ABSTRACT

Ferricretes (hard iron crusts) reaching several meters thickness and consisting of ferruginous ooids and pisolites in an iron-rich argillaceous matrix are extremely widespread in West Africa. They are pedogenic in origin and result from the lateritic weathering of a variety of sedimentary, metamorphic, and igneous rocks.

A characteristic aspect of these ferricretes, developed from Paleocene glauconitic argillaceous sandstones, is analyzed in detail petrographically and geochemically in a weathering profile at Eboinda, Ivory Coast. Ooids and pisolites are generated in place from "glauconite" grains by successive centripetal concentrations and reorganizations of the iron oxides and hydroxides substituted with alumina (aluminous goethite).

The oolitic and pisolitic ferricretes generated by lateritic weathering are identical in all aspects except composition, to the micrites containing vadose ooids and pisolites characteristic of exposure crusts in carbonate rocks. We suggest that whenever such textures are encountered among ancient iron ores they be interpreted as paleoexposure crusts in a tropical climate.