Great barrier reef: Origin, evolution, and modern development

Abstract

The Great Barrier Reef occupies the passive margin of northeast Australia, and is arguably the largest coral reef system to have ever existed on the planet. It represents one of a series of major architectural features forming the margin – the Eastern, Queensland and Marion Plateaux; the Pandora and Bligh Troughs; the Osprey Embay-ment; and the Queensland, Townsville, and Cato troughs. Coral reefs occur on all three plateaux in addition to the Great Barrier Reef. Some 3,600 reefs are estimated to form the Great Barrier Reef. The margin architecture seen today is a consequence of rifting, drifting, subsidence, sea-level/climate change, and continental collision. Coral reefs established on three occasions (1) in the early to late-mid-Miocene; (2) in the uppermost Miocene; and (3) in the Pleistocene. The Great Barrier Reef, as it is known, started in the late Pleistocene, as seen in drill core data from Ribbon 5/Boulder Reefs and at ODP site 820. On the outer shelf at Ribbon 5, drill core data show a reef section (0–96 m) above a rhodolith-dominated section (95–158 m) above a packstone/wachestone section (158–210 m). Initial reef establishment of the coral frameworks in the middle of the rhodolith section dates from some time after 465 ka while the firm establishment of the main reef section above 96 m dates from around 300,000 years. At Boulder Reef on the inner shelf, tentative dating places reef start-up at around 210,000 years. Reef initiation is therefore most probably facies controlled and not karst antecedent controlled. The ages and the succession of reefs at Ribbon 5 (six reefs) and Boulder Reef (four reefs) also testify to the fact that reefs have been destroyed and re-grown again on at least six occasions on the outer shelf and four occasions on the inner shelf. The modern Great Barrier Reef is the last expression of regrowth following the last post-glacial transgression. Almost certainly, antecedence has assumed greater relevance through each new growth phase. The development of the modern reef system is documented through the largest geoscientific reef database for any system in the world (160 drill holes in 50 reefs and 750 dates). Start-up occurred 7–10,000 years BP most often on eroded (karstified) remnants of the previous (125 ka) reef phase. Nearly 60% of all reefs cored and dated reached sea level in the interval 5,000–6,500 years BP meaning that nearly 60% of the reefs studied have been subject to a shallow water energy regime for most of the time that they have been growing; growth therefore reflects this. Different reef growth strategies reflect the interaction of substrate depth, sea-level rate of change, and coral growth rate. A major change in growth and bio-and sedimentary facies occurs when the growing reef approaches sea level – the formation of algal and coral flats reflect biofacies changes and sand sheets and rubble flats reflect sedimentary facies changes. Five broad carbonate facies are recognized; three are framework facies and two are detrital facies: 1. Coralline algal facies. These are laminated crusts, often centimeters thick and/or encrustations on branching or massive corals. The facies is best developed on windward margins, but encrusting corallines coat most dead or dying surfaces in all intertidal reef environments. 2. Branching coral facies. Growth form varies with depth, light, and energy. In shallow water, variations in energy define wide differences in species composition and diversity. Water-depth changes produce similar compositional variations. 3. Massive (head) facies. As with the branching forms, water depth and energy define diversity. 4. Sand facies. These are usually medium to coarse sand comprise coral and corallines, benthic foraminifera, and minor Halimeda. Forms large sand sheets between coral flats and lagoons and also infills lagoons. 5. Rubble facies. These are comprised of massive, platey, and branching corals. Produced either by in situ collapse (in lagoons) or by energy-related breakage and transport. Three growth models are presented, two for mid-shelf platform reefs (One Tree and Stanley Reefs) and one for outer shelf barrier reefs (Ribbon 5). It is concluded that most Holocene reefs in the Great Barrier Reef, with the possible exception of fringing reefs, have undergone the same basic processes of growth from an antecedent platform to sea level and then growth controlled by the direction and energy of the impinging energy. This is common in the three models defined above. The models are however of two totally different reef growth environments – the shelf edge and the mid-shelf platforms. They are almost two end members with different characteristics produced by the same parameters. In both cases, however, the dominant control is substrate, to produce a linear outer shelf feature and crescentic/lagoonal mid-shelf features. However, in the mid-shelf reefs, patterns of reef development are recognized which have specific characteristics which appear to overlap and which are probably related to the depth, size, and shape of the Pleistocene substrate, the timing of reaching sea level, and the total energy within the physical environment. In an evolutionary sense, reefs can be judged to be juvenile, mature, and senile. Thus, crescentic reefs may grow into lagoonal platform reefs and eventually into flat-topped platforms with live coral growth only around the perimeter. Most importantly, the rate of progress in this evolution is a function of the size and depth of the platform and the energy impinging in the still stand system. Thus, small reefs (Wreck in the Capricorns) will progress to senility rapidly while Stanley (large reef) is yet to achieve maturity. Evolutionary progression may occur through more than one highstand growth phase. Predicted future climate changes will accelerate evolutionary progression.