Fringing reef growth and morphology: a review

Transcription

1 Earth-Science Reviews 57 (2002) Fringing reef growth and morphology: a review D.M. Kennedy*, C.D. Woodroffe School of Geosciences, University of Wollongong, Wollongong, NSW 2522, Australia Received 15 February 2001; accepted 15 June 2001 Abstract Fringing reefs are generally not simple veneers of coral growth along tropical shorelines. Extensive research over the past few decades, based on radiocarbon dating of Holocene reef deposits, has indicated that they can develop in a complex variety of ways even though the surface morphology may appear relatively simple. The principal factor that appears to determine the growth and morphology of fringing reefs is the available accommodation space. Sea-level fluctuations are important, primarily because the sea surface determines the absolute accommodation space for a given reef. This means that a reef established during a period of sea-level rise will be able to accrete vertically as space is created above it. If, however, the reef establishes at, or grows to, the sea surface, thereby occupying all the available accommodation space, it can no longer accrete vertically and begins to build laterally. The morphology and chronostratigraphy of a range of Holocene fringing reefs are described, on the basis of which six fringing reef growth models are identified. In model A, the fringing reef is established at depth and primarily accretes vertically towards the sea surface. Reef growth in model B initiates at sea level and due to the lack of vertical accommodation space grows laterally. Model C has a similar morphology to model B; however, the reef progrades over a nonreefal sediment wedge. Episodic lateral and vertical growth occurs in model D, with a stepwise progradation of the reef front. The remaining models are characterised by seaward reef framework behind which unconsolidated sediments accumulate. In model E, reef-crest growth forms a barrier leading to the development of a backreef lagoon. Model F has a similar morphology to model E, except that the reef crest is formed by hurricane rubble accumulation rather than framework accretion, and is periodically reworked. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Holocene; fringing reef; reef growth; sedimentation; stratigraphy 1. Introduction Fringing reefs are simple in terms of their morphology. They consist of reefs that are close to shore, often shore-attached, usually forming a relatively thin veneer of seaward thickening carbonate sediments * Corresponding author. Present address: School of Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand. addresses: David.Kennedy@vuw.ac.nz (D.M. Kennedy), colin@uow.edu.au (C.D. Woodroffe). over non-reefal topography (Steers and Stoddart, 1977). Despite their apparent simplicity, recent morphostratigraphic and geochronological studies of fringing reefs indicate that they may develop in one of several different ways. The objective of this review is to synthesise the results of subsurface investigations of fringing reefs from around the world, and to recognise the principal modes of fringing reef growth. Radiometric dating of the materials which contribute to reef formation enables the representation of gross reef evolution by a series of isochrons (lines of similar age). Six general /02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S (01)

2 256 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) models of fringing reef growth can be recognised from chronostratigraphic studies of reefs in the Holocene, and it may be possible to extend these interpretations to reefs in the Quaternary and pre-quaternary geological record. 2. Reef growth Coral reefs may be classified on the basis of several factors, such as their gross morphology, size, relation to non-limestone rocks and, in some cases, the depth of the surrounding water (Stoddart, 1969). Darwin (1842) devised one of the earliest and most enduring reef classifications, comprising fringing reefs, barrier reefs and atolls. Fringing reefs occur where the reef is close to shore; barrier reefs where the reef is separated some distance from the shore by a lagoon, and atolls are annular reefs surrounding a lagoon with volcanic basement (often presumed) buried at depth (Braithwaite, 1982; McLean and Woodroffe, 1994). This classification is based primarily on the relationship between the reef crest and nonreefal landmass, and it still adequately accommodates the majority of reefs worldwide more than 150 years after its publication. Fringing reefs are morphologically relatively simple, and can be divided into three zones, forereef, reef crest, and backreef. The distinction between individual reef types, however, is not always unambiguous (Davis, 1928). Almost-atolls are intermediate between barrier reefs and atolls, where a residual volcanic outcrop remains (Stoddart, 1975). It is often difficult to discriminate between fringing and barrier reefs, and the term almost barrier-reef has been proposed (Tayama, 1952). This is especially the case along sections the Kenyan coast (Bird and Guilcher, 1982), and may also apply to Pleistocene and Holocene reefs on the uplifting Huon Peninsula, Papua New Guinea where intermediate forms can be detected (Chappell, 1974; Chappell et al., 1996). The threshold between fringing and barrier reef is generally considered to be on the basis of backreef water depth, the term barrier reef being applied to those situations where the lagoon exceeds 10 m water depth (Milliman, 1974). Linear mid- or inner shelf reefs with shallow backreef lagoons, termed bank-barrier reefs, are common in the western Atlantic (Davis, 1928). These may coalesce at headlands to become fringing reefs (Macintyre, 1988). The situation may be further complicated in terms of scale where fringing reefs develop along the mainland shore, or around high bedrock islands, within a broad continental-shelf barrier-reef system, as on the Great Barrier Reef (Hopley, 1982). Various transitional forms of fringing reef have been identified on the basis of the existence of a deeper backreef channel, termed a boat channel between the reef crest and shoreline (Guilcher, 1988). The simplest fringing reefs do not have a boat channel and the reef crest is attached to the shoreline, as occurs in semiarid areas where little runoff or sediment is supplied from the land (e.g., Aqaba, Jordan, and southern Red Sea). An incipient boat channel (about 1.5 m deeper than the reef-flat surface) with minor sediment accumulation and scattered coral growth occurs in similar areas (e.g., south of Aqaba, Elat, Israel, and Mahé in the Seychelles). Fringing reefs with a well-developed boat channel have backreef depressions several metres deeper than the reef flat and up to a few hundred metres wide (e.g., sections of Madagascar). The most complex type described by Guilcher (1988) are fringing reefs with multiple landlocked lagoons; these tend to occur in macrotidal areas characterised by wide intertidal zones within the lagoon (e.g., northwestern Madagascar). Throughout this review the term shallow lagoon rather than boat channel will be used for the backreef depression. A fundamental distinction can be made between backreef areas composed of reef flats and those composed of shallow lagoons. Both are usually submerged at high tide, but in many cases much of the reef flat landward of the crest is exposed at low water. Near-horizontal reef flats that dry at lowest tides are widespread in the Indo-Pacific region. They are frequently described as emergent reef flats; however, the term emergent has become associated with the concept of a mid- to late Holocene fall of sea level. Throughout much of the Indo-Pacific region it appears that reef flats which dry at low water have experienced relative sea-level fall. However, it may be possible for similar reef flats to form through vertical accumulation of reef to its upper limit without any appreciable sea-level adjustment. Shallow lagoons, on the other hand, comprise depressions

3 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) that retain water at low tide, and hence usually have scattered coral colonies and patch reefs. Although these morphologies are distinct, it is also possible that a broad reef flat may be backed by a shallow lagoon. Fringing reefs, barrier reefs and atolls were interpreted by Darwin to represent successive evolutionary stages resulting from maintenance of an actively growing reef crest at sea level while the underlying substrate gradually subsided (Darwin, 1842). Darwin recognised that the most vigorous reef growth occurred at the reef crest, where the highest-energy conditions are experienced. Darwin s deduction of the long-term subsidence of volcanic edifices upon which reefs established has been widely upheld by stratigraphic investigations (Ladd et al., 1948, 1953; Braithwaite, 1982; Guilcher, 1988). The principal modification has been the recognition of the significant role that major fluctuations in sea level have had, particularly on barrier-reef and atoll evolution, both of which have re-established over previous interglacial reefs (Hopley, 1994; McLean and Woodroffe, 1994). Although the subsidence origin of the majority of reefs has been generally accepted, the concept that modern reefs grow upwards and outwards, a legacy of Darwin s theory, needs to be reexamined (Braithwaite et al., 2000). Darwin interpreted fringing reefs as the earliest stage of this long-term process wherein reef growth is initiated around a landmass. Modern fringing reefs, on the other hand, represent Holocene sediment wedges established close to shore, in many cases founded upon bedrock, or less frequently reoccupying reef positions from former interglacial times. They tend to be relatively thin and narrow with dimensions being a function of the underlying slope (Steers and Stoddart, 1977). The accommodation space within which these reefs can form is often limited, which in turn can effect their mode of growth. The adjacent landmass may have an impact on the fringing reefs which are liable to receive terrestrial freshwater runoff, nutrients and sediment. This may inhibit reef growth; in some cases, it may lead to formation or extension of a shallow lagoon (boat channel) as inferred by Guilcher (1988). In other cases, it may totally inhibit reef establishment; for instance there do not tend to be fringing reefs around the larger high islands, or mainland coast in the southern part of the Great Barrier Reef (Hopley, 1982). However, the contribution of sediment from the land is generally small in comparison with the rapid production of calcareous reefal material. Anthropogenic modifications to terrestrial sediment fluxes can modify this balance leading to the demise of already established fringing reefs, such as in Costa Rica (Cortés et al., 1994). Indeed the influence of the hinterland may be infrequent, but cataclysmic and lasting. The reef flats near Mackay, Queensland, appeared luxuriant in photography by Saville-Kent (1893), but corals suffered extensive mortality as a result of freshwater from a storm in 1918, and little of the reef appears to have recovered since (Hopley, 1982). Stream channels draining the land may have topographic expression that continues across a fringing reef as a reef passage or as furrows on the reef front. Fringing reefs in the Gulf of Elat, Red Sea, are particular examples of this expression of terrestrial morphology (Friedman, 1968; Gvirtzman and Buchbinder, 1978). Linear shore-normal depressions traversing the outer parts of fringing reefs may also result from former channels that have influenced the pattern of coral establishment across a reef. The sides of these depressions often become a focus of coral growth. Fringing reef growth would appear to be intimately linked to sea level, growing to, or maintaining a crest at, the sea surface (Buddemeier and Smith, 1988). During sea-level rise, new substrates become available for colonisation and the environmental conditions for coral growth on already submerged substrates change. The rate of this sea-level change will affect the ability of a reef to maintain growth close to the ocean surface. The rate and pattern of sea-level change since the last deglaciation has not been uniform worldwide. In those tropical areas in which reefs flourish (far-field), glacio-isostatic effects which dominate relative sealevel curves from higher latitudes (near-field) are minor, but local hydro-isostatic effects are apparent (Pirazzoli, 1991). In the Caribbean, sea level appears to have approached its modern level at a decelerating rate. The rate of Holocene sea-level rise was around 5 6 mm/year until 5000 years BP when the rate slowed, with modern sea level being attained within the last 2000 years (Lighty et al., 1982; Fairbanks,

4 258 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) ). In the Indo-Pacific, Holocene sea level rose at a rate of around 6 mm/year up to 6500 years BP when it reached a level close to present (Thom and Roy, 1985; Hopley, 1986, 1987; Chappell, 1987; Chappell and Polach, 1991). There are indications in the Pacific that sea level was up to 1 m higher than present during the mid-holocene (Pirazzoli, 1991; Grossman et al., 1998). Ironically, detailed reef-growth studies of fringing reefs from the Caribbean (Panama) where the sea was gradually rising, and the Pacific (Hawaii) where it had reached a level above present and has since fallen, produced reef-growth reconstructions that were remarkably similar (see below, Fig. 1). The significance of the similarity of reef growth, despite the different sea-level history, has been largely lost because the reef-growth data were interpreted to produce a sea-level curve, especially for Hawaii, which was then accepted relatively uncritically. The rate and timing of sea-level rise is likely to directly affect reef growth. Reefs tend to respond in three ways to changing sea level: keep-up, catch-up or give-up. They either keep-up, growing at a rate approximating that of sea-level rise, or lag behind the sea surface but later, when sea level decelerates or is stable, catch-up. In some cases reef growth cannot be maintained and the reef is drowned (gives-up) as has occurred on many of the shelf margins of the Caribbean (Davies and Montaggioni, 1985; Neumann and Macintyre, 1985; Macintyre, 1988). As fringing reefs are established in shallow depths directly on a non-reefal foundation, they would appear less likely to be drowned in the event of sea-level rise, because there is available terrestrial substrate for the reef to back-step to shallower depths. Higher sea level during the mid-holocene, recognised widely across the Pacific (e.g., Eniwetok: Tracey and Ladd, 1974; Buddemeier et al., 1975; Surrarrow: Scoffin et al., 1985; Tuamotus: Pirazzoli et al., 1988), has significant consequences for reef evolution. It means that the surfaces of some reefs have become emergent as the sea has fallen, and in these circumstances the reef top may have been undergoing erosion. These emergent reef flats represent a second type of give-up reef related to sea-level fall (McLean and Woodroffe, 1994). This causes growth to be concentrated on the reef front, which can influence reef morphology as will be illustrated below. The timing of initiation and depth of the reef surface in relation to the sea surface is important in the evolution of lagoons and emergent reef flats. Where the reef grows upwards to the sea surface, acceleration of growth rates on the seaward margin can lead to the development of backreef lagoons. This occurs if the sedimentation rate in the lagoon is less than the rate of sea-level rise and the rate of vertical accretion of the reef crest. If the reef crest is at the sea surface, either by catching-up or initiating at that elevation, then any sea-level fall will cause the emergence of the reef. Fig. 1. Fringing reef section from (A) Hanuama Bay (based on Easton and Olson, 1976) and (B) Galeta Point, Panama (based on Macintyre and Glynn, 1976) showing growth isochrons and sedimentary units. The isochrons are in radiocarbon years (BP) based on dated intervals within the cores.

5 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) Chronostratigraphic studies In the case of barrier reefs and atolls, modern morphology depends on the nature of sea-level change, the antecedent surface and the ecological communities that contribute sediments (Hopley, 1982, 1994; Guilcher, 1988; McLean and Woodroffe, 1994). Subsurface drilling and radiocarbon dating have enabled the growth history of reefs to be reconstructed in considerable detail. Holocene reef carbonates can be radiocarbon dated; the determined age represents the time at which the organism finished secreting its carbonate skeleton (i.e., its time of death). Although this need not be contemporaneous with sediment deposition, there is rarely a large deviation and chronostratigraphic irregularities, such as age reversals, that can indicate which dates are problematic. Accordingly, the history of a reef can be approximated from a reasonable number of radiocarbon dates, and models of development can be derived based on isochrons. Isochrons are imaginary lines through a reef based on interpolation between radiocarbon dates delineating sediments of similar age. They may be used to reconstruct the morphology of the reef through midand late Holocene. Isochron diagrams shown in the following synthesis have been constructed based on original published sections and where possible have been redrawn to a common scale. Ages are reported in radiocarbon years before present (BP) as cited in the original papers, without any attempt to calibrate these to sidereal or calendar years Classic studies of Hanauma Bay and Galeta Point The studies of fringing reefs at Hanauma Bay, Oahu, in the Hawaiian Islands (Easton and Olson, 1976) and Galeta Point, Panama (Macintyre and Glynn, 1976) are classic. They remain almost unsurpassed in the detail of stratigraphy and dating, comprising a sequence of 10 cores with 63 radiocarbon dates, and 13 cores with 32 radiocarbon dates, respectively. These studies provided some of the earliest well-dated fringing reef sequences where the reefal age structure can be effectively reconstructed. The resulting isochron diagrams (Fig. 1) represent the morphology of the reef at various time intervals during its evolution. The fringing reef in Hanauma Bay is 90 m wide and extends from the shoreline to a seaward algal rim elevated 0.3 m above the backreef (Easton and Olson, 1976). Porolithon algae presently dominate the reef. Porites coral growth is abundant on the deeper portions of the reef around 10 m depth. Scattered Pocillopora, Porites and Cyphastrea occur on the reef flat. The reef appears to have been initiated about 7000 years BP over mixed tuffs and basaltic sand (Easton and Olson, 1976). In this case, the volcanic basement is Late Pleistocene in age, and there has not been the opportunity for the modern reef to have inherited its form from a Last Interglacial precursor. The early stages of reef growth between 7000 and 5700 years BP indicate slow vertical accretion with morphology perhaps reflecting a minor undulation in antecedent topography, accentuated by more rapid growth on the fringing reef crest. The main phase of growth occurred between 5700 and 3500 years BP, accreting vertically at a rate of 2.9 mm/year. During this phase of vertical growth, sedimentation was sufficient to begin to mask the antecedent topography. From 3500 years BP to present, only 1 m was added vertically, but the reef prograded seaward slightly at a rate of 22 mm/year (Grigg, 1998). There appears to have been little potential for backreef lagoon development in Hanauma Bay. Coral established uniformly across the underlying tuffs and grew towards sea level as a continuous surface. The relation between reef growth and sea-level rise at Hanauma Bay has been the subject of controversy. A bench just above high water mark around the exposed tuff of the bay was interpreted by Stearns (1935) as evidence that the sea had been higher than present. Easton and Olson (1976), discounting this as evidence of emergence, suggested that the reef keptup with sea level and produced a sea-level curve based upon the coral dates. Montaggioni (1988) suggested that this reef grew in water depths of 2 3 m and that the curve produced was in fact a reefgrowth curve. Recent evidence for a mid-holocene highstand on Oahu (Fletcher and Jones, 1996; Grossman et al., 1998), though dismissed at the time by Easton and Olson (1976), suggests that the higher sealevel interpretation is correct and Hanauma Bay has been a catch-up reef. The rate of sea-level rise in the Caribbean was slower than in the Pacific which allowed the Galeta

6 260 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) Point reef to maintain keep-up growth. This fringing reef is wider than Hanauma Bay, being up to 200 m wide and contains 14.3 m of Holocene reef established over a Miocene calcareous siltstone (Macintyre and Glynn, 1976). Macroalgae and seagrass dominate the surface of the reef, with the main carbonatesecreting organisms being coralline algae and Halimeda. The subsurface facies of the reef, however, are dominated by the staghorn coral, Acropora palmata, and massive corals. Two transects were drilled across the reef, one across the reef front (outer transect) and the second across the backreef area (inner transect) (Fig. 1). As at Hanauma Bay, the reef at Galeta Point commenced around 7000 years BP, with the early stages of accretion veneering the antecedent topography. Massive coral heads dominated the initial phase of growth until 6000 years BP with an accretion rate of around 3.9 mm/year. From 6000 to 3000 years BP, A. palmata dominated the reef, accreting at a rate of between 1.3 and 10.8 mm/year, masking the antecedent topography. Coral heads and detritus replaced the Acropora after 3000 years BP with the modern reef-flat facies developing at around 2000 years BP. Little accretion appears to have occurred after 2000 years BP. The backreef area of Galeta Point contains a large amount of detritus with no reef framework being found in the most landward core. This backreef depression appears to be related to topography on the Miocene carbonates. A large amount of relief is present in the pre-holocene surface, which rapidly deepens below the landward two cores in the inner reef transect. Initial reef growth concentrated on the reef front. This allowed the backreef to become a depositional area for soft sediment. Sediments appear to have started accumulating when the rapid-growing Acropora facies began to dominate the reef. The extensively lithified coralgal forereef pavement implies a hiatus in reef accumulation, which allowed submarine cements to produce a very dense framework Fringing reefs of the Great Barrier reef province The mid-1970s also marked the beginning of intensive research into the reefs of the Great Barrier Reef province of Australia. There are over 2500 reefs in the Great Barrier Reef. Many of these are isolated platform reefs, but the inner-shelf continental islands, and those parts of the coast away from river discharges, support fringing reefs (Hopley, 1982; Chappell, 1983; Chappell et al., 1983). Platform reefs appear to be based upon bedrock prominences. These may initially have been established as fringing reefs around high ground. For example, the few granite boulders that remain on the reef surface of Pandora Reef are an indication of bedrock, and this reef may represent the ultimate stage of a fringing reef before transformation into a platform reef (Hopley, 1982). In the northern section of the Great Barrier Reef, fringing reefs along the mainland coast rarely exceed 250 m wide, whereas to the south (as on Penrith Island in the Keppel Islands), they may be 1.3 km or more wide. In the southern section, there are relatively fewer fringing reefs, though narrow reefs do occur along the mainland, and poorly developed reefs even occur around islands in Moreton Bay (Hopley, 1982). The surface morphology of these fringing reefs is generally dependent on their exposure to the prevailing southeasterly winds (Hopley, 1982). The more exposed the reef the greater the surface zonation and width of the reef flat. Reef-flat zonation also reflects the proximity to the mainland coast as well as the latitude (Endean et al., 1956; Van Woesik and Done, 1997). Those reefs proximal to terrestrial runoff or at higher latitudes tend to have ill-defined benthic community zonation on the reef flat. The surface character of the fringing reefs is also affected by their Holocene evolution. The majority of reef surfaces are old, in excess of 5000 years BP, and have been affected by a relative sea-level fall during the mid- to late Holocene (Hopley, 1982). Investigations of reef-flat ages, based on emergent and extant coral microatolls (Chappell et al., 1983), lead to the development of two broad evolutionary models of fringing reef evolution along the Great Barrier Reef (Fig. 2). The first recognised the gradual landward increase in age and elevation of microatolls across the reef flat. This indicated reef establishment along the shoreline at sea level soon after its stabilisation and progressive lateral accretion associated with a slight sea-level fall (Chappell, 1983). In the second model, the reef established below sea level and caught up to the sea surface as a series of coral pinnacles with the intervening areas between these patch reefs subsequently being infilled with reef-derived sediment. Microatolls across the

7 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) Fig. 2. Fringing reef growth model of Chappel et al. (1983) based on surface microatoll ages of mainland reefs in the Great Barrier Reef, Australia. reef flat tend to have a similar age that corresponds to the time of reef catch up to sea level (Chappell et al., 1983). These models are based on examination of the of the reef surface, and patterns of reef evolution can only be inferred. Seaward reef progradation, as implied in the first model, appears to be occurring today on many modern fringing reefs. Coalescence of a series of patch reefs with the intervening space between the patches infilled with sediment may be occurring on the northern part of the Iris Point reefs on Orpheus Island (described below) or on reefs around the Percy Islands. These models do not take account of variations in reef substrate that can play an important role in reef morphology (Davies and Marshall, 1979; Hopley, 1982). Fringing reefs on the Great Barrier Reef are formed over two major types of substrate. First, there are those that have established upon Pleistocene foundations, such as the reef flats of Magnetic Island, or in the Palm Islands where reefs are particularly diverse in terms of coral assemblages. Second, there are reefs established over unconsolidated sediments, often muds, generally on the leeside of high islands, as is the case on Dunk Island and Rattlesnake Island (Hopley, 1982). Hayman Island, in the central Great Barrier Reef, is one reef which established on a weathered Pleistocene reef around 9500 years BP (Hopley et al., 1978, 1983; Kan et al., 1997a). The modern reef reaches a maximum of 20 m thick and is dominated by Acropora. The upper part of the reef is composed of in situ coral, which overlies lenses of coral shingle, calcareous sand and solid coral. A rate of accretion of 4 5 mm/year occurred beneath the reef crest and 3 mm/year beneath the reef flat. The higher growth rate at its seaward margin has formed a small backreef depression about 3 m deep. Slightly more shingle is present here than in the reef-crest cores, suggesting that framework growth has been less active. Isochrons are generally parallel to the antecedent surface (Fig. 3). The reef caught-up to sea level by 4000 years BP (Hopley et al., 1978, 1983), adopting a growth pattern similar to that shown by the reef at Hanauma Bay. The fringing reefs of Cape Tribulation, part of the high relief of the Australian mainland, form over an antecedent surface of non-carbonate rounded boulders derived from the surrounding hinterland (Partain and Hopley, 1989). The reefs exist in the turbid inshore waters and are between 10 and 150 m wide. Wider reefs contain backreef facies of loose coral rubble with the majority of the crest and backreef areas being exposed at lowest tides. Growth initiated around 7800 years BP making them some of the oldest fringing reefs in the Great Barrier Reef. The maximum period of accretion appears to have occurred between 7800 and 5400 years BP with a vertical accretion rate of between 3.5 and 5.1 mm/year, as the reef kept-up with sea-level rise. Coral growth since 5400 years BP has been restricted to the reef front, partly as a result of a fall in sea level; however, there is little horizontal accretion (Partain and Hopley, 1989). Accretion appears predominantly vertical with the limited data available suggesting that the reefs grew as a relatively horizontal surface. In the southern part of the Great Barrier Reef, reefs are highly detrital with significant framework occurring only in the upper 1 2 m of the reef (Kleypas, 1996). On the continental high islands of Cockermouth, the fringing reefs are established over Pleistocene substrates with calcarenite outcropping in the reef flat. Marble and Digby Reefs also have Pleistocene foundations. The reefs initiated around 8000 years BP and caught up to sea level between 5000 and 6500 years BP. Growth rates are generally around 5 mm/year (range of mm/year). The lack of framework material compared with reefs to the north appears to be related to the effectiveness of

8 262 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) Fig. 3. Fringing reef section from Hayman Island, Great Barrier Reef, showing growth ischrons and sedimentary units (based on Hopley et al., 1978, 1983). The isochrons are in radiocarbon years (BP) based on dated intervals within the cores. the outer reef as a barrier to high-energy conditions (Kleypas and Hopley, 1992). Fringing reefs need not always be established on a pre-holocene substrate of consolidated material. They may develop on Holocene sediment wedges, often boulder beaches, derived from material reworked during the marine transgression or contemporaneous with reef deposition. Orpheus Island, in the Palm Group, in the northern section of the Great Barrier Reef, contains a series of fringing reefs that have developed over a range of antecedent surfaces of different age and sedimentology. Fringing reef growth at Iris Point on Orpheus Island began around 7000 years BP, over terrigenous boulders. The reef grew at a rate of 4 mm/year producing a sediment thickness of 6 m. It reached sea level by 6250 years BP when the inner reef flat developed with individual massive corals adopting a microatoll growth form (Hopley and Barnes, 1985). By 5500 years BP, the reef had begun to prograde seaward developing a second reef flat. The seaward part of the reef is dominated by Porites (60% framework); however, the landward two drill holes contain only 40% framework, being dominated by cemented Acropora shingle (Hopley and Barnes, 1985). The resulting isochron pattern (Fig. 4) indicates that initial growth veneered the antecedent surface with later ones being parallel with the reef front. Seaward progradation therefore appears to have been dominant since 6000 years BP when the reef crest reached sea level. Rates of seaward progradation varied from to 500 mm/year. On another transect at Iris Point, reefs were accumulating by at least 5600 years BP; however, the reef flat did not become established until 3600 years BP (Hopley and Barnes, 1985). This part of the reef accreted at a similar rate of mm/year, but in situ framework accounted for less than 20%. The Fig. 4. Fringing reefs section from (A) northern and (B) southern sites at Iris Point Orpheus Island, Great Barrier Reef (based on Hopley and Barnes, 1985). The isochrons are in radiocarbon years (BP) based on dated intervals within the cores.

9 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) detrital foundation appears to have been derived from the north, which laterally shed sediment to initiate the southern reef. The age structure of the southern part of the reef is therefore different to that of the northern part of the reef. Isochrons dip seaward and older surfaces are buried by younger sediment (Fig. 4). This example demonstrates that the pattern of fringing reef growth observed may be quite local in extent, with parts of the same reef complex showing quite a different pattern of development. On the leeside of Orpheus Island in Pioneer Bay, the reef initiated along the shoreline shortly after 7000 years BP providing a prograding biogenic sand and shingle wedge over which a thin framework unit has developed (Hopley et al., 1983). The reefal sediments have developed over a transgressive muddy sand unit. The vertical rate of growth of the framework units was between 2 and 6 mm/year decreasing seaward; however, the detrital units accumulated at a greater rate of 12 mm/year (Hopley et al., 1983). It is worth noting that on Orpheus Island, fringing reef growth rates were very similar even though they occur on windward (Iris Point) and leeward (Pioneer Bay) sides, as well as initiating at different times and over varying substrates during the Holocene. On adjacent Fantome Island, the reef accumulation rate at its seaward edge is 6.7 mm/year (Johnson and Risk, 1987). The reef was established around 5500 years BP on Pleistocene alluvium close to sea level. However, the main reef structure appears to have prograded seaward over a Holocene muddy-sand unit. This unit contains scattered branching-coral colonies and appears to have been deposited concurrently with the reef, most probably in a forereef environment. As the reef established close to sea level, there was little vertical accommodation space and it has prograded seaward. The resulting age structure is characterised by seaward-dipping isochrons paralleling the reeffront morphology, with older sediments being exposed at the surface of the reef (Fig. 5). The elevation of the reef increases towards the shore which Johnson and Risk (1987) attributed to a mid-holocene sealevel fall. The morphology resembles that of model 1 proposed by Chappell et al. (1983); however, it is important to note that the reefal accumulation has been occurring in association with offshore mud deposition. Some other fringing reefs which have been investigated on the Great Barrier Reef have been shown to be thin, generally less than 7 m thick, often developed over non-reefal sediment (Hopley et al., 1983). Their rate of growth is typically between 1 and 5 mm/year, but with higher rates up to 15 mm/year being associated with high-energy cyclonic events (Hopley and Partain, 1986). A mid-holocene sea-level fall of 1 2 m has had a major influence on the evolution of many reefs, especially those attached to the mainland. This has caused emergent reef platforms to form (e.g., Yule Point; Bird, 1971), and the reef flat to dip seawards (Partain and Hopley, 1989) Fringing reefs in other stable tectonic settings At the northernmost end of the Great Barrier Reef in Torres Strait, luxuriant fringing reef growth occurs in association with strong tidal currents and high turbidity. On Yam Island in the centre of the Strait, the fringing reef established close to the shoreline Fig. 5. Fringing reef section from Fantome Island, Great Barrier Reef, fringing reef (based on Johnson and Risk, 1987). The isochrons are in radiocarbon years (BP) based on dated intervals within the cores.

10 264 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) Fig. 6. Fringing reef section from Hammond Island in Torres Strait (based on Woodroffe et al., 2000). The isochrons are in radiocarbon years (BP) based on dated intervals within the cores and surface microatolls. over a Pleistocene reef limestone at around 7000 years BP. Large coral heads appear to grow vertically in shallow water along the reef front. These are interpreted to coalesce and adopt a microatoll growth form as they reach the sea surface and the intervening area between them is infilled with branching coral debris (Woodroffe et al., 2000). Closer to the Australian mainland there is a large amount of mud deposition and fringing reefs tend to be restricted to the windward (eastern) sides of the islands. However, reef growth can occur in association with limited mud sedimentation such as on Hammond Island. Porites microatolls at the landward edge of the reef are elevated above their modern counterparts. The reefflat height gradually reduces towards its seaward edge as a relatively flat uniform surface dominated by branching corals (Woodroffe et al. 2000). Unlike Yam Island, this reef appears to have prograded over a Holocene muddy base (Fig. 6). The reef initiated around 6000 years BP at sea level and has prograded seaward concurrently with the deposition of the underlying mud (Woodroffe et al., 2000). High turbidity and active mud deposition in the forereef area also occurs at Phuket, Thailand. Here reefs are composed of a framework of Porites corals, which laterally accrete through a process of block toppling (Tudhope and Scoffin, 1994). The reefs initiated close to the land approximately 6000 years BP at sea level and have prograded seaward at an average rate of between 17 and 80 mm/year. The reefs are developing over a muddy substrate, which is still accreting. The incorporation of this mud within the reef causes the high progradation rates. As lateral progradation is dominant, isochrons parallel the reef front, with the oldest ones intersecting the reef surface (Fig. 7). These reefs have also prograded during a mid-holocene sea-level fall, with the reef-flat height decreasing by 0.8 m towards the reef front (Scoffin and Le Tissier, 1998). A relative sea-level fall may also rejuvenate horizontal reef growth. It has been suggested on Mangaia in the Cook Islands that a mid-holocene reef which had caught-up to sea level between 4000 and 3400 years BP was stranded at m by a sea-level fall (Yonekura et al., 1984, 1986, 1988). This emergent reef had been laterally accreting since the reef-flat formation; however, after stranding, growth was concentrated on the reef front. A second reef front, interpreted as a catch-up reef, formed reaching modern sea level at 2000 years BP (Yonekura et al., 1984, Fig. 7. Fringing reef section from Phuket, Thailand (based on Tudhope and Scoffin, 1994). The isochrons are in radiocarbon years (BP) and are inferred from the suggested evolutionary model of the reef development.

11 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) Fig. 8. Fringing reef section from Mangaia Island, Cook Islands (based on Yonekura et al., 1984, 1986, 1988). The isochrons are in radiocarbon years (BP). Isochrons deeper than the drill holes are based on the inferred evolutionary model. 1986, 1988). This created a double reef-crest morphology, each crest with its own backreef area of unconsolidated sediment (Fig. 8). A similar process is suggested to have occurred in the Mariana Islands (Randall and Siegrist, 1988; Kayanne et al., 1988) with emergent reef flats and rubble terraces dating between 2250 and 4750 years BP (Dickinson, 2000). The rates of growth of the emergent crests appear to be much greater than the recent ones, with rates of 3 14 and 1 mm/year, respectively, on Rota Islands, Mariana Islands, even though both reefs are dominated by Acropora humilis and A. digitifera (Kayanne et al., 1988). In areas where the reef crest is still catching-up to the sea surface, a sea-level fall will reduce the available accommodation space and hence shorten the timing of catch-up. Fringing reefs on Reunion and Mauritius Islands in the Indian Ocean initiated around 7500 years BP with vertical growth ceasing around 3000 years BP when the reef reached a level within 2 m of the sea surface (Camoin et al., 1997). The reef flats are a maximum of 300 m wide, while the backreef zone is up to a few kilometres wide but less than 2 m deep. The reefs have grown in catch-up mode at a rate of between 0.9 and 7.0 mm/year (Montaggioni, 1988; Camoin et al., 1997). The highest rates of accretion occurred in the branching-coral units at a rate of around 4.7 mm/year. The initiation facies was dominated by massive corals, and accreted at around 1 mm/year. On Pointe-au-Sable reef on Mauritius, the isochron profiles broadly parallel the modern reef surface (Fig. 9). Sea level does not appear to have been above present levels during the Holocene in this part of the Indian Ocean. The isochrons suggest the prevalence of vertical aggradation over horizontal progradation and the synchroneity of both seaward Fig. 9. Fringing reef section from Mauritius (based on Montaggioni and Faure, 1997). The isochrons are in radiocarbon years (BP) based on dated intervals within the cores.

12 266 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) and shoreward development (Montaggioni and Faure, 1997). Changes in coral communities with time are related to an increase in water agitation and a reduction in accommodation space as the reef grew towards sea level (Montaggioni and Faure, 1997). On Lord Howe Island in the northern Tasman Sea, isochrons appear parallel to the underlying substrate (Fig. 10). The reef crest is up to 200 m wide and is backed by a shallow lagoon up to 2 km wide and on average 1.5 m deep. Live coral growth is virtually absent on the crest, whereas the reef front is dominated by platy Acropora palifera, with columnar Porites lichen, A. palifera and Pocillopora damicornis scattered throughout the lagoon (Kennedy and Woodroffe, 2000). The reef established over Pleistocene calcarenite dunes around 6500 years BP, at depths of between 5 and 10 m. A large proportion of the sediment was deposited between 5500 and 5000 years BP. The lagoon infilled at an average rate of around 5 mm/year but up to 10 mm/year in places. By 4000 years, the reef had grown close to modern levels. Sediments younger than 3000 years BP form a veneer over the older units (Kennedy and Woodroffe, 2000). Some reworking may have occurred on the reef crest as shown by the landward thickening wedge of cemented boulders on the reef crest younger than 3000 years BP; however, it is uncertain whether this is related to a sea-level fall or change in the reef-crest benthic community. The eastern Pacific is associated with poor coral reef development (Veron, 1995). Fringing reefs, however, are found along the Central American coast. On the Panamian coast, the rate of fringing reef growth is similar to many Caribbean reefs. Mean vertical growth rates for the reef flat range from 1.3 to 4.2 mm/year. The maximum growth occurs in the upper forereef slope at a rate of 7.5 mm/year, and possibly as high as mm/year over brief periods (Glynn and Macintyre, 1977). The initiation of these reefs was later than on the Caribbean side occurring between 4500 and 5600 years BP. The reefs also appear to be dominated by Pocilloporid corals (Glynn and Macintyre, 1977). These similar rates are interesting given the different biogeographic zones and sea-level histories of the two areas. On the Costa Rican coast at Punta Islotes, a small fringing reef around 9 m thick occurs. The reef was established around 5500 years BP and grew at rates of mm/year for most of its history (Cortés et al., 1994). The final stage of growth from years BP was characterised by accumulation rates of mm/year. However, the reef at present is virtually dead with very little live coral cover (Cortés et al., 1994). Cores were taken only from the reef crest so the isochron morphology is difficult to ascertain. Catch-up growth of the reef crest appears to have occurred with the backreef elevation lagging slightly behind that of the reef crest Tectonically active settings Vertical tectonic movements, in addition to variations in substrate and sea level, will affect the growth Fig. 10. Fringing reef section from Lord Howe Island, Tasman Sea (based on Kennedy and Woodroffe, 2000). The isochrons are in radiocarbon years (BP) based on dated intervals within the cores.

13 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) of fringing reefs, especially those located close to plate boundaries. These vertical movements can either cause a relative increase or decrease in sea level with respect to individual reefs. Over long time scales of hundreds of thousands of years tectonic uplift can lead to the development of a series of uplifted terraces such as on the Huon Peninsula, Papua New Guinea (Chappell, 1974; Chappell et al., 1996). Individual seismic events may instantaneously emerge a reef, effectively an instant sea-level fall. Generally, however, uplift appears to be gradual, allowing the reefs to respond to the relative sea-level changes. Tectonic uplift and differences in substrate composition have affected fringing reef morphology on New Caledonia. Two phases of Holocene reef initiation occurred, the first around 8500 years BP in the south and the second occurring after 4000 years BP in the northern part of the island (Cabioch et al., 1995, 1999). The lag in reef initiation in the north is attributed to the unfavourable substrate geology of igneous and metamorphic rocks as opposed to the Pleistocene carbonates in the south. The fringing reefs have all grown in catch-up mode with their morphology being strongly influenced by the rate of uplift or subsidence of the island. While the growth isochrons cannot be determined from the reefs based on the drilling results, the thickness of the reef, and degree to which it forms a seaward thickening sediment wedge is directly related to the vertical substrate movement (Fig. 11). In the subsiding northern section, the reefs form a thickening sequence very similar to those observed in the Great Barrier Reef and Hawaii. As subsidence decreases in the south, and in one case uplift occurs, the total reef thickness is markedly reduced until if forms a thin veneer over exposed Pleistocene reef sequences. In areas of most rapid subsidence, the reefs are no longer attached to the shoreline and form barrier reefs (Fig. 11). The Ryukyu Islands, south of Japan, are located on the boundary of the Philippine and Eurasian plates and contain many shore-attached reefs most of which were initiated below modern sea level. On Okierabu Island, some lagoonless narrow reef flats ( < 100 m) occur which have kept-up with sea level since initiation 7050 years BP (Kan et al., 1995). Isochrons mirror the antecedent morphology suggesting that a sub-horizontal reef-flat surface occurred through its entire evolution (Fig. 12). Vertical growth rates were Fig. 11. Fringing reefs of New Caledonia showing the relationship between reef thickness and the rate of vertical tectonic movement (based on Cabioch et al., 1999).

14 268 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) Fig. 12. Fringing reef section from Okierabu Island, Ryukyu Islands (based on Kan et al., 1995). The isochrons are in radiocarbon years (BP) based on dated in situ corals sampled from a channel excavated through the reef crest. around 6 mm/year until 5500 years BP, at which time the reef grew to within 2.5 m of the sea surface. Vertical growth rates then slowed to 0.6 mm/year. Acropora species dominate the reef and there appears to be a lack of surface zonation throughout its evolution (Kan et al., 1995). There was little opportunity for lagoonal development as the reef grew across the entire antecedent surface at much the same rate. Fringing reef development on Minna Island to the north of Okierabu Island initiated offshore from the coast and developed a small lagoon (Kan and Hori, 1993). The reef was initiated around 5000 years BP, and concentrated on the reef crest that grew to sea level creating a backreef lagoon. After attaining sea level, the reef then began to expand laterally forming a reef flat. Vertical reef growth rates are between 1.6 and 2.5 mm/year, while the reef flat has accreted laterally at a rate of mm/year since 5000 years BP. It is during this period of reef-flat accretion that the lagoon was almost completely infilled. Since 4000 years BP, the structure of the reef has been complicated by catch-up growth of seaward reef spurs that have subsequently attached to the reef front (Kan and Hori, 1993). This process of spur attachment has also been described elsewhere; on the Ryukyus (Kan et al., 1997b). Reefs on Kume Island to the south of Minna and Okierabu Island grew faster than the two reefs described above, which led to the development of a small backreef depression (Takahashi et al., 1988; Kan et al., 1991). The reef initiated at 7500 years BP and grew at a rate of 10 mm/year until 6500 years BP with growth mirroring the antecedent topography (Fig. 13). Vertical growth rates after 6500 years BP slowed to 3 mm/year occurring on the seaward edge of the reef. By 5500 years BP, the reef crest had caught up to sea level with a shallow backreef lagoon formed landward of it. This lagoon infilled during the Fig. 13. Fringing reef section from Kume Island, Ryukyu Islands (based on Takahashi et al., 1988; Kan et al., 1991). The isochrons are in radiocarbon years (BP) based on dated intervals within the cores.

15 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) phase of reef-flat formation. Reef growth after 5000 years BP appears to have occurred during a period of relatively higher sea level and is now stranded above present sea level (Takahashi et al., 1988; Kan et al., 1991). A similar growth pattern to that which is seen on Kume Island occurred on Kikai Island, one of the northernmost Ryukyu Islands (Fig. 14). However, there were no differences in growth rates across the reef surface and therefore a shallow lagoon was not formed (Konishi et al., 1983). Emergent Holocene reefs appear to occur all around Kikai Island (Webster et al., 1998). These reefs have vertical accumulation rates of 3 4 mm/year growing in both keep-up and catch-up modes. The different times at which the reef crest reached sea level is attributed to uneven periodic uplift around the island (Webster et al., 1998). This uplift has produced a series of four terraces elevated above modern sea level. This intra-island variation in uplift illustrates the localised nature of tectonic processes on reef development. Some reefs were unable to maintain a reef crest close to sea level and had to catch-up while others grew with the sea surface and were therefore stranded when there was a relative sealevel fall caused by island uplift Rubble-dominated reefs Kan et al. (1997b), in discussing the evolution fringing reefs in the central Ryukyu Islands, stressed the possible importance of high-energy processes in affecting reef evolution. Shinn (1980) noted that on Grecian Rocks, Key Largo, most reefs have a surface cover of coral over a predominantly rubbly substrate. Unconsolidated detritus rather than in situ framework dominates the internal structure of many fringing reefs worldwide. High-energy events can often move large amounts of reef and lagoonal material (Kobluk and Lysenko, 1992; Scoffin, 1993; Dollar and Tribble, 1993; Li et al., 1997). Corals quickly colonise and stabilise these accumulations which can lead to the development of reefs which contain storm-deposited rubble rather than in situ framework (Shinn et al., 1977; Blanchon and Jones, 1997; Blanchon et al., 1997). Rubble-dominated reefs have been described from around Grand Cayman Island (Blanchon and Jones, 1997; Blanchon et al., 1997). They tend to have enough relief that lagoons up to 5 m deep develop behind them. Detailed internal stratigraphy has not been described for these reefs due to a lack of drilling data. From the suggested evolutionary model (Blanchon et al., 1997), they appear to be similar to the keep-up/catch-up reefs in that older material is generally buried within the reef, and not exposed on the surface. Storm processes, where surface coral communities are continually broken and reworked by storms, dominate accretion. The rubble is then stabilised during subsequent quiet periods by the next generation of reef-top flora and fauna. The rate of accumulation of these reefs will therefore be variable, related to storm frequency, and hard to determine Fig. 14. Fringing reef section from Kikai Island, Ryukyu Islands (based on Konishi et al., 1983). The isochrons are in radiocarbon years (BP) based on dated intervals within the cores.

16 270 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) because of the allochthonous nature of the deposited material. Holandes Cay, Panama, is often described as a classic Caribbean algal ridge; however, recent drilling by Macintyre et al. (2000) indicates that the algae play only a minor role in the formation of the reefs relief. The reef is in fact composed of storm deposits dated between 2000 and 3000 years BP with the coralline algae forming only a surface veneer (Macintyre et al., 2000). Braithwaite et al. (2000), in their investigation of the Seychelles in the Indian Ocean, suggest that the geometry of reef accretion is related to an interplay between extreme storms events and fairweather conditions. Storms with a return period of the order of 100 years act to disrupt nascent framework development and thereby reset the accretion process. The surface ecology therefore reflects year-by-year sea conditions while the reef structure and lithology are determined by high-energy events (Braithwaite et al., 2000). 4. Fringing reef growth synthesis Fringing reefs appear simple on the surface. They are attached to the shoreline with the main variation in morphology being the presence and size of the backreef lagoon. Their evolution, however, is far from simple with a range of growth morphologies being possible and a variety of sedimentary units being present below their surface. Reef growth worldwide appears to produce a range of reef structures and morphologies that are affected primarily by relative sea-level movements, coral growth, the morphology of the antecedent surface and the regional storm climate. However, these factors do not appear to be the dominant factors that influence the mode of reef growth. For instance, the fringing reefs of Galeta Point (Macintyre and Glynn, 1976) and Hanauma Bay (Easton and Olson, 1976) appeared to have grown in very similar ways, despite the difference in sea-level history. There does not appear to be a simple relation between growth stage and accretion rates (Davies and Montaggioni, 1985; Montaggioni, 1988; Cabioch et al., 1995) with most fringing reefs having mean growth rates between 2 and 7 mm/year. It is possible to describe the variation between fringing reefs based on the nature of the antecedent surface that they are established over as well as the proportion of framework and detrital sediments within the reef structure (Hopley and Partain, 1986). The first type are simple reefs that are established while sea level is still rising over rocky foreshores. Growth is primarily vertical and composed of framework; however, once the reef surface reaches sea level, as for instance during a stillstand, minor outward growth may occur. Reef-flat development tends to be restricted. The second type of fringing reefs have developed over a gently sloping substrate, particularly older Pleistocene reefs. The reefs were initiated offshore when sea level was lower. The framework units are then attached to the shoreline through interfingered terrigenous and detrital backreef infill. The final fringing reef type is developed over pre-existing sedimentary deposits including fine muds. Progradation of the reef is often rapid over the pre-existing structure with the reef-flat framework forming a thin veneer (generally < 4 m thick) over carbonate and terrestrial debris (Hopley and Partain, 1986). Fringing reefs generally fit within one of these three morphological types, but morphology does not indicate the mode reef evolution. The two very important factors that affect the evolution of the reef are sea-level change and tectonic stability. These two factors are interrelated as vertical tectonic movement is equivalent to a relative sea-level change at a given reef location. A reef will respond to sea-level change by keeping-up, catching-up or giving-up (Davies and Montaggioni, 1985; Neumann and Macintyre, 1985). These different modes of reef development will affect the type of sediments deposited. Shallow-water communities dominate keep-up reefs while give-up reefs will have an initial shallow-water community capped by a deeper-water one. Fringing reefs will develop in this manner, although they are unlikely to be drowned (give-up) given their proximity to the shore and available hinterland substrate to colonise. Sea-level variations will therefore determine the available accommodation space for the reef. Where there is ample space, such as on those reefs established below the sea surface, the primary direction of accretion is vertical, whereas reefs at sea level, and hence with little vertical accommodation space, will accrete laterally. Therefore, as sea level rises, more accommodation space is made available for the reef to occupy hence

17 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) vertical accretion is favoured. A sea-level fall on the other hand will cause a reduction in the available accommodation space. If the reef has already accreted to the sea surface, then some or all of it may be stranded (e.g., McLean and Woodroffe, 1994). For those reefs still vertically accreting, a sea-level fall will decrease the available accommodation space and hence the time taken for the reef to reach the sea surface. Accommodation space can also be reduced by terrigenous sedimentation. These sediments will occupy space otherwise available to be infilled by reef growth. However, fringing reefs can thrive in such turbid environments (Woolfe and Larcombe, 1999), which may in fact enhance reef development (Hopley and Partain, 1986). Terrigenous material can therefore play an important role in determining the timing of the reef reaching sea level, or alternatively the rate of seaward progradation. Six models (A F) of fringing reef development are outlined in Fig. 15 based on the chronostratigraphic data presented above. These models are based on the available accommodation space. In model A, reef growth is established below modern sea level with vertical accretion being dominant, as there is ample accommodation space with rising sea level, growing most rapidly at the reef crest (Fig. 15). The isochrons have a similar appearance to the modern reef morphology. Once the crest has reached sea level, lateral accretion may occur although this will depend on the Fig. 15. Generalised models of fringing reef development. Model A: Reef accretion is vertical growing in keep-up or catch-up mode; model B: Reef accretion is lateral having established at a level with little or no vertical accommodation space; model C: Lateral reef accretion occurs concurrently with deposition of non-reefal mud in the forereef zone; model D: Episodic progradation model; model E: Reef accretion is initially focused offshore creating a shallow landward lagoon; model F: The offshore reef structure is formed by storm processes and is essentially a rubble pile. Some landward movement of this rubble pile may occur as sediment is reworked landward. Isochrons are in thousands of radiocarbon years BP.

18 272 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) productivity of the given reef. Reef initiation may occur any time after flooding. Non-reefal sediment is generally minimal with reef-derived material dominating accretion. Hanauma Bay (Easton and Olson, 1976) and Galeta Point (Macintyre and Glynn, 1976) are examples of this first type of reef growth. As vertical accretion is dominant, older sediments will be found at greater depth within the reef. Material deposited during reef start-up will not be exposed on the surface. The age of the reef flat will correspond to the time that the reef reached sea level. Catch-up reefs will have younger surfaces than keep-up reefs from the same region. Comparisons of surface ages between reef provinces where sea-level histories have differed can be problematic if used to interpret the mode of growth. For example, the reef-flat ages of Hanauma Bay and Galeta Point are similar being around 2000 years BP. Sea-level rise was much faster in the Pacific than the Caribbean (Davies and Montaggioni, 1985; Neumann and Macintyre, 1985), meaning that Galeta Point is a keep-up reef, whereas Hanauma Bay is a catch-up reef. Despite this difference, the isochron morphology is quite similar between the two reefs. Model B relates to reefs that have prograded seaward by lateral accretion (Fig. 15). These reefs were initiated at the shoreline at, or close to, the final elevation of the sea meaning there is little or no vertical accommodation space. This model is very similar to model 1 of Chappell et al. (1983). A relative sea-level fall on these reefs will cause a seaward reduction in reef-flat elevation. Isochrons are generally parallel to the reef front meaning that progressively older sediments will be exposed on the reef surface further landward of the reef crest. Unlike model A, the older sediments are not buried within the reef structure. The primary sediment source in this model is the reef and a range of textures may occur depending on the amount of framework present. Reefs at Iris Point on Orpheus Island (Hopley et al., 1983; Hopley and Barnes, 1985) are examples of this reef type. Model C has an isochron structure similar to the second model, but reef framework is built over a muddy sediment wedge (Fig. 15). Like model B, the reef is initiated close to the shoreline and has prograded laterally as there is with little or no vertical accommodation space. Isochrons therefore parallel the reef front. The non-reefal sediment wedge may be older than, or contemporaneous with, the reef. Reef progradation in this model is therefore dependent on the deposition of non-reefal sediments in the forereef zone to provide a base over which the reef may prograde. Benthic communities in the forereef zone, which may comprise scattered coral, seagrass, or macroalgae, act to filter the finer sediment before it is deposited on the reef crest. This form tends to be confined to areas with a high sediment load. Hammond Island (Woodroffe et al., 2000), Phuket (Tudhope and Scoffin, 1994) and Fantone Island (Johnson and Risk, 1987) are examples of this reef type as are many of the Great Barrier Reef inner shelf reefs and presumably many of the sites described by Chappell et al. (1983). Reef development in the models presented so far is gradual and continual. Accretion rates may vary, especially as a reef crest reaches close to sea level therefore occupying all the accommodation space, but during reef growth there is rarely a prolonged hiatus. Episodic progradation can occur and has been inferred in the reef studies of Guppy (1890) and Umbgrove (1947). Reefs that prograde episodically are grouped into the fourth model, model D (Fig. 15). Progradation occurs through the attachment of linear shore-parallel reefs to the existing reef and sediment infill between them, usually by unconsolidated reef-derived sediment. This mode of reef growth is indicated on Yam Island in Torres Strait (Woodroffe et al., 2000), and has been inferred for other reefs from seismic results by Jones (1995). Lewis (1968) suggested equivalent patterns of progradation for sheltered fringing reefs around Mahé, Seychelles. These prograde by coalescing of offlying coral knolls in the forereef firstly into linear reefs and then were attached to the reef front by sediment infill and framework growth (Braithwaite et al., 2000). A similar pattern of coalescence of an outer reef onto the existing reef flat may account for the pattern of isochrons detected on the Mangaia reef flat (see Fig. 8) by Yonekura et al. (1988). Yonekura et al. (1988) attempted to account for the abandonment of the inner reef crest and inception of the outer keep-up reef as a result of the individual tectonic behaviour of Mangaia. However, there seems little need to invoke movement of the island relative to other islands in the Cook Islands, where similar evidence for higher sea level, and for episodic reef progradation is found (Woodroffe et al., 1990).

19 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) In contrast to these models, models E and F refer to fringing reefs that form as a seaward barrier of reefal material, behind which there is sediment infill (Fig. 15). In model E, reef growth is initiated at depth and keeps or catches up to sea level. In this mode of growth, accretion is concentrated away from the shoreline. The rate of growth on the crest is greater than closer to shore allowing this portion of the reef to reach sea level first, enclosing a landward shallow lagoon. The lagoon will then infill with sediment either laterally shed off the reef crest or produced in situ. The isochron pattern initially mirrors the antecedent surface as coral becomes established across the newly flooded surface. Relatively greater accretion rates on the seaward edge of the reef are then indicated in the isochron pattern that parallels the developing reef surface rather than the antecedent morphology. Once the reef crest has reached sea level, isochrons can parallel the reef front if seaward lateral progradation occurs or mark the beginning of lagoonal infill. Reefs and lagoons on Mauritius (Camoin et al., 1997), Lord Howe Island (Kennedy and Woodroffe, 2000) and the Seychelles (Braithwaite et al., 2000) are examples of this model. This mode of development appears quite common; it may be inferred in the case of Hayman Island reefs (Hopley, 1982), and appears to have occurred with sedimentation behind an algal ridge barrier in reefs in the Bahamas (Macintyre et al., 1996). Model F is similar to model E but is based upon a different process of barrier construction (Fig. 15). Storms rework coral material landward, forming a barrier behind which sediment accumulates. This barrier will then accumulate more coral material both from its surface communities and offshore reefs if present. Although storms are infrequent and brief in duration, storm-generated reef structures may be persistent in the stratigraphic record. Continued storm reworking may shift the barrier seaward as older sediments are excavated from the reef front and transported into the backreef area. Age control on such reefs may be difficult to establish. On the Cayman Islands, the reef shows minimal growth to seaward, indeed it is inferred to have been eroded from its seaward side, in the first instance by wave erosion and in the second by hurricane activity (Blanchon et al., 1997). Reef accumulation therefore is dominated by storm-deposited rubble as seems to have occurred in Holandes Cays, Panama (Macintyre et al., 2000) rather than vertical framework growth. There may be other situations where a fringing reef has developed initially as an offshore reef structure, but sediment has been deposited between that structure and the shoreline. The reef at Yule point on the Queensland coast is probably one such instance where mud from the Mowbray River has welded an offshore reef to the coast (Bird, 1971). 5. Conclusion Holocene fringing reef growth may be described by at least six broad models of evolution. These models are based on stratigraphy and chronology as the reef accreted, reflecting the available accommodation space. Fringing reefs will preferentially accrete vertically. If there is no vertical accommodation space available, either because the reef has established at, or grown to, the sea surface, or due to a relative sea-level fall, then it will prograde seaward. Reef growth may also be episodic, characterised by the growth of coral heads on the forereef which may be joined to the main reef structure as the intervening areas are infilled with sediments. This process can be accelerated if a relative sea-level fall rejuvenates coral growth in the deeper waters of the reef front. For reefs with ample vertical accommodation space, accretion can be relatively faster on the crest and forereef zones leading to the development of a lagoon. This lagoon may be subsequently infilled through a combination of allochthonous reef-derived or terrigenous and in situ sediments. The same lagoon form can also be created by the landward transport of storm rubble during hurricanes. In these reefs, the crest is composed of rubble accumulation rather than framework growth. The critical factor in the final growth form of fringing reefs would appear to be the available accommodation space for the reef to occupy. The water depth over the reef surface will determine the available vertical accommodation space. This will depend on the depth of reef initiation as well as relative sea-level movements. If vertical space is limited, then the reef will prograde laterally which can occur in association with deposition of a forereef muddy wedge or episodically as forereef corals become attached to the reef crest. Sea-level fluctuations will determine the abso-

20 274 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) lute accommodation space for a given reef as reefs cannot grow above the sea surface. This means that a reef established during a period of sea-level rise will be able to accrete vertically as vertical space is created above it. If, however, the reef establishes at the sea surface after a stillstand, it accretes laterally. Vertical growth can still occur under stillstand conditions; however, the reef must initiate at depth. A relative sea-level fall will therefore have a profound effect on reef evolution as it reduces accommodation space even for those reefs already at the surface. The most common response of fringing reefs is to continue lateral accretion but at the new lower sea level. Under these circumstances, reef evolution may switch from a laterally accreting model to a episodic progression model, although a fall in sea level does not have to be the trigger for episodic progradation. Modern sediment supply, especially terrigenous material, is also important in fringing reef evolution as it can occupy accommodation space otherwise available for the reef. These sediments are also important in providing a surface over which reefs can prograde by creating a suitable substrate that may otherwise not be available. Coral and other benthic growth on these sedimentary surfaces may also act as baffles to mud-size material, thereby providing favourable environmental conditions for reef growth landward of these communities. Fringing-reef evolution is therefore complex not merely veneering tropical rocky coasts. Reef forms are determined by the available accommodation space, as defined by sea level and depth of the antecedent surface, and rate of sedimentation. Reefs from many different locations with differing eustatic and tectonic histories may actually develop very similar growth morphologies. These models provide a series of scenarios that may enable reef managers to forecast the likely trajectory of fringing-reef response to global climatic change. They also provide a framework in which the evolution of extant fringing reefs can be assessed and a basis for the investigation of ancient fossil reefs. References Bird, E.C.F., The fringing reefs near Yule Point, North Queensland. Australian Geographical Studies 9, Bird, E., Guilcher, A., Observations préliminaires sur les récifs frangeants actuels du Kenya et sur les formes littorales associées. Revue du Géomorphologie Dynamique 31, Blanchon, P., Jones, B., Hurricane control on shelf-edge-reef architecture around Grand Cayman. Sedimentology 44, Blanchon, P., Jones, B., Kalbfleisch, W., Anatomy of a fringing reef around Grand Cayman Island: storm rubble, not coral framework. Journal of Sedimentary Research 67, Braithwaite, C.J.R., Progress in understanding reef structure. 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21 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) history of an eastern Pacific fringing reef, Punta Islotes, Costa Rica. Coral Reefs 13, Darwin, C.R., The Structure and Distribution of Coral Reefs. Smith, Elder and Co., London, 214 pp. Davies, P.J., Marshall, J.F., Aspects of Holocene reef growthsubstrate age and accretion rate. Search 10, Davies, P.J., Montaggioni, L., Reef growth and sea-level change: the environmental signature. Proceedings of the 5th International Coral Reef Symposium, Tahiti, pp Davis, W.M., The coral reef problem. American Geographical Society, New York. Dickinson, W.R., Hydro-isostatic and tectonic influences on emergent Holocene paleoshorelines in the Mariana Islands, western Pacific Ocean. Journal of Coastal Research 16, Dollar, S.J., Tribble, G.W., Recurrent storm disturbance and recovery: a long-term study of coral communities in Hawaii. Coral Reefs 12, Easton, W.H., Olson, E.A., Radiocarbon profile of Hanauma Reef, Oahu, Hawaii. 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Geobooks, Norwich, pp Hopley, D., Holocene sea-level changes in Australasia and the Southern Pacific. In: Devoy, R.J.N. (Ed.), Sea Surface Studies, A Global View. Croom Helm, Sydney, pp Hopley, D., Continential shelf reef systems. In: Carter, R.W.G., Woodroffe, C.D. (Ed.), Coastal Evolution, Late Quaternary Morphodynamics. Cambridge Univ. Press, Cambridge, pp Hopley, D., Barnes, R., Structure and development of a windward fringing reef, Orpheus Island, Palm Group, Great Barrier Reef. Proceedings of the 5th International Coral Reef Congress, Tahiti, pp Hopley, D., Partain, B., The structure and development of fringing reefs of the Great Barrier Reef Province. In: Baldwin, C.L. (Ed.), Fringing Reef Workshop: Science, Industry and Management. Great Barrier Reef Marine Park Authority, Townsville, pp Hopley, D., McLean, R.F., Marshall, J., Smith, A.S., Holocene Pleistocene boundary in a fringing reef: Hayman Island, North Queensland. Search 9, Hopley, D., Slocombe, A.M., Muir, F., Grant, C., Nearshore fringing reefs in North Queensland. Coral Reefs 1, Johnson, D.P., Risk, M.J., Fringing reef growth on a terrigenous mud foundation, Fantome Island, central Great Barrier Reef, Australia. Sedimentology 34, Jones, M.R., The Torres reefs, North Queensland, Australia strong tidal flows a modern control on their growth. Coral Reefs 14, Kan, H., Hori, N., Formation of topographic zonation on the well-developed fringing reef-flat, Minna Island, the Central Ryukyus. Transactions of the Japanese Geomorphological Union 14-1, Kan, H., Takahashi, T., Koba, M., Morpho-dynamics on Holocene reef accretion: Drilling results from Nishimezaki Reef, Kume Island, the Central Ryukyus. Geographical Review of Japan 64 (Series B), Kan, H., Hori, N., Nakashima, Y., Ichikawa, K., The evolution of narrow reef flats at high-latitude in the Ryukyu Islands. Coral Reefs 14, Kan, H., Nakashima, Y., Hopley, H., 1997a. Coral communities during structural development of a fringing reef flat, Hayman Island, the Great Barrier Reef. Proceedings 8th International Coral Reef Symposium 1, Kan, H., Hori, N., Ichikawa, K., 1997b. Formation of a coral reef spur. Coral Reefs 16, 3 4. Kayanne, H., Yonekura, N., Ishii, T., Matsumoto, E., Geomorphic and geological development of Holocene emerged reefs in Rota and Guam, Mariana Islands. In: Yonekura, N. (Ed.), Sea-Level Changes and Tectonics in the Middle Pacific. Report of the HIPAC Project. Faculty of Science, Univeristy of Tokyo, Tokyo, pp Kennedy, D.M., Woodroffe, C.D., Holocene lagoonal sedimentation at the latitudinal limits of reef growth, Lord Howe Island, Tasman Sea. Marine Geology 169, Kleypas, J., Coral reef development under naturally turbid

22 276 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) conditions-fringing reefs near Broad Sound, Australia. Coral Reefs 15, Kleypas, J.A., Hopley, D., Reef development across a broad continental shelf, Southern Great Barrier Reef, Australia. Proceedings of the 7th International Coral Reefs Symposium, Guam, pp Kobluk, D.R., Lysenko, M.A., Storm features on a Southern Caribbean fringing coral reef. Palaios 7, Konishi, K., Tsuji, Y., Gotoh, T., Tanaka, T., Futakuchi, K., Shallow boring of coral reef: and example of Holocene reef on Kikai Island. Kaiyo 15, (in Japanese). Ladd, H.S., Tracey, J.I., Lill, G.G., Drilling on Bikini Atoll, Marshall Islands. Science 107, Ladd, H.S., Ingerson, E., Townsend, R.C., Russell, M., Stevenson, H.K., Drilling on Eniwetok Atoll, Marshall Islands. American Association of Petroleum Geologists Bulletin 37, Lewis, M.S., The morphology of the fringing coral reefs along the east coast of Mahé, Seychelles. 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23 D.M. Kennedy, C.D. Woodroffe / Earth-Science Reviews 57 (2002) ing reefs in a muddy environment, South Thailand. Journal of Sedimentary Research A 64, Umbgrove, J.H.F., Coral reefs of the East Indies. Geological Society of America Bulletin 58, Van Woesik, R., Done, T.J., Coral communities and reef growth in the southern Great Barrier Reef. Coral Reefs 16, Veron, J.E.N., Corals in Time and Space: The Biogeography and Evolution of Scleractinia. UNSW Press, Sydney, 321 pp. Webster, J.M., Davies, P.J., Konishi, K., Model of fringing reef development in response to progressive sea level fall over the last 7000 years (Kikai-jima, Ryukyu Islands, Japan). Coral Reefs 17, Woodroffe, C.D., Stoddart, D.R., Spence, T., Scoffin, T.P., Tudhope, A.W., Holocene emergence in the Cook Islands, South Pacific. Coral Reefs 9, Woodroffe, C.D., Kennedy, D.M., Hopley, D., Rasmussen, C.E., Smithers, S.G., Holocene reef growth in Torres Strait. Marine Geology 170, Woolfe, K., Larcombe, P., Terrigenous sedimentation and coral reef growth: a conceptual framework. Marine Geology 155, Yonekura, N., Matsushima, Y., Maeda, Y., Kayanne, H., Holocene sea-level changes in the Southern Cook Islands. In: Sugimura, A. (Ed.), Sea-Level Changes and Tectonics in the Middle Pacific. Department of Earth Science, Kobe University, Kobe, pp Yonekura, N., Ishii, T., Saito, Y., Masumoto, E., Kayanne, H., Geologic and geomorphic development of Holocene fringing reefs of Mangaia Island, the South Cooks. In: Sugimura, A. (Ed.), Sea-Level Changes and Tectonics in the Middle Pacific. Department of Earth Science, Kobe University, Kobe, pp Yonekura, N., Ishii, T., Saito, Y., Maeda, Y., Matsushima, Y., Matsumoto, E., Kayanne, H., Holocene fringing reefs and sealevel change in Mangaia Island, Southern Cook Islands. Palaeogeography, Palaeoclimatology, Palaeoecology 68, David Kennedy graduated with a BSc Honours degree from the University of Sydney, Australia and obtained his PhD from the University of Wollongong, Australia. His doctoral research investigated the Holocene reef and lagoonal sedimentation on the southernmost coral reef in the world, Lord Howe Island. David is now based in the School of Earth Sciences at Victoria University Wellington, New Zealand as a lecturer and is an honorary research fellow in the School of Geosciences, University of Wollongong. He previously was a postdoctorial research fellow at Wollongong, investigating reef and shelf environments in the Tasman Sea and northern Australia. Colin Woodroffe is a coastal geomorphologist, who graduated from Cambridge University with a PhD in Geography in He has particular research interests in the morphology, stratigraphy and sedimentary dynamics of tropical coasts, and the application of Geographical Information Systems (GIS) to the study of processes and change in the coastal zone. He is an active research member of the Research Centre for Landscape Change and the Oceans and Coastal Research Centre at the University of Wollongong. Colin has a major research interest in reef studies, having studied reef development and sea-level history in the Cayman Islands, Belize, Tuvalu, Tonga, Cook Islands, Torres Strait, Kiribati, and the Maldives. He has recently completed a multidisciplinary, collaborative project on the Cocos (Keeling) Islands, and a study of Late Quaternary environmental change on Lord Howe Island.