Australia: The Land Where Time Began
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.
The effects of GAB water on soils
- Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003
- Mary E. White, Running Down, Water in a Changing Land, Kangaroo Press, 2000
- Hydrology
- Bass Basin
- Bremer Basin
- Canning Basin
- Carnarvon Basin
- Carpentaria Basin
- Duntroon Basin
- Drainage Patterns
- Eromanga Basin
- Eromanga Sea
- Eucla Basin
- Great Artesian Basin-GAB
- Murray-Darling Basin
- Officer Basin
- Otway Basin
- Perth Basin
- Torquay Basin