Sedimentation on the Distal Reaches of the Okavango Fan, Botswana, and its Bearing on Calcrete and Silcrete (Ganister) Formation

Abstract

The distal reaches or flood plain of the Okavango alluvial fan of northern Botswana are characterized by gently undulating topography with local relief generally less than two meters. The entire area is blanketed by eolian sand. Although the area is semiarid with evapotranspiration exceeding precipitation, the area is subject to seasonal flooding by annual influx of floodwater from subtropical Angola to the north. Distributary 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 load. Higher ground on the flood plain forms islands during the seasonal flood. Elevated tracts arise by displacive, subsurface crystallization of carbonate and silica, which is induced by trees that grow on the higher ground. Sedimentation on lower-lying areas occurs by a combination of: (1) accumulation of fine clastic material derived from the flood water, and phytolithic silica, both of which are mixed into the sandy substrate by illuviation and bioturbation; and (2) precipitation of fine-grained amorphous silica from the groundwater, induced by transpiration 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 association is an abundance of water with low suspended load in an environment with a high evapotranspiration rate. Ganisters in ancient rocks may have a similar origin.