Underlying an area of 430 (lOO km2 onshore and 165000 km2 offshore in Western Australia the Canning Basin contains about 10 000 m of Palaeozoic, Mesozoic, and Cainozoic sedimentary rocks. The Fitzroy Graben is the major structure in the basin. Faults separate it from the Leveque and Lennard Shelves to the north, and a faulted hingeline separates it from the Broome Arch to the south. The basin deepens again to the south between the arch and the Anketell and Tabletop Shelves, on the margin of the Pilbara Block. The deeper parts of this irregular synclinal zone form the Kidson Sub-basin and the Willara Suh basin. The floor of the basin generally deepens offshore towards the edge of the present-day continental shelf, where major depocentres are the Bedout and Rowley Sub-basins-northeasterly trending depressions that owe their origin to continental rifling and seafloor spreading. The Canning Basin sequence has been divided into 11 informal basin-wide intervals, each of which consists of a number of roughly contemporaneous rock units. Each interval contains sediments deposited during one or more transgressions or regressions of the sea, and the boundaries between them are typically unconformities or disconformities. The earliest record of sedimentation is of Lower Ordovician shallow-marine clastic and carbonate rocks (interval 1) in the basin onshore. Earth movements and prolonged erosion preceded the deposition of marine sediments-including widespread evaporites-and continental red beds during the Early and Middle Devonian (interval 2l. Major subsidence of the Fitzroy Graben in the later Devonian coincided with the deposition of turbidites, followed by finer sediments, adjacent to carbonate platforms (interval 3) in an environment of recurrent diastrophism which culminated in earth movements correlated with the Alice Springs Orogeny. Marine clastics-interbedded with glacial debris during the Late Carboniferous to Early Permian and with fluvial deposits in the east during the Late Permian (interval 4)-accumulated, with only a brief hiatus, until the Triassic, when deltaic and fluvial sediments prograded across them (interval 5l. Further earth movements preceded the deposition of deltaic sediments during the Early and Middle Jurassic and marine clastics for much of the Late Jurassic (interval 6l. Late Tithonian to early Neocomian marine sedimentation in the offshore subbasins (interval 7) was interrupted by earth movements that may correlate with the separation of the Indian and Australian continents. Upper Neocomian deltaic sediments were inundated in the Aptian by a basin-wide transgression of the sea, in which sedimentation may have persisted in the offshore sub-basins until the early Cenomanian (interval 8l. The later sedimentary and diastrophic history of the basin is largely confined to the offshore sub-basins, where major transgressions of the sea are recorded in the Turonian to early Paleocene (interval 9), late Paleocene to late Eocene (interval 10), and early Miocene to Recent (interval 11 ). Oil exploration in the Canning Basin must now be regarded as high risk: from eighty petroleum exploration wells drilled up to 1976, the most encouraging show to date is a few litres of oil from Lower Carboniferous sandstone. Offshore, temperatures in the Palaeozoic of the Fitzroy Graben appear to be too high for oil generation to be still continuing, whereas the Mesozoic section has generally not yet reached temperatures adequate for the generation of oil. The best potential onshore rests with the testing of Fairfield Group sediments, Devonian reef trends, and Permian plays in the southeast of the Fitzroy Graben. The unknown and virtually untested offshore Triassic section may also have some potential.
