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Linking source and sink: Evaluating the balance betweenonshore erosion and offshore sediment accumulation

since Gondwana break-up, South Africa

Justine Tinker a,⁎, Maarten de Wit a, Roderick Brown b

a AEON, University of Cape Town, Rondebosch 7701, South Africab Department of Geographical and Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK

Received 24 October 2007; accepted 15 November 2007Available online 4 December 2007

Abstract

The geomorphic origin and evolution of the tectonically unique interior highland of southern Africa, the Kalahari Plateau, and its flanking low-lyingcoastal planes, remain largely unresolved because of a lack of regional quantitative analyses of its uplift and erosion history. Here we focus on thesouthern Cape, South Africa and link onshore denudation, based on new apatite fission track thermochronology results, to offshore sedimentaccumulation, using abundant well data and a seismic reflection profile. We attempt to relate source and sink in order to resolve some first order issuesconcerning timing of the exhumation and development of the topographic features of southern Africa. The volume of sediment accumulated off SouthAfrica's south coast is calculated using 173 wells and a seismic reflection profile. A total, uncompacted, sediment volume of 268,500 km3 accumulatedoff South Africa's south coasts since∼136 Ma, in the Outeniqua and Southern Outeniqua Basins. Accumulation volumes and rates were highest in theearly Cretaceous (48,800×104 km3 at∼8150 km3/Ma from∼136 to 130Ma, and 57,500×104 km3 at 5750 km3/Ma from∼130 to 120Ma) and mid–late Cretaceous (83,700×104 km3 at 3200 km3/Ma from ∼93 to 67 Ma). Volumes and accumulation rates were lowest for the early–mid-Cretaceous(47,400×104 km3 at 1750 km3/Ma from∼120 to 93Ma) and the Cenozoic (31,200×104 km3 at 450 km3/Ma from∼67 to 0Ma). Although our analysisshows that the accumulated volume of offshore sediments does not match the calculated volume of onshore erosion, as quantified through apatite fissiontrack thermochronology (e.g. Tinker, J.H., de Wit, M.J., Brown, R., 2008. Mesozoic exhumation of the 439 southern Cape, South Africa, quantifiedusing apatite fission track thermochronology. Tectonophysics, doi: 10.1016/j.tecto.2007.10.009), the timing of increased sediment accumulation closelymatches the timing of increased onshore denudation. This suggests that the greatest volumes of material were transported from source to sink during twodistinct Cretaceous episodes, and that the processes driving onshore denudation decreased by an order of magnitude during the Cenozoic.© 2008 Elsevier B.V. All rights reserved.

Keywords: Epeirogeny; South Africa; Sediment accumulation; Continental denudation; Basin analysis; Fission track thermochronology

1. Introduction

The Kalahari Plateau, an interior plateau of low relief and highelevation, dominates central southern Africa. It is surroundedby a coastal plane of high relief and low average elevation. Thesteeply-dipping ‘Great Escarpment’ divides these two regions(Fig. 1). How this topography has evolved is part of ongoingdiscourse since the early days of Du Toit (1937, 1954) and King

(1951). Since the break-up of Gondwana between ∼180 and130 Ma, several hundred thousand km3 of rock has been erodedfrom the sub-continent and the products of this erosion are pre-served as accumulated sediments offshore. There is little con-sensus as to the origin, precise timing, duration and magnitudeof the epeirogenic uplift and erosion (e.g. de Wit, 2007; Kounovet al., 2007).

Apatite fission track thermochronology, a low temperaturetechnique, has been used world-wide to attempt to solve geo-morphological problems related to uplift and denudation as re-viewed by Gallagher et al. (1998), Gleadow and Brown (2000)and Reiners and Ehlers (2005). Tinker et al. (2008) appliedthis technique to samples from the southern Cape region and

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⁎ Corresponding author.E-mail addresses: Justine.Tinker@tullowoil.com (J. Tinker),

Maarten.dewit@uct.ac.za (M. de Wit), Roderick.Brown@ges.gla.ac.uk(R. Brown).

0040-1951/$ - see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2007.11.040

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concluded that the area has seen episodic increased denudationin the early and mid–late Cretaceous. Here, we expand thisanalysis to test further whether the timing of increased onshoredenudation matches the timing of maximum sediment accumu-lation along the southern margin since the break-up of Gond-wana. The volume of accumulated sediment is calculated forseveral major unconformity-bounded offshore sequences in theOuteniqua Basin using 173 offshore wells. A seismic reflectionprofile is used to estimate sediment accumulation in the undril-led southern Outeniqua Basin. Further, we test a first orderassumption that what is eroded from the continent during thesetwo periods should be gained on the continental shelf, rise andocean floor, by comparing denudation volumes with that accu-mulated offshore. South Africa's south coast, offshore and in-land, is a particularly good place to test this balance becausehydrocarbon exploration has resulted in a wealth of well data.

2. Offshore sediment accumulation

Dextral shear stress along the Falkland–Agulhas FractureZone (FAFZ) resulted in the separation of the Falkland Plateaufrom theMozambique Ridge and the break-up between southernAfrica and South America during the middle/late Jurassic to∼136Ma. During this time, arcuate rift faults gave rise to a horstand graben structure off the south coast of South Africa. The

Bredasdorp, Pletmos, Algoa and Gamtoos Basins (collectivelycalled the Outeniqua Basin) and their southern extension (herecalled the southern Outeniqua Basin) became the depositoriesfor late Jurassic to Cretaceous sediment eroded from the south-ern African continent (Figs. 1, 2 and 3).

Tectonic readjustments during the break-up of Gondwanahave resulted in several prominent and widespread unconfor-mities within the thick drift succession. These are identifiedand correlated from offshore seismic data and biostratigraphy(Brown et al., 1995; McMillan et al., 1997, McMillan, 2003).The regional unconformities are 1At1 at∼136 Ma (ValanginianStage, the end of most rifting along the south coast), 6At1 at∼130 Ma (Hauterivian Stage, the end of graben infilling andbeginning of shelf progradation), 13At1 at ∼120 Ma (earlyAptian Stage), at ∼103 Ma (Albian Stage), 15At1 at ∼93 Ma(Cenomanian Stage), ∼80 Ma (Campanian Stage) and 22At1 at∼67 Ma (Upper Maastrichtian Stage). The volumes of sedimentbounded by these major unconformities, chosen for their re-gional distribution and known age, are calculated here.

3. Data and methods

Since the 1970's, South Africa's offshore has been exploredfor oil and gas. This exploration has resulted in awealth of seismicand borehole data. A stratigraphic atlas of the south and west

Fig. 1. Simplified geomorphology of the southern Cape and location of Mesozoic basins offshore: Orange Basin (west coast), Bredasdorp, Pletmos, Gamtoos, AlgoaBasins (Outeniqua Basin) and Southern Outeniqua Basin (shaded grey). Dark grey shading = offshore basement highs. Onshore denuded area = dot-dash line. Offshorearea of accumulation = dotted line. Apatite fission track outcrop samples (squares) and boreholes (crosses) (Tinker et al., 2008).

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coast, compiled by Brown et al. (1995) from detailed seis-mic stratigraphic studies, summarizes research along the southcoast and the maps presented here make use of their stratigraph-ic framework. The ages of the regional unconformities have beenadjusted to reflect new age calibrations for the Cretaceous Periodand Cenozoic Era from Gradstein et al., (2004).

The calculations of sediment volume in the Outeniqua Basinuse data from an internal well compilation report of the South-ern Oil Exploration Corp. (Pty.) Ltd. (Soekor) that lists thedepths below seafloor to several regional unconformities for173 boreholes. These boreholes are distributed unevenly off-shore of the south coast, in less than 200 m water depth (Fig. 4).To calculate the sediment volume that has accumulated in theOuteniqua Basin, we assume an offshore area of 75,000 km2 thatis bounded by the shoreline in the north and 200 m isobath thesouth, where the deepwater extension of the Outeniqua Basin, or

Southern Outeniqua Basin, begins (Figs. 1 and 4). The area isbounded in the west and east by two basement highs, theAgulhas–Columbine Arch and the Port Alfred Arch, respec-tively, which would have acted as barriers to sediment transport(Fig. 1). The volume of accumulated sediment is calculated foreach of the 5 time intervals (∼136–130 Ma, ∼130–120 Ma,∼120–93 Ma, ∼93–67 Ma and ∼67–0 Ma) at each boreholelocation by subtracting the depth of each unconformity fromthe depth of the unconformity stratigraphically below (schema-tically shown in Fig. 5). These volumes are then correctedfor compaction using Formula 1. Accumulated sediment thick-nesses at each borehole are transformed into sediment volumeover the specified area by gridding the point data using OasisMontaj 6.0 (Geosoft Inc.). The minimum curvature griddingalgorithm was chosen as it is best suited to randomly distrib-uted data points, fitting the smoothest possible surface to the

Fig. 2. Simplified structure and sediment distribution of the drift succession across the 5 Outeniqua Basins: Bredasdorp (A–A′, B–B′), Pletmos (C–C′), Gamtoos (D–D′)and Algoa (E–E′). Sections from McMillan et al. (1997) and the Petroleum Agency SA.

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given data values. 5 isopach maps of sediment thickness forthe Outeniqua Basin were generated, one for each time interval(Figs. 6a to e).

Vsurf ¼ Vbur þ Psurf � Pburð Þ ð1Þ

Vsurf = volume at surfaceVbur = volume at a given depthPbur = porosity at depthPsurf = porosity at surface

Porosity at depth and surface are estimated by comparison tothe COST B-2 well off the New York continental margin inSteckler and Watts (1978). These data were chosen because ofthe similar tectonic setting (a Jurassic-aged passive margin) andsimilar sediment age.

Accumulated sediment volume in the southern OuteniquaBasin is difficult to calculate due to the absence of well data.Sediment volume is therefore roughly estimated using a seismicreflection profile, interpreted by Roux (1997), Fig. 3. Averagethicknesses of each unconformity-bounded sequence arecalculated from 5 1D intersections along this profile. Thesethicknesses are then multiplied by the basin area of 20,000 km2

(Roux, 1997) to obtain an estimate of accumulated sedimentvolume for each time interval.

4. Results

The total accumulated sediment volume for the Outeniquaand Southern Outeniqua Basins is given in Table 1 and graphedin Fig. 7. The greatest volume of sediment preserved offshoreaccumulated in the mid–late Cretaceous, between ∼93 and67 Ma (83,700 km3). However, the accumulation rate washighest in the early Cretaceous,∼136 to 130Ma (8150 km3/Ma)and ∼130 to 120 Ma (5750 km3/Ma) (Fig. 7). Early Cretaceousoffshore accumulation occurred at more than double the aver-age rate of 1950 km3/Ma calculated for the full time interval of∼136 Myrs. Following a relative lull in accumulation from∼120 to 93 Ma (47,300 km3, at 1750 km3/Ma), sediment ac-cumulation rate increased again in the mid–late Cretaceous(∼93–67 Ma, 3200 km3/Ma). The lowest volume of sediment(31,100 km3) accumulated at the lowest rate (450 km3/Ma) duringthe Cenozoic.

5. Timing of denudation and accumulation fluxes

Tinker et al. (2008) presents new fission track data from 3 deepboreholes, located on the southern Cape coastal plane (Fig. 1).Thermal modelling of these data suggests an increased coolingrate, related to increased denudation of the southern Cape dur-ing the mid–late Cretaceous. Analysis of a fourth well from the

Fig. 3. Distribution of a selection of the 173 offshore boreholes within the Outeniqua Basin. Gridded area = dotted line.

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interior plateau (Brown, 1992) suggests an earlier period of in-creased denudation in the early Cretaceous, though this is, as yet,not quantified. The timing of increases in onshore denudation ratecalculated in Tinker et al. (2008) is plotted against the timing ofoffshore accumulation flux in Fig. 8. Although denudation ratesfar exceed accumulation rates, the timing of relative increases anddecreases in rates matches well. For example, a high denudationrate in the mid-Cretaceous of 125–175 m/Ma (∼100–80 Ma) iscontemporaneous with increased offshore accumulation, appar-ently delayed by a lag of ∼7 Myrs at ∼93–67 Ma (Fig. 8). Thislag may be an artifact of averaging offshore sediment accumula-tion over considerable time intervals (up to ∼27 Myrs). Lowonshore denudation in the Cenozoic (10–15m/Ma) ismirrored bya low rate of offshore sediment accumulation during this time(5 m/Ma). Rate of offshore accumulation is highest in earlyCretaceous (60–85 m/Ma) supporting a period of increased de-nudation evident from apatite fission track evidence from a bore-hole north of the escarpment.

6. Testing the balance between onshore denudation andoffshore accumulation

At least as a first order assumption, the volume of rock de-nuded from the onshore should equal the volume of sedimentaccumulated offshore. To test this balance for the southern Capeand corresponding offshore, we assume all offshore sedimentswere sourced from an onshore of∼140,000 km2 (Fig. 1). Gilchrist

et al. (1994) proposed that synchronous with the break-up ofGondwana, two drainage areas developed, an internal, inwarddraining regional separated from an external, seaward drainingregion by an escarpment. This escarpment either initially formednear, or rapidly retreated to near its present position in the Cre-taceous and has since only experienced limited retreat since rifting

Fig. 5. Schematic of how accumulated sediment thickness is calculated betweenregional unconformities (not to scale).

Fig. 4. A schematic section across the Southern Outeniqua Basin and bounding Northeastern Marginal Fracture ridge (from Roux, 1997).

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Fig. 6. Isopach maps depicting accumulated sediment volume for 5 time intervals; the early Cretaceous (∼136–130 Ma and ∼130–120 Ma), the early–mid-Cretaceous (∼120–93 Ma), the mid–late Cretaceous (∼93–67 Ma) and the Cenozoic (∼67–0 Ma).

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(e.g. Fleming et al., 1999; Van der Beek et al., 2002; Brown et al.,2002; Tinker, 2005; Kounov et al., 2007). The position of thisdrainage divide suggests that only the area south of the GreatEscarpment has been eroded by rivers oriented perpendicularto the coast to eventually be deposited off the south coast. Thearea north of the escarpment has been eroded by inward drainingrivers, for example the Kalahari and Karoo Rivers and laterthe Orange River (de Wit et al., 2000), to be finally deposited offthe west coast. Thus, the estimated source area for south coastsediments (∼140,000 km2) is bounded by the escarpment in thenorth (coinciding with the southern limit of dolerite intrusions),the Cape Fold Belt syntaxis in the west and where the Cape FoldBeltmeets the IndianOcean in the east (Fig. 1) andwas drained bya distinct river system that can be traced to the present day(Stankiewicz and de Wit, 2005a,b).

For this simple test of mass balance, the total offshore ac-cumulated volume (in km3) is converted to an un-eroded onshorerock volume (in km3, using Formula 2). This is then divided by theassumed area of onshore denudation (∼140,000 km2), givingan average denuded thickness across the southern Cape (in m,Table 2). This thickness is then compared to the denudationcalculated directly from fission track low temperature thermo-chronology, above three boreholes located south of the escarpment(Tinker et al., 2008). This comparison is an oversimplification, forthe reasons described below. However, as a back of the envelopecalculation, as this is intended to be, the results are interesting.

Vrock ¼ Vsurf 1� Psurfð Þ ð2Þ

Vsurf = equivalent un-eroded rock volumeVbur = calculated sediment volume at surface (Table 1)Pbur = porosity at surface

Based on estimated offshore sediment volumes (Tables 1 and 2),a calculated average thickness of 860 m has eroded from anestimated onshore source area of ∼140,000 km2 since ∼140 Ma,and 270m in themid–late Cretaceous only (93 to 67Ma) (Table 2).Fission track modelling, however, suggests erosion of ∼2.5 to3.5 km from above 3 onshore boreholes in themid–late Cretaceous(Tinker et al., 2008). Thus the calculated onshore denudationestimated from offshore volumes is around an order of magnitudeless than that calculated directly from the fission track data. If theassumption that what is eroded from the continent is preserved

offshore holds, either onshore denudation is overestimated or off-shore accumulation is underestimated.

7. Onshore and offshore uncertainties

The estimate of onshore denuded volume is uncertainbecause of the difficulties in assessing the extent of the erodedregion. Lateral variation in degree of erosion over an area of140,000 km2 is impossible to resolve from fission track datafrom only three boreholes. More widespread onshore boreholefission track data are required to make a more robust estimate.In addition, dissolution during chemical weathering must havecontributed to the missing volume of sediment between theonshore and offshore, given that a significant portion of theeroded material may have been Karoo basalts (Tinker et al.,2008; de Wit, 2007), rocks that are particularly susceptible tochemical weathering (e.g. Dupré et al., 2003) and that theCretaceous was a time of extreme elevated global temperatures(e.g. Jenkyns et al., 2004).

The calculated offshore sediment volumes are minimumvalues for the following reasons. First, sediment accumulationis always volumetrically less than sediment deposition. It isimpossible to quantify how much sediment has been ‘lost’ ateach of the major (and several minor) offshore unconformities.The onset of current activity in the newly created southern At-lanticOcean (e.g. theAgulhas Current)may have exacerbated thiserosion (e.g. Livermore et al., 2007; Uenzelmann-Neben et al.,2007).

Second, in the early Cretaceous, sediment probably bypassedthe African continental shelf to accumulate in deeper watersoffshore or on the Falkland Plateau. The latter is known to haveaccumulated significant volumes of Cretaceous to recent se-diments (Lorenzo and Mutter, 1988). It is not possible at presentto quantify sediment that may have accumulated beyond theboreholes used here to calculate sediment volume in the innerbasins and beyond the seismic data coverage used to estimateaccumulated volume in the Southern Outeniqua Basin. Sedi-ment may have continued to bypass the continental shelf afterclearing of the Agulhas Bank by the Falkland Plateau and thedevelopment of open ocean circulation (late Albian, Dingleet al., 1983; Uenzelmann-Neben et al., 2007). These sedimentsmust have been transported beyond the shelf by turbidity cur-rents (and thus are likely to be volumetrically significant), oralternatively may have settled out of suspension. However,

Table 1Total sediment volumes and minimum accumulation rates for the Outeniqua and Southern Outeniqua Basins

Boundingunconformities

Age range(Ma)

Time span(Ma)

Measured volume (km3)(Outeniqua Basin)

Estimated volume (km3)(Southern Outeniqua Basin)

Total volume(km3)

Total accumulation rate(km3/Ma)

⁎Total accumulationrate (m/Ma)

1At1–6At1 136–130 6 41,250 7550 48,800 8150 856At1–13At1 130–120 10 51,250 6250 57,500 5750 6013At1–15At1 120–93 27 40,050 7300 47,400 1750 2015At1–22At1 93–67 26 55,750 27,950 83,700 3200 3522At1-seafloor 67–0 67 28,600 2550 31,200 450 5TOTAL 136–0 136 216,900 51,550 268,500 1950

(average over 136 Myrs)20

Volumes are corrected for compaction. Accumulation rates are averaged over period of sediment accumulation. Accumulation rates are given in km3/Ma as calculated,and in ⁎ m/Ma. The latter is calculated by dividing rate in km3/Ma by assumed offshore area, 95,000 km2.

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bypassing sediments in both the early and late Cretaceouswould also have had to clear the Northeastern Marginal FractureRidge (named by Robson and Dingle, 1986, renamed the Diaz

Ridge in Ben-Avraham et al., 1993) and the Agulhas MarginalFracture Ridge (created early in the development of the region,Lorenzo and Mutter, 1988). Basement outcrops may have acted

Fig. 7. Varying sediment accumulation volume (shaded blue, left axis) and rate (red line, right axis) over ∼136 Myrs for (A) the Outeniqua and Southern OuteniquaBasins combined, (B) the Outeniqua Basin and (C) the Southern Outeniqua Basin.

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as a barrier to sediment migration into deeper waters (N3000 mwater depth).

Third, the precision and resolution of the 5 sediment isopachmaps is reliant on the distribution of borehole data. Basins thathave been better explored for hydrocarbons (e.g. the BredasdorpBasin) have more borehole data. Thus variation in sedimentthickness across the Bredasdorp and Pletmos Basins is betterconstrained than across the Gamtoos and Algoa Basins. In theunexplored (undrilled) Southern Outeniqua Basin, sediment vol-ume is less well constrained.

Fourth, sediment that accumulated in the early Cretaceous(between ∼136 and 130 Ma) is rift-bounded and thereforelocally distributed. Locally distributed sediment is difficult togrid accurately due to abrupt changes in sediment thickness.Sediments that accumulated since the early Cretaceous (∼130to 0 Ma) are more regionally distributed and thus can be moreaccurately gridded. The problem of localised sediment distri-bution is compounded when the boreholes are unevenly distri-buted and widely spaced.

Fifth, due to lack of well data our calculations ignore syn-riftsediment accumulation, i.e. the volume that accumulated priorto the rift-drift unconformity (∼136Ma). From offshore seismicprofiles syn-rift sediment appears thick (up to 5000 m, Pletmos

Basin, McMillan et al., 1997), but are highly localised andtherefore syn-rift volumes are difficult to estimate.

8. Conclusion

Accumulated offshore sediment volume from 173 wells anda seismic profile shows that the rate of sediment influx intothe Outeniqua and Southern Outeniqua Basins was highest du-ring the early Cretaceous and increased again in the mid–late

Fig. 8. A comparison of onshore denudation rates (green, from Tinker et al., 2007) and offshore accumulation rates (blue). Offshore accumulation rates are convertedfrom km3/Ma) to m/Ma by dividing by the assumed area of offshore accumulation, 95,000 km2 (Table 1).

Table 2Thickness of average onshore denudation of the southern coastal plain, based onoffshore sediment volumes corrected to an un-eroded rock volume and assumingan onshore source area of ∼140,000 km2

Boundingunconformities

Agerange(Ma)

Timespan(Ma)

Offshoresedimentvolume (km3)

Equivalentrockvolume(km3)

Equivalentaverageonshoredenudation (m)

1At1–6At1 136–130 6 48,800 21,950 1606At1–13At1 130–120 10 57,500 25,900 19013At1–15At1 120–93 27 47,400 21,350 15015At1–22At1 93–67 26 83,700 37,650 27022At1-seafloor 67–0 67 31,200 14,050 100Total 136–0 136 268,500 120,850 860

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Cretaceous. Fission track data from four onshore boreholes pointto two similar episodes of increased denudation in the earlyand mid–late Cretaceous (Tinker et al., 2008). While thereappears nomaterial balance between actual onshore and offshorevolumes, the increase in rate of both onshore denudation andoffshore accumulation is contemporaneous and that bulk trans-portation of materials from onland source to offshore sink wasgreatest in the Cretaceous. These analyses are consistent withmodels of accelerated periods of Mesozoic denudation, andby inference the uplift and major topographic development ofsouthern Africa's inland, Kalahari Plateau and its flankingcoastal planes, as well as the origin of the ‘Great Escarpment’.Further fission track analyses and modelling of additional on-shore and offshore South African deep boreholes, of whichthere are many, may help to refine any calculation of onshore–offshore mass balance.

Acknowledgements

This work was supported by Pioneer Natural Resources,facilitated by M. Ranoszek (JHT), the Upstream Training Trust(JHT) and the NRF (MdW). Access to offshore well data isthanks to I. McLachlan and J. Roux of the Petroleum AgencySA. For their individual contributions we thank David Heimann,Giulio Viola, Alex Kounov, Mike Jones, Mike de Wit, ChristienThiart and Doug Cole. This is AEON contribution number 48.

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