278
Climatic Geomorphology
SiO2: 12.30%, MgO: 3.05%, A1203: 2.12% and Fe203: 2.03%. Those caliches rich in
magnesium are called dolocretes. The percentages of each distinct component vary
enormously from one place to another, as can be seen in the calcretes in southern Australia
(Hutton and Dixon, 1981; Dixon, 1994b). In a study of numerous chemical analyses of
caliches from Oklahoma and New Mexico (Aristarain, 1970) the calcium carbonate,
magnesium, and ferric iron contents were seen to diminish with depth, whereas the silica
content increased significantly in the middle part of the profile. Elsewhere, Dixon (1994b)
examined the role of topography in the chemistry variations in caliches and demonstrated a
higher percentage of calcium lower down in the toposequence and an increase in
magnesium in the upper part. In all studies the calcium content fell with depth.
Calcite and dolomite are the dominant carbonate minerals in caliches and are
accompanied by quartz, opal, and clay minerals. Caliches can also contain minor amounts
of soluble salts (gypsum), glauconite, phosphates, heavy minerals, etc. Mineralogical
investigations have centered around the study of the clay fraction and its origin and, as a
consequence, improving our understanding of the processes involved in the formation of
clay-beating caliches. The dominant clay minerals are palygorskite and sepiolite, although
illite, kaolinite, montmorillonite, inter-layered illite-montmorillonite, and chlorite have
all been recognised (Aristarain, 1970, 1971; Gardner, 1972; Reeves, 1976; Hay and
Wiggins, 1980). The origins of palygorskite and sepiolite have been attributed to
neoformation in an environment containing sufficient magnesium, provided by the
weathering of suitable lithologies (Hay and Wiggins, 1980; Hutton and Dixon, 1981;
Sancho et al., 1992). Other investigators maintain that they arise through the alteration of
montmorillonite and inter-layered illite-montmorillonite (Watts, 1980).
5.2.2. Morphology
Descriptions of caliche morphology are numerous and different classification schemes
have been proposed based on distinct criteria. Some are based on sequential evolution
(Netterberg, 1969), others on degree of hardness of the carbonates (Reeves, 1976). The
degree of calcification also has some bearing on differentiating between soil
characteristics (Gile et al., 1966; Machette, 1985). Some distinguish only a few
morphological types and others, on the contrary, numerous types (Reeves, 1976).
The fundamental macroscopic morphologies (Sancho and Mel6ndez, 1992) are calcified
gravels, which normally appear at the base of a caliche profile, though small levels can be
encountered in the interior of the profile. This is constituted of clasts with a vadose
carbonate coat and a matrix that may contain root casts. Chalky caliche (Figure 12.22) is
formed of loose, clay, and sand-sized carbonate particles with practically no host rock
material. It can be lightly cemented and have a leafy structure. Hardpan caliche occurs in
the upper parts of the profile (Figure 12.23). It consists of centimetre and decimetre scale
plates that stack laterally. They form ooids, basal speleothemic carbonates, root traces, and
dessication cracks. Hardpan caliche can also be found deformed, forming pseudoanticlines (Figure 12.23) generated by expansion and contraction as a consequence of
hydration-absorption of water to a minerals of highly soluble salts, periodic wetting and
drying, calcite crystallisation and subsequent arching. Deformation may also be due to
the presence of expandable clays (Reeves, 1976). Finally, nodular caliche can develop in
fine-grained material, in which the particles are cemented by carbonate and, in turn,
Climatic Geomorphology
SiO2: 12.30%, MgO: 3.05%, A1203: 2.12% and Fe203: 2.03%. Those caliches rich in
magnesium are called dolocretes. The percentages of each distinct component vary
enormously from one place to another, as can be seen in the calcretes in southern Australia
(Hutton and Dixon, 1981; Dixon, 1994b). In a study of numerous chemical analyses of
caliches from Oklahoma and New Mexico (Aristarain, 1970) the calcium carbonate,
magnesium, and ferric iron contents were seen to diminish with depth, whereas the silica
content increased significantly in the middle part of the profile. Elsewhere, Dixon (1994b)
examined the role of topography in the chemistry variations in caliches and demonstrated a
higher percentage of calcium lower down in the toposequence and an increase in
magnesium in the upper part. In all studies the calcium content fell with depth.
Calcite and dolomite are the dominant carbonate minerals in caliches and are
accompanied by quartz, opal, and clay minerals. Caliches can also contain minor amounts
of soluble salts (gypsum), glauconite, phosphates, heavy minerals, etc. Mineralogical
investigations have centered around the study of the clay fraction and its origin and, as a
consequence, improving our understanding of the processes involved in the formation of
clay-beating caliches. The dominant clay minerals are palygorskite and sepiolite, although
illite, kaolinite, montmorillonite, inter-layered illite-montmorillonite, and chlorite have
all been recognised (Aristarain, 1970, 1971; Gardner, 1972; Reeves, 1976; Hay and
Wiggins, 1980). The origins of palygorskite and sepiolite have been attributed to
neoformation in an environment containing sufficient magnesium, provided by the
weathering of suitable lithologies (Hay and Wiggins, 1980; Hutton and Dixon, 1981;
Sancho et al., 1992). Other investigators maintain that they arise through the alteration of
montmorillonite and inter-layered illite-montmorillonite (Watts, 1980).
5.2.2. Morphology
Descriptions of caliche morphology are numerous and different classification schemes
have been proposed based on distinct criteria. Some are based on sequential evolution
(Netterberg, 1969), others on degree of hardness of the carbonates (Reeves, 1976). The
degree of calcification also has some bearing on differentiating between soil
characteristics (Gile et al., 1966; Machette, 1985). Some distinguish only a few
morphological types and others, on the contrary, numerous types (Reeves, 1976).
The fundamental macroscopic morphologies (Sancho and Mel6ndez, 1992) are calcified
gravels, which normally appear at the base of a caliche profile, though small levels can be
encountered in the interior of the profile. This is constituted of clasts with a vadose
carbonate coat and a matrix that may contain root casts. Chalky caliche (Figure 12.22) is
formed of loose, clay, and sand-sized carbonate particles with practically no host rock
material. It can be lightly cemented and have a leafy structure. Hardpan caliche occurs in
the upper parts of the profile (Figure 12.23). It consists of centimetre and decimetre scale
plates that stack laterally. They form ooids, basal speleothemic carbonates, root traces, and
dessication cracks. Hardpan caliche can also be found deformed, forming pseudoanticlines (Figure 12.23) generated by expansion and contraction as a consequence of
hydration-absorption of water to a minerals of highly soluble salts, periodic wetting and
drying, calcite crystallisation and subsequent arching. Deformation may also be due to
the presence of expandable clays (Reeves, 1976). Finally, nodular caliche can develop in
fine-grained material, in which the particles are cemented by carbonate and, in turn,