fringing reef structure – Easton and Olson’s (1976) study
of Hanauma Bay, Oahu, Hawaii (reef flat 90 m wide,
10 cores, 63 radiocarbon dates), and Macintyre and
Glynn’s (1976) study at Galeta Point, Panama (reef flat
200 m wide, 13 cores, 32 radiocarbon dates). At both reefs
initiation occurred around 7,000 years ago – at Hanauma
Bay over volcanic tuffs and basaltic sands, at Galeta Point
over Miocene calcareous siltstones – and was dominated
by massive corals. At both reefs, between 6,000 and 3,000
years ago, vertical accretion by rapidly growing coral
frameworks took place, at Galeta Point dominated by
Acropora palmata, which quickly masked any influence
of the underlying topography (vertical accretion rates of
between 1.3 and 10.8 mm year
À1 occurred during this
phase). At Hanauma Bay, vertical accretion rates during this
main phase of reef growth were slower, averaging around
2.9 mm year
À1
. After about 3,500 years BP at Hanauma
Bay and 2,000 years BP at Galeta Point negligible vertical
accretion occurred – with the reef at the former dominated
by the coralline algae Porolithon and only scattered coral
colonies, and the reef flat at Galeta Point now dominated
by sea grass and Halimeda meadows. Grigg (1998) reports
that the Hanauma Bay reef continues to prograde seaward at
around 22 mm year
À1
. In contrast, despite its impressive
growth between 6,000 and 3,000 years ago, Galeta Point
fringing reef is now effectively senescent.
The similarity of reef growth evident at Hanauma Bay
and Galeta Point is perhaps surprising given that they
experienced different sea-level histories (see section
“Fringing reef growth and sea level”). The dominance of
A. palmata in the framework during the main phase of reef
growth was interpreted by Macintyre and Glynn (1976) as
evidence that Galeta Point was a keep-up reef able to keep
pace with a sea level that rose at a decelerating rate to its
present position. Easton and Olson (1976) also concluded
that Hanauma Bay was a keep-up reef, but this interpretation has been challenged because they did not recognize
evidence of higher sea level during the mid-Holocene
(Grossman et al., 1998), and it is now generally agreed
to be a catch-up rather than keep-up reef (Grigg, 1998).
On the GBR, Hayman Island is the only fringing reef
for which data are available that loosely follows this
model (Hopley et al., 1978). At Hayman Island the fringing reef developed over a gently sloping substrate of
last interglacial limestone, now more than 1 km from the
contemporary shoreline at a depth between À15 and
À20 m, very soon after it was flooded by the transgression
(the earliest dated coral above the substrate is 9,320 years
BP). The interpretation is that original reef was outflanked
by shoreline retreat and shoreline backstepping during the
transgression, stranding it offshore as a catch-up reef.
Once sea level stabilized in the mid-Holocene the reef
caught up, and back-reef infilling and seaward
progradation from the new shoreline combined to construct the reef flat. In this regard Hayman Island reef flat
may also have some affinity to the reef crest and backfill
model depicted in Figure 5e described further below
(section “Reef crest and backfill”).
Progradation out from shore over rocky substrate
Fringing reefs complying with this growth model initiate
at the shoreline where they grow up to sea level and then
prograde seaward (Figure 5b). This model of fringing reef
development is most like the simple model conceived by
Darwin (Figure 1a). However, detailed studies demonstrating that any particular reef has grown this way – over
a hard bedrock shoreline are rare – Rees et al. (2006)
inferred that the windward fringing reef at Lizard Island
on the northern GBR grew directly over a granitic bedrock
foundation and has prograded seaward, but from the three
cores presented it is difficult to confidently determine the
broader growth pattern. In Torres Strait, drilling reveals
that reef initiation occurred close to shore at both
Hammond and Yam Islands (Woodroffe et al., 2000), the
former over a Pleistocene reef limestone at around
7,000 years ago and around 1,000 years later at Hammond
Island, over a granite substrate but with significant
mud deposition occurring during reef growth.
The northern section of the fringing reef at Iris Point,
Orpheus Island, also on the GBR, rises over a rocky but
unconsolidated foundation; drilling suggests the reef
grows over a boulder beach (see Boulder Beach) comprised of corestones winnowed from the regolith. These
boulders vary from 0.4 m to several meters a-axis length,
and would be stable under normal conditions. At Iris Point
reef initiation took place around 7,000 years ago and vertically accreted rapidly (4 mm year
À1
) to be constrained by
sea level by 6,250 years ago, as indicated fossil microatoll
ages (Hopley and Barnes, 1985) (see Microatoll). By
5,500 years BP the reef flat had begun to prograde seaward, and because sea levels were falling post highstand
(Chappell et al., 1983), the reef flat was constrained at
a lower level than the older reef flat to landward. Rates
of seaward progradation at northern Iris Point have been
estimated at between 50 and 500 mm year
À1
. Significantly, another transect at Iris Point located around
1,000 m to the south shows a different (younger) age structure and internal fabric, demonstrating the fact that
a particular grow mode may be very localized, even within
the same reef. The southern part of the Iris Point fringing
reef has grown over detrital material shed from the northern section and distributed alongshore, and in contrast to
the older section which is composed of 40–60% framework, the southern reef contains less than 20% in situ
framework (Hopley and Barnes, 1985).
Although detailed chronostratigraphic studies of fringing reefs with this growth strategy are rare, there are many
narrow fringing reefs attached to rocky headlands that
appear to have formed in this way, at least at some stage
of their evolution.
Progradation over terrigenous sediments
This model of fringing reef growth is characterized by reefs
developed over sedimentary structures including Pleistocene alluvial fans (e.g., Paluma Shoals, GBR [Smithers
and Larcombe, 2003]), transgressionary sediment deposits
438
FRINGING REEFS
of Hanauma Bay, Oahu, Hawaii (reef flat 90 m wide,
10 cores, 63 radiocarbon dates), and Macintyre and
Glynn’s (1976) study at Galeta Point, Panama (reef flat
200 m wide, 13 cores, 32 radiocarbon dates). At both reefs
initiation occurred around 7,000 years ago – at Hanauma
Bay over volcanic tuffs and basaltic sands, at Galeta Point
over Miocene calcareous siltstones – and was dominated
by massive corals. At both reefs, between 6,000 and 3,000
years ago, vertical accretion by rapidly growing coral
frameworks took place, at Galeta Point dominated by
Acropora palmata, which quickly masked any influence
of the underlying topography (vertical accretion rates of
between 1.3 and 10.8 mm year
À1 occurred during this
phase). At Hanauma Bay, vertical accretion rates during this
main phase of reef growth were slower, averaging around
2.9 mm year
À1
. After about 3,500 years BP at Hanauma
Bay and 2,000 years BP at Galeta Point negligible vertical
accretion occurred – with the reef at the former dominated
by the coralline algae Porolithon and only scattered coral
colonies, and the reef flat at Galeta Point now dominated
by sea grass and Halimeda meadows. Grigg (1998) reports
that the Hanauma Bay reef continues to prograde seaward at
around 22 mm year
À1
. In contrast, despite its impressive
growth between 6,000 and 3,000 years ago, Galeta Point
fringing reef is now effectively senescent.
The similarity of reef growth evident at Hanauma Bay
and Galeta Point is perhaps surprising given that they
experienced different sea-level histories (see section
“Fringing reef growth and sea level”). The dominance of
A. palmata in the framework during the main phase of reef
growth was interpreted by Macintyre and Glynn (1976) as
evidence that Galeta Point was a keep-up reef able to keep
pace with a sea level that rose at a decelerating rate to its
present position. Easton and Olson (1976) also concluded
that Hanauma Bay was a keep-up reef, but this interpretation has been challenged because they did not recognize
evidence of higher sea level during the mid-Holocene
(Grossman et al., 1998), and it is now generally agreed
to be a catch-up rather than keep-up reef (Grigg, 1998).
On the GBR, Hayman Island is the only fringing reef
for which data are available that loosely follows this
model (Hopley et al., 1978). At Hayman Island the fringing reef developed over a gently sloping substrate of
last interglacial limestone, now more than 1 km from the
contemporary shoreline at a depth between À15 and
À20 m, very soon after it was flooded by the transgression
(the earliest dated coral above the substrate is 9,320 years
BP). The interpretation is that original reef was outflanked
by shoreline retreat and shoreline backstepping during the
transgression, stranding it offshore as a catch-up reef.
Once sea level stabilized in the mid-Holocene the reef
caught up, and back-reef infilling and seaward
progradation from the new shoreline combined to construct the reef flat. In this regard Hayman Island reef flat
may also have some affinity to the reef crest and backfill
model depicted in Figure 5e described further below
(section “Reef crest and backfill”).
Progradation out from shore over rocky substrate
Fringing reefs complying with this growth model initiate
at the shoreline where they grow up to sea level and then
prograde seaward (Figure 5b). This model of fringing reef
development is most like the simple model conceived by
Darwin (Figure 1a). However, detailed studies demonstrating that any particular reef has grown this way – over
a hard bedrock shoreline are rare – Rees et al. (2006)
inferred that the windward fringing reef at Lizard Island
on the northern GBR grew directly over a granitic bedrock
foundation and has prograded seaward, but from the three
cores presented it is difficult to confidently determine the
broader growth pattern. In Torres Strait, drilling reveals
that reef initiation occurred close to shore at both
Hammond and Yam Islands (Woodroffe et al., 2000), the
former over a Pleistocene reef limestone at around
7,000 years ago and around 1,000 years later at Hammond
Island, over a granite substrate but with significant
mud deposition occurring during reef growth.
The northern section of the fringing reef at Iris Point,
Orpheus Island, also on the GBR, rises over a rocky but
unconsolidated foundation; drilling suggests the reef
grows over a boulder beach (see Boulder Beach) comprised of corestones winnowed from the regolith. These
boulders vary from 0.4 m to several meters a-axis length,
and would be stable under normal conditions. At Iris Point
reef initiation took place around 7,000 years ago and vertically accreted rapidly (4 mm year
À1
) to be constrained by
sea level by 6,250 years ago, as indicated fossil microatoll
ages (Hopley and Barnes, 1985) (see Microatoll). By
5,500 years BP the reef flat had begun to prograde seaward, and because sea levels were falling post highstand
(Chappell et al., 1983), the reef flat was constrained at
a lower level than the older reef flat to landward. Rates
of seaward progradation at northern Iris Point have been
estimated at between 50 and 500 mm year
À1
. Significantly, another transect at Iris Point located around
1,000 m to the south shows a different (younger) age structure and internal fabric, demonstrating the fact that
a particular grow mode may be very localized, even within
the same reef. The southern part of the Iris Point fringing
reef has grown over detrital material shed from the northern section and distributed alongshore, and in contrast to
the older section which is composed of 40–60% framework, the southern reef contains less than 20% in situ
framework (Hopley and Barnes, 1985).
Although detailed chronostratigraphic studies of fringing reefs with this growth strategy are rare, there are many
narrow fringing reefs attached to rocky headlands that
appear to have formed in this way, at least at some stage
of their evolution.
Progradation over terrigenous sediments
This model of fringing reef growth is characterized by reefs
developed over sedimentary structures including Pleistocene alluvial fans (e.g., Paluma Shoals, GBR [Smithers
and Larcombe, 2003]), transgressionary sediment deposits
438
FRINGING REEFS