developed on windward margins, but encrusting corallines coat most dead or dying surfaces in all intertidal
reef environments.
2. Branching coral facies is comprised essentially of various species of Acropora, Pocillopora, and branching
Porites. Growth form varies with depth, light, and
energy. In shallow water, variations in energy define
wide differences in species composition and diversity.
Water-depth changes produce similar compositional
variations.
3. Massive (Head) coral facies is comprised mainly of
various species of Porites, Goniopora, Favia, Leptoria,
Platygyra, Symphyllia, Leptastrea, Goniastrea, and
massive Acropora. As with the branching forms, water
depth and energy defines diversity.
4. Sand facies is usually medium to coarse sand comprising coral, corallines, benthic foraminifera, and minor
Halimeda. Occasionally rudaceous-sized fragments are
found. Sorting varies depending on distance from source
and degree of bioturbation. It forms large sand sheets
between coral flats and lagoons and also infills lagoons.
5. Rubble facies is comprised of massive, platey, and
branching corals varying in size from 20 kg blocks to
finger-like sticks. They are poorly sorted, occupying
large intertidal areas, sometimes cemented, often
bioeroded; also found in spurs and grooves, and at
the base of lagoonal patch reefs. They are produced
either by in situ collapse (in lagoons) or by energyrelated breakage and transport.
Two non carbonate facies have also been identified but
are minor compared with the carbonate facies. These are
siliceous muds and clays derived from rivers inputting
onto the inner shelf (Davies and Hughes, 1983; Smithers
and Larcombe, 2003), and granites as at Lizard Island contributing coarse quartz and felspar to an otherwise carbonate environment, but with corals actually growing on the
granites (Webster, 1994).
Growth models
Three growth models are presented below for three distinctive reef types defined above as lagoonal platforms,
crescent reefs, and ribbon barrier reefs.
The One Tree Reef Model
One Tree Reef (Figure 13) occurs in the Capricorn and
Bunker Reefs of the southern Great Barrier Reef. Surface
sampling and subsurface drilling and coring define One
Tree as the best understood reef in the whole region. Three
dimensional models show the Holocene One Tree Reef
growing atop its Pleistocene foundation and mimicking
its shape. In Figure 16a, the Pleistocene is exposed during
the lowstand. The highest part of the exposed rim is the
algal flat and coral flat because the areas of greatest erosion during the lowstand would likely be the reef front
and the sand flat/lagoon margin (see Antecedent Platforms, this volume). Transgression would initiate growth
on the high rim, after which growth would be to sea level
(Figure 16b). At this point, development of the reef flat,
sand sheets, lagoonal patch reefs, and the rubble sheets
will grow to the stable sea-level position (Figure 13a).
The effect of dominant energy is to produce clear constructional and destructional windward/leeward aspects.
Throughout its history different parts of One Tree
exhibited keep-up or catch-up strategies.
The Stanley Reef Model
Stanley Reef (latitude 19.18S) is a crescent reef in its eastern part and an “almost-atoll” in the west (Figure 16c). All
work conducted was on the eastern part which is both similar and very different to One Tree Reef; similar in having
a distinct edge to the east and southeast and dissimilar in
having a wide open lagoon and a lee edge which is patchy.
The reef is highest around the rim along the eastern margin
where the Pleistocene is at nearly 15–18 m and on the leeward patches at 20–22 m. A lagoonal patch reef in a lee
position showed the Pleistocene at around 20 m. The lagoon
is currently deep (þ20 m), and assuming 5–10 m of sediment, the Pleistocene in the lagoon would be at a depth of
þ30 m. During low sea level, Stanley was a crescent with
a high eastern rim decreasing in height to the west, and
a large open deep basin on the site of a former lagoon and
open lee end. Holocene growth on this antecedent surface
occurred almost simultaneously on windward and leeward
margins (Table 1). At the time of the first dated growth,
palaeowater depths on the windward margins were
10–12 m and on leeward edges 14–18 m. The first phase
of growth was vertically to sea level, after which, growth
of the windward margin replicates that seen on most reefs
(see One Tree and Ribbon 5) with the development of a
windward reef flat and sand flat. The leeward edge arrived
at sea level later than the windward margin. The next phase
of growth for this crescent reef is for the leeward patches to
coalesce to form a continuous lee edge, at which time, the
crescent reef will become a lagoonal platform reef. At this
stage, most sediment derived from the windward margin
will be trapped in the lagoon leading to an acceleration in
the infill of the lagoon (assuming enough time).
The Ribbon 5 model
Ribbon 5 (Figures 11a and 17) is a shelf edge barrier reef
east of Cooktown for which the database is more complete
than any reef in the Great Barrier Reef. It defines both the
Pleistocene and the Holocene evolution. The model,
defined by data in Figures 11a and 17, has the following
elements:
1. Initiation of reef growth on a shelf edge rhodolith bank
following a major climate change around 300,000–
350,000 years BP.
2. Destruction and regrowth of reefs successively, due to
sea-level oscillations, on eroded antecedent surfaces
with each reef initiating and growing on or close to
the position of the previous reef. A stacked succession
of reefs is thus formed (Figure 17b).
528
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT
reef environments.
2. Branching coral facies is comprised essentially of various species of Acropora, Pocillopora, and branching
Porites. Growth form varies with depth, light, and
energy. In shallow water, variations in energy define
wide differences in species composition and diversity.
Water-depth changes produce similar compositional
variations.
3. Massive (Head) coral facies is comprised mainly of
various species of Porites, Goniopora, Favia, Leptoria,
Platygyra, Symphyllia, Leptastrea, Goniastrea, and
massive Acropora. As with the branching forms, water
depth and energy defines diversity.
4. Sand facies is usually medium to coarse sand comprising coral, corallines, benthic foraminifera, and minor
Halimeda. Occasionally rudaceous-sized fragments are
found. Sorting varies depending on distance from source
and degree of bioturbation. It forms large sand sheets
between coral flats and lagoons and also infills lagoons.
5. Rubble facies is comprised of massive, platey, and
branching corals varying in size from 20 kg blocks to
finger-like sticks. They are poorly sorted, occupying
large intertidal areas, sometimes cemented, often
bioeroded; also found in spurs and grooves, and at
the base of lagoonal patch reefs. They are produced
either by in situ collapse (in lagoons) or by energyrelated breakage and transport.
Two non carbonate facies have also been identified but
are minor compared with the carbonate facies. These are
siliceous muds and clays derived from rivers inputting
onto the inner shelf (Davies and Hughes, 1983; Smithers
and Larcombe, 2003), and granites as at Lizard Island contributing coarse quartz and felspar to an otherwise carbonate environment, but with corals actually growing on the
granites (Webster, 1994).
Growth models
Three growth models are presented below for three distinctive reef types defined above as lagoonal platforms,
crescent reefs, and ribbon barrier reefs.
The One Tree Reef Model
One Tree Reef (Figure 13) occurs in the Capricorn and
Bunker Reefs of the southern Great Barrier Reef. Surface
sampling and subsurface drilling and coring define One
Tree as the best understood reef in the whole region. Three
dimensional models show the Holocene One Tree Reef
growing atop its Pleistocene foundation and mimicking
its shape. In Figure 16a, the Pleistocene is exposed during
the lowstand. The highest part of the exposed rim is the
algal flat and coral flat because the areas of greatest erosion during the lowstand would likely be the reef front
and the sand flat/lagoon margin (see Antecedent Platforms, this volume). Transgression would initiate growth
on the high rim, after which growth would be to sea level
(Figure 16b). At this point, development of the reef flat,
sand sheets, lagoonal patch reefs, and the rubble sheets
will grow to the stable sea-level position (Figure 13a).
The effect of dominant energy is to produce clear constructional and destructional windward/leeward aspects.
Throughout its history different parts of One Tree
exhibited keep-up or catch-up strategies.
The Stanley Reef Model
Stanley Reef (latitude 19.18S) is a crescent reef in its eastern part and an “almost-atoll” in the west (Figure 16c). All
work conducted was on the eastern part which is both similar and very different to One Tree Reef; similar in having
a distinct edge to the east and southeast and dissimilar in
having a wide open lagoon and a lee edge which is patchy.
The reef is highest around the rim along the eastern margin
where the Pleistocene is at nearly 15–18 m and on the leeward patches at 20–22 m. A lagoonal patch reef in a lee
position showed the Pleistocene at around 20 m. The lagoon
is currently deep (þ20 m), and assuming 5–10 m of sediment, the Pleistocene in the lagoon would be at a depth of
þ30 m. During low sea level, Stanley was a crescent with
a high eastern rim decreasing in height to the west, and
a large open deep basin on the site of a former lagoon and
open lee end. Holocene growth on this antecedent surface
occurred almost simultaneously on windward and leeward
margins (Table 1). At the time of the first dated growth,
palaeowater depths on the windward margins were
10–12 m and on leeward edges 14–18 m. The first phase
of growth was vertically to sea level, after which, growth
of the windward margin replicates that seen on most reefs
(see One Tree and Ribbon 5) with the development of a
windward reef flat and sand flat. The leeward edge arrived
at sea level later than the windward margin. The next phase
of growth for this crescent reef is for the leeward patches to
coalesce to form a continuous lee edge, at which time, the
crescent reef will become a lagoonal platform reef. At this
stage, most sediment derived from the windward margin
will be trapped in the lagoon leading to an acceleration in
the infill of the lagoon (assuming enough time).
The Ribbon 5 model
Ribbon 5 (Figures 11a and 17) is a shelf edge barrier reef
east of Cooktown for which the database is more complete
than any reef in the Great Barrier Reef. It defines both the
Pleistocene and the Holocene evolution. The model,
defined by data in Figures 11a and 17, has the following
elements:
1. Initiation of reef growth on a shelf edge rhodolith bank
following a major climate change around 300,000–
350,000 years BP.
2. Destruction and regrowth of reefs successively, due to
sea-level oscillations, on eroded antecedent surfaces
with each reef initiating and growing on or close to
the position of the previous reef. A stacked succession
of reefs is thus formed (Figure 17b).
528
GREAT BARRIER REEF: ORIGIN, EVOLUTION, AND MODERN DEVELOPMENT