FLASH-FLOOD SEDIMENTS AND ICHNOFACIES OF THE LATE PLEISTOCENE HOMEB SILTS, KUISEB RIVER, NAMIBIA

The Homeb Silt Formation consists of eroded remnants of fine-grained alluvium that were deposited within the Kuiseb River valley in southern Namibia during the Late Pleistocene. In Holocene times the river deeply incised its valley fill leaving steep walled ''castles'' of relict alluvium-the Homeb Silts. This study is based on the 25 m thick type exposure of Homeb Silts at Homeb and describes flash-flood sediments that were colonised by successive generations of opportunistic arthropods. The stacked flood units each comprise a massive tabular siltstone bed overlain by a narrower interval of rapidly alternating sandstone and siltstone with claystone veneers. These flood units are interpreted as having accumulated under semi-arid climatic conditions by episodic back-flooding of the Kuiseb River into embayments and tributary mouths. Soon after deposition as the floodwaters subsided, the sediment was colonised by burrowing, sediment-ingesting organisms, mostly arthropods, that produced a Taenidium ichnofacies. After the floodwaters had drained, the exposed sediment was colonised by grasses and burrowed by terrestrial arthropods, probably ants and termites, resulting in an overprint of Termitichnus ichnofacies with associated pelletal chambers. Thirteen ''horizons'' of rhizocretions and root tubules occur toward the top of the succession and indicate a gradual reduction in the frequency of flooding. These immature calcic palaeosols suggest that the climate in the Central Namib Desert 20,000 years ago was semi-arid, being wetter and more seasonal than the hyperarid conditions that prevail today.