Australia: The Land Where Time Began

A biography of the Australian continent 

Great Artesian Basin - GAB - The Great Australian Basin

Parts of the Eromanga Basin containing Jurassic and Cretaceous aquifers that form the Great Artesian Basin - GAB. A large intracratonic basin, with an area of 1.7 million square km, about 20 % of the continent, underlying parts of Queensland, Northern Territory, New South Wales and South Australia.

In the Jurassic, about 160 Ma, much of eastern Australia was a vast floodplain, the huge quantities of sediments deposited on this floodplain at that time by freshwater streams forming the GAB. During the Cretaceous, 120 Ma, sea level rise inundated vast areas of the continent, flooding many of the interconnected basins to form an epicontinental sea. The marine deposits from this sea capped the water-bearing freshwater sediments with impermeable layers, forming aquifers that carried water, very slowly, from charging points in the east along the gently sloping beds leading to central Australia. The marine deposits contain very large amounts of salt, the basis for the actual and potential salinity problems over much of present-day Australia.

The presence of the GAB allowed the introduction of livestock to marginal lands around the desert areas of central Australia. The resulting overgrazing has increased the area of the deserts at the expense of the marginal land.

During the Jurassic and Cretaceous Periods the area of the GAB was covered by forests thought to be similar to the modern day Amazon Basin. Under these conditions large amounts of sand were deposited. During the Cretaceous the sea invaded the basin, the Eromanga Sea. During this marine period more sand was laid down, becoming another layer of sandstone. The multiple layers of sandstone account for the aquifers at different depths.

These layers of sandstone slope down from east to west. This accounts for the pressure of the water. The recharge is mostly at the eastern edge of the basin, along the western slopes of the Great Divide. Along the eastern edge of the basin the layers of sandstone are warped up sufficiently to allow water to seep into it from the rain falling on the western slope of the Great Divide. This edge of the sediments total about 15,000 km2. The recharge rate is highest at places of yellow earth that quickly saturate allowing water to run off to be absorbed by red earths further down the slope.

The travel time for the water is very slow. Water seeping in at the ranges east of Longreach takes 2 million years to arrive at Lake Eyre. When the flow of the water is impeded by impervious rock it can reach the surface where it exits as a spring or mound spring, and can form a salt pan. The same sedimentary layers can also hold oil deposits where an impervious cap rock lies above the sandstone layers.

Similarities with Mars

In this paper Rey1 presents a unique set of attributes that explain the formation in such abundance of precious opal has formed in central Australia, occurs almost nowhere else on Earth. The history of the GAB in the Cretaceous is that of a flexural foreland basin at high latitudes that was associated with a Cordillera Orogen that had been built along the Pacific margin of Gondwana. The Eromanga Sea that flooded this basin, that was shallow, cold, not well-connected to the open ocean, and was muddy and stagnant, which is the explanation of the lack of carbonates, acted as a sink for volcaniclastic sediments that were eroded from the volcanic arc of the Cordillera. Sediments that were iron-rich and organic matter-rich, contributed to the development of an anoxic sub-seafloor in which anaerobic bacteria that were pyrite-producers thrived. Lithologies from the Lower Cretaceous, that are rich in pyrite, ferrous iron, feldspar, volcanic fragments and volcanic ash have an acidification potential and pH neutralisation capacity, that is exceptionally large. As a result of this lithologies from the Lower Cretaceous are particularly reactive to oxidative weathering. Australia was still situated at high latitudes between 97 and 60 Ma, and was subjected to a protracted period of uplift, erosion, denudation and crustal cooling. It has been suggested that it is possible that the bulk of precious opal was formed during this period by acidic oxidative weathering. The opalised redox front was preserved by a veneer of Cainozoic sediments, which had been deposited widely. Regional acidic weathering is rare on Earth, the Great Artesian Basin being an example that is found in Australia, and a set if attributes of the GAB is shared with the surface of Mars, such as volcaniclastic lithologies, lack of significant carbonate, similar secondary assemblages that includes opaline silica, similar acidic oxidative weathering that was driven by surfaces drying out that was very similar, and they are both of the same colour. Rey1 suggests the red centre of Australia could possibly be the best regional terrestrial analogue for the surface of Mars.

The effects of GAB water on soils

Sources & Further reading

  1. Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003
  2. Mary E. White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
  3. Rey, P. F. (2013). "Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist." Australian Journal of Earth Sciences 60(3): 291-314.

Author: M. H. Monroe
Last Updated 02/09/2014




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