Reefs and atolls - but not a drop of water in sight
Thursday, 27 July 2000
When the explorer Charles Sturt went looking for an inland sea, he wasn't wrong; just a few million years too late. Oceans once covered great areas of what is now inland Australia. Their traces remain in fossils, minerals and geological formations that reveal much about how Earth has changed over time.
Dr Nicholas Lemon from Adelaide University's National Centre for Petroleum Geology and Geophysics has been field-mapping a Precambrian reef complex in what is now the Flinders Ranges National Park. The work began in 1984 as part of his PhD project. Interest was renewed when Dr Lemon showed the area to a group of international geologists. "They were aware of similar occurrences in North America, but which were not of the same age, and they could see the importance of this outcrop," said Dr Lemon.
Not far from the Brachina Gorge Geological Trail, the reefs at Enorama formed as a fringe around a diapiric island. In geological terms, a diapir is a mobile core containing minerals of low density such as salt. The core pushes upwards, deforming overlying rock to form a dome on the Earth's surface. The ancient diapir at Enorama formed an island in shallow seas, buoyed up by salt. There are similar examples today in the Persian Gulf.
The shallows around the island promoted the growth of bacteria. These precipitated minerals around them, the solid residue growing into a form of stromatolite. Living stromatolites survive at Shark Bay in Western Australia, while fossil forms are common in rocks of the Flinders Ranges.
In microscopic sections, the Enorama stromatolites do not show the usual fine layers, but a succession of thick layers with a clotted texture. They resemble calcareous bacteria well known from later times. These bacteria were colonial, growing as "shrubs" anchored to the sea floor or hanging like branches from rocky outcrops.
The Enorama reefs are built of stromatolitic mounds up to six metres high and ten metres across. They have grown into one another in stacked structures up 40 metres high and 100 metres across.
The Cambrian period began somewhere between 540 and 570 million years ago, and marked an explosion of animal types, many with hard body parts that fossilised well. Relatively complex organisms existed in Precambrian times, but being soft-bodied they were seldom preserved in rocks. The Ediacaran fossils from the Flinders Ranges are exceptions, providing rare insights into a range of ancient jellyfish, worms and other Precambrian animals, many with no counterparts today.
The Enorama reefs were built by organisms older and even more primitive than the Ediacaran fauna, but the lime that they precipitated preserved a record of them. The reefs are surprisingly similar in shape to modern coral reefs which are built in a similar fashion by much more complex organisms.
"The reef was like an atoll with a protected lagoon inside," said Dr Lemon. " Its growth was affected by changes in sea level that are still detectable," he said. " The reef dried out in shallow water, or shed conglomerates when the water was deep. We can see that a series of pulses pushed the diapir up over millions of years. Each movement tilted the surrounding reef, bigger movements causing it to die and become re-established some distance away," said Dr Lemon.
A row of reefs now stands along the edge of the diapir. Lime precipitated by the bacteria has turned to dolomite; far more resistant to weathering than the surrounding green shales. The outcrop is unusual in that all the reefs have been tilted on their sides by the diapir, which continued to move long after the reefs were deposited. It provides a view of ancient reefs not often encountered.
The line of hills can easily be seen from ground level or the surrounding hills, but is best viewed from the air. Dr Lemon has unravelled the complicated story of the growth of the diapiric island and its reef by comparing low- and high-level photographs of the structures and by detailed ground mapping.
"Tape and compass mapping has given the best information, " said Dr Lemon. "If the traverses are closely spaced, I can note all of the rocks, and my children are a great help. They mark out the grid and provide a continuous supply of billy tea," he said.
At Adelaide University's Thebarton campus, the rocks are sliced into thin layers, and analysed under the microscope to see the shapes left by the bacteria as they grew. Limestones and dolomites also record isotopic signatures of carbon and oxygen, two of the building blocks of life. Rocks from the late Precambrian show some of the widest fluctuations in the history of the planet.
"Samples from Enorama fill a gap between some of the highest and lowest carbon isotope values, and these can be traced to the evolution of the atmosphere, the oceans and perhaps to life itself," said Dr Lemon.
Oil and gas are known from similar, younger reef complexes around the world. "The Golden Lane atoll on the east coast of Mexico, and the Golden Spike reef in Alberta Canada are examples of prolific oil producing reefs, " said Dr Lemon. "The Enorama reef does have traces of bitumen still trapped in the rock, but its value lies in the wonderful outcrop which shows geologists an example of what is often buried," he said. "Usually we can only interpret such structures from seismic imaging or a few drill hole intersections."
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