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Palaeodune morphology associated with the Gumare fault of the Okavango graben in the Botswana/Namibia borderland: a new model of tectonic influence

Bosele Investments (Pty) Limited, Box 66, Maun, Botswana.

F.D. Eckardt

Environmental and Geographical Science Department, University of Cape Town, Rondebosch 7701, Cape Town, South Africa. e-mail: eckardt{at}science.uct.ac.za

Correspondence: Corresponding author. e-mail: mcf{at}dynabyte.bw

The degraded linear dunefield of north west Ngamiland, Botswana occurs in a seismically active area, lying to the northwest of the Okavango graben, widely considered to be the tail end of the East African Rift system. To assess the effects of neotectonism on the dunes, an area was selected for examination close to the Gumare fault, which bounds the graben on the northwest side. Digital SRTM data were converted to light shaded representation of the topography. It was found that dune forms only occur along the margin of the rift and on both sides of incising valleys, graded to the graben floor. A selection of long profiles showed dune crests standing some 25 m above the straats at the edge of the rift. The relative relief pinches out away from the fault, towards interfluves that do not depict dune morphology. Landsat satellite imagery shows linear features continuous across the flat interfluves, inviting the incorrect inference from the vegetation that the linear dune forms occur there. A model is proposed that an original linear dune field formed under arid conditions, was entirely flattened under wetter conditions, crest material being washed into the straats, thus obliterating the original dune morphology. Upon rifting and stream incision, these forms are being replicated, an example of equifinality. Replication is suggested to be by the action of infiltrating water, controlled by the groundwater gradients. These observations suggest that the active dunefield significantly predates the tectonic processes. With the additional time required for degradation and replication it would suggest that the dunes are of considerable antiquity. It has already been recognised that the dunefield has a complex history of construction and destruction, to which must now be added a process of base level-controlled replication.

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