6.4 Marine Evaporites
- Primary evaporites. These are precipitated via solar
radiation from a brine pool or brine-filled larger basin
under normal temperature.
- Secondary evaporites. These comprise the bulk of
ancient evaporite beds and are subdivided into three
groups:
(1) Shallow subsurface salt precipitation driven by
solarradiation (e.g. sabkha nodules, described here under the heading ofprimary evaporites).
(2) Evaporites affected by burial diagenesis. Earlier
evaporite beds are partially replaced by other mineral
phases (replacive and cement textures). The subsurface
temperature may be higher than that on the surface.
(3) Subsurface evaporite precipitation as cement or
replacement of non-evaporite matrix.
- Tertiary evaporites. These commonly form afteruplift
and erosion (exhumation) by partial dissolution ofpreexisting evaporite beds. This can occur in a zone of
stagnant water (brine) or active flow (phreatic circulation).
6.4.2 Models for Primary Evaporite Deposition
As a result of an excess in evaporation, the water level
in a basin tends to fall. Simultaneously, both the salt
concentration and the density ofthe water increase. We
distinguish several basic models of primary evaporite
deposition:
Closed sea-water basin.
- Shallow salt lagoon (barred basin or seepage basin).
- Drawdown of water level in deep basin.
Deep-water salt basin.
- Minor evaporite cyc1es controlled by sea-level
changes.
- Coastal sabkhas.
- Combination of different salt basin types.
The first two models are largely based on modem exampIes, whereas the others are deduced from both modem
and ancient examples inc1uding geological reasoning.
The basic processes ofbrine concentration and evaporite
precipitation operate in a similar way in all models.
Main differences result from variations in the tectonic
setting and evolution of the basins. Many ancient
evaporite deposits of relatively small to medium dimension can be interpreted satisfactorily with these models,
but secondary and tertiary processes altering the primary
situation also have to be taken into account (see below).
The Closed Sea-Water Basin
This model is based on the following conditions: (1) the
initial sah content is equal to that of normal sea water;
(2) the full basin is assumed to hold 100 volume units
of water; (3) there is no inflow of sea water or outflow
259
ofbrine (Fig. 6.4a). One can distinguish four stages of
evaporite precipitation:
- Stage 1. The water volume is reduced to about 30%
and its density is raised to 1.126 g/cm 3 • Throughout this
period a small quantity (ab out 0.3% ofthe total salt content) is deposited as biogenic carbonate and later, possibly chemically precipitated, as aragonite. Upon the extraction of Ca, the Mg/Ca ratio of the developing
hypersaline water increases and thus favors the early
diagenetic transformation of aragonite and calcite to
dolomite.
- Stage 2. The brine volume is reduced from about 30%
to 10% (density 1.214 g/cm 3 ) by evaporative drawdoWll.
During this interval, gypsum is precipitated (3 .5% ofthe
total salt content). The formation of anhydrite at normal
field temperatures (:$25 oe) is possible only in the last
phase ofthis period, when the concentration ofthe brine
has nearly reached saturation for halite.
- Stage 3. The brine volume dropsbelow 10% resulting
in the precipitation ofthe maj or part of rock sah (halite).
The halite crystals usually start to grow at the air-water
interface before they settle through the water colurnn.
Precipitation of some anhydrite may continue, particularly in winter time or during periods oftemporary brine
dilution caused by ephemeral fresh water inflow.
- Stage 4. The final stage ofthis "static brine concentration" begins with a volume of about 6% (density 1.257
g/cm 3 ) and is characterized, in addition to continuing
halite precipitation, by the deposition of chlorides and
sulfates of K and Mg (partly bittern salts, 18% of the
total sah contentofthe original sea water). Halite makes
up about 78% of all salts precipitated (cf. Fig. 6.4b).
The areal distribution of the different salts reflects the
evaporative drawdown (Fig. 6.4a). The comparatively
poorly soluble carbonates and calcium sulfates occupy
the margin of the basin, whereas the most soluble salts
are found in the center ofthe basin. Such a facies distribution is referred to as a "bull's eye" pattern. The dry
salt pan may be overlain by wind-blown sand and silt.
An ideal vertical section of such a development is
shown in Figure 6.4e.
This model is seldom verified in nature in its pure
form. Additional influx of sea water may augment the
volume of salts precipitated, but cannot change the contribution of the different evaporites to the total salt
body. However, the salt content of a c10sed basin with
initial normal sea water composition is often also supplemented and modified by inflowing surface and
ground water of differing composition. Thus, the basin
is slowly transformed into a continental salt lake or
playa, where additional salts may gain importance (Sect.
2.5).
- Primary evaporites. These are precipitated via solar
radiation from a brine pool or brine-filled larger basin
under normal temperature.
- Secondary evaporites. These comprise the bulk of
ancient evaporite beds and are subdivided into three
groups:
(1) Shallow subsurface salt precipitation driven by
solarradiation (e.g. sabkha nodules, described here under the heading ofprimary evaporites).
(2) Evaporites affected by burial diagenesis. Earlier
evaporite beds are partially replaced by other mineral
phases (replacive and cement textures). The subsurface
temperature may be higher than that on the surface.
(3) Subsurface evaporite precipitation as cement or
replacement of non-evaporite matrix.
- Tertiary evaporites. These commonly form afteruplift
and erosion (exhumation) by partial dissolution ofpreexisting evaporite beds. This can occur in a zone of
stagnant water (brine) or active flow (phreatic circulation).
6.4.2 Models for Primary Evaporite Deposition
As a result of an excess in evaporation, the water level
in a basin tends to fall. Simultaneously, both the salt
concentration and the density ofthe water increase. We
distinguish several basic models of primary evaporite
deposition:
Closed sea-water basin.
- Shallow salt lagoon (barred basin or seepage basin).
- Drawdown of water level in deep basin.
Deep-water salt basin.
- Minor evaporite cyc1es controlled by sea-level
changes.
- Coastal sabkhas.
- Combination of different salt basin types.
The first two models are largely based on modem exampIes, whereas the others are deduced from both modem
and ancient examples inc1uding geological reasoning.
The basic processes ofbrine concentration and evaporite
precipitation operate in a similar way in all models.
Main differences result from variations in the tectonic
setting and evolution of the basins. Many ancient
evaporite deposits of relatively small to medium dimension can be interpreted satisfactorily with these models,
but secondary and tertiary processes altering the primary
situation also have to be taken into account (see below).
The Closed Sea-Water Basin
This model is based on the following conditions: (1) the
initial sah content is equal to that of normal sea water;
(2) the full basin is assumed to hold 100 volume units
of water; (3) there is no inflow of sea water or outflow
259
ofbrine (Fig. 6.4a). One can distinguish four stages of
evaporite precipitation:
- Stage 1. The water volume is reduced to about 30%
and its density is raised to 1.126 g/cm 3 • Throughout this
period a small quantity (ab out 0.3% ofthe total salt content) is deposited as biogenic carbonate and later, possibly chemically precipitated, as aragonite. Upon the extraction of Ca, the Mg/Ca ratio of the developing
hypersaline water increases and thus favors the early
diagenetic transformation of aragonite and calcite to
dolomite.
- Stage 2. The brine volume is reduced from about 30%
to 10% (density 1.214 g/cm 3 ) by evaporative drawdoWll.
During this interval, gypsum is precipitated (3 .5% ofthe
total salt content). The formation of anhydrite at normal
field temperatures (:$25 oe) is possible only in the last
phase ofthis period, when the concentration ofthe brine
has nearly reached saturation for halite.
- Stage 3. The brine volume dropsbelow 10% resulting
in the precipitation ofthe maj or part of rock sah (halite).
The halite crystals usually start to grow at the air-water
interface before they settle through the water colurnn.
Precipitation of some anhydrite may continue, particularly in winter time or during periods oftemporary brine
dilution caused by ephemeral fresh water inflow.
- Stage 4. The final stage ofthis "static brine concentration" begins with a volume of about 6% (density 1.257
g/cm 3 ) and is characterized, in addition to continuing
halite precipitation, by the deposition of chlorides and
sulfates of K and Mg (partly bittern salts, 18% of the
total sah contentofthe original sea water). Halite makes
up about 78% of all salts precipitated (cf. Fig. 6.4b).
The areal distribution of the different salts reflects the
evaporative drawdown (Fig. 6.4a). The comparatively
poorly soluble carbonates and calcium sulfates occupy
the margin of the basin, whereas the most soluble salts
are found in the center ofthe basin. Such a facies distribution is referred to as a "bull's eye" pattern. The dry
salt pan may be overlain by wind-blown sand and silt.
An ideal vertical section of such a development is
shown in Figure 6.4e.
This model is seldom verified in nature in its pure
form. Additional influx of sea water may augment the
volume of salts precipitated, but cannot change the contribution of the different evaporites to the total salt
body. However, the salt content of a c10sed basin with
initial normal sea water composition is often also supplemented and modified by inflowing surface and
ground water of differing composition. Thus, the basin
is slowly transformed into a continental salt lake or
playa, where additional salts may gain importance (Sect.
2.5).