FerriCrete alteration profile

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). 

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

Figure 3.8 Examples from the ferricrete alteration profile observed at outcrop at Palika Ba (13°28 N, 15°14 W), near the Gambia River, Gambia, West Africa. (A) Nodules of ferricrete developing within Quaternary alluvial sands and silts deposited by the Gambia River. Nodules consist of predominantly goethite with minor haematite (sample PG4 in Table 3.3). (B) Massive iron-cemented ferricrete horizon displaying characteristic tubes (i.e vermiform structure sample PG2 in Table 3.3). Importantly, the iron cement is entirely due to the introduction of allochthonous iron into the pore spaces of the sediment.

The mineralogy of ferricrete alteration profiles can be complex and varied because of the incorporation of mechanically derived materials and the retained importance of host rock composition after the formation of secondary minerals. In general, ferricrete profiles do not display the progression of alteration minerals observed in laterites. Where ferricretes are formed by mechanical accumulation, they can lie disconformably above unaltered bedrock (Bowden, 1987, 1997). In these instances, the ferricrete mineral assemblage will be inherited, in part, from the derived materials, and in part from later cementation processes that involve remobilised iron and alumina deposited as neo-formed oxyhydroxides. In such examples, determining the sequence of mineralogical transformations becomes exceptionally difficult. 

Chemical variations within ferricrete alteration profiles... 

Table 3.3 Geochemical analyses (by XRF) of the ferricrete alteration profile exposed at Palika Ba, Gambia, West Africa, (see figs 3.4C and 3.8)...

Examples from the ferricrete alteration profile observed at outcrop at Palika Ba, near the Gambia River,... 

Geochemical analyses of the ferricrete alteration profile exposed at Palika Ba, Gambia, West Africa. 

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. 

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