A nice easy Earth Cache for beginners!
The Murray–Darling Basin may be a well-recognised geographical feature of the Australian landscape, but its geology is not so neatly defined. The landforms that we see today have taken shape over the last 60 million years, however the foundations of the Basin are hundreds of millions of years old.
The Basin is made up of many geological or tectonic units, some are large in area and others are relatively small. Some of the units formed when Australia was part of Gondwana and long before it separated from other land masses around 500 million years ago. Other units developed more recently when Gondwana split into its eastern and western halves which was about 2 million years ago and then about 140 million years ago when here was more shifting as Australia and Antarctica split from India.
Over 400 million years, the tectonic units that form the foundation of the Basin eroded to a relatively flat land surface, however outcrops of the original rock remained in some regions. Volcanic activity, sometimes extreme, also created distinctive features on the landscape.
Two saucer-like depressions developed across the land, a series of basins that collectively formed the Great Artesian Basin, from 60–250 million years ago, and the Murray Groundwater Basin that formed less than 60 million years ago.
The Murray Groundwater Basin underlies the riverine plains associated with the River Murray and its tributaries.
Ongoing erosion and weathering resulted in sedimentary rocks infilling the basins, while the ancient basement rocks continued to fold and change (metamorphose) at the perimeters and beneath the basins, forming the mountain ranges and outcrops that form the eastern and southern areas of the land surface of the Basin. The south-western rim of the Basin is basement rocks just beneath the surface, which form the Padthaway Ridge in South Australia and separates the Basin from the Southern Ocean.
On the western side of the Murray Groundwater Basin, sea levels rose and retreated several times from 26–2.5 million years ago. The south-western corner of the basin became a sea, which at its peak about 6.0 million years ago, reached Balranald and Kerang. Marine materials were deposited across the landscape in sand sheets until the sea retreated completely.
Localised Volcanic activity in the earth's crust in 'recent' geological time has had significant influences on soil characteristics, groundwater quality and the modern courses of the rivers.
Around 3 million years ago, the uplift of the Pinnaroo Block, near Swan Reach on the lower River Murray, blocked the flow of the river and with time created the massive but shallow Lake Bungunnia. The lake is thought to have covered 50,000 km², from Blanchetown in South Australia, to past Lake Mungo in the north, to Boundary Bend on the Murray in the east, arms extending southwards to near Sea Lake in the Victorian mallee.
The main sedimentary rock deposited in Lake Bungunnia, and underlying much of the Mallee , the Blanchetown Clay provides a barrier between surface water and underlying saline aquifers in modern times. In many areas, the layer slows down leakage of irrigation water into the river.
In the central catchment, the course of the Murray and several other rivers changed about 25,000 years ago as a result of an uplift of land from Echuca to Deniliquin, called the Cadell Tilt. Effectively a large block that stopped the Murray and Goulburn rivers and two very large lakes formed. Over time and with the melting of glaciers in the Great Dividing Range about 20,000 years ago, water headed northwards to create the Edward River, and the Murray created a new course southwards, now called the Barmah Choke, to follow its current course, created by the ancient Goulburn River, to Swan Hill. Green Gully, west of Mathoura is believed to be the former course of the Murray, as are the lower reaches of the Wakool River.
The most significant period the formation of the rivers, dunes and alluvial plains of the Basin was during the glacial cycles of 10,000 to 100,000 years ago, when melting glaciers carried vast amounts of water and sediment from the mountains that formed the eastern highlands.
Australia’s general flatness and mostly arid climate means that the river systems are generally slow flowing, and in some cases, ephemeral. Rather than flowing directly from source to sea.
Today we have you standing where the two great rivers meet, what you can see will depend on what time of the year you are here:
If you are not too chicken take to the nearby viewing platform to help you answer the first question:
To log this Earth Cache we require you to wander around and look at the area, consider the information given and perhaps if you need to do some research of your own, then message us with the following answers to the best of your ability;
1. Where the two rivers meet in front of you, there is a distinctness between them, what is it and what do you think causes this?
2. Go to the waters edge, describe the geology you see, this will vary depending on the height of the rivers, are the rocks and soil you see local to the area, how have they been deposited there?
3. A photo of your team or GPS near GZ. (Optional)
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers within 24 hours. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted.
Source: mdba.gov.au, BOM,
