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The topography of the Okavango Delta, Botswana, and its tectonic and sedimentological implications

Department of Geology, School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits 2050. email: 065mct{at}cosmos.wits.ac.za

C. L. Merry

Department of Geomatics, University of Cape Town, Rondebosch 7700.

A topographic map of the Okavango Delta and environs has been constructed using a combination of elevation data including trigonometric beacons and spot heights from the government of Botswana, surveys of the navigable channels, U. S. Department of Defense data and measurements made during a geophysical survey of the region. The topography provides insight into the local tectonic and sedimentary history. Local tectonics are dominated by uplift and horst formation associated with the Ghanzi Ridge, and an arch to the north of the Panhandle, which appear to represent the tips of incipient rifts which are propagating from the northeast. The Delta has formed in the resulting depression between these arches. The Panhandle has developed along a fault, and may be largely an erosional feature incised into the northern uplift zone. The Delta itself is an alluvial fan of remarkably uniform gradient. There is no evidence of regional tilting of the fan surface. Local highs and lows are developed on the fan, but channel location is relatively insensitive to this local topography. Moreover, marked elevation differences exist between adjacent channels, creating hydrologically unstable conditions. These unusual features of the local hydrology arise because of the confining effect of channel-flanking vegetation. Sedimentation in the Delta appears to be causing crustal sagging of the central Delta, which has: tilted the major palaeo-shoreline of the Mababe Depression to the west; formed a local depression within the Ghanzi Ridge facing the Delta; and detached a sliver of the ridge along the Thamalakane fault. It is suggested that local seismicity also results mainly from sediment loading. The Selinda spillway occupies a marked local depression, which is a graben between the Gumare fault and an extension of the Linyanti fault. It is probable that southwesterly propagation of the uplift zone associated with the incipient rift will ultimately deflect the Okavango River into the Chobe-Zambezi river system via this graben.

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