Geoscience Reference
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break up, with features such as brecciated cobbles and
bedrock and pseudo-anticlines formed (e.g. Price, 1925;
Jennings and Sweeting, 1961; N. L. Watts, 1977, 1978,
1980). This sequence may be complicated if, for exam-
ple, landscape erosion occurs, as this may cause reworking
and a lowering of the carbonate profile by leaching and
translocation.
There are debates over why brecciation occurs (Klappa,
1979c, 1980; Braithwaite, 1983), but it is probably a prod-
uct of root growth (Klappa, 1980) or progressive dis-
placive crystallisation during carbonate accretion (N. L.
Watts, 1978). Displacive crystallisation occurs when the
calcium carbonate content exceeds the host material's vol-
umetric porosity (Gardner, 1972) and is common in non-
carbonate hosts as calcite is unable to form adhesive bonds
with noncarbonate materials (Chadwick and Nettleton,
1990). Chemical replacement rather than physical dis-
placement of host material has been advocated to explain
the purity of some pedogenic calcretes. Hubert (1978),
for example, held that palaeosol calcretes in Connecticut
show signs of 50-95 % replacement; McFarlane (1975)
suggested that a fabric of floating garnet grains in some
Kenyan calcretes represented the residuum of a replaced
host material. Many calcretes exhibit signs of replacement
of clay (Hay and Reeder, 1978), feldspars and quartz
(Burgess, 1961; Chapman, 1974), but the latter may be
diagenetic rather than syngenetic features. Displacive, re-
placive and passive void-filling can all take place (Yaalon
and Singer, 1974; Watts, 1980; Nash, Shaw and Thomas,
1994), but their relative importance varies according to
host lithology and environmental conditions.
Less research has been undertaken into the origins of
nonpedogenic calcretes, despite the fact that the thickest
calcrete accumulations in the world are associated with
carbonate precipitation in the capillary fringe or phreatic
zone. Nonpedogenic calcretes are frequently referred to
as 'groundwater calcretes', a term often considered syn-
onymous with phreatic, channel, valley or alluvial fan
calcrete. These are, however, distinct calcrete types, and
while there is a link between them, it is questionable
whether such terms should be used interchangeably. The
distinction between phreatic and groundwater calcretes is
also unclear in the literature. Groundwater calcretes (
sensu
stricto
) range from thin layers of nodules to large bodies
of massive carbonate (Wright, 2007) and form by pre-
cipitation in the zone of capillary rise directly above the
watertable (although Jacobson, Arakel and Chen, 1988,
suggest that precipitation can also occur below this level).
Phreatic calcretes, in contrast, are those in which cemen-
tation has occurred at or below the watertable (Arakel,
1986; Wright and Tucker, 1991). In both types, carbonate
precipitation may be triggered by mechanisms including
evaporation, evapotranspiration and degassing. Phreato-
phytic rhizocretions and calcified root mats produced by
deep-rooting plants have been described in groundwa-
ter calcretes (Semeniuk and Meagher, 1981), suggesting
that organic agency may be a factor in formation. Some
groundwater calcretes are of economic significance as
they are enriched in ions such as strontium (Kulke, 1974)
or, more unusually, uranium minerals such as carnotite
(Carlisle
et al.
, 1978; Arakel and McConchie, 1982; Briot,
1983).
Calcretes associated with drainage systems may form
ribbon-like bodies extending many hundreds of kilome-
tres in length. Some of the best developed examples in
Australia are over 10 km in width and many metres in
thickness (cf. Arakel and McConchie, 1982; Carlisle,
1983; Arakel, 1986, 1991; Morgan, 1993). Nash and
McLaren (2003) distinguish between valley calcretes,
which cement alluvium within broad, shallow, drainage
courses but do not necessarily occupy the full valley
width (e.g. Carlisle
et al.
, 1978; Mann and Horwitz,
1979; Carlisle, 1983; Reeves, 1983; Arakel, 1986, 1991;
Jacobson, Arakel and Chen, 1988; Arakel
et al.
, 1989), and
channel calcretes, which cement alluvium within confined
impermeable bedrock channels or exhumed palaeochan-
nels and may occupy the full channel cross-section (e.g.
Maizels, 1990; Rakshit and Sundaram, 1998; Nash and
Smith, 2003; McLaren, 2004). Valley calcretes in the
Kalahari (Figure 8.7(c)) comprise sand-sized sediments
bound by massive micritic cements and are suggested
to have formed by relatively rapid calcium carbonate
precipitation in a near-surface setting and close to the
watertable, driven by evaporation or evapotranspiration
(Nash and McLaren, 2003). In contrast, channel calcretes
from southeast Spain (Figure 8.7(d)) are cemented by
micrite and pore-filling sparite, with an increasing per-
centage of euhedral sparite crystals towards the base of
the profile (Nash and Smith, 2003). These cements devel-
oped in conjunction with a fluctuating watertable, with the
downward increase in crystal size caused by the greater
duration of wetting in basal zones. Channel calcretes in
southeast Spain occur most commonly at sites of gradient
change where tributaries feed into main fluvial trunk val-
leys, so carbonate deposition may have been triggered by
the common ion effect at sites of subsurface water mixing
(Nash and Smith, 1998, 2003).
Calcretes preserved within alluvial fans form sheet-like
bodies (Mack, Cole and Trevino, 2000) and reach total
thicknesses of over 200 m (Maizels, 1987). Carbonate
deposition may occur by the common ion effect, espe-
cially where alluvial fans debouch on to playa surfaces
