The Homeb silts in the Namib Desert
The Homeb silts in the Namib Desert
||Dr. Mary. C. Bourke|
|Co-Investigators:||Alex Shaw, School of Geography, University of OxfordAl Child, School of Geography, University of Oxford
Stephen Stokes, St. Catherines Coillege, University of Oxford
Funding was provided in part by the Research Opportunity Fund, Smithsonian Institution (2000-2001).
Fine grained terraces that represent past episodes of valley aggradation are found in many of the world’s arid regions, e.g. the Flinders Ranges of south Australia, the Anti Atlas mountains in Morocco, the Sinai Desert, and the Namib Desert. The sediment texture and facies of these paleo-valley fills contrast markedly with the modern channel and floodplain. Hypotheses for the depositional paleo-environment include lacustrine, wetland and paleoflood.
Figure 1. Aerial photo of Kuiseb River , May 14th 1981. Namib sand sea to the left.
This research focuses on one of the better studied of these terrace sequences, the Homeb Silts in the Kuiseb Canyon, Namibia (Fig. 1). The Kuiseb River (15,500 km2) runs along the northern boundary of the hyper-arid Namib Desert. Twenty-five meter thick sequences of sand, silt and clay are preserved in channel and tributary mouth embayments and now stand up to 45 m above the channel bed (Fig. 2). These remarkable deposits have a well preserved stratigraphy and their facies are typical of slackwater environments. Several depositional models have been proposed, including lacustrine and fluvial. Recently Heine and Heine  suggest that these slackwater deposits were paleo-stage indicators of large Pleistocene floods. The presence of similar vertically stacked, fine-grain deposits in at least three other river systems in northwestern Namibia suggest a regional scale control.
Figure 2: Homeb Silts in Tributary of Kuiseb River.
We have dated the sequence using Optically Stimulated Luminescence and determined that the silts were aggraded in the early to mid Holocene [Bourke et al., 2003]. This revises the Last Glacial Maximum age estimates previously derived using Radiocarbon and Thermoluminescence techniques. These revised age estimates are more in line with regional paleoclimatic data. However, the chronology indicates that the silts aggraded at an extraordinary rate of 7.43 m ka-1, an order of magnitude above rates calculated in other similar depositional environments [e.g., Williams et al., 2001].
We use a suite of techniques (Scanning Electron Microscopy (SEM), Optically Stimulated Luminescence (OSL), geosurveying and facies mapping) to determine the source of sediments, chronology and style of deposition. The data suggest that sediments generated by the aggressive salt weathering regime of the Namib Desert, along with contributions of loess from the Kalahari, were mobilized from the catchment slopes by ‘gentle’ rainfall during the early Holocene. We suggest that while the Homeb Silts are slack water deposits they are not paleostage indicators and support the low energy, valley fill, aggradation model [Bourke and Shaw, in prep; Ollier, 1977; Ward, 1987].
Figure 3: Research on a shoe string. Field vehicle for August 2000 trip!
Bourke, M.C., A. Child, and S. Stokes, Optical age estimates for hyper-arid fluvial deposits at Homeb, Namibia, Quaternary Science Reviews, 22, 1099-1103, 2003.
Bourke, M.C., and A.I. Shaw, The origin and source of the Homeb Silts, Namibia, in prep.
Heine, K., and J.T. Heine, A paleohydrologic reinterpretation of the Homeb Silts, Kuiseb River, central Namib Desert (Namibia) and paleoclimatic implications, Catena, 48, 107-130, 2002.
Ollier, C.D., Outline geological and geomorphological history of the central Namib Desert, Madoqua, 10 (3), 207-212, 1977.
Ward, J.P., The Cenozoic Succession in the Kuiseb Valley, Central Namib Desert, pp. 124, Geological Survey of South West Africa, Namibia, 1987.
Williams, M., J.R. Prescott, J. Chappell, D. Adamson, B. Cock, K. Walker, and P. Gell, The enigma of a late Pleistocene wetland in the Flinders Ranges, South Australia, Quaternary International, 83-85, 129-144, 2001.
Bourke, M.C., A. Child, and S. Stokes, (2003) Optical age estimates for hyper-arid fluvial deposits at Homeb, Namibia, Quaternary Science Reviews, 22 (10-13), 1099-1103.
Bourke, M.C., A. Child, and S. Stokes, (2002) A re-evaluation of the age of the Homeb Silts, Namibia, in 10th International Conference on Luminescence and Electron Spin Resonance Dating, Reno, Nevada.
Bourke, M.C., and A. Shaw, (2003) Slackwater deposits in the Kuiseb Canyon, Namibia, inPaleoflood III, Hood River, Oregon.
Child, A., (2001) A Holocene flood history of the Central Namib Desert established using optical dating, Unpublished undergraduate thesis, University of Oxford.