Discovery of temperate NZ limestone
Sedimentary geologist Professor Cam Nelson’s research has centred on the discovery that limestone can form in colder waters, not just in warm tropical seas as was originally thought. His investigations into these cold-water formations, known as cool-water or temperate limestones, have enabled him to define the key sedimentological and chemical differences between these deposits and those of tropical limestones
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.
Prior to the 1980s, geologists were of the view that limestones were formed in shallow tropical waters with warm sea temperatures (above 23°C) and high carbonate seawater saturation.
However, a careful study conducted by New Zealand geologists such as Cam compared New Zealand limestones with those from tropical regions and revealed many significant and distinctive differences.
For example, the tropical limestones had a high abundance of calcium carbonate in the aragonite crystal form, whereas the New Zealand limestones were predominantly in the calcite mineral form. The New Zealand limestones were also 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 redeposited, 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.
The major limestone-forming epoch in New Zealand was 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 for conversion to limestone. Therefore, the limestones that feature in the Waitomo region are the same as those in the Whangārei area, the Punakaiki rocks of the West Coast, Oamaru stone in Otago and the Clifden Cave system in Southland.
Microscopic analysis of these rocks shows the presence of lithified shell fragments from the same types of invertebrate organisms such as bryozoans, molluscs, gastropods and echinoderms.
Bryomol shell debris
Bryomol shell debris from the seafloor off northern New Zealand. These extensive skeletal remains of bryozoans and molluscs will eventually be converted into temperate limestone.
There are two areas of New Zealand in this modern time where it is possible to see limestone in the making. These areas are rich in the skeletal remains of marine invertebrates such as bryozoans and molluscs that secrete calcite. Eventually, over millions of years, these deposits will transform into limestone with exactly the same characteristics that are found with current New Zealand limestone deposits from the Oligocene.
It is the make-up of the limestones that defines the temperate limestones compared to the tropical limestones, and it was this ground-breaking New Zealand research that led to the acceptance of the temperate model of limestone formation.
