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arid periods in various regions (e.g. Vogt and Corte, 1996)
and might be similar to carbonate crusts currently form-
ing in the Arctic (e.g. Bunting and Christensen, 1980;
Lauriol and Clarke, 1999). Candy (2002) has described
rhizogenic calcretes from Oxygen Isotope Stage 12 de-
posits in Norfolk, UK, probably formed during a period
of climatic amelioration. Local factors may be of greater
importance than macroclimatic variables in some envi-
ronments (Wright, 2007). For example, Strong, Giles and
Wright (1992) describea4mthickHolocene calcrete from
North Yorkshire, UK, that is suggested to have formed as
a result of evapotranspiration promoted by the rapid free
drainage of a host Pleistocene glacial gravel.
Pedogenic calcretes require long periods of time within
a soil profile to reach maturity. As a result, the major-
ity are found on surfaces that are, or were, geomorpho-
logically stable. In southeast Spain, Candy and Black
(2009) have identified that calcretes formed during inter-
glacials/interstadials (when climates were semi-arid and
a garrigue vegetation cover was present) are abundant
compared to calcretes that date to glacial/stadial episodes
(when the climate was more arid and slopes less stable).
Most pedogenic calcretes mantle undulating or gently
sloping terrain, with desert alluvial fans (e.g. Lattman,
1973; Wright and Alonso-Zarza, 1990; Stokes, Nash and
Harvey, 2007), river terraces (e.g. Khadkikar
et al.
, 1998;
Khadkikar, Chamyal, Ramish, 2000; Candy
et al.
, 2003;
Candy, Black and Sellwood, 2004a) and pediments (e.g.
Huser, 1976; Van Arsdale, 1982; Dhir
et al.
, 2004) often
calcretised. The presence of a well-developed calcrete can
influence landscape development since it creates an im-
pervious layer and may promote the erosion of overlying
soil horizons. In some landscapes, pedogenic and ground-
water calcrete layers act as caprocks (Maizels, 1990;
Kaemmerer and Revel, 1991; Nash and Smith, 1998),
while the erosion of noncemented sediments adjacent to
valley calcretes may lead to the creation of inverted relief
(Reeves, 1983; Mann and Horwitz, 1979). The presence of
channel calcretes within fluvial networks has been shown
to have an impact upon both morphology and long profile
development and may limit the depth to which scour can
occur (Nash and Smith, 2003; McLaren, 2004; Nanson
et al.
, 2005).
Extensive plateau calcretes are widespread in North
America (Bretz and Horberg, 1949; Brown, 1956; Reeves,
1970; Machette, 1985), North Africa (Moseley, 1965;
Abdel Jaoued, 1987) and the Middle East (Chapman,
1974). In southern Africa, exposures of calcrete occur
throughout the Kalahari, with greatest thicknesses around
palaeolakes and palaeodrainage features (Mabbutt, 1955;
Netterburg, 1980; Nash, Shaw and Thomas, 1994; Bl umel
Figure 8.6
Idealised pedogenic calcrete profile showing a
range of macroforms (from Wright, 2007).
limits the formation and preservation of most desert car-
bonate crusts. In southwest Australia, calcretes are best
developed where mean annual rainfall reaches 800 mm
but mean annual evaporation is 1900 mm (Semeniuk and
Searle, 1985).
Such environmental boundaries apply mainly to pe-
dogenic calcretes. Nonpedogenic crusts such as ground-
water calcrete may accrete under more arid conditions
and rhizocretionary calcretes in more humid environments
(Mann and Horwitz, 1979; Semeniuk and Searle, 1985),
such that the maximum mean annual rainfall under which
calcretes can develop may be as high as 1000 mm (Mack
and James, 1994). It has also been demonstrated experi-
mentally that the freezing of calcium carbonate-saturated
water leads to calcite precipitation (Ek and Pissart, 1965).
