A second phase of island development has occurred over
the past 1,500 years at contemporary sea level producing
lower sand cay terraces and shingle ridges. Consequently,
the geomorphic complexity of low wooded islands in the
GBR results from the long period during which reefs have
been close to present sea level combined with distinct
changes in sea level position, altering the base level upon
which contemporary processes have been able to control
island formation.
Temporal development and morphological
change
A number of early studies speculated about the developmental sequence of low wooded islands. Spender (1930)
considered the islands to represent equilibrium forms,
whereas Steers (1937), on the basis of the width and extent
of ramparts suggested, they represented an evolutionary
sequence. These ideas were expanded by Fairbridge and
Teichert (1947) into an evolutionary sequence in which
the increase in extent of mangrove was proposed as an
indicator of relative maturity. A similar concept of age
development was proposed by Umbgrove (1947) for the
islands of Jakarta Bay. Shingle ramparts were observed
in different positions across reef platforms and in some
cases fused to the sand cay shoreline. Umbgrove (1947)
proposed a developmental sequence in which shingle
ridges migrate across platforms and fuse with sand islands.
Island maturity was therefore linked to the association of
shingle ridges with the sand cay.
These early theories of island development were largely
based on field descriptions and time-space substitution.
However, these models have been largely dispelled based
on more detailed consideration of the process regime
affecting reefs, the development of chronologies of island
formation, and monitoring studies. For example, monitoring of islands in Jakarta Bay by Zaneveld and Verstappen
(1952) failed to identify the morphological stages proposed by Umbgrove (1947).
Collectively, these latter studies have also provided
a number of insights on the sequence of island development and geomorphic behavior. First, consideration of
the process controls on island formation (Stoddart, 1965)
and radiocarbon evidence (Polach et al., 1978; McLean
et al., 1978; Stoddart et al., 1978b) indicates that shingle
ramparts and sand cays may form independently or contemporaneously, but that mangrove colonization logically
follows formation of shingle ramparts that afford protection from higher energy wave conditions.
Second, decadal scale monitoring and analysis has
identified considerable geomorphic change in low
wooded islands which includes: sand cay migration, rampart building, destruction and migration, and mangrove
expansion and contraction (Verstappen, 1954; Stoddart
et al., 1978c; Rasmussen, 1986; Frank and Jell, 2006).
Of note, such changes display no regular or consistent pattern between islands. The principle causative mechanisms
of these geomorphic changes are decadal scale variations
in climate (Verstappen, 1954), which reconfigure wave
processes on reefs, cyclones, and storms (Moorehouse,
1933, 1936; Stoddart et al., 1978c).
Third, feedbacks are likely to exist that also modify
island morphology. Examples include: shingle rampart
dynamics modulating mangrove contraction or expansion,
the configuration and extent of ramparts controlling wave
refraction patterns on reef surfaces and sand cay shoreline
processes, and mangrove expansion, which may shut
down the process regime influencing shoreline change
on sand cays.
Collectively, these observations suggest that each
island has its own developmental sequence and ultimate
morphological end point, which is determined by its
unique combination of location, exposure, reef geometry
and size, and sediment calibre, which is then modulated
by changes in the boundary process regime.
Summary
Low wooded islands are a specific and unique type of reef
island that occupy the surface of reef platforms. They
occur in a number of moderate energy reef settings where
the combination of wave energy exposure, reef geometry,
and sediment calibre are suitable for island formation. Scientific understanding of low wooded islands is heavily
influenced by detailed investigations in the early to midtwentieth century on islands in the Great Barrier Reef,
Australia, recognized as the most geomorphically complex examples. However, since 1973 there have been
few detailed studies of evolution or process controls on
development of low wooded islands in the Great Barrier
Reef or other reef settings. Consequently, global variations in the process regime controlling island development
and the precise chronology of island formation are poorly
resolved. There remain significant research opportunities
to redress these gaps in understanding and develop
a global model of the formation and distribution of these
distinct island types.
Bibliography
Chappell, J., 1983. Evidence for smoothly falling sea level relative
to north Queensland, Australia, during the past 6000 yr. Nature,
302, 406–408.
Chivas, A., Chappell, J., Polach, H., Pillans, B., and Flood, P., 1986.
Radiocarbon evidence for the timing and rate of island development, beachrock formation, and phosphatization at Lady Elliot
Island, Queensland, Australia. Marine Geology, 69, 273–287.
Fairbridge, R. W., and Teichert, C., 1947. The rampart system at
Low Isles, 1928–1945. The Geographical Journal, 111, 67–88.
Frank, T. D., and Jell, J. S., 2006. Recent developments on
a nearshore, terrigenous-influenced reef: Low Isles Reef, Australia. Journal of Coastal Research, 22, 474–486.
Hopley, D., 1982. The Geomorphology of the Great Barrier Reef:
Quaternary Development of Coral Reefs. New York: John Wiley
and Sons, p. 453.
Hopley, D., 1997. Geology of reef islands of the Great Barrier Reef,
Australia. In Vacher, H. L., and Quinn, T. (eds.), Geology and
Hydrogeology of Carbonate Islands. Developments in Sedimentology, Vol. 54, pp. 835–866.
644
LOW WOODED ISLANDS
the past 1,500 years at contemporary sea level producing
lower sand cay terraces and shingle ridges. Consequently,
the geomorphic complexity of low wooded islands in the
GBR results from the long period during which reefs have
been close to present sea level combined with distinct
changes in sea level position, altering the base level upon
which contemporary processes have been able to control
island formation.
Temporal development and morphological
change
A number of early studies speculated about the developmental sequence of low wooded islands. Spender (1930)
considered the islands to represent equilibrium forms,
whereas Steers (1937), on the basis of the width and extent
of ramparts suggested, they represented an evolutionary
sequence. These ideas were expanded by Fairbridge and
Teichert (1947) into an evolutionary sequence in which
the increase in extent of mangrove was proposed as an
indicator of relative maturity. A similar concept of age
development was proposed by Umbgrove (1947) for the
islands of Jakarta Bay. Shingle ramparts were observed
in different positions across reef platforms and in some
cases fused to the sand cay shoreline. Umbgrove (1947)
proposed a developmental sequence in which shingle
ridges migrate across platforms and fuse with sand islands.
Island maturity was therefore linked to the association of
shingle ridges with the sand cay.
These early theories of island development were largely
based on field descriptions and time-space substitution.
However, these models have been largely dispelled based
on more detailed consideration of the process regime
affecting reefs, the development of chronologies of island
formation, and monitoring studies. For example, monitoring of islands in Jakarta Bay by Zaneveld and Verstappen
(1952) failed to identify the morphological stages proposed by Umbgrove (1947).
Collectively, these latter studies have also provided
a number of insights on the sequence of island development and geomorphic behavior. First, consideration of
the process controls on island formation (Stoddart, 1965)
and radiocarbon evidence (Polach et al., 1978; McLean
et al., 1978; Stoddart et al., 1978b) indicates that shingle
ramparts and sand cays may form independently or contemporaneously, but that mangrove colonization logically
follows formation of shingle ramparts that afford protection from higher energy wave conditions.
Second, decadal scale monitoring and analysis has
identified considerable geomorphic change in low
wooded islands which includes: sand cay migration, rampart building, destruction and migration, and mangrove
expansion and contraction (Verstappen, 1954; Stoddart
et al., 1978c; Rasmussen, 1986; Frank and Jell, 2006).
Of note, such changes display no regular or consistent pattern between islands. The principle causative mechanisms
of these geomorphic changes are decadal scale variations
in climate (Verstappen, 1954), which reconfigure wave
processes on reefs, cyclones, and storms (Moorehouse,
1933, 1936; Stoddart et al., 1978c).
Third, feedbacks are likely to exist that also modify
island morphology. Examples include: shingle rampart
dynamics modulating mangrove contraction or expansion,
the configuration and extent of ramparts controlling wave
refraction patterns on reef surfaces and sand cay shoreline
processes, and mangrove expansion, which may shut
down the process regime influencing shoreline change
on sand cays.
Collectively, these observations suggest that each
island has its own developmental sequence and ultimate
morphological end point, which is determined by its
unique combination of location, exposure, reef geometry
and size, and sediment calibre, which is then modulated
by changes in the boundary process regime.
Summary
Low wooded islands are a specific and unique type of reef
island that occupy the surface of reef platforms. They
occur in a number of moderate energy reef settings where
the combination of wave energy exposure, reef geometry,
and sediment calibre are suitable for island formation. Scientific understanding of low wooded islands is heavily
influenced by detailed investigations in the early to midtwentieth century on islands in the Great Barrier Reef,
Australia, recognized as the most geomorphically complex examples. However, since 1973 there have been
few detailed studies of evolution or process controls on
development of low wooded islands in the Great Barrier
Reef or other reef settings. Consequently, global variations in the process regime controlling island development
and the precise chronology of island formation are poorly
resolved. There remain significant research opportunities
to redress these gaps in understanding and develop
a global model of the formation and distribution of these
distinct island types.
Bibliography
Chappell, J., 1983. Evidence for smoothly falling sea level relative
to north Queensland, Australia, during the past 6000 yr. Nature,
302, 406–408.
Chivas, A., Chappell, J., Polach, H., Pillans, B., and Flood, P., 1986.
Radiocarbon evidence for the timing and rate of island development, beachrock formation, and phosphatization at Lady Elliot
Island, Queensland, Australia. Marine Geology, 69, 273–287.
Fairbridge, R. W., and Teichert, C., 1947. The rampart system at
Low Isles, 1928–1945. The Geographical Journal, 111, 67–88.
Frank, T. D., and Jell, J. S., 2006. Recent developments on
a nearshore, terrigenous-influenced reef: Low Isles Reef, Australia. Journal of Coastal Research, 22, 474–486.
Hopley, D., 1982. The Geomorphology of the Great Barrier Reef:
Quaternary Development of Coral Reefs. New York: John Wiley
and Sons, p. 453.
Hopley, D., 1997. Geology of reef islands of the Great Barrier Reef,
Australia. In Vacher, H. L., and Quinn, T. (eds.), Geology and
Hydrogeology of Carbonate Islands. Developments in Sedimentology, Vol. 54, pp. 835–866.
644
LOW WOODED ISLANDS