Duricrusts

The word was introduced by Woolnough (1927)
who subsequently defined the term thus
(Woolnough 1930: 124–125): ‘The widespread
chemically formed capping in Australia, resting
on a thoroughly leached sub-stratum . . . The
nature of the deposit varies from a mere infiltration
of pre-existing surface rock, to a thick mass
of relatively pure chemical precipitate’.
The mineral matter deposited from solution
falls into three main groups: (1) aluminous and
ferruginous; (2) siliceous; and (3) calcareous and
magnesian. Woolnough believed that bedrock
was an important influence on the distribution of
these three types, which in effect are broadly
equivalent to (1) laterites, bauxites, FERRICRETEs
(see Tardy 1997; Bardossy and Aleva 1990);
(2) SILCRETEs; (3) CALCRETEs, dolocretes.
Because of subsequent work on the individual
duricrust types, the crete-based terminology of
which had been laid down by Lamplugh (1907),
Goudie (1973: 5) proposed a modified definition
which resulted from a synthesis of various definitions
that had already been developed for the
individual types, and stressed their essentially
subaerial and near-surface origin and nature:
A product of terrestrial processes within the
zone of weathering in which either iron and
aluminium sesquioxides (in the case of ferricretes
and alcretes) or silica (in the case of silcrete)
or calcium carbonate (in the case of
calcrete) or other compounds in the case of
magnesicrete and the like have dominantly
accumulated in and/or replaced a pre-existing
soil, rock, or weathered material, to give a substance
which may ultimately develop into an
indurated mass.
Sometimes duricrusts may incorporate characteristics
of more than one type, as with the widespread
calsilcretes of the Kalahari.
To understand the origin and development of
these geomorphologically important materials
some general considerations need to be borne in
mind. First, there is the question of the sources of
the materials which contribute to the make-up of
duricrusts. The primary elements can be derived
from at least four main sources: the weathering of
bedrock and sediment, inputs from dust and precipitation,
plant residues and the dissolved solids
in ground water. Then these sources have to be
translocated and concentrated either by lateral transfers, or by vertical movements, whether
upwards (per ascensum) or downward (per
descensum). Third, the transferred materials need
to be precipitated, and here a very wide range of
processes come into play. Among the most important
of these are changes in chemical equilibria
caused by evaporation, by temperature changes,
by pressure changes in the soil, air and water
systems, by the action of organisms and by miscellaneous
changes caused by interactions of
different solution types.
Models for the origin of duricrusts normally fall
into one of two categories: those involving relative
accumulation and those involving absolute accumulation.
Relative accumulations owe their concentrations
to the removal of more mobile
components, while absolute accumulations owe
their concentration to the addition of materials to
a profile. However, as McFarlane (1983: 20) has
pointed out, the utility of this subdivision depends
on important scale considerations. At one extreme
the accumulation is entirely relative since laterites
would not exist at all were not Fe and Al less readily
mobilized during rigorous chemical weathering.
At the other extreme, in hand specimens even
the residual laterites on interfluves show much
addition of Fe, since samples are enriched
absolutely in materials which originated above
them in the formerly existing column of rock, consumed
to provide the residuum.
Furthermore, laterites and silcretes differ in
that, while ferricretes can result from either relative
or absolute accumulation of iron, silcrete can
only form by absolute accumulation. Weathering
provides the silica and in some cases the material
(a weathering profile, for example) in which the
silica is deposited.