A paleohydrologic reinterpretation of the Homeb Silts, Kuiseb River, central Namib Desert (Namibia) and paleoclimatic implications

摘要

Slack water deposits are usually fine-grained (fine sand and coarse silt) flood sediments deposited in areas of the floodplain that are sheltered from high-velocity flood flows. Slack water deposits have been recognized by many investigators in a wide variety of physiographic and climatic settings. Late Quaternary slack water deposits have not been described from the extremely and Namib Desert of southwestern Africa. Fine-grained flood sediments in the Kuiseb Valley near Homeb, accumulated between 23,000 and 19,000 C-14-years BP, have been interpreted by previous workers as (i) sediments deposited behind dune dams, (ii) river endpoint accumulations, and (iii) flood deposits of an aggrading river controlled either by a base level change in the lower reaches or a change in the hydrological regime in the catchment area. Furthermore, the so-called Homeb Silts could document more and climatic conditions in the Kuiseb catchment area during the last glacial maximum (LGM). Our research shows that the Homeb Silts are slack water deposits and that in many valleys of the Namib Desert, slack water sediments are observed most commonly at the junctions of major rivers and minor tributaries in bedrock canyon settings. Here we present evidence that these slack water deposits accumulated as a result of more humid conditions and/or more intense precipitation events in the upper reaches of the river. They do not document climatic changes in the desert itself as postulated by many researchers.