The botryoidal aragonite cement (Figure 1b and c) was
first described from Holocene sediments of the Persian
Gulf (Shinn, 1969), from Bermuda cup reefs (Schroeder,
1972) and from the Holocene reef of Belize (Ginsburg
and James, 1976). Pleistocene examples have also been
described in details from the Red Sea, Loyalty Islands
(Ouvea), and Mururoa Atoll by Aïssaoui (1985), Aïssaoui
and Purser (1985), and Aïssaoui et al. (1986). The botryoidal aragonite cement forms individual and/or coalescent
knolls of compact fibrous aragonite crystals with
a spherolithic arrangement (Figure 1b and c). These
botryoids vary in size between one to several centimeters
until tens of centimeters in diameter and they have
a typical translucent honey brown color in section. Petrographically, it appears that each fan of aragonite fibers
presents a typical sweeping extinction under crossed
nichols (Figure 1c). The individual crystals forming these
fans are euhedral fibers 2–10 mm in diameter and several
hundreds of microns in length. The botryoidal aragonite
cement occurs in reef open cavities, usually on the reef
wall or reef slope, where it can alternate with internal
marine sediment or even be affected itself by marine
microborings. From the various examples described in
the literature, there is no evidence of substrate control,
as the aragonite botryoids were observed growing on
calcite, including HMC (Figure 1b), or aragonite
substrates. The Sr content of this aragonite cement is
about 8,000–10,450 ppm and hence characteristic of
an aragonite precipitated from normal marine waters.
The composition in other trace elements and stable carbon
and oxygen isotopes analyzed from the Holocene and
Pleistocene botryoidal cements is also in good agreement
with a precipitation from normal marine water (Ginsburg
and James, 1976; Aïssaoui, 1986), although Aïssaoui
(1986) described a Miocene aragonite botryoidal cement
of karstic origin.
High-magnesian calcite (HMC) cements commonly
constitute the most frequent and most volumetrically
important cements in Holocene and Pleistocene reef
frameworks (Figure 1d), and have been largely described
in the literature. HMC cements are morphologically much
more diverse than their aragonite counterparts, with morphologies ranging from fibrous and palissadic types to
sparitic and micritic cements.
The fibrous HMC cements (Figure 1e) form single
or several isopachous successive layers fringing primary
inter- or intra-granular voids. The thickness formed
by several cement layers can reach a few centimeters
in some cases, such as in the Mururoa Atoll rim
(Repellin, 1977; Buigues, 1982; Aïssaoui, 1986). In thin
sections, they show a compact bundle arrangement, each
individual bundle presenting a sweeping extinction
under crossed nichols. Individual crystallites forming
the bundles are not clearly distinguishable, even under
the SEM. They correspond to fibers of several tens to
hundreds of micrometers length, having anhedral shapes
and grouped together in divergent units, several of them
forming the bundle.
Bladelike forms of HMC cements or bladed spar have
also been described, particularly in Belize, Mururoa, and
Loyalty Islands (James and Ginsburg, 1979; Aïssaoui,
1986; Carrière, 1987). They consist of elongated crystals
with angular terminations and triangular sections, which
currently form palissadic layers (Figure 1f ). Individual
calcite crystals are typically of 20–50 mm width for
a few hundreds of microns in length. The bladed HMC
type commonly occurs together with the fibrous HMC
cement in the same facies and even in the same void, in
addition, lateral gradation from one form to the other has
already been described.
HMC sparite or stubby spar cements seem to be less
common and mainly restricted to small intergranular voids
in Belize and Mururoa. This cement forms isopachous
fringes composed of broadly triangular crystals, a few tens
to hundreds of microns large, gradually widening from the
substrate upon which they grow toward the pore center.
Under SEM, each calcite crystal appears composed of
many thin elongated crystallites, with hardly discernible
external limits. This type of cement can be considered as
a variety of the bladed-spar cement.
A different blocky HMC sparite forming mosaics of
equant 20–60 mm crystals has been also described in Bermuda and Bahamas reefs (Schroeder, 1972; Pierson and
Shinn, 1985) and has been also considered as precipitated
from open marine waters. This cement seems, however, to
be relatively rare.
HMC micrite cements are ubiquitous in most Holocene
and Pleistocene reefs although its volumetric importance
is highly variable from one site to the other. The magnesian micrite cement consists of small curved-face rhombs,
a few microns in size, forming a thin layer about10 mm
thick, lining surface grains. They should be distinguished
from micritic envelopes resulting from the algal-microbial
micritized external parts of bioclasts. As underlined by
James and Ginsburg (1979), this cement seems to be, in
most cases, the first to precipitate and may be followed
by any other type of early cement, either calcite or aragonite. It should be noted, however, that when this cement is
abundant, the questionable origin of micrite as cement precipitated from seawater or deposited lime mud, starts to be
a true problem.
Their Mg content of these cements usually varies
between about 10 to more than 15 mol% MgCO 3 , in most
of the Quaternary reefs described in the literature. In the
Mururoa reef facies, there is a good correlation between
the mean Mg content and the type of cement, with cements
formed of fibrous or elongated crystals bearing the highest
Mg content, which confirmed the effect of Mg on the
development of fibrous calcite shown by the experiments
of Badiozamani et al. (1977) on carbonate precipitation
(Aïssaoui, 1986). The geochemical composition of HMC
cements, including trace elements and stable isotopes of
carbon and oxygen, is largely considered as in accordance
with a marine origin.
Low-magnesian calcite (LMC) cements are common
although usually not forming an important volume of the
DIAGENESIS
311
first described from Holocene sediments of the Persian
Gulf (Shinn, 1969), from Bermuda cup reefs (Schroeder,
1972) and from the Holocene reef of Belize (Ginsburg
and James, 1976). Pleistocene examples have also been
described in details from the Red Sea, Loyalty Islands
(Ouvea), and Mururoa Atoll by Aïssaoui (1985), Aïssaoui
and Purser (1985), and Aïssaoui et al. (1986). The botryoidal aragonite cement forms individual and/or coalescent
knolls of compact fibrous aragonite crystals with
a spherolithic arrangement (Figure 1b and c). These
botryoids vary in size between one to several centimeters
until tens of centimeters in diameter and they have
a typical translucent honey brown color in section. Petrographically, it appears that each fan of aragonite fibers
presents a typical sweeping extinction under crossed
nichols (Figure 1c). The individual crystals forming these
fans are euhedral fibers 2–10 mm in diameter and several
hundreds of microns in length. The botryoidal aragonite
cement occurs in reef open cavities, usually on the reef
wall or reef slope, where it can alternate with internal
marine sediment or even be affected itself by marine
microborings. From the various examples described in
the literature, there is no evidence of substrate control,
as the aragonite botryoids were observed growing on
calcite, including HMC (Figure 1b), or aragonite
substrates. The Sr content of this aragonite cement is
about 8,000–10,450 ppm and hence characteristic of
an aragonite precipitated from normal marine waters.
The composition in other trace elements and stable carbon
and oxygen isotopes analyzed from the Holocene and
Pleistocene botryoidal cements is also in good agreement
with a precipitation from normal marine water (Ginsburg
and James, 1976; Aïssaoui, 1986), although Aïssaoui
(1986) described a Miocene aragonite botryoidal cement
of karstic origin.
High-magnesian calcite (HMC) cements commonly
constitute the most frequent and most volumetrically
important cements in Holocene and Pleistocene reef
frameworks (Figure 1d), and have been largely described
in the literature. HMC cements are morphologically much
more diverse than their aragonite counterparts, with morphologies ranging from fibrous and palissadic types to
sparitic and micritic cements.
The fibrous HMC cements (Figure 1e) form single
or several isopachous successive layers fringing primary
inter- or intra-granular voids. The thickness formed
by several cement layers can reach a few centimeters
in some cases, such as in the Mururoa Atoll rim
(Repellin, 1977; Buigues, 1982; Aïssaoui, 1986). In thin
sections, they show a compact bundle arrangement, each
individual bundle presenting a sweeping extinction
under crossed nichols. Individual crystallites forming
the bundles are not clearly distinguishable, even under
the SEM. They correspond to fibers of several tens to
hundreds of micrometers length, having anhedral shapes
and grouped together in divergent units, several of them
forming the bundle.
Bladelike forms of HMC cements or bladed spar have
also been described, particularly in Belize, Mururoa, and
Loyalty Islands (James and Ginsburg, 1979; Aïssaoui,
1986; Carrière, 1987). They consist of elongated crystals
with angular terminations and triangular sections, which
currently form palissadic layers (Figure 1f ). Individual
calcite crystals are typically of 20–50 mm width for
a few hundreds of microns in length. The bladed HMC
type commonly occurs together with the fibrous HMC
cement in the same facies and even in the same void, in
addition, lateral gradation from one form to the other has
already been described.
HMC sparite or stubby spar cements seem to be less
common and mainly restricted to small intergranular voids
in Belize and Mururoa. This cement forms isopachous
fringes composed of broadly triangular crystals, a few tens
to hundreds of microns large, gradually widening from the
substrate upon which they grow toward the pore center.
Under SEM, each calcite crystal appears composed of
many thin elongated crystallites, with hardly discernible
external limits. This type of cement can be considered as
a variety of the bladed-spar cement.
A different blocky HMC sparite forming mosaics of
equant 20–60 mm crystals has been also described in Bermuda and Bahamas reefs (Schroeder, 1972; Pierson and
Shinn, 1985) and has been also considered as precipitated
from open marine waters. This cement seems, however, to
be relatively rare.
HMC micrite cements are ubiquitous in most Holocene
and Pleistocene reefs although its volumetric importance
is highly variable from one site to the other. The magnesian micrite cement consists of small curved-face rhombs,
a few microns in size, forming a thin layer about10 mm
thick, lining surface grains. They should be distinguished
from micritic envelopes resulting from the algal-microbial
micritized external parts of bioclasts. As underlined by
James and Ginsburg (1979), this cement seems to be, in
most cases, the first to precipitate and may be followed
by any other type of early cement, either calcite or aragonite. It should be noted, however, that when this cement is
abundant, the questionable origin of micrite as cement precipitated from seawater or deposited lime mud, starts to be
a true problem.
Their Mg content of these cements usually varies
between about 10 to more than 15 mol% MgCO 3 , in most
of the Quaternary reefs described in the literature. In the
Mururoa reef facies, there is a good correlation between
the mean Mg content and the type of cement, with cements
formed of fibrous or elongated crystals bearing the highest
Mg content, which confirmed the effect of Mg on the
development of fibrous calcite shown by the experiments
of Badiozamani et al. (1977) on carbonate precipitation
(Aïssaoui, 1986). The geochemical composition of HMC
cements, including trace elements and stable isotopes of
carbon and oxygen, is largely considered as in accordance
with a marine origin.
Low-magnesian calcite (LMC) cements are common
although usually not forming an important volume of the
DIAGENESIS
311