several temporal scales (1) through the contribution of
early cements to the rigidity of reef buildups and, hence,
the maintenance of the positive relief above the surrounding seafloor sediments and (2) through the high preservation potential of the reef structure in the fossil record.
Distribution of cements
At the scale of microfacies, the variability of cements distribution both in modern and ancient reefs is well known.
The distributional pattern of cements at that scale is
directly dependent on (1) the amount of water present in
the reef porosity and (2) the petrophysical properties
(i.e., porosity and permeability) of the reef limestone,
while the mineralogical and geochemical characteristics
of cements are mainly controlled by the water chemistry.
The control exerted by the amount of water is well
explained and depends on the position of the considered
site relatively to the water table, i.e., phreatic or vadose
zones. This affects the amount and distribution of cement
being produced: less cement restricted at the lower grain
surface or at the contact between neighboring grains in
the vadose zone, more cement forming isopachous fringes
surrounding grains and lining primary voids in the phreatic zone. The differences in the morphology and distribution of cements occurring at fine scale, even between
adjacent pores or within the same thin section, are
explained by fine-scale changes in porosity and permeability, resulting notably from size and interconnectivity
between voids. They are consequently extremely difficult
to predict or to generalize for modeling. The concept of
microenvironment, as used in diagenetic studies of reefs,
has been created in an attempt to overcome this problem.
The diagenetic microenvironment, defined as the finescale environment within which diagenetic processes are
taking place, is characterized both by its chemical and
petrophysical properties.
At the scale of the reef complex. The intense marine
cementation along the seaward margins of reef complexes
has been largely evidenced in a multitude of modern
examples throughout the world and belonging to various
types of reef settings located both in the Atlantic –
Bermuda (Ginsburg et al., 1971), Belize (James and
Ginsburg, 1979), South Florida (Lighty, 1985), Jamaica
(Land and Moore, 1980), and Galeta Point in Panama
(Macintyre, 1977) – and in the Indo-Pacific – the Great
Barrier Reef (Marshall, 1986), and Mururoa Atoll and
Loyalty Islands (Aïssaoui, 1986; Aïssaoui and Purser,
1985, 1986; Carrière, 1987). The distribution of
synsedimentary marine cementation is strongly heterogeneous at the scale of the reef complex, with intensely
cemented facies on the seaward edges while the leeward
and lagoonal sediments remain few or almost not
cemented. This has been well illustrated both in uplifted
and non-uplifted reef platforms (Aïssaoui, 1986; Carrière,
1987; Figure 2) and conceptualized through the “principle
of maximal cementation” (Aïssaoui and Purser, 1985).
Nonmarine cements, related to the history of the reef
platform in response to the sea-level changes and periods
of emersions, show a very contrasting distribution, concentrated at particular levels following the stratigraphy
and from which the importance of cementation is gradually decreasing downward in the reef limestones. This distributional pattern expresses the control of gravity on the
circulation of interstitial waters from which the cements
are precipitated during the low stands of sea levels
(Figure 2).
Controls of cementation
Control by substrate properties. The properties of substrate being cemented exert undoubtedly a strong control
over the characteristics of the cement itself in some cases,
while there is not apparent control in others. The epitaxial
overgrowths and syntaxial cement of acicular aragonite
are well known to develop preferentially on substrates
composed of fibrous aragonite, mainly corals and gastropods in the reef environment. In this case, both the mineralogy and ultrastructure of the substrate control the
mineralogy and morphology of the cement. Less frequent
examples have also been described for fibrous HMC
cement developing upon fibrous calcitic biogenic substrates such as hyaline foraminifera (Amphistegina).
In thin section, there is usually an optical continuity
between the crystals of the substrate and those of the
cement.
At slightly larger scale, the petrophysical properties of
the substrate seem to control the morphology of HMC
cements. Aïssaoui (1986) has shown that there is a good
correlation between the elongated shape of cement crystals, itself influenced by the Mg/Ca ratio, and the size of
the primary voids being cemented.
Hydrodynamics. The asymmetrical distribution of
cements is explained by several factors favoring the rapid
development of cements along the seaward reef margin,
in particular, the higher-energy conditions and cooler
oceanic water temperatures, together with the higher
permeability and connected porosity of the reef-front
facies compared to the fine-grained sediments of the more
sheltered back reef or leeward side. The higher-energy
conditions on the seaward side are driven by the effect of
waves and tides. The higher hydrodynamics on that side
allows a large volume of water to pass through the reef
framework and an efficient CO 2 degassing to be produced
by the warming and moving of the cool open marine
waters, resulting in the rapid and significant growth of
cements.
Internal sediments
The link between internal sediment and diagenesis is chronological as the internal sediment is defined as being
deposited within the voids of an already lithified substrate.
The internal sediment therefore occurs after the first
cementation. A certain degree of hydrodynamics is
DIAGENESIS
313
early cements to the rigidity of reef buildups and, hence,
the maintenance of the positive relief above the surrounding seafloor sediments and (2) through the high preservation potential of the reef structure in the fossil record.
Distribution of cements
At the scale of microfacies, the variability of cements distribution both in modern and ancient reefs is well known.
The distributional pattern of cements at that scale is
directly dependent on (1) the amount of water present in
the reef porosity and (2) the petrophysical properties
(i.e., porosity and permeability) of the reef limestone,
while the mineralogical and geochemical characteristics
of cements are mainly controlled by the water chemistry.
The control exerted by the amount of water is well
explained and depends on the position of the considered
site relatively to the water table, i.e., phreatic or vadose
zones. This affects the amount and distribution of cement
being produced: less cement restricted at the lower grain
surface or at the contact between neighboring grains in
the vadose zone, more cement forming isopachous fringes
surrounding grains and lining primary voids in the phreatic zone. The differences in the morphology and distribution of cements occurring at fine scale, even between
adjacent pores or within the same thin section, are
explained by fine-scale changes in porosity and permeability, resulting notably from size and interconnectivity
between voids. They are consequently extremely difficult
to predict or to generalize for modeling. The concept of
microenvironment, as used in diagenetic studies of reefs,
has been created in an attempt to overcome this problem.
The diagenetic microenvironment, defined as the finescale environment within which diagenetic processes are
taking place, is characterized both by its chemical and
petrophysical properties.
At the scale of the reef complex. The intense marine
cementation along the seaward margins of reef complexes
has been largely evidenced in a multitude of modern
examples throughout the world and belonging to various
types of reef settings located both in the Atlantic –
Bermuda (Ginsburg et al., 1971), Belize (James and
Ginsburg, 1979), South Florida (Lighty, 1985), Jamaica
(Land and Moore, 1980), and Galeta Point in Panama
(Macintyre, 1977) – and in the Indo-Pacific – the Great
Barrier Reef (Marshall, 1986), and Mururoa Atoll and
Loyalty Islands (Aïssaoui, 1986; Aïssaoui and Purser,
1985, 1986; Carrière, 1987). The distribution of
synsedimentary marine cementation is strongly heterogeneous at the scale of the reef complex, with intensely
cemented facies on the seaward edges while the leeward
and lagoonal sediments remain few or almost not
cemented. This has been well illustrated both in uplifted
and non-uplifted reef platforms (Aïssaoui, 1986; Carrière,
1987; Figure 2) and conceptualized through the “principle
of maximal cementation” (Aïssaoui and Purser, 1985).
Nonmarine cements, related to the history of the reef
platform in response to the sea-level changes and periods
of emersions, show a very contrasting distribution, concentrated at particular levels following the stratigraphy
and from which the importance of cementation is gradually decreasing downward in the reef limestones. This distributional pattern expresses the control of gravity on the
circulation of interstitial waters from which the cements
are precipitated during the low stands of sea levels
(Figure 2).
Controls of cementation
Control by substrate properties. The properties of substrate being cemented exert undoubtedly a strong control
over the characteristics of the cement itself in some cases,
while there is not apparent control in others. The epitaxial
overgrowths and syntaxial cement of acicular aragonite
are well known to develop preferentially on substrates
composed of fibrous aragonite, mainly corals and gastropods in the reef environment. In this case, both the mineralogy and ultrastructure of the substrate control the
mineralogy and morphology of the cement. Less frequent
examples have also been described for fibrous HMC
cement developing upon fibrous calcitic biogenic substrates such as hyaline foraminifera (Amphistegina).
In thin section, there is usually an optical continuity
between the crystals of the substrate and those of the
cement.
At slightly larger scale, the petrophysical properties of
the substrate seem to control the morphology of HMC
cements. Aïssaoui (1986) has shown that there is a good
correlation between the elongated shape of cement crystals, itself influenced by the Mg/Ca ratio, and the size of
the primary voids being cemented.
Hydrodynamics. The asymmetrical distribution of
cements is explained by several factors favoring the rapid
development of cements along the seaward reef margin,
in particular, the higher-energy conditions and cooler
oceanic water temperatures, together with the higher
permeability and connected porosity of the reef-front
facies compared to the fine-grained sediments of the more
sheltered back reef or leeward side. The higher-energy
conditions on the seaward side are driven by the effect of
waves and tides. The higher hydrodynamics on that side
allows a large volume of water to pass through the reef
framework and an efficient CO 2 degassing to be produced
by the warming and moving of the cool open marine
waters, resulting in the rapid and significant growth of
cements.
Internal sediments
The link between internal sediment and diagenesis is chronological as the internal sediment is defined as being
deposited within the voids of an already lithified substrate.
The internal sediment therefore occurs after the first
cementation. A certain degree of hydrodynamics is
DIAGENESIS
313