{{Short description|Cenozoic sedimentary basin in south eastern Australia}} {{about|the geological basin|the Murray Darling River catchment area|Murray-Darling basin}} {{Use Australian English|date=June 2020}} {{Use dmy dates|date=August 2019}} right|thumb|upright=1.80|Sediment layers of Murray Basin The '''Murray Basin''' is a Cenozoic sedimentary basin in south eastern Australia. The basin is only shallow, but extends into New South Wales, Victoria and South Australia. It takes its name from the Murray River 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. This slab was subducted 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 moving north. The sinking slab has a trace in the P-wave velocity in the mantle which is higher from 800 to 1200 km deep below the Murray basin, and also the Lake Eyre Basin. 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.<ref name=Schellart15>{{cite journal|last1=Schellart|first1=W.P.|last2=Spakman|first2=W.|title=Australian plate motion and topography linked to fossil New Guinea slab below Lake Eyre|journal=Earth and Planetary Science Letters|date=July 2015|volume=421|pages=107–116|doi=10.1016/j.epsl.2015.03.036|doi-access=free}}</ref> 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.<ref name=Schellart15/>
==Geologic history== After Australia separated from Antarctica, the Murray Basin was formed. The basin floor only subsided slowly over time.<ref name="GMD">{{cite book|url=http://www.brs.gov.au/mdbsis/mdbgeol.pdf|title=Geology of the Murray-Darling Basin — Simplified Lithostratigraphic Groupings|author=Robert A Kingham|year=1998|publisher=Australian Geological Survey Organisation|isbn=978-0-642-27356-7|access-date=21 November 2010|archive-url=https://web.archive.org/web/20110313034235/http://www.brs.gov.au/mdbsis/mdbgeol.pdf|archive-date=13 March 2011|url-status=dead}}</ref> The basin became filled with up to {{convert|600|m|ft|-1}} of sediments during the Cenozoic Era.<ref>{{cite web |url=http://crcleme.org.au/Pubs/OPEN%20FILE%20REPORTS/OFR%20171-180/OFR180.pdf|title=Combining Geology and Geophysics to Develop a Hydrogeological Framework for Salt Interception in the Loxton Sands Aquifer, Central Murray Basin, Australia|author=T.J. Munday|date=September 2004|publisher=CRC LEME|accessdate=3 November 2010|display-authors=etal}}</ref>
From Paleocene to the Eocene Epoch, the western side was flooded with seawater and deposited the Warina sand.<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=19462|title=Warina Sand|publisher=Geoscience Australia|date=3 February 2009|work=Australian Stratigraphic Names Database}}</ref> The sea withdrew, and later in the Eocene, silt and clay of the Olney Formation<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=14553|title=Olney Formation|publisher=Geoscience Australia|date=25 March 2008|work=Australian Stratigraphic Names Database}}</ref> were deposited. One minor sea incursion in the Late Eocene resulted in the Buccleuch Formation<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=30504|title=Buccleuch Formation|publisher=Geoscience Australia|date=3 February 2009|work=Australian Stratigraphic Names Database}}</ref> in South Australia. The group of deposits is termed the Renmark Group and was earlier known as the Knight Group.<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=16036|title=Renmark Group|publisher=Geoscience Australia|date=29 July 2008|work=Australian Stratigraphic Names Database}}</ref>
The sea level rose again in Late Oligocene to mid Miocene, forming the Murray Group of sediments, with marl and limestone in the deeper locations, and the Geera Clay<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=7090|title=Geera Clay|publisher=Geoscience Australia|date=29 July 2008|work=Australian Stratigraphic Names Database}}</ref> in the shallow waters. The rock units formed in the deeper water included the Ettric Formation (''see cross-section image''),<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=6337|title=Ettric Formation|publisher=Geoscience Australia|date=28 July 2008|work=Australian Stratigraphic Names Database}}</ref> the Winnanbool Formation<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=20250|title=Winnanbool Formation|publisher=Geoscience Australia|date=29 July 2008|work=Australian Stratigraphic Names Database}}</ref> and the Mannum Formation limestone<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=11187|title=Mannum Formation|publisher=Geoscience Australia|date=6 April 2010|work=Australian Stratigraphic Names Database}}</ref> with Gambier Limestone in South Australia.<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=6976|title=Gambier Limestone|publisher=Geoscience Australia|date=7 April 2009|work=Australian Stratigraphic Names Database}}</ref><ref>{{cite web|url=http://www.pir.sa.gov.au/__data/assets/pdf_file/0018/10890/mj27_murray_basin.pdf|title=Northwestern Murray Basin Stratigraphy Sedimentology and Geomorphology|author=Adrian Fabris|publisher=South Australian Office of Minerals and Energy Resources|accessdate=3 November 2010}}</ref> When the sea level fell again in mid-Miocene, the deposited Geera clay and Olney Formation moved westward over the limestone.<ref name=GMD/>
During Upper Miocene to Pliocene, the sea rose and fell several times. The first sea rise formed the Murravian Gulf and resulted in clay and marl in the west, called the Bookpurnong Formation,<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=2203|title=Bookpurnong Formation|publisher=Geoscience Australia|date=23 February 2009|work=Australian Stratigraphic Names Database}}</ref> with the Calivil Formation<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=3322|title=Calivil Formation|publisher=Geoscience Australia|date=3 February 2009|work=Australian Stratigraphic Names Database}}</ref> river and lake sand in the east. When the sea retreated in Early Pliocene, the Loxton Sands,<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=10827|title=Loxton Sands|publisher=Geoscience Australia|date=6 November 2010|work=Australian Stratigraphic Names Database}}</ref> also informally known as Loxton-Parilla Sands, were formed on the beach on the shore of the emergent land. Locally heavy minerals have been concentrated by wave action, including rutile zircon and ilmenite, forming economic mining opportunities.<ref>{{cite journal|title=Mineral Sands Occurrences in the Murray Basin, Southeastern Australia|journal=Economic Geology|date=August 2000|author=Peter S. Roy and John Whitehouse|pages=1107–1128|volume=95|number=5|doi=10.2113/gsecongeo.95.5.1107|url=http://econgeol.geoscienceworld.org/cgi/content/abstract/95/5/1107#SEC2|url-access=subscription}}</ref>
[[File:Lake Bungunnia of the Murray Basin.png|thumb|left|Lake Bungunnia around 1 Mya]] The Murray River became dammed by uplift of over {{convert|250|m|ft|-1}} in the Grampians in Victoria during the Pleistocene about 2.5 Mya. The dam formed Lake Bungunnia, which reached {{convert|40,000|km2|sqmi}} and deposited the Blanchetown Clay therein.<ref>{{cite journal|title=Revised stratigraphy of the Blanchetown Clay, Murray Basin: age constraints on the evolution of paleo Lake Bungunnia|author=S. McLaren|journal=Australian Journal of Earth Sciences|volume=56|number=2|date=March 2009|pages=259–270|doi=10.1080/08120090802547074|hdl=1885/55269|hdl-access=free}}</ref> Higher rainfall of at least {{convert|500|mm|in|0|abbr=off}} per year kept the lake filled at first, but during later times, rainfall was insufficient, and saline lakes formed, depositing dolomite. Even today, some saline lakes remain as a remnant of the vast lake.<ref>{{cite journal|title=Lake Bungunnia — A Plio-Pleistocene megalake in southern Australia|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|author=A.E. Stephenson|volume=57|issue=2–4|date=December 1986|pages=137–156|doi=10.1016/0031-0182(86)90011-8}}</ref> The Murray River carved out a new path to the sea via Murray Bridge, replacing its previous exit at Portland, Victoria. The lake was gone by about 0.7 Mya.
The Pooraka Formation<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=15448|title=Pooraka Formation|publisher=Geoscience Australia|date=22 June 2009|work=Australian Stratigraphic Names Database}}</ref> formed in the north west due to increased erosion, resulting in colluvium depositing. The colluvium forms fans, cones and scree slopes, and often contains clay and breccia. In the flat areas, the Shepparton Formation<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=25474|title=Shepparton Formation|publisher=Geoscience Australia|date=27 February 2009|work=Australian Stratigraphic Names Database}}</ref> also resulted from river deposits of floodplain clay.<ref name=GMD/>
Most of the existing surface dates from Quaternary period. The river deposits from the east have been progressively overlaying the marine deposits further west, as the shore line receded. Within the Pleistocene deposits are three layers of sand that are aquifers, deposited during higher rainfall periods of the interglacials. 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.<ref>{{cite web|url=http://dbforms.ga.gov.au/pls/www/geodx.strat_units.sch_full?wher=stratno=4598|title=Coonambidgal Formation|publisher=Geoscience Australia|date=17 June 2009|work=Australian Stratigraphic Names Database}}</ref> During the dryer glacial periods the area was arid.<ref>S, Pels "The Murray Basin" in ''The Geology of NSW'' p506</ref> ==See also== *Deniliquin multiple-ring feature, detected under the Murray Basin *Darling Sedimentary Basin
==References== {{Reflist}} ==External links== *{{cite web |title=How the Murray Basin was formed {{!}} Murray–Darling Basin Authority |url=https://www.mdba.gov.au/basin/landscapes/how-murray-basin-was-formed |website=www.mdba.gov.au |language=en |date=11 February 2026}}
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Category:Sedimentary basins of Australia Category:Murray River Category:Geology of New South Wales Category:Geology of Victoria (state) Category:Geology of South Australia Category:Landforms of New South Wales Category:Landforms of Victoria (state) Category:Landforms of South Australia