Oligocene (map symbol L)

Distribution
Oligocene rocks are most widespread in the northwest of the South Island, the north and west of the North Island and in Southland and Otago.

Content
The rocks are mainly sedimentary, with limestone, calcareous siltstone and glauconitic sandstone predominant. Volcanic rocks of this age are found in Canterbury and north Otago.

Oligocence

Oligocene deposits are almost entirely of marine origin and are characterised by limestone or other calcareous sediments. In the Te Kuiti area of the North Island, limestone is widespread and there is extensive cave development (e.g. Waitomo) and karst topography. Calcareous sandstone and mudstone are also present in this area. In Northland, limestone is the predominant rock type of this age, whereas glauconitic sandstone, mudstone, and minor limestone are more common on the east coast of the North Island, Marlborough, and north Canterbury. Limestone and calcareous mudstone are widely developed in the west of the South Island, in south Canterbury and Otago, but here a high glauconitic content is typical. Near Lake Wakatipu small slivers of limestone and glauconitic sandstone are preserved, indicating the original wide extent of marine deposition in the Oligocene. Even so, some areas of Central Otago and Southland were above sea level and have terrestrial deposits of coal measures dating from this time and extending into the Miocene, and the lignite resources in these deposits are very extensive.

Paleogeographic conditions
The marine transgression that occurred in the Paleocene and Eocene continued in the Oligocene until the New Zealand area was almost entirely submerged. A shallow marine platform, remote from land-derived sediments and swept by stong currents, was the environment in which many notable fossils such as dolphins and penguins were preserved.

Deformation
The tectonic conditions were generally quiet and original deformation is slight, although some warping occurred in north Otago and south Canterbury.