Geoscience Reference
In-Depth Information
In conclusion, 'the fact that the single term laterite has been applied to materials as
diverse and genetically distinct as iron-cemented colluvial rubble, weathered basalt,
mottled clays, and kaolinized igneous rocks has caused much unnecessary confusion'
(Paton andWilliams,
1972
, p. 55). These authors go on to say that 'perhaps the greatest
misconception relating to laterite is the notion that laterization (an ill-defined and ill-
understood complex of weathering processes) demands a tropical climate' (op. cit.,
p. 55). To do so is to ignore the influence of the other, equally important soil-forming
factors of parent material, topography, drainage and time. Nevertheless, the presence
of laterite or lateritic soils within presently arid regions is clearly indicative of a
previously efficient weathering and leaching regime under a climate that must have
been substantially wetter (Bourman,
1995
; Twidale and Bourne,
1998
). The Eocene
deep weathering profiles of the south-central Sahara are a case in point (Greigert
and Pougnet,
1967
), as are the Mesozoic and Cenozoic laterites of central Australia
(Mabbutt,
1965b
).
15.9 Equivocal paleoclimatic significance of silcrete
Summerfield (
1983
) has provided a comprehensive review of the distribution and
possible origins of silcretes in the Namib Desert. He concluded that silcretes enriched
in titanium occurred today in areas of more humid climate and that those that were
titanium-poor had apparently formed in arid to semi-arid environments. However,
because the underlying bedrock geology also appears to exert a strong control over the
distribution of silcrete, at least in arid inland Australia (Mabbutt,
1965b
), and because
the mode or modes of formation of silcrete remain poorly understood, the presence
of silcrete is not particularly useful as an indicator of past climate. Reprecipitation
of quartz derived from sandstone bedrock during weathering may also play a role
in silicon cycling and the eventual formation of silcrete units (Basile-Doelsch et al.,
2005
).
15.10 Conclusion
Any landscape that supports plant life comprises a surface soil mantle. Soils are
formed as a result of five main factors: parent material, topography, climate, biological
activity and time. In young soils, the first two of these factors (parent material and
topography) exert the dominant influence on soil morphology and soil chemistry, with
immature soils formed on desert dunes being a good example. As time progresses,
soil climate and biological processes within the soil will exert increasingly important
control over the soil's physical and chemical properties. The classic subdivision of
soil profiles into A-, B- and C-horizons is not always a useful basis for classifying
soils, because quite different types of soil profile can develop as a result of the sorting
