The Amspoort Silts, northern Namib desert (Namibia): formation, age and palaeoclimatic evidence of river-end deposits

Abstract

Detailed geomorphological and chronological investigations of the NW-Namibian Amspoort Silt formation show that the sediments are typical river-end deposits. This type of endoreic sediment, occuring only in desert margin areas, provides valuable information about the palaeo-environment. In the Hoanib valley, the fine-grained deposits have buried riverine trees. Radiocarbon dating of the wood and luminescence dating of the sediments allow a detailed reconstruction of the aggradation processes. Accumulation started ∼10 km downstream of Amspoort around the beginning of the 15th century and ended in the 19th century, some kilometres upstream of Amspoort. This upstream shift of sedimentation during the Little Ice Age was caused by gradually decreasing runoff resulting from aridification of the upper part of the Hoanib river catchment lying east of the Namib desert margin ≥1.200 m a.s.l. The Amspoort Silt terrace is evidence of palaeo-hydrological fluctuations in NW-Namibia. At present, the Hoanib river erodes deeply into the silty deposits, indicating that NW-Namibia receives more monsoonal rainfall today than during the Little Ice Age. However, this contradicts the hypothesis of a (continual) natural aridification of NW-Namibia (Damaraland, Kaokoveld) since the mid-19th century in the course of global climatic change. Rather, deposition and erosion of the Amspoort Silts indicate that landscape degradation in NW-Namibia is primarily anthropogenically induced and most probably not accelerated by a decrease in precipitation.