The Perth Basin is a thick, elongated

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 ...

Western Australia Western Australia (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to ...

. It lies beneath the

Swan Coastal Plain The Swan Coastal Plain in Western Australia is the geographic feature which contains the Swan River as it travels west to the Indian Ocean. The coastal plain continues well beyond the boundaries of the Swan River and its tributaries, as a geo ...

west of the

Darling Scarp The Darling Scarp, also referred to as the Darling Range or Darling Ranges, is a low escarpment running north–south to the east of the Swan Coastal Plain and Perth, Western Australia. The escarpment extends generally north of Bindoon, to th ...

, representing the western limit of the much older

Yilgarn Craton The Yilgarn Craton is a large craton that constitutes the bulk of the Western Australian land mass. It is bounded by a mixture of sedimentary basins and Proterozoic fold and thrust belts. Zircon grains in the Jack Hills, Narryer Terrane have b ...

, and extends further west offshore. Cities and towns including

Perth Perth is the list of Australian capital cities, capital and largest city of the Australian states and territories of Australia, state of Western Australia. It is the list of cities in Australia by population, fourth most populous city in Aust ...

Busselton Busselton is a city in the South West region of the state of Western Australia approximately south-west of Perth. Busselton has a long history as a popular holiday destination for Western Australians; however, the closure of the Busselton ...

, Bunbury,

Mandurah Mandurah () is a coastal city in the Australian state of Western Australia, situated approximately south of the state capital, Perth. It is the state's second most populous city, with a population of 107,641 as of the 2021 census. Mandurah's ...

and

Geraldton Geraldton ( Wajarri: ''Jambinu'', Wilunyu: ''Jambinbirri'') is a coastal city in the Mid West region of the Australian state of Western Australia, north of the state capital, Perth. At June 2018, Geraldton had an urban population of 37,648. ...

are built over the Perth Basin.

Geological setting and evolution

The Perth Basin began forming in the Late

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Pale ...

during the breakup of

Gondwana Gondwana () was a large landmass, often referred to as a supercontinent, that formed during the late Neoproterozoic (about 550 million years ago) and began to break up during the Jurassic period (about 180 million years ago). The final st ...

, as the Australian continental plate began rifting away from the African and

India India, officially the Republic of India ( Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the ...

n continental plates. During the Permian, what is now the Perth Basin was the eastern half of a

rift valley A rift valley is a linear shaped lowland between several highlands or mountain ranges created by the action of a geologic rift. Rifts are formed as a result of the pulling apart of the lithosphere due to extensional tectonics. The linear de ...

that formed as the continental plates were pulled apart. This pulling apart, which continued until the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The J ...

, led to the central zone subsiding as a

graben In geology, a graben () is a depressed block of the crust of a planet or moon, bordered by parallel normal faults. Etymology ''Graben'' is a loan word from German, meaning 'ditch' or 'trench'. The word was first used in the geologic contex ...

allowing the sea to enter with the subsequent deposition of transgressive marine sediments. The Perth Basin architecture is dominated by listric,

extensional fault An extensional fault is a fault caused by stretching of the Earth's crust. Stretching reduces the thickness and horizontally extends portions of the crust and/or lithosphere. In most cases such a fault is also a normal fault, but may create a s ...

ing that formed during sedimentation and controlled the distribution of the sediments. The primary mechanism for

sedimentation Sedimentation is the deposition of sediments. It takes place when particles in suspension settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to t ...

was originally subsidence creating accommodation (space for sediments to accumulate), followed by fault extension and more recently, sediment loading, i.e. the basin continuing to subside because of the weight of sediments within it. The eastern boundary of the main Perth Basin is the Darling Fault, topographically expressed as the

. Small outliers of the Perth Basin, such as the Collie Sub-basin, lie east of the Darling Fault.

Stratigraphy

Letters in brackets after the formation name represent shorthand symbols used on geological maps.

Quaternary and recent

The Quaternary and recent sedimentation of the Perth Basin is represented by thin, impermanent sand dune systems, biogenic limestones, sandstones and some shales deposited during the last ~20 million years and during ice ages.

Cretaceous

* Lancelin Formation
Upper Cretaceous glauconitic marl, infrequently developed on top of the Coolyena Group. * Coolyena Group - ''Upper Cretaceous marine sediments''. ** Poison Hill Greensand (Kcp) consists of pale yellow unconsolidated weathered clay, glauconitic sandstone and shale. It conformably overlies the Gingin Chalk and is in turn overlain by unconsolidated Quaternary sands and

alluvium Alluvium (from Latin ''alluvius'', from ''alluere'' 'to wash against') is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluv ...

. ** Gingin Chalk (Kcg) is a weakly consolidated fossiliferous

chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Cha ...

composed of

coccolith Coccoliths are individual plates or scales of calcium carbonate formed by coccolithophores (single-celled phytoplankton such as ''Emiliania huxleyi'') and cover the cell surface arranged in the form of a spherical shell, called a '' coccosphere'' ...

s, which interfingers and conformably overlies the Molecap Greensand. ** Molecap Greensand (Kcm) is a fine to medium grained marine

glauconite Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color which is very friable and has very low weathering resistance. It crystallizes with a monoclinic geometry. Its name is derived from the Greek () ...

sandstone which unconformably overlies the Osborne Formation. ** Osborne Formation (Kco) conformably overlies the Triassic Leederville Formation, and is composed of a marine sandstone, shale and interbedded shale-sandstone sequence from base upwards. * Warnbro Group - ''Early Cretaceous marine sediments representing a transgression''. ** Parmelia Formation (J-Kp) forms the topmost hydraulic unit of the Yarragadee Aquifer. It is composed of 100–200 m thickness of well sorted fluvial sandstone deposited in the latest Jurassic to early Cretaceous. It includes a basal siltstone of lagoonal to lacustrine environment, the Otowiri Siltstone, which acts as an

aquitard An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their charact ...

. ** Leederville Formation (Kll) sandstone, siltstone, shale and mudstone deposited in deeper marine environments, with finer sediments predominant in the upper part of the unit. The unit attains a maximum thickness in the axis of the Yanchep Syncline is ~700 m, and thins eastward to approximately 500 m. The Leederville Formation interfingers and merges with the underlying Parmelia Formation in the north of the Perth Basin. ** Gage Formation (Kwg) is a shallow lagoonal sandstone and shale deposited in shallow marine beach-dune-lagoonal environments.

Jurassic

* Yarragadee Formation (Jy) is one of the thickest units in the Perth Basin and forms a significant freshwater

aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials ( gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteri ...

. It is composed primarily of non-marine fluviatile feldspathic, poorly sorted sandstones which are porous and poorly cemented, hence allowing for considerable groundwater reserves. The Yarragadee Formation grades from a shale-siltstone dominated base to a cleaner sandstone in the upper portions of the formation, probably representing increased subsidence or filling of the basin during the late Jurassic. * Cadda Formation (Jd) conformably underlies the Yarragadee Formation and is composed primarily of shale and siltstone, with occasional beds of very coarse sandstone. The upper portions of the unit are composed of a marl grading into a marine

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 wh ...

, representing stable tectonics at this time. * Cattamarra Coal Measures (Jc) are a sequence of non-marine, probably fluviatile sandstones, shales and silts including

bituminous coal Bituminous coal, or black coal, is a type of coal containing a tar-like substance called bitumen or asphalt. Its coloration can be black or sometimes dark brown; often there are well-defined bands of bright and dull material within the coal seam, ...

, and are up to 300 m thick. The Cattamarra Coal Measures conformably overly the Eneabba Formation. * Eneabba Formation (Je) is a lower-Jurassic terrigenous red-bed unit composed of interbedded sandstone with variably coloured limestone. The Eneabba Formation is conformable with the Lesueur Sandstone in the south of the Perth Basin and unconformable with the Kockatea Formation in the north, suggesting it is transgressive.

Triassic

* Kockatea Shale (Trk) * Lesueur Sandstone (Trl) * Woodada Formation (Trw)

Economic geology

Cenozoic sand dune systems are locally being mined for

mineral sands Heavy mineral sands are a class of ore deposit which is an important source of zirconium, titanium, thorium, tungsten, rare-earth elements, the industrial minerals diamond, sapphire, garnet, and occasionally precious metals or gemstones. Heavy ...

, mainly

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

(a source of

titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...

) and

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 th ...

, by Iluka Resources and other companies. Various formations from Cenozoic to Jurassic age are economically significant freshwater

s. Younger aquifers are an important suppliant to reservoir water in the city of Perth and elsewhere. The Yarragadee Formation, one of the thickest formations in the basin, is a very good aquifer in the southern part of the basin. The Perth Basin contains Permian and Jurassic

coal Coal is a combustible black or brownish-black sedimentary rock, formed as stratum, rock strata called coal seams. Coal is mostly carbon with variable amounts of other Chemical element, elements, chiefly hydrogen, sulfur, oxygen, and nitrogen ...

measures, the most important being mined primarily for electricity generation in the Collie Sub-basin at

Collie Collies form a distinctive type of herding dogs, including many related landraces and standardized breeds. The type originated in Scotland and Northern England. Collies are medium-sized, fairly lightly-built dogs, with pointed snouts. Man ...

in the south-west of the state. The Perth Basin is also prospective for

natural gas Natural gas (also called fossil gas or simply gas) is a naturally occurring mixture of gaseous hydrocarbons consisting primarily of methane in addition to various smaller amounts of other higher alkanes. Low levels of trace gases like carbon ...

and oil, with recent exploration wells, including

Origin Energy Origin Energy an ASX listed public company with headquarters in Sydney. It is a major integrated electricity generator, and electricity and natural gas retailer. It operates Australia’s largest coal-fired power station at Lake Macquarie, N ...

/Arc Energy's ''Hovea 2'', confirming large resources of natural gas, but difficult

reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including control ...

geology and characteristics have prevented the full utilisation of these energy reserves. The oil is sourced from the Kockatea Shale. Currently, a junior minerals exploration company is exploring the basin north of Perth for glauconite to produce potash fertilizer.

Regions

Due to its shape and size, the basin has been surveyed and studied in distinctions such as offshore and onshore, north south, and central

References

Bibliography

Yarragadee Aquifer Geology report, 115kb pdf

External links

Cliff Head Oil Field, Roc Oil

3D-GEO Australia: Multiple Regional Studies completed over the last 10 years. Currently we have a full in-house study.
{{Coord missing, Western Australia Sedimentary basins of Australia Geology of Western Australia Swan Coastal Plain