Geoscience Reference
In-Depth Information
Through-
flow
playa
Discharge
playa
Recharge
playa
Terminal discharge
playa
Low-
permeablility
rock
Low-permeablility rock
Permeable rock
Partly
drained
tributary
area
Undrained
closed
basin
Partly drained
closed basin
(through-flow)
Deflation
surface
(net erosion)
Regional
groundwater
sink
Local system
Regional system
Figure 15.9
Summary diagram of hydrologic classifications based upon groundwater hydrology of playa basins (after Rosen,
1994).
1966). This zonation will also be apparent in the texture
of the pan surface, with a transition from the peripheral
clay floor to a soft mud with surface efflorescence, de-
scribed as 'self-raising ground' (Mabbutt, 1977), which
represents the capillary fringe of the groundwater. This,
in turn, gives way to a salt crust whose thickness is de-
pendent on the frequency of surface flooding and ground-
water characteristics, and to a brine layer if present. The
concentric surface zonation of salts may be mirrored by a
vertical zonation as a result of variations in solubility, with
the most soluble minerals at the surface, or as a response
to subsurface processes, particularly reduction (Neev and
Emery, 1967; Rosen, 1991; Bryant
et al.
, 1994a).
surfaces, with little sign of evaporite accumulation. Con-
versely, saline basins with near-surface water tables have
complex transfers of water and salts along physical and
chemical pathways in three dimensions, leading to the
accumulation of surface crusts and displacive evaporites.
Clay-floored pans are characteristic of regimes with
low groundwater input or where the surface lies above
the influence of capillary rise from the water table, a
depth of usually about 3 metres (Rosen, 1994). Usu-
ally they are composed of a flat clay or sandy clay sur-
face, either as the base of a pan or as a higher surround
to a more saline basin (Mabbutt, 1977). The dominant
sediment is clastic material deposited from suspension
during inundation, although lenses of sand may be de-
posited under higher energy conditions. The clay sur-
face, in turn, forms an impervious layer to groundwater
recharge at the pan centre. Reynolds
et al.
(2007) de-
scribe clay-floored pans as 'dry' playas, which are un-
likely to be susceptible to deflation or dust production
(Figure 15.7(a) or (b)).
Salt input is generally low to basins of this type. Precip-
itation of calcium and magnesium is common, producing
a range of carbonates from calcite to dolomite, dependent
upon the Ca/Mg ratio (Muller, Irion and Forstner, 1972),
as cements, laminates, crusts or other structures (Eugster
and Kelts, 1983). Gypsum efflorescence may follow the
15.2.3
The importance of groundwater:
classification of playa and pan types
The position of the water table has been used as the ba-
sis of classification of playas by Mabbutt (1977), Wood
and Sanford (1990) and Rosen (1994) (Figure 15.9). In
pans where the water table lies at depth, features defined
as recharge playas by Rosen (1994), there will be lit-
tle interaction between the surface and groundwater, and
sedimentation will occur in a shallow surface and sub-
surface layer, with overall transfer of water towards the
