of the tidal cycle, incident wave energy on the windward
margin pumps water onto reef flat and into the lagoon,
while water exits over the leeward reef flat and through
channels. Wave pumping causes lagoon water levels to
remain above the ocean at all tidal phases and to rise significantly during periods of enhanced wave activity (see
also Kraines et al., 1999). As a consequence, exchange is
directly related to the proportion of reef flat that is submerged or dissected by channels and, where tidal channels
are narrow or absent, island formation and sediment deposition on the reef flat can play a key role (Kench and
Mclean, 2004). The impact of water exchange and wave
pumping on lagoonal reef development has been clearly
illustrated in the Tuamotus, where Adjeroud et al.
(2000) reported that reef pinnacles were more abundant
and diverse in bigger atolls, which had either large channels or a high proportion of submerged reef flat. Conversely, they found that smaller atolls with reduced
exchange due to reef-flat island formation were typified
by low pinnacle abundance and diversity.
Cellular reefs
Reef development in lagoons can also be self-limiting due
to a negative feedback between growth and circulation.
For example, prolific development of pinnacles in protected
lagoonal areas of the Houtman Abrolhos reef complex, off
the western Australian coast, has produced a cellular reef
pattern, as shown in Figure 3 (Collins et al., 1996; Wyrwoll
et al., 2006). The reef tops have a dense cover of branching
acroporids, which only extend 7–10 m down the steep
walls of the cells. In some areas, the walls overhang and
show collapse scars, which expose open frameworks of
the same corals; the floors of the cells are between 10and 30-m deep and covered with a thick skeletal sand and
branch-coral gravel. Their water columns are stagnant and
stratified and do not support coral growth below 10 m. Drill
cores and seismic data confirm that these cellular reefs are
composed of branching acroporid frameworks that developed during the last 7–10 ka over a flat Pleistocene reef terrace at $40-m depth (Collins et al., 1996; Wyrwoll et al.,
2006). These studies in the Houtman-Abrolhos show that
the prolific growth of branching acroporids has led to the
coalescence of isolated pinnacles into reticulate structures
that have enclosed deeper cells, causing restricted circulation and self-limitation of coral growth. Reticulate and cellular reef development is therefore one of the several
examples of self-organization in coral reefs and is an emergent property of the synergy between rapid lagoonal reef
growth and circulation.
Formation of lagoonal reefs
The development of lagoonal reefs, particularly cellular
reef forms, has been persistently linked to antecedent karst
due to its similarity to polygonal and cockpit-cone forms
that develop in humid tropical karst terrains (MacNeill,
1954; Purdy, 1974; Purkis, 2010). However, as alluded
to above, drilling and seismic profiling has consistently
shown that reef forms are growth induced, either developing from scratch during the present interglacial or continuing atop development that took place during previous
interglacial stages (Harvey and Hopley, 1981; Hopley,
1982; Smith et al., 1998). On Cocos-Keeling, seismic data
collected by Searle (1994) showed that the modern reticulate reef development was largely growth induced but was
influenced by a combination of fossil reef growth as well
as karst-induced collapse structures in the underlying substrate. In other areas, positive relief provided by previous
episodes of interglacial reef growth has likely played
a negative role in modern lagoonal reef development.
For instance, the rims of Pacific atolls are often
multigenerational reef structures, which developed during
previous interglacials (e.g., Szabo et al., 1985); where
these rims are continuous, they must have prevented
lagoonal reef development for significant time intervals
until the sea level overtopped them and initiated their
modern structure $8 ka ago. This delay in development
may explain the anomalous paucity of reefs in some of
the deeper modern atoll lagoons.
For other forms of lagoonal reef development however,
antecedent substrates have provided little to no guidance
for subsequent events. On John Brewer Reef on the
Great Barrier Reef (GBR), Walbran (1994) obtained seismic profiles showing that both lagoonal patch reefs and
the reef rim are underlain by a flat Pleistocene surface at
20-m depth. The same was also found at Heron and Sykes
reefs in the Capricorn Group by Smith et al. (1998) and in
the Houtman-Abrolhos reef complex by Collins et al.
(1996). Modern reef growth in lagoons and other zones
is clearly capable of generating its own structures with little help from antecedent templates of any kind.
Reef flat
In contrast to the high-relief patches and pinnacles of the
lagoon, reef flats are remarkably level surfaces and their
low-tide elevation varies little for hundreds of meters. If
there is any gradient at all, it tends to slope back from
the outer reef flat toward the inner, and its lagoonal margin
water depths are no more than 1–3 m. In the absence of
islands and storm ramparts, the width of open reef flats
in atoll groups such as the Marshalls and Tuvalu is
between 500 and 1,000 m, but exceeds this in the
Tuamotus and Maldives where flats with widths of 2,000
m are common. The presence of islands and storm
ramparts however tends to significantly reduce reef-flat
width by impeding sediment transport into the lagoon.
As recognized by Hopley (1982), open reef flats consist
of two common subzones: an outer “living zone” composed of an algal rim and a coral algal flat and an inner
sand zone mantled by thin sheets of shifting sediment
(Figure 1).
Algal rims
Algal rims are usually less than 50-m wide and form the
highest part of the reef flat. Two types of algal rim are
GEOMORPHIC ZONATION
473
margin pumps water onto reef flat and into the lagoon,
while water exits over the leeward reef flat and through
channels. Wave pumping causes lagoon water levels to
remain above the ocean at all tidal phases and to rise significantly during periods of enhanced wave activity (see
also Kraines et al., 1999). As a consequence, exchange is
directly related to the proportion of reef flat that is submerged or dissected by channels and, where tidal channels
are narrow or absent, island formation and sediment deposition on the reef flat can play a key role (Kench and
Mclean, 2004). The impact of water exchange and wave
pumping on lagoonal reef development has been clearly
illustrated in the Tuamotus, where Adjeroud et al.
(2000) reported that reef pinnacles were more abundant
and diverse in bigger atolls, which had either large channels or a high proportion of submerged reef flat. Conversely, they found that smaller atolls with reduced
exchange due to reef-flat island formation were typified
by low pinnacle abundance and diversity.
Cellular reefs
Reef development in lagoons can also be self-limiting due
to a negative feedback between growth and circulation.
For example, prolific development of pinnacles in protected
lagoonal areas of the Houtman Abrolhos reef complex, off
the western Australian coast, has produced a cellular reef
pattern, as shown in Figure 3 (Collins et al., 1996; Wyrwoll
et al., 2006). The reef tops have a dense cover of branching
acroporids, which only extend 7–10 m down the steep
walls of the cells. In some areas, the walls overhang and
show collapse scars, which expose open frameworks of
the same corals; the floors of the cells are between 10and 30-m deep and covered with a thick skeletal sand and
branch-coral gravel. Their water columns are stagnant and
stratified and do not support coral growth below 10 m. Drill
cores and seismic data confirm that these cellular reefs are
composed of branching acroporid frameworks that developed during the last 7–10 ka over a flat Pleistocene reef terrace at $40-m depth (Collins et al., 1996; Wyrwoll et al.,
2006). These studies in the Houtman-Abrolhos show that
the prolific growth of branching acroporids has led to the
coalescence of isolated pinnacles into reticulate structures
that have enclosed deeper cells, causing restricted circulation and self-limitation of coral growth. Reticulate and cellular reef development is therefore one of the several
examples of self-organization in coral reefs and is an emergent property of the synergy between rapid lagoonal reef
growth and circulation.
Formation of lagoonal reefs
The development of lagoonal reefs, particularly cellular
reef forms, has been persistently linked to antecedent karst
due to its similarity to polygonal and cockpit-cone forms
that develop in humid tropical karst terrains (MacNeill,
1954; Purdy, 1974; Purkis, 2010). However, as alluded
to above, drilling and seismic profiling has consistently
shown that reef forms are growth induced, either developing from scratch during the present interglacial or continuing atop development that took place during previous
interglacial stages (Harvey and Hopley, 1981; Hopley,
1982; Smith et al., 1998). On Cocos-Keeling, seismic data
collected by Searle (1994) showed that the modern reticulate reef development was largely growth induced but was
influenced by a combination of fossil reef growth as well
as karst-induced collapse structures in the underlying substrate. In other areas, positive relief provided by previous
episodes of interglacial reef growth has likely played
a negative role in modern lagoonal reef development.
For instance, the rims of Pacific atolls are often
multigenerational reef structures, which developed during
previous interglacials (e.g., Szabo et al., 1985); where
these rims are continuous, they must have prevented
lagoonal reef development for significant time intervals
until the sea level overtopped them and initiated their
modern structure $8 ka ago. This delay in development
may explain the anomalous paucity of reefs in some of
the deeper modern atoll lagoons.
For other forms of lagoonal reef development however,
antecedent substrates have provided little to no guidance
for subsequent events. On John Brewer Reef on the
Great Barrier Reef (GBR), Walbran (1994) obtained seismic profiles showing that both lagoonal patch reefs and
the reef rim are underlain by a flat Pleistocene surface at
20-m depth. The same was also found at Heron and Sykes
reefs in the Capricorn Group by Smith et al. (1998) and in
the Houtman-Abrolhos reef complex by Collins et al.
(1996). Modern reef growth in lagoons and other zones
is clearly capable of generating its own structures with little help from antecedent templates of any kind.
Reef flat
In contrast to the high-relief patches and pinnacles of the
lagoon, reef flats are remarkably level surfaces and their
low-tide elevation varies little for hundreds of meters. If
there is any gradient at all, it tends to slope back from
the outer reef flat toward the inner, and its lagoonal margin
water depths are no more than 1–3 m. In the absence of
islands and storm ramparts, the width of open reef flats
in atoll groups such as the Marshalls and Tuvalu is
between 500 and 1,000 m, but exceeds this in the
Tuamotus and Maldives where flats with widths of 2,000
m are common. The presence of islands and storm
ramparts however tends to significantly reduce reef-flat
width by impeding sediment transport into the lagoon.
As recognized by Hopley (1982), open reef flats consist
of two common subzones: an outer “living zone” composed of an algal rim and a coral algal flat and an inner
sand zone mantled by thin sheets of shifting sediment
(Figure 1).
Algal rims
Algal rims are usually less than 50-m wide and form the
highest part of the reef flat. Two types of algal rim are
GEOMORPHIC ZONATION
473