Lateritic weathering profile

Anand, R.R. Butt, C.R.M. (2003) Distribution and evolution of laterites and lateritic weathering profiles, Darling Range, Western Australia. Australian Geomechanics 38, 41-58. 

The manner in which ferricrete alteration profiles evolve differs from laterite weathering profiles in a number of ways a genetic distinction between laterite and ferricrete is adopted here (Figure 3.1). 

Figure 3.1 Schematic diagram showing the laterite-ferricrete genetic relationship, and the natural continuum between the autochthonous (i.e. in situ weathering profiles) and allochthonous end-members. Bidar and MQ (Merces Quarry) are two laterite weathering profiles from India, and PB (Palika Ba) is a fer-ricrete alteration profile from the Gambia (see section 3.3.2).

The division into laterite and ferricrete used in this chapter represents a useful process-based distinction, but the practicality of determining whether mineral components of a profile are allochthonous or autochthonous is problematic because many lateritic weathering profiles are subsequently modified by the introduction of allochthonous materials. Conversely, once formed, ferricretes can be subject to weathering processes in situ and evolve toward more lateritic-type profiles. Nevertheless, the distinction between dominantly autochthonous weathering profiles or allochthonous alteration profiles is an important one because it places constraints upon the processes operating during duricrust evolution, and also upon contemporaneous climatic and geomorphological conditions. 

Figure 3.4 Examples of laterite and ferricrete profiles. (A) Merces Quarry near Panjim, Goa (15°29 N, 73°53 E), where an entire laterite weathering profile developed upon Proterozoic greywacke is exposed. (B) Laterite quarrying at Bidar (17°55 N, 77°33 E), showing recently cut sections (with 1 m rule for scale) where material from the upper parts of the profile (zone IV on Fig. 3) has been extracted for laterite bricks. The Bidar laterite localities were used by Newbold (1846) to argue for laterite as an in situ weathering product. (C) Ferricrete profile exposed in a road cutting to the south of Palika Ba (13°28 N, 15°14 W), near the Gambia River, Gambia, West Africa.

In contrast to lateritic weathering profiles, ferricrete alteration profiles incorporate materials non-indigenous to the immediate locality of duricrust... 

Figure 3.7 Schematic representation of changes in element abundances in a lateritic weathering profile affected by the establishment of a water table. This illustrates how an iron-enriched and silica depleted zone may develop within the weathering profile.

Mineralogical variations within lateritic weathering profiles... 

Figure 3.9 Pathways of formation of secondary minerals in lateritic weathering profiles (after Anand, 2005).

The most widespread of the secondary minerals formed during the development of a laterite weathering profile are iron and aluminium ses-quioxides (Table 3.1). These may form either directly from the alteration of primary minerals, or else via a series of pathways involving the formation of intermediary sheet silicate minerals and clays (e.g. chlorite, illite, smectite, vermiculite and halloysite), which are then themselves broken down, stripped of their mobile ions and silica, and eventually converted to alumina and ferric oxyhydroxide residua (Figure 3.9). It is not possible to describe these mineral transformations in detail, but the key issue is that under tropical-type weathering conditions these transformation pathways lead to... 

A number of principles are illustrated by these two lateritic weathering profiles and by the pattern in which the constituent samples appear on Figure 3.10. The ubiquity of the lateritisation process is confirmed by the fact that comparable patterns of element behaviour occur in two profiles that differ... 

Dating and Palaeoenvironmental Significance of Lateritic Weathering Profiles... 

Mann, A.W. (1984) Mobility of gold and silver in lateritic weathering profiles some observations from Western Australia. Economic Geology 79, 38M9. 

Generalised vertical section through the autochthonous Bidar laterite weathering profile. 

Examples of weathering and lateritic textures at key horizons through the Merces Quarry lateritic weathering profile. 

Schematic representation of changes in element abundances in a lateritic weathering profile affected by... 

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