locations around the world (e.g., Grand Cayman Island –
Blanchon et al., 1997; Seychelles – Braithwaite et al.,
2000). The importance of storm detritus versus in situ reef
framework in fringing reef construction, and natural variability in both vertical and lateral rates of fringing reef
growth are raising challenging questions about measuring
and monitoring reef growth, destruction and condition to
establish baselines in the face of projected environmental
changes (e.g., Smithers et al., 2006).
Fringing reef morphology and processes
Where fringing reefs have formed reef flats, their surface
morphology is usually simple, comprising three main
zones – a forereef, a reef crest, and a backreef. Fringing
reef morphology can vary, however, over a single reef
and from reef to reef, with the greatest complexity usually
developed where the reef flat (see Reef Flats) includes sections formed during and after the mid-Holocene highstand
(see section “Fringing reef growth and sea level”). Hopley
and Barnes (1985) described seven geomorphological
zones (see Geomorphic Zonation) on the northern section
of the reef flat at Iris Point, Orpheus Island, GBR, the
higher inner part of which reached sea level in the
mid-Holocene (based on a 6,250 year old reef flat Favid).
Only five zones were discriminated on the younger (near
surface date: 3,780 years BP) southern section of the reef
flat (Figure 8). Although the descriptions below are
derived from a higher energy fringing reef on the GBR,
the morphological zones discussed below may to varying
degrees and combinations be found on many fringing
reefs worldwide.
High water shingle and debris
An irregular ridge of mainly Acropora shingle to 10 cm
length but also including other reefal debris, pumice and
flotsam is deposited at the highest level of contemporary
storm activity (see Shingle Ridges). This ridge can rise to
over a meter in height, and usually has a steep seaward
face. The lee side is often less abrupt, and can include
small fans or splays. In more protected locations this zone
may be replaced by a sandy beach, as occurs at nearby Pioneer Bay (Hopley et al., 1983).
Terrigenous boulder beach
Rhyolite and basalt boulders form a beach around
35 m wide that lies behind the reef flat. The lower boulder
beach is cemented but the upper beach, which extends to
the highest tidemark, is not. In situ corals indicative of
a higher ($1 m) sea level in the mid-Holocene are attached
to the boulder beach (Hopley and Barnes, 1985) in
a manner comparable to that recently described at Dunk
Island (Perry and Smithers, 2009). In other settings,
a sandy beach, possibly with beach rock exposed, may
replace the boulder beach (see Beach Rock).
Inner moat
A moated pool extends from the base of the boulder beach
seaward to the shingle rampart (see Moats; Moating). At
low tide water 0.3 m deep remains within the moated pool,
>1 m above the open water low tide level. Corals including Porites and Goniastrea microatolls survive in this
moat. A veneer of poorly sorted carbonate sediments
floors much of the moat, with abundant rhodoliths (see
Rhodoliths) accumulating at the landward edge. This zone
is typical of fringing reef flats partly formed during higher
mid-Holocene sea level, but can also occur where uplift
has occurred or high-energy storms are common.
Shingle rampart
A shingle rampart to 0.3 m high and around 10 m wide
was located at the seaward edge of the inner moat when
Hopley and Barnes (1985) mapped the reef flat, but
much of it has since been destroyed. This feature is formed
when rubble is thrown onto the reef flat by storms, and
similar features are common on other fringing reefs
exposed to these events. Subtle bassett edges are sporadically preserved that document the former extent and position of the rampart at Iris Point (see Bassett Edges; Shingle
Ridges).
Algal terraces
An algal pavement forms a relatively smooth feature that
slopes down from the higher inner reef flat and shingle
rampart to the lower outer living coral zone along most
of the northern part of the reef flat at Iris Point. Crustose
coralline algae (see Algae, Coralline) forms patches of this
pavement, but it is mostly composed of Acropora shingle
that has been stabilized with a filamentous turf algae (see
Algae, Turf ). Within this algal turf matting foraminiferans
such as Amphistegina, Calcarina, and Baculogypsina are
produced in great abundance (see Foraminifera). This feature is comparable with the algal-cemented pavements that
are common on fringing reef flat across the Indo-Pacific.
Northern fossil microatolls
A pavement of coalesced fossil microatolls, some
reaching 3 m in diameter, is located on the outer northern
part of the reef flat at Iris Point. The tops of these
microatolls are elevated $0.8 m above their modern open
water counterparts. There is no evidence that these
microatolls have been moated, and they are therefore presumed to have formed at a higher sea level.
Amorphous southern reef flat
This zone is developed where the reef achieves its greatest
width, and essentially consists of a relatively uniform,
gently sloping surface of coral rubble bound by pink crustose coralline algae on its outer half and filamentous
brown algae on the inner half. Radiocarbon dates indicate
that this section of the reef flat was constructed after the
mid-Holocene highstand. Hopley and Barnes (1985)
442
FRINGING REEFS
Blanchon et al., 1997; Seychelles – Braithwaite et al.,
2000). The importance of storm detritus versus in situ reef
framework in fringing reef construction, and natural variability in both vertical and lateral rates of fringing reef
growth are raising challenging questions about measuring
and monitoring reef growth, destruction and condition to
establish baselines in the face of projected environmental
changes (e.g., Smithers et al., 2006).
Fringing reef morphology and processes
Where fringing reefs have formed reef flats, their surface
morphology is usually simple, comprising three main
zones – a forereef, a reef crest, and a backreef. Fringing
reef morphology can vary, however, over a single reef
and from reef to reef, with the greatest complexity usually
developed where the reef flat (see Reef Flats) includes sections formed during and after the mid-Holocene highstand
(see section “Fringing reef growth and sea level”). Hopley
and Barnes (1985) described seven geomorphological
zones (see Geomorphic Zonation) on the northern section
of the reef flat at Iris Point, Orpheus Island, GBR, the
higher inner part of which reached sea level in the
mid-Holocene (based on a 6,250 year old reef flat Favid).
Only five zones were discriminated on the younger (near
surface date: 3,780 years BP) southern section of the reef
flat (Figure 8). Although the descriptions below are
derived from a higher energy fringing reef on the GBR,
the morphological zones discussed below may to varying
degrees and combinations be found on many fringing
reefs worldwide.
High water shingle and debris
An irregular ridge of mainly Acropora shingle to 10 cm
length but also including other reefal debris, pumice and
flotsam is deposited at the highest level of contemporary
storm activity (see Shingle Ridges). This ridge can rise to
over a meter in height, and usually has a steep seaward
face. The lee side is often less abrupt, and can include
small fans or splays. In more protected locations this zone
may be replaced by a sandy beach, as occurs at nearby Pioneer Bay (Hopley et al., 1983).
Terrigenous boulder beach
Rhyolite and basalt boulders form a beach around
35 m wide that lies behind the reef flat. The lower boulder
beach is cemented but the upper beach, which extends to
the highest tidemark, is not. In situ corals indicative of
a higher ($1 m) sea level in the mid-Holocene are attached
to the boulder beach (Hopley and Barnes, 1985) in
a manner comparable to that recently described at Dunk
Island (Perry and Smithers, 2009). In other settings,
a sandy beach, possibly with beach rock exposed, may
replace the boulder beach (see Beach Rock).
Inner moat
A moated pool extends from the base of the boulder beach
seaward to the shingle rampart (see Moats; Moating). At
low tide water 0.3 m deep remains within the moated pool,
>1 m above the open water low tide level. Corals including Porites and Goniastrea microatolls survive in this
moat. A veneer of poorly sorted carbonate sediments
floors much of the moat, with abundant rhodoliths (see
Rhodoliths) accumulating at the landward edge. This zone
is typical of fringing reef flats partly formed during higher
mid-Holocene sea level, but can also occur where uplift
has occurred or high-energy storms are common.
Shingle rampart
A shingle rampart to 0.3 m high and around 10 m wide
was located at the seaward edge of the inner moat when
Hopley and Barnes (1985) mapped the reef flat, but
much of it has since been destroyed. This feature is formed
when rubble is thrown onto the reef flat by storms, and
similar features are common on other fringing reefs
exposed to these events. Subtle bassett edges are sporadically preserved that document the former extent and position of the rampart at Iris Point (see Bassett Edges; Shingle
Ridges).
Algal terraces
An algal pavement forms a relatively smooth feature that
slopes down from the higher inner reef flat and shingle
rampart to the lower outer living coral zone along most
of the northern part of the reef flat at Iris Point. Crustose
coralline algae (see Algae, Coralline) forms patches of this
pavement, but it is mostly composed of Acropora shingle
that has been stabilized with a filamentous turf algae (see
Algae, Turf ). Within this algal turf matting foraminiferans
such as Amphistegina, Calcarina, and Baculogypsina are
produced in great abundance (see Foraminifera). This feature is comparable with the algal-cemented pavements that
are common on fringing reef flat across the Indo-Pacific.
Northern fossil microatolls
A pavement of coalesced fossil microatolls, some
reaching 3 m in diameter, is located on the outer northern
part of the reef flat at Iris Point. The tops of these
microatolls are elevated $0.8 m above their modern open
water counterparts. There is no evidence that these
microatolls have been moated, and they are therefore presumed to have formed at a higher sea level.
Amorphous southern reef flat
This zone is developed where the reef achieves its greatest
width, and essentially consists of a relatively uniform,
gently sloping surface of coral rubble bound by pink crustose coralline algae on its outer half and filamentous
brown algae on the inner half. Radiocarbon dates indicate
that this section of the reef flat was constructed after the
mid-Holocene highstand. Hopley and Barnes (1985)
442
FRINGING REEFS