Cretaceous (map symbol K)

Distribution
Cretaceous sedimentary rocks are widespread, and are exposed in Northland, East Cape, Wairarapa, Marlborough, Westland, Canterbury and Otago. A feature of Cretaceous time was igneous activity, and granite occurs in many places in the South Island, while volcanic rocks are widespread in Northland, East Cape, Marlborough and parts of Canterbury.

Content
The Cretaceous was a time of widely differing depositional environments resulting in varied rock types. In the Marlborough, Wairarapa, and East Cape regions, greywacke deposition continued and plant fossils of late Early Cretaceous time have been found in some of these rocks. Marine fossils of Early to middle Cretaceous age have been found in Marlborough, Wairarapa and East Cape. These rocks, although similar in appearance to the bulk of the greywacke (i.e., sandstone and mudstone), are actually softer and deformed quite differently, with slump folds and internal folding indicating deformation while the rocks were still soft. Other rocks of mid-Cretaceous age in Northland, East Cape, Wairarapa, and Marlborough are undeformed alternating sandstone and mudstone with chert, limestone, and spilite beds. Towards the end of the Cretaceous some areas may have been briefly emergent, but by the end of the Cretaceous there was a return to marine conditions, and the sediments deposited then were sandstone, commonly glauconitic (greensand), and siliceous mudstone.

The only fossils of land-dwelling dinosaurs in New Zealand are preserved in Late Cretaceous rocks of inland Hawkes Bay. Large marine saurian fossils are known from Late Cretaceous rocks of Hawkes Bay, Marlborough, north Canterbury and Otago.

Although the Cretaceous rocks are dominantly marine in the North Island and the northeast of the South Island, the rocks in the rest of the South Island are mainly of terrestrial origin. Areas of coarse conglomerate and breccia occur in Westland and Otago, and are of mid-Cretaceous age. Terrestrial sand and conglomerate of similar age occur at Puysegur Point in southern Fiordland. In the coastal areas and in inland Canterbury coal measures and terrestrial sand and mud are of Late Cretaceous age. Some of New Zealand's biggest coal mines are in these Cretaceous rocks, particularly near Greymouth, but also at Kaitangata, Ohai, and Collingwood.

igneous

Igneous activity was widespread in the Cretaceous. Large areas of granite at Separation Point, the west of the South Island, Fiordland, and Stewart Island were intruded during the Cretaceous. Granite was intruded into older igneous complexes such as those at Lake Rotoroa and Longwood Range, and in Fiordland, causing reheating of rocks and the formation of metamorphic aureoles and hybrid rocks by the assimilation of chunks of older solid rock into the molten magma. Aureoles are also common in sediments. The plutonic activity was mainly acidic except in Marlborough where a basic layered complex of gabbro and diorite in association with extensive dolerite dike intrusion forms Tapuaenuku and surrounding parts of the Inland Kaikoura Range. Basalt rocks nearby are the extrusive phase of activity. Further south in mid Canterbury, rhyolite and dacite are present at Mt Somers and the Malvern Hills, together with minor andesite.

In the North Island, volcanic activity was widespread in Northland and East Cape where basalt (with pillow form) and minor keratophyre are interbedded with sediments. (This activity continued into the Paleocene in Northland and the Eocene at East Cape.) A small gabbroic intrusion associated with serpentine occurs at North Cape and other isolated small dioritic bodies are present in Northland.

Paleogeographic conditions
The Cretaceous was a time of great environmental change. At the beginning of the Cretaceous came the climax of the mountain-building phase of the Rangitata Orogeny and the widespread igneous activity associated with it. Thus the New Zealand area was mainly a mountainous landmass, especially in the southern and western areas. A trough still existed in the east, however, and marine deposition continued. The stirrings of the crust were reflected in the deformation of these soft sediments. As conditions became more stable, the mountainous land was eroded virtually to a flat plain.

Rapid erosion is indicated by the thick breccia deposits of Central Otago and south of the Buller River. Swampy plains developed at the coastal margins and in low-lying areas, collecting and preserving dead vegetation that would eventually become coal. Finally at the end of the Cretaceous, coastal areas were invaded by the sea, especially in the east, and marine conditions again prevailed.

A major event late in the Cretaceous was the gradual splitting off of the New Zealand area from Australia. A spreading ridge developed in the area that was to become the Tasman Sea; this ended New Zealand's long association with the Australian continent, and from the Late Cretaceous New Zealand has had independent geological, faunal, and floral development.

Deformation
The Rangitata Orogeny continued in the Early Cretaceous and deformed earlier rocks and accumulating marine sediments. However, the later part of the Cretaceous was generally a quiet time, and many Cretaceous rocks are not severely deformed. Later tilting, folding, and faulting associated with the Kaikoura Orogeny is responsible for most deformation seen.