Introduction

GEOLOGY – NEW ZEALAND'S GEOLOGICAL HISTORY

by McLintock, Alexander Hare

Introduction

The Pacific Ocean is immensely deep: over millions of square miles the monotonous expanses of ocean floor lying 2–3 miles below the surface are interrupted only where great volcanoes have grown by the eruption of lava on to the bed of the ocean; the largest of these volcanic masses break the surface as Hawaii, Samoa, and other islands of the mid-Pacific. In its south-western part the Pacific is shallower and its floor is more irregular — it is diversified not by volcanoes but by deep trenches, broad ridges, and swells, where the earth's crust has been thrown into huge folds. One vast complex of ridges, rises, and plateaux lies about a thousand miles east of the continent of Australia, separated from it by the deep waters of the Tasman Sea. Much of this great system of folds is submerged, but a small part of it has risen and been shaped into a group of mountainous islands known today as New Zealand.

For hundreds of millions of years there have been land areas of slowly changing size and shape in this part of the Pacific: at times this land must have been large, at times small; it may even have altogether disappeared beneath the sea. A suggestion of the changing form of ancestral “New Zealand” during the later chapters of its long history is given in diagram 7. The evidence for these changes is recorded by the rocks of which this small portion of the earth's crust is built. New Zealand, unlike the wholly volcanic islands of the central Pacific, consists of rocks of a wide variety of types and ages. The intense folding and shearing that they display suggests that this country has long been one of the earth's “mobile belts” — part of a region where the outer part of the earth's crust has been buckling and breaking at a geologically rapid rate. The rocks are cut by innumerable great fractures, called faults, which have displaced them thousands of feet; even young marine sedimentary rocks have been involved in these movements and raised thousands of feet above the level of the sea. Many of the faults have broken the present land surface, showing that the crust blocks adjacent to them have been moving during the past few centuries or thousands of years (diagram 8–9). Even in the short time that Europeans have populated New Zealand, there have been many movements of the crust: shore platforms rose 5 ft from the sea at Wellington during the severe earthquake of 1855; a sudden fault movement raised one part of a road 12 ft above the other at Murchison in 1929, and 5 sq. miles of land rose from the sea at Napier during the Hawke's Bay earthquake of 1931. The presence of active volcanoes is further evidence that New Zealand remains a part of the circum-Pacific mobile belt.

Below, a summary is given of what geologists have learned of New Zealand's geological history: here, as in every part of the world, there is seen in the geological record “no vestige of a beginning, no prospect of an end”. New Zealand's rocks have been formed and its landscape shaped by the continuous interplay during millions of years of the universal geological processes of earth movements, erosion, sedimentation, metamorphism, and igneous activity. The earth's crust in the New Zealand region, yielding to great pressure, has constantly been warping and breaking. Some areas have sunk, others have risen to form land which rain and wind, waves and glaciers have attacked, carrying mud and gravel and sand to the lowlands and out to the sea where this debris has accumulated as thick deposits of sandstone, mudstone, conglomerate, and other sedimentary rocks. These in turn have been raised to form new land.

At times parts of New Zealand were the sites of huge, slowly sinking basins of deposition, called geosynclines, in which sedimentary rock layers accumulated to thicknesses of tens of thousands of feet. In time the geosynclines were compressed and their fill of sedimentary rocks folded, sheared, and raised, to be sculptured by erosion into mountain ranges. The deeper rocks of the geosynclines, subjected for millions of years to great pressure, temperature, and shearing stress, were transformed to slate, schist, gneiss, marble, and other metamorphic rocks: Huge bodies and tongues of molten rock, formed perhaps by melting of deeper rocks of the geosynclines, invaded the roots of the growing mountain ranges, cooling there to form large masses of granite, diorite, gabbro, and other intrusive igneous rocks; volcanoes at times poured out lava and ash and other volcanic rocks, both on to the land and on to the sea floor.

Erosion has constantly been at work removing the outer, younger rocks, revealing again older sedimentary strata and the ancient intrusive and metamorphic rocks that once lay thousands of feet below. The present distribution of New Zealand's rocks, the product of this complex interplay of geological processes during a vast span of time, is summarised by the accompanying small-scale geological maps and by those showing geological cross sections.

Geological History of New Zealand

A regional description of the geology of New Zealand, arranged by land districts, is presented later in this article; but first a summary must be given of what geologists have learned about the geological history of this country — the long sequence of events that has formed New Zealand's rocks and given them their present complex arrangement in the crust.

Events in this history are classified in accordance with the International Geological Time Scale. In this time scale are listed the eras and periods into which geologists have divided the earth's history, with each division representing an enormous span of time. This broad sequence was established in Europe during the nineteenth century. In the light of recent knowledge, the approximate ages of the eras and periods have been assessed in terms of millions of years. This has been achieved by studying the decay of radioactive minerals in certain igneous rocks whose relative position in the time scale was known from stratigraphic evidence. The Cretaceous period, for example, is thought to have lasted some 70 million years: the Paleozoic era may represent more than 250 million years.

Fossils are the keys that make it possible to equate the time of events in the geological history of one country with those of another. In countries like New Zealand which are far from Europe, it is not often possible to match fossils precisely; evolution, though similar, was not exactly parallel the world over. It has therefore been necessary to set up a local time scale, based on the sequence of rocks and fossils preserved in New Zealand, and to key this to the international scale at points where the fossils match closely.

The study of fossils has shown that the South Island has the oldest rocks; further, that rocks of all geological periods since the Cambrian are present in New Zealand, with the exception of the Silurian and Carboniferous. These, however, may in time be recognised here; it is almost certain that some rocks in Nelson are of Silurian age.

The eras and periods are not listed on the legend of the geological maps; instead, broader categories, such as “old sedimentary rocks” are given. Their approximate equivalence with the International Time Scale is as follows:

Alluvium, moraine, sand dunes, etc.: Holocene to mid-Pleistocene.

Young sedimentary rocks: Early Pleistocene to late Cretaceous.

Young volcanic rocks: Cenozoic.

Old sedimentary rocks: Mid-Cretaceous to possibly Precambrian.

Old volcanic and associated intrusive rocks: Cretaceous to possibly Precambrian.

Metamorphic rocks: Mesozoic to possibly Precambrian.

Granitic rocks: Mesozoic to possibly Precambrian.

The major divisions of New Zealand's geological history do not exactly correspond to the eras and periods of the International Time Scale. For simplicity, however, a division of the history into Precambrian and Paleozoic, Mesozoic, and Cenozoic is followed in this article.

Precambrian and Paleozoic Events

The term Precambrian means, broadly, all the vast span of time from the formation of the earth's crust, thousands of millions of years ago, until the beginning of the Cambrian period of the Paleozoic era. It is in the Cambrian rocks of the world that the first well preserved fossils are found. Nothing is known with certainty of the events in this part of the Pacific before the Cambrian. Although the New Zealand islands are small, the expanse of undersea ridges and plateau from which they rise is huge; it includes such features as the Campbell Plateau, an under-sea plateau some 500 miles across immediately to the south of New Zealand. Seismologists deduce from earthquake-wave studies that the material of the earth's crust in and around New Zealand is “sial”; this term sial is a general one given to rocks, such as granite, of high silica and aluminium content, of which the earth's crust in continental areas, but not beneath the deep ocean floors, is composed. Beyond the New Zealand region no sial exists nearer than Australia; the Pacific and Tasman oceans are floored by thin sediments that overlie dark, heavy basaltic rocks — “the sima”. A fundamental problem of New Zealand geology is to know how, and at what time, this extensive, isolated mass of sial was formed — is it a fragment of a “scum” of lighter elements that thousands of millions of years ago floated to the surface of a molten earth to form the continental crust; is it a piece of continental crust that has “drifted” a thousand miles from its parent, Australia; or has some other process brought about this concentration of sialic material within the basin of the sima-floored Pacific?

There is no firm evidence of events even of later Precambian times in New Zealand; no Precambrian rocks are known for sure, although some granites and metamorphic rocks in Westland and Fiordland may be of this age. Some geologists think that the thick sequence of complexly folded, marine sedimentary rocks of the Waiuta and Greenland Groups of Westland and south-west Nelson may possibly be of late Precambrian age. They are predominantly greenish grey, lightly metamorphosed unfossiliferous greywacke and argillites, that probably accumulated in an extensive geosyncline. They have been studied closely because they contain the rich gold-bearing quartz reefs of the Reefton area. Little or nothing has been deduced, however, about the size and shape of the land areas from which these ancient sediments were derived.

Paleozoic

There is a little more evidence of the events of the early part of the Paleozoic era in New Zealand. In west Nelson and Fiordland thick sequences of Cambrian, Ordovician, Devonian, and probably Silurian rocks are preserved. In both areas the rocks include great thicknesses of sedimentary and volcanic rocks that appear to have accumulated in a major geosyncline. Along with these is a wide variety of slates, schists, gneisses, quartzites, and marbles formed by metamorphism of similar sedimentary rocks, and these have been invaded by granites and other intrusive igneous rocks.

In West Nelson deposition in the early Paleozoic geosyncline began with the accumulation of the Cambrian Haupiri Group. The earliest of these rocks was a large thickness of basic volcanic rocks — lavas, agglomerates, and tuffs; later these were covered by grey mudstones with patches of limestone that have yielded New Zealand's oldest fossils — middle Cambrian trilobites, shells, and sponges, discovered in the Cobb Valley. These were succeeded by the great thickness of Ordovician strata of the Aorere and Mount Arthur Groups which include slaty argillites containing fossil graptolites that closely resemble those of Victoria, Australia. The Mount Arthur Group includes the extensive Mount Arthur marble, quarried for many purposes at Kairuru. In the Devonian, sedimentation in the geosyncline slowed down, and quartzites and richly fossiliferous limestones were laid down. Today, fossiliferous Devonian strata are preserved only in the Baton River area near Wangapeka and at Reefton.

In Fiordland the Cambrian sediments, some 9,000 ft thick, have been converted to metamorphic rocks; they appear originally to have been a succession of basic volcanic rocks and sediments that included sandstones, greywacke, and limestone. The Ordovician sediments, more than 20,000 ft thick, have also suffered metamorphism; they appear originally to have been a varied sequence of sandstones, greywackes, mudstones, and tuffs. The Preservation Formation includes abundant graptolites of Lower Ordovician age, in beds of slaty argillite.

Both in Nelson and in Fiordland the geological structure is extremely complex. The intense folding and thrusting of the Lower Paleozoic sediments, their conversion in part to metamorphic rocks, and the invasion of them by large and small masses of intrusive rocks, took place at various times during the Paleozoic. In West Nelson there were several major episodes of deformation. It was perhaps in the Devonian period that Cambrian sediments were transported as great overturned folds (nappes) so that they came to rest on top of younger Ordovician rocks. Further major folding in other directions took place, possibly in the Carboniferous, and again in the Carboniferous and Permian. Ultramafic and basic intrusive rocks, mainly serpentines, peridotites, and dolerites, were intruded, probably in the Cambrian, as a sill; they are exposed now between the lower Cobb and Takaka Valleys. Talc-magnesite and asbestos are minerals won from this ultramafic mass.

Part of the conversion of the Lower Paleozoic sediments of West Nelson to a complex assemblage of metamorphic rocks took place as a result of these foldings. Later, emplacement of the great mass of the Separation Point granite raised the temperatures and increased the degree of metamorphism. The Karamea, Paparoa, and many other granite masses in Nelson and Westland may have been intruded also in late Paleozoic times.

Many minerals of value were formed during Paleozoic metamorphism and intrusion in Nelson and Westland. The gold veins of the Aorere and Wangapeka districts are in Lower Paleozoic rocks: gold and sulphides of lead, zinc, copper, and molybdenum have been worked in Paleozoic rocks at Johnson's United Mine, where the mineralisation apparently occurred along the sole of a Paleozoic thrust plane. The Richmond Hill silver deposits are found in a similar environment. Molybdenite and other sulphide deposits of Mount Radiant, near Karamea, occur in masses of Paleozoic metamorphic rocks within granite. Metamorphism of Mount Arthur marble by Separation Point granite has produced a wollastonite body near Motueka, small deposits of barite and fluorite near Wangapeka, and magnetite iron ore near Canaan, Nelson.

In Fiordland the main phase of metamorphism and plutonic intrusion is thought to have been late Paleozoic; the distribution of the major groups of these rocks is summarised later in the section on Southland. Metamorphism of Fiordland limestone produced marble, some of which is very handsome and suitable for building stone.

The New Zealand Geosyncline

The Lower Paleozoic rocks just discussed are confined to the western South Island; nothing is known of the history of the rest of New Zealand during that time. Rocks of later Paleozoic age are, however, more widespread, and the events of these times better understood. In the late Paleozoic was formed the extensive New Zealand Geosyncline, in which accumulated an enormous thickness of volcanic and sedimentary rocks that now form much of the “basement” (the foundation of old hard rocks) of this country. On the geological maps the rocks of the geosyncline are seen now to make up much of the South Island: most of the expanse of “old sedimentary rocks” extending from East Nelson and Marlborough through Canterbury, Otago, and Southland consists of them; the Haast Schists — the metamorphic rocks are shown in Marlborough, along the western side of the Southern Alps, and in a belt across Otago — have also been formed by metamorphism of these sediments. All the “old sedimentary rocks” of the North Island map were deposited in the geosyncline; they extend as well beneath the cover of younger sedimentary and volcanic rocks that now occupies much of the North Island.

Although the oldest fossils so far discovered in the rocks of the New Zealand Geosyncline are Permian, it is likely that it was in existence in the Carboniferous. Stratigraphically beneath the proven Permian strata are tens of thousands of feet thickness of basic volcanic lavas, agglomerates, breccias, and tuffs. These old volcanic rocks came probably from a chain of volcanoes that lay west of the geosyncline early in its history; two belts of them are shown on the South Island map — in Nelson (Brook Street Volcanics), and in western Southland (Eglinton Volcanics). In Nelson and Southland serpentines, dunites, gabbros, dolerites, and other basic and ultramafic rocks are intimately associated with the Paleozoic volcanic rocks, probably being intruded at that time.

Mesozoic History

Less volcanic debris was supplied to the geosyncline in later Permian and Mesozoic times. The younger Permian strata (Maitai Group) include limestone formations, but the Mesozoic rocks are almost wholly greywackes, argillites, and conglomerates, with occasional beds of red and green volcanic rocks that were erupted into the geosyncline. (The Triassic rocks are known as the Gore and Balfour Series, and the Jurassic as the Herangi, Kawhia, and Oteke Series; the geosynclinal Cretaceous rocks constitute the Taitai, Clarence, and Raukumara Series.) The land from which these thick deposits of sediments were derived is thought to have lain to the west and south of present New Zealand, the reason being that the strata of the New Zealand Geosyncline in the west (marginal facies) are better stratified than those in the east — they are more fossiliferous and of less massive total thickness. This suggests that they accumulated near shore, in the less rapidly sinking margins of the geosyncline. The massive, poorly fossiliferous greywacke deposits (axial facies), typical of the Southern Alps, for example, are thought to be those of the central, rapidly sinking zone of the geosyncline.

The New Zealand geosyncline had its maximum extent in the late Paleozoic and Triassic. It may then have been continuous from as far north as New Caledonia to a long way south of present New Zealand.

Late in the Jurassic there began one of New Zealand's major episodes of crustal compression and mountain building, the Rangitata Orogeny. The sediments of much of the geosyncline were complexly folded, faulted, and raised to form a mountainous land perhaps several times the size of present New Zealand. Geosynclinal sedimentation persisted into the Cretaceous only in the east, where Marlborough and the eastern North Island as far as East Cape are today. The Haast Schists are thought to have been formed by the metamorphism of the thickest sediments of the geosyncline: a gradual transition over a distance of miles from unaltered greywacke through crushed and fissile greywacke to chlorite schist can be observed in many places in Otago and the Alps.

The main structures that were imposed on the thick greywacke and argillite strata of the axial facies are not easy to determine, for the rocks are hard to differentiate lithologically and they contain few fossil marker beds except occasional layers crowded with the Triassic shell Monotis richmondiana. Huge, tightly folded anticlines and synclines have been observed in these rocks in the bare mountains of the Southern Alps: in most other exposures, too, these rocks are seen to be complexly folded. Mapping of the fossiliferous, clearly bedded strata of the marginal facies is easier: major structures recognised in them include the Kawhia Syncline of south-west Auckland, the Nelson Syncline east of Nelson city, and the Southland Syncline, which extends from the Alpine Fault north of Milford Sound across Southland and Otago. Recent study of the Otago Schists suggests that during their formation in the Rangitata Orogeny they were folded into a number of large, nappelike folds which were thrust eastwards.

Vast and mountainous though it must surely have been, the New Zealand land mass raised in the Rangitata Orogeny did not endure long after the Mesozoic. Indeed, rivers and waves had worn it down sufficiently by the late Cretaceous for the sea to begin slowly to spread over it from the east. This gradual submergence continued steadily during the early Cenozoic, becoming almost complete in the early Oligocene.

Late Cretaceous marine sediments (Mata Series), deposited early in this transgression, are preserved in North Auckland (where they make up much of the area of “young sedimentary rocks”) and on the eastern side of New Zealand from East Cape to Otago. In contrast to the thick, geosynclinal sediments of the earlier Cretaceous, those of the Mata Series are predominantly types that accumulate slowly on broad coastal shelves adjacent to lowlands where sluggish rivers bring only fine sediments to the sea. Dark, flinty argillites, white argillites, sulphurous sandstones, bentonitic mudstones, and flint beds are the common late Cretaceous sediments which were deposited adjacent to the land mass that was by then tectonically stable.

Late Cretaceous coal, excellent for metallurgical coke, is mined in the Paparoa mine at Greymouth; poorer Cretaceous coal has also been worked at Puponga coalfields near the base of Farewell Spit and at Kaitangata in Otago. The Cretaceous coal seams were formed from thick masses of vegetation that grew in swamps on the old land surface before the sea advanced over those areas.

Cenozoic History

The term “Cenozoic” is a rather formal one; the Paleocene to Pliocene rocks are usually called the “Tertiary” rocks. In many parts of the world a major break in the succession of strata marks the passage from the Mesozoic Era (“the age of reptiles”) to the Cenozoic Era (“the age of mammals”): New Zealand, however, was one of the regions where sedimentation continued without a break. The diagram opposite shows stages in the slow advance of the sea during the Cenozoic, reaching a maximum in the early Oligocene; followed by a retreat.

Cenozoic strata, although now limited in area in the South Island, are widespread in the North. Economically they are important; they contain almost all of New Zealand's coal and useful limestone; moreover, they and the late Cretaceous sedimentary strata are the only New Zealand rocks that constitute likely sources of petroleum and natural gas.

The late Cretaceous and Cenozoic rocks of this country are wholly sedimentary and volcanic in origin; none of the deep intrusive or metamorphic rocks that may have been forming during this time has yet been exposed at the surface. Volcanic outbursts occurred at intervals during the Cenozoic both in North and South Islands. Some of these volcanic rocks were poured out on to the floors of basins in which the marine sedimentary rocks were accumulating; others were poured out on to land. The huge accumulation of volcanic materials in the central volcanic region was not erupted until very late in the Cenozoic.

Paleocene to Oligocene

In many areas in New Zealand with marine Tertiary deposits the most typical strata are thick deposits of crumbly sandstone or mudstone, often with abundant fossils; conglomerate and coarse-grained limestone beds are common, too. However, the earliest Tertiary deposits (Dannevirke Series — Paleocene and lower Eocene) resemble more the late Cretaceous deposits and include thin-bedded mudstones and light-coloured bentonitic mudstones with greensand bands. In Marlborough, Canterbury, and the eastern Wairarapa area of the North Island hard, smooth, white, fora-miniferal limestone is a distinctive early Tertiary deposit. Some of the early Tertiary bentonite muds of Gisborne, Hawke's Bay, and Marlborough are pure; they are quarried at Porangahau in Hawke's Bay for use as drilling mud.

During this time when the sea was spreading slowly from the east, low land still lay where western New Zealand is today; the bulk of this country's coal deposits accumulated then in swamps on the low-lying land. These younger coal deposits, ranging in age probably through the Paleocene and Eocene periods, are known as the Quartzose Coal Measures; the main coalfields in these rocks are Greymouth, Buller, and the Waikato. Marine sediments buried the coal first in Westland, where the sea in the middle Eocene began to advance from the south-west. The transgression was slow and, in the Waikato coal-field, submergence was delayed until as late as the Oligocene.

The first paleogeographic map of diagram 7 is an interpretation of the distribution of land and sea late in the Eocene period, when the younger strata of the middle and upper Eocene Arnold Series were deposited. In North Auckland greensands, argillaceous limestones, and green and chocolate shales were the earliest Arnold sediments to accumulate, followed by sandstones; in the eastern North Island basins sedimentation continued, light-grey silty mudstones being deposited, for example, in southern Hawke's Bay. The thickest sediments of the Arnold Series accumulated in basins in Nelson and Westland. A miniature geosyncline developed at the site of the present Paparoa range near Greymouth, and a thick sequence of sediments buried and preserved the coal seams; sediments also began to accumulate in the Murchison Basin. Seas had reached far inland over the eastern South Island by the late Eocene. Deposition of Amuri limestone continued in Marlborough. On the stable, shallow shelf of Canterbury and Otago a belt of glauconitic greensand was deposited. This was followed by mudstone in eastern Otago, with limestone in some areas. In the Waiau Syncline of Southland Arnold sandstones, conglomerates, and coal measures accumulated; thicker sediments were deposited there in the Oligocene, and are exposed with the coal measures in some of the mountains that border Te Anau. The extent of the submergence of New Zealand in the early Oligocene period is shown by the second paleogeographic map of diagram 7.

Where the sea floor was shallow and yet far enough from land to be clear of pebbles, sand, etc., the only sediments able to accumulate were made up almost entirely of the skeletons of marine organisms, such as brachiopods, foraminifera, and sea eggs, whole or broken into pieces, large and small, down to microscopic size. These were bound together to form a hard limestone by calcareous cement derived by solution and reprecipitation of the shell substance. Large areas of limestone were formed in this way in the Oligocene in New Zealand. Today these rocks are the main source of lime for agriculture and cement; in some areas they are hundreds of feet in thickness. Cement works at Te Kuiti, Tarakohe in Golden Bay, and Cape Foulwind near Westport are among the largest that use this limestone, which has recently been investigated for possible use as a raw material for a carbide industry. Well known scenic features formed in Oligocene limestone include the Waitomo and Te Anau Caves, and the Punakaiki Pancake Rocks near Westport.

Nearer the diminished land areas, clastic sediments, such as conglomerates, sandstones, and mudstones, were still able to accumulate. Siltstone and sandstone were laid down in limited areas of the sea floor in North Auckland and in the Waikato. Sedimentation continued in the eastern North Island basins and in the Waiau basin, and also began in the Taranaki basin.

Miocene to Present Day

The Oligocene period saw the maximum submergence of New Zealand by the Tertiary sea. Although the complex internal geological structure of this small section of the earth's crust is the product of all that has happened to it during hundreds of millions of years, the landscape is a comparatively recent development, shaped wholly within the 30 million years or so that have elapsed since the Oligocene submergence. Indeed, only in the past few millions of years has New Zealand achieved a coastal outline and surface form broadly resembling that of today: only in the past few tens or hundreds of thousands of years have such features as the present mountain peaks and valleys, the gorges, lakes, harbours, fiords, stream terraces, fault scarps, and recognisable volcanic cones been formed. The slow evolution of New Zealand's form and structure continues in response to tectonic movements, erosion, and other geological processes. This country, millions of years from now, will surely look as different from that of today as did the “New Zealands” of the past.

Vigorous deformation of the earth's crust (the Kaikoura Orogeny), which reached its climax in a major episode of mountain building in the Pliocene and Quaternary, has been the dominant characteristic of New Zealand's history since the Oligocene. During the Kaikoura Orogeny the Southern Alps and other great mountain chains were slowly pushed up and sculptured into their present form by erosion, and all the other major elevations and depressions of New Zealand were roughly blocked out by fault movements.

The later Tertiary sedimentary rocks of New Zealand reflect this increase in the rate of deformation. Even in the late Oligocene a thick sequence of alternating layers of sandstone and mudstone had begun to develop in the Taumarunui area at the northern extremity of the Taranaki basin and, during the Miocene, Taranaki and many other areas received thick deposits of sediments in rapidly deepening basins. The Miocene, Pliocene, and early Pleistocene were the times when most of the thick deposits of soft, grey, fossiliferous mudstones and finegrained sandstones, known colloquially as “papa rock”, were deposited in New Zealand.

The final four maps of diagram 7 are glimpses of the changing outline of New Zealand after the Oligocene period. In earlier Miocene times most of North Auckland and Auckland city were still submerged (the sandstone deposits on which this city is built were then laid down). Andesite volcanoes to the west of Auckland poured out lava and rubble, much of it into the sea, forming the rocks of the Waitakere Ranges. Thick sediments continued to accumulate in the Taranaki and Gisborne basins; sedimentation was interrupted in Central Hawke's Bay.

Although a long belt of land had begun to rise along the axis of the South Island in the early Miocene, marginal sediments still accumulated, the thickest some 12,000 ft of sandstone, silty mudstones, and conglomerates — being laid down in the Murchison basin.

By the late Miocene North Auckland had emerged and has not since been beneath the sea. The Waikato and a broad belt from the Bay of Plenty through the Taupo region to Wellington may also have become land by then: the main areas of sedimentation were still the Taranaki and Gisborne basins. Most of the South Island had emerged by the end of the Miocene. North Canterbury, Westland, and the Waiau basin were still beneath the sea, and a further 6,000 ft of sandstones and conglomerates were deposited in the Murchison basin. Thick gravel deposits worn from rising mountain areas began to accumulate on land.

The latest major marine transgression in New Zealand's history began late in the Miocene and continued into the Pliocene; much land that had recently risen was submerged again. The sea during this time invaded the Auckland and northern Waikato areas and resubmerged much of the southern North Island. It also flooded over most of Canterbury, a little of Marlborough, and over a long, narrow strip of the west coast of the South Island.

During the Pliocene the main North Island areas to receive thick marine sediments were South Taranaki, Wanganui-Rangitikei, and Hawke's Bay-Wairarapa. The Wanganui-Rangitikei region is today the site of a major negative gravity anomaly that trends north-east across the North Island parallel to and south of the Taupo Volcanic Zone. Thick sediments accumulated there in the late Tertiary and Quaternary in a large, rapidly sinking basin. Most of the South Island deposits formed then were thin; one exception is the Grey-Inangahua depression of Westland and south-west Nelson, where some 7,000 ft of sediments accumulated.

Soon after the Pliocene the sea had abandoned its temporary sortie against the land and New Zealand had assumed approximately its present coastal outline. Since then most incursions of the sea have been along only narrow coastal strips.

Events on Land

The slow rise of the land that was going on during this time (the late Tertiary and Quaternary) was achieved mainly by the displacement of large crustal blocks along fractures, called faults. The blocks that rose were carved by streams into a landscape pattern of mountains, hills, and valleys; the sinking blocks were buried by gravels worn from the higher lands.

The Kaikoura Orogeny began, and tapered off, earlier in some areas than others. In Northland it started in the Miocene; today few earthquakes have their origins there, no active faults have been recognised (diagram 8), and erosion has reduced the area to one of low relief. Central and eastern Otago is another region relatively free of earthquakes and active faults. The most powerful mountain-building movements of the Quaternary have been in a “mobile belt” that follows the Southern Alps, Marlborough, and the main North Island mountain chain. The main faults shown by diagrams 8–9 have remained active. All the major faults of this zone are “transcurrent”, that is to say, the crustal blocks adjacent to them slide horizontally along the faults, as well as vertically. Evidence of vertical movement on these faults is shown by the prominent fault scarps that border them: examples are the huge dissected scarp that rises from the Alpine Fault, the scarps of the Wairau, Awatere, and Clarence Faults in Marlborough, and the scarp of the Wellington Fault at Wellington and Lower Hutt. Transcurrent movement is recorded by the displacement of rock groups, by the offsetting of stream courses, and by the presence of fault ponds, shutter ridges, notched spurs, and other minor but revealing landscape features.

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GEOLOGY – NEW ZEALAND'S GEOLOGICAL HISTORY 22-Apr-09 McLintock, Alexander Hare