Geology

The NAR is dominated by two major geological regions; the Yilgarn Craton and the Perth Basin separated by the Darling Fault, which runs up the middle of the region. To the north of the region where the Perth Basin forks, lies the Southern Carnarvon Basin and the Northamton Complex.

These major geological regions are further subdivided into their constituent units and bedrock geology. This is shown in the map below. Additional geology layers can be turned on and viewed in the full page map.

Image Map
The Yilgarn Craton is a large area of Archaean, granitic, stable, continental crust that underlies most of the south-west of Western Australia. It is intruded by swarms of dolerite dykes. Aeons of erosion have resulted in subdued relief. A thick profile of gritty clay saprolite (up to 50 m) derived from in-situ weathering typically mantles the crystalline bedrock (Stuart-Street & Clarke, 2005). It comprises six terranes, two of which underlie the NAR; the Southwest and Youanmi Terranes.

The Southwest Terrane consists of granites and granitic gneisses interlayered with metasedimentary rocks, with a single preserved greenstone belt (Geoscience Australia, 2013).

The Youanmi Terrane is entirely fault-bounded and consists of greenstones (volcanic and sedimentary rocks) deposited at ~2950 Ma to 2690 Ma that have been intruded by felsic magmatic rocks. It also includes several layered mafic-ultramafic intrusions emplaced at ~2800 Ma (Geoscience Australia, 2013).

The Perth Basin is an onshore and offshore sedimentary basin which extends approximately 1300 km up the coast of WA covering around 172,000 sq. km, with a maximum sediment thickness of 15,000 m (Geoscience Australia b, 2014). Structurally the Perth Basin is intensely faulted, with most of the faults having a north-north-westerly trend, although there are crosscutting faults (Stuart-Street & Clarke, 2005).

The basin extends past the length of the region and underlies the western half of the NAR, till just north of Geraldton. Here it splits and the Carnarvon Basin wedges down between the two forks. In this area it contains the Tumblagooda Sandstone partially capped by thin sequences of Mesozoic sediments. The landscape is characterised by a sandplain plateau (Stuart-Street & Clarke, 2005).

Click on the profile lines on the map below to view the cross-sections.

Source: Geoscience Australia
Image Map
The Carnarvon Basin occurs as a wedge in the north-western corner of the NAR between the Northampton Complex and the coast. In this area it contains the Tumblagooda Sandstone partially capped by thin sequences of Mesozoic sediments. The landscape is characterised by a sandplain plateau.
At the eastern boundary of the two basins lies the Northampton Complex (Northampton Block), an inlier of the Pinjarra Orogen, which underlies the Perth Basin. Formed of proterozoic gneissic crystalline bedrock, the Northampton complex is cut by swarms of northeast striking dolerite dykes. The southern and western parts of the Northampton Complex are partially capped by thin sequences of Jurassic sediments forming characteristic mesas and flat topped hills with steep break-away slopes that characterise the Moresby Ranges (Stuart-Street & Clarke, 2005).

Geoheritage

Geoheritage sites comprise those elements of the Earth’s geodiversity that are considered to have significant scientific, educational, cultural or aesthetic value (IUCN, 2014). The NAR is home to twenty such sites, one of which is the Pinnacles. Check out the map below to find the rest.

Click features on the map for more information. View full page map

Data courtesy of the Geological Survey of Western Australia, Department of Mines and Petroleum. © State of Western Australia 2013.
  • Arramall Cave and Lake System

    Arramall Cave is located just off the Brand Highway, around 30 km south of Dongara in ancient aeolian calcarenite limestone. Two major systems have been formed as a result of the overflow of Lake Arramall – River Cave and Arramall Cave, the latter of which contains the largest chambers and extends for approximately 1.8 km. Arramall caves flood infrequently when rains of sufficient volume to flood Lake Arramall are received. Details of the occurrence of selected fauna in Arramall and other caves in the surrounding area can be found on the Western Australian Speleological Group website.

  • Cattamarra Coal Measures

    The Cattamarra Coal Measures was formed during the Jurassic period from volcanic and sedimentary rocks. The formation comprises of siltstone, shale, claystone, coal and sandstone.

  • Coalseam Conservation Park

    A small reserve north of Mingenew that boasts a rich history (it is the site of the first coal mine in WA) and geology.  The Park is surrounded by farmland and provides a sanctuary for wildlife, and is well known for its resident striated cliff faces, marine fossils and wildflowers.

  • Moresby Range

    A remnant of the western edge of the Victoria Plateau, the Morseby Range is a prominent landscape feature to the north and east of Geraldton, with high conservation value. The Range’s distinctive lateritic flat tops extend to around 200 m in height and  run for about 33 km in length, roughly parallel to the coast and an average of 6 km inland. The geology is susceptible to erosion by the action of wind and water. See the Moresby Range Management Strategy and the Moresby Range Management Plan for more information.

  • Vegetation associated with the Northampton Block

    The Northampton Black is a geological formation of the Perth Basin. The vegetation associated with the Northampton Block is rich and contains 3 Critically Endangered, 3 Endangered and 8 Vulnerable flora species, which are threatened by introduced animals and weeds along with inappropriate fire regimes, all of which require management to ensure conservation.

  • Yarragadee Formation

    The Yarragadee formation was formed during the Jurassic period form volcanic and sedimentary rock. The Yarragadee formation consists of fine to coarse grained sandstone with thin interbeds of shale.

  • Zuytdorp Cliffs

    Named after the wreck of a Dutch East Indies trading ship, the Zuytdorp Cliffs originate just north of the Murchison River mouth at Kalbarri, and extend for approximately 150 km north to Steep Point. The Cliffs are part of the Tamala Limestone formation and are of heritage, cultural and geological significance.

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