Feature

A Rocky Riddle Solved

Just how old are New Zealand's oldest rocks?

Hamish Campbell

Scientists at the Institute of Geological and Nuclear Sciences have at last established the significance of some particularly ancient rock formations in eastern Southland.

The formations are exposed in the middle and lower reaches of Titiroa Stream, near Mataura Island, and are best seen in farm quarries, road cuttings and stream banks in the area. They are the oldest rocks known in the Catlins or, more specifically, in the area east of the Mataura River and south of Balclutha. They include fossiliferous limestones and volcanic sandstones of Late Permian age, 258-254 million years old.

The fossils include shells of extinct mussels (bivalves), snails (gastropods) and lampshells (brachiopods) that lived in a shallow sea. Also preserved are microscopic fossil plant spores, and some fossil wood. These fossils enable paleontologists to establish the relative age of these old sedimentary rocks as Late Permian, and other dating techniques tell us just how old the Late Permian is in terms of absolute time.

Paleontology -- the study of ancient life -- is a major activity within GNS and there are at least 15 scientists with paleontological expertise. They work on the most common fossils found in New Zealand rocks: fossil plankton, spores and pollen, and fossil shells. The use of fossils for dating is called biostratigraphy and is one of many ways in which fossils are used to solve geological problems.

"Deep" Time

Like the calendar with future time subdivided into years with 12 named months of known and fixed duration, so too is past time, including "deep" time or geological time. The names of the months are unlikely to change and nor are the names of the geological periods. Everyone should know these names off by heart! Well, maybe not... Even among scientists, only geologists and cosmologists routinely use a timescale involving deep time (1 million years) in their work. The range of precision of geological age measurements is 1-10%, not dissimilar to how precisely we humans use our own calendars.

Originally subdivided on the basis of fossils, geological time is now well calibrated in terms of actual numbers of years. The Permian Period is dated as having lasted from 292 to 251 million years ago. The Triassic Period followed the Permian and is dated as 251-205 million years, and then came the longer Jurassic Period from 205 to 142 million years ago.

Precise age calibration of the major geological time periods is constantly being refined by international research effort. In New Zealand, this work is undertaken by GNS in collaboration with research laboratories in Australia, the United States and the UK. The dating techniques are well established and involve routine measurement of isotopes of the common radioactive elements uranium and potassium that are present in minerals such as zircon and mica respectively.

Isotopes are atoms of an element with all the characteristics and properties of that element but different mass. All isotopes of a particular element have the same number of protons and different numbers of neutrons. Radioactive isotopes such as uranium and potassium decay in a systematic way and at a known rate. Radioactive decay involves the loss of protons and neutrons, in the form of (a-particles, or the conversion of a proton into a neutron (or vice versa). The isotopes formed by these processes are called daughter products. Dating of minerals involves the measurement of the relative amounts of the parent element and the daughter isotope.

Measurement is done in a mass spectrometer. By analogy, this instrument is like a high-tech sheep race. Atoms (sheep) are led down a tube (race) and a powerful magnetic field (the gate) is used to separate out atoms of different mass (the ewes from the lambs), and the atoms are counted as they pass through. Simple! From the count tallies and knowledge of the rate of daughter isotope production, it is easy to calculate how long the decay process has been operating within that particular sample and hence how old it is.

Minerals are ideal for dating because all minerals are crystalline and, as a general rule, all elements, including radioactive elements and their daughter isotopes, are securely trapped and retained within the crystal structure. It is relatively easy to extract appropriate minerals from rock and then analyze them chemically. At GNS, the potassium-argon dating facility routinely measures parent potassium and daughter argon isotopes at a cost of $650 per sample dated.

Most people have heard of radiocarbon dating, a commonly used radiometric dating method. However, this technique can only be used on carbon-bearing molecules and minerals that are less than 50,000 years old. As a tool, it is of immense value to history, archaeology and very young geological applications but obviously is not useful as a tool for dating anything older than 50,000 years. This is simply because the parent carbon decays so rapidly.

Ancient Volcanic Rocks

The old basement rocks of Southland and South Otago are almost exclusively of volcanic origin. Three quite distinct belts (terranes) of rock are recognised, each representing a separate chain of volcanoes. Geologists refer to these three belts as the Central Arc Terranes or CAT. They form a great wedge-shaped area with distinct boundaries against quite different rocks of Fiordland to the west and the schists and greywackes of Central Otago to the north.

The oldest and westernmost and southernmost belt is called the Brook Street Terrane (named after Brook Street in Nelson) and includes the Dunton Range, Takitimu Mountains, the Twinlaw and Longwood Ranges, as well as rocks exposed at Riverton and the Greenhills-Bluff area. The lava flows, scoria and ash beds that make up these rocks erupted in the Permian 285-265 million years ago.

The northernmost belt is called the Dun Mountain-Maitai Terrane (named after Dun Mountain and the Maitai River, again near Nelson) and includes rocks in the Livingstone Mountains, Countess Range, Mount Snowdon, Bare Peak, West Dome and Long Ridge. It also includes all the hard rocks that underlie the Waimea Plains and lowlands eastward of Gore through to Clinton and on to Kaka Point. These rocks are mainly sediments derived from a chain of volcanoes that erupted in the Late Permian and Triassic 285-235 million years ago.

The middle belt is called the Murihiku Terrane (named after Southland) and includes rocks that form Mount Hamilton, Braxton Ridge, Centre Hill, Wairaki Hills, Taringatura Hills, North Range, Hokonui Hills, the Kaihiku, Wisp, Rata, Beresford, McClennan and Forest Ranges and indeed the entire Catlins area. Murihiku Terrane also forms the basement rock beneath the Southland Plains. These rocks are also mainly sediments derived from a particularly long-lived chain of volcanoes that erupted through much of Triassic and Jurassic time, 245-155 million years ago.

Where Did the Volcanoes Go?

Geologists think that these three Central Arc Terranes represent the remains of classic volcanic island arcs. The actual volcanoes are either buried or destroyed; we just don't see them or recognise them in their original form. They would have been similar to the modern-day "ring of fire" volcanic island arcs, such as Vanuatu or the Aleutian Islands of Alaska. In terms of plate tectonics, they were ordinary plate margin subduction-related volcanoes that erupted as a result of the ancient Pacific Plate going under the eastern margin of Gondwana.

The Permian rocks in Titiroa Stream appear to be very different and are so much older than any other known rocks within the Triassic-Jurassic Murihiku Terrane. For these reasons they have long been regarded as probably belonging to either the Brook Street, or Dun Mountain-Maitai Terranes which both include Permian rocks. However, new research has established that this is not the case, and that they are best regarded as part of Murihiku Terrane. This means that Titiroa Stream is the site of the oldest known rocks within the Murihiku Terrane and is the only place where geologists can see what kind of rock underlies the huge thickness (more than 14,000 metres) of Triassic and Jurassic sediments within it.

Simplified geological map of southern South Island. The old basement rocks of Southland and South Otago are almost exclusively of volcanic origin. Three quite distinct belts (terranes) of rock are recognised, each representing a separate chain of volcanoes.
Geologists refer to these three belts as the Central Arc Terranes or CAT. They form a great wedge-shaped area with distinct boundaries against quite different rocks of Fiordland to the west and the schists and greywackes of Central Otago to the north. Titiroa Stream is the site of the oldest rocks known from the Murihiku Terrane.
The unshaded areas denote the extent of post-Jurassic sediments.

The rocks that make up the three Central Arc Terranes provide important information for our understanding of the evolution and history of the New Zealand landmass and indeed the wider (largely submerged) New Zealand "continent". The Central Arc Terranes can be traced the full length of New Zealand, maintaining their integrity as three belts running through the Nelson region, northwards under Taranaki. Rocks of the Murihiku Terrane are exposed in the King Country (between Awakino and Port Waikato) and occur underneath the western side of Auckland and Northland. Counterparts of the Central Arc Terranes are also recognised from submarine dredge samples on the West Norfolk Ridge, and from on-land western New Caledonia.

In so much as rocks, minerals and fossils constitute the memory banks of the Earth, the Central Arc Terranes record the history of much of New Zealand since Permian time began 292 million years ago. Of particular interest to geologists is the relationship between the three chains of volcanoes. How close were they spatially when the volcanoes were active? Why are the three chains so distinctive in terms of the chemical composition of their rocks? And most importantly, what natural resources do they contain that we do not yet know about?

Southland and South Otago are regarded as major farming regions producing fantastic pasture and hence excellent dairy milk, meat and wool. Little wonder -- these regions are exploiting some of the most fertile land in New Zealand. Part of this fertility relates to the volcanic nature of the underlying rocks and in particular the Central Arc Terranes.

Southland and South Otago from space; photo taken from a Landsat satellite, with (from L to R) the Aparima, Oreti and Mataura Rivers clearly visible.
Triassic and Jurassic volcanic rocks of the Murihiku Terrane dominate the landscape of the Southland hill country. The upstanding ridges of the Wairaki Hills, North Range, Taringatura Ranges, Hokonui Hills and Kaihiku Ranges reflect the folded structure of the rocks. The resistant sandstones have been folded into subvertical orientation.
The oldest rocks within the Murihiku Terrane are of Late Permian age and are located in Titiroa Stream, near Mataura Island.

Dr Hamish Campbell works at the Institute of Geological and Nuclear Sciences Ltd in Lower Hutt.