Geoscience Reference
In-Depth Information
Table 5.5 The major types of duricrust found in arid environments. Solubility increases from silcrete to salcrete. (Summarized from Watson & Nash 1997 and references therein.)
Crust type
Main
mineral
Field appearance
Typical profile
thickness
Modes of origin in arid
environments
Distribution
Silcrete
Silica
Brittle, intensely indurated. Colour is variable
with grey brown and green common.
0.5-3 m
Pedogenic and
groundwater.
Found in areas not subjected to late Tertiary and
Quaternary glaciation. Less globally significant than
calcrete. Mantle low gradient slopes, but can also
form beneath basaltic lava flows and in weathering
profiles on a range of lithologies.
Most widespread crust. Petrocalcic horizons cover
20
Calcrete
Calcium
carbonate
Occurs as powdery through to nodular,
laminated to highly indurated. Generally
white, cream or grey with pink mottling and
banding.
1-5 m
Highly alkaline
evaporitic basins or
hydrological or
pedogenic origins.
10 6 km 2 of the globe's landsurface. Typical of
areas where moisture is deficient throughout the
seasons. Pedogenic calcretes mantle undulating or
gently sloping terrain. Can form on most
lithologies.
Typical of warm deserts with very low rainfall
(
×
Gypcrete
Gypsum
Occurs as 1) horizontally bedded crusts;
2) subsurface crusts with large (1 mm -
0.5 m diameter) crystals (desert roses);
3) mesocrystalline (crystals 50
0.1-5 m
Pedogenic,
groundwater and
lacustrine evaporites.
250 mm a −1 ). Pedogenic gypcretes mantle low
gradient surfaces. They are also associated with
hydrological basins. Can develop on a range of
lithologies.
<
m -1 mm)
or 4) surface crusts of alabastrine gypsum
(cystallites
μ
m). Occur as columnar
crusts, powdery deposits or superficial
cobbles. Colour ranges from white /grey to
green or red.
Typically white, but can be pink where
micro-organisms are present.
<
50
μ
Salcrete
Halite
0.1-5 m
Evaporation of waters
in Sabkha settings,
groundwater and
pedogenic (the latter
two being rarer).
Associated with areas of extreme aridity,
particularly in association with saline basins and
rainfalls less than 200 mm a −1 . Can form thin
ephemeral crusts on dune sands.
 
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