West African lateritic pediments: Landform-regolith evolution processes and mineral exploration pitfalls
Résumé
This paper is a contribution to the understanding of surface dynamics of tropical shields over geological timescales. Emphasis is put on the fundamental and applied implications of regolith production and dispersion processes through the formation, dissection and preservation of landforms. It is based on the case study of sub Saharan West Africa, which recorded Neogene (24-3 Ma) stepwise dissection of its topography through the emplacement of three lateritic pediment systems, which still occupy most of its surface. Pediments are erosional/transportation slopes having been weathered and duricrusted. Pediment-regolith associations therefore depend on the parent rock, transport dynamics and preservation of the material having transited on their surface as well as on the intensity of their weathering/duricrusting. Iron oxy-hydroxide-cemented clastic sediments (detrital ferricretes) and unconsolidated clastic sediments are the dominant outcropping material, and as such represent a challenge for mineral exploration that relies on surface geochemical sampling to detect metal concentration in the bedrock. Landform-regolith mapping beyond the scale of modern interfluves combined with paleolandscape reconstitution are relevant to provide exploration guides for (i) interpreting geochemical anomalies on pediments, (ii) tracing their potential source when they have been “transported” on pediments and (iii) targeting suspected ore bodies concealed beneath pediment(s). Past and present latitudinal climatic zonation of pedimentation and weathering patterns suggests a gradation of pedimentation process across the intertropical zone and explains why pediments may have been overlooked in equatorial environments, with implications for mineral exploration. Successive pediment systems adapted to uneven, knickzone bearing river networks, producing a spatially consistent and reduced (<80 m) stepping pattern of pediments independent from elevation or distance to base level. Pediments/pediplains are therefore not proxies of uplift and their preservation over geological timescales typifies regions submitted to <10 m/My erosion rates. The identification and study of lateritic pediments bear important implications on shield sediment routing systems and a better access to the bedrock and its resources, which may still be underestimated in the tropics.
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