orientation was more consistent with the varying destructive intensity of hurricane wave scour and approach angle
(Blanchon and Jones, 1997; Blanchon, 1997). In other
words, these shelf-edge reefs were produced by significant
reef accretion in deeper waters. Although no drilling has
yet proved this around Grand Cayman, cores from an
extensive shelf-edge reef off Carrie Bow Cay, on the
Belize Barrier reef, recovered an 11.6-m-thick section of
domal and branching framework that developed over the
last 5 ka (Macintyre et al., 1981). Similarly, 9 m of domal
framework that had developed in the last 7 ka was encountered on a shelf-edge reef off the south coast of Barbados
(Fairbanks, 1989). Off St. Croix, horizontal drilling into
the front wall of a shelf-edge reef buttress by Hubbard
et al. (1985) recovered 6 m of lateral accretion consisting
of alternating detritus and domal framework. These thicknesses of framework are clearly sufficient to account for
the amplitude of the shelf-edge reef buttresses and spurs
in these areas and therefore support the contention that
the shelf-edge reefs are independent structures controlled
by ambient environmental processes (Macintyre et al.,
1981; Blanchon and Jones, 1997).
Lower reef slope
The upper section of the reef slope to $60-m depth is
dominated by a cover of platy corals associated with
shelf-edge reef development; however, in the lower section of the slope below 60 m, this coral coverage declines
rapidly, and by 100 m, generally forms a cover of less than
1%. In areas investigated by submersibles, this lower
slope usually consists of a near-vertical wall with an irregular surface consisting of discontinuous, sediment-draped
ledges and blind caves dissected by vertical chutes and
gullies (Ginsburg et al., 1991; Grammer and Ginsburg,
1992). This irregular substrate is colonized by a
sciaphyllic assemblage of sclerosponges, crustose coralline algae, microbialites, and platy corals, and their
zonation and distribution are controlled by the light
gradient, falling sediment from the shelf above, and
the margin orientation (Goreau and Land, 1974; Land
and Moore, 1977; Sheppard, 1982; Colin et al., 1986;
Liddell and Ohlhorst, 1988; Ginsburg et al., 1991;
Lesser et al., 2009). By far, the most thoroughly investigated lower reef slope is that along the north coast of
Jamaica (Hartman and Goreau, 1970; Goreau and Land,
1974; Lang et al., 1975; Land and Moore, 1977; Liddel
and Ohlhorst, 1988). In this area, the depth zone
between 60 and 100 m consists of a variable cover of
sclerosponges, with reports ranging from 7% to 25%
and as much as 50% on the walls and ceilings of caves.
In fact, in a cluster of caves between 80 and 90 m,
Lang et al. (1975) reported sclerosponges as large as
1 m in diameter. Furthermore, a half-meter-deep blast
site at 105 m exposed the sclerosponge framework with
interstitial skeletal sediment cemented by highmagnesian calcite (Land and Moore, 1977). Similar
abundances of sclerosponges have also been reported
from the Bahamas, where Ginsburg et al. (1991) estimated a cover of 5–25% in the same depth range.
Formation of the reef slope
There is generally a very little subsurface data available
concerning the development of reef-slope environments.
The tendency of modern reefal margins to form steep precipitous slopes must relate to both accretion and erosion
during rapid sea-level changes during glacial–interglacial
cycles. For example, Blanchon and Jones (1997)
suggested that the pattern of vertical and lateral accretion
in shelf-edge reefs may explain the steep precipitous upper
slope development that is common in many reefal margins. However, the vertical walls of the deeper lower slope
must be related to the long phases of sea-level lowstands at
125 m or more that are associated with glacial stages
(Land and Moore, 1977). Although these cliffs are presently covered by coral and sclerosponge frameworks and
are likely undergoing some small amount of lateral accretion (e.g., Ginsburg et al., 1991), cliff recession rates during the extended duration of glacial lowstands must have
been substantial. Therefore, the basic geomorphology of
these vertical substrates is likely erosional – a claim that
is supported by widespread reports of giant blocks of wall
rock on forereef depositional slopes.
Summary and conclusions
Rough-water coral reefs that develop in trade-wind-,
swell-, and storm-dominated seas consist of five standard
geomorphic zones: a back-reef lagoon, a reef flat, a reef
front, a reef slope, and a fore-reef talus apron. These zones
are separated by simple slope breaks: back-reef lagoons
terminate at the sandslope break, reef flats terminate at
the reef-crest break, reef fronts terminate at the shelf-edge
break, and reef slopes terminate at the fore-reef talus
break.
In lagoons, reef development can be extensive and reefs
develop as dispersed knolls or patches that commonly
merge to form ridges; these in turn can connect with other
ridges to form complex reticulate networks that impede
water circulation and self-limit their own development.
In many lagoons, water circulation is a function of wind
stress and exchange with the open ocean, with the latter
process being controlled by a wave-pumping mechanism
over open reef flats. Atolls with large sections of open reef
flats therefore have better lagoonal reef development than
smaller atolls, where the exchange system is more vulnerable to blockage by island formation in space and time.
Seismic data show that the lagoonal reef form and development is not, as often claimed, inherited from a karstic
substrate but entirely growth induced either from simple
vertical and lateral accretion during the Holocene or from
compound growth over previous interglacials.
Reef flats by contrast are remarkably flat intertidal platforms that can be several kilometers wide. They consist of
an algal rim, an outer coralgal flat, an inner sand flat, and
a variable cover of low-lying detrital islands. The algal
GEOMORPHIC ZONATION
481
(Blanchon and Jones, 1997; Blanchon, 1997). In other
words, these shelf-edge reefs were produced by significant
reef accretion in deeper waters. Although no drilling has
yet proved this around Grand Cayman, cores from an
extensive shelf-edge reef off Carrie Bow Cay, on the
Belize Barrier reef, recovered an 11.6-m-thick section of
domal and branching framework that developed over the
last 5 ka (Macintyre et al., 1981). Similarly, 9 m of domal
framework that had developed in the last 7 ka was encountered on a shelf-edge reef off the south coast of Barbados
(Fairbanks, 1989). Off St. Croix, horizontal drilling into
the front wall of a shelf-edge reef buttress by Hubbard
et al. (1985) recovered 6 m of lateral accretion consisting
of alternating detritus and domal framework. These thicknesses of framework are clearly sufficient to account for
the amplitude of the shelf-edge reef buttresses and spurs
in these areas and therefore support the contention that
the shelf-edge reefs are independent structures controlled
by ambient environmental processes (Macintyre et al.,
1981; Blanchon and Jones, 1997).
Lower reef slope
The upper section of the reef slope to $60-m depth is
dominated by a cover of platy corals associated with
shelf-edge reef development; however, in the lower section of the slope below 60 m, this coral coverage declines
rapidly, and by 100 m, generally forms a cover of less than
1%. In areas investigated by submersibles, this lower
slope usually consists of a near-vertical wall with an irregular surface consisting of discontinuous, sediment-draped
ledges and blind caves dissected by vertical chutes and
gullies (Ginsburg et al., 1991; Grammer and Ginsburg,
1992). This irregular substrate is colonized by a
sciaphyllic assemblage of sclerosponges, crustose coralline algae, microbialites, and platy corals, and their
zonation and distribution are controlled by the light
gradient, falling sediment from the shelf above, and
the margin orientation (Goreau and Land, 1974; Land
and Moore, 1977; Sheppard, 1982; Colin et al., 1986;
Liddell and Ohlhorst, 1988; Ginsburg et al., 1991;
Lesser et al., 2009). By far, the most thoroughly investigated lower reef slope is that along the north coast of
Jamaica (Hartman and Goreau, 1970; Goreau and Land,
1974; Lang et al., 1975; Land and Moore, 1977; Liddel
and Ohlhorst, 1988). In this area, the depth zone
between 60 and 100 m consists of a variable cover of
sclerosponges, with reports ranging from 7% to 25%
and as much as 50% on the walls and ceilings of caves.
In fact, in a cluster of caves between 80 and 90 m,
Lang et al. (1975) reported sclerosponges as large as
1 m in diameter. Furthermore, a half-meter-deep blast
site at 105 m exposed the sclerosponge framework with
interstitial skeletal sediment cemented by highmagnesian calcite (Land and Moore, 1977). Similar
abundances of sclerosponges have also been reported
from the Bahamas, where Ginsburg et al. (1991) estimated a cover of 5–25% in the same depth range.
Formation of the reef slope
There is generally a very little subsurface data available
concerning the development of reef-slope environments.
The tendency of modern reefal margins to form steep precipitous slopes must relate to both accretion and erosion
during rapid sea-level changes during glacial–interglacial
cycles. For example, Blanchon and Jones (1997)
suggested that the pattern of vertical and lateral accretion
in shelf-edge reefs may explain the steep precipitous upper
slope development that is common in many reefal margins. However, the vertical walls of the deeper lower slope
must be related to the long phases of sea-level lowstands at
125 m or more that are associated with glacial stages
(Land and Moore, 1977). Although these cliffs are presently covered by coral and sclerosponge frameworks and
are likely undergoing some small amount of lateral accretion (e.g., Ginsburg et al., 1991), cliff recession rates during the extended duration of glacial lowstands must have
been substantial. Therefore, the basic geomorphology of
these vertical substrates is likely erosional – a claim that
is supported by widespread reports of giant blocks of wall
rock on forereef depositional slopes.
Summary and conclusions
Rough-water coral reefs that develop in trade-wind-,
swell-, and storm-dominated seas consist of five standard
geomorphic zones: a back-reef lagoon, a reef flat, a reef
front, a reef slope, and a fore-reef talus apron. These zones
are separated by simple slope breaks: back-reef lagoons
terminate at the sandslope break, reef flats terminate at
the reef-crest break, reef fronts terminate at the shelf-edge
break, and reef slopes terminate at the fore-reef talus
break.
In lagoons, reef development can be extensive and reefs
develop as dispersed knolls or patches that commonly
merge to form ridges; these in turn can connect with other
ridges to form complex reticulate networks that impede
water circulation and self-limit their own development.
In many lagoons, water circulation is a function of wind
stress and exchange with the open ocean, with the latter
process being controlled by a wave-pumping mechanism
over open reef flats. Atolls with large sections of open reef
flats therefore have better lagoonal reef development than
smaller atolls, where the exchange system is more vulnerable to blockage by island formation in space and time.
Seismic data show that the lagoonal reef form and development is not, as often claimed, inherited from a karstic
substrate but entirely growth induced either from simple
vertical and lateral accretion during the Holocene or from
compound growth over previous interglacials.
Reef flats by contrast are remarkably flat intertidal platforms that can be several kilometers wide. They consist of
an algal rim, an outer coralgal flat, an inner sand flat, and
a variable cover of low-lying detrital islands. The algal
GEOMORPHIC ZONATION
481