Geoscience Reference
In-Depth Information
Table 8.5
Micromorphological classification of silcrete (Summerfield, 1983a, 1983b).
Fabric type
Fabric characteristics
GS-fabric
Grain-supported fabric
- skeletal grains (i.e. grains >30 µm diameter) constitute a self-supporting framework.
Subdivided by cement type:
(a) optically continuous quartz overgrowths
(b) chalcedonic overgrowths
(c) microquartz/cryptocrystalline/opaline silica in-fill
F-fabric
Floating fabric
- skeletal grains comprise >5 % but float in matrix, grain solution or fretting common. Subtypes:
(a) massive (glaebules absent)
(b) glaebular (glaebules present)
M-fabric
Matrix fabric
- skeletal grains comprise
<
5 %. Subtypes:
(a) massive (glaebules absent)
(b) glaebular (glaebules present)
C-fabric
Conglomeratic fabric
- skeletal grains include fractured bedrock, gravel or duricrust fragments > 4 mm
adapted to describe other duricrusts. Milnes and Thiry
(1992) divided silcretes into two groups, also on the basis
of micromorphology, pedogenic (or complex) and ground-
water (or simple), with Thiry (1999) adding a third cate-
gory of silicification associated with evaporites. The clas-
sification shown in Figure 8.11 builds upon this scheme,
subdividing silcretes into pedogenic and 'non-pedogenic'
varieties, with the latter grouped on the basis of geomor-
phological context into groundwater, drainage-line and
pan/lacustrine types.
extensive reference list). Silcretes are exposed in a variety
of geomorphological settings, of which the most impor-
tant are as caprocks on residual hills or escarpments (Fig-
ure 8.12). Exposures of silcrete in low-lying parts of the
landscape are rare, although outcrops have been described
in, or adjacent to, valley floors and ephemeral lakes (Sum-
merfield, 1982; Taylor and Ruxton, 1987; Thiry, Ayrault
and Grisoni, 1988; Nash, Shaw and Thomas, 1994; Nash,
Thomas and Shaw, 1994; Nash, McLaren and Webb, 2004;
Shaw and Nash, 1998). Silcretes also outcrop as layers
or lenses partway up slopes (Mountain, 1952; Young,
1978; Thiry, Ayrault and Grisoni, 1988; Milnes, Thiry
and Wright, 1991), representing exhumed horizons that
formed at earlier stages of landscape evolution.
There is a lack of consensus over the precise controls
upon silcrete distribution. In Australia, many silcretes are
exposed in areas that are less humid than where laterites
are found but not as arid as those where calcretes predom-
inate (Young, 1978; Mann and Horwitz, 1979; Milnes
8.6.2
Distribution
Silcretes are most widespread in Australia and in the Cape
coastal zone and Kalahari basin of southern Africa, and
have been documented in western Europe, North Africa,
the Arabian Gulf and, to a lesser extent, in North and South
America (see Table 4.6 in Nash and Ullyott, 2007, for an
SILCRETE
PEDOGENIC
NON-PEDOGENIC
Microquartz
cement
dominant;
high-Ti,
low-Al/Fe
Opaline
cement
dominant;
low-Ti,
high clay/Fe
Groundwater
Drainage-line
Pan/lacustrine
