Geoscience Reference
In-Depth Information
Table 8.3
Morphological varieties of calcrete (Wright, 2007).
Calcrete type
Characteristics
Calcareous soil
Very weakly cemented or uncemented soil with small carbonate accumulations as grain coatings,
patches of powdery carbonate including needle-fibre calcite, carbonate-filled fractures and small
nodules
Calcified soil
Friable to firmly cemented soil with scattered nodules; 10-50 % carbonate
Chalky or powder
Fine loose powder of carbonate as a continuous body with little or no nodule development, consisting
of micrite or microspar, with etched silicate grains, peloids and root and fungal-related microfeatures
Pedotubule or
rhizocretionary
All or nearly all of the secondary carbonate occurs as root encrustations or calcifications, or around
burrows, having a predominantly vertical structure
Nodular or glaebular
Soft-to-highly indurated concretions of carbonate, or carbonate cemented host material; the nodules
can range in shape from spherical to elongate and typically consist of micrite or less commonly
microsparite
Honeycomb
Partial coalescence of nodules with softer internodular areas produces a honeycomb-like effect
Mottled
Equivalent of nodular features where the host sediment is carbonate-dominated. It consists of irregular
mottles of typically micritic carbonate that has cemented and replaced the original host grains
Hardpan or massive
Consists of a sheet-like indurated layer, which typically has a sharp top and a gradational base into
chalky or nodular calcrete. It may reach thicknesses of >1 m in pedogenic calcretes and many metres
in groundwater forms, and may be blocky or prismatic
Platy
Consists of cm-thick plates, up to tens of cm in diameter, commonly found above hardpan or chalky
layers. Plates may be tabular or wavy in form and may exhibit crude lamination. Some are
fragmented calcified root mats
Laminar
Consists of millimetre-scale sheets of laminated light or dark coloured carbonate. They commonly
occur capping hardpan layers but can also occur within chalky layers or in the host sediment or soil.
Most are only a few cm thick but some forms reach 2 m
Stringer
Closely related to laminar calcretes but not always well laminated. These are sheets of subvertical to
subhorizontal carbonate, usually only a few cm thick, that penetrate into carbonate-rich hosts and
are related to root mats
Pisolitic
Consists of mm-to-cm sized coated grains in layers typically only a few cm thick but up to metres thick
at the bases of slopes. Pisolith laminae are generally micritic. Commonly occur above laminar
calcretes
Breccia or
conglomeratic
Disrupted hardpans or other forms, with fracturing due to mechanical processes and roots and tree
heave
and incorporate complex pedogenic and nonpedogenic
carbonate signatures (Alonso-Zarza, 2003). Furthermore,
the superposition of numerous calcrete horizons can create
complex composite sequences (Watts, 1980; Kaemmerer
and Revel, 1991; Nash and Smith, 1998).
sands of square kilometres of Australia (Wright, 2007) and
southern Africa. Calcic soil horizons develop where there
is an annual net moisture deficit, such that carbonate pre-
cipitated during a drier seaon is not leached away in the
following wetter season. Most contemporary pedogenic
calcretes form in areas with warm to hot temperatures
(mean annual temperature 16-20
◦
C) and low, seasonal
rainfall. Goudie (1983) suggested that a mean annual pre-
cipitation in the range 400-600 mm was typical, while
Royer (1999) concluded from a data set containing 1481
studies that carbonate-bearing soils correlate with a mean
annual precipitation of
8.5.2
Distribution
Calcretes are the most widespread variety of desert duri-
crust. Soils containing calcic or petrocalcic horizons are
estimated to cover some 20 million km
2
or 13 % of the
Earth's land surface (Yaalon, 1988). This figure does not
take into account the large areas over which nonpedo-
genic calcretes outcrop or subcrop. Groundwater calcrete,
760 mm. Annual precipitation
alone, however, does not provide a precise distributional
control. The seasonality of rainfall and average tempera-
ture during the wetter months determine the broad pattern
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