THE DEVONIAN GREAT BARRIER REEF OF THE CANNING BASIN, WESTERN AUSTRALIA

A remarkable example of an exhumed Middle to Late Devonian barrier-reef belt extends for about 350 km along the northern margin of the Canning basin in Western Australia. The reefs form a series of rugged limestone ranges cut by deep river gorges which provide spectacular sections through the reefs and associated facies. The gross morphology of the ranges and intervening valleys closely resembles that of the Devonian seafloor, so that from the air the reefs are displayed much as they were in Devonian time.The Canning basin reef complexes offer exceptional opportunities for carbonate research because of the excellence of exposures and the wide variety of facies represented; moreover the rocks are little deformed, are not dolomitized extensively and are unmetamorphosed. Some facies have undergone significant compaction through stylolitization; however, most structures and textures in the limestones can be shown to have had depositional or early diagenetic origins.The reef complexes developed as reef-fringed limestone platforms flanked by marginal-slope and basin deposits. They were built by stromatoporoids, algae, and corals in the Givetian and Frasnian and by algae in the Famennian. The platform and basin facies were laid down nearly horizontally, whereas the marginal-slope facies accumulated with steep depositional dips away from the platform. Marginal slopes commonly were as high as 35° in loose talus and were up to vertical where algal binding occurred in association with early lithification. Geopetal fabrics quantify depositional and tectonic/compactional components of observed dips for paleobathymetric studies of the complexes and their fossil biotas.Four main types of platform margin are present: retreating, back-stepping, upright and advancing. The advancing type is characteristic of the Famennian platforms, whereas the other three are typical of the Frasnian. Pinnacle reefs developed during periods of rapid subsidence, especially during the middle Frasnian, are associated with back-stepping and retreating platform margins.Very early submarine cementation was widespread around the platform margins and on parts of the marginal slopes, but it was not generally extensive in the platform interiors. Early fracturing of reef limestones along the platform margins, probably associated with earthquakes, resulted in the development of neptunian dikes and the collapse of some sections of the reef as submarine rockfalls. These often initiated massive debris flows, many of which carved channels in and, somewhat deformed, the underlying marginal-slope deposits.