Geoscience Reference
In-Depth Information
15.4.3
Lunette dunes
High
(a)
Potential for
dust emission
Pan
-
margin dunes (Figure 15.4) are important land-
scape features in many regions, including southern Aus-
tralia (Bowler, 1973), the southern Kalahari (Goudie and
Thomas, 1986), Tunisia (Coque, 1979) and Texas (Huff-
man and Price, 1979). Although commonly between
10 and 50 m high, one example in Tunisia rises al-
most 150 m above the basin floor (Goudie and Thomas,
1986). The surfaces of lunettes are frequently vegetated,
which contributes to their development through encour-
aging sediment accretion. Dune size is a function of a
range of factors, but basin size, morphology and sedi-
ment supply are important. Cyclical episodes of lunette
formation have been identified (Thomas
et al.
, 1993;
Dutkiewicz and von der Borch, 1995; Telfer and Thomas,
2007) on the basis of depositional hiatuses and palaeosol
formation.
Individual pans and playas can possess more than one
fringing dune, with as many as three identified on the mar-
gins of some southern Kalahari pans. Differences in mor-
phology, orientation and sedimentology between dunes on
the margins of individual basins have been interpreted as
indicators of changing palaeoenvironmental (wind regime
and hydrological) conditions (e.g. Lancaster, 1978b). In
the southern Kalahari some pans possess an outer quartz
sand lunette and an inner form that has a higher silt and
clay content of between 12 and 20 % by weight (Lan-
caster, 1978b). The former are interpreted as the outcome
of deflation in the initial stages of pan development from
the sandy Kalahari floor, under relatively dry conditions in
an arid environment, in a manner comparable to parabolic
dune development from partially vegetated surfaces. Con-
versely, in Australia, the orientation of quartz-rich fring-
ing dunes reflects wet-season winds. They have therefore
been seen as the outcome of deflation from pan and playa
beach sediments during periods of high or seasonal lakes,
in a manner comparable to coastal dune development (see,
for example, Twidale, 1972).
Importance has been attached to the deflation of clay
pellets from seasonally dry pan surfaces in the devel-
opment of the clay lunettes of Australia (e.g. Bowler,
1973, 1986) and this is also the mechanism that Lan-
caster (1978b) ascribes to the development of the inner
sandy-clay dunes found on pan margins in the southern
Kalahari. As pellet formation is dependent on the break-
down of basin-floor clays by salt efflorescence (Australian
lunettes also contain high percentages of gypsum as well
as clay), clay lunette development is unlikely in extremely
dry or surface water-dominated environments (Bowler,
1986).
Low
Ground
surface
Capillary-
fringe zone
Potentiometric
surface
Unusually
inundated,
moist, or
crusted
Surface
characteristics
Potentialy fluffy,
very soft
Puffy,
soft
Compact,
hard
Lake or
Wetland
Wet Playa
Dry Playa
Setting
(b)
Figure 15.14
Dust emissions from playas. (a) A model, draw-
ing on the classification system of Rosen (1994), which out-
lines the importance of groundwater regime and depth on the
morphology of surface crusts type and dust emission potential
from playas from the southwestern USA (after Reynolds
et al.
,
2007). (b) A dust storm observed emanating from the margins
of Sua Pan, Botswana (photo courtesy of Frank Eckardt).
USA (Figure 15.14(a)). Pelletier (2006) has also been able
to model the relationships that may exist between climate
forcing, groundwater levels and dust emission magnitudes
for one of these playas (Soda Lake, USA), and outlined
similar groundwater-controlled influences on the emis-
sion of dust from these sources. It is apparent that the
success with which playa hydrologic dynamics of this na-
ture are integrated into models that simulate the regional
or global dust cycle will impact significantly on their abil-
ity/inability to predict future dust emission magnitudes
(e.g. Thomas and Wiggs, 2008). In summary, playa type,
size and setting directly influence playa hydrology and
surface sediment characteristics, which, in turn, have been
identified as being important controls on the type and
amount of atmospheric dust emitted from playas. Figure
15.14(b) documents a typical dust storm emanating from
a playa surface (Sua Pan, Botswana).
