420
9 AUTOCHTHONOUS SEDIMENTS
9.2.6 Dolomite
9.2.6.1 Introduction: Chemical Constraints on Dolomite Formation
The term dolomite is applied both to the mineral Ca.Mg(CO3) 2 and to the rock of this
mineralogical composition. The term dolostone is sometimes used for the latter. The
exact genesis of dolomite is still a fruitful field for research despite many years of field
observation and laboratory research (Morrow, 1982a,b; Wells, 1986; Purser et al., 1994).
Figure 9.7 showed that a high ratio of magnesium to calcium is not necessarily a prerequisite for dolomite precipitation. Normal seawater has an Mg: Ca ratio of about 3" 1.
Dolomite forms in supersaline environment where Mg:Ca ratios exceed this value. It
is noteworthy, however, that dolomite may form at the expense of calcite for Mg: Ca ratios less than 1:1 if the salinity is very low (Folk and Land, 1974).
Conditions necessary for dolomite formation appear to include initial permeability
within the host sediment, coupled with sufficient pressure differential to permit pore
fluid movement, an adequate and continuous supply of magnesium ions, and a fluid
which is undersaturated with respect to calcium ions. The last two of these conditions
seem to be fulfilled in the so-called Dorag model of Badiozamani (1973). Seawater and
freshwater may both be saturated with respect to dolomite and calcite, but mixtures
with between 5 and 50% seawater are undersaturated with respect to calcite and supersaturated with dolomite (Fig. 9.13). This suggests that dolomitization may be expected
where marine and fresh waters mix.
Having now examined the theoretical constraints for dolomite formation it is pertinent to examine the observational evidence for dolomite formation. It has long been
known that there are two main types of dolomite, primary or syngenetic, and secondary
or diagenetic. These are considered in turn.
9.2.6.2 Primary Dolomites
Primary dolomites are defined as those which formed at the time of deposition. There
is discussion as to whether genuine direct precipitation of dolomite occurs, or as to
whether it is in fact a replacement of previously formed minerals, that is, penecontemporaneous rather than strictly primary. Recent dolomite deposits have been described
from many arid hypersaline coasts (termed "sabkha" from the Arabic for salt marsh),
and from warm humid coasts too (Budd, 1997). In some examples it is believed that the
dolomite is a direct precipitate within the pore spaces of aragonite mud. Friedman (1979)
has described such a case from marginal pools of the Red Sea, and vonder Borch (1976)
has cited another from the Coorong Lagoon of Australia. Figure 9.14 illustrates a third.
In other instances, however, the modern dolomite has been interpreted as a replacement
of preexisting aragonite or calcite; see, for example, the accounts by Butler (1969) and
McKenzie (1981) of the sabkha dolomite of Abu Dhabi.
In all of these cases there is general agreement that the dolomites are primary, or
penecontemporaneous. These modern examples are all micritic and cryptocrystalline
with a grain size of less than 1-20/zm. Petrophysically they are like chalk, porous, but
of low permeability. Analogs of these Recent primary dolomites occur in ancient carbonate sequences. They too are characterized by a cryptocrystalline texture and low
permeability. Evidence for their primary origin is provided by their bedded concordant
nature, as opposed to the irregular discordant occurrence of secondary dolomites. They
9 AUTOCHTHONOUS SEDIMENTS
9.2.6 Dolomite
9.2.6.1 Introduction: Chemical Constraints on Dolomite Formation
The term dolomite is applied both to the mineral Ca.Mg(CO3) 2 and to the rock of this
mineralogical composition. The term dolostone is sometimes used for the latter. The
exact genesis of dolomite is still a fruitful field for research despite many years of field
observation and laboratory research (Morrow, 1982a,b; Wells, 1986; Purser et al., 1994).
Figure 9.7 showed that a high ratio of magnesium to calcium is not necessarily a prerequisite for dolomite precipitation. Normal seawater has an Mg: Ca ratio of about 3" 1.
Dolomite forms in supersaline environment where Mg:Ca ratios exceed this value. It
is noteworthy, however, that dolomite may form at the expense of calcite for Mg: Ca ratios less than 1:1 if the salinity is very low (Folk and Land, 1974).
Conditions necessary for dolomite formation appear to include initial permeability
within the host sediment, coupled with sufficient pressure differential to permit pore
fluid movement, an adequate and continuous supply of magnesium ions, and a fluid
which is undersaturated with respect to calcium ions. The last two of these conditions
seem to be fulfilled in the so-called Dorag model of Badiozamani (1973). Seawater and
freshwater may both be saturated with respect to dolomite and calcite, but mixtures
with between 5 and 50% seawater are undersaturated with respect to calcite and supersaturated with dolomite (Fig. 9.13). This suggests that dolomitization may be expected
where marine and fresh waters mix.
Having now examined the theoretical constraints for dolomite formation it is pertinent to examine the observational evidence for dolomite formation. It has long been
known that there are two main types of dolomite, primary or syngenetic, and secondary
or diagenetic. These are considered in turn.
9.2.6.2 Primary Dolomites
Primary dolomites are defined as those which formed at the time of deposition. There
is discussion as to whether genuine direct precipitation of dolomite occurs, or as to
whether it is in fact a replacement of previously formed minerals, that is, penecontemporaneous rather than strictly primary. Recent dolomite deposits have been described
from many arid hypersaline coasts (termed "sabkha" from the Arabic for salt marsh),
and from warm humid coasts too (Budd, 1997). In some examples it is believed that the
dolomite is a direct precipitate within the pore spaces of aragonite mud. Friedman (1979)
has described such a case from marginal pools of the Red Sea, and vonder Borch (1976)
has cited another from the Coorong Lagoon of Australia. Figure 9.14 illustrates a third.
In other instances, however, the modern dolomite has been interpreted as a replacement
of preexisting aragonite or calcite; see, for example, the accounts by Butler (1969) and
McKenzie (1981) of the sabkha dolomite of Abu Dhabi.
In all of these cases there is general agreement that the dolomites are primary, or
penecontemporaneous. These modern examples are all micritic and cryptocrystalline
with a grain size of less than 1-20/zm. Petrophysically they are like chalk, porous, but
of low permeability. Analogs of these Recent primary dolomites occur in ancient carbonate sequences. They too are characterized by a cryptocrystalline texture and low
permeability. Evidence for their primary origin is provided by their bedded concordant
nature, as opposed to the irregular discordant occurrence of secondary dolomites. They