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

Abstract

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 wavedominated, 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 graveldominated barrier spits near river mouths along wave-dominated clastic coasts.

Spit bar deposits from the Upper Cretaceous (Cenomanian) transgressive sequence in NE Bohemia (Czechia)

“…Collectively, 5. Characteristic features of spit bar systems, based on Nielsen et al (1988), Rasmussen and Dybkjaer (2005), Lindhorst et al (2008), Shukla et al (2008), Rasmussen et al (2010), Tillman and Wunderlich (2011, Reiman et al (2012), Billy et al (2014), Clemmensen et al (2014), Nehyba andRoetzel (2021), Pellerin Le Bas et al (2022), Shawler et al (2021), andSpaggiari andBordy (2023). 1clay; 2silt (mud); 3sand; 4granule; 5gravel; 6pebble; 7cobble; 8clay intercalations; 9clay intraclasts; 10current ripples; 11wave ripples; 12ripple cross-lamination; 13dune cross-bedding; 14trough cross-bedding; 15low angle cross-bedding; 16large scale foreset cross-bedding; 17hummocky cross-bedding (HCS?…”

“…Localities in which these systems can be directly described are unique. Some sub-fossil spit bars have revealed internal structures in natural outcrops, in basins where locally or regionally sea-level evolution has transitioned from transgressive to regressive (e.g., Lopez 2022;Spaggiari and Bordy 2023).…”

“…At the same time, spit bars are the only known modern coastal system in which both the macroform and current forms (ripples and dunes) migrate towards land, leading to a long-lived, consistent aggradation of the spit bar system. Periodic transverse cutting of the spit bar system during storms, periodic ascent, and/or vegetative overgrowth of the spit bar does not change this tendency until the spit bar is ultimately buried and fossilized (for instance, by being covered by transgressive sediments; e.g., Nielsen et al 1988;Allard et al 2008;Pancrazzi et al 2022;Spaggiari and Bordy 2023). Almost all workers of contemporary and/or subfossil spit bar systems emphasize that the appearance, development and/or preservation of sediments in these systems is contingent upon constant sea level rise, that is, in a regional transgressive regime (cf.…”

“…Spit systems are associated with retreating deltas or river mouths, estuary mouths, bedrock ridges or fault escarpments, or an abrupt change in orientation of shoreline, etc. (compare Leszczyński and Nemec 2015;Nehyba and Roetzel 2021;Spaggiari and Bordy 2023). They often grow in length in the direction of the current and can form lagoons or salt marshes behind them (cf.…”