On some islands lower terraces are characterized by open
areas with creepers, herbs, and grasses. Particularly common in the Great Barrier Reef examples are the colonizers
Thuarea Lepturus and Sporobolus, the succulents
Sesuvium and Portulaca, and creepers Ipomoea and
Canavalia. A similar range of vegetation has been
described from sand cays in the Belize barrier reef and
Jamaica (Steers and Lofthouse, 1940; Stoddart, 1965).
Boulder zone
A boulder zone is a recurrent feature of the leeward reef
edge or sand cay shoreline. Reaching up to 4 m in diameter
the boulders comprise individual coral colonies of storm
deposited origin.
Formation of low wooded islands
The boundary controls on the formation of low wooded
islands are the same as other reef islands. Three factors
typically constrain the development of islands. First,
a physical foundation is required at or near sea level,
which need not be extensive (Hopley, 1997) nor sea level
constrained (Kench et al., 2005). Typically this foundation
is a reef flat (Stoddart et al., 1978a) although some islands
are situated upon lagoonal sediments (Kench et al., 2005).
As noted by Stoddart (1965) the reef structure of low
wooded islands differs between the Great Barrier Reef
and Jakarta Bay. Nevertheless, these structural differences
have no influence on island morphology. Second, a supply
of sediment derived from the surrounding reef platform is
required. Third, a process regime able to entrain, distribute
and deposit sediments at different locations on reef surfaces is necessary (Stoddart and Steers, 1977).
The process controls on formation of each
biogeomorphic unit are relatively well constrained. Shingle ramparts are considered to be storm wave deposited
features with their presence and number on windward reef
margins related to storm frequency and the recovery interval of reefs to generate sediment (Stoddart, 1965; McLean
and Stoddart, 1978). The storm deposition of gravel
deposits has a number of well documented analogues
(Scoffin, 1993) such as the formation of sequential island
ridges in Lady Elliot Island in the southern Great Barrier
Reef (Chivas et al., 1986) and cyclone bank deposition
in Tuvalu (Maragos et al., 1973). Sand islands are developed at leeward depocentres on the reef platform at the
zone of convergence of swell, which refracts around the
reef platform structure and waves that cross the reef flat
at high tide. Their development is therefore, controlled
by reef platform configuration and orientation with respect
to the approach of swell energy and the supply and calibre
of sediment on reef flats (McLean and Stoddart, 1978). Of
note, the deposition of shingle ramparts disrupts the wave
refraction signals propagating across reef surfaces. Consequently, modification of wave processes may have
influenced the leeward depocentre and planform configuration of sand cays. It is generally accepted that mangrove
colonisation of the central reef flat has occurred under low
energy conditions provided by shingle ramparts, which
are conducive to deposition of finer sediments and establishment of mangrove.
Stoddart (1965) proposed a simple process model to
account for the presence and distribution of low wooded
islands and account for the different formative mechanisms and distribution of depositional products on reef
platforms. This model is based on the relative interplay
of exposure to wave energy, reef geometry, and sediment
calibre. The characteristics of exposure include: variability and wind strength, effective fetch, water depth, and
local factors of reef depth, which control incident wave
energy. Low wooded islands occur in settings protected
from high energy swell. They tend to occur where significant fetch length allows development of moderate wave
energy. The moderate energy setting is considered conducive to more fragile branching coral growth forms, therefore, supplying the shingle sediment for ramparts (Steers
and Lofthouse, 1940; Stoddart et al., 1978a). Stoddart
(1965) concluded that wave action is a sufficient mechanism for both shingle ridge and sand cay formation.
Recognition that the process regime is able to account
for the morphological development of low wooded islands
has significant implications for the interpretation of
regional differences in island morphology. In particular,
sea level change can be discounted as a necessary precursor for island formation. However, regional differences in
the patterning of Holocene sea level change and reef
growth are likely responsible for observed differences in
complexity of island morphology.
For example, in the Caribbean, sea level rose continually through the Holocene and slowed significantly over
the past 1,000 years (Toscano and Macintyre, 2003).
Low wooded islands described from this region have subsequently formed in response to the contemporary process
regime at constant sea level position and produced
a simple suite of morphological units. In contrast, Holocene sea level in the Indo-Pacific has been at or close to
present level for the past 6,000 years with a 1 m highstand
approximately 6,000–3,000 years ago (Chappell, 1983).
This differing pattern of sea level has imparted a more
complicated morphological imprint on low wooded
islands.
Results from radiometric dating of reefs and islands in
the Great Barrier Reef provides the most detailed dataset
with which to place an age framework on the evolution
of low wooded islands (Polach et al., 1978; McLean
et al., 1978, Stoddart et al., 1978b).
The platforms which support these islands were developed at modern sea level 6,000–5,000 year B.P. Evidence
from emergent reef platform and fossil microatolls indicate reef platforms accreted to elevations above present
sea level in response to the mid-Holocene highstand.
Radiometric ages on high sand cay terraces, beachrock,
upper promenades and shingle ridges (Figure 1g) indicate
an initial phase of island formation in the period
5,000–3,000 year B.P. with morphological products on
the order of 1.0 m above contemporary counterparts.
LOW WOODED ISLANDS
643
areas with creepers, herbs, and grasses. Particularly common in the Great Barrier Reef examples are the colonizers
Thuarea Lepturus and Sporobolus, the succulents
Sesuvium and Portulaca, and creepers Ipomoea and
Canavalia. A similar range of vegetation has been
described from sand cays in the Belize barrier reef and
Jamaica (Steers and Lofthouse, 1940; Stoddart, 1965).
Boulder zone
A boulder zone is a recurrent feature of the leeward reef
edge or sand cay shoreline. Reaching up to 4 m in diameter
the boulders comprise individual coral colonies of storm
deposited origin.
Formation of low wooded islands
The boundary controls on the formation of low wooded
islands are the same as other reef islands. Three factors
typically constrain the development of islands. First,
a physical foundation is required at or near sea level,
which need not be extensive (Hopley, 1997) nor sea level
constrained (Kench et al., 2005). Typically this foundation
is a reef flat (Stoddart et al., 1978a) although some islands
are situated upon lagoonal sediments (Kench et al., 2005).
As noted by Stoddart (1965) the reef structure of low
wooded islands differs between the Great Barrier Reef
and Jakarta Bay. Nevertheless, these structural differences
have no influence on island morphology. Second, a supply
of sediment derived from the surrounding reef platform is
required. Third, a process regime able to entrain, distribute
and deposit sediments at different locations on reef surfaces is necessary (Stoddart and Steers, 1977).
The process controls on formation of each
biogeomorphic unit are relatively well constrained. Shingle ramparts are considered to be storm wave deposited
features with their presence and number on windward reef
margins related to storm frequency and the recovery interval of reefs to generate sediment (Stoddart, 1965; McLean
and Stoddart, 1978). The storm deposition of gravel
deposits has a number of well documented analogues
(Scoffin, 1993) such as the formation of sequential island
ridges in Lady Elliot Island in the southern Great Barrier
Reef (Chivas et al., 1986) and cyclone bank deposition
in Tuvalu (Maragos et al., 1973). Sand islands are developed at leeward depocentres on the reef platform at the
zone of convergence of swell, which refracts around the
reef platform structure and waves that cross the reef flat
at high tide. Their development is therefore, controlled
by reef platform configuration and orientation with respect
to the approach of swell energy and the supply and calibre
of sediment on reef flats (McLean and Stoddart, 1978). Of
note, the deposition of shingle ramparts disrupts the wave
refraction signals propagating across reef surfaces. Consequently, modification of wave processes may have
influenced the leeward depocentre and planform configuration of sand cays. It is generally accepted that mangrove
colonisation of the central reef flat has occurred under low
energy conditions provided by shingle ramparts, which
are conducive to deposition of finer sediments and establishment of mangrove.
Stoddart (1965) proposed a simple process model to
account for the presence and distribution of low wooded
islands and account for the different formative mechanisms and distribution of depositional products on reef
platforms. This model is based on the relative interplay
of exposure to wave energy, reef geometry, and sediment
calibre. The characteristics of exposure include: variability and wind strength, effective fetch, water depth, and
local factors of reef depth, which control incident wave
energy. Low wooded islands occur in settings protected
from high energy swell. They tend to occur where significant fetch length allows development of moderate wave
energy. The moderate energy setting is considered conducive to more fragile branching coral growth forms, therefore, supplying the shingle sediment for ramparts (Steers
and Lofthouse, 1940; Stoddart et al., 1978a). Stoddart
(1965) concluded that wave action is a sufficient mechanism for both shingle ridge and sand cay formation.
Recognition that the process regime is able to account
for the morphological development of low wooded islands
has significant implications for the interpretation of
regional differences in island morphology. In particular,
sea level change can be discounted as a necessary precursor for island formation. However, regional differences in
the patterning of Holocene sea level change and reef
growth are likely responsible for observed differences in
complexity of island morphology.
For example, in the Caribbean, sea level rose continually through the Holocene and slowed significantly over
the past 1,000 years (Toscano and Macintyre, 2003).
Low wooded islands described from this region have subsequently formed in response to the contemporary process
regime at constant sea level position and produced
a simple suite of morphological units. In contrast, Holocene sea level in the Indo-Pacific has been at or close to
present level for the past 6,000 years with a 1 m highstand
approximately 6,000–3,000 years ago (Chappell, 1983).
This differing pattern of sea level has imparted a more
complicated morphological imprint on low wooded
islands.
Results from radiometric dating of reefs and islands in
the Great Barrier Reef provides the most detailed dataset
with which to place an age framework on the evolution
of low wooded islands (Polach et al., 1978; McLean
et al., 1978, Stoddart et al., 1978b).
The platforms which support these islands were developed at modern sea level 6,000–5,000 year B.P. Evidence
from emergent reef platform and fossil microatolls indicate reef platforms accreted to elevations above present
sea level in response to the mid-Holocene highstand.
Radiometric ages on high sand cay terraces, beachrock,
upper promenades and shingle ridges (Figure 1g) indicate
an initial phase of island formation in the period
5,000–3,000 year B.P. with morphological products on
the order of 1.0 m above contemporary counterparts.
LOW WOODED ISLANDS
643