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

Marine Geology 323–325 (2012) 107–122

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Sedimentation on the narrow (8 km wide), oceanic current-influenced continental shelf off Durban, Kwazulu-Natal, South Africa H.C. Cawthra a,⁎, F.H. Neumann b, c, R. Uken d, A.M. Smith d, e, L.A. Guastella f, A. Yates b, g a

Marine Geoscience Unit, Council for Geoscience, PO Box 572, Bellville, Cape Town, 7535, South Africa Bernard Price Institute for Palaeontological Research, University of the Witwatersrand, Private Bag 3, Johannesburg, 2050, South Africa Forschungsstelle für Paläobotanik am Geologisch-Paläontologischen Institut, Westfälische Wilhelms-Universität Münster, Schlossplatz 9, 48143 Münster, Germany d School of Agriculture, Earth and Environmental Sciences, University of KwaZulu-Natal, Private Bag X54001, Durban, 4000, South Africa e Alan Smith Consulting, 29 Browns Grove, Sherwood, Durban, 4000, South Africa f Department of Oceanography, University of Cape Town, Private Bag X3, Rondebosch, Cape Town, 7701, South Africa g Museum of Central Australia, Araluen Cultural Precinct, PO Box 5321 Alice Springs, NT 0871, Northern Territory, Australia b c

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Article history: Received 18 February 2012 Received in revised form 27 July 2012 Accepted 3 August 2012 Available online 11 August 2012 Communicated by J.T. Wells Keywords: South Africa Continental shelf Holocene sediment wedge Agulhas Current Durban Eddy Seismic stratigraphy Palynology

a b s t r a c t The narrow transform margin of southeast Africa and its associated sedimentological and hydrodynamic setting differs to other documented continental margins. The Durban Bluff continental shelf is extremely narrow and steep (8 km wide with a gradient ranging from 2 to 8°) characterised by a wave- and oceanic current-dominated regime. Seismic Sequence Boundary 2, developed during the Last Interglacial regression, spans the entire shelf separating the Holocene sediment wedge (Seismic Unit H) from underlying Pleistocene deposits. A wave ravinement surface marks the Holocene transgression, comprising a pavement lag of well sorted gravels and bioclastics overlain by inshore reef-derived carbonate rich sediments and offshore by quartzose mid-shelf sands. The shelf sands represent the transgressive Holocene to modern sediment wedge forming a seaward thinning unit stacked against the Pleistocene Blood Reef aeolianite/beachrock substrate. The sediment wedge is dynamic and constantly redistributed by currents associated with the Durban Eddy inshore of the Agulhas Current and bottom surge associated with high swells and marine storm events. These have been instrumental in shaping large-scale shoreface attached and detached sand ridges. The presence of mud lenses in the vicinity of Blood Reef represents deposition from turbid flood waters with preservation facilitated by the morphology of the Durban Bluff and Blood Reef. Palynological results, reflecting the local subtropical vegetation and recently introduced neophytes, together with radiocarbon dates, provide a very recent age for this sediment supporting a terrestrial origin and deposition from a single large flood event. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The narrow shelf flanking the Bluff Ridge on the southeast coast of KwaZulu-Natal (KZN) is strongly influenced by the Agulhas Current, the strongest western boundary current in the world (Lutjeharms, 2006). Globally, narrow sediment starved continental margins bordered by powerful boundary currents are rare and need to be documented. On the basis of the Johnson and Baldwin (1986) sub-division of shelf sedimentation into tide-dominated, storm-dominated and oceaniccurrent dominated sedimentation, the southeast African margin is listed as the best example of the latter (based on the work of Flemming, 1978, 1980, 1981). The only other documented example of a shelf

⁎ Corresponding author. Tel.: + 27 21 943 6700x6715 (office), + 27 82 559 6705 (mobile); fax: + 27 21 946 4190. E-mail addresses: [email protected] (H.C. Cawthra), [email protected] (F.H. Neumann), [email protected] (R. Uken), [email protected] (A.M. Smith), [email protected] (L.A. Guastella), [email protected] (A. Yates). 0025-3227/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2012.08.001

characterised by these processes is the outer Western Sahara Shelf (Newton et al., 1973) which differs significantly to our area of study in morphology of the shelf, patterns of shelf sedimentation and character of the bounding geostrophic current. The Bluff Ridge, flanking the Durban Harbour and associated lowland, lies on the eastern seaboard of South Africa and forms a prominent linear feature rising 120 m above mean sea level (MSL) (Fig. 1A, B). The 8 km wide continental shelf offshore of the Durban Bluff is particularly narrow (Martin and Flemming, 1988) compared to the global average of 78 km (Shepard, 1963; Kennett, 1982) and exhibits a relatively steep gradient of up to 8° (Fig. 1C). The Agulhas western boundary current forms off the northern KZN/Mozambique coast from the confluence of waters which follow complex trajectories in the Mozambique Channel and the area south of Madagascar (Lutjeharms, 1981, 2006; Gründlingh and Pearce, 1990). The Agulhas sweeps poleward with a core generally just offshore of the shelf break (Fig. 1D), affecting the waters on the shelf (Schumann, 1987). As a consequence of the narrow central KZN shelf, the Agulhas Current flows close inshore with a velocity of 2 m s − 1 (Lutjeharms, 2006).

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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, 1981; Flemming 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 (1978, 1980). This paper seeks to investigate the sediment dynamics of the continental shelf off the Durban Bluff using new geophysical, geological, palynological and geochronological data. This study builds on previous work (Belderson, 1961; Hay, 1984; Flemming and Hay, 1988; Richardson, 2005) with the application of high-resolution multibeam echosounder, sub-bottom profiling, side-scan sonar and surficial sediment samples obtained on diver surveys, and one short core. The objectives of this study were to use the above data to describe the stratigraphy, morphology and sedimentary characteristics of the dynamic Holocene sediment wedge. A model for recent sedimentation associated with storm events is presented and the first documentation of muddy marine sediment of the region containing palynomorphs is provided.

2. Regional setting 2.1. Sedimentology and oceanography Flemming (1978, 1980) described the southeast African inner shelf as an active zone of terrigenous deposition and transport. The innerto mid-shelf was characterised by subaqueous dunes and the outer shelf by carbonate-rich terrigenous gravels with subordinate sediment drapes. Sediment dispersal is controlled by the interaction of numerous factors: morphology of the continental margin, wave regime (Cooper, 1991, 1994; Smith 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, 1980, 1981; Flemming 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). Off the east coast of southern Africa, semi-permanent clockwise eddies develop on the inshore region of the Agulhas Current in the lee of large coastal offsets along the coastline (Flemming, 1981; Lutjeharms, 2006). Zones of bedload parting occur (Fig. 1D) particularly where coastal offsets have resulted in the formation of return flows in leeside eddies, moving sediment counter to the prevailing poleward (north to south) Agulhas flow. On the central KZN shelf, the bounding bedload parting zones of sedimentary compartment 2 (Fig. 1D) have been postulated to shift over an approximately 10–20 km wide bedload parting zone from approximately Scottburgh (70 km southwest of Durban) southwards (Flemming and Hay, 1988), depending on where the Agulhas Current meets the shelf at any particular time (Flemming, 1980). This parting zone corresponds to the southern boundary of the Durban Eddy, a semi-permanent, cyclonic eddy that commonly occurs in the lee of the southern end of the KZN Bight between Durban in the north and approximately Scottburgh to Sezela in the south (Fig. 1D).

Hay (1984), Martin and Flemming (1986) and Birch (1996) described several Holocene sediment depocentres along the southern KZN coastline with the Mlazi River depocentre located to the south of the Bluff Ridge (Fig. 1C, D). 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. 2.2. Climate and coastal vegetation In order to evaluate palynological material recovered from a short sediment core, a brief overview of the coastal vegetation is given here. Quaternary palynological material recovered from the coastal plain of KZN (Fig. 1C) potentially dates as far back as the late Pleistocene (Grundling et al., 1998; Mazus, 2000; Finch, 2005; Roberts et al., 2006; Finch and Hill, 2008). Holocene pollen records from the region are available from Lake Sibaya, the Mfabeni peatlands close to St. Lucia, the Mdlanzi swamps and Lake Eteza (Scott and Steenkamp, 1996; Mazus, 2000; Turner and Plater, 2004; Finch, 2005; Finch and Hill, 2008; Neumann et al., 2008, 2010; Walther and Neumann, 2011). For details see Neumann et al. (2010). The climate along the KZN coast is subtropical, characterised by summer rainfall and high air humidity. Average maximum and minimum monthly temperatures at Durban are 32.6 °C and 5.8 °C respectively (Mucina et al., 2006). Rainfall in Durban is ~1000 mm/ year (Jury and Melice, 2000). The coastal strip belongs to the KZN Coastal Belt, which was presumably covered in the past largely by subtropical coastal forest (pre-deforestation) and of which only small patches exist today (Northern Coastal Forest, Mucina and Geldenhuys, 2006). 3. Methods 3.1. Marine geophysical surveys Hydrographic (multibeam bathymetry) and geophysical (sidescan sonar and boomer seismic) data were collected for this study to map offshore Quaternary deposits. The continental shelf was surveyed between depths of 4 m and 52 m below sea level using a 400 kHz Reson Seabat 7125 multibeam echosounder. This ultra-high resolution system uses 256 beams at 0.5° across-track beam width separations to form a swath 128° wide. Vessel motion was corrected using an Applanix POS MV 320 motion reference unit, which achieves ±0.05° accuracy in roll, pitch, heading and heave, and positions were constrained within sub-decimetre resolution by a C-Nav Differential GPS. The survey was navigated using Hypack software. Sound velocity profiles were collected daily within the survey area to correct the multibeam echosounder data for changes in the velocity of sound through the water column. Multibeam bathymetry was used in conjunction with the geophysical data to delineate topographic features at high resolution. Morphometric measurements of sediment bedforms were made from these data within a decimetre-scale resolution and a three dimensional digital terrain model constructed.

Fig. 1. A, location of the study area in relation to Africa–South Africa–KZN. B, light grey block shows the extent of the seafloor survey area using bathymetric, side scan sonar and scuba diving surveys. Diver samples were obtained from within this zone and the locality of the sediment core and the ACEP Inshore mooring are indicated. C, the shelf break, isobaths and localities of interest along the KZN coastline. D, sedimentary compartment “2” of Flemming (1981), of relevance to this study, and associated current dynamics which affect sedimentation (indicated by arrows) from Schumann (1987) and Roberts et al. (2011). Images are modified from Flemming (1981), Martin and Flemming (1988), Gründlingh and Pearce (1990) and Roberts et al. (2011).

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Blood Reef and the surrounding shelf area were imaged using a dualfrequency (500/100 kHz) Klein 2000 side-scan sonar with a 137 m scan range to produce an acoustic texture map. The 100 kHz port and starboard channels were used in data processing. Corrected lines were processed with 25% overlap. Thirty-three line kilometres of single-channel, high resolution seismic reflection data were acquired, covering an area of 80 km 2 of the continental shelf and onto the continental slope offshore of the Durban Bluff. The boomer system was triggered every 500 ms at an energy level of 200 J and the reflected seismic data were acquired by an 8 element Design Projects hydrophone array and recorded on an Octopus 360 seismic processor using a sampling rate of 24 kHz and a sweep period of 75 ms. The Octopus 360 was used to apply band pass filtering of boomer data in the range from 300 Hz to 2 kHz and to store the seismic data on an internal hard-drive in SEG-Y format.

3.5. Ocean current data

3.2. Ground-truthing

4.1. Facies architecture: Sub-bottom profiling

The side-scan sonar mosaic map of Blood Reef delineating various acoustic facies was used to select underwater dive survey sites. Each dive was mapped as a dive track extracted from the GPS system. One hand-operated diver core was collected for palynological and sedimentological analysis and 111 surficial sediment samples were collected from the continental shelf by scuba diving. The sediment samples were analysed using a standard set of stacked sieves. Approximately 100 g of representative sample from each sediment sample site was oven-dried at 100 °C and mechanically sieved through fourteen South African Bureau of Standards (SABS) sieves of known weight with fractions ranging from gravel to mud. The filled sieves were weighed and the amount of sediment in each size fraction was calculated. Carbonate content was determined by the carbonate bomb method, based on the technique initially described by Schink et al. (1979). This procedure relates the amount of CO2 liberated from the dissolution of CaCO3 by dilute HCl to a carbonate percentage. The pressure increase that occurs in a closed system during this reaction is directly related to the mass of carbonate in the sample.

The Holocene sediment seismic unit forms a shore-attached prograding wedge bounded by a basal clinoform attaining a maximum thickness of 22 m on the mid-shelf, where sand ridges terminate against the seaward margin of Blood Reef (Figs. 1 and 2). The uppermost boundary of this seismic unit mostly forms the present seafloor and is sub-divided into two seismic facies, separated by a prominent seismic reflector (Fig. 2). Seismic facies H1 (Fig. 2) is acoustically semi-transparent characterised by low-amplitude divergent reflectors forming the basal component of large-scale submerged sand ridges. Unit H1 is separated in places from the underlying sequences by Sequence Boundary 2 (SB 2), a heavily incised erosion surface, and in the inshore areas H1 is underlain by a wave ravinement surface (WRS). On the inner shelf, Unit H1 conformably overlies the reef seismic facies. Seismic facies H2 is acoustically transparent and massive, capping the sand ridges of the Bluff shelf. On the landward margins of ridges and near the shoreface-attachment, facies H2 forms the entire sequence (Fig. 2). Elsewhere, facies H1 is overlain by the massive, acoustically transparent wedge of H2, separated by reflector h.

Use of acoustic Doppler current profiler (ADCP) data collected as part of the African Coelacanth Ecology Programme (ACEP) II KZN Bight research programme was made to analyse the shelf bottom currents off the Durban Bluff. An RDI Teledyne 300 kHz ADCP was deployed at 50 m depth some 3 km off the Bluff at 29° 54′ 24.00″ S, 31° 4′ 36.00″ E and an RDI Teledyne 75 kHz ADCP was deployed some 45 km offshore at a depth of 530 m at 30° 0′ 0.00″ S, 31° 29′ 60.00″ E for an approximate 18-month period from 24 March 2009 to 5 September 2010. Data were processed using RDI Teledyne support software and locally developed software, corrections being made for magnetic deviation in each case. 4. Results

3.3. Palynological analysis 4.2. Morphology of the continental shelf in the mapped area A standard procedure using KOH, HCl, HF, acetolysis, and heavy mineral separation with ZnCl2 was applied (Faegri and Iversen, 1989; Nakagawa et al., 1998) to process 16 samples (5–10 g) from a short marine sediment core at the Bernard Price Institute (BPI) for Palaeontological Research. Sample distance was 1 cm in the upper part of the core, which was more organic and clayey, and 3–7 cm in the lower section which yielded lighter sandy sediments with abundant mollusc fragments. Generally, palynomorphs are less likely preserved in sandy sediments. Palynomorphs were studied with a light microscope (magnification 1000 ×); measurements and photographs of selected palynomorphs were carried out using a digital camera and the image analysis software AnalySIS 5.1 at BPI Palaeontology. 3.4. Geochronology Selected samples of wood from the core were radiocarbon dated using accelerator mass spectrometry (AMS) at the NSF-Accelerator Mass Spectrometry Laboratory, Arizona State University. At the Arizona Accelerator Mass Spectrometry facility, the radiocarbon ages are determined by measuring the 14C/ 13C ratio in a sample and comparing that ratio with a similar one measured for known standards (Linick et al., 1986). The measured ratios of standards and samples are corrected to values corresponding to δ 13C = −25‰ using 13C/12C ratios measured in a stable isotope mass spectrometer and the “fraction of modern” of the sample is deduced (Donahue et al., 1990).

The continental shelf in the study area narrows considerably with respect to the Durban Bight to the north (Fig. 1B) and can be subdivided into three distinct morphological zones: the inner-, mid-, and outer shelf zones (Fig. 3B). This subdivision is based primarily on the variation in gradient and the consequent change in slope of the constituent zones (Fig. 3A). The maximum angle of slope attained in the study area is 8° on the inner shelf, and this decreases to a gradient of 2° on the outer shelf. Based on this subdivision, the inner boundary of the inner shelf forms the present intertidal zone, its outer boundary occurring at approximately 20 m below MSL. The mid-shelf is defined as the region seaward of the inner shelf zone up to the − 44 m isobaths which marks the beginning of the outer shelf zone that extends to the shelf break. Compared to existing literature on continental shelf structure this shelf is considered particularly narrow when compared to the global average of 78 km (Shepard, 1963; Kennett, 1982). 4.3. Acoustic facies and classification of sedimentary units The various acoustic side-scan sonar facies present were identified by their unique characteristics. The intensity and texture of the image created by the backscattered acoustic pulse are determined by the seafloor composition, allowing the identification of various features or facies types. These are illustrated in Fig. 4 and summarised as follows:

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Fig. 2. Boomer seismic lines collected in the study area, showing the Holocene sediment wedge, prominent horizons and geological substrate. Abbreviations: SB—sequence boundary, WRS—wave ravinement surface, f—fault.

1. Acoustic facies A: Speckled, highly reflective, even-toned to mottled sonar facies, with minor acoustic shadows. 2. Acoustic facies B: Highly reflective, mottled acoustic facies with a granular appearance and minor or no acoustic shadows present. Closely associated/interspersed with low to moderately reflective, even-toned to occasionally granular acoustic facies.

3. Acoustic facies C: Moderate to highly reflective, granular and eventoned acoustic facies, often exhibiting small scale alternating bands of weak reflectance with a more even-toned acoustic reflectance. Complex bedforms abundant. In places associated with facies A and B. 4. Acoustic facies D: Low to moderately reflective, smooth, even toned acoustic facies.

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Fig. 3. A, bathymetric chart of the study area surrounding Blood Reef (extending from −4 m below MSL to −52 m below MSL) and location of the cross section. B, cross section A-A′ and the defined morphological shelf zones (the inner, mid and outer shelf).

Scuba diving observations reveal acoustic facies A (Fig. 4) to be characterised by subdued and rugged relief outcrops comprising beachrock and aeolianite. Acoustic facies B is closely associated with A, being represented by more subdued morphology and being partially covered by a thin veneer of sediment representing scattered outcrop (Fig. 4). Acoustic facies C typically consists of poorly sorted, unconsolidated, bioclastic-rich sediment. The carbonate content averages 33% in the study area and the sediment comprises skeletal detritus originating from the mechanical breakdown and biological destruction of carbonate-secreting organisms such as Mollusca,

Foraminifera, Echinodermata, Bryozoa and Scleractinia. Acoustic facies D corresponds to a sedimentary unit of fine- to coarse grained sand. The composition of the sediment is predominantly quartz, with minor carbonate, feldspar and heavy minerals (dominated by ilmenite and rutile). This unit of unconsolidated siliciclastic shelf sediment is classified as quartzose shelf sand. The Holocene sediment wedge of the Bluff shelf was further subdivided into four Holocene to modern stratigraphic units, each a unique facies. The units were identified on the basis of textures associated with the above acoustic facies, architecture of deposits

Fig. 4. Left, 100 kHz side-scan sonar mosaic showing of the area surrounding Blood Reef. The reef outcrops have a different texture of backscattered pulse to the sandy seafloor. Right, close-up images of the blocks indicated on the side-scan sonar mosaic showing the characteristics of each acoustic facies A–D described above.

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Fig. 5. Bioclastic gravel pavement. A, multibeam point cloud image of bathymetric soundings, illustrating the sharp contact between the bioclastic pavement and quartzose shelf sand. The depth is 20 m below MSL. B and C, photographs of the contact at the same locality as A. D, ripples in the bioclastic field. L = 30 cm; H = 12 cm. E and F, sediment sample of the bioclastic gravel pavement in the vicinity of Blood Reef, illustrating the composition and texture of the gravel pavement. Constituent clasts include very coarse shell material and spheroidal extremely well rounded fragments of aeolianite and beachrock. Fines have winnowed out by extensive reworking.

studied by sub-bottom profiling, sedimentology of surficial samples and position on the modern shoreface to inner-shelf profile. These units are bioclastic gravel pavement, quartzose shelf sand of the modern shoreface, localised muddy sand deposits (though not identified on the side-scan sonar mosaic due to limited exposure of the deposit), and inter-reef bioclastic sediment. These facies are described in chronological order. 4.4. Sedimentary facies 4.4.1. Bioclastic gravel pavement Bioclastic sediment forming the gravel pavement is defined as having a CaCO3-content of > 20% (Tucker, 1982), representing a mixture of biogenically derived debris and well-rounded pebbles of aeolianite and beachrock, and quartzose sand. The bioclastic gravel pavement is widespread, being mainly exposed on the mid- to outer-

shelf in regions starved of quartzose sand (Fig. 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. All mollusc species identified from the sediment core are marine (Fig. 7B), reflecting a typical shallow subtidal environment. The basal portion of the core (30 cm to 17 cm) is dominated by Ranellidae, Turridae, Nassariidae and Venus verrucosa, though V. verrucosa extends through to the 10 cm horizon. Barbatia sp. and Diodora sp. were noted between 20 and 17 cm, and Lucinidae and Acmaeidae from 17 to 14 cm. The upper half of the core contains Turbo sp. and Conus sp. present between 14 and 3 cm and the uppermost 10 cm is characterised by the presence of Septifer sp, Garidae, Mytilidae, Dosinia and Triphoridae. Triphoridae were only found in the uppermost centimetre of the core.

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Though most specimens are water-worn and transported, the turrids, gariids, triphorids, Barbatia, Musculus and Dosinia are well preserved and delicate, and are thus considered autochthonous. The geophysical expression (Fig. 5A, E) of this unit displays a rough surface texture with a linear fabric. Elongate features (oriented ~ 55–235°) are exposed as a pavement in the northwest portion of the study area. Sand ribbons extend from the seaward margin of Blood Reef into the mid-shelf region and have a relief of ~1 m relative to the surrounding seafloor. The sand ribbons, oriented ~ 280–100° and possibly a result of high swell return flow, are superimposed by minor wave ripples. Northwards the shelf becomes increasingly sedimentstarved exposing extensive sections of bioclastic gravel pavement in the Cave Rock Bight (Fig. 1B, Fig. 4).

4.4.2. Quartzose shelf sand This unit volumetrically dominates the shelf and is characterised by a relatively smooth morphology displaying a variety of bedforms (Fig. 6). The sediment is generally fine grained and moderately to well sorted with moderate carbonate content (7–29%).

Three bedform morphological forms are recognised, using the terminology of Ashley (1990): Type 1 Straight-crested and sinuous wave ripples: L = 10–15 cm, H = 2–5 cm Type 2 Dunes: L = 1 m, H = 0.2- 0.5 m Type 3 Sand ridges: L = 320–930 m, H = 9–22 m The dimensionless dune form index L/H (the ratio of wavelength to height) varies substantially between the different bedform morphologies. Type 1 and 2 bedforms have indices less than 2, whilst type 3 sand ridges exhibit values ranging from 35 to 45. Type 1 and 2 bedforms are oriented shore-parallel, while type 3 bedforms are oriented NNE–SSW. Wave form indices indicate that type 3 sand ridges are of low relief relative to their spacing. The quartzose shelf sand of the modern shoreface is generally fine grained and well sorted. Type 1 has straight-crested and sinuous wave ripples with crests containing shell fragments and the troughs infilled with heavy minerals are commonly found inshore of Blood Reef (Fig. 6C, D, E), whereas in the northern portion of the study area

Fig. 6. Dunes of the quartzose shelf sand unit. A, diver sampling sediment 21.5 m below MSL. B, dune consisting of fine-grained quartzose shelf sands. C, dunes 16 m below MSL, L = 40 cm, H = 15 cm. D, diver sampling the quartzose shelf sand in a bifurcating wave-ripple field 26 m below MSL. E, shore-parallel oriented ripples of L = 20 cm, H = 5 cm with superimposed wave ripples. Crests contain shell fragments and troughs are infilled with heavy minerals.

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Fig. 7. A, sedimentary log for the 30 cm sediment core obtained from locality 2. Abbreviations: FS—fine sand, MS—medium sand, CS—coarse sand. B, identified molluscs. C, grain size statistics of the surficial expression of the muddy sediment are provided and abbreviations are as follows: G—gravel; VCS—very coarse sand; CS—coarse sand; MS—medium sand; FS —fine sand and VFS—very fine sand. D, images of the outcropping muddy sand deposit at locality 2, at the selected core locality. The coarse grained material forms the trough of the mid sand ridge and overlies the mud.

Fig. 8. Shortened pollen histogram of the productive section in 0–9 cm depth.

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Fig. 9. Occurrence of Psammobiidae within sand ridge troughs and samples thereof. A, side-scan sonar mosaic draped onto multibeam echosounder data. B, diver retrieving sediment sample from a sand ridge trough 22 m below MSL. C, sample of Psammobiidae 16 m below MSL.

type 1 bedforms are replaced by dunes (type 2 bedforms) with a sharp contact with the underlying bioclastic pavement (Fig. 5). In parts, current scouring has removed the quartzose sediment completely (Fig. 5A) producing a sediment-starved region blanketed only by relict or palimpsest bioclastic gravel. 4.4.3. Muddy sediment Muddy sediment was mapped at two localities on the shelf surrounding Blood Reef. Locality 1 occurs between two ledges of beachrock and locality 2 is exposed in the trough of the middle sand ridge (Fig. 4). The deposits are moderately sorted, negatively skewed, and dominated by muddy fine sand (Fig. 7C). The sediment core (~30 cm) obtained from locality 2 (Fig. 4) was sampled for palynological analysis and wood fragments were collected for AMS radiocarbon dating (Figs. 7 and 8). The core fines upwards with basal gravels of light greyish-brown coarse sediment and shell hash overlain by fine sediment comprising two units separated by a sharp contact. The uppermost 13 cm consists of dark brown finely laminated, fine-grained clay-rich sediment with a mud content ranging from 22.58% to 4.25% (Fig. 7A). Included in this unit are Foraminifera, shelly material and

small wood fragments (b5 g in mass). AMS radiocarbon dates obtained from wood fragments were post bomb, implying an age of less than 50 years and most probably related to the 1987 Natal Flood (Kovacs, 1988). 4.4.4. Palynomorphs Only the upper 9 cm contained sufficient palynomorphs for quantitative analysis, although even there the pollen concentration was comparatively low (120–250 terrestrial pollen per sample counted). Below a depth of 9 cm 0–11 palynomorphs were found in each sample (Fig. 8). Neophytes (Pinus, Ambrosia, Eucalyptus) were identified up to 20 cm depth, but it cannot be excluded that they also occur in deeper layers. Up to 23 cm isolated pollen grains were found. Marine influence is shown by Foraminifer linings and dinoflagellate cysts throughout the core. Although most dinoflagellates are sea dwellers, some taxa are known from lacustrine environments. The coastal vegetation is reflected well in the palynomorph assemblages, including forest elements like Sapotaceae, Phoenix, Podocarpus, Trema, Apodytes, mangrove elements, e.g. Bruguieria, savanna trees, e.g. Spirostachys and Acacia, and grasses. Grasses

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dominate the pollen diagram (~ 50%). Wetland elements are rare, Cyperaceae being an exception (max. 20%). No variation or trend was observed throughout the profile (with exception of high fungal spore values at 2 cm depth); consequently a pollen zonation was not undertaken. 4.4.5. Inter-reef bioclastic sediment This unit is closely associated with the reef, occupying gullies and lying within troughs of sand ridges where they blanket the seaward margins of Blood Reef (Fig. 9A) (Cawthra, 2010). Bedforms within the beachrock/aeolianite complex comprise carbonate-rich gravels and coarse-grained quartzose sand, reworked into large sharp-crested wave ripples (L = 0.6–1 m, H = 10–30 cm). These ripples rest upon gullies and blanket the low-relief reef. Subordinate interference ripples are common (L = 15–20 cm, H = 3–5 cm) (Cawthra, 2010). Thick deposits of Psammobiidae occur within troughs of sand ridges in close proximity to the reef. As the organisms perish, the shells are entrained from the reef into the troughs of the abutting sand ridges (Fig. 9A, B, C). 4.5. Sand ridges The bulk of the quartzose shelf sand of the study area is contained within shoreface attached- and detached ridges (Swift, 1976; Stubblefield and Swift, 1976; Swift et al., 1978; Niedoroda et al., 1985) (Fig. 9A), which dominate the mid-shelf of the Bluff and terminate against the nearshore Blood Reef (Cawthra, 2010) (Table 1). Seaward of these features a thin veneer of quartzose shelf sand extends to the outer shelf. Elongate sediment bodies of high acoustic backscatter occur in the troughs of the subaqueous dunes, comprising coarse gravel sized coral, mollusc shells and other bioclastic material associated with the interreef bioclastic sedimentary facies (Fig. 9B). The mid-shelf is characterised by two shoreface-detached ridges (the middle ridge and outer ridge) and one shoreface-attached ridge (the inner ridge) oriented oblique (~ 30°) to the present day coastline and the bathymetric slope. The landward margins of these ridges become near-asymptotic towards the northwest, where the reef terminates and the bioclastic gravel pavement is exposed. The outer sand ridge (Table 1) is bounded by Blood Reef on the inshore, and a mostly buried aeolianite unit on the offshore side. The middle ridge is bounded by the seaward margin of Blood Reef, similar to that on Aliwal Shoal, some 50 km southwest of Durban (Bosman et al., 2005). The inner shoreface-attached ridge lies on the inner shelf, terminating in the south against Blood Reef.

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The bottom current speeds are mainly in the 0 to 20 cm s− 1 range; however higher speeds are possible, attaining a maximum of 90 cm s− 1. Beyond the shelf break, the south-westward flowing Agulhas current predominates throughout the water column. Bottom currents in this area flow to the south to west sector 45% of the time, with peak frequencies to the south-west (22%), resulting in net south-westward transport. Bottom current speeds in this offshore area are mainly in the 10 to 20 cm s− 1 range, with a maximum of 92 cm s− 1. 5. Discussion 5.1. Configuration of the upper continental shelf Seismic Unit H forms the Holocene sediment wedge initially identified by Martin and Flemming (1986) and the quartzose shelf sand is the surficial expression of this transgressive Holocene to modern deposit. The sediment wedge thins toward the outer shelf; the bulk of this unit forms sand ridges which dominate the inner- to mid-shelf. SB 2 separates the Holocene sediment wedge (Unit H) from underlying units and is interpreted to form the Pleistocene–Holocene boundary as sea level regressed from the Last Interglacial toward the Last Glacial Maximum (Fig. 10). The basal erosion surface of the sand ridges on the Bluff shelf is marked by a high-amplitude laterally continuous reflector and forms the ravinement surface of the Holocene transgression (WRS—Fig. 10) as sea level rapidly transgressed. Green (2009) identified the Holocene Ravinement Surface of the Zululand Basin and this surface can be correlated to the WRS reported here.

4.6. Ocean currents Bottom currents measured from the ACEPII ADCP offshore of the Bluff Whaling station (Fig. 1B) indicate that currents flowing to the north to east sector predominate, occurring 53% of the time, with the highest frequency (28.5%) flowing to the north-east (Fig. 11D). The north to eastward currents inshore (counter to the predominant south-westward flowing Agulhas Current offshore) reflect the presence of the Durban Eddy; thus this would suggest that the Durban Eddy is present in the Blood Reef area over 50% of the time. Net transport is to the north-east (Fig. 11C), although along shelf oceanic current reversal does take place.

Table 1 Dimensions of the three sand ridges. Depths listed are relative to MSL and values represent the maximum thickness along bedform crests. Sand ridge nomenclature

Base depth

Crest depth

Amplitude Wavelength Aspect

Inner Middle Outer

29 m 34 m 43 m

− 17 m − 21 m − 22 m

8m 17 m 21 m

319 m 536 m 926 m

Attached Detached Detached

Fig. 10. Sequence Boundary 2 (SB 2) and the Holocene wave ravinement surface (WRS) superimposed onto the late Quaternary sea level curve of Waelbroeck et al. (2002). Timing relationships can be observed relative to the Last Interglacial 125 000 years BP and the Last Glacial Maximum 18 000 years BP. SB 2 reflects sea level regression, exposing the continental shelf, and WRS rapid transgression toward present sea level (Cawthra, 2010).

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High-resolution sub-bottom profiling across the shorefaceattached ridges of the Bluff reveals two seismic facies. Similar features have been described in ridges on the New Jersey shelf (Stubblefield and Swift, 1976) and these data agree with trends observed in the seismic profiles across the New Jersey shelf which revealed that the modern ridge deposits are separated from older sediments in this region by an erosional unconformity (Dalrymple and Hoogendoorn, 1997). The chaotic appearance of the ridges' basal reflectors (seismic facies H1) and the seaward dip of internal reflectors suggest that they have accreted seaward over time. The upper facies, H2, commonly appears acoustically transparent, which is characteristic of the Holocene marine sediment wedge. 5.2. The Holocene sediment wedge and controls on unconsolidated sedimentary facies distribution 5.2.1. Basal bioclastic gravel pavement and models of bedform accretion Bioclastic gravel deposits on the continental shelf south of Durban described by Hay (1984) and Flemming and Hay (1988) occur as coarse lag deposits on the mid- to outer shelf. These deposits are associated with zones of erosion or non-deposition (Flemming, 1980), and are exposed where high current velocities prevail (Green, 2009). The transition between the relict bioclastic pavement and the inshore sediment wedge on the Bluff shelf is distinctive (Fig. 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. Flemming (1978), Hay (1984) and Flemming and Hay (1988) considered these carbonate deposits relict lag deposits. The acoustically harder surface over which younger bedforms migrate is exposed between successive bedforms. This is observed in the case of dunes, as well as sand ridges of the Bluff shelf (Figs. 5 and 9A). On the basis of the well-rounded beachrock and aeolianite pebbles, coralline material and large amount of coarse shelly detritus extracted from outcrops of this bioclastic gravel offshore of the Bluff, this bioclastic pavement is correlated with the Holocene WRS identified by sub-bottom profiling (Figs. 2 and 10). A comparison of mean grain size, coupled with standard deviation (Folk and Ward, 1957) indicates that the coarse sands of the trough axes become better sorted as they become coarser. A similar relationship was noted by Swift et al. (1972), the relationship evidently being a characteristic feature of relict lag deposits subjected to winnowing. The more prolonged and intense the winnowing, the fewer and coarser are the grains of the residual deposit. These gravel lag deposits thus probably represent the erosion surface over which the sedimentary bedforms have migrated. 5.2.2. Reorganisation of the Holocene sediment wedge Holocene sediments are concentrated predominantly on the midshelf of the Bluff and form a seaward thinning wedge stacked against the Blood Reef aeolianite/beachrock substrate. The narrow continental shelf offshore of the Bluff Ridge is dominated by the southward flowing Agulhas Current, but a component of the Agulhas system in this area is the cyclonic Durban Eddy which causes northward flowing currents in the inshore region (Figs. 1D and 11A, B) and has an intense influence on sediment distribution on this part of the shelf. This mesoscale feature commonly occurs in the area between Durban in the north to Sezela in the south, covering a longshore distance of approximately 60–90 km, extending some 40–50 km offshore, and has an average lifespan of about 8.6 days with a range of 3 to 19 days and an average time between eddy events of 7.7 days (Guastella et al., 2011). Current reversals off the Bluff are frequent, and an expression of the episodic nature of the eddy. The effect of bottom surges from predominant south-southeast swells

(Guastella and Rossouw, 2009), particularly during storm events, may also influence sediment distribution patterns. The mid-shelf zone is dominated by a series of sand ridges (Fig. 11B). This ridge system has a slightly curved configuration relative to the coastline, protruding convexly seaward. The dune belt of KZN described by Birch (1981) is arcuate with respect to the coastline, being closer to the shoreline in the vicinity of Durban than further south as the result of a narrowing shelf. Seaward progradation of dunes only prevails when no obstructions prevent it. In the vicinity of the Bluff Ridge, submerged cordons of aeolianite reef form topographic barriers. These may induce vertical accretion as opposed to horizontal, accounting for the size of the sand ridges surrounding Blood Reef. Sand ridge vertical accretion will also be promoted by the Durban Eddy-produced current reversal as this results in bedform climbing (Rubin and Hunter, 1982). The volume of sediment contained within the sand ridges of the Bluff shelf is substantial (see dimensions in Table 1) and they can be considered as sand ore bodies from a source of coastal erosion. 5.3. Muddy sand deposits, temporal relationships and palynological observations Mud depocentres are rare on the shelf of KZN, the majority of the fines being transported to a deep-sea depositary (Flemming and Hay, 1988). The occurrences of the two small deposits of muddy sand on the Bluff shelf are therefore considered unusual and reveal a recent sedimentation history for this area. The maximum of 50 years in age for the mud deposit and analysis of the marine sediment core demonstrates that the upper section may have been deposited by a single flood event transporting terrigenous, more clayey, organic sediments from the land. It is here proposed that the muddy sediment was most probably derived from the 1987 Natal Flood, as rivers in the vicinity of Durban debouched vast amounts of terrigenous detritus into the sea. Kovacs (1988) reported a 60 year flood for the Mgeni River and a 30 year flood for the Mlazi Rivers associated with this event, and a 6 m swell was running at the time of the event (Guastella and Rossouw, 2009). The geological setting where locality 1 muddy sand outcrops is nestled between two coast-parallel trending units of linear beachrock (Fig. 4). The protection provided by these ledges has provided an environment of quiescence allowing the preservation of the small deposit. The productive section of the marine sediment core (0–9 cm) (Fig. 8) reflects the local vegetation, particularly the KZN coastal belt —the dominance of grasses (~50%) clearly indicating the coastal grassland as the main source. Forest and mangrove elements are also abundant, the pollen composition being comparable to other late Holocene sites, e.g. Lake Eteza and Lake Sibaya (Neumann et al., 2008, 2010; Walther and Neumann, 2011). The pollen content confirms that the origin of the muddy sediment was most likely terrigenous and, as shown by only weak appearance of marine indicators such as foraminifer linings, only to a low degree derived from the marine environment. Sedges show limited influence of wetland indicators. No major fluctuations in the pollen percentages are visible, pointing to either a stable vegetation development during the deposition of the sediments or to a fast sedimentation rate, e.g. deposition during a single flood event. In contrast, high resolution pollen studies at Lake Sibaya indicate rather drastic vegetation changes, e.g. a strong increase of pines during a short time span (Neumann et al., 2008). The only exception from an otherwise uniform pollen record at Blood Reef is a peak in fungal spores at 1.5 cm core depth; the reason for this is unknown but modern contamination cannot be excluded. Neophytes are common: Pinus percentages fluctuate between 5 and 15%. Quercus and Casuarina pollen are rare; Eucalyptus and Ambrosia pollen appear regularly in low numbers. Ambrosia was introduced after the Anglo-Boer war (1899–1902) (see Neumann et al., 2011). Quercus and Pinus were planted since the 17th century in the Cape region (see Neumann et al., 2011), although a more local source of

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Fig. 11. A, regional oceanographic setting of the study area (modified from Schumann, 1987 and Roberts et al., 2011). B, inferred bottom current flow based on current measurements (presented in C and D) and orientation of sand ridge crests identified in this study. C, current rose based on data from the ACEPII inshore ADCP mooring site off the Bluff Whaling Station for the period 24 March 2009 to 5 September 2010. D, direction frequency distribution at 44 m depth at the ACEPII inshore mooring. Data are acknowledged from ACEPII.

those pollens is much more likely, especially in the light of the high pine pollen values. Plantations of exotic trees (pines and eucalypts) were spreading in KZN since 1928/1929 and expanded further after the Second World War (Marwick, 1973). Casuarina pollen signals the introduction of the Australian trees for dune stabilisation in KZN since the 1940s AD (Bruton et al., 1980; Poynton, 1995). The pollen spectra thus point to a very young age for the deposit. Some pollen indicators, especially the high Pinus values and Casuarina, suggest deposition within the last 50 years which is in good agreement with the radiocarbon dates. It has to be emphasised though that the low number of preserved terrestrial palynomorphs per sample limits quantitative interpretations. The fining-upward sequence within the core with a scoured base of gravelly shell hash is indicative of high energy flow. The capping mud horizon may thus represent suspension settling of fluvially derived muds entrained in the Durban Eddy circulation following a high-swell or storm event. A similar model was proposed for shelf sediments of the mid-shelf off the Thukela River mouth by Bosman et al. (2007). In the case of normal river flow and lesser floods, mud is emplaced as flocculation deposits from buoyant freshwater plumes. It was suggested that on the shelf off the Thukela River (Fig. 1C) during extreme floods (>50-year RI), such as the 1987 Natal Flood (Kovacs, 1988), large

quantities of sediment are rapidly deposited (Bosman et al., 2007). During this flood, the Thukela River conveyed six times the annual average sediment discharge to the sea over a few days. Regional studies on storm sedimentation in Oregon (Smith and Hopkins, 1972) and northwest Kimberly, Australia (McGowan et al., 2012) revealed that substantial storm events can have a significant influence on the preserved sedimentological record on an inner shelf.

5.4. Comparison to similar continental shelves and the significance of this sedimentological setting within a global context Globally, narrow continental shelves with comparable widths to the margin of the South African east coast are either divergent, transform or have convergent boundaries (Kennett, 1982). Subsequent evolution of divergent and transform continental margins is dependent on complex interactions between subsidence, sedimentation, climate and ocean circulation (Kennett, 1982). Sediment starved margins (marked by a thin, prograding cover) and mature margins (characterised by a thick (>10 km) wedge of shelf sediment) (Emery, 1980) are commonly a function of the strength of the associated boundary current.

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Swift's (1974) classification of modern continental shelves is based on the distinction between palimpsest (autochthonous) shelf sediments and modern (allochthonous) shelf sediments, in conjunction with the dominant hydraulic regime (tide-, storm- or oceaniccurrent dominated). Swift (1974) identified three types of shelf, namely storm-dominated palimpsest sediment shelf (e.g. Middle Atlantic Bight, USA), tide-dominated palimpsest sediment shelf (e.g. Northwest European shelf) and storm-dominated modern sediment shelf (e.g. Niger shelf). Johnson and Baldwin (1986) add stormdominated palimpsest/modern sediment shelf (e.g. Oregon–Washington), storm-dominated, modern sediment, texturally-graded shelf (e.g. southern Bering Sea) and oceanic-current dominated palimpsest/modern sediment shelf (e.g. South African east coast and Northwest Africa) to the classification of continental shelves from a sedimentological perspective. Although major boundary currents lie seaward of the shelf edge, where they impinge on certain oceanic current-dominated shelves, large eddies form (Johnson and Baldwin, 1986). Current velocities range from a few cm s − 1 to more than 250 cm s − 1. Weaker currents transport only suspended sediment, but the stronger currents transport sand waves, such as on the South African east coast and outer Saharan shelves (Johnson and Baldwin, 1986). The shelf of the South African east coast is considered to be one of the world's narrowest (Martin and Flemming, 1988). Areas with a comparable shelf widths include central Brazil (Natal–Salvador), the African northwest coast (Western Sahara to Morocco), the east coast of Japan, the west coast of North America near California and the east coast of New Zealand. The central Brazil shelf is a divergent/passive, mature margin (Emery, 1980; Kennett, 1982) with an average width of 30 km (ranging from 42 km off Maceió to 8 km off Salvador) (Martins and Counthino, 1981). The sediments on the narrowest section of the Brazil continental shelf, spanning from Cabo São Roque to Belmonte, are characterised by low continental erosion rates and a narrow zone of marine deposition (Martins and Counthino, 1981). In this region the southwestward flowing Brazil Current runs parallel to the shelf, but despite the presence of this boundary current the mid-shelf sediments are fluvially-controlled south of the São Francisco River, and to the north the shelf sedimentation is mainly wave-dominated (Coleman and Wright, 1972). The inner shelf sedimentation is dominated by coralline-algae covered beachrock (Mabesoone and Couthino, 1970). The northwest African shelf (Western Sahara to Morocco) is a divergent/passive margin (Kennett, 1982) characterised by oceanic current-dominated sedimentation (Johnson and Baldwin, 1986). Shelf widths range from a maximum of 140 km off Western Sahara to 25–30 km off Morocco. The shelf exhibits a local vertical relief of up to 10 m, attributed to deposits of Quaternary beachrock and aeolianite outcrops (Summerhayes et al., 1971). Offshore circulation is dominated by the Canary Current (which flows at 25–75 cm s− 1) (US Navy Oceanographic Office, 1965) and has associated gyres over the Saharan Shelf. Summerhayes et al. (1976) identified two distinct sedimentary provinces on the northwest African continental shelf: carbonate-rich sands south of Sidi Ifni (Western Sahara) and more terrigenous, muddy sediments predominate further north (off Morocco). This difference is caused by a low rate of terrigenous sedimentation and a coast-parallel wind system influencing aeolian sedimentation in the south. Off central Western Sahara fine terrigenous sediment supply is low but the absence of fines is attributed to the sufficiently strong current and wave regime in shallow waters, preventing setting (Summerhayes et al., 1976). Though they have comparable shelf widths to the South African east coast, the offshore sedimentary processes on parts of the Japanese, Californian and New Zealand margins differ significantly to the oceanic-current sedimentation along the KZN shelf. Regional sedimentological and oceanographic investigations at modern forearc margins have shown that sediment transport and deposition at the

active margin are controlled by neotectonics and that more than half of the supplied material is transported to the slope (Noda and TuZino, 2010). The narrow Japanese shelf is covered by sandy sediments. Winnowed fines are interpreted by Noda and TuZino (2010) to have escaped to the slope via gravity-driven across-shelf transport or ocean-current-induced along-shelf transport. Sedimentological and stratigraphic variations are thus linked to variations in the physical configuration of the shelf/slope system which is shaped by the local topography in addition to the climatic and oceanographic processes (Noda and TuZino, 2010). These processes are said by Noda and TuZino (2010) to include tsunamis, storms and currents, earthquakes, subsurface gas, basement uplift or subsidence, and the development of submarine channels. Grossman et al. (2006) propose that rapid progradation of clinoforms on the central California shelf may have occurred during transgression due to a unique interaction of modest rates of sediment input and tectonic uplift, variable rates of eustatic sea-level rise and a complex stepped antecedent topography. In the latter portion of the Holocene transgression, uplift rates as great as and sometimes exceeding rates of sea level rise brought the middle and inner shelf into the depths actively reworked by waves and currents, causing progradation of mid-shelf sediments. This shelf is classed as stormand wave dominated (Johnson and Baldwin, 1986). At the flood-dominated Eel margin, California, low sediment trapping efficiency is attributed to the narrow the shelf width, direct bypassing to the Eel Canyon, fluvial input, and the resuspension and redistribution of shelf sediments by waves and currents (Sommerfield and Nittrouer, 1999). These studies showed that sediment transport and deposition at the active Eel margin are controlled by neotectonics, and that more than half of the supplied material is transported to the slope (Noda and TuZino, 2010). Late Pliocene to Recent wave and storm dominated sediments (Carter and Herzer, 1979) on the east coast of New Zealand have been exposed on the shelf by coastal uplift, folding and wave planation (Barnes, 1995). Regional oceanography plays a major role in the seaward dispersal of sediment from the Waipaoa River in the region surrounding Poverty Bay (Orpin et al., 2006). Along the eastern North Island, seaward of the northward flowing Wairarapa Coastal Counter current is the southward flowing East Cape Current, forming part of the Subtropical Inflow to New Zealand and extending almost as far inshore as the shelf break (Heath, 1985). Large eddies formed by complex interactions between the southward-flowing East Cape Current, the Wairarapa Eddy, and the East Cape Eddy (Chiswell, 2005) move southwest along the outer margin. Orpin et al. (2006) stress the important role of tectonics in shelf sediment distribution in New Zealand, as Holocene sedimentation occurred concurrently with fault activity and the creation of accommodation space. Although sedimentary deposits comparable to the central KwaZuluNatal shelf have been documented on the storm-dominated USA east coast (Swift et al., 1971; Duane et al., 1972; Stahl et al., 1974; Niedoroda et al., 1985; Hoogendoorn and Dalrymple, 1986; McBride and Moslow, 1991; Snedden et al., 1994; Dalrymple and Hoogendoorn, 1997), this is considered to be a storm-dominated broad passive shelf (Johnson and Baldwin, 1986). 6. Conclusions 1. The narrow transform margin of southeast Africa and its associated oceanic current-dominated sedimentation differs to other documented continental margins. 2. The sedimentary and oceanographic environment associated with the particularly narrow continental shelf of the South African east coast, in conjunction with the close proximity to the geostrophic Agulhas western boundary current, provides an unusual, if not unique, hydrodynamic shelf setting.

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3. The Bluff continental shelf is a dynamic environment with a thick Holocene sediment wedge. Seismic Sequence Boundary 2 (SB 2) is an extensive surface which separates the Holocene sediment wedge (Seismic Unit H) from underlying Pleistocene units. It is interpreted to form the Pleistocene–Holocene boundary, represented by the sea level regression from the Last Interglacial toward the Last Glacial Maximum. 4. SB 2 is overlain on the inner- to mid-shelf by a Holocene wave ravinement surface (WRS). This surface underlies all bedforms and exposed in areas with high current velocities. The sediment facies comprises well sorted gravels with high carbonate content. The transition between this relict bioclastic pavement and the inshore sediment wedge on the Bluff shelf is a distinctive erosional contact between these units. 5. The quartzose shelf sand is the surficial expression of the transgressive Holocene to modern sediment wedge described by Martin and Flemming (1986) and recognised in the seismic interpretation of this study. The Holocene sediments are concentrated predominantly on the mid-shelf off the Bluff and form a seaward thinning wedge stacked against the Blood Reef aeolianite/beachrock substrate. A significant volume of the sediment is contained within large-scale sand ridges. Though this reworked sediment was argued by Flemming (1977) to have accumulated during the past 6000 years as sea level reached its present position, this study shows that the sediment wedge is dynamic and constantly redistributed. 6. It is proposed that the inshore current reversal, associated with the Durban Eddy of the Agulhas Current, has been instrumental in shaping large-scale shoreface attached and detached sand ridges on the shelf off Durban. This eddy is responsible for the entrainment of muddy sediment and the deposition in a localised low energy sink, protected by the physical features of the Durban Bluff and the adjacent Blood Reef. 7. The presence of mud lenses in the vicinity of Blood Reef on this high energy coastline may be remnant features reflecting high energy swell regimes associated with storm events and the subsequent settling of fine-grained terrigenous sediment which is expected to follow. The fining-upward sequence in the core, in conjunction with palynological evidence, suggests a fluviallyderived clay source. The pollen content reflects the regional vegetation with podocarps, Phoenix, subtropical trees, and grasses. High percentages of neophytes, e.g. Pinus, point to a recent origin of the sediments (b100 years). We suggest that during storms, sediment derived from discharging in Durban and the upper KZN south coast could be entrained within the Durban Eddy and deposited onto the continental shelf off the Durban Bluff. Although the AMS radiocarbon dates were not able to constrain a single event, Pb-210 or Cs-137 dating could be considered for future work.

Acknowledgements The geophysical and sedimentological research was funded by the Council for Geoscience through the Annual Technical Programme of the Marine Geoscience Unit. The Palaeontological Scientific Trust (PAST) granted funds for the two AMS radiocarbon dates. Sediment analysis was funded by the University of KwaZulu-Natal (UKZN) and carried out by R. Leuci (Environmental Mapping and Surveying). P. Chakane processed the samples for pollen analysis. We thank A. Metwally for his help in the BPI lab during withdrawal of the samples for pollen analysis, M. Bamford for her allowance to use BPI facilities and the Marine Geoscience Unit dive team (W. Kidwell, M. MacHutchon, W. van Zyl, P. Young, B. Smith and A. Holmwood) for the offshore mapping and sampling expeditions. ACEPII is acknowledged for the ADCP data. Reviews by Prof. Dr. Burghard Flemming and Dr. Henk de Haas helped to considerably improve this paper.

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