PLATFORM MARGINS AND MARGINAL SLOPES, DEVONIAN REEF COMPLEXES OF THE CANNING BASIN, WESTERN AUSTRALIA

Middle and Late Devonian reef complexes developed in the Canning Basin as reef-fringed limestone platforms flanked by marginal slopes that descended into basins as much as 300 m deep. Terrigenous conglomerate, derived from the mountainous hinterland, interfingered with the carbonates.

The reef complexes consist of two major sequences: the retreating Pillara Sequence (Givetian-Frasnian) and the advancing Nullara Sequence (Famennian). Five third-order sequences, defined by brief episodes of platform emergence and backstepping during the Frasnian, have been precisely dated using conodonts. The Pillara platforms were built by calcimicrobes, stromatoporoids, and corals, whereas the Nullara platforms were built almost solely by calcimicrobes, following a mass extinction of metazoans at the close of the Frasnian, associated with a brief period of platform emergence. Most platform margins are marked by narrow reefs, less than 200 meters wide, constructed primarily by calcimicrobes. The reef frameworks were strongly cemented by micritic and fibrous cements, forming rigid wave-resistant rims to the platforms. The major part of each platform is occupied by biostromal and clastic back-reef deposits.

Microbial binding and precipitation were important in forming and maintaining steep marginal slopes in front of the platforms, with the slopes ranging from more than 50 degrees at the top to less than 5 degrees at the base. The slope deposits include megabreccia debris flows and huge allochthonous blocks of reef, derived from reef-collapse events initiated by contemporary earthquake activity. Frasnian stromatolite-barite-sulfide buildups, up to 3.5 km long, grew in muds at the base of some marginal slopes and along synsedimentary faults. They formed over cool seepages of fluids derived by compaction of anoxic basinal muds.

Synsedimentary normal and strike-slip faulting influenced the development of some reef complexes, but there has been relatively little fault activity since the Early Permian. Successive continental ice sheets covered the area in the mid Carboniferous to Early Permian, planing down the Devonian limestones to an essentially level surface, on which extensive subglacial karst was developed, as caves, karst corridors, solution dolines, tower karst, and tunnels.