THE OKAVANGO FAN AND THE CLASSIFICATION OF SUBAERIAL FAN SYSTEMS

Controversy exists over the classification of fluvially formed subaerial fans. Authors either restrict alluvial fans to debris flow dominated types or extend the spectrum to fans dominated by braided rivers. The Okavango Fan provides an end member which extends this spectrum to fans dominated by meandering and low sinuosity rivers. The fan is a large (150 km radial axis) shallowly sloping (0.00036), highly vegetated subaerial fan, which can be subdivided into four subenvironments. These are: (1) the entry corridor or Panhandle characterized by single and anastomosing meander belts; (2) the upper fan characterized by meander belts diverging from the fan apex, comprising peat-confined meandering channels, with interchannel swamps forming thick peats; (3) the middle fan with highly confined single and anastomosing low sinuosity rivers and less common prograding meander belts whose channels are confined by thick peats with, however, little chance of peat preservation; and (4) the lower fan in which annual floods from relatively unconfined channels spread over the fan surface and interact with pre-existing aeolian and lacustrine deposits. With the recognition of this new fan type, the spectrum of subaerial fan types can be expressed in terms of: (1) debris flow dominated fans of which the Death Valley fans are a member; (2) braided river dominated fans of which the Kosi Fan is a member; and (3) low sinuosity/meandering (losimean) river dominated fans of which the Okavango Fan is a member. This spectrum can be expressed in terms of variation in slope, maximum size and percentage of surface vegetation, but crucial to the evolution of the various fan types is variation in the flashy to continuous nature of the discharge and the degree of channel confinement evident on the fan surface. Comparable ancient examples of the three fan types are recognizable, many of which provide intermediates between the ideal end members. Debris flow dominated and braid dominated fan types are known from throughout earth history. However, the losimean fan type, because of its reliance on confining vegetation, may only have developed after the Devonian Period. The spectrum of subaerial fan types can be expressed on a triangular field of variation with the vertices defined by the relative importance of the processes which shaped a particular fan system be they debris flows, processes associated with braided rivers or processes associated with meandering and low sinuosity rivers.