Geoscience Reference
In-Depth Information
restricted geographical distribution which, like sodium
nitrate crusts, results from the greater susceptibility of
gypsum to dissolution by rainwater. This limits the crusts
to some of the Earth's most arid regions. In addition,
the less widespread occurrence of sources of gypsum
(CaSO
4
·
year. As with calcrete distribution, there also appears to
be an annual rainfall threshold below which gypsum crusts
are less common. In the central Namib Desert, for exam-
ple, there is a gradual transition from gypsum as rainfall
drops below about 25 mm (Watson, 1983b, 1985a); more
soluble halite crusts form preferentially under such hyper-
arid conditions (Hartley and May, 1998).
The main areas of widespread gypsum crusts are in
North Africa, particularly central Algeria and Tunisia
(e.g. Coque, 1955a; Watson, 1979, 1985a, 1988; Drake,
Eckardt and White, 2004). Gypcretes are documented
in the Namib Desert (e.g. Martin, 1963; Scholz, 1972;
Watson, 1981, 1985a, 1988; Eckardt
et al.
, 2001), Aus-
tralia (e.g. Arakel and McConchie, 1982; Warren, 1982;
Jacobson, Arakel and Chen, 1988; Chen, Bowler and
Magee, 1991a, 1991b; Chivas
et al.
, 1991; Magee, 1991;
Milnes, Thiry and Wright, 1991; Chen, 1997) and central
Asia (e.g. Tolchel'nikov, 1962; Evstifeev, 1980). They
are found throughout the Middle East from Egypt (e.g.
Ali and West, 1983; Aref, 2003), Israel (e.g. Dan
et al.
,
1972, 1982; Amit and Gerson, 1986), Jordan (Turner and
Makhlouf, 2005) and Syria (Eswaran and Zi-Tong, 1991)
to Iraq (Tucker, 1978), Iran (Gabriel, 1964), Saudi Ara-
bia (Al Juaidi, Millington and McLaren, 2003) and Kuwait
(El Sayed, 1993). Occurrences in the southwest USA (e.g.
Reheis, 1987; Harden
et al.
, 1991; Buck and van Hoesen,
2002) and South America (Risacher, 1978; Hartley and
May, 1998; Rech, Quade and Hart, 2003) are more spo-
radic. Gypcretes are also reported from Antarctica, where
strong winds promote high rates of evaporation resulting
in evaporite crystallisation (Gibson, 1962; Lyon, 1978).
Gypsum crusts often form an erosion-resistant horizon
at the land surface. Perhaps because of their greater sus-
ceptibility to dissolution, however, gypcretes rarely cre-
ate the mesa-and-butte landscapes associated with more
durable crusts such as silcrete and calcrete. Nevertheless,
in southern Tunisia gypsum crusts mantle several gen-
erations of pediment slopes (glacis), apparently protect-
ing relict surfaces from erosion (Coque, 1955b, 1962).
In many regions, gypsum crusts are found in and around
large hydrological basins (e.g. the chotts of Tunisia and
Algeria, salinas of South America, salt lakes of Australia
and playas in the central Namib). While some crusts are la-
custrine evaporites (Warren, 1982) and others are phreatic
precipitates (Kulke, 1974; Risacher, 1978), many occur on
hill crests and steep slopes beyond the phreatic zone (Fig-
ure 8.5(a)). These are pedogenic crusts, which blanket the
landscape. Such crusts may develop directly on unweath-
ered bedrock such as granite, basalt, marble, limestone
and clay (Watson, 1985a, 1988) or on unconsolidated
sediments such as colluvium, alluvium and dune sand
2H
2
O), compared to calcium carbonate or sil-
ica, hinders crust formation. Nonetheless, gypcretes have
been reported from virtually every continent of the world
(Watson, 1983a).
Three main forms of gypsum crust have been identi-
fied: (a) horizontally bedded crusts; (b) subsurface crusts,
sometimes known as
croute de nappe
, composed either
of large, lenticular crystals (between 1 mm and 0.50 m in
diameter; desert rose crusts) or mesocrystalline material
(crystal diameters from 50 µm to 1.0 mm); and (c) sur-
face crusts (Figure 8.5(a) and (b)), composed mainly of
alabastrine gypsum (crystallites less than 50 µmindiam-
eter), occurring as columnar crusts, powdery deposits or
superficial cobbles (Watson, 1979, 1983a, 1985a). There
is no typical profile. Desert rose crusts (Figure 8.5(c))
can reach thicknesses of up to 5 m (e.g. Kulke, 1974)
and range in colour from white or grey to green or red,
depending on the host material. Columnar surface crusts
are usually 1-2 m thick and white or grey in colour, with
roughly hexagonal columns, 0.25-0.75 m in diameter, ex-
tending through the full thickness. The mesocrystalline,
columnar and cobble forms of gypsum crust are probably
genetically related; morphological and chemical differ-
ences result from diagenesis and, possibly, degradation
of pedogenic crusts. However, just as some calcretes are
nonpedogenic in origin, the bedded and desert rose forms
of gypsum crust are also genetically distinct. This has
profound implications for the significance of these desert
crusts as palaeoenvironmental indicators (Watson, 1988).
8.4.2
Distribution
Gypsum crusts are found in all the Earth's warm deserts,
though their extent varies. Factors such as climate, geol-
ogy, topography and hydrology are critical to their forma-
tion and preservation. Most gypsum crusts are found in
areas where mean annual rainfall is less than 250 mm
(Watson, 1985a). In North Africa, for example, there
appears to be a transition from calcretes to gypsum
crusts as mean annual rainfall drops below this amount
(Pervinquiere, 1903). Rarely, the upper rainfall limit
reaches 300 mm; this is the case in Iraq (Tucker, 1978) and
Rajasthan (Srivastava, 1969). Here, high mean monthly
temperatures, especially during the wetter months, pro-
mote high rates of evaporation and a net monthly soil
