New Zealand limestone origins
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.
Geological timescale section
A section of the geological timescale from 206 million years ago to the present showing the era, period, epoch and age in millions of years (mya).
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.
Discovery of temperate limestones
Professor Cam Nelson explains how his study of limestones in the King Country region led him to conclude that they were formed in cool temperate environments. At that time, it was believed that all shelf limestones had a tropical origin. Cam’s work was instrumental in developing the temperate cooler water limestone model that now sits alongside the tropical warm water model.
Points of interest
Some of the micrographs and samples shown in this clip:
Micrographs a and d: – thin section images (x40) of bryozoan-dominated temperate limestone
Micrograph b: Ōtorohanga limestone thin section showing an abundance of bryozoan and small foraminifera skeletons.
Sample c: Sediment referred to as ‘bryomol’ – rich in the skeletal remains of bryozoans and molluscs. Dredged from the seafloor off Three Kings Islands to the north of New Zealand.
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.
Comparing tropical and temperate limestone
Professor Cam Nelson explains how tropical limestone formation in warm seas involves coral reefs and chemical precipitation. The warm water becomes saturated with calcium carbonate, which chemically precipitates to form solid grains. Temperate limestone forms through a different mechanism. It is the smashed up shells of cool-water sea organisms that provides the material needed to form this type of limestone.
Points of interest
Look for this micrograph
Micrograph a: Thin section of ooids (x12.5), which are spherical balls of inorganically precipitated calcium carbonate.
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.
Limestone in thin section
Thin section of limestone as seen under the microscope. The presence of the shell remains of invertebrate organisms such as bryozoans and molluscs can be readily seen.
Analysing limestone
Dr Steve Hood explains that, by viewing a thin section of limestone rock through a petrographic microscope, detailed information about its origin and composition is revealed. Further information can be obtained by using a technique called cathodoluminescence. In seep carbonate work, it allows the calcium carbonate minerals, aragonite and calcite, to be identified by the colours that they emit.
Point of interest
Look for this micrograph
Micrograph a: Thin section image of a cold-seep carbonate as seen under cathodoluminescent light. The blue regions indicate the presence of aragonite and the red/pink regions calcite.
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?
What is a fossil?
Professor Cam Nelson, on site at a fossil-rich limestone outcrop, defines the term ‘fossil’. This particular limestone outcrop is full of giant oyster fossils, with some 30 cm in size. Cam describes the likely conditions that these oysters once lived in using the present conditions in Foveaux Strait, a rich oyster fishery, as an example.
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.
Modern carbonate sediment locations
The locations to the north and south of New Zealand of thick sedimentary material consisting of fragmented carbonate skeletons. Over time, these sediments will be changed to limestone.
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.
Older limestones
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.
Marble columns
The columns and walls of Parliament House (the central buildings of the parliamentary complex) are made of grey Tākaka marble from Kairuru quarry – marble is metamorphosed limestone.
Image courtesy of the Office of the Clerk/Parliamentary Services’ which is licensed by the Clerk of the House of Representatives and/or the Parliamentary Corporation on behalf of Parliamentary Service for re-use under the Creative Commons Attribution 4.0 International licence.
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.
