Desert crusts and rock coatings - Arid Zone Geomorphology - page 137

Table 8.1 Genesis and post-depositional changes of gypsum sediments (Chen, 1997).

Mode of genesis Example

Primary crystallisation
Surface brine Coastal zone. Large (>2 mm) compact selenite and laminated columnar/prismatic crystals,

commonly occurring with halite and carbonates
Salt lakes. Relatively large (>0.5 mm) prismatic crystals, laminated and commonly interbedded
with mud

Groundwater Large (commonly >5 mm) crystals, near watertable, including sand and mud. Mostly shortened

in c axis, commonly interlocked forming aggregates (e.g. desert rose)

Transportation and redeposition
By water Fractured and corroded crystals or crystal fragments in lacustrine and alluvial sediments

By wind Gypsum sands in dunes along playa lake margins, with crystals relatively well sorted and with

near-horizontal orientation

Post-deposition alteration
Below capillary fringe

Overgrowth and dissolution

Above capillary fringe Dissolution, leaching and recrystallisation, forming very fine crystals ( < 0.1 mm)

Chen (1997) conceptualises the processes involved in
pedogenic gypsum crust formation as a series of genetic
and diagenetic steps (Table 8.1). Prior to accumulating
within a soil profile, gypsum may be precipitated from a
saturated solution such as a surface brine or groundwa-
ter, most commonly via evaporation. Gypsum crystals are
then exhumed, eroded, transported and redeposited at the
ultimate site of gypcrete formation (by either the wind
or water), before being incorporated into the soil profile.
Once within the profile, the gypsum may be subject to
diagenetic alteration either above or below the zone of
capillary rise to generate the pedogenic gypcrete profile.
The main sources of gypsum for pedogenic gypcrete
formation are windblown sand, dust or aerosols (Watson,
1979, 1985a; Dan et al. , 1982; Amit and Gerson, 1986;
Bao, Thiemens and Heine, 2001). Gypsum may be de-
flated from playa surfaces (Coque, 1955a, 1955b, 1962;
Watson, 1985a) or reworked hydromorphic gypsum crusts
(Reheis, 1987; Chen, Bowler and Magee, 1991a, 1991b).
Fog, sea spray, biogenic sulfur and marine evaporite
deposits have been suggested as sulfur sources for gypsif-
erous crusts in Tunisia, Australia, the Atacama and Namib
Desert (Martin, 1963; Carlisle et al. , 1978; Ericksen,
1981; Watson, 1985a; Chivas et al. , 1991; Day, 1993;
Eckardt and Spiro, 1999; Rech, Quade and Hart, 2003;
Drake, Eckardt and White, 2004). However, Eckardt and
Schemenauer (1998) have demonstrated that the ionic
content of Namib fog is too low to act as a major sulfur
source.
Nonpedogenic models of gypcrete formation have been
put forward to describe the deposition of gypsum in lacus-

example, Jacobson, Arakel and Chen (1988) suggested
that the formation of extensive gypsum deposits beneath
central Australian palaeolakes was driven by interactions
between vadose and phreatic groundwaters and infiltrating
meteoric waters. Watson (1985a, 1988) attributed the for-
mation of lacustrine gypcretes in the Namib Desert and
Tunisia to two mechanisms. Bedded, lacustrine evapor-
ites probably precipitated in shallow-water environments.
The presence of size-graded strata indicates that the water
body evaporated to dryness because late in the evaporative
cycle there was minimal ionic migration to the growth
faces on alabastrine crystallites. In contrast, phreatic,
desert rose crusts probably accreted as gypsum precip-
itates from evaporating groundwater (Castens-Seidell and
Hardie, 1984) where the water table was within 1 or 2 m
of the land surface. Precipitation as a result of dilution by
less saline meteoric water (Pouget, 1968) is unlikely be-
cause it does not allow for the prolonged hydrochemical
stability that is required for the growth of large crystals.

8.5 Calcrete

8.5.1 General characteristics

Calcrete is a term coined by Lamplugh (1902, 1907) that is
now used to describe 'a near surface accumulation of pre-
dominantly calcium carbonate, which occurs in a variety
of forms from powdery to nodular, laminar and massive. It
results from the cementation and displacive and replacive
introduction of calcium carbonate into soil profiles, sed-