The Murray Basin is a

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

sedimentary basin Sedimentary basins are region-scale depressions of the Earth's crust where subsidence has occurred and a thick sequence of sediments have accumulated to form a large three-dimensional body of sedimentary rock. They form when long-term subsidence ...

in south eastern

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

. The basin is only shallow, but extends into

New South Wales ) , nickname = , image_map = New South Wales in Australia.svg , map_caption = Location of New South Wales in AustraliaCoordinates: , subdivision_type = Country , subdivision_name = Australia , established_title = Before federation , es ...

, Victoria and South Australia. It takes its name from the

Murray River The Murray River (in South Australia: River Murray) (Ngarrindjeri: ''Millewa'', Yorta Yorta: ''Tongala'') is a river in Southeastern Australia. It is Australia's longest river at extent. Its tributaries include five of the next six longest r ...

which traverses the Basin from east to west.

Formation

One theory for the formation of the basin is that it is dynamic topography due to a sinking slab of

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...

. This slab was

subducted Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

during latest Cretaceous and Early Cenozoic periods to the north of the Australian continent underneath New Guinea. The slab broke off and was gradually overridden by the

Australian plate The Australian Plate is a major tectonic plate in the eastern and, largely, southern hemispheres. Originally a part of the ancient continent of Gondwana, Australia remained connected to India and Antarctica until approximately when India broke ...

moving north. The sinking slab has a trace in the

P-wave A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...

velocity in the mantle which is higher from 800 to 1200 km deep below the Murray basin, and also the

Lake Eyre Basin The Lake Eyre basin ( ) is a drainage basin that covers just under one-sixth of all Australia. It is the largest endorheic basin in Australia and amongst the largest in the world, covering about , including much of inland Queensland, large porti ...

. The velocity anomaly is oriented northwest-southeast and extends from 135°E 26°S to 144°E35°S. The sinking has dragged down the rock above it to make a hollow which is the basin. This hollow has moved south over the last 50 million years. If this theory is correct, then the Murray basin will move south over the next tens of millions of years and disappear under the Southern Ocean. Modelling using the ''Underworld'' program by Moresi indicates that the slab would be sinking at 8.5 mm per year and cause a depression on the surface 200 m deep around 2500 km wide. In the past the basin depression was narrower but deeper in this model.

Geologic history

After Australia separated from

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest contine ...

, the Murray Basin was formed. The basin floor only subsided slowly over time. The basin became filled with up to of sediments during the

Cenozoic Era The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configura ...

. From

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...

to the

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

Epoch, the western side was flooded with seawater and deposited the Warina sand. The sea withdrew, and later in the Eocene, silt and clay of the Olney Formation were deposited. One minor sea incursion in the Late Eocene resulted in the Buccleuch Formation in South Australia. The group of deposits is termed the Renmark Group and was earlier known as the Knight Group. The sea level rose again in Late

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

to mid Miocene, forming the Murray Group of sediments, with

marl Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae. Marl makes up the lower part o ...

and

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

in the deeper locations, and the Geera Clay in the shallow waters. The rock units formed in the deeper water included the Ettric Formation (''see cross-section image''), the Winnanbool Formation and the Mannum Formation limestone with Gambier Limestone in South Australia. When the sea level fell again in mid-Miocene, the deposited Geera clay and Olney Formation moved westward over the limestone. During

Upper Miocene The Late Miocene (also known as Upper Miocene) is a sub-epoch of the Miocene Epoch made up of two stages. The Tortonian and Messinian stages comprise the Late Miocene sub-epoch, which lasted from 11.63 Ma (million years ago) to 5.333 Ma. The ev ...

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Murravian Gulf and resulted in clay and marl in the west, called the Bookpurnong Formation, with the Calivil Formation river and lake sand in the east. When the sea retreated in

Early Pliocene Early may refer to: History * The beginning or oldest part of a defined historical period, as opposed to middle or late periods, e.g.: ** Early Christianity ** Early modern Europe Places in the United States * Early, Iowa * Early, Texas * Early ...

, the Loxton Sands, also informally known as Loxton-Parilla Sands, were formed on the beach on the shore of the emergent land. Locally

heavy minerals In geology, a heavy mineral is a mineral with a density that is greater than 2.9 g/cm3, most commonly referring to dense components of siliciclastic sediments. A heavy mineral suite is the relative percentages of heavy minerals in a stone. Heavy min ...

have been concentrated by wave action, including

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer Polymorphism (materials science), polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest ...

zircon Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of the r ...

and

ilmenite Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printing ...

, forming economic mining opportunities. Lake Bungunnia of the Murray Basin

The

became dammed by uplift of over in the Grampians in Victoria during the

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

about 2.5 Mya. The dam formed

Lake Bungunnia The Murray River (in South Australia: River Murray) (Ngarrindjeri language, Ngarrindjeri: ''Millewa'', Yorta Yorta language, Yorta Yorta: ''Tongala'') is a river in Southeastern Australia. It is List of rivers of Australia, Australia's longest ...

, which reached and deposited the Blanchetown Clay therein. Higher rainfall of at least per year kept the lake filled at first, but during later times, rainfall was insufficient, and saline lakes formed, depositing

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

. Even today, some saline lakes remain as a remnant of the vast lake. The Murray River carved out a new path to the sea via

Murray Bridge Murray Bridge may refer to. *Murray Bridge, South Australia, a city and locality *Rural City of Murray Bridge, a local government area in South Australia *Corporate Town of Murray Bridge, a former local government area in South Australia See also ...

, replacing its previous exit at

Portland, Victoria Portland is a city in Victoria, Australia, and is the oldest European settlement in the state. It is also the main urban centre in the Shire of Glenelg and is located on Portland Bay. As of the 2021 census the population was 10,016, increasing ...

. The lake was gone by about 0.7 Mya. The Pooraka Formation formed in the north west due to increased erosion, resulting in

colluvium Colluvium (also colluvial material or colluvial soil) is a general name for loose, unconsolidated sediments that have been deposited at the base of hillslopes by either rainwash, sheetwash, slow continuous downslope creep, or a variable combinatio ...

depositing. The colluvium forms fans, cones and scree slopes, and often contains clay and

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...

. In the flat areas, the Shepparton Formation also resulted from river deposits of floodplain clay. Most of the existing surface dates from

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...

period. The river deposits from the east have been progressively overlaying the marine deposits further west, as the shore line receded. Within the

deposits are three layers of sand that are aquifers, deposited during higher rainfall periods of the

interglacial An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene in ...

s. The floodplain deposits from the current rivers are the Coonambidgal Formation, however this term is used informally for the older Pleistocene flood deposits as well. During the dryer glacial periods the area was arid.S, Pels "The Murray Basin" in ''The Geology of NSW'' p506

References

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