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Marine Geophysical ResearchAn International Journal for the Study ofthe Earth Beneath the Sea ISSN 0025-3235 Mar Geophys ResDOI 10.1007/s11001-015-9250-3

A sediment budget for the Transkei Basin,Southwest Indian Ocean

Gabriele Uenzelmann-Neben & PeterD. Clift

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ORIGINAL RESEARCH PAPER

A sediment budget for the Transkei Basin, Southwest IndianOcean

Gabriele Uenzelmann-Neben • Peter D. Clift

Received: 20 November 2014 / Accepted: 4 February 2015

� Springer Science+Business Media Dordrecht 2015

Abstract Deep sea sediment budgets can be used to con-

strain erosion rates in the neighboring continents from which

the material was derived. Here we construct a sediment budget

for the Transkei Basin, offshore South Africa using an existing

seismic reflection survey and dated by correlation of seismic

attributes to dated sections in nearby basins. Backstripping of

the sections reveals that sediment accumulation rates fell from

110 to 11 Ma, with a possible period of rapid accumulation

from 36 to 34 Ma that may be driven by strengthening of the

Antarctic Bottom Water (AABW). The long term trend is

linked to erosional degradation of the onshore continental

escarpment, formed as a consequence of continental break-up.

No change is noted at 30 Ma, coincident with proposed uplift

of southern Africa driven by plume activity. The basin shows a

significant increase in sediment accumulation after 11 Ma,

which we interpret to reflect strengthening and rerouting of the

AABW from the south into Transkei Basin, as a far field effect

of the start of closure of the Indonesian Throughflow.

Keywords Reflection seismic � Sediment transport �Decompaction � Bottom currents

Introduction

The history of mass accumulation in any given ocean basin

can be used to understand the history of erosion in

neighboring landmasses, as well as the development of

bottom current activity that may be responsible for the

redistribution of river delivered sediments once they have

reached the deep ocean basins (Rebesco et al. 2014).

Marine sediment depocenters are typically the most com-

plete erosional records because they are more immune

from later erosion and reworking, compared to their con-

tinental equivalents. However, the archives preserved in

marine basins can be susceptible to the influences of

sediment buffering between the source and the sink, which

may lead to significant differences between the deep-sea

sediment record and the original processes acting on the

continental source regions (Simpson and Castelltort 2012;

Armitage et al. 2013). Regional sediment budgets have,

however, been used to understand long-term patterns of

erosion and test models for what might be controlling the

continental erosion over geological timescales (Metivier

et al. 1999; Clift 2006). Nonetheless, over timescales

greater than 1 m.y. and in regions where there are no

substantial basins into which sediment can be sequestered,

the marine sediment record may be our best opportunity to

date the delivery and recycling of sediment in the deep-sea.

In this study we address the deep-water sediments of the

Transkei Basin offshore southern Africa in order to look at

the history of erosion of the neighboring regions of South

Africa, as well as to understand how efficient bottom cur-

rents are at transporting sediment in this critical gateway

region between the South Atlantic and the Indian Ocean.

We exploit a survey interpreted by Schluter and Uenzel-

mann-Neben (2007) by applying standard back stripping

methods to quantify the rate of sediment delivery into the

basin. In particular it has been noted that although sig-

nificant volumes of sediment have been documented on the

continental margins of Southern Africa, in particular the

Outeniqua and Southern Outeniqua Basins, that these are

G. Uenzelmann-Neben

Alfred-Wegener Institute Helmholtz Center for Polar and Marine

Research, Bremerhaven, Germany

P. D. Clift (&)

Department of Geology and Geophysics, Louisiana State

University, Baton Rouge, LA 70803, USA

e-mail: pclift@lsu.edu

123

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DOI 10.1007/s11001-015-9250-3

Author's personal copy

insufficient to account for the volume of eroded rock in the

supposed source areas onshore (Tinker et al. 2008a). Our

estimates suggest that around 176,803 km3 of sediment lies

in the Transkei Basin compared to 268,500 km3 in these

basins. Without accounting for the Transkei Basin it is

clear that no complete understanding of erosion of the

continental margins during the breakup of Gondwana can

be successfully achieved. The new and complementary

observations presented and analyzed here, allow us to

better understand the relationship between climate change,

tectonics, erosion and sedimentation in South Africa.

Geological setting

The offshore region South of South Africa is dominated by

two features, namely the submarine Agulhas Plateau and

the Transkei Basin (Fig. 1a). The Agulhas Plateau is a

deep-sea plateau that rises up to 2000 m above the sur-

rounding oceanic crust seafloor that lies in water depths

around 4500 m. Between the Agulhas Plateau and the

South African continental shelf we find the *50 km wide

Agulhas Passage, which represents the only deep-water

gateway between the Indian and Atlantic Oceans in this

region (Fig. 1).

The Transkei Basin is a small deep-sea basin (*4500 m

water depth, consistent with oceanic crust older than

28 Ma, according to the model of Stein and Stein (1992)),

located East of the Agulhas Plateau (\2000 m water depth)

and Southwest of the Natal Valley (up to 4000 m water

depth; Fig. 1). To the East, the basin is limited by the

submarine Mozambique Ridge. The basin is filled with

sediments with a thickness of up to 1800 m (Schluter and

Uenzelmann-Neben 2007), although typically containing

slightly in excess of 1 km overlying the oceanic basement.

There are three deep-water paths into the Transkei Basin,

namely the Agulhas Passage in the Northwest, the Natal

Valley in the Northeast and a broad opening to the South of

the basin. Determining the age of the Transkei Basin’s

basement is challenging, as it was formed during Creta-

ceous magnetic quiet times (Fullerton et al. 1989). Tectonic

reconstructions indicate that the initial rifting between

South America and Africa took place around 124 Ma

(Goodlad et al. 1982; Tikku et al. 2002).

Based on estimates of plate movement velocities and

directions it is presumed that spreading within the Transkei

Basin stopped around 90 Ma (Martin and Hartnady 1986;

Ben-Avraham et al. 1993). The exact age of the Transkei

Basin sediments unfortunately cannot be determined be-

cause of the lack of any drill core information from the

abyssal regions off South Africa. The only accessible

borehole data provides information about the South Afri-

can continental shelf close to the city of Stanger,

approximately 700 km Northeast of the central Transkei

Basin (Du Toit and Leith 1974). Deep-sea sedimentation

started after the end of seafloor spreading in this region,

i.e., around 90 Ma. The following 90 m.y. of bottom cur-

rent activity and sedimentation history within the Transkei

Basin are mostly unknown.

At present the cold and dense North Atlantic Deep-water

(NADW) and Antarctic Bottom Water (AABW), as well as

the warm Agulhas Current (AC) have to pass the southern tip

of Africa. Off Southwest Africa, the NADW and a branch of

the AABW turn eastward and flow in an eastward direction

through the Agulhas Passage into the Transkei Basin.

Another branch of AABW flows northwestward around the

southern part of the Agulhas Plateau and enters the Transkei

Basin from the South (Figs. 1; 2). The AC enters the

Transkei Basin from the Natal Valley and flows southwest-

ward around South Africa into the Atlantic Ocean.

Data sources

The seismic data, which we used to define the sediment

budget in this study, was collected in 2005 by the Alfred

Wegener Institute Helmholtz Center for Polar and Marine

Research using the research vessel SONNE. These data

have been published and interpreted by Schluter and

Uenzelmann-Neben (2007) and we briefly summarize their

acquisition parameters here. Three GI-guns�, with a total

chamber volume of about 7.2 l, were used as the seismic

source. Each of the GI-guns� consisted of a generator

chamber (0.7 l volume) producing the seismic signal and

an injector chamber (1.7 l volume), which was triggered

with a 33 ms delay to suppress the bubble. The guns were

fired every 10 s (shot-spacing of approximately 25 m),

producing signals with frequencies of up to 300 Hz. The

guns were towed about 20 m behind the ship, 2 m below

the surface. Data were received using a high resolution

seismic data acquisition system (SERCEL SEAL�), con-

sisting of both onboard and in-sea equipment. The total

active streamer length was 2250 m, consisting of 180

channels, and additionally a lead-in cable length of

150–170 m (depending on the seismic profile). Processing

of the seismic reflection data comprised demultiplexing,

geometry definition using the ship’s navigation data (GPS),

and CDP- sorting with a CDP spacing of 25 m followed by

a detailed velocity analysis (every 50 CDP), NMO cor-

rection, stacking and depth migration using an Omega-X

algorithm. Examples of the processed and interpreted

profiles are shown in Fig. 3.

Age control is a significant issue in this area because

there are no nearby scientific drilling sites with which to

correlate the reflectors that can be traced across the basin.

As described by Schluter and Uenzelmann-Neben (2007)

Mar Geophys Res

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−5000

−5000−5000

−5000

−5000

−5000

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−4500

−4500−4000

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−4000

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−3500

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−3000

−3000

−3000

−2500

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−1500

18˚

18˚

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22˚

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26˚

28˚

28˚

30˚

30˚

32˚

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34˚

36˚

36˚

-44˚ -44˚

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-40˚ -40˚

-38˚ -38˚

-36˚ -36˚

-34˚ -34˚

-32˚ -32˚

-30˚ −30˚

Cape Town

Agulhas Bank

South Africa

Agulhas Plateau

Mozambique

A

Transkei

Agulhas Passage

Port Elizabeth Ridge

Basin

Figure 2

−1000

−500

−4500

−4000

−4000

26˚

26˚

28˚

28˚

30˚

30˚

32˚

32˚

-36˚ -36˚

-34˚ -34˚

AWI-20050001

AWI-20050017AWI-20050016

AWI-20050002AWI-20050014

B

AWI-2

0050

012

AWI-2

0050

010

AWI-2

0050

08AW

I-200

5006

AWI-2

0050

04

AWI-2005003

AWI-2005005

AWI-2005007

AWI-2005009

AWI-20050011

Outeniqua Basin

Natal V

alley

AABW

NADW

ACJ(C)-1

Fig. 1 a Bathymetric map of

the research area offshore

southern Africa showing the

bathymetry contoured at 500 m

intervals. Black lines show the

positions of the seismic

reflection profiles that we

consider in this study, which

were previously interpreted by

Schluter and Uenzelmann-

Neben (2008). Bathymetry data

is from Sandwell and Smith

(1997). The arrows show the

schematic flow paths of the

Agulhas Current (orange, AC),

the North Atlantic Deep-water

(magenta, NADW), and

Antarctic Bottom Water

(yellow, AABW). The black box

shows the area displayed in

Fig. 2. b Close-up map showing

the numbers of the lines

discussed in the paper and

showing how they cover the

central part of the Transkei

Basin

Mar Geophys Res

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the seismic reflection data cross one seismic line of Niemi

et al. (2000), who were able to correlate their data with a

borehole on the continental shelf (Du Toit and Leith 1974).

The seismostratigraphic model presented in Schluter and

Uenzelmann-Neben (2007) and used here hence should

represent the best approximation until this can be improved

by direct drilling of the basin, although uncertainties may

be significant because of problems inherent in long dis-

tance correlation of seismic reflectors. The existing

framework allows us to estimate the depositional age of the

different reflection packages based on their seismic ap-

pearance and relationship with overlying and underlying

packages.

Methods

Interpreted seismic reflection profiles were converted to

sediment volumes by converting the sections from a ver-

tical time axis into depth (Fig. 4). Of all stages in the

sediment budget estimation this process was the most

prone to error and results in volume uncertainties of up to

20 % due to uncertainties in the velocity-depth conversion.

Velocities used to make this conversion were taken from

the stacking velocities in the multichannel seismic profiles.

Decompaction methods (Sclater and Christie 1980; Kusznir

et al. 1995) are then applied to the sections in order to

restore each dated sediment body to its original thickness

prior to burial by the next unit. Knowledge of the sediment

type is important to this calculation because shales expe-

rience much greater loss of porosity during burial than do

sandstones (Allen and Allen 2006). Because we have no

information from drilling it is harder to make a correction

based on the lithology although in the deep ocean it is

unlikely that significant volumes of this material would be

Fig. 3 Examples of these seismic reflection data from the Transkei

basin interpreted by Schluter and Uenzelmann-Neben (2007). Data

have been filtered with filter flanks 5/10-200-250 Hz, no gain was

applied. The coloured reflectors show the primary dated horizons on

which we base our sediment budget. B black shale, E late eocene, K-T

cretaceous/tertiary boundary, M middle miocene, O eocene/oligocene

boundary, P early pliocene

cFig. 4 Interpreted and depth converted seismic reflection profiles

showing the overall age structure of the basin. Progressive backstrip-

ping of each sediment package and the compaction of these profiles

provides the basis for the sediment budget we develop here

24°E 26°E 28°E 30°E 32°E

39°S

38°S

37°S

36°S

35°S

34°S

[50 cm/s]

AD

Fig. 2 The modelled averaged present day velocity field in the

bottom layer after Li (2012), which represents the bottom currents. In

water depths 0–1500 m this is interpreted as AC, in water depths

2500–3500 m as NADW, in water depths [3600 m as AABW. The

transparent solid lines mark the isobaths. Computed boundary of the

Agulhas Drift (Schluter and Uenzelmann-Neben 2008) is marked by

black dashed line

Mar Geophys Res

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A

B

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sand sized. For the purpose of this exercise we assume that

the entire section is comprised of silt. Although we rec-

ognize that clay may have a significant and variable con-

tribution to the stratigraphy, this cannot be quantified until

coring has been undertaken in this region. Thus, although

the sediment volumes may be in error because of the silt

assumption used here this is impossible to correct for at the

moment and, since this is a systematic error, it can be

neglected and all the units are treated equally in terms of

making a compaction correction. It seems unlikely that the

entire sediment mass would comprise clay because of the

proximity to southern Africa and constraints from coring

elsewhere offshore this region that indicates that the sedi-

ments are dominantly siliciclastic and derived from the

continent. The prominent drift deposits seen in the shal-

lowest part of the section (Fig. 3; Line AWI-20050008,

Shotpoints 1500–3500) almost require that these sediments

are dominantly silt, since this is a characteristic of these

types of sedimentary structures (McCave et al. 1995).

Distal turbidites of the variety that underlie the contourites

are typically dominated by shale and siltstone outside

canyon systems (Kolla et al. 1992). Deep sea drilling

project (DSDP) Sites 360 and 361, which lie on the SW

continental rise offshore South Africa, recovered shales

between the Cretaceous and the top of the Eocene, with

more chalk in the younger section (Shipboard Scientific

Party 1978). Likewise Ocean Drilling Program Site 1088

on the Agulhas Ridge, located *900 km offshore,

demonstrated a pelagic carbonate cover for the Miocene to

Recent (Shipboard Scientific Party 1999). However, to the

east, on the far side of the Mozambique Ridge, within the

Mozambique Basin DSDP Site 250 revealed that the up-

permost several hundred meters of section are dominated

by silt and clays with only a minor biogenic component

(Shipboard Scientific Party 1974). Because this site is more

comparable to the Transkei Basin in terms of current ac-

tivity we argue that a dominantly fine-grained siliciclastic

character is most likely for the sedimentary cover.

The decompaction process involves accounting for the

loss of porosity of the sediment during burial, which would

otherwise result in an underestimation of deposited vol-

umes for the older, deeper buried sediment packages. After

the original, uncompacted volume of sediment in each

dated interval has been determined, the volume of rock

delivered during that time period can be calculated. The

process of decompaction followed routine basin analysis

methods, whose results are generally considered robust at

the first order level. In this study two-dimensional de-

compaction was calculated using the program Flex-De-

compTM (Kusznir et al. 1995). The areas of the youngest

sediment package on each profile were measured and then

this package was removed, allowing all the underlying

units to be decompacted. The area of the next youngest unit

was then estimated and the procedure repeated until the

entire sedimentary cover had been removed. In doing this

we follow the methods applied by Clift (2006) and Clift

et al. (2004).

Whole basin budgets were then calculated by adding

together the areas of the sediment for each specified unit in

all the profiles and normalizing the rates for each dated

interval in order to match the calculated total volume of the

basin, which we estimate at 176,803 km3, based on the area

(144,000 km2) and the mean 1.23 km thick sediment cover

of the basin. The mean thickness was calculated by looking

at the stratigraphic thickness every kilometer along each of

the profiles (Table 1). It must be assumed that the analyzed

profiles are fully representative of the total sediment flux in

the Transkei Basin. Although the lines do not cover the

whole basin they do provide some image of all of it,

especially the deepest parts with the most sediment.

Moreover, our approach means that the longest lines have

the greatest influence in controlling the final volume bud-

get, which seems reasonable as they account for the

greatest amounts of lateral variation.

Results

The results of the backstripping calculations are shown in

Table 1 and displayed graphically in Fig. 5. Although the

uncertainties are quite significant there is a clear trend to

decreasing sedimentation rates between the Cretaceous

when accumulation rates were high between 110 and

65 Ma, and much lower rates that are recorded between 34

and 11 Ma. It is this trend, rather than the absolute values

that we focus on in this paper. There is a steep increase in

rates after 11 Ma with maximum values being observed

after 5 Ma, although we do note that the major increase

occurs after 11 Ma and only increases slightly, if at all,

after 5 Ma. Very high rates are also observed between 36

and 34 Ma, although it is unclear whether this estimate is

real because the duration of that dated section is extremely

short. This means that only small amounts of sediment are

required to generate apparently high rates of sedimentation.

A modest error in the age model could thus result in very

high predicted rates at 36–34 Ma that may not be a reality.

Until drilling can establish a more robust age model for the

stratigraphy the significance of the 36–34 Ma peak remains

unclear.

Discussion

The temporal variations in sediment accumulation rates we

have reconstructed reflect a number of processes affecting

the continental source areas and neighboring ocean basins

Mar Geophys Res

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since 110 Ma. High rates following rifting of the basin are

a natural consequence of break-up and the erosional

degradation of the uplifted margins of the new rift. High

rates of sedimentation before 65 Ma in the Transkei Basin

are also consistent with independently determined sedi-

ment budgets from the Outeniqua and Southern Outeniqua

Basins, located adjacent to the southern coast of Africa

(Tinker et al. 2008a). Although the extra 176,803 km3 of

sediment identified in the Transkei Basin does help meet

the volume imbalance between estimated erosion depths

onshore in the supposed source areas and the deposited

volumes of 268,500 km3 estimated from the Outeniqua and

Southern Outeniqua Basins by Tinker et al. (2008a), this

extra sediment is insufficient to completely address the

deficit between eroded rock mass and deposited sediment

volume, which was estimated to be on an order of mag-

nitude scale (Tinker et al. 2008a).

Apatite fission track data from South Africa point

to [4.5 km of erosion from the edge of the continent,

seaward of the Drakensberg Escarpment, decreasing to

*2 km inland away from the coast, following the conti-

nental break-up in the middle Cretaceous (Brown et al.

2002; Raab et al. 2006). Modeling of fission-track data

from boreholes in the region of the Cape of Good Hope in

South Africa leads to predictions of increased denudation

in the mid-late Cretaceous (100–80 Ma; Tinker et al.

2008b). In contrast, a lack of Cenozoic apatite ages from

the same area suggests that major cooling was finished by

the end of the Cretaceous and that therefore sedimentation

rates in the offshore would be expected to fall because the

supply of freshly eroded bedrock was reducing. This fall is

seen in the Outeniqua and Southern Outeniqua Basins, as

well as in this new Transkei dataset (Fig. 5).

In western South Africa fission track data and modeling

allow the incision of major valleys cutting the Great Es-

carpment to be dated as having reached a maximum rate

between 120 and 110 Ma, followed by a tapering of rates

as thermal subsidence of the margin became the dominant

tectonic process (Kounov et al. 2008). Similar patterns and

timing of erosion are noted in neighboring regions, such as

Namibia (Cockburn et al. 2000; Raab et al. 2005). Further

north and east fission track data from Madagascar is con-

sistent with rifting of that block from southern Africa after

*155 Ma to form the Mozambique Basin (Reeves and Wit

2000; Seward et al. 2004). In this context, high but de-

creasing values of mass accumulation in the Transkei Basin

can be understood in terms of the erosional degradation of

a retreating escarpment, facing the newly formed passive

margin. Our reconstruction broadly supports models that

indicate higher topography in southern African dating from

Table 1 Summary of the decompacted areas of sediment for each of the backstripped profiles and conversion of the total into deposited

sediment volumes and rates for each of the dated time periods

Line number 0–5 Ma 5–11 Ma 11–34 Ma 34–36 Ma 36–65 Ma 65–110 Ma Mean sediment

thickness (km)

AWI-20050001 28.3 49.5 26.3 14.9 85.3 197.5 1.1785

AWI-20050002 2.2 8.0 13.6 6.5 16.1 67.2 1.1785

AWI-20050003 9.1 16.9 20.5 9.3 32.1 95.4 1.1830

AWI-20050004 9.9 14.7 18.8 5.2 25.2 87.4 1.1771

AWI-20050005 12.7 16.2 15.9 7.9 28.6 87.5 1.1514

AWI-20050006 10.5 11.0 8.0 3.7 18.1 53.3 1.2190

AWI-20050007 18.8 16.8 13.1 3.9 33.9 79.3 1.1951

AWI-20050008 19.8 19.2 12.6 6.0 34.8 85.9 1.2421

AWI-20050009 14.5 13.2 6.8 4.0 27.7 50.2 1.2375

AWI-20050010 20.4 20.6 8.4 5.5 31.7 72.0 1.2080

AWI-20050011 17.8 16.4 8.5 4.6 32.0 63.1 1.2939

AWI-20050012 21.0 22.1 9.7 2.9 44.0 96.1 1.3576

AWI-20050013 4.4 6.5 3.3 1.5 8.5 33.9 1.1317

AWI-20050014 22.1 32.9 19.8 6.7 58.3 131.8 1.1785

AWI-20050015 51.7 42.9 20.1 10.7 74.9 163.8 1.3142

AWI-20050016 50.4 35.8 27.0 10.9 85.1 180.9 1.3984

Grand area total (km2) 313.6 342.7 232.3 104.1 636.4 1545.3

Actual volume (km3) 17,471 19,092 12,939 5802 35,448 86,076

Sedimentation rate (km3/m.y.) 3494 3182 563 2901 1222 1913

Minimum rate (km3/m.y.) 2795 2546 450 2321 978 1530

Maximum rate (km3/m.y.) 4193 3818 675 3481 1467 2295

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the Mesozoic (Brown et al. 2002; Doucoure and de Wit

2003; de Wit 2007), in contrast to those arguing for large

scale uplift in the Oligocene, possibly driven by pinning of

the African Plate in the hot-spot reference frame by a

number of deep-seated mantle plumes at that time (Du Toit

1937; Burke 1996; Burke and Gunnell 2008). In fact, our

data show no evidence for increased sedimentation at

around 30 Ma when the plume-related surface uplift is

believed to have occurred and when the Orange River show

a major increase of sediment flux into the region of its

modern delta (Guillocheau et al. 2012; Paul et al. 2014). A

lack of sediment in the Transkei Basin does not necessarily

mean that regional surface uplift did not start at that time.

Indeed, the fast sediment flux into the Orange Delta argues

that this is likely, especially given evidence that the Orange

River drainage has been stable through the Cenozoic (De

Wit 1999). In this case we infer that sediment supply was

either being diverted into the Orange River at that basin

captured drainage closer to the coast, or that sediment was

reaching the Transkei Basin but was being removed and

redeposited towards the northeast into the Indian Ocean by

bottom current activity.

The final opening of the Drake Passage gateway

*35 Ma (Livermore et al. 2005) resulted in Antarctica

becoming isolated and glaciated, thus leading to an

intensification of AABW inflow into the South Atlantic

(Diester-Haass et al. 1996). Schluter and Uenzelmann-

Neben (2008) argue that it is strengthening of the AABW

that results in a change in sedimentation style in the

Transkei Basin from a quieter, pelagic setting to a more

current-dominated regime after 36 Ma. While this change

in flow regime has the potential to deliver more sediment to

the basin and could explain the start of rapid deposition

from 36 to 34 Ma this mechanism would not account for

the subsequent fall in sedimentation rates after 34 Ma,

which would be expected remain high while the AABW is

strong. Other indicators do not show a short-lived phase of

AABW flow at that time, but rather the start of a long-lived

regime (Billups et al. 2002) that would be expected to

deliver sediment into the Transkei Basin through the

Agulhas Passage, close to the southern African continental

margin (Fig. 2). If the 36–34 Ma pulse sedimentation rate

is an artifact then the lower rates after 34 Ma can be seen

as merely the continuation of slowing sediment supply

after continental breakup. However, it is possible that

continued strong AABW flow would be capable of trans-

porting sediment, potentially derived from South Africa,

out of the Transkei Basin and into the Indian Ocean, de-

pending on the route that the strongest parts of the current

took. Unfortunately we lack data to test whether the sedi-

ment is maybe being transported towards the Northeast out

of the Transkei Basin after 34 Ma.

Our confidence in the authenticity of the 36–34 Ma

spike in sediment delivery is thus somewhat low, unless the

current was only able to deliver modest amounts of mobile

sediment from the deep-water areas to the SW before de-

pleting the supply, or that the phase of faster accumulation

was actually longer lasting, but this deposit was subse-

quently reduced by further bottom current scouring, as

would be consistent with the identification of a regional

disconformity at 34 Ma across the Transkei Basin (Schluter

and Uenzelmann-Neben 2007). Remobilized sediment

could be transported to the NE into the Indian Ocean by the

AABW. It is quite possible that the 36–34 Ma peak in

sedimentation rates is an artifact of the poor age control

and the short duration of the interval concerned, although

the identification of a synchronous spike in sediment ac-

cumulation in Western Africa (Lavier et al. 2001) and

offshore New Jersey (Steckler et al. 1999) raises the pos-

sibility that this is a true event which will have to be

confirmed by local drilling in order to improve the age

model.

In the same way that a short depositional period can

result in an overestimation of sediment flux for that time,

longer periods of deposition are prone to underestimation

and a lack of resolution in identifying shorter periods of

rapid accumulation (Sadler 1981; Gardner et al. 1987;

Sadler and Jerolmack 2014). Thus, the inference that

Age (Ma)

Sedi

men

tatio

n ra

te (k

m3 /

my)

Formation of Agulhas Plateau

0

1000

2000

3000

4000

5000

0 20 40 60 80 100 120

Strengthening of AABW

AABW flow from the south

Stronger AABW

onset of closure of

Indonesian Throughflow

Outeniqua Basin

0

1000

2000

4000

3000

Transkei Basin

Fig. 5 Proposed sediment budget for the Transkei Basin. Note the

progressive long-term foreign sedimentation rates with a sharp

increase since around 11 Ma. Gray shading shows the uncertainty

in the calculation. Lower panel shows sedimentation rates from the

Outeniqua Basin located closer to the South African margin (Tinker

et al. 2008a) with uncertainty estimates of ±20 % added

Mar Geophys Res

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sedimentation was slow between 34 and 11 Ma does not

preclude short periods of rapid sedimentation. The three

longer time periods estimated in our budget are however

resolvably different from one another despite being of

similar longer duration and so we consider the differences

between these to be real.

The increased sedimentation rate since 11 Ma is more

complicated to explain because this change significantly

postdates the 15 Ma weakening of the AABW and its

change in direction to dominantly entering the Transkei

Basin from the south, east of the Agulhas Plateau, after that

time (Schluter and Uenzelmann-Neben 2007). Increased

sedimentation rates are well known on a global basis after

4–3 Ma, a phenomenon that is usually tied to higher con-

tinental erosion rates driven by the rapid switching from

glacial to interglacial conditions since that time (Zhang

et al. 2001), but that climate change is too young to explain

the much more dramatic change after 11 Ma seen in the

Transkei Basin. Although some continental margins and

basins do show increasing sediment flux after around

10 Ma (Paul et al. 2014) the size of that increase is not of

the same magnitude as that observed in this work (Molnar

2004). Indeed, the increase after 11 Ma also contrasts with

the relatively low rates seen in the more landward

Outeniqua and Southern Outeniqua Basins (Tinker et al.

2008a) and which suggest that the sediment pulse may not

be coming directly from the southern African landmass.

We note that deep-water currents favor sediment delivery

from the south. Li (2012) modeled an AABW eddy in the

Transkei Basin that points towards material transport from

the south along the eastern flank of the Agulhas Plateau and

not from South Africa (Fig. 2). Whether the low values for

the Outeniqua and Southern Outeniqua Basins are correct

is open to question because the Tinker et al. (2008a) study

does not have age resolution within the Cenozoic and

would be unable to pinpoint a separate increase in sediment

supply after 11 Ma. We suggest that the increase in sedi-

ment flux after 11 Ma is largely a function of changing

current velocities and pathways accentuated since the onset

of the Pliocene by stronger continental erosion and a

stronger AABW. Some of this sediment may be transported

long distances from the south by bottom current activity.

One possible agent for increasing sediment supply to

Transkei Basin would be the Miocene Indian Ocean

Equatorial Jet (MIOJet), which was established at *14 Ma

as a consequence of the closure of the Indonesian

Throughflow. The MIOJet has been reported to have

strengthened and modified properties of both surface cur-

rents (e.g., the Agulhas Current), and deeper circulation in

the Indian Ocean (Gourlan et al. 2008). This has resulted in

increased reworking of sediment on the continental margin

of southeastern Africa and on the very narrow shelf, as well

as an increase in current-controlled sedimentation

(Uenzelmann-Neben et al. 2011). The MIOJet is recon-

structed as having increased in strength from 14 Ma to

9 Ma, which should have increased the sediment load

transported to the southwestern Indian Ocean, after which

time it remained relatively stable until 4 Ma (Gourlan et al.

2008). Sedimentation rates in Transkei Basin are only re-

constructed to increase after 11 Ma but we do not consider

the time lag between 14 and 11 Ma to be significant. The

sharp difference seen in our sediment budget after 11 Ma

likely reflects the relatively long-duration of the period

from 34 to 11 Ma compared to the following 11 to 5 Ma.

Even if sediment supply increased sharply after 14 Ma this

would not have been sufficient to increase the overall av-

erage rate for the 34–11 Ma period. Consequently, we do

not think it is significant that the increasing sedimentation

rate slightly postdates the onset of the stronger MIOJet.

Nd isotope analysis of ferromanganese crusts form the

southern Mozambique Ridge distinctly resolve the influ-

ence of NADW in water depths [2500 m and AAIW in

shallower water depths after 9 Ma (Heuer 2009; Uenzel-

mann-Neben et al. 2011). A decreasing NADW influence is

inferred for the southern Mozambique Ridge after *5 Ma,

which points to a relocation of its pathway at that time.

This may have been a result of the replacement of the

MIOJet by the seasonal Asian Monsoon as the dominant

wind across the Indian Ocean, with consequences for ocean

currents across the region, including Transkei Basin (Kuhnt

et al. 2004; Gourlan et al. 2008).

Conclusions

In this study we perform a sedimentary budgeting exercise

for the Transkei Basin offshore South Africa. The work

was based on a previously interpreted seismic reflection

survey that covers the main depocenters and is believed to

provide a good estimate of sediment accumulation since

*110 Ma. Standard two-dimensional backstripping meth-

ods were applied to the seismic profiles in order to account

for burial compaction and these were then integrated in

order to provide a volumetric budget for the basin, albeit

one of low temporal resolution caused by the lack of deep

drilling of the stratigraphy. Our analysis reveals generally

decreasing rates of sediment supply from 110 to 11 Ma,

reflecting the erosional degradation of the rifted margin

escarpment onshore in the wake of continental break-up. A

peak in sedimentation rates at 34–36 Ma may be an artifact

of the poor age control but could be linked to a strength-

ening of the AABW at that time. No change is noted in

relation to proposed uplift of southern Africa at *30 Ma,

raising questions as to whether this event really occurred.

Sedimentation rates increased sharply after 11 Ma, which

is too early to be linked to the typical increase seen

Mar Geophys Res

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worldwide and linked to the onset of Northern Hemispheric

Glaciation. Instead we suggest that the increase reflects a

change of bottom current flow to being from the south and

not the west, as before. This change is related to the

strengthening of the atmospheric MIOJet and in turn to the

start of closure of the Indonesian Throughflow.

Acknowledgments PC acknowledges support from the Charles T.

McCord Chair in Petroleum Geology. Alan Roberts and Nick Kusznir

are thanked for letting us use their backstripping software ‘‘FlexDe-

comp’’. The seismic data forming the base of this paper have been

collected as part of project AISTEK-I funded by the German Ministry

of Education and Research (BMBF) under contract No 03G0182A.

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