The Perth Basin is a thick, elongated sedimentary basin in Western Australia. It lies beneath the Swan Coastal Plain west of the Darling Scarp, representing the western limit of the much older Yilgarn Craton, and extends further west offshore. Cities and towns including Perth, Busselton, Bunbury, Mandurah and Geraldton are built over the Perth Basin. Geological setting and evolution The Perth Basin began forming in the Late Permian during the breakup of Gondwana, as the Australian continental plate began rifting away from the African and Indian continental plates. During the Permian, what is now the Perth Basin was the eastern half of a rift valley that formed as the continental plates were pulled apart. This pulling apart, which continued until the Jurassic, led to the central zone subsiding as a graben allowing the sea to enter with the subsequent deposition of transgressive marine sediments. The Perth Basin architecture is dominated by listric, extensional faulting that formed during sedimentation and controlled the distribution of the sediments. The primary mechanism for sedimentation 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 Darling Scarp. 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 FormationUpper 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. 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 unconformably overlies the Osborne Formation. Osborne Formation (Kco) conformably overlies the Triassic Leederville Formation, and is composed of a marine sandstone, shale and interbedded