakaoa area, along with the main ranges of the North and South Islands, was uplifted, folded, and faulted during the Kaikoura Orogeny.

The eastern part of this land district is occupied by a vast thickness of Tertiary sediments filling a continuation of the Hawke Bay–Wairoa trough. The Miocene and Pliocene beds attain a thickness of nearly 4 miles south of Gisborne (see section A of diagram 5) and somewhat less to the north. The sediments have in general been less strongly deformed than in the geologically similar regions to the south of Hawke's Bay. Two main basins may be recognised, the Wairoa and Tutamoe basins, though both are complicated by faulting. They are bordered along the coast by a complicated system of depressions separated by steeply pinched anticlinal zones (for example, at Waitangi and Whangara), where Miocene mudstones and sandstones are broken through by cores of hard Cretaceous rocks to form high jagged peaks similar to the Taipos seen in the Wairarapa-Dannevirke areas.

The Wairoa basin shows a succession of gentle inward-dipping folds in the Tertiary sandstones and mudstones. The western boundary is formed by the fault-bounded greywacke of the Huiarau Range and the eastern by the Mahia Peninsula, where basement rocks are known to approach the surface from geophysical surveys (see section A of diagram 5). Lake Waikaremoana (elevation 1,970 ft) lies on the western flank of the basin and is drained towards Hawke Bay by the Waikare Taheke River. The lake appears to have been formed in Holocene times by landslides damming a gorge cut by the river through the Miocene sandstone of the Ngamoko and Panekiri Ranges immediately to the south. The sandstone rests on soft clays whose erosion and subsequent undercutting have produced the landslides which form a natural rock-fill dam some 1,200 ft high extending 3 miles down the valley from the lake outlet. The lake is the reservoir for three power stations in the Waikare Taheke Valley.

The Tutamoe basin flanks the Raukumara Range and is a shallow saucer-shaped structure with slightly folded Tertiary strata dipping into it, the central resistant Miocene sandstone with conglomerate beds forming a tableland which separates the headwaters of the Waipaoa from those of the Waiapu. These conglomerates are thought to be as much as 7,000 ft thick in places.

The coastline shows a complicated series of structural depressions which may be drowned or filled with fine recent deposits of alluvium layered with thick ash and pumice bands (for example, the Gisborne-Matawhero depression) derived from eruptions in the Taupo-Rotorua area. Four main ash showers have been traced, these may be up to 20 ft in thickness (for example, Te Arai Valley), or they may form prominent escarpments as in the Waimata Valley (Gisborne) where the ash consists of coarse gritty tuffs containing much mica and quartz. All the rocks exposed towards the south are comparatively soft; they show raised beaches where there has been local uplift.

The Cretaceous and Eocene beds of the Gisborne Land District have been faulted and folded along north-west–south-east axes. These lie at a marked angle to the folds and faults in the younger Tertiary sediments, which lie north-east–south-west, parallel to the east coast. Two periods of folding have been suggested–the first trending north-west and the second north-east.

The Wellington Fault system of active faults can be traced to just south-west of Lake Waikaremoana (seediagram 8) but other active faults continue the same trend north-east and traverse the greywacke between the lake and the Bay of Plenty. Many other faults, not active at present but also trending north-east, have been mapped north and east of Lake Waikaremoana.

The Cretaceous and Tertiary rocks inland of the east coast have widespread occurrences of natural gas vents (as at Te Puia), oil seepages, black shales smelling strongly of oil, and carbonaceous mud pools (covering over 10 acres at Hylands, Waimata). The search for commercial quantities of oil has continued in the district since last century, when the first drillings were made in 1874 at Waitangi Hill, 24 miles north of Gisborne. Recently domes have been drilled in the Gisborne area at Mangaone (to 5,085 ft) and Ruakituri (to 9,005 ft), but the only findings were a little gas and salt water. Difficulty was experienced in penetrating the thick sequences of bentonitic clays. The repeated loss of drilling gear and caving of boreholes finally led to the abandonment of drilling activities.

The prevalence of bentonite in this region and, to a lesser extent, of the soft papa clays, is the cause of frequent slips of surface strata after rain. Characteristic landforms of the district are the sharp “taipos” of harder rocks flanked or interrupted by “slumped” country, where landslides which may involve hundreds of acres have occurred. This causes much engineering difficulty with road and rail foundations and is responsible for the acute erosion problems in many localities now cleared for agriculture.

Nelson Land District

Nelson comprises rocks more varied in type than in any other New Zealand land district: in age they represent every geological period since the Cambrian, with the possible exceptions of the Silurian and the Carboniferous. Nelson's rocks include a great diversity of minerals of scientific and possibly, of some economic importance. In Nelson, as in Marlborough, the active Alpine Fault and its probable north-eastern continuation, the Whangamoa Fault (seediagram 8) separates two areas of dissimilar geology. To the east of the fault, for example in the Spenser Mountains, St. Arnaud Range, and Richmond and Bryant Ranges, Upper Paleozoic and Lower Mesozic greywackes and argillites are the predominant rocks exposed. The main part of Nelson lying north-west of the fault is made up chiefly of Paleozoic granites, sediments, and metamorphic rocks, partly obscured by patches of Cretaceous and Tertiary covering strata.

Most of Nelson is mountainous. The largest area of plains and low hill country is the Moutere Depression, which extends from Tasman Bay to the Alpine Fault. It is a major structural depression occupied in its upper levels by thick accumulations of deeply weathered and dissected gravels. The Moutere Depression separates two regions of contrasting geology–East Nelson, which is made up largely of north-east-trending belts of Upper Paleozoic sedimentary, volcanic, and ultramafic rocks; and West Nelson, which comprises mainly granites and Lower Paleozoic sedimentary and metamorphic rocks of exceedingly complex structure. These two regions are shown in section C of diagram 6.

West Nelson

West Nelson contains the largest granite masses in New Zealand. They are disposed in three more or less continuous belts trending approximately north-south. An eastern belt lies immediately to the west of the Moutere Depression and can be traced for 70 miles from Separation Point almost to Murchison. Typically this granite is massive and light coloured, and sand derived from it forms beautiful beaches at Kaiteriteri, Totaranui, and other bays in Tasman Bay. At Canaan and other places a variety of minerals of scientific interest have been formed by contact of this granite with metamorphic rocks; they include a small wollastonite body at present being worked. A central granitic belt, the largest in New Zealand, extends from the northern extremity of Karamea Bight for 120 miles through the Victoria Range to the Ahaura River in Westland. This granite, like the western granite belt, comprises types ranging from pink alkali-granites with large orthoclase crystals to light-coloured, biotite calc-alkali granites. The western belt is exposed discontinuously from the Mokihinui River to the Buller River and from here continuously south through much of the Paparoa Range. These granites are more gneissic than the belts further east. Mention has been made of the Lower Paleozoic basic and ultramafic intrusive rocks of West Nelson. One belt of these, containing useful deposits of serpentine, talc-magnesite rock, and a little asbestos, forms a complex between the lower Cobb and Takaka Valleys. A folded sill or laccolith of gabbro, norite, and amphibolite is exposed in Rameka Creek on the Pikikiruna Range, and has formed skarns by contact with adjacent marble masses.

In south-west Nelson, possibly Precambrian greywackes and argillites of the Waiuta Group form the southern part of the Paparoa Range, and a second belt of them lies east of the Grey-Inangahua Valley, continuing northward to beyond the Mokihinui River. At Reefton and Waiuta these rocks contain gold-bearing quartz veins.

In north-west Nelson Lower Paleozoic sedimentary and metamorphic rocks form a broad wedge, narrowing to the south, between the eastern and central granite belts. The structure is complex: overfolds, thrusts, and nappes have been interpreted there. Broadly, the oldest rocks – the Cambian Haupiri Group – form a central belt that takes in much of the Haupiri, Anatoki, Douglas, Snowden, and Lockett Ranges and continues, displaced by faults, through the Cobb Reservoir area to the Wangapeka River. The Cambrian belt is flanked to the west by Ordovician sedimentary rocks, and to the east by Ordovician, Devonian, and possibly Silurian rocks. Section H shows some of the Cambrian and Ordovician sediments folded and faulted and intruded by granites. Trilobites of Middle Cambrian age, the oldest New Zealand fossils, are known from limestone lenses in the Cobb Valley.

The bulk of the covering strata of Nelson is Tertiary in age, but there are small areas of covering rocks that are older. At the Buller Gorge, Big River, Fox River mouth, and Punakaiki are patches of coarse breccia (the Hawks Crag Breccia) and of nonmarine sandstones and shales, that are probably Jurassic or Cretaceous in age. The special interest of these rocks is that the Hawks Crag Breccia is the host rock of New Zealand's only known uranium deposits. At the base of Farewell Spit an expanse of Cretaceous non-marine conglomerates, sandstones, and mudstones has yielded coal from mines at Puponga and Mangarakau. The Tertiary limestones at Golden Bay and Cape Foulwind are used for cement.

The Tertiary rocks of Nelson are the remnants of a once continuous cover. On a peneplain cut in later Cretaceous and early Eocene times small areas of coal measures accumulated in south-west Nelson in the Eocene; these have been mined at Ngakawau, Stockton, Millerton, Denniston, and other mining areas of the Buller coalfield. The largest areas of marine Tertiary rocks in Nelson are the Murchison Basin, the Karamea Syncline, and the Grey-Inangahua Depression (shown in section I). In northern Nelson most of the Tertiary rocks have been removed except for a few strips preserved in tectonic depressions, such as the Aorere Valley and the Takaka Valley.

East Nelson

In East Nelson Triassic and Upper Paleozoic sedimentary and marine volcanic rocks of the New Zealand Geosyncline form north-east-trending belts that extend from D'Urville Island to the Wairau Fault. Associated with them is a belt of basic and ultramafic intrusive rocks – the so-called Nelson “mineral belt”. Youngest of these basement rocks are fossiliferous Triassic sandstones and conglomerates, which are preserved by faulting as a narrow strip adjoining the northern part of the Moutere Depression; they are well exposed in the Wairoa Gorge. A major fault separates the Triassic rocks from a belt of Permian strata (Maitai Series) which form a synclinal fold to the east. The Maitai Series include a number of formations, the most distinctive of which is a finely banded red and green sediment. The Maitai rocks rest in some areas on “mineral belt rocks”, in others on marine volcanic and sedimentary rocks of the Te Anau Group. The “mineral belt” consists largely of serpentine masses, but includes two large masses of dunite, an almost pure olivine rock. Dun Mountain is the massif that gives this rock its name; it also forms the Red Hills to the south. Chromium and copper ores were mined last century from many small mines at Dun Mountain and other areas in the “mineral belt”, but with little commercial success. South, east, and north of Nelson city is a belt of Paleozoic volcanic rocks (the Brook Street Volcanics); west of these, between McKays Bluff and Pepin Island, is a strip of syenites and similar intrusive rocks that has yielded the hard boulders that form the Nelson Boulder Bank.

The Nelson Upper Paleozoic and Triassic sedimentary and volcanic rocks together form the Nelson Syncline, a regional feature complicated by faulting which can be matched with similar rocks in the Southland Syncline. It has been suggested that both synclines were formerly continuous and have been displaced horizontally by the Alpine Fault. This implies a horizontal movement of 300 miles, perhaps since late Jurassic and early Cretaceous times. Further support for this theory stems from the fact that the plutonic and metamorphic rocks of west Nelson can be matched with those of Fiordland and the Marlborough schists (flanking the Nelson Paleozoic strata to the east) and with the Otago schists.

A small area of Tertiary sedimentary rocks, including thin coal measures, lies south of Nelson city. The most prominent exposure of them is in the waterfront cliffs of the Port Hills.

The known active faults in Nelson district are shown in diagram 8. Earthquakes associated with movement on the White Creek Fault in 1929 produced much damage in the Murchison area and spectacular landslides occurred along much of the West Nelson coastline.

Westland Land District

The Alpine Fault running north-east – south-west divides Westland into two distinct parts. To the east stand the high schist and greywacke ranges of the Southern Alps; westward at lower level lie extensive areas of Pleistocene rocks, underlain by economically important areas of Cretaceo-Tertiary rocks (mainly to the north) which cover ancient folded basement rocks of possible Precambrian age broken by plutonic intrusions.

The Alpine Fault itself is a major topographic as well as geological feature of the region. The scarp forms a natural western boundary to the Alps and its course is marked in many places by a prominent fault-line valley. In places the precipitous western downthrow side falls 5,000 ft, forming a massive natural rampart. Both lateral and vertical movements have occurred along the fault; rocks to the west have been displaced toward the north-east and it has been suggested that a total horizontal shift of 300 miles has occurred, probably mainly in the late Jurassic-early Cretaceous Rangitata Orogeny (see Nelson district above). Horizontal movements continued on the fault in the Kaikoura Orogeny, but upward displacement of over 11 miles occurred simultaneously, forming the Southern Alps as they are known today.

Past movements along the fault have produced bands of cataclastic rocks, that is, rocks that have been deformed by the severe mechanical stress associated with the faulting. In these rocks individual minerals have sometimes been rolled and crushed so that frequently the entire rock has been milled to form a mylonite. More recent movements are marked by fault pugs, zones of greenish or purplish clayey, or gritty comminuted rock. In many instances the faulting movements have pushed the older schists or granite over Pleistocene morainic material. This is seen very clearly at Gaunt Creek, Waitangitaona River, near Whataroa. Such faulting and displacement of young terraces show that movement is still continuing.

The mountainous terrain east of the fault includes the highest ranges of the country, 8,000–12,000 ft; the rugged Mount Cook region is the most impressive as it is the centre of the main present-day glaciers. The highest part of the axial range is composed of intensely folded greywackes and argillites of Triassic and perhaps Permian age in this region. These grade westwards into their metamorphosed equivalents, schists. The metamorphism produced new minerals, first chlorite and then, where it was more intense, garnet and oligoclase. Rocks with these minerals were probably formed at great depths and have since been exposed by uplift and erosion near to the Alpine Fault.

Nephrite, the commonest type of greenstone used by the Maoris, is an ultrabasic rock found in some West Coast rivers as boulders carried downstream from bands occurring in the Alpine schists, notably those of the Griffin Range east of Hokitika. In south Westland, near the Moeraki River, mica has been mined from pegmatite veins in the schist.

West of the fault lie folded ancient greywacke and argillite (Greenland and Waiuta Groups) of possible Precambrian age and plutonic intrusions (mainly alkali-granites of late Paleozoic or Mesozoic age). Over much of Westland these are covered by younger sediments but are exposed in some areas, as, for instance, in the Paparoa Range and south of Ross. The Greenland and Waiuta Groups are unfossiliferous and have been intensely folded, contorted, and fractured. Some of the alluvial gold that has been won from the sands and gravels of Westland may have come originally from reefs in the Greenland greywacke, but much probably came from thin gold-bearing veins in the schists.

Section D (diagram 6), drawn through the Haast area, shows the Alpine Fault separating the Greenland and Waiuta Groups (with their associated plutonic intrusions) from the schists of Otago.

Towards the end of Mesozoic times the older rocks of Westland became greatly reduced by erosion; breccias and conglomerates accumulated locally on the eroded surface, in places reaching a thickness of several thousand feet. Deposition of coal-bearing rocks in local basins followed in late Cretaceous and early Tertiary times, the Paparoa Coal Measures immediately after the breccias, and the Quartzose or Brunner Coal Measures slightly later. In both groups of coal measures the seams are extensive and of good bituminous grade in north Westland. In south Westland only thin discontinuous seams are known.

Section I shows coal beds and late Tertiary sediments lying on the Precambrian basement rocks. To the east is the Grey-Inangahua Depression (which continues into south-west Nelson) filled by a great thickness of late Tertiary and early Pleistocene sediments, with thin coal beds at considerable depth. Late Tertiary and early Pleistocene deformation has faulted both the basement rocks and covering strata.

In the Tertiary (seediagram 7) subsidence allowed the sea to transgress, and thick sequences of marine sedimentary rocks were laid down on top of the coal measures. The mid-Tertiary limestones deposited at this time are quarried locally for agricultural lime. Marine erosion of a thinly bedded type has formed the Pancake Rocks at Punakaiki. Following the limestone, thick siltstone and fine sandstone were deposited, these being extensively preserved beneath the glacial gravels north-east from Ross to the northern boundary of Westland. The movements of the Kaikoura Orogeny folded the Tertiary rocks and at the same time raised the main ranges from which erosion produced vast quantities of gravels. The folding has produced structures which in north Westland may contain oil, the oil seepage at Kotuku being encouraging. Uplift of the ranges coincided with the Pleistocene ice age: as the mountains were uplifted they were several times subjected to attack by glaciers. Erosion by ice produced enormous quantities of glacial moraine and outwash gravel, which form a prominent feature of the country between the Alps and the sea and obscure many of the older rocks. What is believed to be the most ancient glacial deposit in New Zealand may be seen at Ross, after which the earliest glacial period in the country is named. Close to the moraines, as at Kumara and Kaniere, the outwash gravels proved to be a rich source of alluvial gold, and the action of the sea also concentrated gold in rich leads, not only along the present beaches, but also along old beaches now uplifted hundreds of feet.

Southland Land District

The western part (Fiordland) consists of a huge, broadly domed glaciated mountain mass of plutonic and metamorphic rocks. Between Fiordland and the Otago schists late Paleozoic and Mesozoic rocks are folded into the Southland Syncline, a major structural feature which curves in an arc through the district from the Hollyford area to the Mataura Valley and thence through south-east Otago to the coast. In Southland the Alpine Fault runs from Lake McKerrow (in Otago) to the entrance of Milford Sound and then probably continues just off shore, thus accounting for the spectacularly cliffed Fiordland coast and the deep water close inshore. The southern part of Stewart Island contains granites correlated with those of south-west Fiordland, whereas Ruapuke Island and the northern part of Stewart Island contain rocks correlated with the volcanic and igneous rocks associated with the late Paleozoic rocks of the Southland Syncline.

The mountain summits of Fiordland rise northward along the main divide from 3,600 ft to over 6,000 ft until they terminate in the Darran Range at 9,000 ft. The main rock mass was originally composed of quartzite, greywacke, tuffaceous greywacke, and limestone between 6 and 8 miles thick and probably ranging in age from Cambrian to Devonian, or possibly Carboniferous. The sequence later became highly metamorphosed, probably in the Carboniferous; the most altered rocks are now sillimanite gneisses and garnet-hypersthene gneisses. In a coastal strip in the extreme north-west of Southland near Milford Sound a narrow belt of these rocks has been intensely deformed by movements along the Alpine Fault, and mylonites similar to those in Westland have been formed.

In the later Carboniferous and Permian, the Fiordland rocks were invaded by a sequence of syenitic and granitic dykes and batholiths. The granite batholiths of the southern part of Stewart Island were formed at about this time. The oldest fossiliferous rocks known from Fiordland are Ordovician graptolite slates found at Preservation Inlet, but some of the adjacent metamorphic rocks are probably Cambrian.

Although Fiordland's ancient dome originally had radial drainage, Pleistocene glaciation has created a complicated rectangular network, the most impressive features of which are the elongated, high-level lakes gouged out in pre-existing valleys by the ice. Lake Te Anau, the largest, has an area of 132 sq. miles. Other residual evidences of glaciation abound, chief of which is the magnificent forested fiord terrain extending for some 170 miles round the south-west coast; a succession of drowned U-shaped valleys in the resistant rocks which have retained their precipitous sides, frequently plunging thousands of feet into the sea along the whole of their 10–20 mile branched courses inland. Freeze and thaw acting on the bleak uplands during the Pleistocene have produced extensive scree slopes, filling in many of the topographic irregularities.

Numerous small deposits of metallic minerals in Fiordland, such as copper, zinc, lead, and molybdenum ores, are of only marginal economic value. Marble occurs in quantity in Doubtful and Caswell Sounds, mica was once mined at Mount Elwood, and the Maoris used to obtain greenstone from the ultrabasic rocks of Anita Bay in Milford Sound.

The most extensive structural feature of the land district is the Southland Syncline, which formed the southernmost extension of the Permian-Jurassic New Zealand Geosyncline. The Upper Paleozoic and Triassic rocks of the Southland Syncline are closely similar to those of the Nelson Syncline and it is thought that both structures were once continuous, but have been moved horizontally by the Alpine Fault.

The oldest sediments in the Southland Syncline are Permian tuffaceous greywackes which delimit the northern margin of the syncline flanking the Otago schists and extend across Southland as a broad south-east-curving belt from the Humboldt Mountains through the Thomson Mountains to the Waikaka area (and thence through Otago to the Kaitangata coast). These sediments become more strongly folded and partly schistose as they approach the Otago schists which are at least in part their metamorphosed equivalents.

Rocks of similar age form the other margin of the Southland Syncline flanking Fiordland and extending from the Hollyford Valley southwards and southeastwards through the Takitimu Mountains to Riverton, Bluff, Ruapuke Island, and the northern part of Stewart Island (where they flank Fiordland-type granites to the south). These rocks are dominantly volcanic – basalts, tuffs, agglomerates – but with thin sediments, and attain a thickness of about 14 miles, attesting to the most intense and prolonged period of volcanicity known in the New Zealand region. Linear intrusions of igneous rocks are associated with these volcanic rocks (for example, Longwood Range, Bluff Peninsula, Ruapuke Island, and the northern part of Stewart Island) and are also known along the northern margin of the syncline (as at Livingstone Mountains, Otama Hills, and the Clinton-Waipahi area of Otago). Triassic and Jurassic rocks form the core of the syncline and strike across eastern Southland to the south-east Otago coast. Subdivisions of the New Zealand Permian and Triassic rocks are based on the fossil sequences exposed in the Takitimu Ranges (Permian) and Hokonui Hills (Triassic). The Jurassic rocks contain many fossils a famous locality is Curio Bay near Waikawa, where a fossil forest is preserved with petrified tree trunks reaching up to 40 ft in length.

Section J shows Permian and Triassic rocks of the Southland Syncline flanked by the Otago schists to the east and Fiordland igneous and metamorphic rocks to the west. The schists shown are representative of those found in northern and eastern Southland and throughout Otago. They are characterised by complex folding, shown diagrammatically in the cross section. The igneous and metamorphic rocks shown in the Te Anau area are representative of those found throughout Fiordland – granites, diorites, and gneisses. In the Te Anau area the older rocks are mantled by Tertiary sediments, remnants of probably more extensive deposits elsewhere removed by erosion.

The Rangitata Orogeny folded the sediments of the Southland Syncline, very steeply on the northeast side where the beds become overturned towards the Otago schist belt. This land was quickly eroded to low relief and the bituminous coal measures now mined at Ohai were formed from late Cretaceous swamps. Similar conditions prevailed during the Eocene (seediagram 7) when the thick lignite deposits of Mataura Valley and eastern Southland were formed. Contemporary freshwater sandstones and conglomerates (Arnold Series) were laid down around Lake Te Anau.

During the Oligocene (seediagram 7), the sea was able to transgress over much of Southland and extensive limestone was laid down. This is quarried for agricultural use and a particular type occurring in the Waiau Valley is valuable in cement manufacture. Much of the mid-Tertiary marine mudstones and sandstones which underlie the widespread recent alluvium in the lowland areas was also deposited. Towards the end of the period the sea slowly retreated to a small bight in the lower Waiau Valley, where shelly sands and deltaic deposits are now preserved.

The lowlands of Southland comprise the huge Waiau Valley west of the Takitimu-Longwood barrier and the Plains area to the east, consisting of the Central Plains extending over some 1,000 sq. miles and the smaller more dissected Waimea Plain to the north. Although Pleistocene ice did not extend more than about 15 miles from the southern and eastern limits of the present lake basins, the remainder of Southland was subjected to a rigorous cold climate with the production of much coarse detritus. This material, together with glacial outwash gravels, forms most of the filling for the Southland Plains. The valuable lowland soils, however, on which the economy is mainly based, are formed on a blanketing deposit of fine yellow silt (loess) which originated during the last glacial period of the Pleistocene when clouds of dust were blown from the river beds of the region and trapped by vegetation on the surrounding plains. Alluvial gold is widespread in the Pleistocene deposits, but only at Te Waewae Bay and Preservation Inlet are the deposits likely to be economic. Gold was, until recently, won by sluicing at Round Hill (west of Riverton).

A number of earthquakes are known from the Fiordland coast and the neighbouring sea, and these are probably related to the Alpine Fault, lying just off shore for most of the length of western Fiordland. An earthquake accompanied by uplift was recorded in the Doubtful Sound area by early whalers.

Otago Land District

Four main geological divisions may be made in the Otago Land District. North-west Otago consists of steeply eroded alpine ranges of Fiordland-type rocks. Central Otago is underlain by peneplaned schists and is a range-and-basin terrain with flat-topped or rolling block-faulted mountain ranges, separated by broad alluviated basins. South-east Otago is low rolling country with elongated north-west – south east fold ridges of Upper Paleozoic and Mesozoic sedimentaries. Eastern and north-eastern Otago comprises rolling hills of Cretaceous and Tertiary and late Tertiary volcanics. Many of the rocks and structures seen in Otago are continuations of similar features in Southland.

The steep ranges of north-west Otago support permanent snowfields and glaciers and are dissected by deep glaciated valleys occupied by freshwater lakes, the largest of which are Lakes Wakatipu, Wanaka, and Hawea. This area consists of Fiordland metamorphic and igneous rocks flanked to the east by late Paleozoic volcanics and sedimentaries of the Southland Syncline. These rocks are well exposed in the Hollyford Valley and in the adjacent Humboldt and Ailsa Mountains. The seaward margin of this mountain region is formed by the still active Alpine Fault. This continues through from Westland to separate the ranges from the south-western extension of schistose greywackes of Paleozoic or Precambrian age (Greenland and Waiuta Groups), with their mid-Tertiary cover rocks, which compose the low-lying coastal strip of Martins Bay — Awarua Bay area. To the east the Southland Syncline rocks are separated from the Otago schists and schistose greywackes by the Livingstone Fault, which branches off the Alpine Fault and has probably infaulted a strip of Upper Jurassic rocks that is known from the Pyke River area.

By far the most extensive area in Otago is underlain by chlorite schists, forming the basement to the east and outcropping over much of Central Otago. They are high stress metamorphic rocks, originally greywackes and argillites of the New Zealand Geosyncline, probably Upper Paleozoic or Triassic in age and attaining a possible thickness of up to 9 miles. During the later stages of metamorphism they were intensely folded on a grand scale into a series of recumbent or semi-recumbent nappe-folds. This gave rise to subhorizontal schistosity and lamination over many hundreds of square miles, and steeply dipping schistosity near the edges of the schist. Metamorphism and folding of the schists is thought to have occurred during the Rangitata Orogeny (late Jurassic — early Cretaceous). Erosion rapidly stripped the overlying strata, exposing the schists towards the end of the Cretaceous; long-continued erosion extending into the Tertiary reduced the land to a peneplain; the covering forests are now preserved as lignites in central Otago. At this time beds of quartzose conglomerate, sand, and white clays were laid down by slow-flowing rivers and in shallow inland lakes. In many of these were concentrated deposits of valuable alluvial gold, originally from quartz veins in the schist.

The numerous flat-topped ranges of central Otago (for example, Dunstan Mountains, Raggedy Range) arose mainly by block faulting in the Kaikoura Orogeny (late Tertiary and Pleistocene). Stripping of the Tertiary covering beds from the Otago peneplain and exposure of the underlying schist led to the formation of the curious tor topography of the schist region. Warm climate conditions produced zones of deep weathering of the schist. These were subsequently more easily removed by soil creep during the glacial stages of the Pleistocene, leaving relatively unweathered residuals standing as tors.

To the north-west the Otago schist continues into Westland where it is cut by the Alpine Fault and, it is thought, displaced north-eastwards into Marl-borough. To the north-east and south-west the schist is flanked by tuffaceous greywackes thrown into large steeply dipping folds close to the schist. The north-eastern greywackes are mainly of Triassic age and continue into Canterbury to form the Southern Alps and foothills. The south-western greywackes are Permian and form the north-eastern limb of the Southland Syncline, continued in a sweeping arc through Southland to reappear in north-west Otago in the Hollyford and Pyke areas before being cut by the Alpine Fault. The Triassic and Jurassic rocks forming the core of the Southland Syncline, and seen in eastern Southland, continue to the south-eastern Otago coast. Particularly good sections are seen along the Catlins coast. The rocks of the Southland Syncline were strongly folded in the Rangitata Orogeny, probably at about the same time as the Otago schists were formed, the region being elevated above sea level and eroded to form hilly land. During this erosional phase talus fans and alluvium filled the valleys, and small areas of such deposits of Cretaceous age are now preserved at Kyeburn and Henley. On the east coast of Otago, quartz sands and conglomerates were deposited with coal beds in a swampy margin of the Cretaceous sea. These now comprise the Kaitangata, Green Island, and Shag Point coalfields.

During the middle and late Tertiary times, inland Otago remained above sea level seediagram 7) and was occupied by shallow lakes and clothed with forest. In eastern Otago mid-Tertiary sandstones, greenstones, and limestone were deposited; in the Oamaru district mudstones, greensands, and bryozoan limestones were interbedded with basaltic flows and volcanic ash layers; in the western Otago area shelly mudstones and limestones were deposited. During the late Tertiary the north-western and eastern coastal districts emerged slowly above sea level while, on the east coast, volcanic eruptions centering on the Dunedin district produced cones, extensive lava flows, and bouldery mud flows. These andesitic and basaltic volcanic rocks form the hills in the Dunedin district and are also scattered throughout eastern and northern Otago.

During the Pleistocene the ranges were heavily glaciated in the west by valley glaciers and in central Otago by high-altitude cirque-glaciers. Around the margins of the glaciated ranges of western Otago rigorous periglacial conditions caused mass-slumping of rock and development of characteristic solifluxion slopes. Throughout central Otago and eastern districts a covering of loess was deposited during cold-climate periods of the Pleistocene. Intermontane valleys of central Otago were filled with auriferous river alluvium and alluvial fans which were later dissected into terraces by the main rivers. Uplift of the ranges appears to have continued during deposition of the early Pleistocene alluvium, and many parts of the main rivers are now incised across rock barriers in deep gorges (for example, the lower Taieri gorge). In the last major glacial period of the Pleistocene valley glaciers occupying the present lake basins terminated at about the southern limits of the lakes and during the earlier stages had advanced only a few miles further down the valleys. At the same time the two major rivers (Clutha and Waitaki) built extensive deltas and plains of alluvium at their mouths on the east coast.

Excluding a region in the north-west, close to the Alpine Fault, Otago is seismically relatively quiet and, as may be seen in diagram 8, only a few recent fault traces are known — some from central Otago and one in the Milton-Taieri area.

Gold was formerly extensively worked in Otago – including both reef gold (for instance, in the Shotover district) and alluvial gold, which occurred almost everywhere, especially in the Arrowtown and Bannockburn areas. Today the most important economic minerals of Otago are mainly non-metallic and include limestone (Oamaru), coking coal (Shag Point), bright coal (Kaitangata, Green Island), sands for foundry and other industrial purposes, and clays for pottery manufacture (Benhar). Oilshale deposits in the Nevis Valley have proved of doubtful economic value.

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GEOLOGY – LAND DISTRICTS OF NEW ZEALAND 22-Apr-09 Geoffrey Conrad Shaw, B.SC., New Zealand Geological Survey, Lower Hutt and Graeme Roy Stevens, M.SC.(N.Z.), PH.D.(CANTAB.), Paleontologist, New Zealand Geological Survey, Lower Hutt.