Art. IX.—The Birth and Development of New Zealand.
[Read before the New Zealand Science Congress, Palmerston North, 26th January, 1921; received by Editor, 2nd February, 1921; issued separately, 27th June, 1921.]
Though it contains in its fabric rocks of great antiquity, considered as a geographical unit New Zealand is, geologically speaking, very young. We must ever bear in mind that, though it may be built of stones of great antiquity, a house is not older than its builder. And so it is with New Zealand. Its framework is composed of an extraordinary pile of Palaeozoic and Mesozoic rocks, but it was not till the Cretaceous epoch that these rocks were built up into the mountain-chains and other land-forms familiarly known to us by the geographical name “New Zealand.”
The Palaeozoic sediments were derived from the denudation of a land area that formerly occupied the greater part of the Southern Hemisphere. This ancient continent certainly existed throughout the whole of the Palaeozoic era, and eventually became submerged some time in the Mesozoic. Like the larger continents of the present day, this Archaean land was dominated by mountain-chains, tablelands, and plains, and its coasts were deeply indented with bays and estuaries. Though no trace of this Pacific continent now remains, the pile of sediments derived from the wear-and-tear of its surface tells us that it was no arid land, but possessed an abundant rainfall. Moreover, there is evidence that in the Cambrian, Devonian, and Permian epochs its alpine chains were covered with an ice cap from which tongues of ice reached down the mountain-glens towards the sea.
The great rivers which drained the highlands built up mighty deltas along the ancient strands, covering the floor of the seas where New Zealand now stands with sands and muds many thousand feet in thickness. But we must not assume that deposition was continuous in the New Zealand area throughout the whole of the Palaeozoic. No rocks of Devonian or Carboniferous age are known in New Zealand, and from this we infer that during a great hiatus, the exact limits of which are not yet definitely ascertained, there was a cessation of deposition on the floor of the seas covering the area now contained within our borders. The cessation of deposition on a sea-floor may arise from the profound submergence of the land area providing the sediments, or from the uplift of the sea-floor as a consequence of crustal folding or a recession of the sea. By submergence the scene of deposition is shifted landward, and by uplift seaward. The absence of Devonian and Carboniferous rocks leaves a tremendous gap in the geological history of New Zealand, and is ascribed to crustal folding that raised the sea-floor, thereby enlarging the borders of the ancient Palaeozoic continent.
In the late Carboniferous there began a general transgression of the sea that submerged the coastal lands and permitted the deposition of the Permo-Carboniferous Maitai series on the folded rocks of the Silurian and
older epochs. The succeeding Permian was an epoch characterized by earth-movement, and the intrusion of granitic and dioritic magmas on a gigantic scale.
The Palaeozoic formations contain an abundant marine fauna that in many respects shows a curious resemblance to the contemporary faunas of Europe and North America; but of the Psilophytales—the rootless and leafless land-plants of pre-Devonian Europe—and of the prolific Carboniferous flora of the greater continents there is no trace in New Zealand. For an explanation of this we must look to the land-movements that brought about the great Devonian-Carboniferous hiatus. And this leads to the surmise that the ancient continent on the shores of which the Palaeozoic sediments of New Zealand were laid down had no direct land connection with the Palaeozoic land areas of the Northern Hemisphere—a surmise further strengthened by the absence of the typical Glossopteris flora of the hypothetical Gondwanaland of the South Pacific.
But to return to the New Zealand area. After the cessation of the Permian diastrophic movements already described, normal deposition continued without interruption throughout the Triassic and Jurassic epochs, the sediments being derived from the denudation of the ancient continent, which was now larger in area, having been augmented in size by the addition of the uplifted Palaeozoic rocks of the New Zealand area. The Mesozoic sediments consist mainly of alternating beds of deltaic sands and muds, in places intercalated with marine beds containing a rich assemblage of fossil molluscs that in general facies bears a striking resemblance to the contemporary Mesozoic marine faunas of Europe. It is noteworthy that, though beds of limestone are common in all the Palaeozoic formations no limestones occur among the Juro-Triassic strata of New Zealand. This circumstance may possibly be ascribed to the prevailing deltaic conditions of deposition, which, as we know, would not favour the growth of limestone-building organisms.
Up till the close of the Jurassic epoch New Zealand had not come into existence, but for a million centuries rocky materials had been accumulating on the site it was destined to occupy. The early Cretaceous witnessed the birth of the new land. At this time there began two syntaxial crustal movements that folded and ridged the Mesozoic and older rocks into great chains. Of these, the Rangitatan, a north-east and south-west movement, produced the main alpine chains, and the Hokonuian the northwest and south-east transverse chains. These movements built up and gave definite form to the framework of New Zealand as we now know it. They were accompanied by rock-shattering and faulting, and the extrusion of igneous magmas, mostly of basic and ultra-basic types. This period of intense crustal movement also brought about the foundering and submergence of the ancient continent that had existed in the south from Archaean times, shedding the materials of which the Palaeozoic and Mesozoic rocks were mainly composed. The disappearance of the parent continent was doubtless a consequence of the process of crustal adjustment, or compensation, arising from the emergence of New Zealand from the floor of the ocean.
The aphorism of Plato that a country is only as old as its mountains contains more than a grain of truth, and in the case of New Zealand is actually true. The mountain-chains came into existence in the early Cretaceous, and it was in that epoch that the real history of New Zealand as a geographical unit began.
The Cretaceous and Tertiary formations are marginal deposits mainly composed of materials derived from the wear-and-tear of the axial chains. The post-Albian history of New Zealand is a chronicle of denudation, submergence, uplift, faultings, vulcanicity, glaciation, and river erosion, all of which have taken an active part in modifying and shaping the topographical forms with which we are familiar.
In the early Cretaceous the foothills, transverse chains, and even the lower parts of the axial chains became worn down to a peneplain that bordered the coast on all sides. When the peneplain became submerged by the mid-Cretaceous transgression of the sea, the area of the dry land was correspondingly diminished. It is probable that the New Zealand of this period was represented by a long narrow island, or by a chain of islands, of moderate relief, deeply indented with bays and sounds, and drained by numerous small streams. The submerged peneplain was now a sea-floor, and on it accumulated the marginal pile of Cretaceous and Tertiary sediments, which as partially consolidated and elevated strata may now be seen fringing many parts of the sea-coast in both Islands, and as down-faulted blocks on the flanks of the alpine chains. When we speak of the marginal pile of Cretaceous and Tertiary strata we do not wish it to be inferred that deposition was continuous. As a matter of fact, we know that it was broken by a considerable hiatus in the early Eocene.
The Cenomanian transgression was preceded by the deposition of deltaic and estuarine silts and muds, on the emergent surface of which there grew a dense jungle vegetation. The vegetable remains were subsequently buried by the sediments laid down by the advancing sea, and afterwards formed the coal-seams of our Upper Cretaceous measures. The Upper Cretaceous strata constitute what is called the Waipara formation.
At the close of the Cretaceous there was a general uplift which lasted well into the Eocene. During this uplift the greater part of the newly formed Cretaceous sediments was removed by denudation, thereby uncovering the pre-Albian peneplain.
Towards the close of the Eocene there took place another transgression of the sea, which was preceded by the deposition of deltaic sediments on the surface of the recently uncovered peneplain. A jungle vegetation flourished for some time on the emergent deltaic flats. Afterwards the marine sediments laid down by the advancing Miocene sea covered and preserved the vegetable remains.
The late Cretaceous and older Tertiary movements were unaccompanied by crustal folding, and as a consequence the stratigraphical break between the Upper Cretaceous and Lower Tertiary formations is generally insignificant. But in New Zealand, as elsewhere, the close of the Cretaceous witnessed momentous faunal changes.
The Miocene coal-measures and the associated marine strata that cover them constitute the well-known Oamaruian formation.
In the central, or Cook Strait, area deposition continued uninterruptedly till the close of the Pliocene, but in the north Auckland region and in that part of the South Island lying to the south of the Trelissick Basin marine strata of Pliocene age are absent. So far as we know, the Tertiary succession of marine strata in the far north and south of New Zealand ended with the deposition of the Awamoan beds, of Upper Miocene age. The Awamoan is the closing member of the great Oamaruian formation as developed in north Auckland, south Canterbury, Otago, and Southland. The abrupt
cessation of marine deposition in these parts is, I think, a good reason for considering the Oamaruian a distinct geological unit in the chronological succession of the Tertiary formations.
A cessation of deposition, or even an abrupt faunal change, is merely the expression of a geographical change. The absence of marine Pliocene deposits in the north and south of New Zealand must be regarded as a consequence of a differential elevation that raised the sea-floor in these parts till it became dry land, but did not affect the central region till late in the Pliocene.
At the close of the Pliocene the differential uplift became general throughout the length and breadth of New Zealand, the movement being more rapid along the axis of the main chains than towards the coasts to the east and west. The unequal upward movement raised the marginal cover of Tertiary strata high up on the flanks of the upraised axial chains, and at the same time subjected the rocks composing these chains to stresses that found relief by the formation of powerful faults. The major faults run more or less parallel with the trend of the folded chains. Thus in western Southland and in Otago they run north and south; in Canterbury, Marlborough, Wellington, Hawke's Bay, and south Auckland, north-east and south-west; in eastern Southland, north Nelson, and north Auckland, north-west and south-east.
The ancient peneplain, Tahora, which for a hundred thousand generations had exercised a powerful influence on the arrangement and distribution of the younger formations, now became deeply dissected, and for the most part almost obliterated, by the intense pluvial and glacial erosion of the Pleistocene period. In almost all cases the lines of dissection followed fault-planes, along which the rocks were, as a rule, shattered, and hence incapable of offering an effective resistance to the turbulent mountain-streams and the ponderous advance of the Pleistocene ice-sheet. The dissection of the uplifted peneplain was preceded by the removal from its surface of the covering strata, except along the coast and in the trough-faulted mountain-basins, where they were in some measure protected from the full activities of subaerial denudation.
The Pliocene uplift, elsewhere called the Ruahine movement,* gave the finishing-touches to the structure of New Zealand. The crustal dislocations and faultings which accompanied it determined the lines of the great trunk rivers, already well established in their courses when the Pleistocene refrigeration began. The Pleistocene glaciers descending from the alpine chains ploughed out and deepened the valleys, smoothed the contours of the mountain-slopes, and wore down to rounded hummocks the rocky ridges lying in their path. Before their final retreat they piled up vast moraines that will always remain as imperishable monuments of the iron grip in which in the near past the great Ice King held this now sunny land.
[Footnote] * See head of p. 67 of this volume.