13.1 Introduction

The South African coastline is >2700 km in length and covers a wide range of climate zones, wave/tidal regimes and physiographic settings (Fig. 13.1). The Great Escarpment forms the backdrop to the coastal plain on the west coast which is ~60 km wide around Cape Town and ~190 km wide near the Orange River mouth. Off the south coast lies the extensive Agulhas Bank where the relatively flat continental shelf extends up to 130 km in a water depth of <200 m. On this coastline, the coastal plain is generally narrower at <40 km wide (Flemming, Reference Flemming1981) but broadens to the west due to the Agulhas Bank. It is also narrow in KwaZulu-Natal but widens northwards reaching 440 km width in Mozambique (Armitage et al., Reference Armitage, Botha, Duller, Wintle, Rebêlo and Momade2006).

Fig. 13.1. Map of the South African coastline, the paths of the Agulhas and Benguela Currents and the extensive coastal Quaternary deposits. Note also the extensive shallow offshore Agulhas Bank, parts of which would have been exposed during Quaternary sea-level lowstands.

The South African shoreline is composed of approximately 29% rocky coast, 39% sandy coast, 32% mixed coast (Harris et al., Reference Harris, Nel and Schoeman2011), defined here as ‘part-lithified sandy intertidal shorelines’. The distribution of coastline types is strongly governed by the highly variable geological substrate (Musekiwa et al., Reference Musekiwa, Cawthra, Unterner and van Zyl2015). In places, resistant lithologies (such as the quartzites of the Palaeozoic Cape Supergroup on the west and south coasts and the Natal Group and Msikaba Formation sandstones on the east coast) extend to the coast and form the basis for relatively steep sea-cliffs or resistant rocky shorelines (Roberts et al., Reference Roberts, Cawthra, Musekiwa, Martini and Wanless2014) (Fig. 13.2).

Fig. 13.2. Examples of coastline types found on the southern Cape coast. (A) Cliffed coastlines in Table Mountain Group sandstone at Pondoland, (B) beachrock dominated foreshore with active back beach dunes at Reebok, (C) barrier dunes and backbarrier lagoons at Wilderness

(photos: Hayley Cawthra).

This chapter concentrates on sandy coasts, including part-lithified sandy intertidal shorelines composed of cemented Quaternary deposits. Volumetrically, calcareous aeolian sediments dominate the coastal successions, with an age range of Palaeogene to Recent. Where lithologies are resistant, coastal topography is steep and thus limits sediment accommodation space for beaches. As beaches constitute the major sediment source of coastal dunes (Tinley, Reference Tinley1985; Porat and Botha, Reference Porat and Botha2008; Roberts et al., Reference Roberts, Cawthra, Musekiwa, Martini and Wanless2014), the paucity of beaches and the steep topography limit the size and create obstructions for inland transgression of dunes. Broad coastal plains tend to develop in areas of softer lithological substrates (typically shale-dominated) through planation by subsequent Cenozoic sea-level highstands. In these regions, palaeoshorelines may ingress up to tens of kilometres inland (Pether et al., Reference Pether, Roberts, Ward, Partridge and Maud2000; Roberts et al., Reference Roberts, Botha, Maud, Pether, Johnson, Anhausser and Thomas2006, Reference Roberts, Cawthra, Musekiwa, Martini and Wanless2014). The aeolian deposits along the west coast are dominated by dune plumes, whereas their counterparts along the south and east coasts comprise coast-parallel trending dune barriers or cordons (Roberts et al., Reference Roberts, Cawthra, Musekiwa, Martini and Wanless2014) (Fig. 13.1).

13.2 Regional setting

In terms of coastal climate, winds along the southern coastline between Cape Town and Port Elizabeth are mainly coast-parallel (Kruger, Reference Kruger2004), turning more north-northeasterly (summer) and southwesterly (winter) east of Port Elizabeth (Tinley, Reference Tinley1985; Ramsay, Reference Ramsay1996). Sea surface temperatures (SST) are largely controlled by the cold Benguela Current and the warm Agulhas Current (Fig. 13.1), with nearshore upwelling of the former giving rise to a high marine productivity. Waves along the South African coastline have a long fetch and are of high energy. On the southern coastline, the waves are southwesterly (Heydorn and Tinley, Reference Heydorn and Tinley1980) and on average 2.1 m high and 11 s apart (Whitfield, Reference Whitfield1983; Cooper, Reference Cooper2001). On the east coast, waves are dominantly from the south (Smith et al., Reference 218Smith, Mather, Bundy, Cooper, Guastella, Ramsay and Theron2010) with average heights of 2.07 m and spacing of 9 s (Cooper, Reference Cooper2001). The tidal range along most of the South African coast is microtidal and semidiurnal, with spring tidal ranges typically 1.8–2.0 m and neap tidal ranges falling within 0.6–0.8 m (Cooper, Reference Cooper2001). Longshore drift of sediment is northwards on the west coast (1.4 km³/a^‐1; de Decker, Reference de Decker1988) and eastward/northeastward (0.8 km³/a^‐1; Schoonees, Reference Schoonees2000) on the southern and eastern coastlines (Davies, Reference Davies1980).

Currently, the nearshore area on the west coast/western part of the south coast receives little terrestrial sediment (Dingle et al., Reference Dingle, Birch, Bremner, de Decker, Du Plessis, Engelbrecht, Fincham, Fitton, Flemming, Gentle, Goodland, Martin, Mills, Moir, Parker, Robson, Rogers, Salmon, Siesser, Simpson, Summerhayes, Westall, Winter and Woodborne1987). In contrast, 78% of the sediment on the continental shelf on the eastern coast is of fluvial origin (Dingle et al., Reference Dingle, Birch, Bremner, de Decker, Du Plessis, Engelbrecht, Fincham, Fitton, Flemming, Gentle, Goodland, Martin, Mills, Moir, Parker, Robson, Rogers, Salmon, Siesser, Simpson, Summerhayes, Westall, Winter and Woodborne1987). On the west coast only the Orange, Olifants and Berg rivers contribute sediment offshore, which is largely mud (Meadows et al., Reference Meadows, Rogers, Lee-Thorp, Bateman and Dingle2002). Littoral sediment varies slightly around the coastline with coarser sediments found in KwaZulu-Natal and the west coast, compared to the south coast (Cooper, Reference Cooper2001).

The west coast is heterogeneous, with contrasting rocky cliffs and extensive sandy beaches, sheltered bays and exposed straight coasts (Harris et al., Reference Harris, Nel and Schoeman2011) (Fig. 13.2) and distinctive dunes associated with river mouths (Roberts et al., Reference Roberts, Bateman, Murray-Wallace, Carr and Holmes2009). West coast beaches are generally steep and narrow with high berms (Cooper, Reference Cooper2001; Harris et al., Reference Harris, Nel and Schoeman2011). In the southwestern Cape, a dominantly rocky coast is interspersed with smaller sandy beaches (Harris et al., Reference Harris, Nel and Schoeman2011; Musekiwa et al., Reference Musekiwa, Cawthra, Unterner and van Zyl2015). This coastline, due to a combination of fine sediments and high wave energy, is dominantly composed of flat and wide dissipative beaches with multiple offshore bars and a wide surf zone (Harris et al., Reference Harris, Nel and Schoeman2011). The south coast comprises a cliffed or rocky coastline interspersed with a sequence of log-spiral bays that are largely structurally controlled by a series of half-grabens which trend SE–NW and extend onto the continental shelf (Partridge and Maud, Reference Partridge and Maud2000). In the bays of Wilderness and Nature’s Valley, beaches are mostly dissipative, although less well developed than on the west coast (Harris et al., Reference Harris, Nel and Schoeman2011). In many bays of the Eastern Cape, sand has commonly been blown over the rocky headland through mobile dunes, in the form of headland-bypass dune systems (Illenberger and Burkinshaw, Reference Illenberger, Burkinshaw and Lewis2008; Harris et al., Reference Harris, Nel and Schoeman2011). The 50 km-long, 2 km-wide Alexandria dunefield in Algoa Bay (Fig. 13.1) is one of the largest active coastal dune fields in the world (McLachlan et al., Reference McLachlan, Siebe and Ascaray1982). The Wild Coast, which lies between the south- and east coasts, is predominantly rock or cliff, interspersed with pocket and embayed beaches (Harris et al., Reference Harris, Nel and Schoeman2011). In the southern part of the 550 km-long KwaZulu-Natal coast, between Port Edward and Durban, beaches are predominantly part-lithified sandy, as well as rocky, intertidal shorelines with numerous small temporary open and closed estuaries. In northern KwaZulu-Natal, the coastal plain is of lower relief and wider, and sandy beaches become more dominant. These beaches tend to be reflective, being narrow and steeply sloping with a limited surf zone. The wave-dominated marine environment and relatively arid hinterland along much of the southern and Eastern Cape coasts has also produced a number of ‘wave dominated’ estuarine systems, often characterised by temporary connections to the Indian Ocean and the formation of perched estuaries (Cooper, Reference Cooper2001; de Lecea et al., this volume).

13.3 Lithified Quaternary sediments

Carbonate-cemented Quaternary deposits are relatively common along the South African coastline where they create part-lithified sandy intertidal shorelines, and on the continental shelf, where they form prominent reefs (Martin and Flemming, Reference Martin and Flemming1987; Cooper and Flores, Reference Cooper and Flores1991; Ramsay and Cooper, Reference Ramsay and Cooper2002; Roberts et al., Reference Roberts, Botha, Maud, Pether, Johnson, Anhausser and Thomas2006; Carr et al., Reference Carr, Bateman, Roberts, Murray-Wallace, Jacobs and Holmes2010; Bosman, Reference Bosman2012; Cawthra et al., Reference Cawthra, Uken and Ovechkina2012). These deposits volumetrically dominate the part-lithified sandy intertidal shorelines. The oldest reported Quaternary cemented coastal sand deposits in South Africa (~390 kyr BP) crop out near Mossel Bay (Jacobs et al., Reference Jacobs, Roberts, Lachlan, Karkanas, Marean and Roberts2011; Roberts et al., Reference Roberts, Karkanas, Jacobs, Marean and Roberts2012) and Richards Bay (Porat and Botha, Reference Porat and Botha2008), and the youngest (<100 yr BP) on the Durban Bluff (Cawthra and Uken, Reference Cawthra and Uken2012).

As carbonate-cemented Quaternary deposits studied along the South African coast lack organic structures (Cooper and Flores, Reference Cooper and Flores1991; Cawthra et al., Reference Cawthra, Uken and Ovechkina2012; Roberts et al., Reference Roberts, Karkanas, Jacobs, Marean and Roberts2012) (Fig. 13.3), the mechanism behind the cementation is inorganic. The most common modes of cementation are through the evaporation of interstitial seawater in response to climatic conditions, or dissolution of shell fragments by meteoric water. Beachrocks are consolidated sedimentary formations, consisting of beach material bonded together by in situ precipitated carbonate cements (calcite and/or aragonite) (Bathurst, Reference Bathurst1975; Vousdoukas et al., Reference Vousdoukas, Velegrakis and Plomaritis2007). Lithification occurs in the intertidal and/or supratidal zones either on the beach surface, or beneath a thin veneer of unconsolidated sediment (Neumeier, Reference Neumeier1999; Vousdoukas et al., Reference Vousdoukas, Velegrakis and Karambas2009). Constituent particles include clastic, biogenic and authigenic sands and gravels, as well as human artefacts (Milliman, Reference Milliman1974; Cawthra and Uken, Reference Cawthra and Uken2012). Aeolianite (palaeodune) cementation is dominated by calcite spar, generally equant in form, infilling voids between constituent grains (Flügel, Reference Flügel2004). This process occurs in the vadose diagenetic zone and is facilitated by the percolation of rainwater through unconsolidated sand dunes.

Fig. 13.3. (A) ‘Dog tooth’ calcite spar infilling a void, Blood Reef (15 m below sea level), transmitted light microscope image, (B) blocky equant calcite spar cement, Durban Bluff, transmitted light microscope image, (C) isopachous rim cement (micrite) and equant spar, Blood Reef (23 m below sea level), scanning electron microscope image, (D) micritic isopachous rim cement, Blood Reef (19 m below sea level), scanning electron microscope image, (E) minor isopachous micrite rims surrounding clasts, prominent aragonite radiating into voids, Durban Bluff, transmitted light microscope image, (F) magnified micrite rims and aragonite crystals, Durban Bluff, transmitted light microscope image

(photos: Hayley Cawthra).

Carbonate diagenesis has been used to infer relative ages of aeolianite on palaeo-coastlines, as carbonate overprinting, replacement and re-cementation potentially provide insight into diagenetic zones. As an example, cementation of the wind-blown sands off the Durban Bluff and adjacent continental shelf (Ramsay and Cooper, Reference Ramsay and Cooper2002; Cawthra et al., Reference Cawthra, Uken and Ovechkina2012) initially occurred in the interstadial of Marine Isotope Stage (MIS) 7a. In response to a rise in sea level, the water table gradually migrated towards the landward margin of the basin. The raised deposits are characterised by a predominance of equant or drusy calcite spar, but a minor influence of micrite precipitation attests to the last interglacial (MIS 5). Offshore, MIS 4 deposits (forming Blood Reef), in thin section, reveal multiple cementation episodes, argued to be remnant of MIS 3 and the Holocene transgression of MIS 1.

13.3.1 Erosional features

The reliability of erosional coastal features of former sea level depends on identification and understanding of the processes that were active in developing the features (Guilcher, Reference Guilcher1988). The most reliable proxies of former sea levels include sedimentary (e.g. beachrock/aeolianite) and erosional features (e.g. notches and erosional terraces) (e.g. Laborel et al., Reference Laborel, Morhange, Lafont, Le Campion, Laborel-Deguen and Sartoretto1994; Elias, Reference Elias and Elias2007; Lambeck et al., Reference Lambeck, Woodroffe, Antonioli, Anzidei, Gehrels, Laborel, Wright, Church, Woodworth, Aarup and Wilson2010). In the case of sandy carbonate-cemented coastal deposits, most erosion occurs on the seaward margins of deposits (Miller and Mason, Reference 217Miller and Mason1994) due to the interaction of freshwater with sea water dissolving carbonate cements and mechanical erosion of outcrops by wave action.

In a South African context, erosional features of the intertidal environment provide relatively reliable sea-level indicators where associated with cemented Quaternary deposits. Ramsay (Reference Ramsay1996), Bosman (Reference Bosman2012) and Cawthra et al. (Reference Cawthra, Uken and Ovechkina2012) have recognised that the dominance of remnant erosional features on seafloor outcrops of the east coast relate to the Holocene transgression. Above present sea level, a well-developed palaeo-cliff-line occurs on the seaward side of the dune cordon on the north side of the Wilderness embayment (Illenberger, Reference Illenberger1996). Erosion of pre-existing beachrock/aeolianite has also resulted in the incision of intertidal potholes, solution basins, undercuts and the development of joints/fractures (Marker and Holmes, Reference Marker and Holmes2010) (Fig. 13.4) as well as impressive aeolianite cliffs, e.g. 180+ m-high barrier dunes near Wilderness (Fig. 13.1).

Fig. 13.4. (A) Erosional gullies on a rocky shoreline, trending in a coast-perpendicular orientation, Durban Bluff, (B) multibeam echosounder data from Blood Reef showing submerged erosional gullies 17 m below sea level, of comparable dimensions to (A), (C) fractured joint sets on an outcrop of beachrock, Durban Bluff, (D) intertidal zone potholes, Reebok (Mossel Bay), (E, F) erosional potholes, now submerged 18 and 24 m below sea level respectively, Blood Reef, (G) wave-cut platform in the Msikaba Formation, Pondoland, (H) coastal sea-cliffs and incised overhands, Pondoland

(photos: Hayley Cawthra).

13.4 Quaternary coastal and sea-level change

Raised beach deposits, aeolianites and erosive features have provided evidence for at least two sea-level highstands during the middle to late Quaternary. On the west coast these two highstands form part of a series of marine terraces with transgressive maxima incorporating the Langebaan and Springfontyn Formations, and span back to the late Pliocene (Roberts et al., Reference Roberts, Matthews, Herries, Boulter, Scott, Dondo, Mthembi, Browning, Smith, Haarhoff and Bateman2011). The earlier highstand is thought to have occurred during MIS 11 around 400 kyr BP. Similar highstands have been reported from Mossel Bay where a wave-cut platform extends 1 km inland from the present coast (Roberts et al., Reference Roberts, Karkanas, Jacobs, Marean and Roberts2012) and around the Alexander Bay area (Gresse, Reference Gresse1988). Along the east coast, mid-Quaternary highstands are represented by coastal lake muds (Port Durnford Formation) and aeolian sands (Kosi Bay Formation; Botha and Porat, Reference Botha and Porat2007). The latter on the Tshongwe–Sihangwane ridge underlie late Quaternary dunes and are an example of the buried former coastal aeolian sand landscape (Cooper and Kensley, Reference Cooper and Kensley1991).

Evidence for the 6–8 m highstand attributed to MIS 5e is more widespread, such as relict estuarine sediments on the south coast (Carr et al., Reference Carr, Bateman, Roberts, Murray-Wallace, Jacobs and Holmes2010). Preserved raised beach deposits along the KwaZulu-Natal coast have been dated using U-series to MIS 5 (Ramsay and Cooper, Reference Ramsay and Cooper2002). At the Nahoon hominid footprint site, shelly beach deposits at 6 m elevation date to MIS 5 (Jacobs and Roberts, Reference Jacobs and Roberts2009). Within the MIS 5e highstand there is some stratigraphic evidence that more than one transgressive event took place (Carr et al., Reference Carr, Bateman, Roberts, Murray-Wallace, Jacobs and Holmes2010), which fits with recent findings from a variety of far-field locations (Hearty et al., Reference Hearty, Hollin, Neumann, O’Leary and McCulloch2007).

Dune formation was not limited to highstands of MIS 11 and 5e. Bateman et al. (Reference Bateman, Carr, Dunajko, Holmes, Roberts, McLaren, Bryant, Marker and Murray-Wallace2011), working in the Wilderness dunes on the south coast, suggested barrier dune formation started in MIS 9 with directly dated dunes (using the luminescence method) spanning back to MIS 7. Both highstands and lowstands have significantly altered the configuration and sediment dynamics of all parts of the coastline in the past (Fig. 13.5). This is particularly evident on gently sloping sections of the continental shelf. Modelling has demonstrated how these changes would have transformed the coastal plain over the last 420 kyr, and influenced how and where early humans would have lived (Fisher et al., Reference Fisher, Bar-Matthews, Jerardino and Marean2010). Given the tectonic stability of the coastline and its far-field position, relative to Quaternary ice sheets, the association of dunes to sea levels reflects a eustatic control on shoreline position. Evidence for drowned dunes relating to sea-level lowstands during MIS 4–2 have been reported from the Wilderness area (Cawthra et al., Reference Cawthra, Bateman, Carr, Compton and Holmes2014). Two distinct cemented sandy shorelines at ‐100 m and ‐60 m have been mapped on the narrow but steep KwaZulu-Natal shelf (Salzmann et al., Reference Salzmann, Green and Cooper2013) where they have been associated with a preserved barrier dune complex which was overstepped during rapid sea-level transgressions around 11.5 kyr BP (Green et al., Reference Green, Cooper, Leuci and Thackeray2013).

Fig. 13.5. Phases of dune building from western, southern and eastern South African coastlines. (A) Late Quaternary dunes (shaded bars) and their association with eustatic sea level (adapted from Bateman et al., Reference Bateman, Holmes, Carr, Horton and Jaiswal2004), (B) summarised regional patterns of Holocene coastal dunes (data from Illenberger and Verhagen, Reference Illenberger and Verhagen1990; Compton and Franchescini, Reference Compton and Franceschini2005; Carr et al., Reference Carr, Thomas and Bateman2006) and based on Compton’s (Reference Cooper2001) South African Holocene sea-level curve.

Holocene coastal dunes appear to have developed after the mid-Holocene sea-level highstand around 6700 years BP (Compton, Reference 215Compton2001) (Fig. 13.5). Most of the Holocene dune fields are not lithified and are a series of mobile parabolic dunes forming plumes moving inland (Roberts et al., Reference Roberts, Bateman, Murray-Wallace, Carr and Holmes2009), or are stacked parabolic dunes remaining semi-permanently fixed behind beaches or atop older aeolianite dunes (e.g. Dana Bay and Wilderness).

13.5 Summary

The coastline of South Africa is dominated by sand, both in terms of the present-day beaches and coastal dune fields, but also through cementation with preserved beachrocks and aeolianites. The latter two contain a range of Quaternary sedimentary and geomorphic features that record changes in sea level and coastal configuration. These show that Quaternary sea levels around South Africa have varied from +13 m to ‐120 m relative to present in response to global climate change and regional eustatic adjustments. As a result of these changes, coastlines shifted and land was exposed or inundated. The extent of this movement was dependent on the gradient of onshore and nearshore coastal platforms.