surface by as much as 1 m and clearly represent former
positions of the algal rim (Ladd et al., 1950; Scoffin
et al., 1985). Also protruding are boulders of coral framework eroded from the reef front and thrown up onto the
reef flat during cyclones. These become welded to its surface by corallines and often develop an intertidal notch
produced by intense bioerosion (Stoddart, 1969).
On other swell-dominated reefs, troughs are absent and
the coralgal flat behind the rim consists of a very distinctive striated zone, especially in areas with high tidal ranges
such as the GBR and western Indian Ocean. This consists
of elongate strips of reef flat consolidated by corallines,
sometimes colonized by corals, and separated by furrows
of unconsolidated skeletal sand and gravel, which deepen
and widen inshore (Hopley, 1982). Although coral cover
varies, the furrows are persistent features, which can
extend across the entire coralgal zone. At Mayotte, sand
and gravel stripes clearly continue in alignment with the
reef-front spur and groove, and have a height differential
of 10–50 cm. At Mahé in the Seychelles, 2–3-m-high stabilized gravel ridges narrow inshore and are separated by
1–2-m-deep furrows, which widen and deepen inshore
(Guilcher, 1988).
Sand flat
The innermost sides of open reef flats, adjacent to the
lagoon, are dominated by shifting sand that is either
washed back from the seaward rim or, when winds switch
direction during storms or monsoons, from the lagoon
itself. The predominant movement of sediment in this
zone occurs during tropical cyclones when large amounts
of sand covering islands in the reef-front reservoir is
remobilized and deposited as spill-over lobes, which prograde into the lagoon, forming a distinctive sand slope.
This general sediment movement into the lagoon is
interrupted by islands, which provide shelter from sedimentation and foster patch-reef development along this
boundary. These patches can become well developed and
form a linear reef with a steep lagoon-side scarp.
Islands and storm deposits
Low-lying sand islands, and their associated channels
(hoas) and gravel ramparts, constitute a large proportion
of swell-dominated reef flats and form features which
are dynamic in both space and time. Although these features are formed by a combination of storm and
fairweather processes (Kench and Brander, 2006), storms
assume a greater importance because they generate gravel
through the destruction of corals, remobilize inactive sediment deposits on islands and in sub-wavebase reservoirs,
and provide a continuous force on geological timescales.
For these reasons, storms likely play a major role in initiating island formation and causing major subsequent
changes (Baines et al., 1974; Bayliss-Smith, 1988).
Reports of the impacts of tropical cyclones indicate that,
in addition to erosion, three kinds of reef-flat deposit are
common: outer-flat gravel bars, coastal ramparts along
existing islands, and aerially extensive gravel sheets over
both the flat and islands (McKee, 1959; Blumenstock,
1961; Baines et al., 1974). The outer-flat gravel bars are
composed of cobble to boulder-sized coral clasts, which
are eroded from the reef front and typically deposited
20–30 m inshore from the algal rim. Under fairweather
conditions, these gravel bars rapidly migrate inshore and
are eventually left as marine cemented lenses of gravel
spread out over the reef flat $100 m from the outer rim;
these are known as conglomerate platforms (Hopley,
1982). Such immobile lenses commonly provide a locus
for subsequent sedimentation, and storms can add extensive shore-attached gravel ramparts for considerable distances along and between platforms (Baines et al.,
1974). However, the most volumetrically important reefflat deposits consist of thin but aerially extensive sheets
of finer-grained cobble to pebble gravels that can become
rapidly stabilized either in island settings by marine
vadose cements to form “cay rock,” or on open reef flats
themselves by crustose corallines (Newell and Bloom,
1970; Marshall and Jacobson, 1985). Successive deposition and cementation of gravel lenses and sheets can produce island platforms that reach 2 m or more above
present mean sea level. Dating of these platforms in several areas has yielded anomalously old ages and, as
a result, it has been claimed that they represent widespread
reef-flat development during a regionally higher midHolocene sea level (e.g., Dickinson, 2004) despite the fact
that clasts in these deposits have large age ranges (e.g.,
Scoffin et al., 1985).
Formation of reef flats
The paucity of active coral growth and the flatness and
low-tide elevation of many Indo-Pacific reef flats have
been a continued source of confusion and, as alluded to,
the age and origin of gravel deposits have provoked particular controversy (e.g., Newell and Bloom, 1970).
A persistent idea is that reef flats have been eroded to their
present low-tide level by a slight fall in regional sea level
over the last several 1,000 years (e.g., Cloud, 1952).
A variation of this has been advocated by Pirazzoli and
Montaggioni (1988) and Woodroffe (2008), who
suggested that the relative sea-level fall during past few
millennia has produced many emergent reef flats, which
are developed at elevations too high to be suitable for coral
growth. This implies that reef flats were constructed by
coral framework prior to the inferred sea-level fall; yet,
drill cores through reef flats consistently show that they
are not composed of thick framework but of detrital
deposits (Henny et al., 1974; Marshall and Jacobson,
1985). Hence, the present reef-flat surface is a reflection
of its interior, and an absence of corals and low-tide elevation does not require an erosional explanation. In addition,
elevation data used to infer higher sea level have not
accounted for elevated water levels on reef flats due to
wave pumping and ponding of water behind storm
ramparts, and elevations used from relict algal ridges are
GEOMORPHIC ZONATION
475
positions of the algal rim (Ladd et al., 1950; Scoffin
et al., 1985). Also protruding are boulders of coral framework eroded from the reef front and thrown up onto the
reef flat during cyclones. These become welded to its surface by corallines and often develop an intertidal notch
produced by intense bioerosion (Stoddart, 1969).
On other swell-dominated reefs, troughs are absent and
the coralgal flat behind the rim consists of a very distinctive striated zone, especially in areas with high tidal ranges
such as the GBR and western Indian Ocean. This consists
of elongate strips of reef flat consolidated by corallines,
sometimes colonized by corals, and separated by furrows
of unconsolidated skeletal sand and gravel, which deepen
and widen inshore (Hopley, 1982). Although coral cover
varies, the furrows are persistent features, which can
extend across the entire coralgal zone. At Mayotte, sand
and gravel stripes clearly continue in alignment with the
reef-front spur and groove, and have a height differential
of 10–50 cm. At Mahé in the Seychelles, 2–3-m-high stabilized gravel ridges narrow inshore and are separated by
1–2-m-deep furrows, which widen and deepen inshore
(Guilcher, 1988).
Sand flat
The innermost sides of open reef flats, adjacent to the
lagoon, are dominated by shifting sand that is either
washed back from the seaward rim or, when winds switch
direction during storms or monsoons, from the lagoon
itself. The predominant movement of sediment in this
zone occurs during tropical cyclones when large amounts
of sand covering islands in the reef-front reservoir is
remobilized and deposited as spill-over lobes, which prograde into the lagoon, forming a distinctive sand slope.
This general sediment movement into the lagoon is
interrupted by islands, which provide shelter from sedimentation and foster patch-reef development along this
boundary. These patches can become well developed and
form a linear reef with a steep lagoon-side scarp.
Islands and storm deposits
Low-lying sand islands, and their associated channels
(hoas) and gravel ramparts, constitute a large proportion
of swell-dominated reef flats and form features which
are dynamic in both space and time. Although these features are formed by a combination of storm and
fairweather processes (Kench and Brander, 2006), storms
assume a greater importance because they generate gravel
through the destruction of corals, remobilize inactive sediment deposits on islands and in sub-wavebase reservoirs,
and provide a continuous force on geological timescales.
For these reasons, storms likely play a major role in initiating island formation and causing major subsequent
changes (Baines et al., 1974; Bayliss-Smith, 1988).
Reports of the impacts of tropical cyclones indicate that,
in addition to erosion, three kinds of reef-flat deposit are
common: outer-flat gravel bars, coastal ramparts along
existing islands, and aerially extensive gravel sheets over
both the flat and islands (McKee, 1959; Blumenstock,
1961; Baines et al., 1974). The outer-flat gravel bars are
composed of cobble to boulder-sized coral clasts, which
are eroded from the reef front and typically deposited
20–30 m inshore from the algal rim. Under fairweather
conditions, these gravel bars rapidly migrate inshore and
are eventually left as marine cemented lenses of gravel
spread out over the reef flat $100 m from the outer rim;
these are known as conglomerate platforms (Hopley,
1982). Such immobile lenses commonly provide a locus
for subsequent sedimentation, and storms can add extensive shore-attached gravel ramparts for considerable distances along and between platforms (Baines et al.,
1974). However, the most volumetrically important reefflat deposits consist of thin but aerially extensive sheets
of finer-grained cobble to pebble gravels that can become
rapidly stabilized either in island settings by marine
vadose cements to form “cay rock,” or on open reef flats
themselves by crustose corallines (Newell and Bloom,
1970; Marshall and Jacobson, 1985). Successive deposition and cementation of gravel lenses and sheets can produce island platforms that reach 2 m or more above
present mean sea level. Dating of these platforms in several areas has yielded anomalously old ages and, as
a result, it has been claimed that they represent widespread
reef-flat development during a regionally higher midHolocene sea level (e.g., Dickinson, 2004) despite the fact
that clasts in these deposits have large age ranges (e.g.,
Scoffin et al., 1985).
Formation of reef flats
The paucity of active coral growth and the flatness and
low-tide elevation of many Indo-Pacific reef flats have
been a continued source of confusion and, as alluded to,
the age and origin of gravel deposits have provoked particular controversy (e.g., Newell and Bloom, 1970).
A persistent idea is that reef flats have been eroded to their
present low-tide level by a slight fall in regional sea level
over the last several 1,000 years (e.g., Cloud, 1952).
A variation of this has been advocated by Pirazzoli and
Montaggioni (1988) and Woodroffe (2008), who
suggested that the relative sea-level fall during past few
millennia has produced many emergent reef flats, which
are developed at elevations too high to be suitable for coral
growth. This implies that reef flats were constructed by
coral framework prior to the inferred sea-level fall; yet,
drill cores through reef flats consistently show that they
are not composed of thick framework but of detrital
deposits (Henny et al., 1974; Marshall and Jacobson,
1985). Hence, the present reef-flat surface is a reflection
of its interior, and an absence of corals and low-tide elevation does not require an erosional explanation. In addition,
elevation data used to infer higher sea level have not
accounted for elevated water levels on reef flats due to
wave pumping and ponding of water behind storm
ramparts, and elevations used from relict algal ridges are
GEOMORPHIC ZONATION
475