Geoscience Reference
In-Depth Information
Underlying the Av-horizon is a cambic (Bw) or argillic
(Bt) horizon enriched in silt and clay and commonly con-
taining pedogenic CaCO
3
(Bk-horizon) and soluble salts
(By- or Bz-horizon) (Reheis
et al.
, 1995). These deeper
horizons represent dilutions of the original parent material
by dust and may contain products of in situ weathering.
The B-horizon is virtually gravel-free. The C-horizon has
a higher stone content than either the A- or B-horizons
(Cooke, 1970). It may contain introduced fines and salts
(chlorides, carbonates, gypsum) and gravel shattered by
mechanical weathering (Amit and Gerson, 1986).
Variations in soil chemistry are related to ecohydro-
logic conditions. Nitrate levels in soils beneath pavements
of the Mojave Desert may be one to two orders of magni-
tude higher than found in nearby soils that lack pavement
(Graham
et al.
, 2008). Nitrate accumulates in soils via
atmospheric deposition under very arid conditions, where
infiltration and leaching are minimal. Graham
et al.
(2008)
found that the highest concentrations of nitrate beneath
pavements are within 0.1-0.6 m in depth. The depth distri-
bution trends of chloride and nitrate are similar, reflecting
minimal leaching in the upper half-metre of the soil (Fig-
ure 9.11). These trends are consistent throughout widely
separated locations within the Mojave Desert. The high
salinity of pavement soils and the lack of available water
limits root access to the stored nitrate. Cooke (1970) notes
that although pavement soils in the Atacama and Mojave
Deserts are broadly similar in character, greater aridity
causes the Atacama soils to have calcium carbonate at
higher levels in the profile.
rings are often referred to as carbonate collars (Sena
et al.
,
1994). They occur as a coating of carbonate that forms as
a belt, typically orientated parallel to the ground surface,
but truncated both at the ground line and at a relatively
shallow depth below it (McFadden
et al.
, 1998). The rings
may be displaced or tilted with respect to the ground sur-
face (Wood, Graham and Wells, 2002). In the Providence
Mountains, California, the number of tilted clasts, mea-
sured as the degree of concordance with the surface of
both varnish lines and carbonate collars, has been shown
to increase with age (Sena
et al.
, 1994). In extreme cases,
coarse pavement particles may be cemented together by
calcium carbonate, as described by Cooke (1970) for the
Atacama Desert.
9.7.2
Pitting
Some surface clasts show pitting, but this characteristic
appears to be less universal than others. Pitting is rarely
recorded for pavements of the American Southwest. In
the Negev Desert, Israel, on the oldest desert pavement
yet recorded (
2.5 Ma), only slight to moderate pitting
was recorded (Matmon
et al.
, 2009). However, pitting is
extensive on exposed limestone gravel in reg soils in the
Dead Sea region of Israel, and the degree varies greatly
according to the age of the surface, with pitting much
more marked on older pavements. Pitting depth varies,
with Amit and Gerson (1986) recording a range from
0.1-0.5 cm (low values) to 1.5-2.5 cm (very high values).
≥
9.7.3
Development of varnish
9.7
Secondary characteristics of pavement
surfaces and regional differences in
pavement formation
Desert varnish (Chapter 8) on pavement clasts is common
(Figures 9.4 and 9.9). Heavily varnished clasts are often
found on very old pavements (Matmon
et al.
, 2009) and,
in general, the progressive darkening of varnish is a use-
ful measure of the age or maturity of a surface (Al-Farraj
and Harvey, 2000). However, this relationship is not uni-
versally applicable, as in some areas varnish development
shows complex patterns related to weathering (Al-Farraj
and Harvey, 2000), with varnish inhibited in areas subject
to intense salt shattering or wind abrasion. The lithology
of the clasts may also play a role, with varnish absent on
the feldspars of granitic detritus, but very dark in tone on
pavements composed of metamorphic and volcanic clasts
(Christenson and Purcell, 1985).
Pavements have many secondary characteristics that ac-
count for local or regional differences. These include
changes to the surface texture or colour, coatings of car-
bonate, clast rubification, pitting, wind abrasion features
or varnish, as well as variations in clast axis orientation
and the degree to which they are embedded in the surface.
In some regions, human artefacts are also present on the
pavement surface.
9.7.1
Presence of calcium carbonate and
carbonate collars
9.7.4
Embedded clasts
