Paleocene to Oligocene
In many areas in New Zealand with marine Tertiary deposits the most typical strata are thick deposits of crumbly sandstone or mudstone, often with abundant fossils; conglomerate and coarse-grained limestone beds are common, too. However, the earliest Tertiary deposits (Dannevirke Series — Paleocene and lower Eocene) resemble more the late Cretaceous deposits and include thin-bedded mudstones and light-coloured bentonitic mudstones with greensand bands. In Marlborough, Canterbury, and the eastern Wairarapa area of the North Island hard, smooth, white, fora-miniferal limestone is a distinctive early Tertiary deposit. Some of the early Tertiary bentonite muds of Gisborne, Hawke's Bay, and Marlborough are pure; they are quarried at Porangahau in Hawke's Bay for use as drilling mud.
During this time when the sea was spreading slowly from the east, low land still lay where western New Zealand is today; the bulk of this country's coal deposits accumulated then in swamps on the low-lying land. These younger coal deposits, ranging in age probably through the Paleocene and Eocene periods, are known as the Quartzose Coal Measures; the main coalfields in these rocks are Greymouth, Buller, and the Waikato. Marine sediments buried the coal first in Westland, where the sea in the middle Eocene began to advance from the south-west. The transgression was slow and, in the Waikato coal-field, submergence was delayed until as late as the Oligocene.
The first paleogeographic map of diagram 7 is an interpretation of the distribution of land and sea late in the Eocene period, when the younger strata of the middle and upper Eocene Arnold Series were deposited. In North Auckland greensands, argillaceous limestones, and green and chocolate shales were the earliest Arnold sediments to accumulate, followed by sandstones; in the eastern North Island basins sedimentation continued, light-grey silty mudstones being deposited, for example, in southern Hawke's Bay. The thickest sediments of the Arnold Series accumulated in basins in Nelson and Westland. A miniature geosyncline developed at the site of the present Paparoa range near Greymouth, and a thick sequence of sediments buried and preserved the coal seams; sediments also began to accumulate in the Murchison Basin. Seas had reached far inland over the eastern South Island by the late Eocene. Deposition of Amuri limestone continued in Marlborough. On the stable, shallow shelf of Canterbury and Otago a belt of glauconitic greensand was deposited. This was followed by mudstone in eastern Otago, with limestone in some areas. In the Waiau Syncline of Southland Arnold sandstones, conglomerates, and coal measures accumulated; thicker sediments were deposited there in the Oligocene, and are exposed with the coal measures in some of the mountains that border Te Anau. The extent of the submergence of New Zealand in the early Oligocene period is shown by the second paleogeographic map of diagram 7.
Where the sea floor was shallow and yet far enough from land to be clear of pebbles, sand, etc., the only sediments able to accumulate were made up almost entirely of the skeletons of marine organisms, such as brachiopods, foraminifera, and sea eggs, whole or broken into pieces, large and small, down to microscopic size. These were bound together to form a hard limestone by calcareous cement derived by solution and reprecipitation of the shell substance. Large areas of limestone were formed in this way in the Oligocene in New Zealand. Today these rocks are the main source of lime for agriculture and cement; in some areas they are hundreds of feet in thickness. Cement works at Te Kuiti, Tarakohe in Golden Bay, and Cape Foulwind near Westport are among the largest that use this limestone, which has recently been investigated for possible use as a raw material for a carbide industry. Well known scenic features formed in Oligocene limestone include the Waitomo and Te Anau Caves, and the Punakaiki Pancake Rocks near Westport.
Nearer the diminished land areas, clastic sediments, such as conglomerates, sandstones, and mudstones, were still able to accumulate. Siltstone and sandstone were laid down in limited areas of the sea floor in North Auckland and in the Waikato. Sedimentation continued in the eastern North Island basins and in the Waiau basin, and also began in the Taranaki basin.