Geoscience Reference
In-Depth Information
Figure 9.6
A pavement scraped clear during construction, which illustrates (1) the role of wash processes in the recovery of
disturbed pavements through the redistribution of particles over short distances and (2) ped boundaries in the A-horizon. The
A-horizon is commonly characterised at the surface by a small-scale polygonal pattern of cracks, which form a three-dimensional
ped system of vertical columns. Fine sands, silt and solutes are transported to depth through the polygonal cracks. Location:
Father Crowley overlook, Panamint Valley, California.
conditions and extensive freezing does not occur. Fur-
thermore, continuous upward migration over time should
produce stones with different ages at the surface, and cos-
mogenic dating and varnish studies indicate that pavement
stones are of a similar age (McFadden, Wells and Jerci-
novich, 1987; Wells
et al.
, 1995; Liu and Broecker, 2008).
The saturation and desiccation of desert soils causes ex-
pansion and contraction features, such as polygonal struc-
tures in the A-horizon and the development of gilgai in
Australia. This process is most effective where the clay
content is high.
terraces. In their model, the stones originally present on
a landform are maintained at the surface as the landscape
evolves. The surface rises and smoothes owing to the de-
velopment of cumulate soils beneath the clasts in response
to the incorporation of aeolian silts and clays. In Israel,
Amit and Gerson (1986) also detailed how gravelly allu-
vial terrains evolve as dust and salts are introduced into
the Dead Sea region. In recent years, new research has
shown that this model may be extended to areas beyond
western North America, including the Libyan Plateau of
Egypt (Adelsberger and Smith, 2009). Figure 9.4 shows
pavement development by this process in western China.
The accretionary mantle model of pavement formation is
the basis for most modern research on pavements. Addi-
tional details are provided in the following sections.
9.3.4
Accretion of aeolian fines
Whereas models that emphasise deflation or lowering by
surface wash imply that pavements are largely zones of
erosion, the accretionary mantle model considers them
to be zones of deposition. This concept, first outlined in
detailed papers in the 1980s, represents a significant de-
parture from earlier studies. Based on studies of pavement
formation on volcanic fields in the Mojave Desert, Wells
et al.
(1985) and McFadden, Wells and Jercinovich (1987)
proposed a model of upward surface growth, which has
been subsequently applied to other landforms with pave-
9.3.5
Desert pavement formation by aeolian
aggradation and development of an
accretionary mantle
Desert pavements are highly complex systems. Their de-
velopment can be understood by considering key factors
that affect soils in desert regions, including vegetation
type and density, faunal activity, the nature and rate of
