Geoscience Reference
In-Depth Information
STRUCTURE
SILICA MINERALOGY
SAND
2
PSEUDO-
BRECCIA
COLUMNAR
1
GRANULAR
CLAYS
0
Figure 8.14 Schematic representation of a 'typical' pedogenic silcrete profile, showing silcrete structures and the vertical
arrangement of silica cements (after Milnes and Thiry, 1992; Thiry, 1999).
evapotranspiration from seasonal pools (Shaw and Nash,
1998). Massive silcrete layers within the channel allu-
vium are suggested to have developed in response to salin-
ity shifts associated with movements of the wetting front
during flood events.
Pan/lacustrine silcretes develop within, or adjacent
to, ephemeral lakes, pans or playas (Goudie, 1973).
Phases of silica mobility and precipitation are driven
by changes in pH and salt concentration during cycles
of flooding and evaporation (Summerfield, 1982). The
zone of maximum mixing and precipitation occurs around
the lake margin and immediately above the water ta-
ble (Thiry, 1999), which may explain why silicification
is often linked to lake regression (Ambrose and Flint,
1981; Bustillo and Bustillo, 1993, 2000; Armenteros,
Bustillo and Blanco, 1995). Biological fixing of silica
may also occur. Sua Pan in Botswana contains sheet-like
silcretes developed from desiccated colonies of the silica-
fixing cyanobacteria Chloriflexus (Shaw, Cooke and Perry,
1990). Pan/lacustrine silcretes may contain a variety of
minerals in addition to silica, depending upon the chem-
istry of the cemented lake sediments. Glauconite-illite
occurs within green pan silcretes in the Kalahari (Sum-
merfield, 1982). Silica replacement is commonplace in
such environments and may lead to the development of
intergrade cal-silcrete and sil-calcrete crusts (Nash and
Shaw, 1998).
8.7
Desert rock coatings
8.7.1
General controls on formation
Thin rock coatings occur on almost all exposed rock sur-
faces found in deserts. Over a dozen varieties have been
identified (Table 8.7), some of which exist in different
types. Many coatings interdigitate, producing complex
microstratigraphic sequences (Dorn, 2007). Some, such
as iron films and rock varnish, appear similar but have
contrasting origins and hence palaeoenvironmental sig-
nificance. The most common coatings found in deserts
are rock varnish, silica glaze and iron films. Carbonate
skins and salt crusts are also important in some localities.
For further information see Dorn (1998, 2007, 2009).
For any rock coating to form, a bare and stable rock
surface is required. In some cases, coatings may develop
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