Platform-Margin and Marginal Slope Relationships and Sedimentation in Devonian Reef Complexes of Canning Basin, Western Australia: ABSTRACT

Abstract

Devonian limestone platforms in the Canning basin were generally rimmed by reef-margin and reef-flat deposits, constructed by stromatoporoids, algae, and corals in the Givetian and Frasnian, and by algae in the Famennian. However, some platforms were low-relief banks with little or no reef development. End_Page 516------------------------------ Reef rims were flanked by steeply dipping marginal-slope deposits that descended to depths up to several hundred meters. These deposits include debris flows, huge allochthonous reef blocks, scheck breccias, and reefal tongues and bioherms built primarily by sponges and algae. Platform margins are classified as advancing, retreating, upright backstepping, and roll-over types. Advancing margins are typical of the Famennian reef complexes; the others occur principally in the Frasnian and Givetian, where they are often associated with platform-margin unconformities resulting from submarine erosion or margin collapse. The reefs and slowly deposited parts of the marginal-slope facies were subject to pervasive early submarine cementation by fibrous high-magnesium calcite (now radiaxial spar). The strongly cemented reef limestones formed rigid wave-resistant rims to the platforms. Fracturing of these limestones, probably largely associated with earthquake shaking, gave rise to extensive networks of neptunian dikes and sills, and to the collapse of some sections of the margins. Such collapses in turn initiated debris flows and the deposition of allochthonous reef blocks on the adjoining marginal slopes. The reef complexes are being explored extensively for lead-zinc deposits in outcrop and oil in the subsurface. A significant oil discovery was made in a Famennian platform margin (the Blina field) in 1981. End_of_Article - Last_Page 517------------