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.