Sand-spits systems from Benguela region (SW Angola). An analysis of sediment sources and dispersal from textural and compositional data

Sand spits are important coastline features in western Angola, but only limited knowledge on their recent evolution and sediment sources were obtained so far. The present study is focused on the Baia Farta and Lobito spits of coastal Benguela that develop to the north (i.e. downdrift) of the Coporolo and Catumbela river outlets. We used grain-size distributions, heavy-mineral suites and clay -mineral assemblages of sediments in the Coporolo-Bala Farta and Catumbela-Lobito coastal stretches to characterize the main depositional units and investigate sediment provenance. From the combined grain -size and mineralogical variability in mud and sand samples it is possible to infer sediment sources and dispersal in the two coastal stretches. Kaolinite is mainly derived from the Angola hinterland, and is particularly common in finer grained floodplain sediments from the Catumbela River. Expansive clays (smectite and illite-smectite mixed layers) are inferred to be mainly sourced by Meso-Cenozoic units of the Benguela Basin, being abundant in coarser grained fluvial deposits and in lagoonal deposits near Baia Farta. Sand supplied by the sedimentary units from Benguela Basin and their basement rocks tends to be enriched in epidote associated with blue green hornblende. The Coporolo River sand is progressively diluted during the longshore northward transport by sand supplied by coastal units. Conversely, beach deposits in the Catumbela-Lobito coastal stretch are mainly sourced by the Catumbela River. A divergent longshore transport from Catumbela river-mouth occurs at Catumbela delta. Sand spit morphology and evolution reflect the patterns of dispersal of bedload and suspended load in settings of contrasting orography and human influence. (C) 2016 Elsevier Ltd. All rights reserved.