Deposition occurs where refracted waves converge, either
along the center of the reef or in a nodal area toward the
lee. However, on very large reefs, waves that are distant
from the reef front may become incompetent to transport
even fine sediments that remain spread over the reef flat.
Leeward sand cays thus form where there is both
a centripetal convergence of refracted waves and sufficient wave power to transport sediments to a focal point.
On large reef flats, sediment transport by refracted waves
may be augmented by locally generated short-period wind
waves, with their influence growing as rising tides
increase fetch over broad reef platforms (Samosorn and
Woodroffe, 2008). In high-energy areas and/or on smaller
reef platforms, sediments may be swept entirely off the
reef. Because maximum wave height over a reef flat is
constrained by water depth (which is tidally modulated)
and wave attenuation is a function of friction, a reef energy
window index can be calculated from the ratio of the reef
flat depth at mean spring high tide and reef width, which
indicates potential geomorphic activity over a particular
reef platform (Kench and Brander, 2006).
Cay sediments
The textural characteristics of reef sediments play an
important role in the bimodal distribution of reef flat sediment deposition observed on many reefs. Because shingle
and rubble tend to break down into sands of approximately
2ø (Orme, 1977), a distinctly bimodal distribution of
sediments and depositional environments develops
under the wave-energy conditions described above (see
Sediments, Properties; Sediment Dynamics; Reef Flats).
Nonetheless, the basic division of reef islands into windward shingle cays and leeward sand cays is very much
oversimplified.
Studies of GBR cays (Maxwell et al., 1961, 1964;
McLean and Stoddart, 1978) suggest that for each cay
type, either sand or shingle, sediments textures are
remarkably uniform. McLean and Stoddart (1978) examined sediments of cays on the northern GBR and found
that shingle is relatively homogeneous in composition
(mainly Acropora clasts) but varies in size and shape,
whereas sand cay sediments are mainly well-sorted
(<1ø) medium to coarse sands (0–1.5ø) derived from
a wider range of biota and are compositionally very similar to reef flat sediments. Where winds blow from one
prevailing direction, the beaches on sand cays contain
the coarsest sediments, particularly on the windward
shore. Winnowing by the wind transports finer sand to
the cay interior or berm, though dune structures are rare
or poorly developed on most cays. McLean and Stoddart
(1978) found that the finest sediments were associated
with soils, either as a component of the active soil profile
or within buried soil horizons.
The broad patterns established for GBR cay sediments
are generally applicable elsewhere. Although sand cay
sediments occur within a narrow textural range, small
but distinct variations between cays develop due to
differences in proportions of constituent components
(which may be related to the nature of the reef flat), differences in distances, modes, and rates of transport from
source area to cay (dependent on the size of the reef and
the location of the cay upon it), and variations in residence
time since deposition. Coral cays may include minor
amounts of siliciclastic sediment; pumice fragments
floated in from volcanic source areas are common and
can form distinctive strata up to 30 cm thick on some cays.
Pumice fragments up to 0.5 m in diameter have been
found, but more commonly they range between 1 and
5 cm in size. Occasionally, continental rocks reach cays
in the roots of floating trees.
Criteria used in classification
Several coral cay classification schemes have been proposed to accommodate and in some cases explain the variation in morphological and compositional diversity
outlined above. Steers (1929) suggested three classes of
reef islands based on his detailed work on the GBR in
the 1920s and 1930s: sand cays, shingle cays, and low
wooded islands. Spender (1930) proposed a general classification of reefs and low islands that recognized five classes of reef, four of which support reef islands (1) a sand
cay; (2) a sand cay and unvegetated rampart; (3) a sand
cay and vegetated rampart without extensive mangroves
across the reef platform; and (4) a sand cay, vegetated rampart, and mangrove-forested reef flat. Spender referred to
this latter class as “island reefs” but they are synonymous
with Steers’s low wooded islands by which they are better
known today (see Low Wooded Islands).
Fairbridge (1950) identified five classes of island based
on sediment type, vegetation cover, and the occurrence of
emergent reef flat – a function generally of late Holocene
relative sea-level history that can vary geographically
(Hopley et al., 2007). Fairbridge’s cay classes were
(1) unvegetated sand cays; (2) vegetated sand cays;
(3) shingle cay, with or without vegetation; (4) sand cay
with shingle ramparts, vegetated or unvegetated islands
and with mangrove swamp over reef top; and (5) island
with exposed platform of older emergent reef, sometimes
fringed by more recently deposited sediments. Hopley
(1982, 1997) reviewed the literature on GBR reef islands
and their classification and concluded that four criteria
can be used to define a reef-island classification equally
applicable to coral cays formed in other reef regions.
The four key criteria that form the basis of reef-island classification are as follows:
Criterion 1: Sediment type. Sand, shingle, or a mixture of
both can dominate reef islands. In areas of moderate
energy and a prevailing wind direction, shingle is typically located toward the windward edge of the reef platform, and sand cays generally form near the leeward
margin due to hydrodynamic sorting. This distinction
may be less clear in areas rarely affected by storms where
shingle deposits may be lacking or on reefs affected
by frequent high-energy storms where successive ridges
CORAL CAY CLASSIFICATION AND EVOLUTION
239
along the center of the reef or in a nodal area toward the
lee. However, on very large reefs, waves that are distant
from the reef front may become incompetent to transport
even fine sediments that remain spread over the reef flat.
Leeward sand cays thus form where there is both
a centripetal convergence of refracted waves and sufficient wave power to transport sediments to a focal point.
On large reef flats, sediment transport by refracted waves
may be augmented by locally generated short-period wind
waves, with their influence growing as rising tides
increase fetch over broad reef platforms (Samosorn and
Woodroffe, 2008). In high-energy areas and/or on smaller
reef platforms, sediments may be swept entirely off the
reef. Because maximum wave height over a reef flat is
constrained by water depth (which is tidally modulated)
and wave attenuation is a function of friction, a reef energy
window index can be calculated from the ratio of the reef
flat depth at mean spring high tide and reef width, which
indicates potential geomorphic activity over a particular
reef platform (Kench and Brander, 2006).
Cay sediments
The textural characteristics of reef sediments play an
important role in the bimodal distribution of reef flat sediment deposition observed on many reefs. Because shingle
and rubble tend to break down into sands of approximately
2ø (Orme, 1977), a distinctly bimodal distribution of
sediments and depositional environments develops
under the wave-energy conditions described above (see
Sediments, Properties; Sediment Dynamics; Reef Flats).
Nonetheless, the basic division of reef islands into windward shingle cays and leeward sand cays is very much
oversimplified.
Studies of GBR cays (Maxwell et al., 1961, 1964;
McLean and Stoddart, 1978) suggest that for each cay
type, either sand or shingle, sediments textures are
remarkably uniform. McLean and Stoddart (1978) examined sediments of cays on the northern GBR and found
that shingle is relatively homogeneous in composition
(mainly Acropora clasts) but varies in size and shape,
whereas sand cay sediments are mainly well-sorted
(<1ø) medium to coarse sands (0–1.5ø) derived from
a wider range of biota and are compositionally very similar to reef flat sediments. Where winds blow from one
prevailing direction, the beaches on sand cays contain
the coarsest sediments, particularly on the windward
shore. Winnowing by the wind transports finer sand to
the cay interior or berm, though dune structures are rare
or poorly developed on most cays. McLean and Stoddart
(1978) found that the finest sediments were associated
with soils, either as a component of the active soil profile
or within buried soil horizons.
The broad patterns established for GBR cay sediments
are generally applicable elsewhere. Although sand cay
sediments occur within a narrow textural range, small
but distinct variations between cays develop due to
differences in proportions of constituent components
(which may be related to the nature of the reef flat), differences in distances, modes, and rates of transport from
source area to cay (dependent on the size of the reef and
the location of the cay upon it), and variations in residence
time since deposition. Coral cays may include minor
amounts of siliciclastic sediment; pumice fragments
floated in from volcanic source areas are common and
can form distinctive strata up to 30 cm thick on some cays.
Pumice fragments up to 0.5 m in diameter have been
found, but more commonly they range between 1 and
5 cm in size. Occasionally, continental rocks reach cays
in the roots of floating trees.
Criteria used in classification
Several coral cay classification schemes have been proposed to accommodate and in some cases explain the variation in morphological and compositional diversity
outlined above. Steers (1929) suggested three classes of
reef islands based on his detailed work on the GBR in
the 1920s and 1930s: sand cays, shingle cays, and low
wooded islands. Spender (1930) proposed a general classification of reefs and low islands that recognized five classes of reef, four of which support reef islands (1) a sand
cay; (2) a sand cay and unvegetated rampart; (3) a sand
cay and vegetated rampart without extensive mangroves
across the reef platform; and (4) a sand cay, vegetated rampart, and mangrove-forested reef flat. Spender referred to
this latter class as “island reefs” but they are synonymous
with Steers’s low wooded islands by which they are better
known today (see Low Wooded Islands).
Fairbridge (1950) identified five classes of island based
on sediment type, vegetation cover, and the occurrence of
emergent reef flat – a function generally of late Holocene
relative sea-level history that can vary geographically
(Hopley et al., 2007). Fairbridge’s cay classes were
(1) unvegetated sand cays; (2) vegetated sand cays;
(3) shingle cay, with or without vegetation; (4) sand cay
with shingle ramparts, vegetated or unvegetated islands
and with mangrove swamp over reef top; and (5) island
with exposed platform of older emergent reef, sometimes
fringed by more recently deposited sediments. Hopley
(1982, 1997) reviewed the literature on GBR reef islands
and their classification and concluded that four criteria
can be used to define a reef-island classification equally
applicable to coral cays formed in other reef regions.
The four key criteria that form the basis of reef-island classification are as follows:
Criterion 1: Sediment type. Sand, shingle, or a mixture of
both can dominate reef islands. In areas of moderate
energy and a prevailing wind direction, shingle is typically located toward the windward edge of the reef platform, and sand cays generally form near the leeward
margin due to hydrodynamic sorting. This distinction
may be less clear in areas rarely affected by storms where
shingle deposits may be lacking or on reefs affected
by frequent high-energy storms where successive ridges
CORAL CAY CLASSIFICATION AND EVOLUTION
239