but he acknowledged that less stable “unpinned” reef
islands are common on reef flats that remain flooded by
lower tides. These higher foundations that confer stability
to overlying islands are not always composed of Holocene
reef. Cays composed of or attached to Pleistocene reefal
deposits, or occasionally eolianites (see Eolianite), are common in the Bahamas and offshore from British Honduras
(Milliman, 1973).
Establishing precise chronologies for reef-island formation is difficult as radiocarbon dates age the death of the
contributing organism and not the age of the deposit; in
many settings, these two ages can be centuries or even
millennia apart. If the radiocarbon chronologies are correct,
rapid sand production, delivery, and cay deposition
occurred on many planar reefs of the GBR between 4,000
and 3,000 years ago, with only relatively minor modifications since. Similar histories are reported from elsewhere
(Kench et al., 2005; see Woodroffe, 2008), including for
Warraber in Torres Strait where bulk sand ages suggest
a growth chronology very similar to those of the GBR cays
described above. However, AMS radiocarbon dating of
specific skeletal components suggests that the midHolocene age may be an artifact of age determination on
bulk sands; ages of molluscs indicate sustained incremental
accretion of Warraber over the past 3,000 years (Woodroffe
et al., 2007). This pattern of incremental development is
observed on many reef islands across the Indo-Pacific,
rather than rapid deposition in discrete periods or phases
(Woodroffe, 2008). It is important to note that progressive
sea-level change and island growth may modify sediment
production, transport efficiency over the reef platform,
and depositional nodes. For example, where sea level has
fallen during the late Holocene, reduced reef flat depths
may significantly reduce sediment delivery to leeward cays
(Kench and Brander, 2006).
The central, oldest areas of cays may be indicated by
the presence of mature vegetation and greater soil development (see previous section), although Woodroffe and
Morrison (2001) found no clear relationship between soil
development and age at Makin Island, Kiribati. Reefisland formation has not ceased, and new reef islands will
form if reef growth and sediment production continues.
Hopley et al. (2007) present average estimates of the time
required to progress through this sequence on the GBR
and emphasize that small shallow lagoonal reefs can transform into planar reefs in as few as 250 years. Thus, where
sediment supply is adequate and reefs of suitable elevation, geometry, and energy exposure exist, reef islands
may form quite rapidly.
Factors influencing cay stability
There are many records of cays, including those with
a mature vegetation cover, disappearing completely, both
during storms and over longer periods of time. Lines of
beach rock on numerous reef flats attest to the previous
presence of a cay even where there may be no historical
record. A number of factors are involved which may also
require consideration when assessing the future impacts
of global climate change. For the 300 reef islands of all
types found within the Great Barrier Reef Marine Park,
Aston (1995) carried out a statistical analysis of the factors
that contributed to stability. They varied with island type
but included:
(a) Location on reef flat and size of reef. The position of
the cay on the reef flat is determined by wave refraction patterns, which on most small to moderate size
reefs is toward the lee side. On large reefs and irregularly shaped reefs, the area of sand delivery may be
less focused and broad areas of sediment deposition
in the central reef flat may result. Any cay that forms
will be highly mobile as small weather changes result
in contrasting wave refraction patterns.
(b) Size of cay. Reef islands may vary from a few tens of
meters in diameter, to several kilometers. As it takes
a considerable period for large bodies of sediment to
accumulate, the larger islands are indicative of stability, requiring major changes in energy conditions to
produce significant changes to the cay shoreline.
(c) Shape of cay. Cays vary from oval to elongate. Oval
cays represent a sediment body that has been
constructed by the same energy conditions over
a period of time and are the most stable. Linear cays,
often with highly mobile spits on both ends, are by
far the least stable with location and orientation of
the spits often changing seasonally especially in monsoonal climates (Figure 8).
(d) Vegetation. A degree of stability is required for
a mature vegetation to take hold on sand cays. Very
important is the availability of a freshwater lens associated with cays with a minimum width of $120 m
(see Coral Cays – Geohydrology). Vegetation is not
only indicative of at least some degree of stability
but also through the binding action of roots, the addition of organic matter to soils and protection from
heavy tropical storms by the canopy, it adds further
to the island stabilization process.
(e) Cementation. Several cementation processes produce
hard rock outcrops which retard erosion. Intertidally
beach rock can form very quickly (Figure 6). Similar
intertidal cementation can occur on shingle islands
forming conglomerate outcrops. On older islands,
which have had a mature vegetation for some time
and which have been used as nesting or roosting sites
for sea birds, the leaching of guano into the soil can
result in the formation of phosphatic cay sandstone
(Figure 7) at the water table.
(f) Sediment budgets. Over time the delivery of sediments
to the reef flat, and ultimately to the reef cay, can
change. Initially when the reef first reaches sea level,
its high proportion of coral cover may be producing
calcium carbonate at rates up to 10 kg/m
2
/yr
À1
, though
much of this goes into the infilling of irregularities in
the maturing reef flat. Ultimately, the reef flat may
become totally sediment covered with delivery of
CORAL CAY CLASSIFICATION AND EVOLUTION
247
islands are common on reef flats that remain flooded by
lower tides. These higher foundations that confer stability
to overlying islands are not always composed of Holocene
reef. Cays composed of or attached to Pleistocene reefal
deposits, or occasionally eolianites (see Eolianite), are common in the Bahamas and offshore from British Honduras
(Milliman, 1973).
Establishing precise chronologies for reef-island formation is difficult as radiocarbon dates age the death of the
contributing organism and not the age of the deposit; in
many settings, these two ages can be centuries or even
millennia apart. If the radiocarbon chronologies are correct,
rapid sand production, delivery, and cay deposition
occurred on many planar reefs of the GBR between 4,000
and 3,000 years ago, with only relatively minor modifications since. Similar histories are reported from elsewhere
(Kench et al., 2005; see Woodroffe, 2008), including for
Warraber in Torres Strait where bulk sand ages suggest
a growth chronology very similar to those of the GBR cays
described above. However, AMS radiocarbon dating of
specific skeletal components suggests that the midHolocene age may be an artifact of age determination on
bulk sands; ages of molluscs indicate sustained incremental
accretion of Warraber over the past 3,000 years (Woodroffe
et al., 2007). This pattern of incremental development is
observed on many reef islands across the Indo-Pacific,
rather than rapid deposition in discrete periods or phases
(Woodroffe, 2008). It is important to note that progressive
sea-level change and island growth may modify sediment
production, transport efficiency over the reef platform,
and depositional nodes. For example, where sea level has
fallen during the late Holocene, reduced reef flat depths
may significantly reduce sediment delivery to leeward cays
(Kench and Brander, 2006).
The central, oldest areas of cays may be indicated by
the presence of mature vegetation and greater soil development (see previous section), although Woodroffe and
Morrison (2001) found no clear relationship between soil
development and age at Makin Island, Kiribati. Reefisland formation has not ceased, and new reef islands will
form if reef growth and sediment production continues.
Hopley et al. (2007) present average estimates of the time
required to progress through this sequence on the GBR
and emphasize that small shallow lagoonal reefs can transform into planar reefs in as few as 250 years. Thus, where
sediment supply is adequate and reefs of suitable elevation, geometry, and energy exposure exist, reef islands
may form quite rapidly.
Factors influencing cay stability
There are many records of cays, including those with
a mature vegetation cover, disappearing completely, both
during storms and over longer periods of time. Lines of
beach rock on numerous reef flats attest to the previous
presence of a cay even where there may be no historical
record. A number of factors are involved which may also
require consideration when assessing the future impacts
of global climate change. For the 300 reef islands of all
types found within the Great Barrier Reef Marine Park,
Aston (1995) carried out a statistical analysis of the factors
that contributed to stability. They varied with island type
but included:
(a) Location on reef flat and size of reef. The position of
the cay on the reef flat is determined by wave refraction patterns, which on most small to moderate size
reefs is toward the lee side. On large reefs and irregularly shaped reefs, the area of sand delivery may be
less focused and broad areas of sediment deposition
in the central reef flat may result. Any cay that forms
will be highly mobile as small weather changes result
in contrasting wave refraction patterns.
(b) Size of cay. Reef islands may vary from a few tens of
meters in diameter, to several kilometers. As it takes
a considerable period for large bodies of sediment to
accumulate, the larger islands are indicative of stability, requiring major changes in energy conditions to
produce significant changes to the cay shoreline.
(c) Shape of cay. Cays vary from oval to elongate. Oval
cays represent a sediment body that has been
constructed by the same energy conditions over
a period of time and are the most stable. Linear cays,
often with highly mobile spits on both ends, are by
far the least stable with location and orientation of
the spits often changing seasonally especially in monsoonal climates (Figure 8).
(d) Vegetation. A degree of stability is required for
a mature vegetation to take hold on sand cays. Very
important is the availability of a freshwater lens associated with cays with a minimum width of $120 m
(see Coral Cays – Geohydrology). Vegetation is not
only indicative of at least some degree of stability
but also through the binding action of roots, the addition of organic matter to soils and protection from
heavy tropical storms by the canopy, it adds further
to the island stabilization process.
(e) Cementation. Several cementation processes produce
hard rock outcrops which retard erosion. Intertidally
beach rock can form very quickly (Figure 6). Similar
intertidal cementation can occur on shingle islands
forming conglomerate outcrops. On older islands,
which have had a mature vegetation for some time
and which have been used as nesting or roosting sites
for sea birds, the leaching of guano into the soil can
result in the formation of phosphatic cay sandstone
(Figure 7) at the water table.
(f) Sediment budgets. Over time the delivery of sediments
to the reef flat, and ultimately to the reef cay, can
change. Initially when the reef first reaches sea level,
its high proportion of coral cover may be producing
calcium carbonate at rates up to 10 kg/m
2
/yr
À1
, though
much of this goes into the infilling of irregularities in
the maturing reef flat. Ultimately, the reef flat may
become totally sediment covered with delivery of
CORAL CAY CLASSIFICATION AND EVOLUTION
247