SEDIMENTATION ON THE DISTAL REACHES OF THE OKAVANGO FAN, BOTSWANA, AND ITS BEARING ON CALCRETE AND SILCRETE (GANISTER) FORMATION

The distal reaches or flood plain of the Okavango alluvial fan of nort hern Botswana are characterized by gently undulating topography with l ocal relief generally less than two meters, The entire area is blanket ed by eolian sand, Although the area is semiarid with evapotranspirati on exceeding precipitation, the area is subject to seasonal flooding b y annual influx of floodwater from subtropical Angola to the north, Di stributary channels on the flood plain are poorly defined and consist of sinuous depressions lacking normal fluvial characteristics such as levees, bars, or incision. The flood water has very little suspended l oad, Higher ground on the flood plain forms islands during the seasona l flood, Elevated tracts arise by displacive, subsurface crystallizati on of carbonate and silica, which is induced by trees that grow on the higher ground, Sedimentation on lower-lying areas occurs by a combina tion of: (1) accumulation of fine elastic material derived from the fl ood water, and phytolithic silica, both of which are mixed into the sa ndy substrate by illuviation and bioturbation; and (2) precipitation o f fine-grained amorphous silica from the groundwater, induced by trans piration by aquatic grasses and sedges, Accumulation of silica in the soil profile produces a proto-silcrete, This sequence grades laterally into carbonate-dominated island soils, The ultimate cause of this ass ociation is an abundance of water with low suspended load in an enviro nment with a high evapotranspiration rate. Ganisters in ancient rocks may have a similar origin.