Spatial variations in sediment production and surface transformations in subtropical fluvial basins (Caculuvar River, south-west Angola): Implications for the composition of sedimentary deposits

Abstract

Several compositional features of sedimentary deposits can be used to reconstruct environmental conditions of source areas. In this research, bulk X-ray diffraction mineralogy, heavy and clay mineral assemblages and geochemistry obtained for modern deposits of the Cunene River and its tributaries Caculuvar–Mucope are integrated with geological/geomorphological characteristics of respective catchment areas to evaluate to what extent sediment production is spatially variable and source materials are differently affected by exogenous transformations. Detrital sources can be classified into four main types based on sediment composition: felsic, mafic, recycled and mixed. Source contributions obtained with unmixing models using distinct input data reveal some disagreements, with heavy mineral assemblages pointing to higher mafic contribution and bulk XRD-mineralogy favouring the recycled sedimentary component. However, the three datasets coincide showing a lower supply from the Kalahari Basin than the large outcropping areas of its sedimentary units would suggest, which is attributed to the relatively low rainfall and relief in this region. Where multiple depositional cycles are involved in sediment production the composition of the river deposits, even their clay fractions, will not reflect coeval weathering transformations and an under-estimation of the mafic component probably occurs. This research reveals how exogenous processes may deviate the composition of the produced sediment from a simple weighted by outcropping area average of the source units and compromise (palaeo)environmental interpretations based on sediment composition.