limestone mass in Quaternary reefs. They are restricted to
particular horizons in the reef rock and usually considered
as resulting from a diagenesis related to subaerial environment. They are, therefore, linked to some degree to subaerial exposure of the reef body during emersions and,
hence, can be precipitated either directly on emerged surface substrates or at depth within the reef rock in relationship with circulation of interstitial fluids. In both case,
they have a nonmarine origin as shown by their lowmagnesium content and their isotopic signature. The most
frequent morphological types include needlelike calcite
cement and a great diversity of sparite cements.
The needlelike or whisker calcite cement (Figure 1h) is
developed in calcrete (caliche) pedogenetic facies and is
often associated with the characteristic alveolar texture
formed by networks of interconnecting micrite walls
representing rhizoconcretions or rhizoliths. This calcite
is composed of single fibrous crystals often forming
a loose meshwork and displays a variety of habits and
arrangements of individual crystals. The precise origin
of this typical cement has been questioned for past
decades to know if it results from direct precipitation
in close relation with the high degrees of supersaturation
caused by physicochemical changes in soil profile or if
it has a biogenic, possibly microbial origin, and this is
still a matter of debate (Goudie, 1996; Cailleau et al.,
2009).
The mosaic sparite cement consists of large size (from
100 mm to more than 1 mm) limpid crystals either with
a thick and irregular shape or slightly more elongated with
euhedral terminations. It fills up partially or completely
primary or secondary intra- and inter-granular voids.
The elongated-crystal sparite or brown calcite cement
(Figure 1g) is formed by individual elongated crystals
widening toward the center of the cavity within which
they develop, and having pyramidal terminations. The
sometimes important development of these cements in
relationship with emersion surface and karstic processes
clearly demonstrate their subaerial origin, also corroborated by their trace element and isotopic composition.
Early cements in ancient reefs
Ancient reefs commonly have undergone successive
steps of various diagenetic processes, including diverse
phases of cementation, linked to the changing physicochemical conditions and supersaturation state in pore
fluids during their geological history. As a result, in this
chronology, each diagenetic phase provides new diagenetic products and affects in some degree the preexisting
materials of sedimentary, biological, and diagenetic origins. While the relative chronology of cements in ancient
reefs can be established from detailed petrographical analyses, their original mineralogy and geochemical composition have to be deduced through interpretation of
available data.
Botryoidal cements in ancient reefs and carbonate platforms are relatively common and have been described
from Precambrian to Cenozoic formations. While their
morphology and size are similar to their counterparts
occurring in recent reefs, they differ significantly in terms
of their current mineralogy and petrography. Ancient botryoidal cements are formed by LMC sparite resulting from
the calcitization of the original fibrous aragonite, as shown
by the frequent presence of aragonite relics included in the
spar crystals (Mazzulo and Cys, 1979; Mazzulo, 1980;
Aïssaoui, 1985). Diverse origins have been demonstrated
for these ancient botryoidal aragonite cements.
Fibrous cements represent frequent synsedimentary
cements in ancient reefs and non-reef carbonate platforms.
They usually form isopachous layers of fibrous crystals,
parallel or in fanlike arrangement, in primary voids and
often present a sweeping extinction when seen in thin section under crossed nichols. They are usually rich in dark
micro-inclusions and depending on their diagenetic alteration, a sparitic structure can be overprinted upon the original fibrous arrangement. Most ancient fibrous cements
have not preserved their original mineralogy and are
formed by LMC. In the lower part of the carbonate formation in Mururoa Atoll, Berbey (1989) has shown that the
early marine fibrous HMC cements have undergone
a diagenesis leading to the gradual loss of their Mg content, probably by progressive Ca–Mg substitution in the
calcite lattice.
Sparite cements are generally the most abundant
cements in ancient reef facies and usually consist of
LMC. Several types can be differentiated from the
arrangement and morphologies of crystals and usually
the relative chronology of cements can be established for
petrographical analysis. However, origins of these LMC
sparite cements can be highly diverse, including early diagenesis from surface waters to late burial diagenetic
cementation. As there is no a priori reliable criterion either
geochemical or petrographical, which allows surface/
near-surface sparite from deep burial sparite to be distinguished, the precise origin of these cements together with
the quality of their parent waters remain extremely difficult to infer, depending mainly on the degree of knowledge of the geological context.
Cementation in reefs
Role of cementation
Numerous studies have shown that rapid lithification
through synsedimentary marine cementation is
a widespread feature common to many, if not all, modern
tropical corals reefs (Macintyre, 1977; Friedman, 1998)
and the development of early cements in ancient reefs is
also well known. Therefore, one of the main characteristics of both recent and ancient reefs is that they are being
cemented at the time of their growth.
Although the first primary actors in the reef development are undoubtedly the reef-building organisms edifying a rigid mineralized biological framework, early
lithification of sediment and cementation of the framework can exert also an important role in the edification
of reefs (Purser and Schroeder, 1986). This occurs at
312
DIAGENESIS
particular horizons in the reef rock and usually considered
as resulting from a diagenesis related to subaerial environment. They are, therefore, linked to some degree to subaerial exposure of the reef body during emersions and,
hence, can be precipitated either directly on emerged surface substrates or at depth within the reef rock in relationship with circulation of interstitial fluids. In both case,
they have a nonmarine origin as shown by their lowmagnesium content and their isotopic signature. The most
frequent morphological types include needlelike calcite
cement and a great diversity of sparite cements.
The needlelike or whisker calcite cement (Figure 1h) is
developed in calcrete (caliche) pedogenetic facies and is
often associated with the characteristic alveolar texture
formed by networks of interconnecting micrite walls
representing rhizoconcretions or rhizoliths. This calcite
is composed of single fibrous crystals often forming
a loose meshwork and displays a variety of habits and
arrangements of individual crystals. The precise origin
of this typical cement has been questioned for past
decades to know if it results from direct precipitation
in close relation with the high degrees of supersaturation
caused by physicochemical changes in soil profile or if
it has a biogenic, possibly microbial origin, and this is
still a matter of debate (Goudie, 1996; Cailleau et al.,
2009).
The mosaic sparite cement consists of large size (from
100 mm to more than 1 mm) limpid crystals either with
a thick and irregular shape or slightly more elongated with
euhedral terminations. It fills up partially or completely
primary or secondary intra- and inter-granular voids.
The elongated-crystal sparite or brown calcite cement
(Figure 1g) is formed by individual elongated crystals
widening toward the center of the cavity within which
they develop, and having pyramidal terminations. The
sometimes important development of these cements in
relationship with emersion surface and karstic processes
clearly demonstrate their subaerial origin, also corroborated by their trace element and isotopic composition.
Early cements in ancient reefs
Ancient reefs commonly have undergone successive
steps of various diagenetic processes, including diverse
phases of cementation, linked to the changing physicochemical conditions and supersaturation state in pore
fluids during their geological history. As a result, in this
chronology, each diagenetic phase provides new diagenetic products and affects in some degree the preexisting
materials of sedimentary, biological, and diagenetic origins. While the relative chronology of cements in ancient
reefs can be established from detailed petrographical analyses, their original mineralogy and geochemical composition have to be deduced through interpretation of
available data.
Botryoidal cements in ancient reefs and carbonate platforms are relatively common and have been described
from Precambrian to Cenozoic formations. While their
morphology and size are similar to their counterparts
occurring in recent reefs, they differ significantly in terms
of their current mineralogy and petrography. Ancient botryoidal cements are formed by LMC sparite resulting from
the calcitization of the original fibrous aragonite, as shown
by the frequent presence of aragonite relics included in the
spar crystals (Mazzulo and Cys, 1979; Mazzulo, 1980;
Aïssaoui, 1985). Diverse origins have been demonstrated
for these ancient botryoidal aragonite cements.
Fibrous cements represent frequent synsedimentary
cements in ancient reefs and non-reef carbonate platforms.
They usually form isopachous layers of fibrous crystals,
parallel or in fanlike arrangement, in primary voids and
often present a sweeping extinction when seen in thin section under crossed nichols. They are usually rich in dark
micro-inclusions and depending on their diagenetic alteration, a sparitic structure can be overprinted upon the original fibrous arrangement. Most ancient fibrous cements
have not preserved their original mineralogy and are
formed by LMC. In the lower part of the carbonate formation in Mururoa Atoll, Berbey (1989) has shown that the
early marine fibrous HMC cements have undergone
a diagenesis leading to the gradual loss of their Mg content, probably by progressive Ca–Mg substitution in the
calcite lattice.
Sparite cements are generally the most abundant
cements in ancient reef facies and usually consist of
LMC. Several types can be differentiated from the
arrangement and morphologies of crystals and usually
the relative chronology of cements can be established for
petrographical analysis. However, origins of these LMC
sparite cements can be highly diverse, including early diagenesis from surface waters to late burial diagenetic
cementation. As there is no a priori reliable criterion either
geochemical or petrographical, which allows surface/
near-surface sparite from deep burial sparite to be distinguished, the precise origin of these cements together with
the quality of their parent waters remain extremely difficult to infer, depending mainly on the degree of knowledge of the geological context.
Cementation in reefs
Role of cementation
Numerous studies have shown that rapid lithification
through synsedimentary marine cementation is
a widespread feature common to many, if not all, modern
tropical corals reefs (Macintyre, 1977; Friedman, 1998)
and the development of early cements in ancient reefs is
also well known. Therefore, one of the main characteristics of both recent and ancient reefs is that they are being
cemented at the time of their growth.
Although the first primary actors in the reef development are undoubtedly the reef-building organisms edifying a rigid mineralized biological framework, early
lithification of sediment and cementation of the framework can exert also an important role in the edification
of reefs (Purser and Schroeder, 1986). This occurs at
312
DIAGENESIS