Information about the geology of the
Great Artesian Basin

The Great Artesian basin consists of layers of sedimentary rock (see the diagram below). At its deepest point, Permian shale and Triassic mudstones/siltstones form it’s watertight (aquitard) base. At the end of the Permian ice age – around 250 million years ago - large amounts of sediments contained in glacial ice were deposited above earlier palaeobasins.

During the following Triassic period these sediments sagged down under their own weight. To balance this sagging the country around the edge, largely made of sandstone was uplifted. This sandstone gradually eroded away and was carried into the basin, resulting in a deposit of sandy sediments almost 3000 meters deep. This sandstone was porous.

During the Jurassic, the whole area was covered with a freshwater lake called Lake Walloon, which was over 800,000km² in size and covered most of the eastern side of central Australia. Later during the Cretaceous, around 110 million years ago the Australian continent slumped in the middle and the ocean poured in, forming the Cretaceous Sea and splitting Australia in two. In this sea there swam plesiosaurs and great white sharks. The floor of the sea was muddy, and over 20 million years transformed into a dense mudstone, and 95 million years ago when the sea retreated a cap was effectively laid over the porous sandstone.

For the next 30 million years the basin was an interregnum – nothing much happened, it was just a large flat plain with little sedimentation, until 60 million years ago the eastern edge of the basin on the Great Dividing Range experienced uplift. Since then rain clouds have been crossing the continent, uplifting and falling as rain on the western slopes of the Great Divide. The water percolates down into the porous aquifer of the basin. Its rate of flow is estimated to be only 1-5 meters per year, gradually building up pressure under the weight of further incoming water.

Across the basin there are natural breaks in the Cretaceous capping resulting in natural springs, the most marked a natural line of mound springs that the Oodnadatta Track follows from the Flinders Ranges to Alice Springs. The Cretaceous cap is also broken by man-made bores.

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