Geoscience Reference
In-Depth Information
in the Middle East (see Chapter 9). Two concurrent pro-
cesses operate to produce desert pavements: mechanical
weathering of the clasts exposed on the surface and the
accumulation of windblown dust. Several theories have
been advanced for the origin of desert pavements (Cooke,
1970), aeolian winnowing of fine material and wetting
and drying of the fines, causing swelling and contraction,
which in turn causes the clasts to rise through the fine ma-
terial, but the general consensus now is that mechanical
weathering of the clasts occurs concurrently with the ae-
olian accumulation of dust to form the pavement and the
underlying Av soil horizon (McFadden, Wells and Jerci-
navich, 1987; McFadden
et al.
, 1999). Pavement develop-
ment is time-dependent (Yaalon, 1970; Dan
et al
., 1982;
Amit and Gerson, 1986; Amit, Gerson and Yaalon, 1993).
Any initial depositional morphology, e.g. bar forms, is
obliterated as the pavement develops. The clasts them-
selves are reduced in size and become more angular with
progressive fracturing through mechanical weathering (Al
Farraj and Harvey, 2000).
The clasts on desert fan surfaces commonly carry a
rock varnish (desert varnish) of iron and manganese salts
(Chapter 8), which increases in thickness and darkens with
age to a deep red on the undersides of the clasts and a dark
brown to black above. Again, there is debate concerning
the formation of desert varnishes (see Dorn and Ober-
lander, 1981, 1982), but their age-related characteristics
have allowed the relative dating and correlation of fan
surfaces (e.g. Hunt and Mabey, 1966; Harvey and Wells,
2003). It has been argued that trace carbon content in var-
nish may allow the radiocarbon dating of desert surfaces
(Dorn
et al
., 1989) and that other detailed analytical tech-
niques may not only reveal age information (Dorn, 1983)
but also give other palaeoenvironmental signals (Dorn,
1984; Dorn, De Niro and Ajie, 1987). There is, however,
some controversy about this methodology.
Many studies in arid and semi-arid regions have used
soil characteristics (Chapter 7) as indicators of surface age
(see Birkeland, 1985). Some have used characteristics of
the soil profile as a whole (Harden, 1982; Harden and
Taylor, 1983; McFadden, Ritter and Wells, 1989; Har-
vey and Wells, 2003), while others have focused on the
B-horizon, dealing with soil colour (see Hurst, 1977),
pedogenic iron oxides (see Alexander, 1974; White and
Walden, 1997) or magnetic mineral behaviour (White and
Walden, 1994; Harvey
et al
., 1999, 2003; Pope, 2000;
Pope and Millington, 2000).
Perhaps the best-known aspect of arid and semi-arid
area soils is the accumulation of pedogenic carbonate.
The stages of carbonate accumulation defined by Gile
et al.
(1966) and elaborated by Machette (1985) have been
dating of geomorphic surfaces, including alluvial fan sur-
faces. On exposure pedogenic carbonate indurates to form
calcrete (caliche) (Chapter 8) and undergoes a complex
sequence of brecciation and recementation. Many geo-
morphic surfaces in arid and semi-arid regions, including
but not exclusively alluvial fan surfaces, are crusted by
various forms of calcrete (Butzer, 1964; Lattman, 1973;
Goudie, 1983; Wright and Alonso-Zarza, 1990; Alonso-
Zarza
et al
., 1998; Nash and Smith, 1998; Stokes, Nash
and Harvey, 2007). As well as providing the basis for cor-
relation and relative dating of geomorphic surfaces (Du-
mas, 1969; Harvey, 1978), absolute dating is also possible
using the uranium/thorium methodology (Ku
et al
., 1979;
Candy
et al
., 2003; Candy, Black and Sellwood, 2004) (see
below, Section 14.5.2.3). Calcrete is not only important
for its correlation or dating potential but it also modifies
geomorphic processes on fan surfaces by modifying infil-
tration behaviour (McDonald
et al
., 1997) and erosional
resistance (Harvey, 1978, 1987; Van Arsdale, 1982).
A final point to make in relation to alluvial fan
sediments relates to interactions with nonfan sediments,
particularly aeolian and lacustrine sediments. Interactions
between alluvial fan and aeolian sediments may have
palaeoclimatic implications (e.g. Nanson, Chen and Price,
1995; Enzel, Wells and Lancaster, 2003; Al Farraj and
Harvey, 2004). Interaction with lacustrine sediments, and
particularly with dated lacustrine shorelines of pluvial
lakes, does, especially in the Basin and Range area of the
USA, provide the basis for calibrating relatively dated fan
surfaces with a known Quaternary chronology (see Har-
vey and Wells, 1994, 2003; Adams and Wesnowsky, 1998,
1999; Harvey, Wigand and Wells, 1999; Ritter, Miller and
Husek-Wulforst, 2000; Harvey, 2005; Garcia and Stokes,
2006).
14.2.3
Alluvial fan sediment sequences
and spatial variations
Assemblages of depositional facies produce characteris-
tic vertical and spatial variations within alluvial fan de-
posits. A common vertical sedimentary sequence is that
described by Miall (1978) as the 'Trollheim' type, named
after the Trollheim fan in California, northwest of Death
Valley (Hooke, 1967). It comprises a sequence dominated
by alternating debris-flow deposits, massive sheet and
channel gravels (Figures 14.4(f) and 14.5(a), showing fa-
cies Gms, Gm, Gt, after Miall, 1978). Further downfan
or on fluvially dominant fans the sequence may be of the
'Scott' type (Figures 14.4(b) and 14.5(a)), dominated by
sheet and channel gravels (facies Gm, Gp, Gt, after Miall,
