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.
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Continental rift basins have long been of interest to sedimentologists. Of all the terrestrial settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. These records are a product of a complex interplay between several factors that include geological structure and tectonic activity, volcanism, climate and its temporal variability, hydrology, biology and time. Sedimentation in Continental Rifts is a timely update on this exciting interdisciplinary field and presents new approaches and insights into tectonic and structural controls of sedimentation. Other topics included are lacustrine and fluviatile depositional environments and some lesser-known settings, such as springs, wetlands, and paleosols. Several papers consider the behavior of silica in rift lakes, particularly the roles of microorganisms in silica precipitation, whereas others examine the paleoenvironmental importance of freshwater carbonates. The contents of the volume show that sedimentological research in rift basins has progressed beyond basic facies description and general models, and is now focused on understanding the integrative effects of physical, chemical and biological processes in rifts.