Geoscience Reference
In-Depth Information
groundwater outflow and are commonly associated with
accumulations of evaporite deposits (e.g. Clarke, 1994).
The role of groundwater as a factor in pan forma-
tion and development has been most closely considered
through studies in Australia (e.g. Bowler, 1986; Torg-
ersen
et al.
, 1986; Jacobson, Ferguson and Evans, 1994;
Dutkiewicz and von der Borch, 1995; Boggs
et al.
, 2006;
Cupper, 2006; Harrington, Herczeg and La Salle, 2008),
the Americas (e.g. Osterkamp and Wood, 1987; Paine,
1994; Rosen, 1994; Risacher, Alonso and Salazar, 2003;
French
et al.
, 2006; Risacher and Fritz, 2009; Scuderi,
Laudadio and Fawcett, 2010) and in the Kalahari (e.g.
Butterworth, 1982; Farr
et al.
, 1982; Thomas
et al.
, 1993;
Eckardt
et al.
, 2008), with other basins fed by ground-
water seepage across North Africa described by Boulaine
(1954), Coque (1962), Glennie (1970), Bryant (1999),
Bryant and Rainey (2002) and Hamdi-Aissa
et al.
(2004).
Groundwater can operate as a factor in playa devel-
opment in three main ways. First, percolating groundwa-
ter can lead to pan-floor subsidence by direct dissolution
processes (Baker, 1915; Judson, 1950; Osterkamp and
Wood, 1987), as discussed in more detail in Chapter 15.
Second, a long-term reduction in groundwater head levels
can lead to a change in the status of a playa, e.g. from dis-
charge to recharge as groundwater flow is focused towards
other lower-lying playas in a region. Such a change may
be associated with a concomitant decrease in the surface
salinity, in turn promoting an increase in the vegetation
cover and ultimately leading to a change in the distribu-
tion of discharge playas by a process of playa capture
(Jacobson and Jankowski, 1989). Third, the subsurface
watertable can also act as a base level for wind deflation
(Bowler, 1986; Thomas
et al.
, 1993). Groundwater dis-
solution and playa capture will now be considered, with
the links between groundwater and aeolian deflation dis-
cussed in the final section of this chapter.
The processes of deep-weathering and bedrock disso-
lution outlined in the previous section have also been
proposed as possible mechanisms (along with deflation,
biogenic activity, volcanism, tectonism and meteorite im-
pact; see Goudie and Thomas, 1985, 1986; Goudie and
Wells, 1995; Sanchez
et al.
, 1998) in the formation of
playas. Osterkamp and Wood (1987) and Wood and Os-
terkamp (1987) have proposed a lithologically specific
groundwater solution model for the development of clay-
floored basins based upon observations in the Southern
High Plains of Texas and New Mexico, substantiated by
mass-balance calculations (see Chapter 15). These authors
suggest that deepening and expansion of a playa-floor area
occurs essentially by dissolution and removal of material
beneath the playa surface. The infiltration, weathering and
ter (Zartman, Evans and Ramsey, 1994), along with the
removal to the subsurface of clastic material along solu-
tional pipes, is suggested to lead to the gradual subsidence
of the playa surface. Subsidence is a particularly impor-
tant mechanism in the Southern High Plains in areas where
many playas are underlain by evaporite-bearing Permian
bedrock (Paine, 1994). The dissolution of the Permian
strata is suggested to have been continuous throughout
the deposition of later formations during the Neogene and
may be occurring today.
In addition to operating directly as a factor in playa-
floor dissolution, groundwater may also be an important
control on the hydrological and sedimentological charac-
teristics of a playa through time. The depth of the wa-
ter table beneath a playa surface will vary in response
to seasonal and longer-term drought and also due to re-
gional climatic change. A long-term reduction in the level
of groundwater head has been suggested to cause the
migration of playas by a process termed
playa capture
(Jacobson and Jankowski, 1989). This process, illustrated
by Figure 16.9, is broadly analogous to river capture and
results from a shift in subsurface groundwater flow aris-
ing from the combination of a fall in regional groundwater
tables and the preferential deepening of one playa floor
relative to adjacent basins. The model is based upon stud-
ies of discharge playas near Curtin Springs, central Aus-
tralia, an area where groundwater head is known to have
decayed over several thousand years (Jacobson, Arakel
and Chen, 1988). In this region, Samphire Lake contains
thick deposits of groundwater-derived gypsum, indicat-
ing that it was a previously active discharge playa. Low-
ering of the water table has reduced the levels of saline
groundwater outflow on to the playa surface, allowed the
encroachment of vegetation on to the playa and focused
groundwater discharge into the neighbouring Spring and
Glauberite Lakes. The dry playa surfaces are thus rendered
susceptible to both aeolian sediment deflation as well as
alluviation by sediments from around the playa periphery.
This may, in part, explain the occurrence of groundwater
discharge-derived playa sediments now buried by aeolian
material. The decay in groundwater head therefore results
in a variation in the spatial distribution of active and aban-
doned groundwater discharge playas through time (Jacob-
son and Jankowski, 1989). Glauberite Lake will become
the primary focus of groundwater activity in this region,
ultimately forming a regional groundwater sump.
16.4
Groundwater and aeolian processes
