Devonian Great Barrier Reef"of Canning Basin, Western Australia"

Abstract

Abstract A well-preserved Middle to Upper Devonian barrier-reef belt is exhumed as a series of limestone ranges for 350 km along the northern margin of the Canning basin. The reefs are of international importance for reef research because of the excellence of exposures and the lack of extensive dolomitization or structural deformation. They are also known in the subsurface, where they are regarded as prime objectives for oil exploration. The reef complexes developed as reef-fringed limestone platforms standing tens to hundreds of meters above the surrounding seafloor, and flanked by marginal-slope and basin deposits. The platforms were built by stromatoporoids, algae, and corals in the Givetian and Frasnian, and by algae in the Famennian. The platform and basin deposits were laid down nearly horizontally, whereas the marginal-slope deposits accumulated on steep depositional slopes. Geopetal fabrics, which quantify depositional and tectonic-compactional dip components, provide paleobathymetric data concerning the reef complexes and their fossil biotas. Reef limestones, both frame built and clastic, formed narrow, discontinuous, wave-resistant rims to the platforms. The reef limestones were subject to strong submarine cementation, resulting in very early porosity destruction, whereas the back-reef deposits of the platform interiors remained largely uncemented and retained most of their primary porosity. Stylolitization and associated compaction were greatest in limestones whose primary porosity was not destroyed by early submarine cementation. Consequently the platform interiors have compacted more than the margins, resulting in the typical concave shape of many platforms. Cementation concomitant with stylolitization destroyed most of the porosity that remained in the limestones after early submarine diagenesis. The most porous rocks now are dolomites having secondary moldic porosity. Around the platform margins, early fracturing of reef limestones, probably resulting from earthquake tremors, led to the development of neptunian (sedimentary) dikes and the collapse of some sections of the reef rims as submarine rockfalls, in turn often initiating massive debris flows down the marginal slopes.