Geology

The Murray–Darling Basin has been hundreds of millions of years in the making — geologically speaking.

The 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.

Gondwana

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 (about 500 million years ago). Other units developed more recently (around 2 million years ago) as Gondwana split into its eastern and western halves; and then about 140 million years ago when 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.

Basin development

Harsh sandy cliffs at Lake Mungo in New South Wales.
The ancient landscape of Lake Mungo in south-west New South Wales. Photo by Brayden Dykes, MDBA.

Over this ancient landscape, 2 saucer-like depressions developed — 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.

Over time, more basins formed partly or entirely over the Great Artesian Basin, including the Darling, Surat and Eromanga basins. The southern part of the Great Artesian Basin underlies much of the northern Basin, where the Darling River and its tributaries run.

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.

The climate of 40–60 million years ago was much wetter than present. The basins contained large swamps and bogs, and thick sediments that were laid down in broad valleys. As streams and rivers ran from the eastern highlands, new valleys were eroded and sand and gravel were deposited as rivers fanned out across the plains. Great quantities of water accumulated in the Great Artesian Basin, which maintained connections to the ground surface or shallow aquifers. Up to 600 m of sediment was deposited in the Basin.

Rising and retreating sea levels

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.

Compared with other continents, Australia has been remarkably free of volcanic or mountain-building activity in recent time. The Great Dividing Range in eastern Australia, the Flinders Ranges in South Australia and the extensive plains in between have been present for at least 20 million years.

Earth moving changes

Localised 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 and arms extending southwards to near Sea Lake in the Victorian mallee. Salt lakes through the region, such as Lake Tyrrell, are believed to be remnants of the ancient Lake Bungunnia.

Lake Bungunnia existed for 2 million years at a time when the climate was much wetter or there was considerably less evaporation, conditions were humid and the landscape was heavily vegetated. About 600,000 years ago, the Pinnaroo Block was breached and the ancient and much wider River Murray cut a deep gorge through sediment to makes its way to the sea.

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. However, as the river carved a new course, new aquifers formed that provided a direct connection between the salt store in the soil and the Murray.

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 Barmah Choke has resulted in regular flooding of a large area of the floodplain in the area, enabling the establishment of the largest stand of river red gums in Australia in the Barmah–Millewa Forest. The choke provides a challenge for water management as it limits flow to 10 ML/day, so downstream supply of irrigation or urban water needs to be scheduled to avoid unseasonal flooding of the red gum forests.

River formation

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, the Basin's rivers generally meander across the giant floodplains of the interior, making many twists and turns on their way to the ocean.