When the Australian 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.