A ground-penetrating radar survey of late Holocene fluvial sediments in NW Namibian river valleys: Characterization and comparison

Fine-grained terraces and coarse-grained channel fills that represent episodes of late Holocene valley aggradation are used as geoarchives for palaeohydrological reconstructions in many arid regions of the world (e.g. southwestern USA, Australia, Israel and the Namib Desert). In Namibia, previous work on the sediment texture and facies of these valley fills has led to diverse interpretations of the depositional environment, including river-end, palaeoflood and floodout. Here we show, using ground-penetrating radar (GPR) profiles across four complex river channels to image sedimentological and stratigraphical structures, that the GPR images can be grouped into eight 'radar facies' (reflection patterns). A combination of radar facies analysis and groundcheck (exposures, boreholes) has been used to interpret the radar profiles. By considering the geomorphological-sedimentological features and age determinations of the fluvial deposits described by earlier researchers and supplemented by our own investigations, we explain the late Holocene fine-grained terraces and coarse-grained channel fills as deposits accumulated during floods. Although the GPR results alone cannot conclusively resolve the fine-grained terrace deposits either as river-end or as slackwater accumulation, the novel combination of GPR, geomorphological findings and results from the literature provides evidence for our interpretation.