Die Dünen-Namib

Abstract

English summary by author:

Many features of the Namib dune field are also found in other sand seas of the world and suggest a similar history (4. 2.), for example the sharp boundaries of ergs may be caused by underlying sandstone plateaus. Giant pyramidal or stellate dunes, often associated with horse-shoe depressions in Arabian, Persian and Saharan ergs, may have positions similar to the Namib equivalents and may have their origin in a similar fluvio-erosional topography beneath. Interdune valleys in several Saharan ergs seem to correspond with fluvial channels between lace dunes in the eastern Namib erg. Widely distributed in sand seas are linear dune systems of the same dimensions as in the Namib suggesting the same mechanism of formation. On the whole the Edeyen of Mursuk in the Sahara comes closest to the Namib erg with respect to sands, dune types and dimensions and their distribution within the sand sea, and also features like calcretes in interdune valleys and sand-covered cliffs. In many sand seas, as in the Namib, dune topography and morphology cannot be explained by aeolian activity alone; water has played an important part

The Postafrican Planation Surface of the Namib Plain is also present in the sandstone beneath the Namib dune field (2. 2. 1., map 3). Marine terraces are found at levels of 40-50 m, 80-100 m, 130-150 m and 200 m (2. 2. 2.). Between the bordering rivers, Kuiseb to the north and Koichab to the south, five great river systems have been active on this ancient landsurface: Tsondab-Dieprivier, Tsauchab (-Tsams), Bushman Hill-Uri Hauchab, Nam, and a northern branch of the Koichab (2. 2. 3.). The fresh-water places near the coast do not correspond to their mouths but to still older sub sedimentary channels in the basement (2. 6.). The first recognizable stage of river development within the area of the northern erg is represented by the high terraces of Kuiseb and Tsondab (2. 4. 3., map 7), the latter covered by the first generation of calcretes (2. 4. 1., map 6). Traces of a more efficient local run-off on sandstone are found on the low watersheds between and partly on the meanwhile abandoned high terraces, for example, the parallel channels and the eastward drained Tsams basin (2. 10. 2., map 14). Smaller channels incising the Tsondab high terrace were discharging into the Kuiseb (high terrace) south of Gobabeb (2. 4. 1.). According to radiocarbon analysis of calcretes this second stage of river development (2. 4. 3.) has been dated at older than 27. 000 years B. P. (2. 4. 2.). More than 20 000 years ago, during the third stage of river development, the incising Kuiseb lost these tributaries (second generation of calcretes). A third generation of calcretes has been dated ± 17. 000 years B. P. (2. 4. 1.). Thus there seems to be a correlation with the northern Kalahari (2. 4. 2.). During the High Würm period the main valleys of all river systems seem to have developed and considerable erosion of sandstone occurred along the Great Escarpment in the east. Remnants of this ancient low level of erosion are the basins, often with pans, beneath dune covered sandstone cliffs at the eastern edge of the erg (for example near Awasib), which today are the lowest parts of their surroundings but formerly were drained towards the east (2. 10. 2.). Decreasing run-off led to the accumulation of large alluvial fans, decreasing in size from north (Kuiseb and Tsondab combined) to south (Tsauchab fan still reaching the ocean) and still farther south (Nam with interior fan) (2. 7. and 2. 9., map 12). Only the Tsondab river succeeded in a large-scale shift towards the northwest and had its last course north of Narabeb (2. 4. 3. and 2. 10. 2.). In the northern erg successive sand supply and deposition cycles resulted in five different sand provinces from east to west and can be studied in detail (2. 9. 2.). Here, patina (2. 1.) and sorting (2. 7. 1.) of quartz sand grains increasing from the coast to the east do not reflect aeolian transport from the coast but fluvial transport from the east with increased bleaching and mixing of sands (2. 9. 2., fig. 5)

The alluvial fans were the source of blown sands and the centres for erg development. These sands were moulded into the high longitudinal dune ridges of the interior erg by Taylor-Görtler-movement in a more efficient wind system of the past (2. 10. 3. 2. 2., map 16). The marine terraces, disturbing the vortices of this motion, were covered separately by transversal dunes (2. 10. 3. 1.). In the zone of erosion east of the alluvial fans the fluvial features were blurred by decaying sandstone. Here the complex dune topography is given by a small-scale fluvial relief where there are no true dunes but fluvio-aeolian forms (2. 10. 3. 4.). The most spectacular dunes of the Namib (near Sossus Vlei, pyramidal dunes) belong to this type. After the end-Pleistocene shift of the wind regime to the south the erg was modelled by a weaker, bimodal wind system consisting of trade winds (S - SW), sea breezes (SW), and eastern katabatic winds (3. 2. 2. and 3. 2. 5.). Only the coastal. transversal dunes are still in equilibrium with the sea winds (2. 10. 3. 1.). The interior longitudinal dune ridges have been and are reshaped according to the given topography. Thus fourteen dune types have developed out of the original form becoming progressively lower and covered by smaller secondary dunes (2. 10. 3. 2. 3., fig. 6). In the southern erg with less alluvial sands the longitudinal ridges are only weak undulations beneath a zibar-dune system (2. 10. 3. 2. 1.). The actual process in the Namib erg is the formation of small silks: sinuous dunes growing downwind and curving eastward during normal dry years and westward during wet years (3. 1. 1. and 3. 1. 2.). Conspicuous features like dune pits, three-horned barchan dunes, and horse-shoe depressions near pyramidal dunes may all be explained by the same mechanism (3. 1. 4.). On the whole the role of water has proved most important for the history of the Namib dune field, not only for sand supply but partly also for dune types including height. Alternating erosion and sand accumulation were the origin for the "erg from alluvial fans", but just erosion was the origin for the "erg of decaying sandstone" in the eastern part