Geoscience Reference
In-Depth Information
from solution and redeposited either through the agency of infiltrating waters or from
the escape of carbon dioxide from vadose water (Lamplugh,
1902
; Bates and Jackson,
1987
, p. 94). It follows from these definitions that ferricrete, silcrete and calcrete are
secondary features formed by the precipitation of allochthonous inputs of iron, silica
or calcium carbonate within coarse-grained alluvium. In the case of certain iron- and
silica-rich formations, this is not always true. Gypcrete, as the name implies, is a hard
gypsum crust. Ferricrete in Lamplugh's sense of the term is broadly equivalent to
certain forms of detrital laterite, discussed in
Section 15.8.2
.
Gypsum crusts are less widespread than calcrete, silcrete or ferricrete are because
of the greater ease with which gypsum can be dissolved or removed by deflation.
Gypcrete tends to be confined to playa and sebkha margins in hyper-arid regions, as in
the northern Sahara, the coastal regions around the Persian Gulf, the Lake Eyre Basin
in central Australia and a number of the saline lakes in the more arid regions of South
America (Watson,
1983
,
fig. 5.1
). As a broad generalisation, calcretes and silcretes
tend to occur in the semi-arid to arid regions of Africa and Australia in particular,
while ferricretes occupy the more humid sectors of the desert margins (Mabbutt,
1977
). The value of silcretes and ferricretes as indicators of past climates is limited
by the difficulty of obtaining precise and reliable ages for these formations (Bourman,
1993
; Twidale and Bourne,
1998
) and, in the case of silcrete especially, by the lack
of convincing modern analogues.
15.8.1 Pedogenic, biogenic and groundwater calcretes
Calcretes have been widely investigated in many parts of the semi-arid world (Goudie,
1983
; Lekach et al.,
1998
; Khadkikar et al.,
2000
;Amitetal.,
2007
;Amitetal.,
2010
; Singhvi et al.,
2010
), not least because they are often hosts for economic-
ally important minerals such as uranium, copper and gold. Calcretes have attracted
considerable recent interest from the mineral exploration community, most notably
in Australia, where a Cooperative Research Centre for Landscape Environments and
Mineral Exploration involving the mining industry, universities and government agen-
cies has been pioneering new methods of detecting such minerals as uranium, copper
and gold in calcrete deposits formed within desert dunes, fossil river channels and
desert lake margins (Lintern,
2001
; Keeling,
2004
; Schmidt Mumm and Reith,
2004
;
Wittwer et al.,
2004
). Uranium-bearing calcretes in Mauritania and Namibia have
also aroused the interest of exploration geologists but have so far been less intensively
investigated than their Australian counterparts.
Calcareous soils are actively forming today on a variety of rock types in areas
with a mean annual rainfall of 200-500 mm, although they may occur in wetter areas
where the underlying parent rock is limestone, dolomite or eolianite. A series of stages
has been identified in the development of calcretes in the drier regions of the United
States, north-west and southern Africa, north-west India, Israel, peninsular Arabia,
