New constraints on the age and origin of the sandy to clayey surface cover horizon in Gabon (Central Africa)
Résumé
Independently of their underlying basement, the superficial formations of Gabon display a constant organization, showing the superposition of two units: (i) a lower breccia known as the Stone line; (ii) an upper sandy to clayey layer, the Cover Horizon. This latter is generally 1-2 metres-thick but locally reaches 7-8 metres. Available archeological and geochronological data constrain the age of the Stone line and Cover Horizon as being younger than c. 50 000 BP and 10 000 BP respectively. Local interlayering shows that the emplacement of the two units was partly contemporaneous. The allochtonous vs. autochtonous origin of the Stone line - Cover Horizon association has been abundantly discussed. Field observations discard an autochtonous (in situ) origin so that, most authors conclude to a "sub-autochotonous" origin, many of them considering the Cover Horizon as the result of termite activity. New data obtained during the course of a geological mapping program sponsored by the E.U. bring important constraints on the age and mode of emplacement of those superficial formations. The 14C datings obtained on the Cover Horizon at five sites confirm its very recent age (< 3000 BP), which is consistent with a 5290 ± 40 BP age obtained on coal fragments sampled in the Stone line. At one site, a bottom to top decrease of the 14C ages from 2830 to 1880 BP implies an emplacement rate of 1.5 m/ky. Granulometric analyses attest for a more or less pronounced bimodality. The granulometric mode is almost constant at a given site, but strongly differs from one site to another, with large variations in the clay content (15-90%). Irrespective of the grain-size, the geochemical data attest for the homogeneity of the geochemical signatures and evidence a clear gap between them and those of any formation of the basement. These data lead us to conclude for an aeolian origin and a historical age of the Cover Horizon. Preliminary isotopic analyses show that the deposit could have originated from the Congo Basin just following a major environmental crisis at c. 3000 BP.