Laterite profiles

Figure 11. Distribution with depth of U/Th normahzed to the bedroek in two lateritic profiles of the Kaya toposeqnenee, about 300m apart (Burkina Faso) (Deqnineey et al. 2002 submitted). One profile is located downhill (Kaya 5) and the other one at the top of a residual hill (Kaya 1). The laterite consists of an uppermost fermginous hardtop, an intermediate pink clay nnit and a lowest pistachio unit. For Kaya 5 profile, U/Th distribntion shows a relative enrichment of U in the nppermost horizon and depletion in the lower part of the profile. This kind of distribution is quite conunon in weathering profiles bnt is not systematic as illnstrated by the Kaya 1 profile. In the latter, a relative depletion of U is observed in the npper part and a U-enriched level in the intermediate horizon. This lateral difference in U distribution is explained by vertical redistribntion of U from the ferruginons top to the nnderlying horizons, whose intensity is controlled by the evolntion of the iron oxides from the nppermost horizons (Dequincey et al. snbmitted).

Theveniaut, H. Freyssinet, Ph. (1999) Paleo-magnetism applied to lateritic profiles to assess saprolite and duricrust formation pro-... 

U. (1994) Mineralogy and stratigraphy of three deep lateritic profiles of the Jos Plateau (Central Nigeria). Catena 21 195—214 Zeien, H. Briimmer, G.W. (1989) Chemische Extraktionen zur Bestimmung von Schwer-metallbindungsformen in Boden. Mitted. 

Braun, J. J., Pagel, M., Herbillon, A. Rosin, C. 1993. Mobilization and redistribution of REEs and thorium in a syenitic lateritic profile A mass balance study. Geochimica et Cosmochimica Acta, 57, 4419-4434. 

Marker, M.E., McFarlane, M.J. Wormald, R.J. (2002) A laterite profile near Albertinia, Southern Cape, South Africa its significance in the evolution of the African Surface. South African Journal of Geology 105, 67-74. 

Figure 3.2 Examples of mesa-like remnants of a Late Cretaceous lateritised palaeosurface developed on Deccan basalt from widely separated localities across the Maharashtra Plateau, western India. On the eastern reaches of the Deccan (A) Bidar (17°55 N, 77°33 E) (B) Bidar area (17°53 N, 77°36 E) on the western reaches of the Deccan c. 400 km from Bidar, (C) Panchgani (17°56 N, 73°49 E) (D) Patan (17°23 N, 73°56,E). In all these areas, the mechanically resistant upper layers of the laterite profile typically form a protective capping to the less altered materials beneath and, following erosion, producing a characteristic cliff-like morphology.

Any silica not combined as kaolinite typically goes into solution as silicic acid, and may then be lost from the system. However, precipitation of secondary silica is not uncommon in some laterite profiles, where it can become part of pore or void infillings. 

Kaolinite is an important constituent of many lateritic profiles, and a common product of hydrolysis it tends to be abundant where there is free-drainage. The reaction progressively separates silica in aqueous solution from aluminium, which remains in the solid phase (as kaolinite or gibbsite). 

Table 3.2 Geochemical analyses (by XRF) of autochthonous laterite profiles (A) Developed on Deccan basalt exposed at Bldar, India (see Figures 3.2A and 3.3)... 

For lateritic profiles, routine thin-section analysis can readily be achieved on those materials that represent the lower degrees of alteration... 

One of the perennial problems of laterite study is the determination of their age (Bourman, 1993). Until recently, stratigraphical techniques have provided the only method. However, since the protolith may be considerably older than any laterite profile developed upon it, the value of such techniques is limited. It is clear that many lateritic duricrusts are of considerable antiquity because their current distribution places them in climatic, geomorphological or tectonic settings that otherwise would have been inimical to their formation. Establishing when the lateritisation process that formed them began, or the point at which they ceased to form, remain polemic issues, but a number of recent studies have provided some insights. 

Finally, since the formation of laterite profiles depends upon prevailing climate, the distribution of laterites, and to a lesser extent ferricretes, of different ages have become increasingly used as proxies for palaeoclimate... 

Kisakurek, B., Widdowson, M. James, R.H. (2004) Behaviour of Li isotopes during continental weathering the Bidar laterite profile, India. Chemical Geology 212, 27-44. 

McFarlane, M.J. (1983b) A low level laterite profile from Uganda and its relevance to the question of parent material influence on the chemical composition of laterites. In Wilson, R.C.L. (Ed.) Residual Deposits Surface Related Weathering Processes and Materials. Special Publication 11, Geological Society of London, pp. 69-76. 

Moura, M.L. (1987) The establishment of an international interdisciplinary collection of reference laterite profiles. Zeitschrift fur Geomorphologie N.F., Supplement Band 64, 111-118. 

Ollier, C.D. (1991) Laterite profiles, ferricrete and landscape evolution. Zeitschrift fiir Geomorphologie N.F. 35, 165-173. 

Sharma, M., Clauer, N. Toulkeridis, T. (1998). Rhemium-omsium systematics of an ancient laterite profile. Abstract Goldschmidt Conference, Toulouse, 1998. Mineralogical Magazine 62A, 1373-1374. 

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