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(a)
(b)
Figure 15.2 Playas and pans with contrasting groundwater depths: (a) Chott el Djerid, Tunisia, a discharge playa with groundwater
at the surface (foreground) and evaporate mineral accumulation (background); (b) Silver Lake, USA, a clay-floored recharge playa,
with little or no evaporate mineral accumulation at the surface.
Table 15.1
Origins of closed arid zone basins.
and Lyons, 1991; Jankowski and Jacobson, 1989; Bryant
et al. , 1994a, 1994b). Not only is the role of groundwater
important in pan and playa formation, but there is again
a spectrum of conditions and subsequent effects present.
These range from pans and playas where the groundwater
table intersects the basin surface ( Figure 15.2(a)), accom-
panied by surface evaporite accumulation and evapora-
tive effects, as in the Chotts of Tunisia (e.g. Roberts and
Mitchell, 1987; Bryant et al. , 1994) , to those where the
water table lies at depth (Figure 15.2(b)). These latter fea-
tures are usually clay-floored and percolating groundwater
plays a major role in deep weathering and eluviation. Such
variations are thus a function of topography and geology
rather than climate.
Surfaces are usually vegetation-free, particularly at
their lowest elevations. Episodic flooding, vertisol or
solonchak formation and salt accumulation discourage
vegetation growth, although halophytic plants and shallow
rooting grasses may be established. Grassed pans exist
alongside bare clay surfaces, as in the Kalahari (Boocock
and Van Straten, 1962), suggesting small variations in soil
alkalinity and wind action. Butterworth (1982) has propos-
ed a cycle of development linking grassed and clay pans.
(a) Structural controls
Faulting and rifting
Downwarping
Fracture lines
Intrusions
Differential weathering of adjacent rock types
(b) Erosional controls
Deflation
Subsurface solution and karstic development
Animal scouring
(c) Ponding
Ephemeral or abandoned drainage lines
Floodplain
Interdune troughs
Interstandline troughs
Coastal sedimentation
Drainage disrupted by tilting
(d) Dramatic
Meteor impact
Volcanic crater development
15.1.3 Origins and development of pans
and playas
(Table 15.1 and Figure 15.3). A few have more dramatic
origins: Pretoria Saltpan (South Africa), Zuni Salt Lake
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