Geological History

Evidence from fossils indicates that the history of the Murray-Darling Basin began about 65 million years ago

65 - 32 million

- Australia was separated from Antarctica and began to move slowly northwards. The south-eastern highlands of Australia lifted and caused the Murray Basin to subside and take shape.

- The climate at this time was wetter, more humid and less seasonal and the Basin was covered with an evergreen rainforests, lakes and wetlands.

32 -12 million

- Sea levels rose and the western half of the Murray Basin was covered in sea. At its maximum extent, about twenty million years ago, it reached beyond the present site of Swan Hill.

- As the eastern highlands eroded, large rivers deposited sediments in the Murray basin. Forming the river plains of today.

- Shallow marine limestone was deposited by the sea and laid down beneath today’s Mallee Region of South Australia and western Victoria. This limestone forms the Murray Group Aquifer System.

- The sea then retreated about 12 million years ago due to a gradual uplift of the land combined with a receding world sea level. The Murray Basin was then exposed and become subject to erosion.

6 - 2 million

- Around six million years ago the western part of the Murray basin was again invaded by a shallow sea. Vast sheets of sand were deposited and formed one of the aquifers beneath the Murray Basin Today.

- After several advance and retreats the sea retreated to the south-west between four and two million years ago.

2 - 500,000 million

- A giant fresh water lake, Lake Bungunnia , formed about two million years ago when the lower Murray was dammed by an earth uplift. The lake covered some 33,000 square kilometres.

- The natural dam breached around 700,000 years ago and the lake emptied.

- The climate also becomes drier.

The last 500,000 years

- In the last 500,000 years the climate has remained relatively dry, with four or five dry to wet oscillations occurring which produced the dunes, lakes and river channels that are present today.

The Murray Basin area can be described as a shallow flat-lying saucer (500 km across and about 600 metres deep in the centre), made of rock over 350 million years old, and almost filled with material that has been deposited over the last 65 million years. The deposits of sand and fine clay have made the Murray Basin relatively flat. It is only 200 metres above sea level.


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