Geoscience Reference
In-Depth Information
5.3.1 Desert lakes, playas and sabkhas
catchments and fan environments. Fine mater-
ials are also supplied by aeolian transport. In
addition evaporitic deposits may accumulate,
sourced by evaporation of flood water or ground-
water sources and the resulting concentration
and accumulation of salts (Fig. 5.10). The flat
morphology typically associated with playas is
a function of infrequent inundation by water,
which evens out the microtopography through
a combination of deposition and dissolution.
Playas with a highly irregular topography (e.g.
from salt growth or sand-dune development in
the centre) suggest extremely infrequent inunda-
tion by water, although an uneven topography
from alluvial fans and drainage courses around
the margins is not unusual (Fig. 5.10). To main-
tain a playa depression it is essential that accu-
mulation does not outweigh erosion (typically
dominated by deflation). Deflation is particularly
effective, as the sediments are highly erodible
when exposed due to a combination of dis-
persive agents (sodium), lack of protection from
vegetation and availability of fine sediments
(section 5.2.2.2).
Lake basins act as sediment sinks, particularly in
arid environments. Many arid zone areas either
lack integrated drainage systems or are dominated
by endoreic (internal) systems. Thus, topographic
lows act as foci for runoff collection and associ-
ated sediment deposition. For example, erosion
hollows generated by deflation may occur where
crusts (section 5.3.5) are broken in arid regions.
These hollows can later act as collection areas
for runoff, forming playas. In most cases these
water bodies tend to be supersaturated with salts
(
5000 mg L
−1
) and are ephemeral. The associ-
ated landform is known as a 'playa' or 'pan'. The
playas and pans vary in size from a few square
metres to several thousand square kilometres.
Their spatial significance varies regionally, but
accounts for only 1% of drylands (Shaw &
Thomas 1997). There are a wide range of ter-
minologies used for the same feature, with the
term 'pan' used for small areas originating from
geomorphological (e.g. a depression between
alluvial fans) rather than geological processes
(Goudie & Wells 1995) and 'playa' for a depres-
sion with a saline surface (Rosen 1994).
Sediments trapped in pans and playas are
typically fine-grained and represent the suspended
sediments of distal flood inputs from adjacent
>
5.3.2 Ephemeral streams
Ephemeral systems (both sand and gravel bed)
tend to act as temporary stores for sediment
Fig. 5.10
Death Valley playa with well
developed salt accumulation in the
foreground. Note the irregular surface.
The background consists of alluvial fans
feeding into the edge of the playa.
Polygons are approximately 1 m across.
