Miocene (map symbol M)

Distribution
Miocene rocks are well preserved throughout New Zealand, although previously they were even more extensive. The greatest extent of sedimentary rocks is in the west of the North Island, but there are large areas on the east coast too. Miocene sediments occur in patches right around the South Island, but are particularly extensive in the northwest and far south. Volcanic rocks of Miocene age are found in Northland and Coromandel, and they also form Banks and Otago peninsulas and several small peaks areas in eastern Otago.

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Content
Most Miocene sediments are marine in origin, particularly in the North Island. The most typical rocks are sequences of alternating sandstone and mudstone, particularly in the north and west of the North Island. Around Auckland, the Miocene is typically represented by rather soft alternating mudstone and sandstone, with more massive beds of sandstone, and minor occurrences of conglomerate, grit, and limestone. There are large areas of Miocene rocks from south of the Waikato River to north Taranaki. These rocks are mainly alternating sandstone and mudstone sequences, massive mudstone, and rare limestone. Coal measures are rare. The Upper Miocene is characterised by the development of concretionary mudstone and more common conglomerate. Tuffaceous bands are common in the Upper Miocene, particularly in the northern districts. On the east coast of the North Island, Miocene rocks include massive blue-grey sandstone and mudstone (sometimes known as papa), mudstone with minor sandstone and conglomerate, and alternating sequences of sandstone and mudstone.

In Northland and at East Cape, mixtures of Cretaceous to Miocene sedimentary and igneous rocks were emplaced in the Early Miocene, by sliding over the underlying sedimentary rocks. These are referred to as the Northland and East Coast allochthons, and they consist of sheets of rock separated by faults and transported by tectonic processes some distance from their original sites of deposition.

miocene

Miocene deposits in southern Wairarapa and northeastern South Island are characterised by large amounts of marine conglomerate, particularly in the Wairau and Awatere valleys. Calcareous mudstone and sandstone are more common nearer the coast. In inland Canterbury non-marine deposits of sandstone, conglomerate, and mudstone are typical of the Late Miocene whereas marine mudstone and coarse sandstone are representative of earlier Miocene times, as well as being more common nearer the coast and further south into north Otago. A phosphatic sandstone is the only marine sediment south of Dunedin, with all other Miocene deposits in the southeast being non-marine, including coal measures and oil shale. Marine deposition, however, continued in the Waiau River area of Southland, with a sequence of mudstone, limestone, micaceous sandstone, and coarse sandstone.

The northwest of the South Island from Greymouth through to Nelson is an area of extensive Miocene deposition which is characterised by great thicknesses of brown or blue-grey mudstone and sandstone, locally in alternating sequences. The youngest Miocene rocks here are non-marine conglomerate, sand, and coal measures. Further south near Jackson Bay, a small area of limestone, conglomerate, and alternating sandstone and mudstone is of Miocene age.

Volcanic activity levels were high in the Miocene. Andesite, basalt, and dacite with volcanic breccia in Northland are but remnants of huge volcanoes that existed off the present west coast. The Waitakere Ranges, west of Auckland and the hills near Tokatoka, further north, are composed of andesite, volcanic breccia, and volcanic sediments, while basalt is present near Waipoua. Andesite, dacite, and rhyolite around Whangarei probably represent separate volcanic outlets. In the Coromandel area andesite and volcanic breccia are the most common Miocene rocks, although there are some areas of basaltic andesite as well as areas of rhyolite and dacite. Many of the offshore islands of Coromandel and Northland are of Miocene volcanic origin. Minor plutonic intrusions were associated with the volcanic activity, including the granite and diorite of Doubtless Bay, and diorite south of Cape Colville and that making up Cuvier Island. In the South Island, Miocene volcanoes erupted mainly basaltic lavas to build the Otago and Banks peninsulas. The two harbours of Banks Peninsula are both breached volcanic craters. The Otago Peninsula is made up of basalt with a high alkali content, and the harbour here is the result of later downwarping. Miocene basalt and tuff also occur in inland Otago and mid Canterbury.

Paleogeographic conditions
At the end of the Oligocene the New Zealand area was almost entirely under water. At the beginning of the Miocene, a new round of tectonic activity and mountain building started. This had the effect of uplifting the central part of the South Island, so that coastal areas were still experiencing marine conditions while the environment inland became progressively non-marine. The vast quantities of conglomerate in the Marlborough area result from rapidly rising and eroding ground. The tectonic movements, however, were not simply uplift; there were varying degrees of uplift, down warping, and buckling caused by the tectonic stresses. Thus in the west of the South Island a series of parallel basins developed which were continually downwarping and collecting vast quantities of sediment, especially around Murchison. In the Waiau River area in Southland another trough, which had developed at the end of the Eocene, continued to deepen and collect sediment throughout the Miocene. In central Otago, a large basin which developed during this time was progressively filled with lake sediments, quartz sand, silt and peat which became lignite.

In the North Island the pattern was one of isolated basins continually developing, subsiding, and filling, a pattern which was particularly well developed in the east coast area where the rocks can vary markedly in type and thickness within a few kilometres. There is no one place which has an unbroken record of the Miocene (an indication of the general mobility); terrestrial deposits interbedded with marine deposits confirm this. The sediments would all have been derived from the eroding centre of the North Island which was above sea level, and mainly deposited in fairly shallow water. In Northland, and probably the East Cape area too, the tectonic movements were sufficiently violent to disrupt the already deposited, mainly consolidated rocks of pre-Miocene age, in such a way as to transport them from their original depositional site and emplace large blocks in a chaotic manner in another site. This may have taken place by large-scale submarine sliding.

Volcanic activity is generally associated with crustal movement and the Miocene is no exception. The effect of the volcanism was widespread, because many of the sediments have interbedded tuff layers.

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Deformation
The Miocene saw the start of a new phase of crustal activity and mountain building - the Kaikoura Orogeny. The movements continued throughout the Miocene and on through the Pliocene to the present day. The effects, however, varied from place to place. In some areas such as eastern Southland, tilting was gentle, while elsewhere, as described above for Northland and East Cape, tremendous movement took place. In general, deformation was more severe in the east than the west so that local folding and faulting from Wairarapa to north of Gisborne is very marked. Soft-sediment deformation is also common, as seen in slump folding within bedded units. In the west of the North Island beds are folded and faulted, but not so severely, and locally horizons can be followed for some distance. In the South Island the rapidly rising Southern Alps caused marked tilting, and debris flows are common, particularly in the northeastern area. On the western side the activity appears to have been more gentle, with broad open folds, although there are local areas of more intense deformation, particularly associated with fault movement.