Perth basin
From Wikipedia, the free encyclopedia
The Perth Basin is a thick sedimentary basin developed offshore of the Yilgarn Craton in several stages beginning in the Late Permian.
Contents |
[edit] Geological setting and evolution
The Perth Basin began forming in the Late Permian during the breakup of Gondwana, as the Australian continental mass began rifting away from the African and Indian continental segments.
During the Perman, the Perth Basin was marginal to a rift basin. This resulted in the deposition of transgressive marine sediments, filling in the graben areas of the basin, with continued extension until the Jurassic. The Perth Basin architecture is dominated by listric, extensional faulting during prolonged sedimentation.
The primary mechanism for sedimentation was originally subsidence creating accommodation, followed by fault extension and, more recently, sediment loading. The principal detachment and eastern boundary of the Perth Basin is the Darling Scarp, which essentially prepresents the extent of the Darling Fault
[edit] Stratigraphy
[edit] Quaternary and Recent
The Quaternary, Cainozoic 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.
[edit] 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 unconsolidate Quaternary sands and alluvium.
- Gingin Chalk (Kcg) is a weakly consolidated fossiliferous chalk composed of coccoliths, which interfingers and conformably overlies the Molecap Greensand.
- Molecap Greensand (Kcm) is a fine to medium grained marine glauconite sandstone which uncconformably 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.
- 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.
[edit] Jurassic
- Yarragadee Formation (Jy) is one of the thickest units in the Perth Basin and forms a significant freshwater aquifer. 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 decreasd subsidence or filling of the basin dring 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, 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, and are up to 300 m thick. The Cattamarra Coal Measures conformably overly the Eneabba Formation.
- Enneabba Formation (Je) is a lower-Jurassic terrigenous red-bed unit composed of interbedded sandstone with variably coloured limestone. The Enneabba Formation is conformable with the Lesueur Sandstone in the south of the Perth Basin and unconforable with the Kockatea Formation in the north, suggesting it is transgressive.
[edit] Triassic
- Lesueur Sandstone Formation (Trl)
- Woodada Formation (Trw)
- Kockatea Shale (Trk)
[edit] Economic geology
The quaternary and cainzoic sand dune systems are currently being mined for titanium as rutile and zircon in mineral sands mines, some of which are controversial for environmental reasons, by Iluka Resources and other companies.
The Perth Basin contains significant Jurassic coal measures, which have been worked at Collie in the south-west of the state, primarily for electricity generation.
The Perth Basin is prospective for natural gas, with recent exploration wells, by Origin Energy/Arc Energy's Hovea 2 well, confirming large resources of natural gas, but difficult reservoir geology and characteristics have prevented the utilisation of these energy reserves.
Onshore, the Perth Basin is also highly prospective for oil and petroleum exploration, with minor successes struck recently. The oil producing unit is the Kockatea Shale.
