Publication Name: The Canning Basin, W.A.
Authors: Phillip E. Playford
Date Published: December 1984
Number of Pages: 37
Reference Type: Book Section
Abstract:
Most Devonian reef complexes in the Canning Basin developed as high-relief reef-fringed limestone platforms,flanked by steep marginal slopes which descended to water depths of up to several hundred metres. However, some platforms were low-relief banks having little or no reef around their margins, and relatively low marginal slopes.
The platform-margin to marginal-slope zone is the most complex part of the carbonate sequence, being characterized by abrupt biological, lithological, compactional, and diagenetic changes, depositional iscordances, early fracturing, and complex cavity systems. The zone is also of particular interest to petroleum and mineral explorers.
During the Givetian to Frasnian, reef-fringed limestone platforms of the Pillara Cycle were constructed by stromatoporoids, corals, and cyanobacteria. A mass extinction of shallow-water metazoans occurred at the close of the Frasnian, so that the succeeding Famennian platforms of the Nullara Cycle were built primarily by cyanobacteria.
Reef-margin and reef-flat limestones were subject to pervasive early cementation, and formed rigid averesistant
rims around most platform margins. Organically bound and early-cemented reefal-slope limestones immediately below the platform margins accumulated on very steep marginal slopes (up to vertical), which flattened progressively passing down into deeper water in fore-reef limestones. Maximum depositional slopes in loose talus were up to about 35?. The marginalslope deposits include debris flows, huge allochthonous reef blocks, adnet breccias, and reefal tongues and bioherms built primarily by cyanobacteria and sponges.
Platform margins are divided into upright-scarp, upright-rollover, retreating, backstepping, advancing, and pinnacle-reef types. Advancing margins characterize the Famennian reef complexes; the other types occur primarily in the Frasnian, where they are often associated with platform-margin unconformities resulting from submarine erosion.
Early submarine cementation was of fundamental importance in evolution of the complexes, both during growth and after burial. Extensive early cementation by high-magnesium turbid fibrous calcite and cloudy microcrystalline calcite largely destroyed the high initial porosities and permeabilities of the reef and reefal-slope deposits. The most porous carbonates remaining at the time of burial were in the back-reef and fore-reef
deposits, although some parts of those deposits were also cemented at an early stage.
After burial the remaining porous deposits were subject to mechanical and pressure-solution compaction. Dolomitization is most widespread in those carbonates that retained significant porosity and permeability at the
time of burial, prior to pressure-solution compaction, which destroyed most of the remaining pore space. The
highest porosity now found in the carbonates is secondary moldic porosity in dolomite.
Fissuring of early-cemented limestones in the reef and reefal-slope subfacies and parts of the fore-reef and back-reef subfacies gave rise to extensive networks of neptunian dykes and sills, and to the collapse of some sections of the platform margins and marginal-slope deposits. Such collapse in turn led to debris flows and the accumulation of reef talus (including huge isolated blocks) on the marginal slopes.
The reef complexes are being explored for lead-zinc deposits in outcrop and oil in the subsurface. A small but significant oil discovery was made at Blina during 1981 in a Famennian platform margin.