The Opal Sea

David Cook — 6 June 2019
Camper's resident offroad-loving geologist takes a closer look at Australia's magnificent interior lands.

Elsewhere in this issue of Camper we took a look at the great advantages bestowed on this nation by the once great Eromanga Sea, which a hundred million years ago flooded across one fifth of the landscape of modern Australia. It left behind a unique sequence of sandstones and shales which have formed a great bowl to capture rainfall along the wetter northern and eastern margins of the continent and deliver it to the now parched and dry centre.

That flow of water has, over the past thousands of years, sustained small tribes of aboriginal nomads and, in the past 150 years, has made possible a pastoral industry that has helped make Australia one of the great food exporters to a hungry world. And it has sustained settlements of everything from individual families up to those of several hundred people in size, allowed livestock to be moved around with some safety and determined where roads and railways were built.

Such seas, like the Eromanga Sea, are known as epeiric or epicontinental seas, and are much less common than in the days of the dinosaurs. However, we can find examples in Hudson’s Bay in Canada, the Persian Gulf, the Gulf of Carpentaria and the Arafura Sea between Australia and New Guinea.

And no less significant for some has been the discovery that underground there were riches to be discovered in the form of the brilliant sparkle opals, a consequence of much more subtle movement of water.

Opal is a hydrated silica which has formed thin layers through what were once fissures in the ground rock, or which has replaced what were once the surface or even all of the remains of ancient life embedded in the rock. This includes bones, teeth, shells and woody tissues of prehistoric plants and animals whose remains were buried in the sediments, almost all of them in the early Cretaceous Period, about 100 to 125 million years ago. 

The opal was formed from silica rich water moving through the rock. The silica comes from the sandstone, which in itself is porous enough to permit the movement of the water. Wherever the water remained stationary for a while or evaporated it would leave a deposit of the silica. In most gems the stone is formed from a strictly regulated almost mathematical three dimensional lattice, but in opal the gem is made up of microscopic “balls”, each of which acts individually and in concert with those around it to diffract light, giving the multi-coloured and seemingly random brilliance which makes them so attractive. The smaller balls favour colours towards the blue end of the spectrum and the larger balls favour colours at the red end of the spectrum. The more orderly the balls in size and arrangement, the less the colour; the greater the range of sizes and the more irregular the pattern, the greater the amount of colour.

It is interesting that these not uncommon requisite conditions for the formation of opal are not more readily present elsewhere in the world. Though there has been a consistent search elsewhere there are still only very few known localities which produce gem quality opal, and Australia still supplies between 80 and 90 per cent of the world’s opals. A recent study has indicated that there appear to be very specific requirements for the formation of opal, with the formation of the basic deposit in quite shallow marine sediments followed by a prolonged sequence of erosion. This followed sampling from 1,036 known opal localities in Australia. Based on these conclusions the requisite conditions are met at 10 per cent of the Great Artesian Basin, which opens the area for future search.

The early Cretaceous was a time when the dinosaurs dominated life on land, and flying reptiles populated the skies and huge reptiles swam in the seas. Thus we find fossilised bones and even very rare complete skeletons of all these organisms, as well as extremely rare pieces of dinosaur egg shell and skin, along with less spectacular remains of snails and shellfish and other contemporaneous life, all beautifully preserved in sparkling colour.

Based on the many pieces of bone it appears the most common dinosaurs living around the coastal margins of the Eromanga Sea were small dromaeosaurs. These were bipedal animals of between one and 11 metres in length which walked on two of the three main toes on their feet, with the third toe being held off the ground and carrying a large sickle-shaped claw which could have been used for hunting, fighting and/or climbing trees. The best known of the dromaeosaurs was Velociraptor, made famous in the movie Jurassic Park. Based on evidence from elsewhere in the world it is now considered likely that all dromaeosaurs were covered with feathers.

While random bits of unidentifiable bone are often found it is the more usual denizens of the shallow seas – the clams and other shellfish – that are the most common. The Eromanga Sea was connected with the open ocean at Australia’s northernmost margin, and so there are many places where the more archetypal denizens of those dinosaur-era seas are to be found, creatures like ammonites – coiled molluscs (shell-coated relatives of the squid and the octopus) that are a universal marker for marine deposits of those times, as well as belemnites – the ancient equivalent of a cuttle bone, from more primitive relatives of modern cuttlefish. Few of these are found in opal deposits as the seas were a bit too shallow for them but are often to be found in deeper water deposits not far away.

These opal deposits are mined at many small towns across the margins of this ancient sea, the main locations being at Winton, Quilpie, Lightning Ridge, White Cliffs, Andamooka, Coober Pedy and Mintabe, though there are hundreds of lesser centres of activity. Lightning Ridge has been the source of the most significant fossil finds, from what was once a rich estuarine and lake environment with prolific forests growing in the area.

Next time you’re looking at a tray of gaudily sparkling opals just realise you’re quite likely to be lusting after what was once a dead shellfish or animal bone.

Tags

opal sea geology geography desert

Photographer

David Cook