Anatomy of a diamondiferous gravel barrier spit at the palaeo-Orange River mouth, south-western Namibia

The sedimentary facies architecture of a Plio-Pleistocene diamond-bearing gravel barrier spit within the palaeo-Orange River mouth in south-western Namibia has been revealed during onshore mining activities. Four sedimentary facies associations build this ancient coastal landform and have been interpreted, from a seaward to landward direction, as sediments in: (i) the foreshore, represented by an intertidal beach; (ii) a subtidal spit platform on which the spit developed; (iii) storm-constructed spit head recurves on the landward side of the spit; and (iv) an estuarine back-barrier. This sedimentary record reflects an evolving spit that grew in response to a combination of marine and fluvial processes in a wave-dominated, high-energy coastal regime. Here, the southern Atlantic Ocean aggressively dispersed sediment delivered by a Plio-Pleistocene Orange River, ensuring that the fine-grained sediments were removed to allow only the gravel fraction to build up along the coastline. This extreme dispersal of sediment also precluded the construction of a classic delta; hence sediment storage offshore of the river mouth was limited and spit growth was dependent on the direct supply of river sediment. In response to the intermittent supply of sediment, spit growth and decay alternated during periods of sediment abundance and starvation, respectively. During positive sediment budget, the spit extended northward and cut off parts of the mainland to form an estuarine back-barrier. Foreshore and spit head progradation was sustained under these conditions, and formed a beach-ridge plain with a topographic expression of several coast-parallel ridges that curved at the spit head into the back-barrier. Spit extension and recurve development were controlled by a combination of high sediment supply, a vigorous longshore drift and powerful waves approaching from various directions. This Plio-Pleistocene coastal landform in Namibia offers a depositional model relevant to gravel-dominated barrier spits near river mouths along wave-dominated clastic coasts.