erosion would be that the algal-dominated reef flat and the
coral flat will stand up as a high rim. During the ensuing
submergence, a new reef will first occupy the high ground
on the previous reef and eventually grow at a still stand
sea level with bio and sediment facies distribution similarly
distributed to the reef below. Note that some modern patch
reefs grow on top of earlier patch reefs, indicating a quite
specific control on re-growth; that is, the reef is not growing
on conical karst but on earlier patch reefs. Thus, in the Pleistocene, dominated by oscillating sea levels, a stacked succession of reefs will form through the combined effects of
sea-level change, subsidence, and sub-aereal erosion (see
Davies et al., 1988). In the case of the southern Great Barrier
Reef, the periods of reef growth have been short (<10,000
years) compared with the periods of sub-aereal erosion of
the previous reef (around 100,000 years). The driving force
for the amount of reef stacking is clearly the frequency and
magnitude of sea-level change.
The likelihood that this model also applies to Pacific
atolls is high because they have a similar shape and facies
distribution to that described above for the reefs of the
southern Great Barrier Reef. The effects of a sea-level fall
on an atoll would be to particularly expose the highly
porous reef front, much of which would be removed and
re-deposited down slope. The algal flat however would
resist rapid erosion and form a protective cap above the
catch-up coral facies below. The lagoon would remain
a depositional low, although depending on the magnitude
of the sea-level fall, it may contain a low stand lake or
a saline pool connected to the ocean through the body of
the exposed reef. Subsequent reef growth would eventually
stack above the previous reef. Expansion of the facies laterally would be impossible on the outside of the reef because
of the steep slopes. An example of such growth is Mururoa
(Buigues, 1985; Guilcher, 1988), in that most atolls have
similar characteristics to Mururoa (reef top biofacies distribution and steep external slopes inhibiting facies migration
on the front), and most would also react to sea-level oscillations in the same way. Thus, in the Pleistocene at any rate,
the high stand growth of platform reefs and oceanic atolls
reflect two factors: (1) a high stand facies control of low
stand produced inheritance features and (2) growth controlled largely by the direction and amount of energy input
(See Great Barrier Reef: Origin, Evolution, and Modern
Development; also Davies, 1983; Davies and Hopley,
1983). It is important to note however that in this model
rapid sea-level oscillations are important. Change the oscillation pattern and a different effect will occur. For example,
increasing the length of the low stand period may obliterate
the effects of facies control. Increasing the lengths of the
high stand will serve only to produce flat topped reefs.
Non-karst antecedence
Reef growth has been shown to occur on antecedent surfaces that are not limestones. In the Pacific, Murray’s original ideas (1880, 1887, 1889) that volcanics form a surface
off which reefs have grown holds true. Perhaps less
obvious, however, are Choi and Holmes (1982) and Choi
and Ginsburg (1982) work that show that late Pleistocene
and Holocene reefs are growing on siliciclastic sediments,
which have an alluvial and delta-like morphology. The
residual relief of these coastal plain sediments localized
initial coral growth; favored sites for such growth were
elevations such as levees and bars of remnant channels
and deltaic lobes. Thus, the unique rhomboid and long sinuous lagoon reefs are derived from the early Pleistocene
river morphology and not from a fault dominated karst
morphology as suggested by Purdy (1974).
Conclusions
A consideration of reefal platforms began with Darwin’s
startling hypothesis that subsidence was paramount and
drove fringing reefs to become barrier Reefs and ultimately atolls. In opposition the role of substrate was raised
early, and then in conjunction with glacial control and subaereal erosion, it has risen to a position of general acceptance as a dominating process in the evolution of barrier
reefs and atolls. Such antecedence is generally limestone
based, and in some cases that antecedence has been shown
to be facies controlled. However, antecedence has also
been shown to occur off siliclastic foundations. There
can be little doubt, however, that much antecedence is produced by low sea-level erosion.
Atolls are underlain by subsiding volcanic basements
with steep peripheral slopes and surrounded by deep
water. The limestones comprising the atoll platform can
be tropical or temperate depending on their latitudinal
position. During periods of oscillating sea level, the platform surface may be exposed, and karstic erosion set in,
the effects of which will be the development of a raised
rim which may or may not be breached, a central depression and conical karst features within the lagoon. Subsequent sea-level rise will allow growth preferentially such
that it mimics the positive underlying topography, thus
inheriting the eroded shapes.
In the formation of Barrier Reefs, a critical feature is the
formation of a “karst marginal plain” – effectively an
exposed shelf, carbonate dominated in the outer part and
siliceous or non-carbonate in the inner part (effectively
most shelves) – exposed as a consequence of a fall in
sea level. Consequent sub-aereal erosion would occur to
a maximum at the boundary of carbonate/non-carbonate
rocks effecting the formation of tower-karst. Elsewhere
in the carbonate-dominated area, a raised rim would form
proximal to the outer steep slope (a la atolls) and conicalkarst would form over the outer carbonate shelf (Figure 2).
Following the subsequent rise in sea level, platform reefs
would develop over the tower-karst and a barrier reef
would develop over the outer shelf rim.
Bibliography
Asano, D., 1942. Coral Reefs of the South Sea Islands. Tokyo Imperial University, Geological and Palaeontological Institute
Reports, 39, 1–19.
ANTECEDENT PLATFORMS
45
coral flat will stand up as a high rim. During the ensuing
submergence, a new reef will first occupy the high ground
on the previous reef and eventually grow at a still stand
sea level with bio and sediment facies distribution similarly
distributed to the reef below. Note that some modern patch
reefs grow on top of earlier patch reefs, indicating a quite
specific control on re-growth; that is, the reef is not growing
on conical karst but on earlier patch reefs. Thus, in the Pleistocene, dominated by oscillating sea levels, a stacked succession of reefs will form through the combined effects of
sea-level change, subsidence, and sub-aereal erosion (see
Davies et al., 1988). In the case of the southern Great Barrier
Reef, the periods of reef growth have been short (<10,000
years) compared with the periods of sub-aereal erosion of
the previous reef (around 100,000 years). The driving force
for the amount of reef stacking is clearly the frequency and
magnitude of sea-level change.
The likelihood that this model also applies to Pacific
atolls is high because they have a similar shape and facies
distribution to that described above for the reefs of the
southern Great Barrier Reef. The effects of a sea-level fall
on an atoll would be to particularly expose the highly
porous reef front, much of which would be removed and
re-deposited down slope. The algal flat however would
resist rapid erosion and form a protective cap above the
catch-up coral facies below. The lagoon would remain
a depositional low, although depending on the magnitude
of the sea-level fall, it may contain a low stand lake or
a saline pool connected to the ocean through the body of
the exposed reef. Subsequent reef growth would eventually
stack above the previous reef. Expansion of the facies laterally would be impossible on the outside of the reef because
of the steep slopes. An example of such growth is Mururoa
(Buigues, 1985; Guilcher, 1988), in that most atolls have
similar characteristics to Mururoa (reef top biofacies distribution and steep external slopes inhibiting facies migration
on the front), and most would also react to sea-level oscillations in the same way. Thus, in the Pleistocene at any rate,
the high stand growth of platform reefs and oceanic atolls
reflect two factors: (1) a high stand facies control of low
stand produced inheritance features and (2) growth controlled largely by the direction and amount of energy input
(See Great Barrier Reef: Origin, Evolution, and Modern
Development; also Davies, 1983; Davies and Hopley,
1983). It is important to note however that in this model
rapid sea-level oscillations are important. Change the oscillation pattern and a different effect will occur. For example,
increasing the length of the low stand period may obliterate
the effects of facies control. Increasing the lengths of the
high stand will serve only to produce flat topped reefs.
Non-karst antecedence
Reef growth has been shown to occur on antecedent surfaces that are not limestones. In the Pacific, Murray’s original ideas (1880, 1887, 1889) that volcanics form a surface
off which reefs have grown holds true. Perhaps less
obvious, however, are Choi and Holmes (1982) and Choi
and Ginsburg (1982) work that show that late Pleistocene
and Holocene reefs are growing on siliciclastic sediments,
which have an alluvial and delta-like morphology. The
residual relief of these coastal plain sediments localized
initial coral growth; favored sites for such growth were
elevations such as levees and bars of remnant channels
and deltaic lobes. Thus, the unique rhomboid and long sinuous lagoon reefs are derived from the early Pleistocene
river morphology and not from a fault dominated karst
morphology as suggested by Purdy (1974).
Conclusions
A consideration of reefal platforms began with Darwin’s
startling hypothesis that subsidence was paramount and
drove fringing reefs to become barrier Reefs and ultimately atolls. In opposition the role of substrate was raised
early, and then in conjunction with glacial control and subaereal erosion, it has risen to a position of general acceptance as a dominating process in the evolution of barrier
reefs and atolls. Such antecedence is generally limestone
based, and in some cases that antecedence has been shown
to be facies controlled. However, antecedence has also
been shown to occur off siliclastic foundations. There
can be little doubt, however, that much antecedence is produced by low sea-level erosion.
Atolls are underlain by subsiding volcanic basements
with steep peripheral slopes and surrounded by deep
water. The limestones comprising the atoll platform can
be tropical or temperate depending on their latitudinal
position. During periods of oscillating sea level, the platform surface may be exposed, and karstic erosion set in,
the effects of which will be the development of a raised
rim which may or may not be breached, a central depression and conical karst features within the lagoon. Subsequent sea-level rise will allow growth preferentially such
that it mimics the positive underlying topography, thus
inheriting the eroded shapes.
In the formation of Barrier Reefs, a critical feature is the
formation of a “karst marginal plain” – effectively an
exposed shelf, carbonate dominated in the outer part and
siliceous or non-carbonate in the inner part (effectively
most shelves) – exposed as a consequence of a fall in
sea level. Consequent sub-aereal erosion would occur to
a maximum at the boundary of carbonate/non-carbonate
rocks effecting the formation of tower-karst. Elsewhere
in the carbonate-dominated area, a raised rim would form
proximal to the outer steep slope (a la atolls) and conicalkarst would form over the outer carbonate shelf (Figure 2).
Following the subsequent rise in sea level, platform reefs
would develop over the tower-karst and a barrier reef
would develop over the outer shelf rim.
Bibliography
Asano, D., 1942. Coral Reefs of the South Sea Islands. Tokyo Imperial University, Geological and Palaeontological Institute
Reports, 39, 1–19.
ANTECEDENT PLATFORMS
45