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Recent and Pleistocene Coral Reefs of Australia

1950; University of Chicago Press; Volume: 58; Issue: 4 Linguagem: Inglês

10.1086/625751

1537-5269

Rhodes W. Fairbridge,

Marine and fisheries research

The contemporary and Pleistocene coral reefs of the Australian shelf regions illustrate reef development in a relatively stable epicontinental environment-the most common facies in geology. All reef forms-fringing, barrier, atoll, and platform reefs- and five varieties of coral island are found here. The vertical distribution of corals is controlled mainly by light, a function of turbidity, and the areal distribution mainly by temperature (minimum is about 18° C. average for the coldest month). The shape of coral reefs is generally controlled by wind, wave, and current, although complex forms due to old geomorphologic or structural controls later modified by wind, etc., are also common. Normally, a small reef patch, in a region of a single dominant wind and current, will grow into a horseshoe form, then to an atoll, and will eventually fill up to become a large platform. Coral islands are due either to accumulation of sand or shingle, in which case they tend to have an oval or streamlined shape, or to a Recent eustatic drop in sea level, leading to emerged coral limestones that tend to be eroded into irregular scalloped patterns. "Negroheads" may be formed by such erosion and by jetsam-like accretion on the reef margin. Reef borings and structural and geomorphologic features show that ecologic, tectonic, and eustatic factors may all play a part in reef development. The reality of Pleistocene-Recent eustatic oscillations is proved on geological and physiographic grounds, regardless of coral reefs. Most of the reefs in the Australian area are believed to have grown up from initial coastal fringing reefs during eustatic lows, but the foundations of many of the deeper reefs (in the Great Barrier Reef and on the outer Sahul Shelf) must have subsided tectonically. Physiographic evidence shows that an antecedent platform (a continental shelf) existed prior to the main development of the reefs. Sedimentation is extremely rapid in the enclosed Great Barrier Reef lagoon but is slow on the exposed northern and western shelves. In both living and ancient reefs the proportion of actual colonial corals grown in situ is extremely small in relation to the enormous quantities of "coralline" sedimentary debris. The ratio of reef structures to areas of "normal" shelf sedimentation, even in the most prolific coral areas, is also very small. The thickness of reef structures in the Great Barrier Reef lagoon has been proved by boring not to exceed 500 feet. Physiographic evidence shows that most of the near-shore reefs are not more than 60 feet thick. Similar evidence shows that in extreme marginal areas (which probably underwent Quaternary subsidence) certain reefs may exceed 1,500 feet in thickness (e.g., on the outer Sahul Shelf). The Great Barrier Reef borings and those of Funafuti, Bikini, etc., show that tectonic subsidence was fairly rapid in all of them. Concentrations of $$MgCO_{3}$$ of about 5-16 per cent are evidence of stillstands; lower percentages are evidence of rapid submergence. Submarine dolomite formation requires reducing conditions-an environment present in the closed atoll ring of Funafuti but absent in the Barrier Reef. Lack of dolomite in Queensland borings is thus not surprising.