Desert surfaces: pavements, patterned ground, varnishes and crusts
283
Goudie (Figure 12.27). They can form close to the phreatic level by evaporation in the
capillary zone and also beneath the phreatic level as a consequence of a fall in CO2 content
(Land, 1970). This type of caliche can arise through carbonate-rich flood waters, which
can reach thicknesses of more than 10 m. They may develop rapidly, because they occur in
very young alluvium (Machette, 1985), although this origin constitutes a special case.
5.3. Siliceous crust
Also known as silcrete, this is a grey, tan or green coloured, hard and fragile rock,
comprised of quartz grains cemented by a microcrystalline and amorphous matrix.
Common thicknesses vary between 1 and 3 m. It forms in arid environments, although it
can form in more humid zones. It is a product of replacement of surface materials
(regolith, sediment, soil) by silica, in which silification takes place at a low temperature
unrelated to volcanism, plutonism, or metamorphism (Summerfield, 1983). Siliceous
crusts abound in Australia and South Africa, although they have been observed elsewhere.
They can occupy the tops of hills due to their resistance to erosion. Precious opals can
occur in some silcretes.
In outcrop silcretes commonly exhibit a massive structure accompanied by columnar
joints. Plate, botroidal and pillowy structures can also be recognised (Wopfner, 1978).
The micromorphology reflects the characteristics of the host rock (grain size and shape,
mineralogy and fabric) and the silicification processes. This includes micro- and
cryptocrystalline quartz with boundaries indented in the mineral grains, which confirms
the idea of replacement (Milnes et al., 1991). Chemical analyses indicate that they are
usually more than 95% SiO2 by weight and contain minor amounts of aluminum, iron and
titanium. The aluminum is commonly associated with the clay minerals, whereas the
titanium appears to be related to the bedrock.
In silcrete formation it is necessary to consider the silica source and its dissolution,
transport and precipitation. The primary silica source can be derived from in situ
dissolution, silicate weathering and aeolian dust. In fluvial and lacustrine environments,
diatoms may play an important role. Dissolution and transport are produced by water at
alkaline pH, moving vertically and laterally (Summerfield, 1983). The lateral movement
model is used in fluvial and lacustrine environments. Vertical transfer has similar aspects
to the per descensum and ascensum models used by Goudie (1973b). Finally, precipitation
is produced by evaporation or a fall in pH.
Where siliceous crusts occur in great thicknesses, their age, obtained from diverse
places around the world, go back to Mesozoic times, and so are considered relict accumulations, possibly generated in different climatic regimes (Cooke et al., 1993).
5.4. Gypsum crusts
These are found in numerous arid zones, but over much reduced areas (Coque, 1962).
They develop in deserts with annual precipitations of less than 250 mm (Watson, 1983).
They are defined as accumulations of gypsum found in the uppermost 10 m, having a
thickness of 0.1 to 5 m, a gypsum content of greater than 15% by weight and a minimum of
5% gypsum in the underlying substrate (Watson, 1985, 1989a).
283
Goudie (Figure 12.27). They can form close to the phreatic level by evaporation in the
capillary zone and also beneath the phreatic level as a consequence of a fall in CO2 content
(Land, 1970). This type of caliche can arise through carbonate-rich flood waters, which
can reach thicknesses of more than 10 m. They may develop rapidly, because they occur in
very young alluvium (Machette, 1985), although this origin constitutes a special case.
5.3. Siliceous crust
Also known as silcrete, this is a grey, tan or green coloured, hard and fragile rock,
comprised of quartz grains cemented by a microcrystalline and amorphous matrix.
Common thicknesses vary between 1 and 3 m. It forms in arid environments, although it
can form in more humid zones. It is a product of replacement of surface materials
(regolith, sediment, soil) by silica, in which silification takes place at a low temperature
unrelated to volcanism, plutonism, or metamorphism (Summerfield, 1983). Siliceous
crusts abound in Australia and South Africa, although they have been observed elsewhere.
They can occupy the tops of hills due to their resistance to erosion. Precious opals can
occur in some silcretes.
In outcrop silcretes commonly exhibit a massive structure accompanied by columnar
joints. Plate, botroidal and pillowy structures can also be recognised (Wopfner, 1978).
The micromorphology reflects the characteristics of the host rock (grain size and shape,
mineralogy and fabric) and the silicification processes. This includes micro- and
cryptocrystalline quartz with boundaries indented in the mineral grains, which confirms
the idea of replacement (Milnes et al., 1991). Chemical analyses indicate that they are
usually more than 95% SiO2 by weight and contain minor amounts of aluminum, iron and
titanium. The aluminum is commonly associated with the clay minerals, whereas the
titanium appears to be related to the bedrock.
In silcrete formation it is necessary to consider the silica source and its dissolution,
transport and precipitation. The primary silica source can be derived from in situ
dissolution, silicate weathering and aeolian dust. In fluvial and lacustrine environments,
diatoms may play an important role. Dissolution and transport are produced by water at
alkaline pH, moving vertically and laterally (Summerfield, 1983). The lateral movement
model is used in fluvial and lacustrine environments. Vertical transfer has similar aspects
to the per descensum and ascensum models used by Goudie (1973b). Finally, precipitation
is produced by evaporation or a fall in pH.
Where siliceous crusts occur in great thicknesses, their age, obtained from diverse
places around the world, go back to Mesozoic times, and so are considered relict accumulations, possibly generated in different climatic regimes (Cooke et al., 1993).
5.4. Gypsum crusts
These are found in numerous arid zones, but over much reduced areas (Coque, 1962).
They develop in deserts with annual precipitations of less than 250 mm (Watson, 1983).
They are defined as accumulations of gypsum found in the uppermost 10 m, having a
thickness of 0.1 to 5 m, a gypsum content of greater than 15% by weight and a minimum of
5% gypsum in the underlying substrate (Watson, 1985, 1989a).