Geoscience Reference
In-Depth Information
Figure 9.7
A tightly interknit pavement surface in which the clasts are formed of rounded pebbles rather than angular fragments.
There is little secondary modification to the surface of this Holocene beach ridge. Location: Death Valley, California.
9.5
Processes of pavement formation
and Pleistocene gravelly beach deposits to produce a sin-
gle massive soil structure beneath the desert pavement
(Sauer, Schellmann and Stahr, 2007). Entrapment rates
increase as the rock fragments become smaller, more flat-
tened and elongated, and the total cover density increases
(Goossens, 2006). Pavement stones trap dust brought in
by the wind from surrounding plains. The fine dust, of-
ten rich in salt, accelerates mechanical weathering of the
surface rocks and accumulates within the pore spaces be-
tween clasts. As the aeolian layer grows in thickness, clay
neogenesis proceeds until the soil has the strength and
shrink-swell potential to displace stones upwards (Blank,
Young and Lugaski, 1996). The surface gravels continue
to trap dust and thicken the B-horizon, and the deposition
of aeolian fines creates an accretionary mantle, in which
the clasts undergo syndepositional lifting with the surface
(Figures 9.8 and 9.9) (Wells
et al.
, 1985; Wells, McFadden
and Dohrenwend, 1987).
The accretionary mantle model of pavement formation
implies that many pavements are 'born at the land sur-
face', with pavement gravels exposed at the surface rather
than having been deeply buried in the underlying soil and
excavated by wind or water erosion (McFadden, Wells and
Jercinovich, 1987). This theory has been supported by a
number of studies on the age of surface clasts. Cosmo-
genic ray surface exposure dating in the Cima volcanic
field (Wells
et al.
, 1995) showed that the ages of pave-
ment clasts were similar to those of their source bedrock.
The establishment of a stone pavement requires a geo-
morphologically stable area where the processes of stone
sorting, surface creep and clast disintegration are domi-
nant over concentrated wash and fluvial processes. Mature
stone pavement surfaces are characterised by low relief,
as lower areas are gradually infilled, and the entire sur-
face rises by infiltration of aeolian material. On alluvial
fan surfaces, for example, the original bar and swale mi-
crotopography is reduced as a smooth vegetation-free sur-
face develops (Matmon
et al.
, 2009). In most cases, the
surface clast size becomes progressively smaller as rocks
weather (Figure 9.8). Salt-rich aeolian fines accumulate in
clast fractures, and wetting and drying result in volumetric
changes related to salt crystal growth and/or shrinking and
swelling of clay. Over time, clasts fracture and are verti-
cally and laterally displaced. Additional aeolian fines, as
well as weathered granules (grus), are deposited between
the fragments, further enhancing their separation.
The formation of smooth pavement surfaces is related
to the entrapment and infiltration of dust. Landscapes that
initially have surface obstructions, such as plants or rocks,
trap dust, beginning a period of pedogenesis, which re-
sults in an upwardly thickening aeolian mantle (Blank,
Young and Lugaski, 1996). Rocky surfaces trap several
tens of times more dust than adjacent pebble-free surfaces.
In Patagonia, for example, fine aeolian particles have
