The Great Artesian Basin covers 1,711,000 square kilometres, about one fifth of Australia. It holds enough water to fill Sydney Harbour 130,000 times. Its depth ranges from 100 metres to 3000 metres.
GEOLOGICAL FORMATION:
This geological phenomenon began to develop between 140 and 117 million years ago when shallow seas covered much of the continent and it was still part of Gondwana.
Over time sediments sagged down under their own weight and settled under the shallow seas, then were compressed and cemented into layers of rock. 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 metres deep. Sandy layers became porous sandstone while layers of mud and clay formed impermeable mudstone and siltstone.
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. The seas withdrew and cleared the continent by 99 million years ago. In the same period, turbulence under the earth’s surface resulted in volcanic activity along eastern Australia. The land was squashed, folded and further uplifted so that layers of rock which exposed their edges became subject to the forces of erosion.
WATER ROCKS:
Much of the water in the GAB entered when the climate was wetter. Where porous sandstone was exposed, rain water leaked into the rock over millions of years. Sandwiched between impervious mudstone and siltstone the water became trapped in sandstone aquifers. Water still enters the sandstone aquifers along higher exposed edges, mostly along the Great Dividing Range. This recharges the aquifers and the water continues to slowly flow downhill from the uplifted edges, mostly towards the inland.
UNDER PRESSURE:
Because the sandstone aquifer is confined between two impermeable layers the water is under constant pressure. When a bore is drilled into the aquifer the water rises and escapes. Water that flows to the surface through a bore without pumping is an artesian bore. In other areas the water needs to be pumped to the surface. There are also mound springs where the aquifer flows naturally to the surface. Mound springs often feature in Aboriginal stories and remain culturally significant. They are also important to wildlife.
An 11 metre windmill can be seen in Prairie Park as you cross the bridge on Flinders Highway heading north. It is typical of a windmill found on many of the GAB bores.
As a matter of interest, if you are intending to visit Porcupine Gorge, there is a WHISTLING BORE on the side of the road you can check out on the way. Its coords are: S20 24.003 E144 25.403.
DID YOU KNOW? The term "artesian" is derived from the locality of Artois (France) where water under pressure was used as early as 1126 AD.
You may log this Earthcache straight away but then please email your answers to the questions to the CO. We will contact you ONLY if your answers need extra work. Logs with no answers sent will be deleted. The answers can be found by reading the information boards and perhaps a bit of research. Please do not post your answers in your log.
Q1. What does impermeable mean?
Q2. How old has some of the water been dated in the SW of the basin?
Q3. How many million mega litres does the GAB hold?
Q4. What kind of bore is one where the water has to be pumped?
Q5. What is the average depth of bores in the basin?
It would be nice to see a photo of you at Federation Rotunda, the windmill or even the Whistling Bore. You are welcome to post photos but no spoilers please. We hope you enjoy your time learning about the Great Artesian Basin of Australia.