The Okavango Delta—Semiarid Alluvial-Fan Sedimentation Related to Incipient Rifting
T. S. Mccarthy, N. D. Smith, W. N. Ellery, T. Gumbricht, 2002. "The Okavango Delta—Semiarid Alluvial-Fan Sedimentation Related to Incipient Rifting", Sedimentation in Continental Rifts, Robin W. Renaut, Gail M. Ashley
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The Okavango “Delta” is a large (25,000 km2), low-gradient (1:3400) alluvial fan which has developed in a depression between two basement arch-horst systems which mark the tips of incipient rifts. Base flow in the Okavango River sustains about 4000 km2 of permanent wetland in the Delta, while the seasonal flood can expand the area of inundation to more than 12,000 km2. The climate over the Delta is semiarid, and only 2% of combined inflow plus rainfall (ca. 15 km3/yr) leaves as surface flow, the remainder being lost to the atmosphere by evapotranspiration. Sedimentologically, the system is virtually closed, and about 600,000 tonne of sediment is deposited annually in the Delta, resulting in a very low average sedimentation rate. Two distinct types of sedimentation occur on the fan: clastic on the upper fan and chemical on the lower fan. In both, biotas play a key role. Although the region is tectonically active, there is little active faulting within the Delta itself, and apart form the rising arches associated with the propagating rifts, the major tectonic process in the Delta appears to be crustal sagging, induced by sediment accumulation. This has caused localized depression of the southeastern arch and associated faulting. Seismicity is localized to the area of the fan, and is also attributed to gravitational loading. Continued propagation of the rift is likely to result in diversion of the Okavango River into the Zambezi River.