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The Holocene growth of fringing and nearshore reefs on the GBR is examined. A review of data from 21 reefs indicates that most grow upon Pleistocene reef, boulder, and gravel, or sand and clay substrates, with no cored examples growing directly over rocky headlands or shores. Dated microatolls and material from shallow reef-flat cores indicate that fringing and nearshore reefs have experienced several critical growth phases since the mid-Holocene: (1) from initiation to 5500 YBP, optimum conditions for reef and reef-flat growth prevailed; (2) from 5500–4800 YBP, reef-flat progradation stalls in almost 50% of the reefs examined; (3) of reefs prograding post-4800 YBP, approximately half ceased active progradation around 3000–2500 YBP; (4) reefs prograding to present do so at rates well below mid-Holocene rates; (5) a group of nearshore reefs has established since 3000 YBP, in conditions traditionally considered poor for reef establishment and growth. Importantly, many of the reefs that appear to ha...

Several modes of coral reef growth are found along the edge of Australia's Great Barrier Reef (GBR), determined by the morphology and slope of the shelf edge, especially between -50 m and -100 m, and the velocity of tidal currents near the surface. The simplest forms are the flood tide deltaic reefs and ribbon reefs of the far north. The shelf margin of the central GBR is characterized by lines of submerged reefs that continue on the ocean (northeastern) side of the Pompey Reefs, which are the largest and most complex in the entire GBR. Combining interpretations of reef evolution from the simpler marginal reefs with data collected from the Pompey Complex, a model of evolution on a stepped continental shelf margin is developed, involving initiation as ribbon reefs, formation of both ebb and flood tide deltas (which have formed the foundation for further reef growth), and incorporation of at least one line of previously submerged reefs on the open ocean side by progradation of the deltaic structures to form large lagoonal reefs. Although the reefs cover a smaller area than the extensive reefs of the continental shelf, which could have grown only at higher Quaternary sea levels, the smaller area of shelf-marginal reefs may contain a longer record of coral growth than that of the better-known shelf reefs.

The platform and fringing reefs of Torres Strait are morphologically similar to reefs of the northern Great Barrier Reef to the south, except that several are elongated in the direction of the strong tidal currents between the Coral Sea and the Gulf of Carpentaria. Surface and subsurface investigations and radiocarbon dating on Yam, Warraber and Hammond Islands reveal that the initiation and mode of Holocene reef growth reflect antecedent topography and sea-level history. On the granitic Yam Island, fringing reefs have established in some places over a Pleistocene limestone at about 6 m depth around 7000 years BP. Emergent Holocene microatolls of Porites sp. indicate that the reefs have prograded seawards while sea level has fallen gradually from at least 0.8 m above present about 5800 years BP. On the Warraber Island reef platform drilling near the centre indicated a Pleistocene limestone foundation at a depth of about 6 m over which reefs established around 6700 years BP. Reef growth lagged behind that on Yam Island. Microatolls on the mature reef flat indicate that the reef reached sea level around 5300 years BP when the sea was around 0.8–1.0 m above present. On the reef flat on the western side of Hammond Island bedrock was encountered at 7–8 m depth, overlain by terrigenous mud. A progradational reef sequence of only 1–2 m thickness has built seaward over these muds, as sea level has fallen over the past 5800 years. Reef-flat progradation on these reefs is interpreted to have occurred by a series of stepwise buildouts marked by lines of microatolls parallel to the reef crest, marking individual coalescing coral heads. Detrital infill has occurred between these. This pattern of reef progradation is consistent with the radiocarbon dating results from these reefs, and with seismic investigations on the Torres Reefs.

Only a small proportion of funding for global change research programs has been directed at planning for sea level rise through mitigation projects. Although recent estimates of the rise by 2050 (25–50 cm) are reduced compared to previous predictions and are now within the ranges achieved in the geological past, considerable changes will still take place in the coastal zone, especially when greater and more storm surge incidence and rainfall intensity factors are considered. Adjustments available include: no action; coastal defence based on present rates of sea level rise; retreat with planning and social engineering; protection; and abatement of global change processes. Actual decisions for specific reactions will vary according to risk and impact assessment. Production of Coastal Vulnerability Indices (CVI) using Geographical Information Systems (GIS) is advocated at several scales for the identification of regional response planning strategies.

Introduction Fringing reefs and nearshore reefs occur close to mainland or high island shores. Fringing reefs are often shore-attached, with reefal sediments extending from the fore reef to shoreline, although back reef areas can be shallowly submerged to form flooded depressions up to several hundred meters wide. However, along most of the inner Great Barrier Reef (GBR) where the majority of fringing reefs occur, relative sea level has fallen by ∼1.5 m since the mid-Holocene highstand (Section 3.4.3), and it is more typical for back reef areas to be higher than the reef crest (discussed further in Sections 7.4 and 7.5). Nearshore reefs are very close but not directly attached to mainland or high island shores, and are separated from these shorelines by back reef areas dominated by non-reefal sediments that are shallower than 10 m (Larcombe et al ., 2001). Following Darwin's (1842) classification, fringing reefs are interpreted as the first stage in a long-term evolutionary sequence, in which reef growth is initiated over a substrate on the shoreline, but later becomes separated from the shore as the underlying substrate gradually subsides and a leeward lagoon develops. Fringing reefs are thus often considered as characteristic of stable or uplifting coasts, where the geomorphological transition from fringing to barrier reef is suppressed by a lack of subsidence. However, as discussed in this chapter, the influence of slow geological subsidence on modern fringing reef growth on the Great Barrier Reef and elsewhere is often relatively small compared to that of more rapid changes in relative sea level through the Holocene.

Introduction For much of its length even the innermost reefs of the Great Barrier Reef (GBR) are beyond sight of the mainland. The main reef tract occupies the outer 30% to 50% of the shallow ( et al ., 1983), and as Lloyd (1977) noted, the geology of the shelf is closely related to the onshore geology of Queensland. For a large part of their history the GBR foundations have been part of the mainland as, during major glaciations of the Quaternary, sea level dropped to expose the shelf edge. Today, the direct influence of the mainland (including Papua New Guinea) in the form of runoff, sediment, and nutrients may be restricted to the Reef north of 18° S but along the entire coastline south to Fraser Island the sedimentary and geomorphological record of the last 8000 years or so greatly augments our understanding of the final phases in the evolution of the GBR which are the focus of this book.

Introduction Two distinctive types of islands occur on the Great Barrier Reef (GBR), the continental or high islands (hereafter referred to as high islands), and the low or reef islands (hereafter referred to as reef islands). Earlier workers defined high islands as those composed of elevated reef limestones or of non-limestone lithologies (Stoddart and Steers, 1977); on the Great Barrier Reef they are essentially continental outcrops that were separated from the mainland as the Holocene transgression flooded the continental shelf (Chapter 3). The high islands exhibit a range of topography and morphology similar to that observed along the mainland coast, which in large part reflects lithology and structure. Some high islands such as Stone Island (20° 03′ S, 149° 15′ E) are relatively low and gently sloping, but others such as Hinchinbrook Island (18° 20′ S, 146° 15′ E) are high, rugged, and drop steeply to the sea (Fig. 2.6). A good account of the relationships between mainland and high island geology and morphology is provided by Stoddart (1978). Reef islands are, in contrast, usually relatively low-lying accumulations of reef-derived sediments, portions of which may be lithified. On the GBR, reef islands have formed on reef platforms that have reached sea level after the mid-Holocene (see Sections 3.4.3 and 11.7), and which focus incident wave trains at a consistent locus over which deposition and accretion can take place (Fig. 4.11).

This 2007 book reviews the history of geomorphological studies of the Great Barrier Reef and assesses the influences of sea-level change and oceanographic processes on the development of reefs over the last 10,000 years. It presents analyses of recently attained data from the Great Barrier Reef and reconstructions of the sequence of events which have led to its more recent geomorphology. The authors emphasise the importance of the geomorphological time span and its applications for present management applications. This is a valuable reference for academic researchers in geomorphology and oceanography, and will also appeal to graduate students in related fields.

Introduction: shelf-edge morphology The gently sloping continental shelf on which the Great Barrier Reef (GBR) is situated starts to steepen at depths between 40 and 50 m. In the far north the change of slope is rapid and coral reefs are located on the very edge of the drop-off. Along the central GBR the slope is more gentle and this section of the outer shelf is dominated by submerged reefs which only rarely reach the sea surface. The outermost reefs of the south central GBR are the massive Pompey Reefs which appear to be located on the shelf edge but which have been shown more recently (Hopley, 2006) to be on the highest of a series of shelf-edge steps descending to ∼80 m, at which depth the shelf drops away steeply. Distinctive shelf-edge reefs are not found around the Swain Reefs nor opposite the southernmost Bunker–Capricorn reefs. A feature of all the shelf-edge reefs is that they are linear and parallel to the contours of the shelf. Even the large Pompey Reefs are aligned parallel to the shelf-edge bathymetry, have a series of submerged linear reefs to seawards, and contain linear features which are persistent along the entire complex. The best known of the shelf-edge reefs are the ribbons of the northern GBR. Fairbridge (1950, 1967) suggested that they represent the upgrowth of fringing reefs established at lower sea levels and all the evidence suggests that this is very probable, not just for the ribbons but also for all the shelf-margin linear reefs which are discussed in this chapter.

Introduction The evolutionary classification for coral reefs of Hopley (1982), although applied elsewhere, was initially devised specifically for the reefs of the Great Barrier Reef (GBR). The classification (Figs. 5.6 and 5.7, Table 5.3) shows antecedent platforms being recolonized, with reef growth during initial juvenile stages being largely vertical, enhancing the original relief of the antecedent surface. A mature phase after reefs reach or catch up with sea level follows, during which both growth and sedimentation infill and obliterate the previously constructed relief. The senile phase consists of flat-topped or planar reefs. At the time the classification was devised there were few drill holes which had penetrated the Holocene reef on the GBR and great reliance was put on seismic refraction survey to identify the depth of the Holocene (Harvey, 1977a, b, 1980; Harvey et al ., 1979) and approximately 102 radiocarbon dates from surface or near-surface samples from 24 reefs (Hopley, 1982, table 9.2). A number of intuitive presumptions were also made at this time and these formed many of the questions asked by the drilling projects on the GBR subsequent to about 1980. These studies have built up a considerable data bank on the age and structure of mid-shelf reefs (Table 8.1). Outer shelf or shelf marginal reefs such as the ribbons of the northern GBR, some of the outermost reefs of the central GBR, and the unique Pompey Reefs of the stepped shelf margin of the south central GBR are discussed in Chapter 9 whilst Chapter 11 synthesizes the structural and growth data of all reefs, fringing, mid-shelf, and outer shelf.