The bedrock on dam site composes of sequences of marl, mudstone with reddish brown and gray sandstone with silty-marl and greenish silt related from upper red formation (URF), while the abutments covered with old alluvial terrace. The stratification orientation is northwest-southwest.
One of the current projects, on the Akposso Plateau, is a search for diamonds, which have been recovered by small miners from the alluvial terraces. The geotectonic setting, the Atacorian mobile belt, could suggest the presence of a diamond-lamproite association.
Two of these three studies were at sites, La Selva and the Toba valley, that contain infertile upland areas adjacent to young alluvial terraces that together span a wide range of chemical and physical soil characteristics.