lower, and here dolomite can be formed at lower
Mg
2þ
=Ca
2þ ratios because there is very little sulphate.
However, freshwater contains little magnesium, so
Mg
2+ must be supplied by mixing with seawater and
large amounts of seawater must circulate through the
limestone. A steady percolation of freshwater alone
will not lead to any great degree of dolomitisation,
because freshwater will displace the magnesium-rich
salt water, and the mixing zone will be too small.
Seawater is the only source of magnesium for large
scale dolomitisation, except in evaporite sequences
where Mg-rich evaporite minerals can dissolve and
add Mg
2+ to the porewater. The seawater which
circulates inside atolls and reefs is reducing and therefore low in sulphate, at the same time having abundant
magnesium. Reef and atoll facies therefore offer
favourable
conditions
for
dolomitisation.
Dolomitisation of large volumes of limestones would
require vary large sources of magnesium and it is only
very near to the surface that seawater can flow with
sufficient fluid fluxes to supply enough Mg
2+ to cause
pervasive dolomitisation. Freshwater has a low
sulphate content which makes it easier to precipitate
dolomite but it does not explain the supply of such
large quantities of Mg
2+ .
Hydrothermal circulation of seawater will cause
dolomitisation but that will be limited to the flow
path controlled by fractures and permeable layers.
Very little of the flow will penetrate tight rocks, i.e.
micritic limestones.
Massive and extensive dolomites are likely to be
formed at shallow depth near the surface or seafloor in
marine basins. This is favoured by high salinity and
evaporitic conditions.
Dolomite showing fluid inclusion and textural evidence of having formed at higher temperatures may
have been recrystallised at greater depth.
5.7.10.2 Evaporite Model for Dolomitisation
Dolomite is often associated with evaporite
environments. The first definite example of dolomite
being formed today was found in evaporating
sediments in a supratidal environment in the Bahamas
in about 1960. It is clear that when seawater
evaporates, and aragonite and also gypsum
CaSO 4 Á 2H 2 O
ð
Þare precipitated, the composition of
the fraction still in solution will become increasingly
enriched in magnesium, and dolomite will only be
precipitated at high Mg
2þ
=Ca
2þ ratios. In addition
magnesite (MgCO 3 ) may form. This explains why
dolomite is important in most evaporite sequences.
However, evaporite minerals are very soluble, particularly chlorides, and often are not preserved; gypsum
may also dissolve and be replaced by carbonate. Dolomite might therefore have been deposited in an evaporite environment despite the fact that we do not find
any of the original highly soluble salts preserved. For
this reason it is important to look for indirect evidence
of evaporite conditions and solution, including
replacement of evaporite minerals.
The most important indicators of evaporite
conditions are:
1. Absence of ordinary marine fossils, apart from
stromatolites which can tolerate high salinity. In
Palaeozoic and younger deposits stromatolites are
typical of evaporites, because under normal marine
conditions cyanobacteria have too much competition from other organisms.
2. Breccias which may have been formed through
solution of underlying salt deposits so that beds
collapse and form a collapse breccia. Such breccias
are characterised by angular fragments from an
overlying bed, for example of carbonate, which
have fallen down into a solution cavity.
3. Pseudomorphosis (replacement) of evaporite
minerals, e.g. halite (NaCl) and gypsum
CaSO 4 Á 2H 2 O
ð
Þ . Evaporite minerals, which are
disseminated through a matrix of less soluble
minerals such as carbonates, are often replaced
through pseudomorphosis so that the crystal form
may reveal the original mineral. The cubic halite
crystals are typical, and the characteristic swallowtail twins of gypsum crystals are easily recognised
even if they have been replaced by other minerals.
4. Chickenwire structure. Anhydrite often forms very
characteristic nodules or continuous layers which
look like chicken wire. Even if the anhydrite layers
are converted to calcite, these structures may be
preserved.
5. Authigenic quartz and feldspar. Evaporites are often
associated with microcrystalline quartz (chert) and
chalcedony. A high content of authigenic feldspar
and zeolites is also typical of many evaporites. With
low-grade metamorphism (200–300
C) zeolites will
dissolve and be replaced by feldspar.
5 Carbonate Sediments
207
Mg
2þ
=Ca
2þ ratios because there is very little sulphate.
However, freshwater contains little magnesium, so
Mg
2+ must be supplied by mixing with seawater and
large amounts of seawater must circulate through the
limestone. A steady percolation of freshwater alone
will not lead to any great degree of dolomitisation,
because freshwater will displace the magnesium-rich
salt water, and the mixing zone will be too small.
Seawater is the only source of magnesium for large
scale dolomitisation, except in evaporite sequences
where Mg-rich evaporite minerals can dissolve and
add Mg
2+ to the porewater. The seawater which
circulates inside atolls and reefs is reducing and therefore low in sulphate, at the same time having abundant
magnesium. Reef and atoll facies therefore offer
favourable
conditions
for
dolomitisation.
Dolomitisation of large volumes of limestones would
require vary large sources of magnesium and it is only
very near to the surface that seawater can flow with
sufficient fluid fluxes to supply enough Mg
2+ to cause
pervasive dolomitisation. Freshwater has a low
sulphate content which makes it easier to precipitate
dolomite but it does not explain the supply of such
large quantities of Mg
2+ .
Hydrothermal circulation of seawater will cause
dolomitisation but that will be limited to the flow
path controlled by fractures and permeable layers.
Very little of the flow will penetrate tight rocks, i.e.
micritic limestones.
Massive and extensive dolomites are likely to be
formed at shallow depth near the surface or seafloor in
marine basins. This is favoured by high salinity and
evaporitic conditions.
Dolomite showing fluid inclusion and textural evidence of having formed at higher temperatures may
have been recrystallised at greater depth.
5.7.10.2 Evaporite Model for Dolomitisation
Dolomite is often associated with evaporite
environments. The first definite example of dolomite
being formed today was found in evaporating
sediments in a supratidal environment in the Bahamas
in about 1960. It is clear that when seawater
evaporates, and aragonite and also gypsum
CaSO 4 Á 2H 2 O
ð
Þare precipitated, the composition of
the fraction still in solution will become increasingly
enriched in magnesium, and dolomite will only be
precipitated at high Mg
2þ
=Ca
2þ ratios. In addition
magnesite (MgCO 3 ) may form. This explains why
dolomite is important in most evaporite sequences.
However, evaporite minerals are very soluble, particularly chlorides, and often are not preserved; gypsum
may also dissolve and be replaced by carbonate. Dolomite might therefore have been deposited in an evaporite environment despite the fact that we do not find
any of the original highly soluble salts preserved. For
this reason it is important to look for indirect evidence
of evaporite conditions and solution, including
replacement of evaporite minerals.
The most important indicators of evaporite
conditions are:
1. Absence of ordinary marine fossils, apart from
stromatolites which can tolerate high salinity. In
Palaeozoic and younger deposits stromatolites are
typical of evaporites, because under normal marine
conditions cyanobacteria have too much competition from other organisms.
2. Breccias which may have been formed through
solution of underlying salt deposits so that beds
collapse and form a collapse breccia. Such breccias
are characterised by angular fragments from an
overlying bed, for example of carbonate, which
have fallen down into a solution cavity.
3. Pseudomorphosis (replacement) of evaporite
minerals, e.g. halite (NaCl) and gypsum
CaSO 4 Á 2H 2 O
ð
Þ . Evaporite minerals, which are
disseminated through a matrix of less soluble
minerals such as carbonates, are often replaced
through pseudomorphosis so that the crystal form
may reveal the original mineral. The cubic halite
crystals are typical, and the characteristic swallowtail twins of gypsum crystals are easily recognised
even if they have been replaced by other minerals.
4. Chickenwire structure. Anhydrite often forms very
characteristic nodules or continuous layers which
look like chicken wire. Even if the anhydrite layers
are converted to calcite, these structures may be
preserved.
5. Authigenic quartz and feldspar. Evaporites are often
associated with microcrystalline quartz (chert) and
chalcedony. A high content of authigenic feldspar
and zeolites is also typical of many evaporites. With
low-grade metamorphism (200–300
C) zeolites will
dissolve and be replaced by feldspar.
5 Carbonate Sediments
207