Geoscience Reference
In-Depth Information
15.1.1
Playa and pan terminology
The term
coastal sabkha
or
sebkha
(Glennie, 1970)
originally applied to saline flats in arid areas that occur
above the level of high tide, but nevertheless receive pe-
riodic marine incursions and associated sediments. They
have many of the features and processes of inland playas
(Yechieli and Wood, 2002); indeed there is a recognised
continuum between coastal flats and inland playas which
receive sea water (Bye and Harbison, 1991). However,
coastal sebkhas are not considered in this chapter.
Arid and semi-arid lacustrine basins have a rich terminol-
ogy, particularly in Asia and the Middle East (Shaw and
Thomas, 1997). The most commonly used terms, pan and
playa, are interchangeable, with an increasing tendency
to use pan for small basins formed by arid zone geo-
morphological, rather than geological, processes (Goudie
and Wells, 1995) and playa for a depression with a saline
surface (Rosen, 1994). However, the many descriptors in
regional usage and with approximately equivalent mean-
ing has led to confusion in terminology. For example, at-
tempts to restrict the application of terms such as sebkha
to the coastal environment have not been widely adhered
to, while recent attempts to introduce broad terms such as
hemi-arid basin (Currey, 1994) have done little to clarify
the issue. A major concern has been the permanence and
provenance of the lacustrine system.
In a review of terminology, Briere (2000) attempted
to group terms into the categories of playa, playa lake
and sabkha (Figure 15.1) and offers the following revised
definitions:
15.1.2
General characteristics
Despite the variability of pans and playas, a number of
common characteristics emerge. Most obvious is that pans
occupy topographic lows, though not necessarily the low-
est areas in enclosed drainage basins, because small pans
can develop almost anywhere in relatively flat arid land-
scapes, e.g. along deranged drainage lines (Osterkamp
and Wood, 1987). With larger playas there is inevitably
a strong geological control on form (e.g. Salama, 1994).
Topographic position and geological framework may both
influence a groundwater regime and hence the forma-
tive processes. This is nowhere more apparent than in
the
boinkas
(groundwater discharge areas) of southeast
Australia (Jacobson, Ferguson and Evans, 1994), where
contemporary playas are nested in the topographic lows
of larger groundwater-controlled landscapes and geolog-
ically older lake basins.
In terms of surface hydrology they are essentially
'closed' systems, having no surface outflow. The dom-
inance of potential evaporation (PE) over precipitation
(P) and other inputs is the essential contributory factor to
closed status. Hydrological inputs may be direct precipi-
tation, surface or subsurface inflow, or any combination of
the three. Standing surface water is ephemeral, account-
ing for the distinct morphological and sedimentological
differences between arid basins and those of more humid
areas. The extent, frequency and length of surface water
occupancy depends on climatological and hydrological
regimes, and is a major source of pan and playa variabil-
ity. Overall, arid and humid closed basins can be viewed
as part of a climate-based spectrum ranging from 0 to 100
% surface water occupancy at its extremes, each with a
distinct morphology and processes (Bowler, 1986).
Much research has focused on the role of groundwater
in pan formation and function, particularly in terms of in-
teraction with surface processes (Fryberger, Schenk and
Krystinik, 1988; Osterkamp and Wood, 1987; Torgersen,
1984; Torgersen
et al.
, 1986; Rosen, 1994, Yechieli and
Wood, 2002; Reynolds
et al.
, 2007) and of the origins of
Playa
. An intercontinental arid zone basin with a negative
water balance for over half of each year, dry for
over 75 % of the time, with a capillary fringe close
enough to the surface such that evaporation will cause
water to discharge, usually resulting in evaporates
(Briere, 2000, p. 3).
Playa lake
. An arid zone feature, transitional between
playa and lake, neither dry more than 75 % of the time
nor wet more than 75 % of the time. When dry, the
basin qualifies as a playa (Briere, 2000, p. 3).
Sabkha
. A marginal marine mudflat where displacive and
replacive evaporate minerals form in the capilliary zone
above a saline water table (Briere, 2000, p. 4).
These definitions have some merit, particularly as they
eliminate the need for the definition of a continental or
inland sabkha, a step not met with approval by some (e.g.
Barth, 2001). They fail, however, to cover clay-floored
pans with little hydrological input such as the features of
Texas and the Kalahari.
A broader definition of
pans
or
playas
can be arid zone
basins of widely varying size and origin, which, although
generally above the present groundwater table, are subject
to ephemeral surface water inundation of variable period-
icity and extent. Their basal and marginal sediments often
display evidence of evaporite accumulation, aeolian de-
