Sedimentary Geology 309 (2014) 15–32

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Sedimentary Geology

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Pliocene–Pleistocene climate change, sea level and uplift historyrecorded by the Horingbaai fan-delta, NW Namibia

Harald Stollhofen a,⁎, Ian G. Stanistreet b, Christoph von Hagke c, Anna Nguno d

a GeoZentrum Nordbayern, Friedrich-Alexander-University (FAU) Erlangen-Nürnberg, Schloßgarten 5, 91054 Erlangen, Germanyb Dept. of Earth, Ocean & Ecological Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3GP, UKc Geological and Planetary Sciences, California Institute of Technology, 1200 E California Blvd., Mailcode 100-23, Pasadena, CA 91125, USAd Geological Survey of Namibia, Private Bag 13297, Windhoek, Namibia

⁎ Corresponding author. Tel.: +49 9131 85 22617 (direE-mail addresses: harald.stollhofen@fau.de (H. Stollho

istanistreet@btconnect.com (I.G. Stanistreet), vonhagke@canguno@mme.gov.na (A. Nguno).

http://dx.doi.org/10.1016/j.sedgeo.2014.05.0080037-0738/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 February 2014Received in revised form 23 May 2014Accepted 24 May 2014Available online 2 June 2014

Editor: J. Knight

Keywords:Alluvial fanMarine terracesClimate changeUpliftSea levelNamibia

Its location on a tectonically relatively stable passive margin and its degree of interaction with the sea make theHoringbaai fan-delta, NWNamibia, an exceptional record of coastal activity, providing insights into the responseof ephemeral fluvial systems to changes in climate, sea level and continent-scale uplift. The fan comprises upper,middle and lower segments. The upper fan and middle fan are dominated by a braided river system; only in theupper fan are fluvial sediments interleaved with hyperconcentrated flow deposits. Plio–Pleistocene sea levelhighstands have leftmarine terraces on the fan surfacewhich enable correlation with the offshoremarine recordand provide timelines to constrain fan growth. The bulk of fan-delta progradation took place at ∼2.7–2.4 Ma, be-tween the formation of a widespread erosional surface incising the middle Pliocene Karpfenkliff and KambergFormations, but prior to the emplacement of the warm-water fauna-bearing late Pliocene “Oyster Terrace”(∼2.4–2.2 Ma), an equivalent of the +30 mP (marine terrace package) in coastal southwestern Africa. Majorfan progradation is contemporaneous with widespread regional uplift (~12 ± 5 m/Ma) and climate change insouthwestern Africa, the latter associated with intensification of northern hemisphere glaciations. Younger fangrowth phases are weaker and constrained by b10 m asl marine terrace bodies that yield mostly cold-waterfauna, corresponding to the onset of strong glacial/interglacial climatic fluctuations superimposed on a generalaridification trend and the introduction of colder sea-surface temperatures after 2.2 Ma.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Alluvial fans are important geomorphological features along theSkeleton Coast, NW Namibia, because they provide insights into theCenozoic geomorphological and climatic history of southern Africa.Although work has been undertaken on Skeleton Coast fan geomor-phology (Van-Zyl and Scheepers, 1993; Scheepers and Rust, 1999;Blümel et al., 2000), Skeleton Coast river hydrology (Jacobson et al.,1995), and variations of fluvial style along the coast (e.g., Stanistreetand Stollhofen, 2002; Krapf et al., 2003; Svendsen et al., 2003), little re-search has been undertaken on subaerial fan characteristics and associ-ated palaeodrainage. The first systematic sedimentological study of anindividual fan, the Koigab fan (Fig. 1A), was that of Krapf et al. (2005)who characterized it as a tropical latitude, ephemeral braided-riverdominated fan system. A remaining knowledge gap, however, is thetiming of fan progradation, its causes and its links to the marine sedi-mentary record through time.

ct); fax: +49 9131 85 29295.fen),altech.edu (C. von Hagke),

The aim of this paper is to describe fan-marine interactions of theHoringbaai fan-delta (Fig. 1B), located 140 km SE of the Koigab fan.There, associated marine terraces and relative dating of containedfaunal elements enable the setting of a time-stratigraphic frameworkand correlation with other offshore and onshore strata and contempo-raneous climate changes. This provides the opportunity to constrainphases of fan progradation despite the lack of reliable time markers inthe fluvial fan deposits themselves.

2. Morphotectonic and hydrological setting

The dominant morphotectonic feature of the Skeleton Coast area inNW Namibia is the Great Escarpment (Fig. 1A), the formation ofwhich relates to continental uplift and mainly pre-end-Eocene denuda-tion (Cockburn et al., 2000), following break-up and opening of theSouth Atlantic during the early Cretaceous. The escarpment separatesa low elevation coastal strip, varying in height between sea level and400m, from a mountainous hinterland, typically at 900–1300 m height(Partridge and Maud, 2000). As with all the Skeleton Coast fans, theHoringbaai fan-delta is situated seaward of the Great Escarpment,which in the study area achieves a height of about 1000–1500m inlandfrom the modern coastline by a distance varying from 50 to 150 km

Fig. 1.Map locating Skeleton Coast ephemeral rivers and fan systems. (A) Catchment areas of larger ephemeral rivers in NWNamibia in the context of average annual rainfall distribution(based on Jacobson et al., 1995). Indicated is the position of the escarpment and the offshore ODP Site 1082 (asterisk); frame shows position of B. (B) Detail of the coastal stretch betweenCape Cross and the Ugab River, showing topography and positions of Messum, Horingbaai and Orawab fan systems and their drainage.

16 H. Stollhofen et al. / Sedimentary Geology 309 (2014) 15–32

(Partridge and Maud, 1987). Along the Skeleton Coast, the escarpmentis particularly close to the South Atlantic, apart from a pronounced ero-sional gap that occurs inland betweenWalvis Bay and c. 25 km north ofHenties Bay (Fig. 1A). The low-lying coastal areas preserve a record ofNeogene and Quaternary fluvial and linear marine terraces (Miller,1988, 2008), which extend for long distances along the Namibianand South African coasts. Some of the latter are diamond bearing(Schneider and Miller, 1992; Spaggiari et al., 2006), with numerousexploration trenches providing an opportunity to access and analyzethe terrace sediments.

Skeleton Coast hydrology is underpinned by a pronounced, topo-graphically related, climatic gradient away from the coast (Atlas ofNamibia Project, 2002) (Fig. 1A). Because of the strong cooling and pre-cipitative effects of the cold polar Benguela Current offshore from theNamibian coastline (Tyson and Preston-Whyte, 2000), Atlantic weatherfronts rarely advance into the area, but rather fogs are developed abovethe locus of that current (Oliver, 1995). At the coast mean annual rain-falls of only 0–50 mm are experienced, constituting hyper-arid condi-tions. In contrast, only 200 km inland, at the escarpment and inthe interior of northern Namibia, semi-arid annual rainfall values of200–500mmare recorded, associatedwith the activity of the Intertrop-ical Convergence Zone (ITCZ), which particularly affects the northernhalf of the Namibian interior (Shannon et al., 1986). This setting leadsto the fact that, apart from the Kunene, Skeleton Coast rivers are allephemeral, prone to flash flooding (Jacobson et al., 1995; Krapf et al.,2003), and several of them, particularly those with relatively smallcatchments (b2500 km2) on the seaward flank of the escarpment areassociated with active fan systems at their downstream end (Fig. 1B).

Majorfloods of the Skeleton Coast ephemeral rivers are promoted bySouth Atlantic El Niño-equivalent warm-water events, also referred toas Benguela Niño, during which the ITCZ progresses farther south thannormal, initiating exceptional rainfall events there (Shannon et al.,

1986). Only then, can river flash floods break through the naturaldune dams of the Namib Sand Sea, as for example described from theHunkab (Ward and Swart, 1997) and Hoanib Rivers (Stanistreet andStollhofen, 2002). Braided river dominated fan systems, such as theKoigab fan (Krapf et al., 2003) and the Horingbaai fan-delta also experi-ence active floods of variable magnitudes, both in main and subsidiarydistributary channels. Major floods are preferentially recorded betweenFebruary and April whereas minor river flows may occur during thecourse of the southern hemisphere summer between November andApril and contribute to minor but regular modifications of the fansurface, in particular winnowing of fines, weathering and pore fluidflow near the sediment surface.

3. Methods

The entire Horingbaai fan-delta surface and associated marine ter-races were mapped during successive fieldseasons from 2007 to 2010on a 1:50,000 topographic map base (Sheet 2113 DB Horingbaai), aerialstereo photographs acquired in 2000, and a 1995 Landsat TM-5 image.To constrain facies architecture and composition, a total of 32 rivercuts and exploration trenches and pits of fan and terrace sedimentswere examined and logged down to cm-scale accuracy, out of whichthree representative sections are reproduced and graphed in thispaper. To resolve downfan changes in channel gradient and width/depth ratios, longitudinal sections (n = 3) and cross sections (n = 8)of the fan-delta surface and of the marine terrace morphology (n = 4)were surveyed in 2007 and 2009, using a laser theodolite systemwork-ing at mm-scale accuracy.

In addition to direct modal clast and mineral analyses in thefield, a representative set of catchment lithologies (n = 28), fan-delta (n = 29), terrace (n = 36) and modern Horingbaai channelsediments (n = 12) was sampled and further analyzed by optical

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microscopy. Statistics on downfan changes in clast sizes and composi-tion are based on 19 sampling points at regular 500m intervals, consid-ering macroscopic inspection and grain size determination by countingand sieving. However, due to obvious effects of marine reworking andmixing with marine terrace materials, these data are not consideredclosely in this paper.

Along with Horingbaai fan-delta analysis, the Orawab, Messum,Koigab, Salt and Sout and various (n = 6) unnamed Skeleton Coastfans were examined to update comparison of fan gradients, fan faciesand the degree of marine interaction, following the research of Krapfet al. (2005). Fieldwork also included examination and surveying ofsedimentological sections in the Kuiseb (n = 5), Gaub (n = 3) andTsauchab (n = 2) river valleys c. 200–250 km southwest of Windhoek,an area which was originally studied by Ward (1987). Also time-equivalent Skeleton Coast marine terraces were studied at localitiesRooikop, Wlotzkasbaken, Henties Bay, Cape Cross, Ugabmond, Toscanini,Terrace Bay and Möwe Bay, involving the setup of detailed faunal lists,heavy mineral statistics and single-grain geochemistry, together withelectron spin resonance (ESR) and optically stimulated luminescence(OSL) dating.

To simplify comparison with published data, the pre-2009 strati-graphic table (Gradstein et al., 2004) has been used.

4. Description of the Horingbaai fan-delta

The Horingbaai fan-delta (Fig. 2A) presently measures 10 km fromapex to coast (not counting its subaqueous fan) and 13 km across itsmaximum lateral extent. On the scale of Skeleton Coast fans it is in-termediate in size between larger systems, such as the Koigab fan(15 × 23 km) and smaller bodies, such as the Salt fan (3.5 × 4.5 km)and the Sout fan (4 × 5 km) (Krapf et al., 2005). Because of the hyper-aridity of the area, the fan surface is almost devoid of vegetation.Wind effects, both prevailing south-southwesterly (Lancaster, 1982)and less frequent easterly hot Berg winds from the interior (Tlhalerwaet al., 2012), produce extensive deflation surfaces in all areas not pres-ently affected directly by the active fluvial system. There is enoughmoisture, both from sparse rainfall and coastal fogs only in the upperreaches of the system, to allow growth of sporadic plants, for exampleWelwitschia mirabilis (tree tumbo), a plant adapted to survive underthe conditions of aridity and seamist (Seely, 1978). Additionally, inland

Fig. 2. (A) Satellite image and (B) geological map (compiled from Miller, 1988; Mi

incursions of coastal mists induce evaporation and precipitation of ped-ogenic gypsum crusts through aerosol activity, to indurate the fan sur-face to varying degrees (Eckardt and Spiro, 1998; Bao et al., 2001).This allows the distinction of those fan areas that are relatively inactive,showing high encrustation, from those that are active, with almostnone.

River flows are very ephemeral with flashy discharge, and flashflows are initiated every few years, notably more frequently over thelast two decades (cf., Stanistreet and Stollhofen, 2002; Krapf et al.,2003; Srivastava et al., 2005) by heavy rainstorms on the escarpmentand near interior. Such storms can fall in one or two specific drainagesystems, and then flash floods flowing down specific river systemsmight reach as far as the coast, where no rainfall will usually havebeen experienced. Thus the Horingbaai fan-delta is an example of theprovision of river flow into more arid regions from a drainage areasited under much more humid conditions, as is the case with theOkavango Fan (McCarthy et al., 1991; Stanistreet and McCarthy,1993), but in this case producing a very different fan type.

The slope of the Horingbaai fan-delta surface (Fig. 3) is fairly consis-tent throughout its length, averaging 0.01 and classifying towards thegentle end of the “middle slope category” of braided dominated fans(Saito and Oguchi, 2005). It is remarkably similar to that of otherSkeleton Coast fans such as the Koigab (0.011), Orawab (0.0086), andMessum (0.0087); only the smaller Salt (0.015) and Sout (0.014) fans(Fig. 1A) are pronouncedly steeper. Where the Horingbaai and Koigabfan systems do differ markedly is the preponderance on the Horingbaaifan-delta of beach terraces on the lower fan surface, an areawhich in thepast would have been affected by marine sedimentary processes. Theterraces provide a relative time framework against which to comparethe phases of fan progradation. In contrast, the terraces of the Koigabfan lie outside of the fan surface (Krapf et al., 2005) and therefore pro-vide only a minimum date for fan progradation phases.

5. Sediment sources and drainage feeding the Horingbaai fan-delta

The presently active Horingbaai fan-delta catchment is notablysmall, covering only ∼260 km2, narrow, and asymmetrically oval-shaped (Fig. 1B), reflecting the structural grain of faults paralleling themajor NE–SW Autseib Lineament (Milner, 1997) (Fig. 2B). The fanextends ∼45 km inland, where highest elevations are up to 731 m asl

lner, 1997, and own data) of the Horingbaai fan-delta and its catchment area.

Fig. 3.Downfan profiles surveyed (I) in the main active channel (dotted line) and (II) on the adjacent Horingbaai fan-delta surface (solid line) revealing a rather constant gradient of 0.01from fan apex to toe. Black arrows mark side channel confluences.

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(above high-tide sea level), at the southeastern flank of the MessumCrater (Fig. 1B), a Cretaceous igneous complex (Ewart et al., 2002)which in this sector exposes mainly Tafelkop basalts and Gobobosebquartz latites (Etendeka Supergroup, Awahab Formation), Messumquartz monzonites and the less common Messum gabbros (Milner,1997). Widespread in the catchment are also meta-greywackes andschists of the Neoproterozoic Amis Formation (Damara Sequence)associated with pegmatites, abundant quartz veins and veinlets, andPermian Gai-As pelitic red beds (Karoo Supergroup). Annual rainfall inthe fan-delta catchment is generally below 100 mm (Atlas of NamibiaProject, 2002).

Where the feeder channel of theHoringbaai fan-delta cuts through aridge area of fault blocks, 9–12 kmeast of the coastline, aminor gorge ofabout 4.5 m depth is incised. The fault blocks form the southeastwardcontinuation of the Albin Ridge, regarded as a remnant of the pre-South Atlantic rift shoulder (Clemson et al., 1999).

6. Description and segmentation of the fan surface

6.1. The fan-delta surface

Along its total length of 45 km, the Horingbaai River declines by∼420m. This makes an overall river gradient of about 0.0093 comparedto a regional gradient of about 0.016, with both values remarkablysimilar to those calculated for the Koigab River drainage (Krapf et al.,2005). Leaving its catchment area through the Albin Ridge Poort, theHoringbaai River enters the fan surface (Fig. 2A) through an entrenchedfan apex at a height of 100 m asl, then flows down to the sea over a dis-tance of 10 km and debouches into an elongate coastal lagoon that is atabout 2–3m asl andwhich is occasionally flooded bymarine and/or flu-vial processes. Only at the distal northern edge of the fan, does elevatedrelief associated with Cambrian granites and Cretaceous dolerite dykespartly hinder the sediment flux to the ocean.

A braided drainage network characterizes the fan surface (Fig. 2A),bearing very little surface vegetation apart from isolated shrubs and li-chen near the fan toe. Channel sinuosity is about 1.1, braidingparameteris 6–7 and width-depth ratios range 75–331, with a downstream trendtowards higher numbers. Five major active channels have been recog-nized of which the main one bisects the fan close to its central axis(Fig. 4A). The Horingbaai drainage network is not all presently activebut comprises a considerable number of long-term abandoned and in-active channels which notably concentrate in the northern half of thefan-delta area. In contrast to active channels, such abandoned channelscommonly show less well defined cut walls due to erosion, less bankfailure and infill by aeolian sand and granule ripples (Fig. 4B), consisting

predominantly of Etendeka volcanic grains. The latter are deflated fromunconsolidated channel and fan sediments,migrate over the fan surfaceand then tend to accumulate in topographic depressions, such asinactive river channels. In fan surface areas with a long-term lack ofnet sediment accumulation, gypsum encrustation is a common phe-nomenon, stabilizing widespread armored deflation surfaces (Fig. 4C).A NNW–SSE topographic profile surveyed across the fan-delta surface(Fig. 5) illustrates the typical convex-upward buildup of the fan andalso the distribution of active and inactive channels.

Highstand terraces can bemapped across the surface (Fig. 6), and arecontinuous beyond the fan surface with similarly aged terraces alongthe Namibian and South African coastline (Schneider and Miller, 1992;Pether et al., 2000). The most distal parts of the Lower Fan are subma-rine presently, however GEBCO_08 Grid bathymetric data (IOC et al.,2003) show that the fan-delta extends a further 1.5–2 km offshore,where the fan body connects with background continental shelf sedi-mentation. It is this coastal stretchwhere the subaerial part of the trian-gular shaped fan surface attains its maximum width of 13 km (Fig. 6)and interacts with the sea over almost its entire length.

6.2. Characteristics of the upper fan

Considering downfan changes in facies architecture, the Horingbaaifan-delta surface subdivides into upper, middle and lower segments.The upper fan segment rests almost entirely upon Damara schists andonly towards the apex of the fan does it sit on Etendeka volcanics(Fig. 2B). Multiple incision surfaces have been cut into the volcanicsand into earlier incised valley fills. Fig. 7A–F shows a set of topographicprofiles measured by laser survey across the main active channel alongits course from fan apex to toe, defining the upper fan (Fig. 7A–D) as thearea with a maximum of 3–4 m incision over 50–75 m width into thefan surface. This is a far less pronounced fan-head entrenchment, thanthat displayed by the Koigab fan (Krapf et al., 2005) farther north. Inthe upper fan themore active channels occupying the fan center appearto be associatedwith a slight suprafan bulge or lobe, in contrastwith themore evenly arranged half-cone contours farther downfan. This mightindicate a recent renewal of sediment input onto the area of the upperfan, and indeed there is a darker satellite image signature coincidingwith that area and indicating “fresher” less incrusted sediment cover.

Within the main active channel of the upper fan area the depositedsediment comprises predominantly plane-bedded coarse to very coarsepebbly sand sheets up to 8 cm thick, with clasts up to 4 cm diameter.These are interspersed with longitudinal gravel bar forms b20 cmhigh, 5–15 m wide and 50–150 m long, in which there are clasts upto 6 cm diameter. Such sediment characteristics are produced by

Fig. 4. Horingbaai fan-delta field photographs illustrating (A) the main active channel(westward downstream view), (B) an inactive channel, partly infilled by aeolian sedi-ments with a granule rippled surface and (C) a gypsum-encrusted deflation surface.

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deposition from “usual” flash flows such as have been witnessed inSkeleton Coast rivers during previous decades (e.g., Ward andSwart, 1997; Stanistreet and Stollhofen, 2002; Krapf et al., 2003),rather than the more unusual event that may have the capacity toinitiate mass flows, such as the hyperconcentrated flow depositsdocumented by Svendsen et al. (2003) from the Uniab River farthersouth.

Characteristics of Horingbaai fan fluvial facies are summarized inTable 1. A representative measured section along walls at either sideof the incision through the upper fan (Fig. 8A) shows all thefluvial faciesdistinguished on the Horingbaai fan-delta as a whole, and this full spec-trum has only been recognized in the upper fan area. Pebbly coarse tovery coarse sandstone sheetswith pebble to cobble conglomerate lenses(Fig. 8B) record themore usual flash floods. These are interbeddedwithclast-supported, but matrix-rich boulder-bearing pebble to cobble con-glomerate units (Fig. 8C), massive to crudely horizontally bedded with

clast sizes up to 24 cmdiameter and even 40 cm farther upfan of the sec-tion. The latter suggest that the Horingbaai fan-delta has experiencedcatastrophic hyperconcentrated flow events that may have initiated asrockfall avalanches and debris flows more proximal to their sourceregion. The transition between the two flow types may occur whereflow properties change due to downstream segregation of coarsergrain sizes in highly concentrated debris flows, or due to the dilutionof debris flows by mixing with water or less concentrated flows(Pierson and Costa, 1987; Smith and Lowe, 1991).

A survey of the southern incised wall of the active channel (Fig. 9)reveals the organization of depositional units in the upper fan. Fluvialchannel bodies infilled laterally persistent incision surfaces which iden-tify particularly well where they cut down through subhorizontal,sheet-like deflation lags. It appears that each of these incision infills isintroduced by the provision of coarse pebble to boulder-sized debristhrough fluvial or hyperconcentrated flows and then fines upward,and is succeeded by deposition of repeated sheet flood units. The typicalupper fan clast assemblage comprises 60–70% Etendeka volcanics witha dominance of basalt over quartz latite, 25–35% vein quartz, ∼5%meta-sedimentary rock fragments and 2–5% pegmatitic feldspar,although these proportions are not constant throughout the section.They are different in particular in the hyperconcentrated flow unit andthe fluvial deposits infilling immediately above incision surfaces(Fig. 9), with the latter bearing up to 70% vein quartz and only 20%volcanics.

6.3. Characteristics of the middle fan

The boundary between upper andmiddle fan segment is taken at the40 m asl contour (Fig. 6), at the maximum down-fan extent of ancienthyperconcentrated flow deposits and a change towards higher width/depth ratios of the main active channel in the middle fan. We suspectthat the distinction between Middle and upper fan areas is due to thedistance from catchment tributary streams within the hilly areas tothe north and southwest of the Upper Fan, as well as upstream fromit, which debouch directly onto the upper fan and are the likely sourcefor the hyperconcentrated (±75% sediment concentration) flow de-posits found there. In contrast, when the same flows reach the middlefan they would have already deposited much of their sediment loads,and would proceed with lower sediment concentrations (±10%).They would then take on the characteristics of “normal” stream flows,depositing gravelly sand facies and longitudinal gravel bars.

The middle fan also rests largely on the Damara metamorphic base-ment with only its western to northwestern toe covering Cambrian bi-otite granite (Fig. 2B).Most significant however are its age relationshipstomarine terraces, one of the highest and oldest of which is fortuitouslyvery distinguishable, containing fossil oyster shells and a very distinctclast assemblage. This “Oyster Terrace”, sits at a variable height of15–24 m asl on the outer limits of the middle fan and on a wave-cut,gullied platform developed on granite basement. Thus it marks theboundary between the lower and middle fan segment. The distal partsof the middle fan have also entirely cut out gravel sheet bundles ofpre-Oyster terraces which are only preserved north and south of thefan body, typically at heights of 25–28 m asl. Constraining the ages ofthese different terrace generations provides an opportunity to fix thetiming of periods of major fan progradation in between these marinetransgressions.

A topographic profile across the middle fan surface (Fig. 7E) revealsthat incision is slighter here (2–3 m) than on the upper fan. In concertwith reduced incision, the width of the resulting shallow valley in-creases to 100–130 m and widens seaward across the upper to middlefan transition. This is a typical feature of all the active drainage systems,particularly in the southeastern half of the fan apron. The incised wallsshow the same gravelly sand sheets and interspersed gravel bar lensesas the upper fan, but the hyperconcentrated flow facies is lacking. Max-imum clast sizes are 7–10 cm, significantly less than on the upper fan.

Fig. 5.NNW–SSE section across the central part of theHoringbaai fan-delta illustrating its convex-upmorphology. Distinguished are active and inactive channels (arrows). In the fan centerand south of it heavily encrusted terrace sediments crop out. The base of the fan body is only exposed in its marginal areas in a few deep exploration trenches; the incisional surface un-derlying the fan center is therefore speculative.

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The clast assemblage is characterized by Etendeka derived volcanicclasts (~40–70%) andDamara sourced vein quartz (~25–50%) although,amongst the volcanics, quartz latites are more frequent than basalts.Compared to the upper fan there is also a reduction in the proportionof meta-sedimentary rock fragments from about 5% to almost zerowhereas pegmatitic feldspar maintains an equally low abundance of~2–5%. Remarkable are layered concentrations of vein quartz (~50%)and calcrete peds (~10–50%) in which proportions of Etendeka volca-nics are reduced to b30%. Also in themiddle fan area, deflation lag layersare less maturely developed compared to the upper fan, with the sandymatrix frequently comprising quartz grains showing aeolian graincharacteristics.

6.4. Characteristics of the lower fan

The lower fan is easily distinguishable, because it includes suprafanbeach and shoreface sediments, as well as the presently subaqueousfan. Incision in the lower fan is reduced to 0.5–1.5 m depth and thewidth of the incision surface further increases to 150–180 m (Fig. 7F).All the facies, apart from hyperconcentrated flows described from far-ther upfan, are present in the lower fan but joined by additional marinefacies. The stratigraphic position of the Oyster Terrace is of crucial im-portance as it exhibits a marine facies association sandwiched betweenolder, pre-terrace and younger post-terrace fluvial deposits (Fig. 5). Ayounger series of marine terraces located below 10m asl is present far-ther downfan, incised into the lower fan surface (Fig. 6).

6.4.1. Oyster Terrace characteristicsDiscriminant characteristics of Oyster Terrace shallow marine

lithofacies are summarized in Table 2. Fig. 10 illustrates the architectureof the Oyster Terrace at location 2 (Fig. 6), comprising successive, sea-ward (westward) dipping sheets of pebble to boulder conglomerateunits with interleaved bioturbated, coarse to very coarse, mostly wellsorted tabular sand lenses. A distinct warm water faunal assemblageincludes the oyster Striostrea margaritacea and the zonal fossils Donaxrogersi and Fissurella glarea. The facies association represents lowershoreface to upper foreshore facies with the latter bearing dominantlywell rounded, platy components imbricated both towards 213–263°(seawardbackwash) and 48–76° (landward swash). The facies architec-ture reveals a prograding, regressive nearshore architecture withthe shoreface and foreshore facies pinching out up-dip against the un-derlying transgressive unconformity. Polymict composition and well

roundedness of the Oyster Terrace conglomerates differ markedlyfrom the surrounding fluvial units. Contained in conglomerates arebrownish phosphatized silt- to sandstone, frequently bored clasts(∼50%), Etendeka quartz latites (∼15–30%), vein quartz (∼10–25%),as well as quartzite, granite, granodiorite, rare banded brownish iron-stone, chert, jasper, dark blue-gray riebeckite minerals and a variety ofsedimentary rock fragments.

6.4.2. Characteristics of the suprafan terraces below 10 m aslThe seaward dipping gravel sheet architecture of the terraces is pres-

ent at all levels but is particularly well exposedwhere themain channelcuts the Oyster Terrace. Elsewhere only ridges, sitting with positive re-lief on the fan surface (Fig. 6), are taken as evidence of gravel sheet bun-dles and are well recorded in survey lines across the terraces (Fig. 11),perpendicular to the coastline. There are 5 marine terrace levels identi-fiable on top of the lower fan body, at heights of 2–10 m asl, eachfingerprinted by characteristic clast and faunal assemblages and relativeheight positions above high-tide sea level (Table 3). From lowest tohighest these are named the Lower Donax, FLV (Felania–Loripes–Venerupis), Upper Donax, and Lower and Upper Vein Quartz Terraces,with the most distinct ones mapped in Fig. 6. The Lower and UpperDonax Terraces both bear cold-water faunal elements including Donaxserramollusk shells and are further characterized by Etendeka volcanicsdominated clast assemblages. The same D. serra faunal assemblagesoccur in the Lower and Upper Vein Quartz Terraces but the clast assem-blages comprise fewer volcanics with both, vein quartz and Etendekavolcanics in rather equal proportions. The FLV Terrace (bracketed bythe Lower and Upper Donax Terrace levels) likewise also shows veinquartz as its prevailing clast type and warm-water taxa (Tankard,1975) that comprise Felania diaphana, Loripes liratula, and Venerupiscorrugata as key elements.

In a corridor south of themain channel the Oyster Terrace is entirelyremoved by later fluvial activity and the younger Vein Quartz Terracesare frequently dissected by drainage channels, leaving only isolated,patchy terrace remnants on the fan. In contrast, themost recent terraces(Lower/Upper Donax) are fairly continuous across the toe of the fan(Fig. 6).

To avoid local erosional effectswewill consider the terrace stratigra-phy gathered from two subparallel laser theodolite surveys (Fig. 11) ofthe lower fan and the area of interaction with the sea. Cross-section 1is located close to the southern margin of the Horingbaai fan, whereasSection 2 is located c. 3 km farther south, outside the reach of the fan

Fig. 6. Map of the Horingbaai fan surface, showing active and inactive channel networks and the distribution of Plio–Pleistocene marine terraces that constrain the timing of fanprogradational phases prior to and after emplacement of the Oyster Terrace. Indicated are locations of measured sections (Loc. 1: Figs. 8, 9 and Loc. 2: Fig. 10) and the positions ofNE–SW cross-sectional surveys that constrain suprafan terrace positions (Fig. 11). Lower/Upper Vein Quartz Terraces and FLV/Upper Donax Terraces cannot be separated at the scaleof the map, but are easily distinguished in cross-sectional surveys (Fig. 11).

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Fig. 7. Surveyed downfan cross-sectional profiles at positions A to F across the main active channel of the Horingbaai fan-delta. Inset locates cross-sectional profiles from fan apex to toe.

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(Fig. 6). Starting at the modern beach/shoreface high-tide level, bothcross-sections (Fig. 11) record the modern storm beach at up to 4.5 masl, succeeded landwards by the Lower Donax Terrace at c. 3.3 m aslwhich is heavily reworked and in places buried (cross-section 2), dueto modern washover into the lagoon (Fig. 12). The lagoonal area is50–350 m wide and forms a rather even surface at c. 3 m asl. A gentle0.4 m step at about 300 m east of the coastline occurs in cross-section1 (Fig. 11) where a distal toe of the fan progrades into the lagoon. Thenext higher (and older) FLV terrace is almost indistinguishable fromthe Upper Donax Terrace in the map (Fig. 6) and satellite images(Fig. 12B). However, in the high-resolution cross-sectional surveys(Fig. 11) the characteristic terrace surfaces are clearly distinguished atabout 3.8–4.0 and4.5–5.0masl, forming themost completely preservedterrace deposits. Two higher terraces, the Lower and Upper Vein QuartzTerraces, occur farther inland at 6.5–7.0 m asl and 8.5–9.5 m asl, butamalgamate into one body at the scale of the geological map shown inFig. 6. Despite the fact that they are heavily dissected southeast of themain channel, this older suprafan terrace series can be traced over adistance of N8 km and correlates well with more continuous terracesknown from outside of the active Horingbaai fan, including CapeCross, Toscanini and Terrace Bay areas (cf., Davies, 1973; Schneiderand Miller, 1992). Considering their suprafan position, the Vein QuartzTerraces are younger than the vast majority of the underlying fan-delta body and register a period of largely reduced and discontinuousfan progradation.

The terraces also affect the modern fan drainage, for example, somefluvial channels are deflected or dammed up to the east of the UpperDonax Terrace and incision of drainages into the fan surface starts justupstream of the terrace (Fig. 12A). Channel grades are also lessenedjust upstream of the terrace, causing braided channels to meander, lo-cally, at their approach to this feature. Only the main active channeland the channel to its south have thoroughly breached the UpperDonax Terrace and, once they have passed it, they tend to diverge,developing a more sheetflood style of deposition (Fig. 12A). Becauseof the localized reworking of marine terrace materials and associatedadmixture of pebble to boulder sized material, conventional downfanvariations in grain size and clast compositional assemblages are notapplicable to the fluvial facies of the lower fan-delta.

7. Marine terraces: facies and age constraints

The Skeleton Coast is a highwave energy,microtidal coastline affect-ed by substantial changes in relative sea level ranging from an earlyEocene high of c. 180 m above present, down to a late Pleistocene lowof c. 120 m below present (Pether, 1986; Corbett, 1996; Bluck et al.,2001). The marine system affected Horingbaai fan development duringmultiple transgressive episodes, which are recorded as successive gen-erations of marine deposits interleaved with fluvial facies and at thefan surface. Mapping of the Horingbaai fan body (Fig. 6) revealed itsage-relationships with marine bodies of essentially three differentgenerations: (I) Terraces older than the Oyster Terrace are entirely cutby the body of the middle fan and are thus older than parts of thisfan segment; (II) the Oyster Terrace, interleaved between middle fanand lower fan, marks termination of the major phase of overall fanprogradation, and (III) the suprafan Vein Quartz Terrace, providing aminimum age for the latest fan progradational phases. Table 3 summa-rizes typical heights, clast assemblages, faunas and age constraints ofindividual terraces in the Horingbaai area.

7.1. The Oyster Terrace marker

Correlatives of the Horingbaai Oyster Terrace form a very wide-spread marker unit along the southern African west coast, originallytermed “Oyster Line” (Merensky, 1927) and “Ostrea Bed” (Haughton,1928, 1932). Considering faunal assemblages, clast composition andthe range of topographic elevation, the Oyster Terrace correlates with

the “Rooikop gravels” east of Walvis Bay (Miller and Seely, 1976;S.A.C.S., 1987), and the “Old Beach” of Davies (1973) extending fromnorth of Walvis Bay. It also corresponds to the coastal “+30 m” raisedmarine terrace package (+30 mP) (Ward, 1987; Spaggiari et al.,2006), the D, E and F beaches (Murray et al., 1970), as defined in south-west Namibia and along theNamaqualand coast of South Africa (Pether,1986; Schneider andMiller, 1992), and theD. rogersi package of Pickfordand Senut (1999). Typically along this coast, a seaward thickeningsuite of shallow marine deposits was laid down during regressionfrom the 30 m asl transgressive maximum with the lower shorefaceunits often preserved at 17–21 m asl (Pether, 1986). Heights of this ter-race package may vary laterally due to the westward (seaward) de-creasing elevation of individual facies units during a steadily fallingsea level and due to variations in contemporaneous continental marginuplift and localized faulting (Partridge and Maud, 1987).

Previously, correlative D–F beaches in southern Namibia were con-sidered Miocene in age (Stocken and Campbell, 1982). Pether (1986,1994) later correlated them with an early Pleistocene highstand of theQ2 sea level cycle of Vail and Hardenbol (1979). The most recentsynthesis on age–altitude relationships of Namaqualand and Namibonshore marine deposits by Pickford and Senut (1999) considers alate Pliocene age for the +30 mP and places it at 3.0–2.5 Ma.

Tsondab, Karpfenkliff and Kamberg Formation sedimentary rockfragments observed in Oyster Terrace conglomerates at Horingbaai pro-vide a maximum age for the Oyster Terrace, as these onshore strata areconsidered to be late Miocene to middle Pliocene in age (Pickford andSenut, 1999; White et al., 2009) with an interpreted depositional 21Neage of 2.81 ± 0.11 Ma obtained from Karpfenkliff Formation pebblesof the Kuiseb Canyon area (Van derWateren andDunai, 2001). Remainsof the extinct Cape zebra (Equus capensis) are reported from the +30mP (Pickford, 1998; Pickford and Senut, 1999), which in a southernAfrican context are unlikely to be older than 2.36 Ma (Behrensmeyeret al., 1997; Bernor and Armour-Chelu, 1999; Churcher, 2006). Faunalelements and phosphatized material of Oyster Terrace equivalentsboth suggest warm-water environments at the time of formation. Thisin turn implies a terrace age older than 2.2 Ma, because after this timedecreasing sea-surface temperatures were introduced at the Namibiancoast due to increased upwelling (Dupont et al., 2005).

Integrating all available age constraints, the Oyster Terrace and itsequivalents should be placed between 2.4 and 2.2 Ma in the latest latePliocene (pre 2009 stratigraphic nomenclature) or early Pleistocene,on the basis of the recently set Plio/Pleistocene boundary at 2.588 Ma(International Commission on Stratigraphy, 2009). Most probably thisterrace package relates to one or several of the Marine Isotope Stages(MIS) 95 to 85, placed at ~2.4–2.2 Ma (Lisiecki and Raymo, 2005).Thus the Oyster Terrace and its correlatives are presently the bestmarker we have along the southern African coast close to the Pliocene–Pleistocene boundary.

Two to three terrace levels which are older than the Oyster Terraceand underlie the Horingbaai fan body (Fig. 6), typically at heights of25–28 m asl, are tentatively correlated with MIS G7, G5 and G3, placedby Lisiecki and Raymo (2005) at ~2.76–2.65 Ma.

7.2. The suprafan terrace markers below 10 m asl

The warm-water fauna contained in the Oyster Terrace contrastswith the overall colder water assemblages characterized by D. serramollusk shells of the younger palaeo-shorelines below 10 m asl of themodern Atlantic coast (Pether, 1994; Pether et al., 2000). There are atleast three widely recognized marine terrace levels at 8–10 m, 5 mand 2 m asl in southern Africa (Corvinus, 1983; Kensley and Pether,1986), which are considered as equivalents of the C-A beaches in south-western Namibia (Miller, 2008) and of the Lower/Upper Vein Quartz,the FLV and the Lower/Upper Donax Terraces at Horingbaai, respective-ly. At Alexander Bay, South Africa and in southern Namibia (Corvinus,1983), the b10 m asl terrace package overlies and truncates strata

Table 1Lithofacies, sedimentary fabrics and process-related interpretation of fluvial Horingbaai fan-delta deposits.

Lithofacies Sedimentary characteristics Interpretation

Boulder-bearing pebble to cobble conglomerate Flat-lenticular geometry, massive to crudely plane-bedded, matrix-rich butclast-supported, bearing angular boulders b40 cm

Hyperconcentrated flow

Cobble conglomerate Flat-lenticular geometry, massive to crudely plane-bedded, clast imbrication,b30 cm thickness

Longitudinal bar

Pebbly very coarse sandstone to pebble conglomerate Lenticular to flat-lenticular geometry, scoured base, trough cross-bedded, clastimbrication, fining-upward

Minor channel fill or transverse bar

Pebbly very coarse sandstone Flat-lenticular geometry, scoured base, epsilon cross-bedded, clast imbrication Side barPebbly coarse to very coarse sandstone to pebble conglomerate Sheet-like geometry, faint plane-bedded, fining-upward, 5–10 cm thickness Sheet floodPebble to cobble conglomerate Sheet-like geometry, massive to plane-bedded, clast imbrication, matrix con-

tent very low or lacking, b5 cm thick, may develop in top part of all other fa-cies, gypsum-encrustation to variable degrees

Sieve/deflation lag

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which contain Acheulian artifacts; a prehistoric industry that is bracket-ed between 1.76 Ma (Lepre et al., 2011) and 0.511–0.435 Ma in anAfrican context (Herries, 2011), although the lower boundary is slightlyyounger in other African locales. More particularly, terrestrial faciesyounger than and contemporary with equivalents of the Vein QuartzTerrace deposits have yielded abundant faunas and Middle Stone Agearcheological remains (Pickford and Senut, 1999), confirming thatthey are younger than 511 ka, the maximum age considered for theMiddle Stone Age in southern Africa (Herries, 2011). Consequently,the 8–10 m (Upper Vein Quartz Terrace equivalent) and the 6–7 m(Lower Vein Quartz Terrace equivalent) might relate to Middle Pleisto-cene interglacials, associated with MIS 11 (424–374 ka) and MIS 9(337–300 ka) of Lisiecki and Raymo (2005). In line with that reasoning,Pether (1986) correlates the 8–10 m beach with Middle Pleistocene

Fig. 8. Characteristic facies types of theHoringbaai Upper fan-delta area in ameasured section ofof discriminant facies characteristics. (A) Photo of stacked sheet flood deposits with interleavedflow deposit, bearing volcanic clasts.

cycles Q3/Q4 of Vail and Hardenbol (1979) and the ∼5 m (FLV Terraceequivalent) and ∼2 m shorelines (Donax Terraces equivalent) are con-sidered to be Late Pleistocene (Eemian) and Holocene respectively.

The FLV Terrace and its equivalents contain a remarkable thermo-philic mollusk assemblage (Table 3) comprising F. diaphana, L. liratula,and V. corrugata as key elements. The extension of these “WestAfrican” species down to the southwestern Cape and the associatedtransgressive maximum has been related to the last interglacial(MIS 5e; Pether, oral comm. 2007) at about 120 ka (Tankard, 1975;Tankard and Rogers, 1978) which provides an excellent time-stratigraphic marker within the b10 m asl terrace package. TheUpper and Lower Donax Terraces at Horingbaai bracket the FLV Terrace(MIS 5e) and are therefore related toMIS 7 (243–191 ka) of Lisiecki andRaymo (2005) and the Early Holocene. Electron spin resonance (ESR)

themain channel cutwall at Locality 1 (see Fig. 6 for location). Table 1 provides a summarydeflation lag gravels; (B) Photo of matrix-rich, crudely plane-bedded hyperconcentrated

Fig. 9. Line drawing of the northward facing cutwall of themainly activemodernHoringbaai fan-delta channel (Loc. 1; see Fig. 6 for location), illustrating the tabular toflat lenticular faciesarchitecture of the upper fan and the importance of incision surfaces.

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dating of Lower Donax Beach shells at Terrace Bay revealed datesof 8 ± 0.4 ka and 5 ± 0.4 ka (Uffmann, 2008), confirming its asso-ciation with the early Holocene sea level rise, recently dated at11,650–7000 cal. years BP (Smith et al., 2011).

8. Controls on and timing of fan growth

8.1. Major emplacement of the Horingbaai fan-delta

Major Horingbaai fan progradation occurred prior to deposition ofthe Oyster Terrace marker (Fig. 13), begging the question as to itscause. In situ cosmogenic isotope analyses constrain very low long-term denudation rates in the range of ∼0.5–10 m/Ma since at least5 Ma for the central Namibian margin (Bierman and Caffee, 2001; Vander Wateren and Dunai, 2001). However, the presence of incisionsurfaces in the late Neogene to Quaternary sedimentary record suggeststhat themarginwas also subject to intermittent pulses of rapid denuda-tion and incision. One of these erosional events was originally recog-nized by Ward (1987) for the Kuiseb Valley (Fig. 1A), just afterdeposition of the Karpfenkliff and Kamberg Formations (Fig. 13),which are now placed in the middle Pliocene (Van der Wateren andDunai, 2001; White et al., 2009). That major incision shaped the centralNamibian continentalmargin prior to deposition of the Oswater Forma-tion, ephemeral braided conglomerates and interleaved aeolianites,which progressively infilled and covered the incised surface (Ward,1987; Miller, 2008). From field relationships we infer that it was upona correlative of that eroded surface that the pre-Oyster terraces, theHoringbaai Fan and other Skeleton Coast fans were laid down. Thetiming of major Horingbaai Fan progradation is thus well constrained(Fig. 13): It is younger than post-Karpfenkliff/Kamberg incision and de-position of the pre-Oyster terraces (~2.65–2.76 Ma) sitting thereuponbut older than the Oyster Terrace (~2.4–2.2 Ma).

Table 2Lithofacies, sedimentary fabrics and process-related interpretation of Oyster Terrace shallow-m

Lithofacies Sedimentary characteristics

Boulder conglomerate Seaward (westerly) dipping (~14°) sheets, thickly layered (seaward and landward clast imbrication, dominantly platybrachiopod shells

Pebble to cobble conglomerate Seaward (westerly) dipping (10–14°) weakly plane beddesupported framework, coarse sand matrix, mostly sphericashells mostly broken and rounded

Pebbly coarse sandstone Seaward (westerly) dipping (8–14°) plane laminated, rarel(b4 cm), heavy mineral-rich, few mussel and gastropod sh

Medium sandstone Seaward (westerly) dipping (8–14°), mainly plane beddedisolated pebbles (b3 cm), heavy mineral-rich laminae and

Aggradation following incision led to deposition of theMeso-OrangeTerrace III in the Orange River (Pickford and Senut, 1999; Jacob et al.,2006), the Oswater Formation in the Kuiseb River (Ward, 1987) andequivalent deposits in many other coastal river systems (Miller, 2008).As noted by Miller (2008), the Oswater Formation conglomerates tendto thicken downstream (seaward). Considering that initial depositionof the Oswater Formation is contemporaneous to a sea level rise thatfinally led to emplacement of the Late Pliocene +30 mP (Pickford andSenut, 1999) and its Rooikop Gravel and Oyster Terrace equivalents atabout 2.4–2.2 Ma, major Horingbaai fan-delta progradation predatesand overlaps with the initial aggradation of Oswater Formation con-glomerates (Fig. 13).

Causes of post-Karpfenkliff/Kamberg incision and the supply ofcoarse-grained debris were originally related to a phase of end-Neogene isostatic (epeirogenic) uplift (Ward, 1987), but this maynot be the only explanation. On a southern African scale, Partridgeand Maud (1987) and Partridge et al. (1995) inferred uplift duringthe Pliocene from the examination of river profiles, raised marinebeach deposits, and offshore sedimentation rates, particularly on theeastern side of the subcontinent, but to a lesser degree also on itswesternside. Although not corrected for contemporaneous eustatic sea level fluc-tuations, the overall regular age–altitude relationships of onshoremarineterraces in southwest Africa (Fig. 14) strongly support an uplift compo-nent. The oldest, late middle Eocene marine deposits (BuntfeldschuhFormation) are found at the highest elevations, at up to 168 m asl(Fig. 14), while younger terraces systematically follow at lower levelsdown to the modern coastline (Pickford and Senut, 1999; Pether et al.,2000; Miller, 2008). Unfortunately the precise age and the palaeo waterdepth at the time of deposition of the marine Eocene are controversial(Siesser and Salmon, 1979; Pickford and Senut, 1999; Miller, 2008) anddo not allow a reliable calculation of uplift rates. Considering present-day elevation of the Oyster Terrace (+24 m asl), its original depositionat a palaeo water depth of ~5 m and peak eustatic sea levels around

arine lithofacies, Horingbaai fan-delta.

Interpretation

~10–30 cm), clast- tomatrix-supported framework, coarse sandmatrix,components, polymict clast assemblage, oyster, mollusk, inarticulated

Upperforeshore

d sheets containing flat lenses of plane laminated sand, clast- to matrix-l to platy components, vein quartz dominated clast assemblage, oyster

Lowerforeshore

y trough cross-bedded units, patches of gravel, scattered (~10%) pebblesells, fish vertebrae and spines, crab fragments, sporadic bioturbation

Uppershoreface

and low-angle cross-stratified layers, well sorted sandstone withlenses, bioturbated intervals

Lowershoreface

Fig. 10. Measured section at Locality 2 (see Fig. 6 for location), illustrating regressive, seaward prograding gravel sheets of the marine Oyster Terrace and its contact relationships withbracketing braided fluvial facies units of the Horingbaai fan-delta. (A) Photo of plane bedded, well sorted lower shoreface medium sandstones; (B) photo of lower foreshore pebble conglom-erate; (C) photo of upper foreshore boulder conglomerate, length of scale is 8.5 cm. Table 2 summarizes distinguishing characteristics of themarine lithofacies observed in the Oyster Terrace.

Fig. 11. Laser theodolite surveys illustrating topographic positions of marine terrace levels in two NE–SW trending sections (note vertical exaggeration). (A) Cross-section 1, close to thesouthern margin of the Lower Horingbaai fan-delta and (B) cross-section 2, 2.5 km south of the fan-delta. See Fig. 6 for locations of surveyed cross-sections.

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Table 3Distinguishing characteristics of individual marine terrace levels, which are associated with the Horingbaai fan-delta. Marine Isotope Stages (MIS) and ages based on Lisiecki and Raymo(2005); see main text for discussion of terrace ages.

Terrace Heightasl

Clast assemblage Fauna Age

Lower Donax ~3 m 80% quartz latite, 10% vein quartz, 5% basalt, agate Cold water: Donax serra Early HoloceneFLV (Felania–Loripes–Venerupis) ~4 m Polymict assemblage: 60–70% vein quartz, 10–20% quartz

latiteTemperate water: Felaniadiaphana, Loripes liratula,Venerupis corrugata, Lutrarialutraria, Turritella atlantica

Late Pleistocene (Eemian) MIS 5e ~120 ka

Upper Donax 4.5–5 m 70–80% quartz latite, 10–20% vein quartz, 5% basalt, 5%percussion-marked quartzite

Cold water: Donax serra,mytilus

Middle Pleistocene MIS 7 243–191 ka

Lower Vein Quartz 6.5–7 m 60% vein quartz, 40% quartz latite Cold water: Donax serra Middle Pleistocene MIS 9 337–300 kaUpper Vein Quartz 8.5–9.5 m 60% vein quartz, 40% quartz latite Cold water: Donax serra Middle Pleistocene MIS 11 424–374 kaOyster 15–24 m Polymict assemblage: ~50% phosphatized, bored silt- to

fine sandstone, 15–30% quartz latite, 10–25% vein quartz,percussion-marked quartzite, granite, granodiorite, rarebrown banded ironstone, chert, jasper, Karpfenkliff- andTsondab Formation rock clasts, (Kamberg) calcrete, darkblue-gray riebeckite

Warm water: oysterStriostrea margaritacea,Donax rogersi, Fissurelaglarea, phosphatized sharkteeth

Late Pliocene MIS 95-85 2.4–2.2 Ma

Pre-Oyster 25–28 m Polymict assemblage: 15–30% quartz latite, 30–55% veinquartz, b10% phosphatized, bored silt- to fine sandstone,percussion-marked quartzite, granite, granodiorite, rarebrown banded ironstone, chert, jasper,

No fossils found Late Pliocene MIS G7-G3 2.76–2.65 Ma

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2.7–2.2 Ma at −10 m (Miller et al., 2011) to +10 m (Abreu andAnderson, 1998), compared to the present, suggests ~19–39m of coastaluplift since the late Pliocene. The calculated uplift rate of ~12±5m/Ma iswithin the uppermost range of an uplift rate of 6 ± 3 m/Ma since 4 Ma,inferred for the De Hoop coastal plain section in South Africa (Robertset al., 2012), although this area is N1800 km southsoutheast of thestudy area. Not only the evidence for vertical movements on the onehand, but also the coincidence in the timing and pattern of SouthAfrican and western Australian sea level curves on the other, suggestedto Pickford and Senut (1999) an interplay between eustatic and tectonicprocesses.

Instead of being driven by uplift and sea level change, Van derWateren and Dunai (2001) favor climate change, in particular an in-crease in southwestern African humidity and annual precipitation as amajor control of accelerated denudation and incision during the lateNeogene. A late Pliocene offshore high-resolution pollen record(Ocean Drilling Program, ODP Site 1082; Fig. 1A) indicates, that desertand semidesert biomes in adjacent onshore areas of NW Namibiawere restricted before 2.7 Ma, but after this date percentage valuesfluctuate more strongly and at a higher level, indicating increased cli-matic variability and extension of arid vegetation types (Dupont et al.,2005). This timing of climate change in southwestern Africa impliesan overlap with the onset of major northern hemisphere glaciation(DeMenocal, 2004) at 3.15 Ma and its culmination in the great “climatecrash” at marine isotope stage G6 (2.74 Ma) (Bartoli et al., 2005).Dupont et al. (2005) inferred from variation pattern, values andtypes of pollen at ODP Site 1082, that between 2.7 and 2.2 Ma glacial/interglacial shifts became more prominent and that the winter rainfallarea extended northward, in particular during, but not restricted to,the glacial stages after 2.7 and 2.2 Ma. The ODP section 1082 registersenhanced offshore sedimentation rates from 3.1 to 2.2 Ma (Dupontet al., 2005) with a maximum centered around 2.6 Ma (Fig. 13). It waswithin this timeframe (2.7–2.4 Ma) that major progradation of theHoringbaai fan-delta occurred (Fig. 15).

8.2. Latest Pliocene to Pleistocene fan-delta sedimentation

Following Oyster Terrace formation, Horingbaai fan growth contin-ued at a much lower accretion rate. During subsequent sea levelhighstands younger marine terraces, now mainly characterized bycold-water faunas (Table 3) were laid down as veneers on the fan-delta surface, each underlain by a shallow incision generated duringthe lowstand and subsequent marine transgression (Fig. 15). The

contemporaneous pollen record of ODP Site 1082 offshore NWNamibia registers not only strong fluctuations, comparable to the latePleistocene glacial/interglacial variation (Shi et al., 2001), but also ageneral aridification starting at 2.2Ma, associatedwith the introductionof ephemeral river discharge and colder sea-surface temperatures alongthe Namibian coast (Marlow et al., 2000; Dupont et al., 2005).

Further evidence for climate change is recorded by changes in fansediment composition. In association with the overall higher aridity,fan sediments postdating the Oyster Terrace show on average lowercompositional maturities and on top of this generalized trend also agreater compositional heterogeneity than fan sediments predating theOyster Terrace. Despite their source proximity, some of the fan sectionsinclude interbeds containing remarkably low percentages of Etendekavolcanics and Kuiseb schists, the dominant rock types of the catchmentarea. Instead, these thin (5–15 cm) interbeds are characterized by con-siderable proportions of vein quartz. An example is illustrated by thesection measured at Loc. 1 (Fig. 9) in the upper fan segment. Thebases of the lower three units (marked by asterisks) that cover incisionsurfaces are relatively poor in Etendeka volcanic components (20–50%)but rich in vein quartz (50–70%). This contrasts markedly with thecomposition of the under- and overlying units, dominated (N70%) byEtendeka volcanic components. As there is no evidence for a change ofthe catchment area during deposition, it is speculated that the observedcompositional changes may record variable intensities of chemicalweathering due to climatic variation.

9. Conclusions

Marine terraces associated with the Horingbaai fan-delta linkPliocene–Pleistocene terrestrial and marine sedimentary records withclimate, sea level and uplift history (Fig. 13). During a period of verylow long-term denudation rates (Van der Wateren and Dunai, 2001),the fan-delta registers intermittent periods of erosion and episodiccoarse debris supply, timing and controls of which provide a frameworkfor the reconstruction of the geomorphological history of the Namibianpassive margin and add complementary data to the more commonuse of fluvial archives in reconstructing landscape (Bridgland andWestaway, 2012), climate and uplift history (Westaway et al., 2009)in the Cenozoic.

Major Horingbaai fan-delta progradation during the upper Pliocene(∼2.7–2.4 Ma) and high sedimentation rates offshore NW Namibia(Dupont et al., 2005) are recorded at about the same time (Fig. 13)that widespread uplift (Partridge et al., 1995), sea level and climate

Fig. 12. Details of the interaction of the Horingbaai fan-delta with marine terraces close to the coastline. (A) Once the active channel had breached the Vein Quartz Terraces it started tospread, developing farther downfan a sheedflood style of deposition. (B) The area ∼ 2.5 km south of the active fan, showing awell preserved FLV/Donax terrace level preserved outside thereach of the fan. See Fig. 6 for location of detailed images. White pins mark marine terrace levels, black arrows trace southern fan margin, and white arrows mark washover fans.

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change in southwest Africa occurred, the latter reflecting the globallyrecognized cooling step at ∼2.7–2.5 Ma, associated with the intensifica-tion of the northern hemisphere glaciations (DeMenocal, 2004). Al-though this is considered as a time of globally accelerating erosionrates, probably because the sedimentary systems have to attain new

equilibria with rapid changes in sea level, precipitation, weatheringand vegetation (Peizhen et al., 2001), the overall regular age–altituderelationships of marine terraces (Fig. 14) suggest that besides climateand sea level, a considerable uplift component of ~12 ± 5 m/Ma isinvolved.

Fig. 13. Summary diagram placing Horingbaai fan-delta growth within the context of marine terraces (thicknesses not to scale), other onshore Cenozoic strata and contemporaneous cli-mate changes. Fan growth between Oyster Terrace and Upper Vein Quartz Terrace markers schematically drawn. Offshore sedimentation rates (SR) of ODP Site 1082 after Dupont et al.(2005). Marine Isotope Stages (MIS) of Lisiecki and Raymo (2005), based on a stack of 57 globally distributed benthic foraminiferal δ18O records and conversion to a global sea level byNaish and Wilson (2009). To simplify comparison with published data, the pre-2009 stratigraphic table (Gradstein et al., 2004) has been used with recent modifications shown.

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Because of its global background,major Horingbaai fan progradationmay be seen as representative of the timing of activity of other, yetundated alluvial fans (e.g., Miller, 2008) that exit from the front of theGreat Escarpment in Namibia and issue on a time-equivalent post-Kamberg erosional surface. Substantial fluxes of immature gravelsfrom the escarpment area attest to a sediment delivery system thatwasmuchmore vigorous than it is at present. Voluminous debris supplymost probably provided the prerequisites for the aggradation ofMeso-Orange III fluvial terrace conglomerates (Pickford and Senut,1999; Miller, 2008) and widespread gravel spits, barrier beachesand prograding gravel shorelines (Jacob et al., 1999; Bluck et al., 2001;Spaggiari et al., 2006) that constitute the Oyster terrace equivalent

+30 mP, preserved along much of the Namibian and South Africancoastline (Pether et al., 2000).

Succeeding Horingbaai fan growth phases younger than 2.2 Ma areconstrained by b10m asl marine terrace bodies (Fig. 15), dated as Mid-dle Pleistocene (Upper/Lower Vein Quartz and Upper Donax Terraces),Late Pleistocene (FLV Terrace) and Holocene (Lower Donax Terrace).These attest to substantiallyweaker andmore episodic fan progradationafter 2.2 Ma, corresponding to a general aridification trend and theonset of glacial/interglacial climate fluctuations recorded by marineclimate proxies (Dupont et al., 2005).

Induced by glacial/interglacial changes in sea level, fluvial baselevels, sediment and water supply, the apex of the fan would have

Fig. 14. Age–altitude relationships of Neogene and Quaternary marine terraces (white circles) in coastal Namibia (compiled from Pickford, 1998; Pickford and Senut, 1999; Pether et al.,2000; Miller, 2008) and of Orange river terraces (black circles), based on Jacob et al. (1999) andMiller (2008). Marine terrace levels refer to meters above sea level (m asl), river terracesrefer to meters above present river bed.

30 H. Stollhofen et al. / Sedimentary Geology 309 (2014) 15–32

been repeatedly incised in the course of successive falls in sea level, andrepeatedly infilled during sea level rises. However, available OSL chronol-ogies of ephemeral river deposits in coastal Namibia, which indicate wet-ter conditions, shownopreferred associationwith interglacials or glacials(Stone and Thomas, 2012). Based on terrigenous grain size distributionsoffshore Namibia, Stuut et al. (2002) inferred relative aridity duringthe interglacial stages, which runs counter the results of Pichevin et al.(2005), who inferred that the last glacial periods were concurrent with

Fig. 15. Schematic model (not to scale) of the Horingbaai fan-delta and its relationship to Skeleplacement of the Late Pliocene “Oyster Terrace” (∼2.4–2.2Ma)but after the formation of awidesfan-delta progradation, postdating “Oyster Terrace” formation is overall weaker andmore irregdeposited during interglacial marine highstands.

higher aridity. There is currently no unanimous opinion as to whetheraridity is controlled by variations in winter (e.g., Stuut et al., 2002) orsummer rainfall (e.g., Brook et al., 1996), although the latter receivesgrowing support (Brook et al., 2006; Stone and Thomas, 2012). Potential-ly the short-headed drainages feeding the coastal plain alluvial fans areparticularly sensitive to even small variations in climate, in contrast tolarger river systems, catchments of which are larger, extend fartherinland, and thus benefit from higher humidity there.

ton Coast marine terraces. The bulk of fan-delta progradation took place prior to the em-pread erosional surface overlain bypre-Oyster Terraces (∼2.7–2.6Ma). A younger phase ofular, accompanied byMiddle Pleistocene, Upper Pleistocene and Holocenemarine terraces

31H. Stollhofen et al. / Sedimentary Geology 309 (2014) 15–32

Acknowledgments

This study was funded by the German Science Foundation (DFGgrant STO 275/7-1) and the German Academic Exchange Service(DAAD) which is gratefully acknowledged. Special thanks are due tothe Namibian Geological Survey, in particular its Director Dr. GabiSchneider, for generous help with logistics, to the Ministry of Minesand Energy and to the Ministry of Environment and Tourism as wellas the game wardens of the Skeleton Coast Park for their permissionand support to work in the area. The paper benefited greatly from on-site discussionswith John Pether and Frank Preusser. Christoph Krämer,Benjamin Hagedorn, Anna Uffmann, Henning Schulz and BastianWirthare acknowledged for field assistance during surveys of fan and marineterrace profiles. The manuscript was greatly improved by commentsfrom David Bridgland and an anonymous reviewer and the work of Jas-per Knight, the editor.

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