528
Climatic Geomorphology
3.8.3. The laterite profile
Laterite forms part of a group of weathered materials that are organized into zones or
horizons, forming what is known as the laterite profile. An ideal profile was elaborated
by Millot (1964) (Figure 20.23), inspired by differentiations made by Walther (1915) and
reported by Thomas (1974) and Faniran and Jeje (1983) in their respective treatises.
Above the lateritic horizon there lies a brownish loose material, slightly humic and with
ferruginous concretions and fragments. The laterite, with a dark ochre colour, presents a
thickness ranging between some metres to 60 m with dominant vermiform and pisolithic
structures. Below the lateritic horizon there appears the mottled clay zone with red
ferruginous blots 1 to 9 m thick, although in some profiles it is lacking. The transition to
the laterite can be sharp or gradual (McFarlane, 1976). The lowest horizon is the pallid
zone, also named lithomarge, constituted by kaolinitic clays and quartz grains, and of
variable thickness (5 to 60 m). Its white colour is due to iron leaching and, much like the
motted clay zone, it is absent in some profiles. In general, if the pallid zone presents an
important thickness, the laterite thins and vice versa (Loughnan et al., 1962). Downwards
there is a transition zone of variable thickness, constituted by weathered material rarely
containing fresh rock blocks, which lies upon the parent rock.
This standard or ideal profile serves as a guide for recognizing the different zones that
can exist in lateritic profiles although, as mentioned above, some horizons may be
missing in a given profile. Laterites or bauxites of great thickness in some cases lie
almost directly on fresh rock, whereas thin laterites commonly rest on very thick
regoliths (McFarlane, 1983).
PROFILE TRUNCATED AFTER EROSION
,\
LATERITIC PROFILE WITH CRUST UNDER FOREST
~umic
horizon ___~0
:":' ~.:..' .' .' .' ~."i' :' :.:;"."~".:.:".::::.' .:
Beige gravelly soil I
I
;;L.~:~,:.".,,' :' .:~".' ;;:' ~-' ~.' .,;,' :~. aedconcretionary i
i" 1
so,
i t
-5
Mottled clay
..~
White clay
I
l- 7
(lithomarge)
I
I " E
\ \\\ ,,
-8
\\
\
. .
'\ \ 6\ (~\\ \.\ Gr'tw'thc~
I
I~ ~
\\~.\\.\ \ \~,x,
I
i -
~--~~~~
Parentrock !
rll
BO. HO,
Figure 20.23. Lateritic profile under shade forest, and its equivalent truncated by erosion in Sudan (after
Millot (1964)).
Climatic Geomorphology
3.8.3. The laterite profile
Laterite forms part of a group of weathered materials that are organized into zones or
horizons, forming what is known as the laterite profile. An ideal profile was elaborated
by Millot (1964) (Figure 20.23), inspired by differentiations made by Walther (1915) and
reported by Thomas (1974) and Faniran and Jeje (1983) in their respective treatises.
Above the lateritic horizon there lies a brownish loose material, slightly humic and with
ferruginous concretions and fragments. The laterite, with a dark ochre colour, presents a
thickness ranging between some metres to 60 m with dominant vermiform and pisolithic
structures. Below the lateritic horizon there appears the mottled clay zone with red
ferruginous blots 1 to 9 m thick, although in some profiles it is lacking. The transition to
the laterite can be sharp or gradual (McFarlane, 1976). The lowest horizon is the pallid
zone, also named lithomarge, constituted by kaolinitic clays and quartz grains, and of
variable thickness (5 to 60 m). Its white colour is due to iron leaching and, much like the
motted clay zone, it is absent in some profiles. In general, if the pallid zone presents an
important thickness, the laterite thins and vice versa (Loughnan et al., 1962). Downwards
there is a transition zone of variable thickness, constituted by weathered material rarely
containing fresh rock blocks, which lies upon the parent rock.
This standard or ideal profile serves as a guide for recognizing the different zones that
can exist in lateritic profiles although, as mentioned above, some horizons may be
missing in a given profile. Laterites or bauxites of great thickness in some cases lie
almost directly on fresh rock, whereas thin laterites commonly rest on very thick
regoliths (McFarlane, 1983).
PROFILE TRUNCATED AFTER EROSION
,\
LATERITIC PROFILE WITH CRUST UNDER FOREST
~umic
horizon ___~0
:":' ~.:..' .' .' .' ~."i' :' :.:;"."~".:.:".::::.' .:
Beige gravelly soil I
I
;;L.~:~,:.".,,' :' .:~".' ;;:' ~-' ~.' .,;,' :~. aedconcretionary i
i" 1
so,
i t
-5
Mottled clay
..~
White clay
I
l- 7
(lithomarge)
I
I " E
\ \\\ ,,
-8
\\
\
. .
'\ \ 6\ (~\\ \.\ Gr'tw'thc~
I
I~ ~
\\~.\\.\ \ \~,x,
I
i -
~--~~~~
Parentrock !
rll
BO. HO,
Figure 20.23. Lateritic profile under shade forest, and its equivalent truncated by erosion in Sudan (after
Millot (1964)).