Perth Basin facts for kids

The Perth Basin is a large, long area in Western Australia where many layers of sediment have built up over millions of years. It's found under the Swan Coastal Plain, which is west of the Darling Scarp. This basin also stretches out into the ocean. Many cities and towns, like Perth, Busselton, Bunbury, Mandurah, and Geraldton, are built on top of the Perth Basin.

How the Perth Basin Formed

The Perth Basin started to form a very long time ago, in the Late Permian period. This was when the supercontinent Gondwana began to break apart. The Australian land plate started to move away from the African and Indian land plates. This movement is called plate tectonics.

During the Permian, the area that is now the Perth Basin was part of a huge rift valley. This valley formed as the land plates were pulled apart. This pulling continued until the Jurassic period. As the plates moved, the central part of the valley sank down, creating a deep ditch called a graben. The sea then flowed into this graben, and layers of marine sediments (stuff from the sea) were laid down. The Perth Basin's shape is mostly due to large extensional faults. These are cracks in the Earth's crust where the land pulled apart. These faults formed as the sediments were being laid down and controlled where they ended up.

The main reason sediments built up here was because the land was sinking, creating space for them. Later, the pulling apart of the land and the sheer weight of the sediments themselves caused the basin to keep sinking.

The eastern edge of the main Perth Basin is marked by the Darling Fault. You can see this on the surface as the Darling Scarp, which is a steep slope. Some smaller parts of the Perth Basin, like the Collie Sub-basin, are found east of the Darling Fault.

Layers of Rock in the Basin

The Perth Basin is made up of many different layers of rock, called formations. These layers tell us about the Earth's history.

Recent Layers (Quaternary)

The newest layers in the Perth Basin are thin systems of sand dunes. There are also limestones (rocks made from sea creatures), sandstones, and some shales. These layers were laid down over the last 20 million years, including during ice ages.

Cretaceous Period Layers

• Coolyena Group
  • Poison Hill Greensand (Kcp): This layer is made of pale yellow clay, sandy rock, and shale. It sits on top of the Gingin Chalk.
  • Gingin Chalk (Kcg): This is a soft, chalky rock made of tiny fossils called coccoliths. It lies over the Molecap Greensand.
  • Molecap Greensand (Kcm): This is a sandy rock found over the Osborne Formation.
  • Osborne Formation (Kco): This layer is made of sandstone and shale. It sits on top of the Triassic Leederville Formation.
• Warnbro Group
  • Parmelia Formation (J-Kp): This layer is mostly well-sorted sandstone, about 100–200 meters thick. It was laid down by rivers. It includes a silty layer called the Otowiri Siltstone, which acts like a barrier to water flow.
  • Leederville Formation (Kll): This layer has sandstone, siltstone, shale, and mudstone. It was laid down in deeper ocean environments. It can be up to 700 meters thick in some places.
  • Gage Formation (Kwg): This is a sandy and shaly layer that formed in shallow lagoons, beaches, and dunes.

Jurassic Period Layers

• Yarragadee Formation (Jy): This is one of the thickest layers in the Perth Basin. It's a very important source of fresh water, known as an aquifer. It's mostly made of non-marine sandstone that lets water flow through easily.
• Cadda Formation (Jd): This layer is mostly shale and siltstone, with some coarse sandstone. The top part has a soft, chalky rock called marl that turns into marine limestone.
• Cattamarra Coal Measures (Jc): These are layers of non-marine sandstones, shales, and silts. They also contain bituminous coal and can be up to 300 meters thick.
• Eneabba Formation (Je): This lower-Jurassic layer is made of interbedded sandstone and different colored limestone.

Triassic Period Layers

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

What We Get from the Basin

The Perth Basin is important for many reasons, especially for its natural resources.

• Mineral Sands: The sand dune systems from the Cenozoic era are mined for valuable minerals. These include rutile (which is a source of titanium) and zircon. Companies like Iluka Resources extract these.
• Fresh Water: Many layers of rock, from the Cenozoic to the Jurassic age, hold important supplies of fresh water. These are called aquifers. The younger aquifers help supply water to cities like Perth. The Yarragadee Formation, which is very thick, is an excellent aquifer in the southern part of the basin.
• Coal: The Perth Basin has layers of coal from the Permian and Jurassic periods. The most important coal is mined in the Collie Sub-basin near Collie. This coal is mainly used to generate electricity.
• Natural Gas and Oil: The Perth Basin also has natural gas and oil. Recent drilling, like the Hovea 2 well by Origin Energy and Arc Energy, has found large amounts of natural gas. However, the way the rock layers are structured makes it hard to get all the oil out. The oil comes from the Kockatea Shale layer.
• Potash Fertilizer: Currently, a company is exploring the basin north of Perth for glauconite. This mineral can be used to make potash fertilizer, which helps plants grow.

Parts of the Basin

Because the Perth Basin is so big and long, it's often studied in different sections. These include offshore (in the ocean) and onshore (on land) parts, as well as northern, southern, and central areas.