Desert crusts and rock coatings - Arid Zone Geomorphology - page 142

Table 8.4 Stages in the morphogenetic sequence of carbonate deposition during calcrete formation (Bachman and Machette,

1977).

Stage Diagnostic carbonate morphology

I Filaments or faint carbonate coatings, including thin discontinuous coatings on the underside of pebbles

II Firm carbonate nodules few to common but isolated from one another. The matrix in between nodules may include

friable interstitial carbonate accumulations. Continuous pebble coatings present

III Coalesced nodules in disseminated carbonate matrix

IV Platy, massive indurated matrix, with relict nodules visible in places. The profile may be completely plugged with

weak incipient laminar carbonate coatings on upper surfaces. Case hardening is common on vertical exposures

V Platy to tabular, dense and firmly cemented. Well-developed laminar layer on upper surfaces. Scattered incipient

pisoliths may be present in the laminar zone. Case hardening common

VI Massive, multilaminar and brecciated profile, with pisoliths common. Case hardening common

8.5.3 Micromorphology and chemistry

comparatively few descriptions from Asia, though cal-
cretes can be common in both pedogenic contexts (e.g.
Tandon and Narayan, 1981; Dhir et al. , 2004, 2010;
Khadkikar, 2005; Durand et al. , 2007) and in associa-
tion with alluvial deposits (e.g. Khadkikar et al. , 1998;
Khadkikar, Chamyal and Ramesh, 2000). In Australia,
many calcretes have been interpreted as nonpedogenic
crusts (Mann and Horwitz, 1979; Arakel, 1986, 1991; Ja-
cobson, Arakel and Chen, 1988), but pedogenic varieties
are also extensive (Warren, 1983; Semeniuk and Searle,
1985; McQueen, Hill and Foster, 1999).
The development of both pedogenic and nonpedogenic
calcretes often bears little relationship to the materials
upon which they accrete. Calcretes can occur on most
sediments and rock types, both fresh or weathered, rang-
ing from undifferentiated metamorphics (Durand et al. ,
2007), granites (Scholz, 1972) and volcanics (Hay and
Reeder, 1978) to gypsum bedrock (Lattman and Lauffen-
burger, 1974), alluvium (Khadkikar et al. , 1998), dune
sand (Warren, 1983; Dhir et al. , 2004, 2010) and loess
(Reeves, 1970; Gustavson and Holliday, 1999). It has been
suggested that pedogenic calcretes form preferentially on
basic rocks (Lattman, 1973; Wells and Schultz, 1979).
However, this presupposes that the source of carbonate
was from bedrock as opposed to surface inputs (Capo
and Chadwick, 1999; Chiquet et al. , 1999). Arakel (1986)
suggested that rock type is important in determining the
distribution of pedogenic and groundwater calcretes in
central Australia. There are numerous examples of cal-
cretes formed by in situ alteration of limestone (Blank and
Tynes, 1965; El Aref, Abdel Wahab and Ahmed, 1985)
or chalk (Yaalon and Singer, 1974). Clearly, an immedi-
ate supply of calcium carbonate is conducive to calcrete
formation but hydrological or atmospheric inputs from

Calcretes contain a variety of microtextures (Figure 8.9),
which provide insights into the way the crust developed.
Calcium carbonate in the majority of calcretes is in the
form of micrite - aggregates of calcite crystallites less
than about 100 µm in diameter. Micrite may occur in a
variety of forms (Figure 8.9(a)), including 2-3 µmmicro-
crystalline interflorescences, rounded calcite grains, cal-
cans, neocalcans, neocalcitons surrounding pore spaces,
concretions and nodules or glaebules (Sehgal and Stoops,
1972; Rabenhorst, Wilding and Girdner, 1984; Drees and
Wilding, 1987; West et al. , 1988). Nodules exist in or-
thic, disorthic and allothic forms; orthic nodules appear
to be similar to the surrounding matrix and developed
in situ, disorthic nodules exhibit evidence of displace-
ment and allothic nodules are relict forms inherited from
a pre-existing soil (Wieder and Yaalon, 1974). The pres-
ence of micrite can indicate that precipitation was driven
by rapid evaporation. However, micrite formation is not
always syngenetic with calcretisation, since sparry cal-
cite may alter to micrite through dissolution and repre-
cipitation (Kahle, 1977). Other common calcite crystal
textures include microspar (Figure 8.9(b)), flower spar
and both random and tangential fibres or needles (James,
1972), which may crystallise from carbonate-saturated
water (Knox, 1977). The presence of well-developed
rhombic microspar and sparry calcite crystals may indi-
cate that carbonate precipitation occurred under relatively
moist microconditions (cf. Nash and Smith, 2003). Nee-
dle fibre calcite and calcified filaments often occupy void
spaces within powder, nodular, platy and hardpan cal-
crete types (Phillips and Self, 1987; Phillips, Milnes and
Foster, 1987). Calcified filaments include biogenic struc-