LATERITIC WEATHERING

The nature of the regolith is influenced by bedrock lithology and structure, tectonic history and topographic position, past
and present climate, local rates of erosion and deposition, variations in groundwater composition and activity of regolith
biota over time. The thickness and preservation of the regolith profile are determined by the rate of erosion. A thick,
weathering profile will only develop where the rate of erosion is less than the downward advance of the weathering front.
Where erosion has been minimal during and since weathering, a complex lateritic regolith profile may develop and be
preserved. Where erosion rates are high, or have increased (e.g. following uplift or climate change), the profile may be thin
or even absent. Sediments deposited contemporaneously with weathering may become incorporated in the profile, whereas
later sediments may overlie the weathered profile.

Above the unweathered bedrock (protolith), a lateritic profile has two major components, the saprolith and the pedolith,
distinguished by their fabrics (Figures 5, 6). The base of the saprolith is the boundary with fresh rock, the weathering front,
and the boundary between saprolith and pedolith is the pedoplasmation front.

The saprolith is weathered bedrock in the lower part of the profile that has retained the fabric originally expressed by the
arrangement of the primary minerals of the protolith. It consists of a saprock overlain by saprolite.

Saprock is compact, slightly weathered rock of low porosity with less than 20% of the weatherable minerals altered,

whereas saprolite is more porous and has more than 20% of the weatherable minerals altered. Weathering commences along
mineral boundaries, cleavages, shears, joints and fractures; it may affect only a few specific minerals. The first signs of
weathering are commonly dissolution of carbonates, oxidation of sulfides and hydrolysis of some easily weathered Fe-Mg
silicates. Weathering may also locally extend along fractures and shears many tens of metres below the depth at which most
rock appears fresh. The weathering front and the saprock-saprolite boundary may be sharp or gradational; they may vary
markedly in depth and separation (a few centimetres to several metres), over short distances and with only minor lithological
changes. Both saprock and saprolite are products of nearly isovolumetric weathering, with the weathering products retaining
the fabric and structure of the parent rock, pseudomorphically replacing the primary minerals. In the upper saprolite, most of
the weatherable minerals have been altered to kaolinite, goethite and hematite. Only the most resistant primary minerals,
such as quartz, zircon and chromite, remain. Saprolite may progressively lose its fabric upwards as porosity and the
proportion of clay increases, leading to collapse, and as Fe oxyhydroxides (and, later, secondary silica, aluminosilicates and
carbonates) form secondary structures. Conversely, in places, cementation by such materials may preserve the fabric (Figure
6a).

The pedolith is the upper part of the profile, above the pedoplasmation front, that has been subjected to surficial and soil-
forming processes. These processes result in loss of the parent rock fabric and the development of new fabrics through non-
isovolumetric weathering, collapse and the precipitation of introduced materials. The principal horizons of the pedolith are
the plasmic and arenose zones, the mottled zone and lateritic residuum.

The plasmic zone is a mesoscopically homogeneous component of a weathering profile developed on quartzpoor rocks,
composed dominantly of clay or silty clay. It has neither the fabric of saprolite nor any significant development of secondary
segregations such as mottles, nodules and pisoliths. It is a transitional zone of settling and consolidation above the saprolite
produced by loss of fabric without significant chemical and mineralogical change. The loss of lithic fabric is caused by
solution and authigenesis of minerals and mechanical processes such as shrinking and swelling of clays and settling of
resistant primary and secondary minerals through instability induced by leaching. Although lithological contacts may be
preserved, there is generally some distortion.

The arenose zone is the equivalent of the plasmic zone on quartz-rich rocks such as granite; it is composed of angular
quartz, with a grain-supported fabric. The loss of lithic fabric results from the solution and removal of kaolinite, and settling
of quartz and other resistant minerals. The plasmic and arenose zones are not present in all profiles. On some mafic rocks,
for example, the saprolite more commonly becomes increasingly ferruginous and brecciated, before merging into a lateritic
gravel or duricrust.

The mottled zone is that part of a weathering profile having macroscopic segregations of subdominant colour different from
that of the surrounding matrix (Figure 6b). Although typically described, as here, as part of the pedolith, similar mottling
may occur in the saprolite. Ferruginous mottling is the most common; the boundaries of mottles may be sharp, or diffuse,
with a goethiterich halo. Some represent ferruginisation of the matrix, without change in fabric, and others may form
secondary structures, with a new fabric. Mottling may also be due to bleaching of ferruginous clays or saprolite. Some
ferruginous mottles, typically developed in clays, especially in paleochannel sediments, tend to be cylindrical with a crudely
vertical orientation. These may have originally formed around root structures and can exceed 200 cm in length and 40 cm
diameter (Figure 6c); mottles 420 cm long are termed megamottles (Ollier et al. 1988).

Lateritic residuum is the upper ferruginous zone of a lateritic profile, composed dominantly of secondary oxides and
oxyhydroxides of Fe (goethite, hematite, maghemite), hydroxides of aluminium (e.g . gibbsite, boehmite) and kaolinite, with
or without quartz. It consists of a cemented duricrust and/or loose pisolitic or nodular gravel (Figure 6d–f). Where both units
are present, lateritic gravel commonly overlies lateritic duricrust. Lateritic residuum has a broad genetic and compositional
relationship with the substrate. It is residual rather than in situ because there has possibly been some minor (generally 5–50
m) lateral movement during its formation as well as vertical collapse.
Lateritic duricrust may be massive but, more commonly, it consists of various secondary segregations such as nodules,
pisoliths and ooliths, structures such as open and in-filled vermiform voids, or angular fragments of ferruginised saprolite,
older duricrust and vein quartz, accumulated by vertical collapse. Some examples of lateritic residuum formed on residual
regolith and colluvium are illustrated in Figure 7.

Lateritic gravel consists of loose ferruginous segregations (nodules, pisoliths and mottle fragments) and may have formed
by disintegration of lateritic duricrust (Figure 6f). The gravels are commonly grain-supported and may have a clay-rich or
sandy matrix. Nodules and pisoliths generally have an outer cortex or skin (cutan). Some pisoliths, especially authigenic
pisoliths in paleochannel clays, have multiple concentric cutans. 2

Lateritic residuum may retain diagnostic textural, mineralogical and chemical signatures of underlying bedrock, including
mineralisation, and can be an excellent exploration sample medium. Few, if any, soils originally overlying the developing
lateritic profiles are preserved. Most existing soils represent the present episode of weathering and are developed from their
immediate substrate.