Evidence for an aeolian origin of the Holocene lateritic surface cover of Gabon (Central Africa)

Field, granulometric and geochemical data are reported for a set of twenty-one samples collected at six sites spread over Gabon in the extensive lateritic clayey to sandy layer (Cover Horizon) which constitutes the upper part of the profiles of surficial formations. Granulometric analyses show that this material of homogeneous aspect is a mixture of two fractions: a clay fraction and a medium- to coarse-grained sandy fraction. Relative proportions of these two constituents show strong variations from one site to another but are almost constant from the bottom to the top of a given profile. The coarse-grained fraction includes lithic elements of local provenance. Irrespective of their underlying basement, the samples display many common geochemical features including a lack of Ca, Na and Mg. Their major element compositions may be accounted by the combination of a discrete number of mineralogical phases, namely quartz, kaolinite, gibbsite, Fe-oxides/hydroxides and subordinate K-minerals (micas). The normative contents of aluminous and ferrous phases regularly increase with the weight-percent of clays. Conversely, the quartz regularly decreases. This inverse relationship is consistent with mixing between an Al- and Fe-bearing clay fraction and a coarse-grained fraction dominated by quartz. Correlations between most immobile trace elements and the weight-percent of clays indicate a control of those elements by Al- and Fe-bearing phases. The main exception is for Zr and Hf, as those elements are probably associated with zircons in the coarse fraction. Because of the control of the trace element distribution by secondary phases, the geochemical signatures of all samples are clearly distinct from those of the upper Continental Crust and any basement rocks. Thus, the trace element characteristics of the Cover Horizon carry no sign of any source-material, being the sole consequence of mineralogical sorting prior to deposit.From field, granulometric and geochemical data, an aeolian origin appears as the most probable for the Cover Horizon. Geochronological constraints, including archaeological data and 14C dating, indicate that this aeolian event took place during the Holocene and probably occurred in less than 3000 y. Additional works will be necessary to assess the global context which made possible this very fast and extended event over large areas of Central Africa.