CHEMICAL SEDIMENTATION IN THE SEMIARID ENVIRONMENT OF THE OKAVANGO DELTA, BOTSWANA

The Okavango basin of northern Botswana is an area of active diagenetic carbonate and silica accumulation. The basin is formed by grabens which represent an extension of the East African Rift system. The Okavango Delta, a large, marshy, alluvial fan (22,000 km2), occupies most of the surface area of the basin. Evapotranspiration exceeds precipitation by a factor of 3 and approximately 96% of incoming water is lost to the atmosphere. Evaporative concentration of dissolved solids occurs and they precipitate in the capillary fringes of islands. The less soluble components CaCO3 and SiO2 precipitate in the soils, which results in a significant volume increase which has important implications for the topography of the area and the character of the sediment fill. The more soluble salts, mainly alkali carbonates, accumulate at the surface as efflorescent crusts but are leached into the soil during the rainy season. This salt accumulation destroys vegetation. Swamp abandonment leads to leaching to greater depth, and, with reflooding, to the removal of these salts, possibly to form a deep saline groundwater. Mass-balance calculations indicate that this form of chemical accumulation is the dominant aggradational process occurring in the delta at present.