Desert lakes: playa lakes and sebkhas
397
in suspension forming a whitish fog in the littoral areas. Winds transport this fog inland
and the salt crystals are finally deposited during rainy events. Daily temperature changes
provide another mechanism for crystal deposition. At night, increased relative humidity
causes water molecules to adhere to the hygroscopic salt crystals, increasing their size and
forcing them to settle. During stronger rain events, the salt crystals disperse in the surface
dissolve and are transported to the interior of the sebkhas. Finally, waters evaporate and
salts are precipitated (Tricart, 1956). Marine spray is a very important solute source in
littoral arid regions with strong wave action. The amount of salt of marine origin rapidly
decreases at distances farther away than 100 km from the coast (Mabbutt, 1977).
Another significant solute source is chemical weathering. One example of such types of
chemical reactions is the dissolution of salt-beating rocks, such as the Triassic and Tertiary
formations of some Spanish basins. Salt recycling has been common through geological
time, as it has been shown using isotopic signatures of some of the elements. The final
water compositions after chemical weathering reactions take place depends on the existing
rock minerals, their relative abundance, the degree of weathering, the type of chemical
reactions, and the specific drainage conditions (Eugster and Hardie, 1978).
Finally, another salt source is saline crusts from beaches and littoral sebkhas subjected
to deflation. Wind transport salt mineral particles considerable distances.
Chemical composition and concentration of lake waters from desert lakes are very
variable. The actual mineral composition of the precipitated salt depends on the brine
composition, but other factors are also important. Such factors include for example, brine
concentration, gas diffusion, particularly CO2, mixing of water of different composition,
and temperature changes. The interplay of all these processes on different brine types is
responsible for the precipitation of different mineral assemblages (Table 16.1) (Eugster
and Hardie, 1978).
With increasing chemical concentration in water, the precipitation sequence commonly
starts with Ca-carbonates, and is followed by Ca-Mg-carbonates with increasing Mg 2+
content. Sulphates are the next to precipitate, and as a result, the Mg/Ca ratio increases, and
the brine becomes more conducive to the formation of protodolomite and magnesite. After
carbonate and sulphate precipitation, water concentrations increase by evaporation until
they reach saturation in the most soluble salt minerals (chlorides). As chemical
concentration increases, the different solubility of the salt minerals controls the
precipitation and, as a result, a concentric pattern is common in many saline deposits
(Figure 16.14) (Hunt et al., 1966). Calcium and magnesium carbonates occur in the
outermost areas, sulphates the intermediate, and the inner core is occupied by chlorides
(Figure 11.8).
Saline deposits alternate with fine-grained sediments, as fluvial and aeolian sand, silt,
clay, and organic-rich beds (kerogens) (Figure 16.15). Saline lacustrine sequences reflect
the complex sedimentary evolution of the basins, with flooding and desiccation periods
and a large spatial and temporal variability of the depositional processes (aeolian, fluvial).
The changes in water content in playa lakes define a saline cycle, composed of a
flooding stage, evaporative concentration, and desiccation phases (Bryant et al., 1994).
Other consequences of flooding events are the reworking of the playa lake surfaces, redissolution of salts, and mobilization of fine particles.
The micromorphology of the saline lake surface comprises a large number of
microforms, mostly controlled by the chemical composition and the depth of the water
397
in suspension forming a whitish fog in the littoral areas. Winds transport this fog inland
and the salt crystals are finally deposited during rainy events. Daily temperature changes
provide another mechanism for crystal deposition. At night, increased relative humidity
causes water molecules to adhere to the hygroscopic salt crystals, increasing their size and
forcing them to settle. During stronger rain events, the salt crystals disperse in the surface
dissolve and are transported to the interior of the sebkhas. Finally, waters evaporate and
salts are precipitated (Tricart, 1956). Marine spray is a very important solute source in
littoral arid regions with strong wave action. The amount of salt of marine origin rapidly
decreases at distances farther away than 100 km from the coast (Mabbutt, 1977).
Another significant solute source is chemical weathering. One example of such types of
chemical reactions is the dissolution of salt-beating rocks, such as the Triassic and Tertiary
formations of some Spanish basins. Salt recycling has been common through geological
time, as it has been shown using isotopic signatures of some of the elements. The final
water compositions after chemical weathering reactions take place depends on the existing
rock minerals, their relative abundance, the degree of weathering, the type of chemical
reactions, and the specific drainage conditions (Eugster and Hardie, 1978).
Finally, another salt source is saline crusts from beaches and littoral sebkhas subjected
to deflation. Wind transport salt mineral particles considerable distances.
Chemical composition and concentration of lake waters from desert lakes are very
variable. The actual mineral composition of the precipitated salt depends on the brine
composition, but other factors are also important. Such factors include for example, brine
concentration, gas diffusion, particularly CO2, mixing of water of different composition,
and temperature changes. The interplay of all these processes on different brine types is
responsible for the precipitation of different mineral assemblages (Table 16.1) (Eugster
and Hardie, 1978).
With increasing chemical concentration in water, the precipitation sequence commonly
starts with Ca-carbonates, and is followed by Ca-Mg-carbonates with increasing Mg 2+
content. Sulphates are the next to precipitate, and as a result, the Mg/Ca ratio increases, and
the brine becomes more conducive to the formation of protodolomite and magnesite. After
carbonate and sulphate precipitation, water concentrations increase by evaporation until
they reach saturation in the most soluble salt minerals (chlorides). As chemical
concentration increases, the different solubility of the salt minerals controls the
precipitation and, as a result, a concentric pattern is common in many saline deposits
(Figure 16.14) (Hunt et al., 1966). Calcium and magnesium carbonates occur in the
outermost areas, sulphates the intermediate, and the inner core is occupied by chlorides
(Figure 11.8).
Saline deposits alternate with fine-grained sediments, as fluvial and aeolian sand, silt,
clay, and organic-rich beds (kerogens) (Figure 16.15). Saline lacustrine sequences reflect
the complex sedimentary evolution of the basins, with flooding and desiccation periods
and a large spatial and temporal variability of the depositional processes (aeolian, fluvial).
The changes in water content in playa lakes define a saline cycle, composed of a
flooding stage, evaporative concentration, and desiccation phases (Bryant et al., 1994).
Other consequences of flooding events are the reworking of the playa lake surfaces, redissolution of salts, and mobilization of fine particles.
The micromorphology of the saline lake surface comprises a large number of
microforms, mostly controlled by the chemical composition and the depth of the water