The 4-D land
Before the end of this decade, New Zealand will have a new map of its geological self. The project is known as QMAP, short for “quarter million” and indicative that the scale of the new map is 1:250,000.
The Institute of Geological and Nuclear Sciences (GNS) commenced the work in 1994 and is about half way through the project. To cover the whole country, 21 map sheets are required. The latest to be produced is that of the Wellington area, and its release happens to mark a significant anniversary year for geological mapping.
In 1751, Jean Etienne Guettard, a doctor of medicine, produced the first geological map and memoir of an entire nation or country: France. The map showed the distribution of rocks, minerals and fossils using shading and symbols comparable to those used in modern geological maps.
New Zealand has been mapped at 1:250,000 before, a project that occupied the New Zealand Geological Survey through much of the 1960s until it was completed in 1971.
Why map the country again? Only 30 years have elapsed. Can the geology of New Zealand have changed very much in that time? Certainly not, but scientific understanding of geological processes has increased dramatically, as has knowledge of the subsurface and the ability to determine the age of rock formations.
Unlike the older map, QMAP embraces plate tectonics, the unifying theory that explains the motion of the Earth’s crust. This theory emerged only in the late 1960s. Geophysics, another relatively new discipline, has played a major role in determining the nature of the subsurface through investigations of the gravitational, magnetic, electrical and wave properties of the Earth’s crust.
Unlike the 1:250,000 map, QMAP is able to attribute accurate geological ages to all New Zealand rock formations. The application of age determination using fossils (referred to as biostratigraphy), and radiometric dating of minerals and rocks (referred to as geochronology), has advanced markedly over the past 50 years, and for NewZealand much of this dating work has been accomplished only within the last 30 years.
Whereas the older map provided an accurate two-dimensional picture of the surface geology of New Zealand—the “skin” of our landmass—QMAP offers a much more thorough and satisfying interpretation of the “body” itself; the third and fourth dimensions are greatly filled out. The corporeal essence of our national real estate is now much better understood. So, too, are our natural resources—and hazards.
On the hazard side, the new map shows that Cook Strait—a central feature in the new Wellington sheet—sports several submarine pinnacles lying at water depths of less than 20 metres at low tide. During conditions of exceptionally low tides and stormy weather, Fisherman’s Pinnacle and Hunters Pinnacle must constitute serious threats to shipping.
Fisherman’s Pinnacle lies in a central position between Mana Island and The Brothers. According to fishers, it is shear-sided and about the size of the BNZ building in downtown Wellington. This spike of rock rising off the seafloor is an attractive habitat for fish and a Mecca for those who would catch them. It is shown on the Wellington QMAP sheet as belonging to the same rock formation as Arapawa Island in the Marlborough Sounds, and different from that of the Wellington area of the North Island.
Hunters Pinnacle is located halfway between Mana Island and Kapiti Island. This rock formation is also like that of Arapawa Island, but, more importantly, it is identical to rocks on Kapiti.
New analytical data acquired since the QMAP Wellington sheet was published suggests that Kapiti is a drowned extension of a South Island rock formation, and that, although composed of greywacke, it is quite different from the greywacke of the Wellington area.
Detailed studies of New Zealand greywackes by GNS scientists have established that the various belts of greywacke that make up so much of New Zealand can be fingerprinted on the basis of their strontium isotope signature, and it is this new technique that has been used in the Kapiti analysis.
It has long been recognised that there is a distinctive rock called phyllonite on the eastern side of Kapiti. It has a distinctive platy texture generated by shearing at depth within the crust, such as would be expected within a fault zone, and has always been attributed to a fault within the Wellington greywacke.
This phyllonite is now seen as denoting a much more significant boundary—a major fault, though not necessarily an active one, between Kapiti and the North Island. Kapiti appears to be a remote finger of the South Island.
The Wellington and Marlborough greywackes clearly have different histories and may have started life as sedimentary basins quite separate in space and time which have been subsequently shunted into juxtaposition by plate tectonic processes.
Knowledge of New Zealand’s submarine geology is steadily accumulating from the ongoing Taranaki-based oil and gas exploration industry. It turns out that there are substantial sedimentary basins in the Cook Strait area that may ultimately become attractive targets for oil and gas exploration. Deep-water drilling platforms in Cook Strait are far from impossible.
New information on the seafloor around New Zealand is also being gained as telecommunications cables are buried in the seabed, and the National Institute of Water and Atmospheric Research studies our oceans.
QMAP is incorporating much of this new information on the submarine geology around New Zealand.
QMAP will be complete by 2009, if not sooner. After that, perhaps we can expect a map of the submarine geology of the entire extended economic zone.