Page 105 - Sedimentology and Stratigraphy

92 Continents: Sources of Sediment

6.4.3 The products of weathering In places where chemical weathering is subdued,
lithic fragments may form an important component of
Material produced by weathering and erosion of mate- the detritus generated by physical processes. The na-
rial exposed on continental land masses is referred to as ture of these fragments will directly reflect the bedrock
terrigenous (meaning derived from land). Weathered type and can include any lithology found at the
material on the surface is an important component of Earth’s surface. Some lithologies do not last very
the regolith that occurs on top of the bedrock in most long as fragments: rocks made of evaporite minerals
places. Terrigenous clastic detritus comprises miner- are readily dissolved and other lithologies are very
als weathered out of bedrock, lithic fragments and fragile making them susceptible to break-up. Detritus
new minerals formed by weathering processes. composed of basaltic lithic fragments can form around
Rock-forming minerals can be categorised in terms volcanoes and broken up limestone can make up an
of their stability in the surface environment (Fig. 6.6). important clastic component of some shallow marine
Stable minerals such as quartz are relatively unaf- environments.
fected by chemical weathering processes and physical
weathering simply separates the quartz crystals from
each other and from other minerals in the rock. Micas 6.4.4 Soil development
and orthoclase feldspars are relatively resistant to
these processes, whereas plagioclase feldspars, amphi- Soil formation is an important stage in the transfor-
boles, pyroxenes and olivines all react very readily mation of bedrock and regolith into detritus available
under surface conditions and are only rarely carried for transport and deposition. In situ (in place) physical
away from the site of weathering in an unaltered and chemical weathering of bedrock creates a soil
state. The most important products of the chemical that may be further modified by biogenic processes
weathering of silicates are clay minerals (2.4.3). A (Fig. 6.7). The roots of plants penetrating into bedrock
wide range of clay minerals form as a result of the can enhance break-up of the underlying rock and the
breakdown of different bedrock minerals under differ- accumulation of vegetation (humus) leads to a change
ent chemical conditions; the most common are kaoli- in the chemistry of the surface waters as humic acids
nite, illite, chlorite and montmorillonite. Oxides of
aluminium (bauxite) and iron (mainly haematite)
also form under conditions of extreme chemical
weathering.
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Fig. 6.7 An in situ soil profile with a division into different
Fig. 6.6 The relative stability of common silicate minerals horizons according to presence of organic matter and degree
under chemical weathering. of breakdown of the regolith.