Mass and hyperconcentrated flow deposits record dune damming and catastrophic breakthrough of ephemeral rivers, Skeleton Coast Erg, Namibia

Channel-shaped deposits of well sorted sand with only 2-4% fine-grained material, being either massive and structureless or upward fining with basal lag, are found interbedded with aeolian sand in the Skeleton Coast Erg, Namibia. Detailed analyses of the channel-shaped deposits suggest that they are formed as hyperconcentrated flows within the erg. Grain-size analysis and whole rock geochemical modeling revealed that some of the fluvial sediments contain up to 70% aeolian sand, interpreted as a result of dune collapse into the fluvial system. In certain cases, this instantaneous supply of sand resulted in generation of sandy mass flows with laminar flow behaviour. The presence of smectite as dominant clay mineral proved to be of crucial importance in formation of mass flow deposits. These mass flows had no erosional capacity, and drape the palaeotopography. They are comparable to those generated by catastrophic collapse of dunes, described in the literature. This paper suggests that all these deposits should be termed intra-erg mass flows, as several of them carry little, if any debris. Based on their origin, intra-erg mass flows can be divided into two groups: (1) Attached intra-erg mass flows, which are formed and found attached to the dune that sourced the flow; (2) Detached intra-erg mass flows, which are not found adjacent to the dune sourcing the flow, and often carry material sourced outside the dune field. A five-stage model is proposed, involving dune damming to explain the observed deposits and the degree of reworking. When the flood basin behind the erg overtops the threshold area, water pours into the erg, giving rise to a complex fluvio-aeolian setting. The setting in the Skeleton Coast erg is comparable to recent as well as ancient settings, and reveals valuable information for reservoir characterization and hazard assessment. (C) 2002 Elsevier Science B.V. All rights reserved.