Geostrophic sand ridge, dune fields and associated bedforms from the Northern KwaZulu‐Natal shelf, south‐east Africa

Abstract

Subaqueous dunes are formed on the KwaZulu‐Natal outer‐shelf due to sediment transport by the Agulhas Current (geostrophic current). These dunes occur within two dune fields at depths of − 35 to − 70 m. The net sediment transport direction is south, but short‐period reversals form northward‐migrating bedforms. The dune fields are physically bounded by late Pleistocene beachrock and aeolianite ledges. A bedform hierarchy has been recognized in the dune fields comprising a system of three generations of climbing bedforms. The outer dunefield has given rise to a sand ridge (H=12 m; L=4 km; W=1.1 km; and an 8° lee slope) whereas the inner dune fields have achieved large‐scale dune status. Bedload parting zones within the dune fields occur where the sediment transport direction switches from north to south due to reversals in the geostrophic flow; these zones occur at depths of − 60, − 47 and − 45 m. An interpretative stratigraphic model is presented on what such geostrophite deposits would look like in the ancient sedimentary record.

Sedimentation on the narrow (8 km wide), oceanic current-influenced continental shelf off Durban, Kwazulu-Natal, South Africa

“…5), indicating an erosional contact between these units (Cawthra, 2010). The relationship is also apparent in northern KZN as indicated by Green (2009) and Ramsay et al (1996), suggesting a regional distribution of these facies. The occurrence of these sediments bolsters the facies relationship described by Flemming (1978), where he described an inner terrigenous and outer relict carbonate facies in KZN.…”

“…Sediment dispersal is controlled by the interaction of numerous factors: morphology of the continental margin, wave regime (Cooper, 1991(Cooper, , 1994Smith et al, 2010), wind-driven circulation, influence of the Agulhas Current, eddies and sediment supply (Flemming, 1981;Bosman et al, 2007). The Agulhas Current generates large-scale subaqueous dunes (H > 17 m) in the unconsolidated shelf sediment along the outer continental shelf with a dominant southwesterly transport direction (Flemming, 1978(Flemming, , 1980(Flemming, , 1981Flemming and Hay, 1988;Ramsay, 1994;Ramsay et al, 1996;Green, 2009). The amalgamation of subaqueous dunes is considered to give rise to the formation of shore-detached sand ridges in northern KZN (Ramsay et al, 1996).…”

“…Here unconsolidated Holocene sediments cover the seaward margin of the Bluff Ridge and form deposits of recent sand accreted as beaches and dunes along a narrow coastal belt (Roberts et al, 2006). The bulk of the Holocene sediment is found offshore blanketing the inner to mid shelf, forming the Holocene sediment wedge (Flemming, 1981;Martin and Flemming, 1986;Flemming and Hay, 1988;Birch, 1996;Ramsay et al, 1996). Offshore of the Bluff Ridge, Birch (1981) suggested that the sediment that mantles the shelf attains a thickness of up to 25 m. This reworked sediment is argued by Flemming (1977) to have accumulated during the past 6000 years.…”

“…5A, B, C). The sediment represents skeletal detritus originating from the mechanical breakdown and biological destruction of carbonate-secreting organisms such as Mollusca, Foraminifera, Echinodermata, Bryozoa and Scleractinia corals (Ramsay et al, 1996) as well as aeolianite and beachrock reef.…”

“…Marine Geology 323-325 (2012) [107][108][109][110][111][112][113][114][115][116][117][118][119][120][121][122] The combination of the morphology of this extremely narrow continental shelf and the powerful Agulhas Current influences the distribution of shelf sediments and has resulted in the formation of large-scale sand ridges (Fig. 1D) (Flemming, 1978(Flemming, , 1981Flemming and Hay, 1988;Ramsay et al, 1996). Re-organisation of the Holocene sediment wedge associated with the western boundary flow of the Agulhas Current has been the focus of east coast continental shelf sedimentological studies since the quantification of this environment by Flemming (1978Flemming ( , 1980.…”