2 – Geomorphology of Coral Reefs with Special Reference to the Great Barrier Reef
15
of the Pompeys. A steep drop-off occurs around the
northern margin, but to the south and west adjacent to
the Capricorn Channel the slope is far gentler.
The Bunker-Capricorn Group. The southernmost
reefs of the GBR are set back from the shelf margin,
which is poorly defined. The reefs grow over shallow
Pleistocene foundations and are either planar or lagoonal with many cays. Corals are not found in the
shelf sedi ments south of 24°S and it is possible that
there was no reef growth at this latitude at the height of
the glacial periods.
Coral islands
Wave action accumulates sediments produced by
coral reefs in particular areas of the reef flat to form
various types of coral islands or cays. Reef flat is
required and as some can be up to 6000 years old, reef
islands are all younger than this. As reefs of the central GBR are relatively immature, no vegetated reef
island exists between Green Island (16°45S) and
Bushy Island (25°57S). Centripetal action of refracted
waves moves sediment over the reef, deposition occurring when the waves no longer have the capacity
to carry it further. Capacity to carry shingle is quickly
lost in normal weather conditions and shingle cays
and ramparts are found on the windward side of reefs.
Sand may be carried greater distances and sand cays
are usually found towards the lee of the reef, the shape
depending on the wave refraction pattern. On oval
reefs the depositional area may be fairly compact and
the cay is oval. On elongate reefs the depositional area
where wave trains from each side of the reef meet is
linear and the resulting island is longer and narrower,
such a shape being susceptible to changing location
and erosion in response to changes in weather patterns. On stable, older cays, mature soils, perhaps with
guano deposits, have had time to form and a climax
woodland vegetation, for example, Pisonia grandis,
may exist but because of constant changes all stages
of the vegetational succession may be present on a
single island.
Classification and description of reef islands is
based on sediment type (sand or shingle) and vegetation cover (from unvegetated to highly complex including reef flat mangroves). Island shape and size
are also important as there is a threshold island width
of about 120 m to support a freshwater lens on which
mature vegetation depends. Some reefs have more
than one island, a windward shingle cay and a leeward sand cay but the most complex are the low
wooded islands of the northern Reef. These consist of
windward shingle ridges and cemented ramparts that
give protection to the reef flat over which mangroves
can become established (in some instances over almost the entire reef top). Sand cays are usually found
on the leeside of these reefs.
There is a distinctive pattern in the distribution of
island types. Unvegetated islands occur throughout
the GBR but are least common on the central GBR,
where vegetated cays are totally absent. Vegetated cays
are most numerous in the far south and far north and
occur on six of the Swain Reefs. Low wooded islands
are limited to the inner shelf north of Cairns. Within the
GBRMP are 213 unvegetated cays, 43 vegetated cays
and 44 low wooded islands (total 300) though changes
are continually taking place, especially to those with
ephemeral vegetation.
N GEOMORPHOLOGY AND THE FUTURE
OF THE GREAT BARRIER REEF
The GBR has changed dramatically over the last 10 000
years and, in terms of coral cover and ecological variety,
was probably at its peak about 6000 years ago. Since
then reefs have progressed towards more senile stages.
Global climate change may accelerate this trend. Even
a simple rise in sea level may see a renewal of carbonate
productivity over reef flats that have been at sea
level for 5000 years or more, but with more efficient
movement of sediment as water levels deepen. However, increases in coral mortality from bleaching,
diseases, and fragility of many surviving corals as
oceans become more acidic are all going to accelerate
sediment production and infilling of lagoons. These
same processes may counteract the rise of sea level
against reef islands. Geomorphological studies show
that far from disappearing as some have suggested,
islands may increase in size. Tropical cyclones, which
may increase in frequency and magnitude, can both
erode and build island structures largely dependant on
15
of the Pompeys. A steep drop-off occurs around the
northern margin, but to the south and west adjacent to
the Capricorn Channel the slope is far gentler.
The Bunker-Capricorn Group. The southernmost
reefs of the GBR are set back from the shelf margin,
which is poorly defined. The reefs grow over shallow
Pleistocene foundations and are either planar or lagoonal with many cays. Corals are not found in the
shelf sedi ments south of 24°S and it is possible that
there was no reef growth at this latitude at the height of
the glacial periods.
Coral islands
Wave action accumulates sediments produced by
coral reefs in particular areas of the reef flat to form
various types of coral islands or cays. Reef flat is
required and as some can be up to 6000 years old, reef
islands are all younger than this. As reefs of the central GBR are relatively immature, no vegetated reef
island exists between Green Island (16°45S) and
Bushy Island (25°57S). Centripetal action of refracted
waves moves sediment over the reef, deposition occurring when the waves no longer have the capacity
to carry it further. Capacity to carry shingle is quickly
lost in normal weather conditions and shingle cays
and ramparts are found on the windward side of reefs.
Sand may be carried greater distances and sand cays
are usually found towards the lee of the reef, the shape
depending on the wave refraction pattern. On oval
reefs the depositional area may be fairly compact and
the cay is oval. On elongate reefs the depositional area
where wave trains from each side of the reef meet is
linear and the resulting island is longer and narrower,
such a shape being susceptible to changing location
and erosion in response to changes in weather patterns. On stable, older cays, mature soils, perhaps with
guano deposits, have had time to form and a climax
woodland vegetation, for example, Pisonia grandis,
may exist but because of constant changes all stages
of the vegetational succession may be present on a
single island.
Classification and description of reef islands is
based on sediment type (sand or shingle) and vegetation cover (from unvegetated to highly complex including reef flat mangroves). Island shape and size
are also important as there is a threshold island width
of about 120 m to support a freshwater lens on which
mature vegetation depends. Some reefs have more
than one island, a windward shingle cay and a leeward sand cay but the most complex are the low
wooded islands of the northern Reef. These consist of
windward shingle ridges and cemented ramparts that
give protection to the reef flat over which mangroves
can become established (in some instances over almost the entire reef top). Sand cays are usually found
on the leeside of these reefs.
There is a distinctive pattern in the distribution of
island types. Unvegetated islands occur throughout
the GBR but are least common on the central GBR,
where vegetated cays are totally absent. Vegetated cays
are most numerous in the far south and far north and
occur on six of the Swain Reefs. Low wooded islands
are limited to the inner shelf north of Cairns. Within the
GBRMP are 213 unvegetated cays, 43 vegetated cays
and 44 low wooded islands (total 300) though changes
are continually taking place, especially to those with
ephemeral vegetation.
N GEOMORPHOLOGY AND THE FUTURE
OF THE GREAT BARRIER REEF
The GBR has changed dramatically over the last 10 000
years and, in terms of coral cover and ecological variety,
was probably at its peak about 6000 years ago. Since
then reefs have progressed towards more senile stages.
Global climate change may accelerate this trend. Even
a simple rise in sea level may see a renewal of carbonate
productivity over reef flats that have been at sea
level for 5000 years or more, but with more efficient
movement of sediment as water levels deepen. However, increases in coral mortality from bleaching,
diseases, and fragility of many surviving corals as
oceans become more acidic are all going to accelerate
sediment production and infilling of lagoons. These
same processes may counteract the rise of sea level
against reef islands. Geomorphological studies show
that far from disappearing as some have suggested,
islands may increase in size. Tropical cyclones, which
may increase in frequency and magnitude, can both
erode and build island structures largely dependant on