Greywacke

This rock type, or association of rocks, is found in many areas of New Zealand and spans from Carboniferous to Early Cretaceous in age. A general summary is given in this section. Greywacke rocks in the South Island are subdivided into major blocks known as the Caples and Torlesse terranes, while those of the the North Island are divided into Torlesse and Waipapa terranes.

Content
The dominant rock types are hard grey sandstone and darker coloured mudstone (argillite). These generally form alternating sequences with the sandstone predominant, and together the sandstone and mudstone are often referred to jointly as "greywacke". Minor rock types found interbedded with, or faulted into, the greywacke include limestone, chert, conglomerate, spilite, and chaotic units of melange and broken formation.

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Metamorphic rocks present in the South Island, west coast of Wellington and the Kaimanawa Mountains are thought to be metamorphic equivalents of greywacke, and of the same age range. The boundary between greywacke and schist is transitional and in the less metamorphosed part of the sequence, sedimentary features from the parent rock can still be seen. These sedimentary features become obliterated as the metamorphic grade increases. Metamorphosed volcanic bands become a characteristic green colour (greenschist). Most of the schist in Otago and Marlborough falls into the chlorite metamorphic zone, which itself is divided into three sub-zones depending on the metamorphic minerals which develop in the rock as the metamorphic grade increases. The highest metamorphic grades (biotite, garnet, and oligoclase) are restricted to the western parts of Otago and the Southern Alps.

Although the greywacke rocks contain few age-diagnostic fossils there does seem to be a general pattern of age distribution. In north Otago and south Canterbury, rare Carboniferous fossils are recorded. Permian fossils are present in the same general area including the Waitaki valley, and it appears that this area has the oldest greywacke. Further west and north into mid Canterbury, Triassic fossils are present, while further north again into north Canterbury and Marlborough, Jurassic and Early Cretaceous fossils occur, demonstrating a general younging trend to the north and east.

In the North Island, the main Tararua, Ruahine, and Kaimanawa ranges appear to be mainly Triassic to Jurassic in age while the Wairarapa and Raukumara greywacke ranges to the east and northeast are younger (Jurassic to Early Cretaceous). Northland, Auckland and Coromandel have greywacke of Triassic to Jurassic age.

Deformation
Greywacke rocks are commonly extremely deformed, fractured, and veined, and mudstone in particular is commonly very sheared. Some deformation took place within the sedimentary pile, but the severe folding, faulting, and shearing were caused by the mountain-building episode of the Rangitata Orogeny, and the rocks were further deformed by the later Kaikoura Orogeny. In many areas it is difficult to see original sedimentary features because of severe deformation.