Geoscience Reference
In-Depth Information
In Israel, the terminal soil thickness was reached early
in pavement development, with soils of several hundred
to about one thousand years old being the same thickness
as those 14 000 years in age (Amit and Gerson, 1986). By
contrast, other locations, including Oman (Al-Farraj and
Harvey, 2000) and the Providence Mountains, California
(Sena
et al.
, 1994), showed an increase in soil thickness
over time: these locations are of much greater age, how-
ever, and have probably experienced several cycles of dust
deposition.
presence of Middle and Upper Palaeolithic artefacts sug-
gests that the surface has seen 100 000 years of stability.
Clast parameters, such as size and orientation, show no
distinguishable changes over space and so do not allow
pavements of different ages to be distinguished (Adels-
berger and Smith, 2009).
The very great age of pavements in areas of the
Sahara-Arabian Deserts is further supported by the re-
search of Matmon
et al.
(2009), who concluded that aban-
doned alluvial surfaces of the Paran Plains in the southern
Negev Desert, Israel, represent one of the longest-lived
landforms on Earth. Heavily varnished chert clasts cover
more than 95 % of the surface, which is thought to have
not been disturbed since emplacement. The pavement and
associated reg soils began to form at the latest 1.5-2.0 Ma
ago. The surface has remained exceptionally stable, ow-
ing to long-term hyper-aridity that limits surface runoff,
vegetation and bioturbidity. Indeed, the erosion rates are
the lowest that have been documented to date on Earth.
9.10.2
Pavement characteristics and geomorphic
surface ages
Although it is known that pavement changes systemati-
cally with time, it is a more difficult step to assign ages to
surfaces based on these assumptions. In general, fan sur-
faces of Early Holocene age are thought to exhibit partial
pavement development and Late Pleistocene surfaces are
characterised by smooth, well-armored, tightly knit and
heavily varnished pavements (Christenson and Purcell,
1985; Bull, 1991; Al-Farraj and Harvey, 2000), indicative
of long-term surface stability. Amit and Gerson (1986)
suggest that at least 100 000 years is required for the devel-
opment of a mature smooth pavement surface consisting
of fully shattered small clasts. There are exceptions, how-
ever, with some Early to Middle Holocene alluvial sur-
faces exhibiting well-developed pavements (Quade, 2001)
and VML dates indicating that pavements in California
can form in less than 10 000 years (Liu and Broecker,
2008). Clearly, the development of the pavement and its
underlying soil are influenced by dust and salt deposition
rates, which not only vary with time but also are higher
near source areas, such as playas. Thus, it is likely that
rates of pavement development vary spatially (Pelletier,
Cline and DeLong, 2007). In addition, Wood, Graham and
Wells (2002) caution that pavements often demonstrate a
mosaic of disjunct fabrics and textures within short dis-
tances, although they may appear monotonous, flat and
uniform at first glance. As such, caution must be used
when applying rock weathering of pavement as a relative
age dating technique.
Furthermore, while pavement development character-
istics may be useful in assigning relative ages to young
pavements or separating young from old pavements, very
old pavements have often attained an equilibrium form
and show little in the way of distinguishable changes
from place to place. Varnish microlamination dating by
Liu and Broecker (2008) confirm that once a pavement
forms, it may survive for 75 000 years or longer without
9.10.3
Pavement surfaces as a tool in
geomorphic assessment
Given the paucity of numerical dates in arid environments,
there is often an emphasis on relative age dating and the
use of soil development to understand geomorphic devel-
opment and to establish a paleoseismic fault chronology.
Where dated surfaces are available, a chronosequence of
soils may be established on alluvial fans and the displace-
ment of these soils used to study active tectonism.
A determination of surface ages is critical in studies of
the key controlling factors in alluvial fan development,
such as climate, climate change and tectonism. In charac-
terising the age of alluvial fan surfaces, a variety of mor-
phostratigraphic and weathering characteristics are used,
including the relative development of desert pavement.
Key characteristics include the degree of disintegration of
surface clasts, the preservation of bar and swale morphol-
ogy (changes in microtopography), the percent coverage
of the surface by pavement, the degree of soil maturity and
the development of rock varnish (Christenson and Purcell,
1985; Amit
et al.
, 1996; Spelz
et al.
, 2008).
Changes in soil properties and pavement development
with time are used as a basis for tectonic studies, including
estimations of fault scarp ages and slip rates. Pavements
and their associated soils provide a valuable resource ow-
ing to the difficulty of applying radiometric methods in
extremely arid environments. In desert areas of the Amer-
ican southwest and the Middle East (Amit
et al.
, 1996),
many faults displace alluvial fan surfaces. Indeed, allu-
