Most of New Zealand’s limestone deposits were formed in the Oligocene epoch some 23–34 million years ago. The properties of these limestones led New Zealand geologists to propose that their origins were temperate and not tropical.
Tropical versus temperate
Prior to the 1980s, geologists were of the view that limestones formed only in shallow tropical waters with warm sea temperatures (above 23°C) and high carbonate seawater saturation levels. However, studies conducted by New Zealand geologists that compared New Zealand limestones with those from tropical regions revealed many significant and distinctive differences.
For example, the tropical limestones tend to have a high abundance of calcium carbonate in the aragonite crystal form, whereas the New Zealand limestones are dominated by the calcite mineral form. The New Zealand limestones were formed at much deeper levels than the tropical ones – depths at which a process called ‘pressure dissolution’ could occur. This process allows calcium carbonate to be dissolved out of the shell debris and reprecipitated, cementing the shell fragments together. A denser and harder limestone is formed as a result of this.
This New Zealand research added considerable weight to the global understanding of the formation of temperate limestone.
New Zealand’s limestone deposits
The major limestone-forming period in New Zealand was during the Oligocene, about 22–30 million years ago. At that time, New Zealand was covered almost entirely by a shallow sea. Many invertebrate organisms with calcareous shells thrived in this sea, and over time, a thick sedimentary layer of shell fragments developed. Subsidence followed by deep burial of these sediments provided the right conditions to convert them to limestone. Therefore, the limestones that feature in the Waitomo region are the same age as those in the Whangārei area, the Punakaiki rocks of the West Coast, Oamaru stone in Otago and the Clifden Cave system in Southland.
Petrographic analysis of these rocks shows the presence of lithified shell fragments from the same types of invertebrate organisms, principally bryozoans, bivalve molluscs, foraminifera and echinoderms.
Dating limestone using fossils
Limestone contains many fossils, and these can be used to date the rock so that the geological period of time in which the limestone formed can be determined. The fossils can also give clues as to the environment in which the limestone formed. For example, was the water shallow or deep? Marine or estuarine? Agitated or calm?
Modern New Zealand limestone
Off the northern (Three Kings Islands) and southern (Snares Islands) coastline of New Zealand, there are extensive shallow-marine platforms (<250 m deep) covered with the fragmented remains of bryozoans, molluscs and foraminifera. These deposits are >70% calcium carbonate and range in age from about 20 000 years ago to modern times. Environmental conditions in these two regions such as freedom from land-derived sediments, rugged rocky and gravelly shelf floors and energetic sea movements have permitted the shelly sediment build-ups to occur.
It is in these two regions in particular that modern New Zealand limestone is in the process of being made. Given that most on-land deposits of New Zealand limestone dating from the early Cenozoic era have a similar shell debris composition, these skeletal carbonate sediments will require deep burial over long time periods to be transformed into limestone.
The other time in New Zealand’s geological history when limestone was prominent was in the Ordovician era 450–500 million years ago. Today, Ordovician limestone is widespread in north-west Nelson. Over time, granite intrusions pushed into regions of these deposits, heating and pressurising them and effecting their conversion into marble.
Tākaka marble from the Kairuru quarry was used in the construction of the Houses of Parliament in Wellington, with a total of 5000 tonnes being used on the completion of the buildings in 1922.
Nature of science
Scientific knowledge is never absolute or certain. As geologists, particularly in New Zealand, discovered more about the formation of shelf limestones, it became clear that the tropical model was at odds with evidence being collected in temperate regions. As a result, two models now exist to explain shelf limestone formation – the tropical model and the temperate model.