The Perth Basin

Australia: The Land Where Time Began

Perth Basin

The Perth Basin, adjacent to the Yilgarn Craton, on the Western Australia margin, is an elongate rift basin that is oriented to the northwest, covering an area of about 100,000 km^2, between 27^o S and 34^o 30 S, between the Yilgarn Craton, from the Archean in the east and west to the edge of the continental shelf. The thin Cainozoic cover of the Perth Basin, mostly of non-marine origin, overlies deposits up to 12 km deep from the Ordovician to Middle Carboniferous to Cretaceous sediments. The first indications of possible subsurface sections were from pre-Cainozoic outcrops in the northern part of the basin, about 31^o30 N, as a result of a comprehensive outcrop study (Campbell, 1910). Ridges in the middle of the basin, the Leeuwin Complex and the Northampton Complex are fault-bounded ridges are outcrops of the Pinjarra Orogen the lies beneath the Perth Basin (Source 3).

The Perth Basin, on the west coast of the continent, extends 800 km along the coast. Its onshore portion ranges from 15-90 km wide, the basin being bounded on the east by the Darling Fault and Scarp. Unconfined aquifers comprise most of the groundwater in the Perth Basin, being usually about 30 m deep, but reaching up to 70 m. The aquifers are in the Quaternary Complex sand, silt and clay sediments that accumulated during the Pleistocene glacial phase when the sea level was rising and falling, large quantities of sand being blown about by the high strength winds of the dry periods at this time in Australia. This large unconfined aquifer in the basin supplies much of the water to Perth, as well as being of importance to the conservation of wetlands. Rainfall is its main source of recharge, with an additional more minor source of recharge being from the confined aquifers in sandstone units deposited in the Upper Jurassic and Lower Cretaceous. Deep artesian bores are required to access these deep aquifers. Some water is also provided by dams on the Darling Plateau.

The aquifers supplying water to Perth receive only about 11.5 % of the rainfall of the area, a mean annual of 869 mm, while the potential evaporation from the area is 1972 mm. The water table is high in spring and low in autumn. The groundwater is usually good, but the unconfined aquifer is susceptible to contamination with pollutants.

In the region of winter rainfall, in the southwest corner, the rainfall declined over the last half of the 20th century, and predictions based on the effects of global warming and climate change, it is expected the rainfall of the region could drop another 40 % by 2040, with an additional burden on the water supply of increasing evaporation resulting from increased temperatures. The implications of this are potentially serious, for increased salinisation as well as less water being available. The problems will be added to by the further penetration of salt water up the Swan, already reaching many kilometres up the river (White, 2000).

On the coastal plain there are many swamps, wetlands and shallow lakes, in places where the unconfined water table reaches the surface. Urban sprawl and agriculture have meant many have been reduced, modified or destroyed (White, 2000).

Situated along the western margin of the Australian continent, this a major tectonic province. North-striking faults that were formed during the rifting in the Permian, and reactivated in tectonic events, especially those associated with the breakup of Gondwana during the Late Jurassic-Early Cretaceous. The basin was compartmentalised by transfer structures, that include those normal and oblique to the major faults, into segments, each with a distinctive character. Throughout the rift stage of development of the basin a number of east-west transfer faults were active, perpendicular to the trend of the basin. These are only recognised in the on-shore, northeastern part of the basin. This corresponds with the depocentre where sediments accumulated during the Permian. These east-west structures terminate, or change the character of, normal faults that are northerly trending (Source 3).

Deformational features formed during the breakup of the Late Jurassic-Early Cretaceous are influenced by the NW striking transfer zones. In the sedimentary sequences no continuous fault plane has been identified with these zones. Termination and/or swings of major normal faults at the transfer zones characterise them. Across the Abrolhos Transfer Zone, on the basis of offset in the Beagle Fault System trend, similar strike-slip movement, of at least 16 km, has been recognised. Transfer faults in the adjacent Indian Ocean are similar in orientation, activation age and position, which suggests the 2 structures are contiguous (Source 3).

Clastic rocks, of Middle Carboniferous-Early Cretaceous age, are the main components of the northern section of the basin. The sediment forming these rocks were deposited in a rift system associated with the breakup of Gondwana in the Cretaceous. There were 2 major phases that have been recognised, extension in a south-westerly direction in the Permian and in the Early Cretaceous, during the breakup, transtension to the northwest (Source 3).

Along major north-striking faults at these times, but especially as Gondwana was breaking up, sinistral and dextral movements, respectively, are inferred, causing horizontal displacements, anticlines that were wrench-induced, as well as further faulting (Harris, 1994; Mory & Iasky, 1996; Song & Cawood, 2000). In the northernmost section of the Perth Basin, extending to 28^o 45 S, redbed facies of Ordovician age, that are typically associated with the southern Carnarvon Basin, are believed to probably be associated with the a rift phase that opened to the north at an earlier period (Source 3).

In West Australia, the post-breakup phase of late Early Cretaceous and Cainozoic is characterised by marine margin deposits of a transgressively phase, that are close to being undeformed in the onshore section of the Pert Basin. Deformation associated with the breakup of Gondwana during the Early Cretaceous dominates the structural configuration of the northern Perth Basin of the present, which is reflected in the present structural subdivisions. The Dandaragan Tough, a large half graben, and a ridge the extends along the present coast (Beagle Ridge - Dongara Saddle - Greenough Shelf) dates from the Early Permian. To the east of the Dandaragan Trough the Irwin Terrace is bounded by the Urella Fault, is implied by much lower maturity of the Permian succession to the east, when compared to that in the west, to have undergone significant movements during the Middle Triassic. Subsurface data is largely the basis for most other subdivisions, reflecting the the large block faults that were active at breakup (Source 3). Structures on a much smaller scale are believed to have been produced by tectonism from an earlier period. Where there is good quality data, such structures become evident, such as faults from the Middle Jurassic. Outcrops are the basis for only a few the entire delineations of a structural divisions, and it is not common for major faults to be exposed. Gasfields and oilfields that have been described have been associated with fault blocks that have been rotated and anticlines (Owad-Jones & Ellis, 2000; Buswell et al., 2004). (Source 3).

Deposits that are mostly sand or ferruginous duricrust from the Cainozoic cover most of the region. Weathering of underlying units in situ is suggested to be the origin of the sand (Newsome, 2000; Tapsell et al., 2003). From the 1970s to the mid-1990s a number of authors had suggested the sand had been brought to the site by aeolian transport over great distances. Along the Gingin Scarp, mostly in strandlines, heavy mineral sand deposits are present. The dating of the sands is not well known (Baxter, 1974). The duricrust that covers much of the higher parts of the region is believed to probably be from the Miocene. Aeolian carbonate sands from the Pleistocene is found to cover an area up 20 km inland from the coast (Source 3).

Sources & Further reading