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Widdowson, M.
(2003).
URL: http://www.routledge.com/books/Encyclopedia-of-Geo...
Abstract
Ferricrete is a horizon, at the land surface, made up of the cementation of near surface materials by iron oxides, and often forming a resistant duricrust. Typically between 1 – 20 m in thickness, it can form laterally extensive sheets which may extend over a few, to hundreds, or even thousands of km2. Consequently, is perhaps the most widespread of all the duricrust materials. At outcrop it comprises a massive, interlocking fretwork of iron, and often aluminium compounds (i.e. sesquioxides) that bind together other lithological and pedogenic components.
Ferricrete has a long history of study by geologists, geomorphologists, pedologists and agronomists. Considerable effort has been directed toward determining the conditions under which it forms, and this has proved crucial in advancing many aspects of tropical geomorphology. Moreover, chemical and physical durability of ferricrete has meant that it has often played a prominent role in evolution of tropical, and sub-tropical landscapes.
In its broadest sense, the term ferricrete can be used to describe any duricrust material in which the dominant bulk components are iron-rich compounds. However, whilst this may seem a straightforward definition, difficulties arise because the term has been employed to describe a wide range of terrigenous weathering and alteration products resulting from differing processes of formation. Therefore, it becomes important to understand the differences and, where possible, make distinction between genetically different types of iron-rich duricrust.
Since the nineteenth and early twentieth centuries, the terms ferricrete (‘an iron-rich crust’) and laterite (‘a highly weathered material rich in secondary forms of iron and/or aluminium’) have been used interchangeably to describe iron-rich duricrusts of various genetic origins. This has led to considerable confusion. However, the problems of co-ordinating laterite and ferricrete description stem not only from investigation by a variety of different scientific disciplines, but also from the development of extensive anglophone and francophone descriptive terminologies. Nevertheless, it is evident from field studies that the majority of iron-rich duricrusts can be adequately described in terms of two genetically distinct types. Recently, authors have begun to distinguish between those duricrusts in which an absolute iron enrichment occurs (i.e. those which receive a net input of iron), and those which attain their elevated iron contents through residual enrichment within the profile (i.e. no net input of iron).
Ferricretes are those duricrusts which incorporate materials non-indigenous to the immediate locality in which the duricrust formed. In many instances the transported materials can be readily identified as pebbles or clasts derived from adjacent lithological terranes, or as fragments from indurated layers of earlier generations of laterite or ferricrete. Importantly, the term ferricrete should also be extended to those materials whose constituents have been substantially augmented by the precipitation or capture of elements and compounds from allochthonous fluids (i.e. those derived during the breakdown and mobilisation of materials outside the immediate locality of ferricrete formation). Although it is the allochthony of the constituent materials of the ferricrete which justify its appellation, determining whether the introduction of such fluids has taken place, and confirming their allochthony is often problematic. However, since ferricretes may develop as ferruginous foot slope accumulations or within topographic depressions, they can often be distinguished by the fact that they display an obvious discordance with the underlying substrate lithologies. In effect, they do not display the progressive weathering profile characteristic of many laterite profiles, and instead the ferricrete horizon sits upon relatively unaltered bed rock.
By contrast, laterites are iron-rich duricrusts which have formed directly from the breakdown of materials in their immediate vicinity, and so do not contain any readily identifiable allochthonous component. Lateritic duricrusts are typically manifest as the uppermost layers of in-situ weathering profiles. Where these profiles are fully exposed, such as the widespread examples developed on basalt in Western India, they consist of an uninterrupted progression from unaltered bedrock, through the weathering front into saprolite (in which structure and crystal pseudomorphs of the parent rock may still be recognized), and then upward through increasingly altered and iron enriched zones that culminate as a highly indurated ‘tubular’ laterite at the top of the alteration profile.
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