Morphology and fluvio-aeolian interaction of the tropical latitude, ephemeral braided-river dominated Koigab Fan, north-west Namibia

摘要

The Koigab Fan is the largest of the active fan systems formed by some of the west-south-west flowing ephemeral river systems of the Skeleton Coast area, north-west Namibia. Issuing from the volcanic Etendeka Plateau, the Koigab River flows towards the Atlantic Ocean across a considerable climatic gradient from semi-arid summer rainfall in the mountainous catchment, to hyperarid in the coastal depositional setting. The morphology of channels can be discerned over the whole fan surface (gradient 1.011), the majority of which appears as a vast deflation surface on which lithic and heavy mineral grains are concentrated by aeolian removal of fines. The Koigab catchment restricts source-rock lithologies to flood basalts and interleaved quartz latites of the Etendeka Plateau, so components that unequivocally relate to a volcanic source (e.g. volcanic lithics, Ti-magnetite, pyroxenes) indicate fluvial transport, whereas grains reflecting a metamorphic basement source (e.g. garnet, muscovite, staurolite) must be aeolian derived. Both heavy mineral and grain-size data were used to estimate the amount of fluvio-aeolian interaction at the Koigab Fan surface. This aspect is significant because it comprises not only winnowing of the fan surface and of 'fresh' sandy channel deposits but also fluvial recycling of aeolian material. The contribution of aeolian-derived grains to river deposits increases from 5% in the fan apex area to as much as 50% in the distal fan reaches. In the spectrum of fan types, the Koigab Fan takes an intermediate position both in size and in terms of the braided river style between debris flow and low sinuosity meandering fan systems. Within the braided fluvially dominated fan class itself the Koigab Fan is also intermediate in size, but its ephemeral channels contrast sharply with those of perennial glacial outwash fans previously described from the sub-Arctic. Within low-latitude fan systems, the Koigab also contrasts with other highly vegetated fans in the tropics, for example the sub-aerial portion of the Yallahs Fan-delta, Jamaica. Thus, the Koigab Fan is important as a potential analogue for Precambrian and early Palaeozoic low-latitude fan systems that lacked surface vegetation prior to the evolution of land plants.