Incised meanders along the mixed bedrock-alluvial Orange River, Northern Cape Province, South Africa

Following the late Jurassic breakup of the Gondwanan supercontinent, the extensive Orange River system has undergone long-term incision into bedrock but also has formed the principal conduit for sediment transport across southern Africa. River valley and channel morphology thus reflect the interactions between bedrock erosional and clastic sedimentary processes, as exemplified in a ∼125 km long reach downstream of the Vaal River confluence where large-scale meanders (average wavelength ∼ 12 km) have developed in weakly-lithified and weathered tillite. The modern channel possesses steep cutbanks formed in tillite but point bars, levees and narrow (< 200 m) flanking floodplains have developed on the inner bends and at inflections. Organic-rich horizons illustrate the stable nature of these modern levee-floodplain complexes, despite a highly variable flow regime characterised by occasional large floods. Alluvial terraces up to ∼160 m above the modern river are also present in many locations, and typically consist of ∼4-6 m thick fining upwards successions of gravel, sand and silt that form near-continuous sheets several kilometers long. In the prevailing semi-arid climate, the terrace alluvium has become strongly cemented by calcium carbonate, and is commonly blanketed with a layer of aeolian sand derived from the periodically-exposed channel bed. Formation of the meanders and associated terraces is a result of base level control exerted by resistant banded ironstones and quartzitic successions cropping out downstream of the study reach. Relatively slow erosion of these resistant rocks has retarded vertical incision into the tillite, with lateral erosion consequently increasing in relative importance. Vertical incision since at least the Miocene thus has occurred concomitantly with lateral meander migration, which in turn has led to cutbank erosion on the outer bends, and to formation of gently-sloping, fining-upwards, point bar and floodplain deposits on the inner bends. Periodic knick-point migration through the study reach, possibly related to the breaching of resistant rock barriers downstream, has resulted in phases of accelerated vertical incision, and abandonment of these alluvial deposits to form terraces. The broad, asymmetrical valley, strongly ingrown meanders, and alluvial floodplains/terraces in the study reach contrast strongly with the narrower valleys, straighter course, and limited alluviation characteristic of reaches eroded in more resistant lithologies upstream and downstream. The findings thus show: 1) how the forms and processes of mixed bedrock-alluvial rivers such as the Orange are often closely adjusted to the variable geology which they traverse; and 2) how sedimentary features such as point bars, levees and floodplains are able to form and persist over short to medium timescales, even while bedrock erosion is ongoing.