Geoscience Reference
In-Depth Information
marker for defining slip rates in North America (Spelz
et al.
, 2008).
of incorporation of aeolian fines has been slow enough
to permit cumulate soil development and to cause uplift
of pre-existing soil pavements. Varnish data from alluvial
fans in the southwestern United States suggest that varnish
development has occurred on stones exposed continuously
since abandonment of the fan surface (no stones reflect the
emergence of clasts during the Holocene into Late Pleis-
tocene pavements, as would develop if there was upward
migration of gravel). This suggests that desert pavements
(1) are born at the land surface and (2) remain at the land
surface via aeolian deposition and simultaneous devel-
opment of cumulate soils beneath the pavements. Pave-
ments appear to require some degree of stability for their
full development, occurring beyond the limits of modern
drainage or standing above active channels through dis-
section or tectonic processes. Formation times are related
to rates of salt and dust accumulation, a function of the
distance from source areas (particularly playas) and the
prevailing wind direction (Pelletier, Cline and DeLong,
2007). Leaching rates also influence soil formation. Thus,
stone pavement development is strongly linked to long-
term climate change.
Desert pavements play an important ecohydrologic role
in deserts. Mature pavements form a strongly interlock-
ing carapace that sheds water rapidly. Plants are largely
absent from pavements owing to salts within the soil and
to rapid runoff from the impermeable surface. However,
vegetation in adjacent wadis or dune areas benefits from
the enhanced water supply provided by the runoff. If pave-
ment surfaces are extensive, they tend to increase the flood
potential of a region.
Stony surfaces remain an important focus of geologic,
geomorphic, hydrologic and archaeologic studies in arid
lands. To date, detailed studies of pavements have largely
focused on the American southwest and Israel. The recent
expansion of research into new regions has increased our
understanding of the diversity of pavement types and pro-
cesses, but a more systematic approach to pavement and
hamada studies is required in order to understand the sim-
ilarities and dissimilarities of stony surface development
across the desert realm.
9.11
Conclusions
Contemporary desert landscapes reflect the integration of
geologic conditions and geomorphic processes operating
over long time frames. They thus reflect a succession
of former events and environments. Stony surfaces are
widespread and show a range in forms associated with
geologic conditions, age and process. A continuum of
rocky or gravelly surfaces develops from hamada through
desert pavement. There is potentially a range of processes
responsible for their formation. Cooke (1970) suggested
that pavements may illustrate the concept of equifinality,
with similar forms produced in different areas by a variety
of processes operating on different initial conditions. It is
likely that a single set of processes cannot explain all
occurrences of gravel-covered surfaces.
In sandy soils, deflation may play a role. Deflated sur-
faces are recorded from gobi (China), gibber plains (Aus-
tralia) and possibly the Martian landing sites. Unlike ac-
cretionary pavements, however, there has been little de-
tailed study of the pavement of deflation. The presence of
sand in the groundmass between separated clasts is not
sufficient to claim that the pavement has formed by wind
erosion.
Where sand is largely absent, pavements appear to de-
velop by a variety of geomorphic processes that result
in stone sorting, surface creep and stone displacement,
clast disintegration and syndepositional lifting of surface
clasts by the accumulation of salt- and carbonate-rich ae-
olian fines (Dohrenwend, 1987). These processes operate
at different levels of relative importance as the pavements
evolve. Aeolian processes have significantly influenced
pavement soil development. The silt, clay, carbonates and
soluble salts accumulated in desert soils are attributable
largely to the incorporation of windblown dust rather than
to chemical weathering of soil parent materials. Although
Quaternary aeolian activity has been episodic, the rate
Box 9.1
Pavements of the Mojave Desert: the Cima and Pisgah Volcanic Fields
The lava flows of the Mojave Desert, California, have provided the basis for some of the most detailed research
on the relationship between aeolian deposition, pedogenesis and pavement formation (Dohrenwend
et al.
, 1984;
Wells
et al.
, 1985, 1995; McFadden, Wells and Jercinovich, 1987; Anderson, Wells and Graham, 2002; Helms,
McGill and Rockwell, 2003; Williams and Zimbelman, 1994; Wood, Graham and Wells, 2002; Graham
et al.
,
2008; Meadows, Young and McDonald, 2008). The best studied of these volcanic deposits is the Cima Volcanic
Field in the east-central Mojave Desert, which consists of around 40 cinder cones and 60 associated basalt flows,
