rim, generated by the surf-adapted coralline Porolithon
onkoides, forms the highest part of the reef flat and consists of either an elevated ridge fronting islands along the
most exposed reef sections or a lower mound along more
open flats. Behind the rim is an intertidal coralgal platform, veneered by a rapidly decreasing cover of corals
and an increasing proportion of coral gravel stabilized by
coralline algae. In its outer part, this platform can be punctuated by one or multiple remnants of former algal ridges
whose age increases inshore or by large blocks of reef
framework thrown up from the reef front during storms.
In addition, the coralgal flat is commonly dissected by
surge channels, which extend back into the reef flat in-line
with reef-front grooves. Some flats are crossed by corals
organized into narrow, parallel ridges that are separated
by gravel-filled furrows, which also align with reef-front
grooves. These various types of coralgal flats either grade
into an inner flat dominated by shifting sands or pass
directly into cobble rampart-fronted islands composed of
a mix of storm-lain sand and gravels that can reach
4 m or more above the surrounding flat. Although stabilized by marine and meteoric cements, these gravel islands
yield clasts with a large spread of ages, suggesting that
they are commonly reworked by storms.
In explanations of reef-flat development, it has been
argued that elevated islands, remnant algal ridges, and
the extremely flat, barren nature of reef flats are a combined result of a late Holocene regional fall in sea level,
which led to erosional planation of the surface to its present level. Several authors, however, have noted that these
same features are consistent with processes that are presently active with a stable sea level. In fact, many of the
key features of reef flats, such as their monotonous flatness, aligned features, and remnant ridges, can be
accounted for by the simple seaward progradation of the
algal rim out over the reef-front spur-and-groove zone
through time.
Spur-and-groove development in shallow, rough-water
reefs consists of two types: one with spurs covered by
crustose coralline algae and the other with spurs covered
by corals. On swell-dominated reefs, algal spurs extend
seaward from the algal rim parallel to wave orthogonals
and form high-frequency, steep-sided spur arrays, whose
length is proportional to wave energy and the gradient of
the seaward shelf. Coral spurs have a similar form and frequency but develop in lower wave-energy regimes either
downslope from algal spurs or in an equivalent position
along the leeward margin. All subsurface evidence collected so far point to a growth origin for both spur types,
with algal spurs nucleating on downslope coral growth
causing a seaward progradation through time, and coral
spurs developing downslope from the crest and
retrograding upslope through time.
The platform downslope of the shallow spur-andgroove terminates at about À10 m in a mid-shelf slope
break that commonly takes the form of a small notched
scarp (where it has not been covered by coral spurs or
sedimentation). This scarp forms the riser to a distinct
sand-covered terrace, known as the 10-fathom terrace,
which slopes from 20 to 30 m out to the shelf edge. This
10-fathom terrace and its scarp has been reported from
all oceans and even around uplifting islands such as
Barbados, and has become a depocenter for sediment
accumulation and reef development during the Holocene.
Discoveries of several early-Holocene breakwater reefs on
the terrace prove that the notched scarp and terrace couplet
represents a cliffed shoreline that formed around the same
time and was drowned by a meter-scale sea-level jump
$8 ka ago.
The shelf break at the terrace edge is commonly a site of
extensive submerged reef development, which has produced an array of sand chutes and steep-sided buttresses
that form parallel to wave orthogonals. The buttresses,
which can rise to 15 m and have amplitudes of 10 m or
more, extend back over the terrace as a series of tapering
spurs. To seaward, the subvertical face of the buttresses
extends down the reef slope to $60 m and has a dense
cover of platy corals. Covariance between buttress area,
amplitude, spur frequency, and local margin orientation
indicates that the architecture of these shelf-edge reefs is
controlled by hurricane wave scour and approach angle.
Other suggestions that they developed over drowned
breakwater reefs are not supported by drilling, which
shows that shelf-edge reefs have produced greater than
12 m of vertical growth and 6 m of lateral accretion in
the last 5 ka.
The reef slope beneath shelf edge reefs is commonly
vertical below 60 m and is covered by an assemblage of
sciaphyllic sclerosponges, crustose corallines, and
microbialites. Although this assemblage is actively producing framework, the morphology of the deep reef slope
has been significantly influenced by cliff collapse and
mass wasting during the long intervals of glacial
lowstands of sea level.
The remarkable similarity of reef geomorphology and
architecture between and within oceans is a clear testament to their capacity for self-organization (Hatcher,
1997; Dizon and Yap, 2006). In other words, many of
the features of these large-scale structures have emerged
spontaneously from ecological processes operating at
smaller scales. For example, the negative feedback
between the simple vertical and lateral reef accretion and
water circulation has produced extensive networks of cellular reefs in many lagoonal systems. Likewise, the simple
interaction between wave scour and the colonization and
growth of corals and crustose coralline algae has produced
well-ordered spur systems with similar spacings at both
local (windward–leeward), regional (trade or swell) and
inter-oceanic scales. Finally, the interaction between herbivory and coralline accretion along the seaward edge of
algal spurs may be responsible for intermittent seaward
advances of reef flats through time.
Perversely, many of those responsible for interpreting
large-scale reef development still cling to the paradigm
that modern reefs are somewhat of a geological oddity
and that their form and architecture are merely
482
GEOMORPHIC ZONATION
onkoides, forms the highest part of the reef flat and consists of either an elevated ridge fronting islands along the
most exposed reef sections or a lower mound along more
open flats. Behind the rim is an intertidal coralgal platform, veneered by a rapidly decreasing cover of corals
and an increasing proportion of coral gravel stabilized by
coralline algae. In its outer part, this platform can be punctuated by one or multiple remnants of former algal ridges
whose age increases inshore or by large blocks of reef
framework thrown up from the reef front during storms.
In addition, the coralgal flat is commonly dissected by
surge channels, which extend back into the reef flat in-line
with reef-front grooves. Some flats are crossed by corals
organized into narrow, parallel ridges that are separated
by gravel-filled furrows, which also align with reef-front
grooves. These various types of coralgal flats either grade
into an inner flat dominated by shifting sands or pass
directly into cobble rampart-fronted islands composed of
a mix of storm-lain sand and gravels that can reach
4 m or more above the surrounding flat. Although stabilized by marine and meteoric cements, these gravel islands
yield clasts with a large spread of ages, suggesting that
they are commonly reworked by storms.
In explanations of reef-flat development, it has been
argued that elevated islands, remnant algal ridges, and
the extremely flat, barren nature of reef flats are a combined result of a late Holocene regional fall in sea level,
which led to erosional planation of the surface to its present level. Several authors, however, have noted that these
same features are consistent with processes that are presently active with a stable sea level. In fact, many of the
key features of reef flats, such as their monotonous flatness, aligned features, and remnant ridges, can be
accounted for by the simple seaward progradation of the
algal rim out over the reef-front spur-and-groove zone
through time.
Spur-and-groove development in shallow, rough-water
reefs consists of two types: one with spurs covered by
crustose coralline algae and the other with spurs covered
by corals. On swell-dominated reefs, algal spurs extend
seaward from the algal rim parallel to wave orthogonals
and form high-frequency, steep-sided spur arrays, whose
length is proportional to wave energy and the gradient of
the seaward shelf. Coral spurs have a similar form and frequency but develop in lower wave-energy regimes either
downslope from algal spurs or in an equivalent position
along the leeward margin. All subsurface evidence collected so far point to a growth origin for both spur types,
with algal spurs nucleating on downslope coral growth
causing a seaward progradation through time, and coral
spurs developing downslope from the crest and
retrograding upslope through time.
The platform downslope of the shallow spur-andgroove terminates at about À10 m in a mid-shelf slope
break that commonly takes the form of a small notched
scarp (where it has not been covered by coral spurs or
sedimentation). This scarp forms the riser to a distinct
sand-covered terrace, known as the 10-fathom terrace,
which slopes from 20 to 30 m out to the shelf edge. This
10-fathom terrace and its scarp has been reported from
all oceans and even around uplifting islands such as
Barbados, and has become a depocenter for sediment
accumulation and reef development during the Holocene.
Discoveries of several early-Holocene breakwater reefs on
the terrace prove that the notched scarp and terrace couplet
represents a cliffed shoreline that formed around the same
time and was drowned by a meter-scale sea-level jump
$8 ka ago.
The shelf break at the terrace edge is commonly a site of
extensive submerged reef development, which has produced an array of sand chutes and steep-sided buttresses
that form parallel to wave orthogonals. The buttresses,
which can rise to 15 m and have amplitudes of 10 m or
more, extend back over the terrace as a series of tapering
spurs. To seaward, the subvertical face of the buttresses
extends down the reef slope to $60 m and has a dense
cover of platy corals. Covariance between buttress area,
amplitude, spur frequency, and local margin orientation
indicates that the architecture of these shelf-edge reefs is
controlled by hurricane wave scour and approach angle.
Other suggestions that they developed over drowned
breakwater reefs are not supported by drilling, which
shows that shelf-edge reefs have produced greater than
12 m of vertical growth and 6 m of lateral accretion in
the last 5 ka.
The reef slope beneath shelf edge reefs is commonly
vertical below 60 m and is covered by an assemblage of
sciaphyllic sclerosponges, crustose corallines, and
microbialites. Although this assemblage is actively producing framework, the morphology of the deep reef slope
has been significantly influenced by cliff collapse and
mass wasting during the long intervals of glacial
lowstands of sea level.
The remarkable similarity of reef geomorphology and
architecture between and within oceans is a clear testament to their capacity for self-organization (Hatcher,
1997; Dizon and Yap, 2006). In other words, many of
the features of these large-scale structures have emerged
spontaneously from ecological processes operating at
smaller scales. For example, the negative feedback
between the simple vertical and lateral reef accretion and
water circulation has produced extensive networks of cellular reefs in many lagoonal systems. Likewise, the simple
interaction between wave scour and the colonization and
growth of corals and crustose coralline algae has produced
well-ordered spur systems with similar spacings at both
local (windward–leeward), regional (trade or swell) and
inter-oceanic scales. Finally, the interaction between herbivory and coralline accretion along the seaward edge of
algal spurs may be responsible for intermittent seaward
advances of reef flats through time.
Perversely, many of those responsible for interpreting
large-scale reef development still cling to the paradigm
that modern reefs are somewhat of a geological oddity
and that their form and architecture are merely
482
GEOMORPHIC ZONATION