commonly no higher than their modern counterparts. And
even where they are, possible changes in wave climate in
controlling their elevation rather than sea level are not
taken into account. Moreover, in all of the reef-flat studies
claiming regional sea-level highstands, the error range of
the sea-level indicator is the same magnitude as the sealevel change itself (Woodroffe and Horton, 2005). Thus,
claims that modern reef flats are somehow palimpsest
and have been modified by falling sea level lack adequate
support.
Other analyses of reef-flat form and zonation have
assumed that flats did not develop until reefs caught up
with sea level, despite the fact that the evidence for reef
catch-up in most areas is equivocal due to inadequate core
coverage (Blanchon and Blakeway, 2003). For example,
Hopley (1982) proposed a retrogressive model of reef-flat
development on the Great Barrier Reef in which presumed
catch-up reefs progressively developed a zoned reef flat
through time as reef framework and sediment migrated
into the lagoon. A prediction of this hypothesis is that
deposits should get younger toward the lagoon; yet, drill
cores consistently show the opposite, with reef-flat ages
increasing toward the lagoon in almost all the areas studied in sufficient detail (Easton and Olson, 1976; Pirazzoli
et al., 1987; Randall and Siegrist, 1988; Takahashi et al.,
1988; Kan et al., 1997a; Scoffin and Le Tissier, 1988;
Woodroffe et al., 2000).
This widely documented pattern of inshore reef-flat
ageing is more consistent with progradation, whereby
the algal rim builds seaward out over the reef front through
time (Ladd et al., 1950; Pirazzoli et al., 1987). Indeed,
such a pattern would not only explain why reef flats are
flat, with no space for framework accretion, but it would
also account for many of their morphological features.
For example, the widespread occurrence of back-rim
troughs and surge channels extending 100 m or more into
the reef flat interior is a key indication of this process
(Figure 4). Imagery from the Marshall Islands in particular
indicate that back-rim troughs represent shallow spur and
groove that were abandoned due to algal-rim development
along their leading edges (see Bikini and Rongelap, in
Figure 4). The jump in the rim position to the spur front
consequently left a shallow trough between the old and
new rim that provided space for coral growth. In many
areas, coral growth and gravel deposition rapidly filled
this trough and only surge channels remain as evidence
of its existence. In this way, reef flats can prograde rapidly
and produce an extensive flat surface devoid of coral
growth that is punctuated only by abandoned algal rims
and boulder thrown up during storms. Why algal rims
are periodically abandoned and jump seaward is related
to the dynamics of spur-and-groove development, which
is outlined below.
Reef front
The reef front zone extends from the reef crest to the shelf
break which, in many areas, occurs between depths of
20 and 50 m. This shallow zone is perhaps the most distinctive of all reef environments and consists of narrow
annular subzones, which show remarkable concordance
between regions. As shown in Figure 1, reef fronts on
swell- and trade-dominated reefs generally consist of three
common subzones: a shallow spur-and-groove zone,
which extends down to a mid-shelf slope break around
10 m, followed by a relatively low-gradient sand terrace
at 15–25 m, which passes into a chute and buttress zone
developed over the shelf break and down the reef slope.
In all of these subzones, reef development is molded by
wave scour during both fairweather and storms and forms
a characteristic cross-shelf sequence of elongate spur systems. In shallow water, scour is also responsible for sculpturing exposed bedrock surfaces into ridges and furrows,
with ridges becoming enhanced by coral colonization
(Allen, 1982; Blanchon and Jones, 1995). Unfortunately,
due to the rigors of collecting data in such wave-swept
areas, reef development in these subzones is poorly known
and core data exists only for the greater Caribbean with no
reports for any Indo-Pacific reef fronts.
Spur and groove
Spur and groove is a general term for regularly spaced
arrays of buttresses and channels, which extend across
the shelf (Figure 5). Two types of shallow spur and groove
are common: one dominated by coralline algae and the
other by corals. These systems are not mutually exclusive
however and their distribution and form is controlled by
wave energy (Munk and Sargent, 1954; Roberts, 1974;
Sheppard, 1982; Blanchon and Jones, 1997; Storlazzi
et al., 2003). Both coral and algal spurs can be present
along the same high-energy swell-dominated reef front:
a shallow high-energy surf-zone system, dominated by
crustose coralline algae (see Ujelang and Oeno,
Figure 5), and a lower-energy coral-dominated system that
either develops downslope in slightly deeper waters (see
Nonouti and Diego-Garcia, in Figure 5) or in an equivalent
position along the leeward margin (Sheppard, 1981;
Glynn, 1996). Furthermore, the frequency of spurs shows
a correlation with water depth (and hence wave energy):
surf-zone systems have a spacing of one spur every
6–10 m, whereas in deeper-water systems, such as the
chute and buttress zone (see below), spurs are spaced
every 30 m or more, largely as a result of greater spur
widths and wavelengths (Blanchon and Jones, 1997;
Storlazzi et al., 2003). Surf-zone spur systems extend
seaward from the reef crest for 50–100 m dependent upon
the shelf slope, whereas deeper water spur systems can be
more extensive, and submerged terraces can be covered by
coral spurs for several kilometers.
In high-energy swell-affected reefs, shallow spur-andgroove systems are dominated by coralline algae and
extend seaward from algal rims parallel to wave orthogonals (Munk and Sargent, 1954; Sargent and Austin,
1954). Spurs are commonly 4–8-m wide and have steep,
sometimes overhanging, sides; intervening grooves are
476
GEOMORPHIC ZONATION
even where they are, possible changes in wave climate in
controlling their elevation rather than sea level are not
taken into account. Moreover, in all of the reef-flat studies
claiming regional sea-level highstands, the error range of
the sea-level indicator is the same magnitude as the sealevel change itself (Woodroffe and Horton, 2005). Thus,
claims that modern reef flats are somehow palimpsest
and have been modified by falling sea level lack adequate
support.
Other analyses of reef-flat form and zonation have
assumed that flats did not develop until reefs caught up
with sea level, despite the fact that the evidence for reef
catch-up in most areas is equivocal due to inadequate core
coverage (Blanchon and Blakeway, 2003). For example,
Hopley (1982) proposed a retrogressive model of reef-flat
development on the Great Barrier Reef in which presumed
catch-up reefs progressively developed a zoned reef flat
through time as reef framework and sediment migrated
into the lagoon. A prediction of this hypothesis is that
deposits should get younger toward the lagoon; yet, drill
cores consistently show the opposite, with reef-flat ages
increasing toward the lagoon in almost all the areas studied in sufficient detail (Easton and Olson, 1976; Pirazzoli
et al., 1987; Randall and Siegrist, 1988; Takahashi et al.,
1988; Kan et al., 1997a; Scoffin and Le Tissier, 1988;
Woodroffe et al., 2000).
This widely documented pattern of inshore reef-flat
ageing is more consistent with progradation, whereby
the algal rim builds seaward out over the reef front through
time (Ladd et al., 1950; Pirazzoli et al., 1987). Indeed,
such a pattern would not only explain why reef flats are
flat, with no space for framework accretion, but it would
also account for many of their morphological features.
For example, the widespread occurrence of back-rim
troughs and surge channels extending 100 m or more into
the reef flat interior is a key indication of this process
(Figure 4). Imagery from the Marshall Islands in particular
indicate that back-rim troughs represent shallow spur and
groove that were abandoned due to algal-rim development
along their leading edges (see Bikini and Rongelap, in
Figure 4). The jump in the rim position to the spur front
consequently left a shallow trough between the old and
new rim that provided space for coral growth. In many
areas, coral growth and gravel deposition rapidly filled
this trough and only surge channels remain as evidence
of its existence. In this way, reef flats can prograde rapidly
and produce an extensive flat surface devoid of coral
growth that is punctuated only by abandoned algal rims
and boulder thrown up during storms. Why algal rims
are periodically abandoned and jump seaward is related
to the dynamics of spur-and-groove development, which
is outlined below.
Reef front
The reef front zone extends from the reef crest to the shelf
break which, in many areas, occurs between depths of
20 and 50 m. This shallow zone is perhaps the most distinctive of all reef environments and consists of narrow
annular subzones, which show remarkable concordance
between regions. As shown in Figure 1, reef fronts on
swell- and trade-dominated reefs generally consist of three
common subzones: a shallow spur-and-groove zone,
which extends down to a mid-shelf slope break around
10 m, followed by a relatively low-gradient sand terrace
at 15–25 m, which passes into a chute and buttress zone
developed over the shelf break and down the reef slope.
In all of these subzones, reef development is molded by
wave scour during both fairweather and storms and forms
a characteristic cross-shelf sequence of elongate spur systems. In shallow water, scour is also responsible for sculpturing exposed bedrock surfaces into ridges and furrows,
with ridges becoming enhanced by coral colonization
(Allen, 1982; Blanchon and Jones, 1995). Unfortunately,
due to the rigors of collecting data in such wave-swept
areas, reef development in these subzones is poorly known
and core data exists only for the greater Caribbean with no
reports for any Indo-Pacific reef fronts.
Spur and groove
Spur and groove is a general term for regularly spaced
arrays of buttresses and channels, which extend across
the shelf (Figure 5). Two types of shallow spur and groove
are common: one dominated by coralline algae and the
other by corals. These systems are not mutually exclusive
however and their distribution and form is controlled by
wave energy (Munk and Sargent, 1954; Roberts, 1974;
Sheppard, 1982; Blanchon and Jones, 1997; Storlazzi
et al., 2003). Both coral and algal spurs can be present
along the same high-energy swell-dominated reef front:
a shallow high-energy surf-zone system, dominated by
crustose coralline algae (see Ujelang and Oeno,
Figure 5), and a lower-energy coral-dominated system that
either develops downslope in slightly deeper waters (see
Nonouti and Diego-Garcia, in Figure 5) or in an equivalent
position along the leeward margin (Sheppard, 1981;
Glynn, 1996). Furthermore, the frequency of spurs shows
a correlation with water depth (and hence wave energy):
surf-zone systems have a spacing of one spur every
6–10 m, whereas in deeper-water systems, such as the
chute and buttress zone (see below), spurs are spaced
every 30 m or more, largely as a result of greater spur
widths and wavelengths (Blanchon and Jones, 1997;
Storlazzi et al., 2003). Surf-zone spur systems extend
seaward from the reef crest for 50–100 m dependent upon
the shelf slope, whereas deeper water spur systems can be
more extensive, and submerged terraces can be covered by
coral spurs for several kilometers.
In high-energy swell-affected reefs, shallow spur-andgroove systems are dominated by coralline algae and
extend seaward from algal rims parallel to wave orthogonals (Munk and Sargent, 1954; Sargent and Austin,
1954). Spurs are commonly 4–8-m wide and have steep,
sometimes overhanging, sides; intervening grooves are
476
GEOMORPHIC ZONATION