396
Climatic Geomorphology
In mountain deserts and tectonic basins, playa lake models are commonly characterized
by a topographically high margin, which corresponds to the raised block (Figure 16.8). At
the foothills, alluvial fans develop, with coarse gravel and sand proximal facies grading
into silty and clayey distal facies, commonly associated with salts. Finally, in the
topographic lows, a shallow lake develops and evaporites form (Eugster and Hardie, 1975;
Hardie et al., 1978; Anad6n, 1989). Most of these depressions are located in tectonicallyactive areas, with high subsidence rates, and consequently, thick sediment series
accumulate. Many examples occur in the western United States deserts.
Ephemeral lakes located in platform and shield deserts commonly have very low
topographic gradients and thin sediment sequences composed of sand, silt, and clay with
some evaporites. In many cases, the rocky substrate crops out in the middle of the
depressions. These lakes show large surface area expansions during the recharge periods.
During dry periods, aeolian activity is the dominant process and, if the basin is large
enough, dune systems can develop in different areas, as has happened in Lake Chad
(Central Africa).
4.1. Clayey closed depressions
These are flat areas dominated by clay sediments with minor amounts of sands and silts.
They occur at the bottom of the lake or surrounding the inner evaporite-dominated areas of
the lake (Figure 16.8). Finer particles remain in suspension and deposit more slowly,
whereas coarser silt and clay particles only reach the inner part of the depression during
flooding episodes. Surface waters have commonly low solute concentrations and dissolved
solutes are transported into the saline core of the lake or are mixed with the finer clay
fraction. Desiccation of the muddy sediments form polygon systems characterized by
concave cells if they lack salts, or with convex cells if salts precipitate with the fine clastic
sediments.
An interesting example of formation small clayey closed depressions has been described
by Osterkamp and Wood (1987) in the Texas and New Mexico High Plains. First, small
depressions form by deflation and are partially filled with water. Waters percolate and
slowly dissolve underlying calcrete horizons, which causes subsidence and a widening of
the depressions. These small basins are filled with fine clastic material without salts.
4.2. Saline lakes
Saline lakes are natural laboratories where many active processes can be studied. They
also provide useful modern analogue environments to interpret fossil sedimentary records
(Reeves, 1968). From a geological point of view, they have been intensively studied
during the last two decades, mostly because of their economic significance through their
association with valuable oil and evaporite minerals. They also contain a detailed archive
of past climatic, hydrological, sedimentological, geochemical, and biochemical conditions
(Eugster and Kelts, 1983).
One of the first problems to solve in saline lakes studies is the origin of the salts. In the
Senegal Delta, the marine spray caused by waves is the main salt source for some of the
sebkhas. Sprayed waters evaporate and small salt crystals precipitate and remain floating
Climatic Geomorphology
In mountain deserts and tectonic basins, playa lake models are commonly characterized
by a topographically high margin, which corresponds to the raised block (Figure 16.8). At
the foothills, alluvial fans develop, with coarse gravel and sand proximal facies grading
into silty and clayey distal facies, commonly associated with salts. Finally, in the
topographic lows, a shallow lake develops and evaporites form (Eugster and Hardie, 1975;
Hardie et al., 1978; Anad6n, 1989). Most of these depressions are located in tectonicallyactive areas, with high subsidence rates, and consequently, thick sediment series
accumulate. Many examples occur in the western United States deserts.
Ephemeral lakes located in platform and shield deserts commonly have very low
topographic gradients and thin sediment sequences composed of sand, silt, and clay with
some evaporites. In many cases, the rocky substrate crops out in the middle of the
depressions. These lakes show large surface area expansions during the recharge periods.
During dry periods, aeolian activity is the dominant process and, if the basin is large
enough, dune systems can develop in different areas, as has happened in Lake Chad
(Central Africa).
4.1. Clayey closed depressions
These are flat areas dominated by clay sediments with minor amounts of sands and silts.
They occur at the bottom of the lake or surrounding the inner evaporite-dominated areas of
the lake (Figure 16.8). Finer particles remain in suspension and deposit more slowly,
whereas coarser silt and clay particles only reach the inner part of the depression during
flooding episodes. Surface waters have commonly low solute concentrations and dissolved
solutes are transported into the saline core of the lake or are mixed with the finer clay
fraction. Desiccation of the muddy sediments form polygon systems characterized by
concave cells if they lack salts, or with convex cells if salts precipitate with the fine clastic
sediments.
An interesting example of formation small clayey closed depressions has been described
by Osterkamp and Wood (1987) in the Texas and New Mexico High Plains. First, small
depressions form by deflation and are partially filled with water. Waters percolate and
slowly dissolve underlying calcrete horizons, which causes subsidence and a widening of
the depressions. These small basins are filled with fine clastic material without salts.
4.2. Saline lakes
Saline lakes are natural laboratories where many active processes can be studied. They
also provide useful modern analogue environments to interpret fossil sedimentary records
(Reeves, 1968). From a geological point of view, they have been intensively studied
during the last two decades, mostly because of their economic significance through their
association with valuable oil and evaporite minerals. They also contain a detailed archive
of past climatic, hydrological, sedimentological, geochemical, and biochemical conditions
(Eugster and Kelts, 1983).
One of the first problems to solve in saline lakes studies is the origin of the salts. In the
Senegal Delta, the marine spray caused by waves is the main salt source for some of the
sebkhas. Sprayed waters evaporate and small salt crystals precipitate and remain floating