Australia: The Land Where Time Began

A biography of the Australian continent 

Great Australian Basin - Opalisation

In this paper the authors¹ present the unique set of attributes that explain the reason precious opal formed so abundantly in central Australia like almost nowhere else on Earth. In the Early Cretaceous the Great Australian Basin was a flexural foreland basin associated with a Cordillera Orogen at at high latitudes built along the Pacific margin of Gondwana. The Eromanga Sea flooded the basin that was a sink for the volcaniclastic sediments that eroded from the cordillera's volcanic arc. The absence of significant carbonates from the Eromanga sea is explained by this sea being shallow, cold, with poor connections to the ocean, and the water of which was muddy and stagnant. Pyrite-producing anaerobic bacteria thrived in the anoxic sub-seafloor that resulted from the sediment being rich in iron and organic matter. Lithologies from the Lower Cretaceous have an exceptionally large acidification potential and a pH neutralisation capacity as they are rich in pyrite, ferrous iron, feldspar, volcanic fragments and volcanic ash. This results in lithologies from the Lower Cretaceous being particularly reactive to oxidative weathering. Australia was at high latitudes from 97 Ma to 60 Ma and experienced a protracted period of uplift, erosion, denudation and cooling, the authors¹ suggesting that it may have been during this period that most of the precious opal formed by acidic oxidative weathering. After the end of uplift about 60 Ma the opalised front was preserved by a widespread deposition of a veneer of sediments during the Cainozoic. Though regional acidic weathering is rare on Earth, acidic oxidative weathering has been found on the surface of Mars that intriguingly shares a set of attributes with the Great Australian Basin. These include absence of significant amounts of carbonate, similar secondary assemblages that include opaline silica, similar oxidative weathering that is driven by surface drying out that is very similar, and the same colour. According to the authors¹ this suggests the red centre of Australia could be the best regional analogue of the surface of the red planet.

Sources & Further reading

1. Rey, P. F. "Opalisation of the Great Artesian Basin (Central Australia): An Australian Story with a Martian Twist." Australian Journal of Earth Sciences 60, no. 3 (2013/04/01 2013): 291-314.