Geoscience Reference
In-Depth Information
9.2.2.2
Reg or serir
heave processes play a role in the formation of the Sturt
Stony Desert.
In North Africa and the Middle East, the terms 'reg' or
'serir' are used to describe extensive sheets of gravel
with little or no vegetation (Evenari, Yaalon and Gut-
terman, 1974).
Reg
is an Arabic term meaning 'becoming
smaller', and surfaces of finer gravel produce trafficable
desert pavements in Algeria, Israel and areas of the Sa-
hara.
Serir
replaces reg in the central Sahara (Libya and
Egypt) (Mabbutt, 1977). Saharan regs cover vast expanses
of low-angle surfaces in the desert, extending for immense
distances from residual mountain ranges (Symmons and
Hemming, 1968). The Tibesti Serir, for example, is an
alluvial plain of low gradient (0.1 to 0.2%) and little relief
(less than 50 cm) (Furst, 1965). The rounding of some
stones in regs suggests either transport over a consider-
able distance or alluvial stones of a residual nature, derived
from beds rich in pebbles (Peel, 1968).
Desert soils that form on stable, well-drained, sandy
to coarse gravelly alluvial parent material are referred to
as reg soils (Amit and Gerson, 1986; Amit and Yaalon,
1996). In hyper-arid parts of the Negev Desert, these soils
show characteristics similar to desert pavements in North
America, with high amounts of soluble salts in the soil
(mainly halite and gypsum), a vesicular A-horizon and
desert pavement at the surface. Salt plays a major pe-
dogenic role, with salt-shattered gravel in the B- and C-
horizons and gypsic and sometimes petrosalic horizons
developed (Dan
et al.
, 1982; Amit and Gerson, 1986;
Amit and Yaalon, 1996).
9.2.2.4
Desert or stone pavements
The terms desert or stone pavement are used in southwest-
ern North America, particularly in the Mojave and Sono-
ran Deserts, where such surfaces are common and well
developed. This nomenclature is becoming more widely
applied, however, and has recently been used to describe
stony surfaces in other areas of the world, including Chile
and Argentina (Bailey
et al.
, 2007; Sauer, Schellmann and
Stahr, 2007), Israel (Matmon
et al.
, 2009), the UAE and
Oman (Al-Farraj and Harvey, 2000) and Egypt (Adels-
berger and Smith, 2009). Pavement surfaces have also
been reported in southern Africa (Francis
et al.
, 2007).
Desert pavements have received detailed and systematic
study and, relative to other surfaces described earlier, are
well defined. They are discussed in more detail later in
this chapter.
9.3
General theories concerning stony
surface formation
Four main theories have been proposed to explain the con-
centration of surface stones common to gobi, reg, gibber
plains or pavement surfaces: (1) deflation; (2) concentra-
tion by rain beat and surface wash; (3) upward migration of
stones by heave processes; and (4) upward displacement
of clasts by aeolian aggradation of fine materials beneath
the pavement surface. Of these, the most oft-cited intro-
ductory textbook explanation relates pavement formation
to the deflation of fines from the surface. Research over
the past few decades, however, has shown that pavement
formation is highly complex and cannot be explained by
simple deflationary processes. Indeed, some of the best-
studied pavements, most notably those of the American
southwest, appear to have evolved in conjunction with
long-term aeolian deposition rather than erosion.
Although a truly global perspective on stony surface
formation is still lacking, research to date suggests that
several different processes may act in concert, with ae-
olian aggradation, for example, as the principal pro-
cess, but surface wash and subsurface heave contributing
to long-term development. Additionally, numerous sec-
ondary processes - chemical, physical and biological -
influence the ultimate surface and subsurface expression
of the pavement. The following sections summarise the
four major theories of pavement formation. Following this
introduction, the best studied of these, the aeolian aggra-
9.2.2.3
Gibber plains or stony mantles or stony desert
Extensive stony plains in Australia are termed gibber
plains, stony mantles or stony desert. The term stony table-
land soil has been applied to fine-textured, saline soils
with a high gypsum content and an abundance of stones
in the upper part of the profile and on the surface (Jessup,
1960).
Gibber plains vary in character according to local
sources of bedrock, with limestone gibbers, for exam-
ple, adjacent to the northern Flinders Ranges (Twidale,
1994) and silcrete gibbers mantling a deflated surface in
the Sturt Stony Desert (Thomas, Clarke and Pain, 2005).
The silcrete fragments are derived from the cappings of
dissected, silcrete-capped plateaus (Twidale, 1994).
Multiple processes may be involved in gibber plain
formation. According to Twidale (1994), vertical sorting
and churning by the wetting and drying of clays in the soil
(gilgai action) concentrate stones at the surface. Thomas,
