Laterite and Ferricrete

Abstract

The term laterite, in its general sense, is given to a range of iron-rich, subaerial weathering products that develop as a result of intense, substrate alteration under tropical or sub-tropical climates. Physically, many laterites are rock-like, yet they cannot be easily placed into any of the major petrological groupings. Neither do they lend themselves readily to description as 'soils', other from the fact that they are the products of processes operating at the atmosphere -substrate interface. They are perhaps best considered to be metasomatic rock materials; i.e. rocks with chemical, mineralogical and physical characteristics that have been substantially changed by lowtemperature and pressure alteration processes such as those operating under subaerial conditions. The conditions under which lateritic profi les form are primarily:• a favourable climate, typifi ed by seasonal, high annual rainfall (e.g. a monsoon-like climate) -high humidity and high mean annual temperatures further promote chemical weathering and mineral alteration; • a favourable geomorphological environment, characterised by limited runoff and lack of aggressive erosion -the ingress of rainfall and/or the establishment of a water table may promote element enrichment and depletion processes; • relative tectonic stability, characterised by minimal uplift and crustal deformation.The manner in which ferricrete alteration profi les evolve differs from laterite weathering profi les in a number of ways: a genetic distinction between laterite and ferricrete is adopted here (Figure 3.1).

Origin of ferricretes in fluvial-marine deposits of the Lower Cenomanian Bahariya Formation, Bahariya Oasis, Western Desert, Egypt

“…The presence of siderite is consistent with the interpretation that the muds were deposited in organic-rich lagoonal or estuarine environments. Widdowson (2007) observed that multiple, or stacked ferricrete zones in profiles are not unusual, as the iron concentration is localized at the water table, which changes with time. Indeed, Goldbery and Beyth (1984) interpreted the multiple sheet crusts in the Triassic Budra Formation at Gebel Mussaba Salama as the record of still-stands in the rising water table on an aggrading alluvial plain.…”

“…Conversely, Phillips (2000), following Goudie (1985) and Pain and Ollier (1992), restricted the term ferricrete to refer to regolith in an indurated state due to cementation by iron in the zone of water table fluctuation, thus distinguishing ferricrete from laterite, as earlier defined by McFarlane (1983), Ollier (1991), Aleva (1994) and Widdowson (2007). Widdowson (2007), for example, described ferricrete as an ''alteration profile," formed by the accumulation of allochthonous iron. Pickett (2003) commented usefully on the importance of recognizing features of pedogenic profiles to distinguish laterites from ferricretes.…”

“…Pickett (2003) commented usefully on the importance of recognizing features of pedogenic profiles to distinguish laterites from ferricretes. This paper follows the usage of Widdowson (2007) and earlier workers in considering ferricretes as iron-cemented regolith formed primarily through non-pedogenic processes. The broader term ironstone is used in reference to all iron-enriched sedimentary strata, regardless of the mode of enrichment.…”

Elemental weathering fluxes and saprolite production rate in a Central African lateritic terrain (Nsimi, South Cameroon)

“…However, the integrated effects of intrinsic changes in mineral properties and abundances, as well as extrinsic long-term changes in climate and vegetation, soil permeability and topography, commonly only reach maturity over periods of tens of thousands to millions of years (Nahon, 1991;Tardy, 1993;Thomas, 1994;Tardy and Roquin, 1998). Moreover, most of these lateritic covers have developed in those regions where prevailing transportlimited conditions have been favourable to deep and prolonged chemical weathering (Nash and McLaren, 2007;Retallack, 2010;Tardy and Roquin, 1998;Widdowson, 2003Widdowson, , 2007.…”

A reconstruction of the palaeoecology and environmental dynamics of the Bahariya Formation of Egypt

“…Enhanced continental weathering resulted in Bahariya developing layers of iron ore, ranging from 5 to 7 meters in thickness in some areas (Floegel & Hay, 2004;Meyer & Kump, 2008;Tanner & Khalifa, 2009;Baioumy & Boulis, 2012). Unsurprisingly, ferricretes are also common at Bahariya (Tanner & Khalifa, 2009) and usually form in large numbers in swamps when the soluble iron in groundwater meets salt water (Widdowson, 2007).…”