Geoscience Reference
In-Depth Information
fragments of indurated pedogenic carbonate. The clasts
are smaller than those of the surrounding pavement. Other
than ephemeral plants and very young creosote bushes
(
Larrea tridentata
), vegetation is absent from the mounds.
The mounds are thought to represent a temporal stage that
follows the disappearance of large perennial plants, be-
low which burrow systems and bioturbation mounds were
once common. Burrowing rodents originally brought the
small clasts to the surface. Following the death of the
plant, deflation removed fine sediment, leaving behind a
concentrated lag of small clasts.
Why did creosote bushes once grow on pavements
that are largely vegetation-free today? McAuliffe and
McDonald (2006) hypothesise that the more effective Late
Pleistocene precipitation not only provided more moisture
to the plants but also leached the soils to greater depths,
reducing the salinity of the substrate. Today, under con-
ditions of contemporary drought, the mortality of modern
creosote bush on pavement is greater in the central pave-
ment area than peripheral areas, owing to the more xeric
soil moisture environment.
In North America, the percent shrub cover seems to be
strongly correlated with the percent clast cover, decreasing
as pavement cover increases. Within a given pavement,
vegetation is most likely in areas with the highest amount
of bare soil exposed. In areas of contemporary pavement
where shrubs cluster, the hydrologic character of the soil
differs considerably from nonshrub surfaces. Aeolian sand
is trapped by the shrubs and later incorporated into the soil
horizons, the organic fraction increases and macropore
channels from shrub roots promote deeper infiltration of
water. Leaching beneath shrubs removes soluble salts to
below the 50-cm depth (Wood, Graham and Wells, 2005).
on the oldest and highest fluvial terraces. Pavement char-
acteristics have been used to map a number of Quater-
nary surficial deposits, including alluvial fans (Shlemon,
1978; Christenson and Purcell, 1985; Al-Farraj and Har-
vey, 2000), stream terraces, shorelines and beach ridges
(Sauer, Schellmann and Stahr, 2007; Al-Farraj, 2008),
and lava flows (Wells
et al.
, 1985; McFadden, Wells and
Jercinovich, 1987; Williams and Zimbelman, 1994).
Changes in the chemical composition of varnish and the
presence of organic carbon in varnish coating gravel of
pavements have been employed to estimate the age of
the underlying materials on which the pavement formed
(McFadden, Wells and Jercinovich, 1987; Dorn, 1988; Liu
and Broecker, 2008).
9.10.1
Changes in surface characteristics
The relative dating of surfaces and the correlation of ge-
omorphic surfaces by pavement development is possible
because pavement characteristics tend to change in a sys-
tematic way within any given region as the surface ma-
tures and becomes older. Common surface modifications
include a reduction in clast sizes, better particle sorting,
increased angularity and an increase in the surface area oc-
cupied by interlocking smooth pavement. In Israel, Amit
and Gerson (1986) studied pavement evolution by examin-
ing 15 terraces on a vast sublacustrine delta formed by the
Holocene drop of Lake Lisan, the precursor of the Dead
Sea. Similar to the findings of Cooke (1970), pavement
maturity, as determined by percent cover of the terrace
surface, sorting, shape of fragments, length of fragments,
degree of pitting and percent cover, was greatest on the
oldest terrace and least on the youngest. In Oman, a sim-
ilar progression was noted on fans and wadi terraces of
different ages (Al-Farraj and Harvey, 2000). With time,
the topography is modified and smoothed, with a sys-
tematic decrease in bar-and-swale microtopography and
a rounding of gully and terrace edges on older surfaces
(Pelletier, Cline and DeLong, 2007).
Soil development proceeds concurrently with pavement
evolution and involves increases in soil thickness, fines
content (silt plus clay), redness of the B-horizon and
carbonate accumulation, as well as progressive horizon
development (Al-Farraj and Harvey, 2000; Young
et al.
,
2004). For early pavement formation in many areas, vesic-
ular A-horizons constitute the initial soil horizon, weakly
developed colour B-horizons are present below the vesic-
ular A-horizon of Middle to Early Holocene soils and
a weak to moderately strong and usually nongravelly
argillic horizon is present below the vesicular A-horizon
9.10
Relative and absolute dating of
geomorphic surfaces based on
pavement development
Desert pavements are commonly examined in order to
understand other geomorphic processes. These include
the age of surfaces, rates and processes of erosion and
deposition, the development of alluvial fans and faulting
and tectonism.
Pavements and their underlying soils have been widely
used as a relative age dating tool owing to the pronounced
changes that characterise their development over time (Al-
Farraj and Harvey, 2000; Al-Farraj, 2008). The differen-
tiation of surfaces into age groups based on pavement
development indices has been used to correlate geomor-
phic surfaces within a region. Cooke (1970), for example,
