260
Shallow Salt Lagoon
(Barred Basin or Seepage Basin)
This basin type is connected with the open sea by a
small opening (Fig. 6.4c). Lowering of the lagoonal
water level by intensive evaporation leads to influx of
surface water from the open sea. Salt concentration and
density of the inflowing water inciease landward and
cause the surface water to sink and flow seaward as
underflow and over the sill back into the sea (brine reflux, cf. Chap. 4, arid adjacent seas). Consequently, salts
are transported continuously into and out of the lagoon.
Because the influx of salts is greater than their reflux,
part ofthe salts can be precipitated in the lagoon. Such
a situation can be described as a "dynamic system of
evaporite deposition".
The type of salts precipitated depends on the salt
concentration which is reached in different parts ofthe
lagoon. If the entrance to the lagoon is relatively large,
the water exchange between open sea and lagoon is little
hampered, and the concentration of the lagoonal water
remains low. With decreasing area of inflow, the salt
concentration in the lagoon increases.
Thus, a certain opening in conjunction with the surface area
ofthe basin may cause a brine concentration which leads to
the precipitation of gypsum in large parts of the lagoon, but
not yet to the crystallization ofhalite. In this case, the ratio of
the areas ofthe evaporite basin and its inlet is often between
10 7 and 10 8 (Lucia 1972). Under these conditions, NaCI is
returned by brine reflux to the open sea before it reaches saturation. Such a situation can be maintained for a considerable
time period and thus enables the deposition of a thick layer of
gypsum or anhydrite.
Consequently, the lagoonal sediments may start with the
preferential deposition of carbonate, followed by gypsum and anhydrite. Then the halite stage is reached and
finallypotash salts (chlorides and sulfates ofK and Mg)
can be precipitated. However, in contrast to the closed
basin model, the thickness or volume of these different
evaporites is no longer a function of normal sea water
composition; it rather depends on how long a certain
stage of evaporite deposition is maintained.
Figure 6.4f shows a vertical section of such a lagoonal evaporite sequence, in which the carbonate and
calcium sulfate stages lasted much longer than the halite
stage and therefore generated comparatively thick layers. In this example, the stage of potash salt precipitation was realized for a short period, but in many cases
in nature neither this nor the halite stage were ever
reached.
Deepening or widening of the inlet, or a relative sealevel rise (e.g. due to rapid subsidence, also see below)
cause again increased water circulation in the lagoon
and thus dilution ofthe brine. Consequently, the development of the depositional system can be reversed with
the result that stages ofhigh salt concentration are followed by the precipitation of lower soluble evaporites
(upper part of section in Fig. 6.4f).
Chapter 6 Special Depositional Environments
Particulady in shallow lagoons, the concentration of
the brine mayaiso change laterally and become higher
in alandward direction. As a result, different evaporites
can form simultaneously as shown in Fig. 6.4c, i.e.,
carbonates precipitate near the entrance of the lagoon,
while rock salt and possibly K and Mg salts are deposited at its landward end. In plan view, such a facies distribution is caHed a "tear drop pattern".
A famous recent exarnple described in many textbooks is the
Kara Bogas Goi (lagoon) on the eastem side ofthe Caspian
Sea. It covers an area of approximately 20 000 km 2 but has a
maximum depth of only 8 m. Although the Caspian Sea has
a salinity of only 1.3% and its salt composition differs from
that of normal sea water, this lagoon inspired Ochsenius in
1877 to propose a silled lagoon as model for the generation
of marine evaporites. This is the "bar theory" (in German
"Barren-Theorie", see, e.g., Sonnenfeld 1984; Müller 1988).
Carbonate and gypsum are precipitated near the entrance of
the KaraBogas lagoon; glauberite (Na2S04.CaS04) andhalite
follow landward.
The sill between the open sea and the lagoon or shallow
adjacent sea is often controlled by tectonic movements, but
mayaiso be generated by sedimentary processes such as reef
structures, nearshore sand bars, or barrier islands (Sect. 3.2).
In the ancient record, shelf carbonates and reefs of different
nature (e.g. algal structures) in front of a reeflagoon are frequently associated with evaporites, because warm climate
favors both carbonate production including reef growth and
high evaporation.
Modifications of the shallow lagoon model are the seepage basin and the synsedimentary subsidence basin.
See page basin. The lagoon is separated from the open
sea by a permeable subaerial bar or ridge (Fig. 6.4d).
Similar to the processes in the silled basin, evaporative
drawdown of the water level in the closed lagoon then
causes sea water to flow underground into the lagoon
and denser brine to flow back into the ocean. Due to the
restricted inflow and reflux, this system creates a transitional situation between a hydrologically closed and an
open silled basin. In contrast to the silled basin model,
the ratios ofthe different salts precipitated tend to deviate less from those given by the composition of normal
sea water. Consequently, the predominant salt in a seepage lagoon will often be halite.
A weil studied example ofthis type ofbasin is the MacLeod
evaporite basin in western Australia (Logan 1987), where
carbonates, gypsum, and halite were precipitated in the Holocene.
The brine reflux in both the subaeriaHy separated and
the barred basin ceases when the permeability of the
underground ridge becomes too poor or the water level
in the lagoon drops too deep to enable reflux. The latter
case is verified for the KaraBogas GoI (see above). In
this manner, a kind of semi-closed system is established
which, in contrast to the fuHy closed basin, is fed by
inflowing sea water for a certain period oftime (also see
below).
Shallow Salt Lagoon
(Barred Basin or Seepage Basin)
This basin type is connected with the open sea by a
small opening (Fig. 6.4c). Lowering of the lagoonal
water level by intensive evaporation leads to influx of
surface water from the open sea. Salt concentration and
density of the inflowing water inciease landward and
cause the surface water to sink and flow seaward as
underflow and over the sill back into the sea (brine reflux, cf. Chap. 4, arid adjacent seas). Consequently, salts
are transported continuously into and out of the lagoon.
Because the influx of salts is greater than their reflux,
part ofthe salts can be precipitated in the lagoon. Such
a situation can be described as a "dynamic system of
evaporite deposition".
The type of salts precipitated depends on the salt
concentration which is reached in different parts ofthe
lagoon. If the entrance to the lagoon is relatively large,
the water exchange between open sea and lagoon is little
hampered, and the concentration of the lagoonal water
remains low. With decreasing area of inflow, the salt
concentration in the lagoon increases.
Thus, a certain opening in conjunction with the surface area
ofthe basin may cause a brine concentration which leads to
the precipitation of gypsum in large parts of the lagoon, but
not yet to the crystallization ofhalite. In this case, the ratio of
the areas ofthe evaporite basin and its inlet is often between
10 7 and 10 8 (Lucia 1972). Under these conditions, NaCI is
returned by brine reflux to the open sea before it reaches saturation. Such a situation can be maintained for a considerable
time period and thus enables the deposition of a thick layer of
gypsum or anhydrite.
Consequently, the lagoonal sediments may start with the
preferential deposition of carbonate, followed by gypsum and anhydrite. Then the halite stage is reached and
finallypotash salts (chlorides and sulfates ofK and Mg)
can be precipitated. However, in contrast to the closed
basin model, the thickness or volume of these different
evaporites is no longer a function of normal sea water
composition; it rather depends on how long a certain
stage of evaporite deposition is maintained.
Figure 6.4f shows a vertical section of such a lagoonal evaporite sequence, in which the carbonate and
calcium sulfate stages lasted much longer than the halite
stage and therefore generated comparatively thick layers. In this example, the stage of potash salt precipitation was realized for a short period, but in many cases
in nature neither this nor the halite stage were ever
reached.
Deepening or widening of the inlet, or a relative sealevel rise (e.g. due to rapid subsidence, also see below)
cause again increased water circulation in the lagoon
and thus dilution ofthe brine. Consequently, the development of the depositional system can be reversed with
the result that stages ofhigh salt concentration are followed by the precipitation of lower soluble evaporites
(upper part of section in Fig. 6.4f).
Chapter 6 Special Depositional Environments
Particulady in shallow lagoons, the concentration of
the brine mayaiso change laterally and become higher
in alandward direction. As a result, different evaporites
can form simultaneously as shown in Fig. 6.4c, i.e.,
carbonates precipitate near the entrance of the lagoon,
while rock salt and possibly K and Mg salts are deposited at its landward end. In plan view, such a facies distribution is caHed a "tear drop pattern".
A famous recent exarnple described in many textbooks is the
Kara Bogas Goi (lagoon) on the eastem side ofthe Caspian
Sea. It covers an area of approximately 20 000 km 2 but has a
maximum depth of only 8 m. Although the Caspian Sea has
a salinity of only 1.3% and its salt composition differs from
that of normal sea water, this lagoon inspired Ochsenius in
1877 to propose a silled lagoon as model for the generation
of marine evaporites. This is the "bar theory" (in German
"Barren-Theorie", see, e.g., Sonnenfeld 1984; Müller 1988).
Carbonate and gypsum are precipitated near the entrance of
the KaraBogas lagoon; glauberite (Na2S04.CaS04) andhalite
follow landward.
The sill between the open sea and the lagoon or shallow
adjacent sea is often controlled by tectonic movements, but
mayaiso be generated by sedimentary processes such as reef
structures, nearshore sand bars, or barrier islands (Sect. 3.2).
In the ancient record, shelf carbonates and reefs of different
nature (e.g. algal structures) in front of a reeflagoon are frequently associated with evaporites, because warm climate
favors both carbonate production including reef growth and
high evaporation.
Modifications of the shallow lagoon model are the seepage basin and the synsedimentary subsidence basin.
See page basin. The lagoon is separated from the open
sea by a permeable subaerial bar or ridge (Fig. 6.4d).
Similar to the processes in the silled basin, evaporative
drawdown of the water level in the closed lagoon then
causes sea water to flow underground into the lagoon
and denser brine to flow back into the ocean. Due to the
restricted inflow and reflux, this system creates a transitional situation between a hydrologically closed and an
open silled basin. In contrast to the silled basin model,
the ratios ofthe different salts precipitated tend to deviate less from those given by the composition of normal
sea water. Consequently, the predominant salt in a seepage lagoon will often be halite.
A weil studied example ofthis type ofbasin is the MacLeod
evaporite basin in western Australia (Logan 1987), where
carbonates, gypsum, and halite were precipitated in the Holocene.
The brine reflux in both the subaeriaHy separated and
the barred basin ceases when the permeability of the
underground ridge becomes too poor or the water level
in the lagoon drops too deep to enable reflux. The latter
case is verified for the KaraBogas GoI (see above). In
this manner, a kind of semi-closed system is established
which, in contrast to the fuHy closed basin, is fed by
inflowing sea water for a certain period oftime (also see
below).