began in earnest in the Mesozoic and has increased dramatically ever since (see entry Bioerosion).
Geologic history of reefs
The Phanerozoic witnessed major turnovers of reef biotas,
mass and minor extinction events, and profound changes
in the chemistry of seawater. The evolving history of reefs
has been reviewed extensively by Fagerstrom (1987),
Wood (1999), Stanley (2001), Kiessling et al. (2002), and
James and Wood (2010), and is summarized in Figure 3.
Reefs have broadly evolved from dense, stromatolitedominated reefs of the Archean, with the appearance of
thrombolites and calcimicrobes in the Neoproterozoic,
forming the first reef cavities, to the successive rise of various
skeletal metazoans from the earliest Phanerozoic onward.
During geological periods when large calcareous skeletal and microbial structures were common, reefs grew as
fringing reefs landward, as patch reefs across platforms,
and as barrier reefs along platform margins. Coeval
mounds developed in either quiet water settings across
the platform, or on ramps, or on the slope.
The formation of large, shallow-water, flat-topped
platforms coincide with the periods of skeletal reef development because skeletal reefs are characterized by wellcemented carbonate buttresses along their seaward
margin. These ramparts absorbed waves and swells, thus
allowing more tranquil, protected lagoonal facies to
develop across the platform interiors. When skeletal reefs
were absent, carbonate ramps were the norm, where reef
mounds and mud mounds were the only buildups, which
either grew leeward of sand shoals or more commonly
on deep-water ramps and slopes.
The great diversity of ancient reefs present a continuum
of shared ecologies and sedimentary characteristics, but
they can be divided into the broad end members of (1) skeletal reefs, (2) skeletal–microbial reefs, (3) microbial reefs,
and (4) mud mounds (James and Wood, 2010; Figure 4).
Skeletal reefs
Shallow reefs
Skeletal, shallow reefs, where skeletons dominate most of
the rock volume, are restricted in geologic time to the
(1) middle Ordovician to late Devonian, (2) late Jurassic,
and (3) Cenozoic.
Although synsedimentary cements, microbes, and
automicrite were present, they are usually of subsidiary
importance, except during the intervals of the Palaeozoic.
Internal cavities, which may comprise as much as 30% of
the reef volume, can be sites of diverse cryptic communities,
calcimicrobial growth, and internal sediment. Automicrite is
present on, around, and between skeletons. Synsedimentary
cement can be spectacular, with the largest amounts found
in the reefs at the platform margin. Such reefs were built
by calcified sponges (stromatoporoids), tabulate corals, or
scleractinian corals.
These reefs, with their large skeletons and relatively
rapid growth rates, are amongst the most extensive in the
rock record. Diversity is lowest in reef crest and deepwater environments, with both environments favoring
sheet-like skeletal morphologies. The highest diversity
of skeletons and shapes, and therefore rock types, occur
at intermediate depths.
Growth forms and skeletal diversity are strongly controlled by hydrodynamic energy. Unlike modern corals,
Palaeozoic stromatoporoid sponges and tabulate corals,
even though they often grew as laminar forms, sat on or
were anchored within the sediment (Figure 5). Large, tabular forms that were solidly rooted in the sediment
inhabited high-energy zones such as the reef crest, but
since stromatoporoids did not typically have an encrusting
habit, it is doubtful that they were successful builders in
the surf zone. Tabular stromatoporoids, locally bound
together by calcimicrobes, automicrite, or cement, are also
known to have occurred in energetic waters. Domal, bulbous, and dendroid forms occupied quiet water zones,
either below wave base or in sheltered areas of the back
reef and the lagoon (such as delicate stick-like
amphiporoids). These skeletons were locally reworked
during storms and redeposited as rubble units associated
with peritidal settings.
Deep-water skeletal reefs
Today, deep-water skeletal reefs grow worldwide in water
depths of between 250 and 1,500 m, where temperatures
range from 4
C to 12
C (Freiwald and Roberts, 2005).
They are constructed by scleractinian corals that lack
photosymbionts, especially the branching form Lophelia
pertusa. These reefs are muddy and unlithified and range
up to 5 km long and 40 m high, with generally steep sides
up to 160 m thick. Many are located in areas of elevated
nutrients such as oceanic fronts or upwelling; others are
sited on the top of cold hydrocarbon seeps.
Cold-seep reefs are part of a lineage of similar structures that extends back to the Silurian, but there are very
few corals from methane seep communities older than
the late Eocene. The rock record of such skeletal reefs is
not extensive. The oldest known is Triassic, but they are
not numerous until the Cenozoic, where an especially
good record exists in north-west Europe and the Mediterranean region. Pre-Cenozoic deep-water reefs seem to be
either skeletal–microbial mounds or mud mounds.
Mixed skeletal–microbial reefs
Many ancient reefs bear two important constituents: small
or delicate skeletal metazoans that are either heterotrophs
or autotrophs (e.g., phylloid algae) and a microbial component of calcimicrobes or inferred microbialite (Webb,
1996; Wood, 1999). These structures with a mixed consortium of organisms are unlike most modern skeletal reefs
and have been called reef mounds or biogenic mounds
(James and Bourque, 1992). Such reefs often contain
extensive framework cavities, with their own distinctive
cryptic biotas and abundant synsedimentary cements
(Figure 6b).
GENERAL EVOLUTION OF CARBONATE REEFS
455
Geologic history of reefs
The Phanerozoic witnessed major turnovers of reef biotas,
mass and minor extinction events, and profound changes
in the chemistry of seawater. The evolving history of reefs
has been reviewed extensively by Fagerstrom (1987),
Wood (1999), Stanley (2001), Kiessling et al. (2002), and
James and Wood (2010), and is summarized in Figure 3.
Reefs have broadly evolved from dense, stromatolitedominated reefs of the Archean, with the appearance of
thrombolites and calcimicrobes in the Neoproterozoic,
forming the first reef cavities, to the successive rise of various
skeletal metazoans from the earliest Phanerozoic onward.
During geological periods when large calcareous skeletal and microbial structures were common, reefs grew as
fringing reefs landward, as patch reefs across platforms,
and as barrier reefs along platform margins. Coeval
mounds developed in either quiet water settings across
the platform, or on ramps, or on the slope.
The formation of large, shallow-water, flat-topped
platforms coincide with the periods of skeletal reef development because skeletal reefs are characterized by wellcemented carbonate buttresses along their seaward
margin. These ramparts absorbed waves and swells, thus
allowing more tranquil, protected lagoonal facies to
develop across the platform interiors. When skeletal reefs
were absent, carbonate ramps were the norm, where reef
mounds and mud mounds were the only buildups, which
either grew leeward of sand shoals or more commonly
on deep-water ramps and slopes.
The great diversity of ancient reefs present a continuum
of shared ecologies and sedimentary characteristics, but
they can be divided into the broad end members of (1) skeletal reefs, (2) skeletal–microbial reefs, (3) microbial reefs,
and (4) mud mounds (James and Wood, 2010; Figure 4).
Skeletal reefs
Shallow reefs
Skeletal, shallow reefs, where skeletons dominate most of
the rock volume, are restricted in geologic time to the
(1) middle Ordovician to late Devonian, (2) late Jurassic,
and (3) Cenozoic.
Although synsedimentary cements, microbes, and
automicrite were present, they are usually of subsidiary
importance, except during the intervals of the Palaeozoic.
Internal cavities, which may comprise as much as 30% of
the reef volume, can be sites of diverse cryptic communities,
calcimicrobial growth, and internal sediment. Automicrite is
present on, around, and between skeletons. Synsedimentary
cement can be spectacular, with the largest amounts found
in the reefs at the platform margin. Such reefs were built
by calcified sponges (stromatoporoids), tabulate corals, or
scleractinian corals.
These reefs, with their large skeletons and relatively
rapid growth rates, are amongst the most extensive in the
rock record. Diversity is lowest in reef crest and deepwater environments, with both environments favoring
sheet-like skeletal morphologies. The highest diversity
of skeletons and shapes, and therefore rock types, occur
at intermediate depths.
Growth forms and skeletal diversity are strongly controlled by hydrodynamic energy. Unlike modern corals,
Palaeozoic stromatoporoid sponges and tabulate corals,
even though they often grew as laminar forms, sat on or
were anchored within the sediment (Figure 5). Large, tabular forms that were solidly rooted in the sediment
inhabited high-energy zones such as the reef crest, but
since stromatoporoids did not typically have an encrusting
habit, it is doubtful that they were successful builders in
the surf zone. Tabular stromatoporoids, locally bound
together by calcimicrobes, automicrite, or cement, are also
known to have occurred in energetic waters. Domal, bulbous, and dendroid forms occupied quiet water zones,
either below wave base or in sheltered areas of the back
reef and the lagoon (such as delicate stick-like
amphiporoids). These skeletons were locally reworked
during storms and redeposited as rubble units associated
with peritidal settings.
Deep-water skeletal reefs
Today, deep-water skeletal reefs grow worldwide in water
depths of between 250 and 1,500 m, where temperatures
range from 4
C to 12
C (Freiwald and Roberts, 2005).
They are constructed by scleractinian corals that lack
photosymbionts, especially the branching form Lophelia
pertusa. These reefs are muddy and unlithified and range
up to 5 km long and 40 m high, with generally steep sides
up to 160 m thick. Many are located in areas of elevated
nutrients such as oceanic fronts or upwelling; others are
sited on the top of cold hydrocarbon seeps.
Cold-seep reefs are part of a lineage of similar structures that extends back to the Silurian, but there are very
few corals from methane seep communities older than
the late Eocene. The rock record of such skeletal reefs is
not extensive. The oldest known is Triassic, but they are
not numerous until the Cenozoic, where an especially
good record exists in north-west Europe and the Mediterranean region. Pre-Cenozoic deep-water reefs seem to be
either skeletal–microbial mounds or mud mounds.
Mixed skeletal–microbial reefs
Many ancient reefs bear two important constituents: small
or delicate skeletal metazoans that are either heterotrophs
or autotrophs (e.g., phylloid algae) and a microbial component of calcimicrobes or inferred microbialite (Webb,
1996; Wood, 1999). These structures with a mixed consortium of organisms are unlike most modern skeletal reefs
and have been called reef mounds or biogenic mounds
(James and Bourque, 1992). Such reefs often contain
extensive framework cavities, with their own distinctive
cryptic biotas and abundant synsedimentary cements
(Figure 6b).
GENERAL EVOLUTION OF CARBONATE REEFS
455