diagenesis probably play a crucial role in the stability of
skeletal carbonates (Perrin and Smith, 2007a, b). This is
expressed by the fact that elements of same mineralogy
often show various degrees of dissolution depending on
the nature and origin of the structure considered: e.g., in
some cases aragonite cement may be more resistant to dissolution than their aragonite skeletal substrate (e.g.,
Scherer, 1986).
Dolomite in reefs
Reefs are highly susceptible to dolomitization compared
to other carbonate facies, as shown by the numerous
ancient, including Quaternary, reef bodies and reef platforms which are completely or partially affected by
dolomitization. As already emphasized by Purser and
Schroeder (1986), the high susceptibility of reefs to dolomitization is the result of a combination of favorable factors such as (1) the important primary porosity and
permeability of reef facies, (2) their early history frequently dominated by sea-level changes and therefore
rapid evolution of the chemistry of interstitial waters,
(3) a high content in metastable minerals, and (4) the presence of organic material.
Petrography of dolostones
Dolomite may result from different diagenetic processes
comprising cementation, replacement of sedimentary or
diagenetic precursors, and recrystallization of dolomitic
precursors, the three processes occurring often concomitantly in reef facies (Buigues, 1982; Purser and Aïssaoui,
1985; Carrière, 1987; Berbey, 1989). The identification
of these processes from petrographical analysis of thin
sections permit various dolomite petrotypes to be defined
and characterized from geochemical analysis, their distribution within the reef platform to be mapped, and beyond,
the interpretation of diagenetic fluids, and the development of dolomitic bodies to be reconstructed.
Cementation
In thin section, these dolomite cements are dolosparites
characterized by limpid crystals of relatively large size
(a few tens to more than 100 mm), developed in primary
and secondary voids, which they may fill completely
(mosaic of cement) or partially (fringe of cement).
A quite frequent particular case corresponds to dolomite rhomboedral crystals, typically a few tens of microns,
consisting usually of a central darker microcrystalline part
resulting from the replacement of a non-dolomitic precursor, and a limpid border, interpreted as the further growth
of the same crystalline unit within a void (cementation).
Replacement
The replacement dolomites include most of the microcrystalline dolomites, which preserve the detailed shape of
bioclasts or previous cements, and most of their texture
and consist of small rhomboedral crystals a few microns
to 10 mm in size. These replacement dolomites often show
a relatively high intercrystalline porosity.
Recrystallization
The recrystallization dolomites are generally represented
by large-sized (a few hundreds of microns) rhomboedral
to subrhomboedral crystals, which often show inclusions
of much smaller rhomboedral dolomite crystals. This,
together with the disappearance of any textural remain of
precursor suggests successive phases of recrystallization
of previous dolomitic elements (Buigues, 1982; Purser
and Aïssaoui, 1985). Within a given reef complex, the
oldest dolomites of recrystallization have been shown to
be those having the highest Mg content, this increase in
Mg/Ca ratio compared to the other dolomite types may
be due to a gradual equilibration toward a stoichiometric
composition, characteristic of many ancient dolomites in
fossil reefs (Buigues, 1982; Berbey, 1989; Koch and
Shorr, 1986).
Distribution of dolomite in reefs
The study of diagenesis in Holocene and Pleistocene reefs
constitutes a major contribution to our general understanding of dolomitization of carbonate platforms because
ancient platforms have generally undergone a complex
diagenetic history including several phases of dolomitization including synsedimentary and burial diagenetic
stages.
As shown in recent Cenozoic reefs, although the different types of dolomite (cementation, replacement, and
recrystallization) may occur in the same reef facies, some
broad general trends have been described showing the
dominance of some processes in some parts of the reef
complex (Figure 3). The dolomitic cements tend to dominate the external margins of the reef complex where primary porosity is higher due to coarser facies and
circulation of diagenetic fluids more efficient. By contrast,
the replacement dolomites are usually better developed in
the lagoonal or back-reef finer sediments while the recrystallization dolomites are more characteristic of the oldest
basal parts of the dolomitic body (Buigues, 1982;
Aïssaoui, 1986; Purser et al., 1994).
Dolomitizing fluids
Isotopic approach
The stable isotopic composition of dolomites may reveal
the marine or nonmarine signature of dolomitizing fluids
providing that the studied dolomites have not undergone
a too complex diagenetic history. In that sense, the partially dolomitized facies frequently occurring on the top
and margins of the dolomite body under modern reef platforms have been shown to be the most informative for the
understanding of the modalities of dolomitization.
Although several models of dolomitization of carbonate platform have been considered, the prevalent model
for explaining the dolomitization of reefs is the “mixing
zone” system or Dorag model (Badiozamani, 1973),
mainly deduced from the isotopic signature of dolomites
suggesting a certain mixture of marine and meteoric
waters (Buigues, 1982; Purser and Aïssaoui, 1985;
316
DIAGENESIS
skeletal carbonates (Perrin and Smith, 2007a, b). This is
expressed by the fact that elements of same mineralogy
often show various degrees of dissolution depending on
the nature and origin of the structure considered: e.g., in
some cases aragonite cement may be more resistant to dissolution than their aragonite skeletal substrate (e.g.,
Scherer, 1986).
Dolomite in reefs
Reefs are highly susceptible to dolomitization compared
to other carbonate facies, as shown by the numerous
ancient, including Quaternary, reef bodies and reef platforms which are completely or partially affected by
dolomitization. As already emphasized by Purser and
Schroeder (1986), the high susceptibility of reefs to dolomitization is the result of a combination of favorable factors such as (1) the important primary porosity and
permeability of reef facies, (2) their early history frequently dominated by sea-level changes and therefore
rapid evolution of the chemistry of interstitial waters,
(3) a high content in metastable minerals, and (4) the presence of organic material.
Petrography of dolostones
Dolomite may result from different diagenetic processes
comprising cementation, replacement of sedimentary or
diagenetic precursors, and recrystallization of dolomitic
precursors, the three processes occurring often concomitantly in reef facies (Buigues, 1982; Purser and Aïssaoui,
1985; Carrière, 1987; Berbey, 1989). The identification
of these processes from petrographical analysis of thin
sections permit various dolomite petrotypes to be defined
and characterized from geochemical analysis, their distribution within the reef platform to be mapped, and beyond,
the interpretation of diagenetic fluids, and the development of dolomitic bodies to be reconstructed.
Cementation
In thin section, these dolomite cements are dolosparites
characterized by limpid crystals of relatively large size
(a few tens to more than 100 mm), developed in primary
and secondary voids, which they may fill completely
(mosaic of cement) or partially (fringe of cement).
A quite frequent particular case corresponds to dolomite rhomboedral crystals, typically a few tens of microns,
consisting usually of a central darker microcrystalline part
resulting from the replacement of a non-dolomitic precursor, and a limpid border, interpreted as the further growth
of the same crystalline unit within a void (cementation).
Replacement
The replacement dolomites include most of the microcrystalline dolomites, which preserve the detailed shape of
bioclasts or previous cements, and most of their texture
and consist of small rhomboedral crystals a few microns
to 10 mm in size. These replacement dolomites often show
a relatively high intercrystalline porosity.
Recrystallization
The recrystallization dolomites are generally represented
by large-sized (a few hundreds of microns) rhomboedral
to subrhomboedral crystals, which often show inclusions
of much smaller rhomboedral dolomite crystals. This,
together with the disappearance of any textural remain of
precursor suggests successive phases of recrystallization
of previous dolomitic elements (Buigues, 1982; Purser
and Aïssaoui, 1985). Within a given reef complex, the
oldest dolomites of recrystallization have been shown to
be those having the highest Mg content, this increase in
Mg/Ca ratio compared to the other dolomite types may
be due to a gradual equilibration toward a stoichiometric
composition, characteristic of many ancient dolomites in
fossil reefs (Buigues, 1982; Berbey, 1989; Koch and
Shorr, 1986).
Distribution of dolomite in reefs
The study of diagenesis in Holocene and Pleistocene reefs
constitutes a major contribution to our general understanding of dolomitization of carbonate platforms because
ancient platforms have generally undergone a complex
diagenetic history including several phases of dolomitization including synsedimentary and burial diagenetic
stages.
As shown in recent Cenozoic reefs, although the different types of dolomite (cementation, replacement, and
recrystallization) may occur in the same reef facies, some
broad general trends have been described showing the
dominance of some processes in some parts of the reef
complex (Figure 3). The dolomitic cements tend to dominate the external margins of the reef complex where primary porosity is higher due to coarser facies and
circulation of diagenetic fluids more efficient. By contrast,
the replacement dolomites are usually better developed in
the lagoonal or back-reef finer sediments while the recrystallization dolomites are more characteristic of the oldest
basal parts of the dolomitic body (Buigues, 1982;
Aïssaoui, 1986; Purser et al., 1994).
Dolomitizing fluids
Isotopic approach
The stable isotopic composition of dolomites may reveal
the marine or nonmarine signature of dolomitizing fluids
providing that the studied dolomites have not undergone
a too complex diagenetic history. In that sense, the partially dolomitized facies frequently occurring on the top
and margins of the dolomite body under modern reef platforms have been shown to be the most informative for the
understanding of the modalities of dolomitization.
Although several models of dolomitization of carbonate platform have been considered, the prevalent model
for explaining the dolomitization of reefs is the “mixing
zone” system or Dorag model (Badiozamani, 1973),
mainly deduced from the isotopic signature of dolomites
suggesting a certain mixture of marine and meteoric
waters (Buigues, 1982; Purser and Aïssaoui, 1985;
316
DIAGENESIS