PROCESSES OF HOLOCENE FERROMANGANESE-COATED GRAINS (ONCOLITES) IN THENEARSHORE SHELF OF CAMEROON

Brown Fe and black Fe-Mn coated grains are present at a water depth of 25-30 m in the subsurface reworked deposits of the Sanaga Pleistocene delta (Cameroon); most are former fecal pellets with concentric layer ing composed of one to four lavers, Brown grains are composed of a med ium to well crystallized Hurling hurling goethite, Black grains common ly have a metallic luster and show todorokite microcrusts on the surfa ces of the goethite rims. Cyanobacterial microstructures that develope d in the voids between the nuclei and the cortices of tile grains as w ell as between successive layers within the cortices suggest a weakly microlaminated oncolite and biomediation of the oxidation. These grain s were generated between c. 9600 and 8000 gr BP by successive centripe tal coatings, The most likely environment is thought to he oxygenated seawater over offshore bars near the mouth of the Sanaga river, Succes sive steps are involved in the buildup of Fe-Mn coatings: (1) upward f lux of Mn2+ and Fe2+ from the sediment column to the sediment-water in terface, (2) Fe3+ precipitation as Fe oxyhydroxide and catalyzed depos ition of Mn4+ on the oxyhydroxide iron surface, and (3) bacterial biom ediation of the Pe-Mn oxidation, Modern nearshore sediments on the Cam eroonian shelf and modern and Quaternary sediments on other West Afric an shelves contain only goethite-coated grains; the todorokite formati on off the Sanaga mouth appears to have no modern analog. On the basis of the present-day factors-high sea-water temperature (> 30-31 degree s C), low salinity (24-28 parts per thousand), high bacterial activity , and high concentration of MnO (0.27%) in the suspended matter of the upstream Sanaga waters-we consider the formation of the analyzed coat ed grains to have resulted from the same enhanced factors associated w ith the early Holocene transgressive environment.