Geoscience Reference
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Erosional processes, such as deflation, contribute to
the formation of the larger structural basins, as in the
Qattara and Siwa Depressions, Egypt (Gindy, 1991), but
are especially important in the genesis of smaller local
or subregional-scale features. Both aeolian deflation and
removal of material by solution during deep weathering
have been proposed as erosional mechanisms, but, as the
debate on small depressions in, for example, Texas and
New Mexico (Reeves, 1966; Carlisle and Marrs, 1982;
Osterkamp and Wood, 1987; Wood and Osterkamp, 1987)
shows, there is a strong case for a polygenetic origin for
many small pans.
Deflation has often been cited as an originator or con-
tributor in pan development, e.g. Egypt (Haynes, 1980),
the Kalahari (Lancaster, 1978a), Australia (Hills, 1940;
Bowler, 1973), Texas (Reeves, 1960), the Argentine Pam-
pas (Tricart, 1969) and Zaire (de Ploey, 1965). For de-
flation to be effective the criteria necessary for aeolian
entrainment must be satisfied, while a near-surface
groundwater table in a playa can act as a base-level control
on the depth of deflation. Of special importance is the sus-
ceptibility of surfaces to deflation (Goudie and Thomas,
1985), both in terms of material susceptibility and in the
absence of a protective vegetation cover. The latter may be
effected by concentration of salts (Le Roux, 1978) or sea-
sonal surface inundation (Bowler, 1986). In this respect
Osterkamp and Wood's (1987) observation that any slight
depression in an otherwise flat surface has the potential to
develop into a pan or playa should be noted.
The role of deflation in playa and pan development
may be indicated by the presence of fringing transverse
or lunette (Hills, 1940) dunes on the downwind margin of
the depression, or, indeed, by orientation of the pan trans-
verse to prevailing winds (see Le Roux, 1978; Goudie
and Thomas, 1985; Bowler, 1986). Some authors (e.g.
Wood and Osterkamp, 1987) have opposed the deflation-
ary hypothesis on the grounds that the volume of sand
in the fringing dune does not represent the volume re-
moved from the pan. However, deflated sediment can be
transported beyond the margins of depressions and into
the atmospheric circulation (Reheis, 2006). A more se-
rious objection is the observation that, in many Kalahari
pans, the material comprising the lunette have different
sediment characteristics to the pan surface (Goudie and
Thomas, 1986).
Solution, piping and subsurface karstic collapse may
be locally important mechanisms in areas underlain by
carbonate and other sedimentary lithologies. Wood and
Osterkamp (1987) propose a model for the formation of
small clay-floored pans based on studies of the Texas
High Plains, where pan development has taken place in
a variety of sedimentary strata in response to lowering of
the regional water table. During initial development, de-
pressions originate by various means, including deflation,
drainage ponding and along structural lineaments. Proto-
basins act as sites of seasonal runoff concentration on the
relatively flat plains surface and through which groundwa-
ter recharge occurs. This results in sub-basin locations in
the unsaturated zone becoming foci for oxidation and car-
bonate dissolution, leading to piping development and the
disintegration of the calcrete, thus contributing to basin
enlargement.
This model accords well with other regions of small
pans unaffected by groundwater inputs, such as the Kala-
hari, although here percolating water rarely reaches the
water table under present climatic conditions, leading to
the precipitation of fresh calcrete at depth. The contention
of Wood and Osterkamp (1987) that pans are capable of
enlargement by peripheral weathering is backed by the
observation of Farr et al. (1982) that some pans in the
Kalahari are capable of migration over a long period of
time.
The excavations and trampling of animals were seen as
important factors in forming depressions by early investi-
gators in Texas (Gilbert, 1895) and the Kalahari (Allison,
1899; Passarge, 1904). While clearly inapplicable to the
evolution of larger basins, animal activity has been ob-
served to contribute to depression development in areas
of seasonally limited water supplies (Weir, 1969; Ayeni,
1977; see Thomas, 1988, for a review). Termites have
also been implicated in the formation of small, highly
saline pans on islands in swamp ecosystems, such as
the Okavango Delta (McCarthy, McIver and Cairncross,
1986).
The mechanisms proposed require suitably suscepti-
ble surfaces. In southern Africa, pans are preferentially
found on lithologies that readily break down to fine-
grained sediments or which are generally poorly consol-
idated (Goudie and Wells, 1995). Susceptibility may be
enhanced in lithologies that contain significant amounts
of sodium sulfate, which enhances salt weathering and re-
tards plant growth, or clays such as bentonite, which have
high coefficients of expansion on hydration. Extensive
low-relief terrain also seems to favour pan development,
as in Texas and southern Africa. Such surfaces limit the
potential to develop integrated drainage and promote the
concentration of both moisture and fine-grained clastic
material into surface depressions.
Playas are also aggradational features, deriving sed-
iment through episodic inflows or aeolian inputs. The
sediments that are received are almost exclusively fine
grained, which can be explained in three ways. First,
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