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Bedrock
Yardangs
Shoreline
features
Alluvial fan
Ephemeral streams
and floodplains
Fringing dunes
(lunette)
Dry mudflat
(Efflorescent crust)
Spring deposits
(tufa) and ponds
Saline mudflat (wet)
polygonalised ground
Playa margin
(seepage zone)
Sandflat
Springs
(salt-karst chimneys)
Aquifer flow
Ancient evaporites
and lake sediments
Saline pan
(Ephemeral/perrenial lake)
thick salt crust
Figure 15.4 Idealised diagram of the depositional subenvironments that can occur in closed arid zone playa basins (after Hardie,
Smoot and Eugster, 1978; Chivas, 2007).
sediments are likely to be transported to the basin as dis-
charge generally decreases downstream in arid channels.
Second, where playas occur in a tectonic basin closely
bordered by uplands, alluvial fans at the basin-mountain
interface usually act as buffers in the fluvial system, trap-
ping sediment. Third, aeolian sediments are, by their very
nature, no coarser than sand.
Surface evaporation plays a major role in pan evolu-
tion, together with the complex processes involved in salt
and water transfer at the groundwater-surface water in-
terface, leading to the accumulation of evaporite deposits
on and within near-surface deposits. Where bedrock is
close to, or outcrops at, the playa surface, high rates of
evaporation may favour rapid breakdown by salt weather-
ing (Goudie and Thomas, 1985). The various geomorphic
and hydrologic depositional processes that operate in the
basins are neither mutually exclusive in space nor time
(Bowler, 1986). Consequently, a series of depositional
subenvironments may be present (Hardie, Smoot and Eu-
gster, 1978) (Figure 15.4), including concentric zonation
of salts and sediments (e.g. Rosen, 1991) that may be iden-
tifiable as facies (Magee, 1991). Any individual basin may
possess only some of these environments at any given time
The aggradational attributes of playas contributes to
their usually flat, horizontal surfaces, especially in the
subenvironments subject to inundation. Given the fine-
grained nature of the sediments, any irregularities, includ-
ing those derived from evaporite growth (see Lowenstein
and Hardie, 1985), are smoothed out by water movement
and dissolution when surface water occupies the basin.
Playas with highly infrequent (possibly not recorded in
historical times) surface water inundation may develop
uneven surfaces through evaporite growth or sand dune
development. The extension of dunes (Bowler, 1986) and
fluvial distributaries (Townshend et al. , 1989) on to playa
surfaces from surrounding areas may also lead to uneven
margins.
15.2
Pan hydrology and hydrochemistry
Several sedimentary subenvironments exist in playas and
pans. The processes involved can be grouped into those
resulting in deposition on the basin floor, the basin sub-
surface and the basin margins. Deposition in basin mar-
gin locations is not necessarily directly related to the
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