Desert crusts and rock coatings - Arid Zone Geomorphology - page 149

(a) (c)

(b)

Figure 8.12 Examples of silcrete profiles: (a) in situ pedogenic silcrete profile developed within deeply weathered bedrock, near
Inniskillin, Western Cape, South Africa; (b) massive drainage-line silcrete at Samedupe Drift in the floor of the ephemeral Boteti
River, Botswana; (c) superposed groundwater silcrete lenses developed within Fontainebleau Sand, Bonnevault Quarry, Paris basin,
France (with Medard Thiry for scale).

and Thiry, 1992). This prompted the suggestion that
Australian silcretes originated under semi-arid or
markedly seasonal, wet and dry climates. However, even
authors that accepted this view had differing opinions as to
whether the crusts were the result of weathering (Bruck-
ner, 1966; McGowran, Rutland and Twidale, 1977), hy-
drological processes (Smale, 1973, 1978) or pedogenesis
(Hutton, Twidale and Milnes, 1978; Callen, 1983). It was
also argued that many silcretes were relict horizons from
deep chemical weathering profiles which formed under
humid or seasonally wet, tropical conditions (Woolnough,
1930; S. H. Watts, 1977, 1978a, 1978b; Butt, 1985) and
were preserved under subsequent aridity (Twidale, 1983).
In contrast, silcretes in the Kalahari are found in asso-
ciation with calcretes (Summerfield, 1982; Shaw and De
Vries, 1988; Nash, Shaw and Thomas, 1994), and there
are numerous sites where silica has replaced calcrete ce-
ments (Nash and Shaw, 1998; Nash, McLaren and Webb,

lateritic (Wright, 1963; Alley, 1977; Langford-Smith and
Watts, 1978) or calcrete profiles (Summerfield, 1982) may
suggest formation under tropical or semi-arid conditions
respectively, such occurrences do not necessarily prove
simultaneous development (Smale, 1973).

8.6.3 Micromorphology and chemistry

Silcretes are chemically simple (Table 8.6), by definition
comprising >85 % (and more often >95 %) silica with mi-
nor amounts of titanium, iron and aluminium oxides and
resistate trace elements (Summerfield, 1983a). Titanium
is usually present as anatase and may be disseminated
throughout the matrix or concentrated within geopetal
structures. TiO 2 may be enriched to >1 % (Summerfield,
1979, 1983d; Thiry and Millot, 1987) and exceed 20 %
in silcrete 'skins' (Hutton et al. , 1972). TiO 2 content may