different reef systems. Deep-water mud mounds are commonly found in groups, suggesting that their formation
was environmentally mediated. Several environmental
triggers have been proposed: the episodic formation of
nutrified water masses reduced sediment supply during
platform drowning or localized oxygen depletion of seawater. Some mud mounds are aligned along faults or fissures, which may have acted as conduits for
hydrothermal fluids. For example, the microspars and brachiopods of Muleshoe mound have been shown to have
higher d
13 C values than non-mound samples, possibly
related to methanogenic fermentation.
Mud-mound formation began in the Palaeoproterozoic
(probably Neoproterozoic) and extended until the Miocene, but they are mostly a Palaeozoic phenomenon, with
widespread mud-mound formation occurring during the
early Cambrian, the lower Ordovician, the late Devonian,
and the Mississippian, which was dominated by
Waulsortian mounds.
Origin of the modern coral reef ecosystem
The modern coral reef ecosystem is geologically very
young. Scleractinian corals appeared in the mid-Triassic
and had almost certainly acquired photosymbionts by the
late Triassic at the latest (Stanley and Swart, 1995). The
ancestral stock of the modern reef biota appeared in
the middle Jurassic but was largely suppressed by the prolific growth of calcitic rudist bivalves in the Cretaceous,
only to rebound after the end-Cretaceous extinction. There
is considerable debate, however, as to whether this coraldominated biota was able to construct spectacular large
reefs like those of the modern day before the Cenozoic.
Most modern coral genera appeared in the Eocene–
Miocene, and many extant species extend back no further
than the Pliocene. Modern reef fish appeared in the Eocene,
but the oldest record of parrotfish (scarid) remains are from
Miocene sediments dated at 14 million years ago (Ma).
During the Oligocene, the compression of climatic belts
and the rise of the Isthmus of Panama created two distinct
regions of reef growth to the Caribbean and Indo-Pacific.
As a probable result of climatic cooling or habitat loss, a
major episode of coral faunal turnover ensued between
4and 1 Ma in the Caribbean (Budd et al., 1994). Extinction
of genera in the Pocilloporidae and Agaricidae was most
marked, but many of these genera continued to persist in
the Indo-Pacific. A similar differential extinction coincident with corals, removed many fish, including all large
excavating scarids, herbivorous siganids, and plantivorous
caesionids from Atlantic reefs (Bellwood, 1997).
Although acroporid corals appeared in the Eocene,
pocilloporids appear to have dominated Caribbean reefs
from 5to 6 Ma, but following a 1-million year (Myr) transition period of mixed acroporid–pocilloporid assemblages, acroporids became dominant in reef communities
in the early Pleistocene (approx. 1.6 Ma). Acroporids
may not, however, have achieved levels of extreme
abundance until the late Pleistocene (approx. 0.5 Ma)
(Budd and Kievman, 1994). With this as yet unexplained
rise to dominance of branching Acropora, and a
corresponding decline in massive, domal corals, coral reef
communities with a completely modern aspect appeared
about 0.5 Ma. Except for the extinction of Pocillopora in
the Caribbean at about 60,000 years (ka), the patterns of
community membership and dominance of coral species
appear to have been highly predictable for at least the past
125,000 years (Kyr) (Pandolfi and Jackson, 2001).
Response to long-term environmental change
Climate and changing seawater chemistry are important
controls on the history of reef growth, in terms of both
complex feedback mechanisms that govern skeletal mineralogy and hence promote some groups over others, and
controlling the styles of early lithification.
Tectonics ultimately drives many aspects of reef growth
such as sea level, circulation patterns, climate, and evolving seawater chemistry change over long timescales
(Figure 10). Reefs and carbonate platforms commonly initiate on preexisting highs such as horst blocks or salt
domes during the early stages of rifting. Reefs are most
numerous on passive continental margins at low latitudes
where they typically form barrier or fringing reefs. Similar
carbonate shelves with reefs can form around the margins
of shallow intracratonic basins, where they can be
interbedded with evaporites. Such basins may be prone
to isolation from the open ocean, leading to infilling by
extensive evaporites.
Rates of thermal subsistence on passive margins are
slow and predictable and are similar to those found in
intracratonic basins. Reef geometries are thus generally
compound to progradational. By contrast, reef-forming
toward the cratonward side of foreland basins is relatively
rapid and reef geometries more aggradational. Finally,
reefs forming in strike-slip basins and on thrust complexes
are affected by the vagaries of local tectonic movements,
which may be highly episodic but involve substantial displacements and are thus highly unpredictable.
Epeiric seas, those that formed by the extensive
flooding of continents during globally high sea levels,
have negligible basin floor topography. Water depths
rarely exceeded 10 m, such that shallow subtidal and intertidal sediments dominate, with many episodes of exposure. Epeiric seas could support patch reefs, often
elongated because of wind waves, storms, or tides. Due
to the limited accommodation space on epeiric platforms,
progradation is the dominant depositional process,
resulting in stacked shallowing-upward sequences or
biostromes.
Climate
Climate is a major controlling force on the evolution of reefs
on both short and long timescales, as the latitudinal range
of carbonate-producing species is largely governed by temperature and carbonate supersaturation. Reef growth therefore shows cyclicity at all scales in response to short-term
462
GENERAL EVOLUTION OF CARBONATE REEFS
was environmentally mediated. Several environmental
triggers have been proposed: the episodic formation of
nutrified water masses reduced sediment supply during
platform drowning or localized oxygen depletion of seawater. Some mud mounds are aligned along faults or fissures, which may have acted as conduits for
hydrothermal fluids. For example, the microspars and brachiopods of Muleshoe mound have been shown to have
higher d
13 C values than non-mound samples, possibly
related to methanogenic fermentation.
Mud-mound formation began in the Palaeoproterozoic
(probably Neoproterozoic) and extended until the Miocene, but they are mostly a Palaeozoic phenomenon, with
widespread mud-mound formation occurring during the
early Cambrian, the lower Ordovician, the late Devonian,
and the Mississippian, which was dominated by
Waulsortian mounds.
Origin of the modern coral reef ecosystem
The modern coral reef ecosystem is geologically very
young. Scleractinian corals appeared in the mid-Triassic
and had almost certainly acquired photosymbionts by the
late Triassic at the latest (Stanley and Swart, 1995). The
ancestral stock of the modern reef biota appeared in
the middle Jurassic but was largely suppressed by the prolific growth of calcitic rudist bivalves in the Cretaceous,
only to rebound after the end-Cretaceous extinction. There
is considerable debate, however, as to whether this coraldominated biota was able to construct spectacular large
reefs like those of the modern day before the Cenozoic.
Most modern coral genera appeared in the Eocene–
Miocene, and many extant species extend back no further
than the Pliocene. Modern reef fish appeared in the Eocene,
but the oldest record of parrotfish (scarid) remains are from
Miocene sediments dated at 14 million years ago (Ma).
During the Oligocene, the compression of climatic belts
and the rise of the Isthmus of Panama created two distinct
regions of reef growth to the Caribbean and Indo-Pacific.
As a probable result of climatic cooling or habitat loss, a
major episode of coral faunal turnover ensued between
4and 1 Ma in the Caribbean (Budd et al., 1994). Extinction
of genera in the Pocilloporidae and Agaricidae was most
marked, but many of these genera continued to persist in
the Indo-Pacific. A similar differential extinction coincident with corals, removed many fish, including all large
excavating scarids, herbivorous siganids, and plantivorous
caesionids from Atlantic reefs (Bellwood, 1997).
Although acroporid corals appeared in the Eocene,
pocilloporids appear to have dominated Caribbean reefs
from 5to 6 Ma, but following a 1-million year (Myr) transition period of mixed acroporid–pocilloporid assemblages, acroporids became dominant in reef communities
in the early Pleistocene (approx. 1.6 Ma). Acroporids
may not, however, have achieved levels of extreme
abundance until the late Pleistocene (approx. 0.5 Ma)
(Budd and Kievman, 1994). With this as yet unexplained
rise to dominance of branching Acropora, and a
corresponding decline in massive, domal corals, coral reef
communities with a completely modern aspect appeared
about 0.5 Ma. Except for the extinction of Pocillopora in
the Caribbean at about 60,000 years (ka), the patterns of
community membership and dominance of coral species
appear to have been highly predictable for at least the past
125,000 years (Kyr) (Pandolfi and Jackson, 2001).
Response to long-term environmental change
Climate and changing seawater chemistry are important
controls on the history of reef growth, in terms of both
complex feedback mechanisms that govern skeletal mineralogy and hence promote some groups over others, and
controlling the styles of early lithification.
Tectonics ultimately drives many aspects of reef growth
such as sea level, circulation patterns, climate, and evolving seawater chemistry change over long timescales
(Figure 10). Reefs and carbonate platforms commonly initiate on preexisting highs such as horst blocks or salt
domes during the early stages of rifting. Reefs are most
numerous on passive continental margins at low latitudes
where they typically form barrier or fringing reefs. Similar
carbonate shelves with reefs can form around the margins
of shallow intracratonic basins, where they can be
interbedded with evaporites. Such basins may be prone
to isolation from the open ocean, leading to infilling by
extensive evaporites.
Rates of thermal subsistence on passive margins are
slow and predictable and are similar to those found in
intracratonic basins. Reef geometries are thus generally
compound to progradational. By contrast, reef-forming
toward the cratonward side of foreland basins is relatively
rapid and reef geometries more aggradational. Finally,
reefs forming in strike-slip basins and on thrust complexes
are affected by the vagaries of local tectonic movements,
which may be highly episodic but involve substantial displacements and are thus highly unpredictable.
Epeiric seas, those that formed by the extensive
flooding of continents during globally high sea levels,
have negligible basin floor topography. Water depths
rarely exceeded 10 m, such that shallow subtidal and intertidal sediments dominate, with many episodes of exposure. Epeiric seas could support patch reefs, often
elongated because of wind waves, storms, or tides. Due
to the limited accommodation space on epeiric platforms,
progradation is the dominant depositional process,
resulting in stacked shallowing-upward sequences or
biostromes.
Climate
Climate is a major controlling force on the evolution of reefs
on both short and long timescales, as the latitudinal range
of carbonate-producing species is largely governed by temperature and carbonate supersaturation. Reef growth therefore shows cyclicity at all scales in response to short-term
462
GENERAL EVOLUTION OF CARBONATE REEFS