Composition and origin of the clay cover on North Brazilian laterites

The Belterra Clay in Northern Brazil is a uniform, yellowish, unstratified kaolinitic clay with variable contents of gibbsite. It is generally 5–10 m thick and covers a lateritic weathering crust on remnants of a dissected table-land. Complete profiles with Belterra Clay, the underlying bauxite/laterite and saprolite/bed rock were sampled in three areas of the Amazon region. The chemical and mineralogical data show a distinct affinity between the Belterra Clay and the underlying layers above all with the saprolite. Variations in the composition of the underlaying layers are reflected by the Belterra Clay. Among the structural and textural features important for the discussion of the origin of the Belterra Clay are the sharp contact between the clay cover and the underlying laterite as well as conglomeritic/brecciate textures and occasional bedding structure in the upper part of the lateritic crust. The detailed analyses do not permit definite conclusions on the origin of the Belterra Clay. Three hypotheses ar discussed:

1. In-situ formation of the clay cover by weathering processes.
2. Clay deposition after lateral transport of saprolitic material coming from former elevations inside or outside the present laterite areas.
3. Vertical transport of the saprolitic material to the surface by termites. However, thicknesses up to 20 m of the Belterra Clay cannot be explained solely by this process.

The compositional relations of the Belterra Clay with the underlying layers are of practical relevance for geological mapping and for mineral exploration.