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Accueil du site → Doctorat → Allemagne → Ephemeral river systems at the Skeleton Coast, NW-Namibia : sedimentological and geomorphological studies on the braided river dominated Koigab Fan, the cenozoic succession in the Uniabmond area and comparative studies on fluvio-aeolian interaction between ephemeral rivers and the Skeleton Coast erg

Bayerischen Julius Maximilians Universität Würzburg (2003)

Ephemeral river systems at the Skeleton Coast, NW-Namibia : sedimentological and geomorphological studies on the braided river dominated Koigab Fan, the cenozoic succession in the Uniabmond area and comparative studies on fluvio-aeolian interaction between ephemeral rivers and the Skeleton Coast erg

Krapf, Carmen

Titre : Ephemeral river systems at the Skeleton Coast, NW-Namibia : sedimentological and geomorphological studies on the braided river dominated Koigab Fan, the cenozoic succession in the Uniabmond area and comparative studies on fluvio-aeolian interaction between ephemeral rivers and the Skeleton Coast erg

Auteur : Krapf, Carmen

Université de soutenance : Bayerischen Julius Maximilians Universität Würzburg

Grade : Doktorgrade der Naturewissenschaftlichen 2003

Résumé
The Skeleton Coast forms part of the Atlantic coastline of NW Namibia comprising several ephemeral rivers, which flow west-southwest towards the Atlantic Ocean. The area is hyper-arid with less than 50 mm average annual rainfall and a rainfall variability of 72%. Therefore, the major catchment areas of the rivers are about 100-200 km further inland in regions with relatively high annual rainfall of 300-600 mm. The coastal plain in the river downstream areas is characterized by a prominent NNW trending, 165 km long belt of 20-50 m high, locally compound, barchanoid and transverse dunes. This dune belt, termed Skeleton Coast Erg, starts abruptly with a series of barchans and large compound dunes 15 km north of the Koigab River and extends from 2-5 km inland sub-parallel to the South Atlantic margin of NW Namibia over a width of 3-20 km. As the SSE-NNW trending dune belt is oriented perpendicular to river flow, the dunefield dams and interacts with the west-southwestward flowing ephemeral river systems. This study focused on three main topics : 1) investigation and classification of the Koigab Fan, 2) the investigation of the Cenozoic succession in the Uniabmond area and 3) comparative studies of fluvio-aeolian interaction between five ephemeral rivers and the Skeleton Coast Erg. Sedimentological and geomorphological investigations show that the Koigab Fan represents a yet undocumented type of a braided fluvial fan system, which operates in an arid climatic, tropical latitude setting, is dominated by ephemeral mixed gravel/sand braided rivers, lacks significant vegetation on the fan surface, has been relatively little affected by human activity, is a perfect study site for recording various types of fluvio-aeolian interaction and thereby acts additionally as a model for certain Precambrian and Early Palaeozoic fan depositional systems deposited prior to the evolution of land plants. The Cenozoic succession in the Uniabmond area consists of three major unconformity-bounded units, which have been subdivided into the Red Canyon, the Whitecliff, and the Uniabmond Formation. The Tertiary Red Canyon Fm. is characterized by continental reddish sediments documenting an alluvial fan and braided river to floodplain depositional environment. The Whitecliff Fm. displays a wide variety of continental and marine facies. This formation provides the possibility to examine fluvio-aeolian interactions and spectacular, steep onlap relationships towards older sediments preserved in ancient seacliffs. The Whitecliff Fm. has been subdivided into four sedimentary cycles, which resulted from sea level changes during the Plio- to Middle Pleistocene. The following Uniabmond Fm. provides a unique insight into the depositional history of the NW Namibian coast during the Last Pleistocene glacial cycle. The formation has been subdivided into four units, which are separated by unconformities controlled by sea level changes. Unit 1 represents deposits of an Eemian palaeo-beach. The overlying Units 2-4 build up the sedimentary body of the Uniab Fan, again a braided river dominated fan, which is nowadays degraded and characterized by deeply incised valleys, deflation surfaces and aeolian landforms. The Uniabmond Fm. is overlain by the dunes of the Skeleton Coast Erg, whose development is related to the Last Glacial Maximum (LGM). The damming of river flow by aeolian landforms has been previously recognized as one of several principal types of fluvio-aeolian interaction. Five ephemeral rivers (from S to N : Koigab, Uniab, Hunkab, Hoanib, Hoarusib), which variously interact with the Skeleton Coast Erg, were chosen for the purpose of this study to consider the variability of parameters within these fluvio-aeolian systems and the resulting differences in the effectiveness of aeolian damming. The fluvio-aeolian interactions between the rivers and the dune field are controlled by the climate characteristics and the geology of the river catchment areas, the sediment load of the rivers, their depositional architecture, the longitudinal river profiles as well as the anatomy of the Skeleton Coast Erg. Resulting processes are 1) aeolian winnowing of fluvially derived sediments and sediment transfer into and deposition in the erg ; 2) dune erosion during break-through resulting in hyperconcentrated flow and intra-erg mass flow deposits ; 3) the development of extensive flood-reservoir basins caused by dune damming of the rivers during flood ; 4) interdune flooding causing stacked mud-pond sequences ; and 5) the termination of the erg by more frequent river floods

Mots clés : Skelettküste ; Geomorphologie ; Fluviale Sedimentation ; Äolische Sedimentation

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Page publiée le 24 mars 2008, mise à jour le 5 janvier 2019