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Journal of African Earth Sciences 111 (2015) 288e295

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Journal of African Earth Sciences

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Sedimentary budgets of the Tanzania coastal basin and implicationsfor uplift history of the East African rift system

Aymen Said a, *, Christoph Moder a, Stuart Clark a, Mohamed Mansour Abdelmalak b

a Kalkulo AS, Simula Research Laboratory, Martin Linges vei 25, Fornebu, 1364, Norwayb Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway

a r t i c l e i n f o

Article history:Received 1 July 2015Received in revised form12 August 2015Accepted 14 August 2015Available online 18 August 2015

Keywords:Sediment fluxTanzania coastal basinUplift historyEast African rift system

* Corresponding author.E-mail address: aymen@simula.no (A. Said).

http://dx.doi.org/10.1016/j.jafrearsci.2015.08.0121464-343X/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Data from 23 wells were used to quantify the sedimentary budgets in the Tanzania coastal basin in orderto unravel the uplift chronology of the sourcing area located in the East African Rift System. We quan-tified the siliciclastic sedimentary volumes preserved in the Tanzania coastal basin corrected forcompaction and in situ (e.g., carbonates) production. We found that the drainage areas, which suppliedsediments to this basin, were eroded in four episodes: (1) during the middle Jurassic, (2) during theCampanianePalaeocene, (3) during the middle Eocene and (4) during the Miocene. Three of these higherosion and sedimentation periods are more likely related to uplift events in the East African Rift Systemand earlier rift shoulders and plume uplifts. Indeed, rapid cooling in the rift system and high denudationrates in the sediment source area are coeval with these recorded pulses. However, the middle Eocenepulse was synchronous with a fall in the sea level, a climatic change and slow cooling of the rift flanksand thus seems more likely due to climatic and eustatic variations. We show that the rift shoulders of theEast African rift system have inherited their present relief from at least three epeirogenic uplift pulses ofmiddle Jurassic, CampanianePalaeocene, and Miocene ages.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

The volumes of siliciclastic sedimentary rocks preserved in themarginal basins represent good archives of past uplifts and to-pographies over the continent, and are therefore a key to constrainthe timing and amplitude of vertical movements, and consequentlythe mechanism(s) responsible of epeirogenic movements. Here weaim to unravel the vertical motion history of the catchment arealocated at the East African Rift System which supplies sediment tothe Tanzania coastal basin. Only few estimates of sedimentationrates in the Tanzania coastal depocentres fed by the East African RiftSystem are published (Macgregor, 2010). They are based on a singlecross-section and have therefore a low level of confidence. Based onan interpolation between 23 exploratory wells, we carried out avolumetric study of the sediments preserved in the Tanzaniacoastal basin from the mid-Jurassic onwards. The presented resultsprovide a better estimate of the sediment fluxes. We then investi-gate the implications of our findings for the uplift timings and thetopographic building of the East African rift system.

2. Geological setting

2.1. Rifting and uplift history of the East African rift system

The East African rift system (EARS) is a major topographicfeature in East Africa. It is composed of two branches: the easternbranch and the western branch separated by the East AfricanPlateau (alternatively known as the Tanzanian plateau; Fig. 1). Themechanistic links between topography and rifting as well as thetiming and magnitude of the uplift of the East African Plateau aredebated (Wichura et al., 2010). Available geological and geophysicaldata support the presence of limited relief of less than 1000m priorto the onset of volcanism in the EARS (Smith, 1994). The earliestrecorded volcanic activity took place ~40e45 Ma in the northernTurkana depression (Fig. 1; e.g., Ebinger et al., 1993; George et al.,1998; Knight et al., 2003; Furman, 2007). This was followed bythe formation of the Gulf of Aden and Red Sea rifts, which separatedthe Nubian Plate from Arabia at approximately ~35e27 Ma(Schilling et al., 1992; Pik et al., 1999). Intracontinental rifting inEast Africa is believed to have commenced in the region sur-rounding Lake Turkana at ~25 Ma, with volcanism and fault prop-agation moving southward through the Eastern and Western Riftbranches into Mozambique, while simultaneously propagating

Fig. 1. Topographic/bathymetric map of the East African rift system (ETopo1; Amante and Eakins, 2009) showing its two branches (the Western branch and the Eastern branch)separated by the East African Plateau. The figure shows also the location of the Tanzania coastal basin and the boreholes used in this study. EARS: East African rift system; EAP: EastAfrican Plateau.

A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295 289

northwards into Ethiopia (Watson et al., 2012). The northwardspropagation connected the East African Rift Systemwith the Gulf ofAden and Red Sea spreading centers, creating the Afar TripleJunction at ~11 Ma (Smith, 1994; Chorowicz, 2005; Furman, 2007;Watson et al., 2012). The formation of this Triple junction waspreceded by the arrival of the Afar plume at the start of theOligocene (Ebinger and Sleep, 1998).

In the western rift of the EARS, Apatite fission track analysisshow 3 periods of accelerated cooling and denudation (Van derBeek et al., 1998). The oldest denudation event is recorded ataround 250e200 Ma and corresponds to late Karoo erosion. Asecond denudation phase, corresponding probably to a renewedrifting, was recorded in Late JurassiceEarly Cretaceous (~150 Ma).The youngest denudation event seems to start since ~40 Ma with amajor part taking place since 20 Ma. In the Malawi and Rukwa riftflanks (Fig. 1), the denudation of the Late Karoo and the LateJurassiceEarly Cretaceous events is estimated to 2.0 ± 0.4 km each,however the Cenozoic denudation is estimated to less than1.2 ± 0.2 km (Van der Beek et al., 1998).

In the eastern branch of the EARS, thermal history modeling ofapatite fission-track and helium data (Spiegel et al., 2007) shows arapid rock cooling during Late Cretaceous (~85e40 �C) and LateNeogene (40 �C to surface temperature). During this later period,the average denudation rates range between ~0.2 and 0.4 km/Myr.In eastern Tanzania, Noble et al. (1997) show a wide variation inapatite fission track age from 221 ± 11 to 48 ± 3 Ma, and identifythree periods of accelerated cooling: during the Early Cretaceous,Late CretaceouseEarly Paleogene, and Late EoceneeEarly Oligo-cene, In northern Tanzania, Mbede (2001) identifies three periodsof rapid rock cooling: during the Early Jurassic (~195e159 Ma), LateCretaceous (~81e75 Ma), and Early Cenozoic (~64e48 Ma).

It is widely admitted that the EARS developed above a mantle

plume initiated in southwestern Ethiopia and sourcing the volca-nism in the region (e.g., Duncan and Richards, 1991; Ebinger andSleep, 1998; Ring, 2014). Most of these studies admit that the up-lift of the EARS is mainly related to the plume effect. However, mostof the episodes of fast rock cooling are correlated with periods ofextension in the African plate and development and reactivation ofhigh-angle faults (Noble et al., 1997; Foster and Gleadow, 1992,1996). They are also synchronous with accelerated deposition ofsediments in the East African basins like in the Selous basin(Wopfner and Kaaya, 1991). It seems that the uplift of the EARSoriginated from both volcanism and rifting.

2.2. Stratigraphy in the Tanzania coastal basin

The Tanzania marginal basin is composed of the Tertiary sedi-ments associated to the fluvio-deltaic system, the underlying syn-rift Mesozoic sediments associated to the East Africa passivemargin onset and the older pre break-up system (Fig. 2). Thissedimentary column is characterized by several megasequencesbounded by major unconformities recognized at Base Pliocene,Base Miocene, Base Middle Eocene, Base Palaeocene, Base LateCretaceous, and Base Middle Jurassic (Break-Up Unconformity;Fig. 2). The stratigraphy of the coastal basin of Tanzania has beendescribed by a number of authors (Kent et al., 1971; Kajato, 1982;Mpanda, 1997; Kapilima, 2003). The Triassic to Early Jurassic sedi-ments in the coastal basin of Tanzania consists of mainly fluviatilesandstones (Fig. 2). They unconformably overlie the Precambriangneissic basement in both Selous-Tanga and Mandawa sub-basins(Fig. 3). These two sub-basins contain Karoo rift sequences corre-sponding to different depositional environments. In the Selous-Tanga sub-basin, the Karoo rift sequence is represented by fluvia-tile sandstones resting on basal conglomerates. In the Mandawa

Fig. 2. Composite stratigraphic column of the Tanzania coastal basin (modified after Orca Exploration Group Inc, 2010).

A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295290

sub-basin, the Karoo rift sequence is formed by evaporitic sequence(gypsum, halites and anhydrite) with silty shales. This lithofaciesvariation reflects difference in rates of subsidence and sedimentarydepositional environments (Kapilima, 2003). The evaporiticsequence developed in a more rapidly subsiding part of the basinallowing a restricted marine incursion (Sabkha environment;Mpanda, 1997). The coastal basin of Tanzania became a continentalshelf during the AalenianeBathonian (174e166 Ma) when the firstmarine transgression overlapped the continental Karoo. Westward,the transgression was limited by the Tanga and Lindi faults(Mpanda, 1997; Fig. 3). The depositional environment varied fromshallow marine to neritic. In some localities, the Middle Jurassicsediments rest unconformably on the basement. The Late Jurassic ismarked by accumulation in a low energy sedimentary environment(mostly marine shales) in the Mandawa basin. The deepening ofdepositional environments is evidenced by the occurrence of morepredatory marine organisms such as ammonites and nautilus(Kapilima, 2003). The thickness of the sediments in general de-creases southward. During the ValanginianeMiddle Aptian(~140e120 Ma), a regression cycle started, accompanied by the

deposition of fluvial sediments along the shoreline, forming pre-dominantly sandstones. In the Aptian-Albian period, a transgres-sional cycle started (Mpanda, 1997). The Late Cretaceous wasmarked by a major transgression (Mpanda, 1997). An argillaceousfacies, associated with a deep marine domain, dominates thisperiod (Fig. 2), and probably results from a regional rise in the sealevel (Haq et al., 1987). The Transgression series which started inthe Aptian-Albian continued in the Palaeocene (Fig. 2). The Palae-ocene deposited sediments are mostly deep marine, consisting ofclay and claystone with thin bands of fine sandstone. However, thedeltaic type of sedimentation continued along the track of thepresent Rufiji River (Fig. 5). Between Middle Eocene and Oligocene,a regressive phase occurred in the region (Kent et al., 1971; Kentand Pyre, 1973). The Lower Eocene sediments mainly consist ofshales, sandstones and carbonate rocks. The Middle and LateEocene comprises a sandy deposit transported from the erodedhigh relief basement areas in the west. It constitutes the gasreservoir in the Zanzibar, Pemba and Mafia grabens (Fig. 3). Asubsidence process allowed marine water to intrude the area andprovided convenient conditions to establish the Late Eocene

Fig. 3. Major structural features of coastal Tanzania with the main faulting zones and ages (modified after Kajato, 1982).

A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295 291

carbonate platform. The Oligocene marine regressional sedimentsare of relatively restricted occurrence in the coastal belt of Tanzania.During the Neogene, the reactivation of the old Tanga and Lindifaults was coeval with the opening of Eastern branch of the EARS(Mpanda, 1997). Consequently, the maximum subsidence in thePemba, Zanzibar and Mafia channels occurred at that time (Fig. 3).

3. Data and methods

The Tanzania coastal basin covers the Selous-Tanga and theMandawa sub-basins (Fig. 3). The boundaries of the Tanzaniacoastal basin have been refined based on geophysical (gravimetry)and geological (isopachs and structural features) data (Fig. 4). Thebasin is limited to the West by the Tanga and the Mandawa faultswhich separate it from the basement (Figs. 3 and 4b). To the East,the basin is bordered by a relatively low gravimetric feature parallelto the Davie Fracture Zone and situated to the east of the isopach6 km (Fig. 4b). The Tanzania coastal basin is laterally adjacent to theLamu basin in the North and to the Ruvuma basin in the South.Presently, this basin is connected to the Rufiji catchment basinwhich supplies its central part with sediments. The southern part ofthe basin is fed by the Ruvuma catchment (Fig. 5).

Data from 23 wells drilled in the Tanzania coastal basin wereused to estimate the volume of sediment accumulated fromMiddleJurassic to Present times. To do so, we followed a methodologicalapproach, which defines 13 time-intervals corresponding todifferent litho-tectonic units, as reported in the stratigraphic col-umn in Fig. 2. From the youngest to the oldest, the time-intervalsare reported in Table 1.

To determine the evolution of the sedimentary flux and calcu-late changes in the sedimentation rate, the volume of sedimentpreserved in each formation was quantified using interpolationbetween the well data. The quantification of terrigenous sedimentvolumes requires two main corrections: a correction for carbonatesproduced in situ; and a correction for the sediment compactionexerted by the overlying sediment layers (Said et al., 2015).

- Compaction correction

The sediment decompaction was calculated based on theporosity-depth law in Allen and Allen (2013). This law assumes thatdensity changes of a sedimentary unit are caused only by changesin the pore space (ignoring, for example, diagenesis) and that theporosity, f (the ratio between pore space and sediment), is

Fig. 4. (a) Data used in the delineation of the Tanzania coastal basin boundaries: isopachs and faults (provided by Statoil AS) and free air gravity of Sandwell and Smith (2009). (b)Zoom on the Tanzania coastal basin. Davie FZ: Davie Fracture Zone.

A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295292

decreasing exponentially with the depth, z:

fðzÞ ¼ f0 expð�c$zÞ;

where f0 is the porosity at the surface and c is a lithology-dependent coefficient that determines the slope of the porosity-depth curve. In the well-logs used in this study, the following li-thologies are present: sandstone, shale, limestone and shalysandstone. For each lithology, we used the f0 and c values asdescribed by Sclater and Christie (1980).

The total pore volume of a sedimentary layer is given by inte-gration over the depth interval:

Dzpore¼Zztop

zbottom

f0expð�c$zÞdz¼f0c

�exp

��cztop��expð�czbottomÞ

�:

The decompacted thickness of a layer is its net thickness plusthis depth-dependent pore volume. The decompacted thickness isdetermined by an iterative approach: decompacting a layer,removing it and then decompacting the layers beneath it.

- Carbonate correction

To calculate the terrigenous sediment volume, the non-terrig-enous contribution is removed from the total sediment in the basin.In our case, carbonate deposits are the main non-terrigenous sed-iments present and we assume all carbonates are produced in situ,given the shallow water depths associated with the carbonates inthe well-logs. The thickness of carbonate layers for each time in-terval at each borehole were estimated based on information fromthe available well-logs. The well data was interpolated to a gridcovering the studied region at 0.01 resolution, excluding valuesoutside the defined limits of the Tanzania coastal basin. The

Fig. 5. Map of East African drainage system and major related drainage basins. Pink dots are location of the boreholes used in this study. (For interpretation of the references tocolour in this figure legend, the reader is referred to the web version of this article.)

A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295 293

interpolation is a two-stage process: first combining the well-datafor each 0.2 block as a barycentric approach on a course grid with0.2 grid spacing, using aweighted combination of the data from thenearest three available wells or boundary points. In the case of thesediment thickness, the thickness was assumed to be zero on theboundary and the barycentric interpolation included at most twoboundary values. In the second stage, the course grid was refined to0.01 resolution using a Delaunay triangulation.

4. Results

The sedimentary flux is the ratio between a volume of sedimentand the time period required for its deposition. Fig. 6 show aMiddleJurassic high sedimentation rate (4.9 � 103 km3/Myr) whichdropped during the Late Jurassic to ~1.9 � 103 km3/Myr. The EarlyCretaceouseSantonian (T3 to T5) was characterized by low sedi-mentation rates (~1.2 � 103 km3/Myr). After this long sedimentstarvation, three periods of high sedimentation rate have beenrecorded. The first is of CampanianePalaeocene age (T6eT7) and ismarked by the presence of basal conglomerates (Fig. 6). The

Table 1Time periods used in the sedimentary budgets analysis.

Label Epoch From age (Ma) To age (Ma)

T13 Plio-Pleistocene 5.3 0 (Present)T12 Miocene 23 5.3T11 Oligocene 33.9 23T10 Late Eocene 38 33.9T9 Middle Eocene 47.8 38T8 Early Eocene 56 47.8T7 Palaeocene 66 56T6 CampanianeMaastrichtian 83.6 66T5 ConiacianeSantonian 89.8 83.6T4 CenomanianeTuronian 100.5 89.8T3 Early Cretaceous 145 100.5T2 Late Jurassic 163.5 145T1 Middle Jurassic 174.1 163.5

maximum of sedimentation rate in this period was reached duringthe Palaeocene (~3.2 � 103 km3/Myr). The second period (T9) wasrecorded during the Middle Eocene (~3.9 � 103 km3/Myr), and thethird one (T12) was recorded during the Miocene (~2.5 � 103 km3/Myr; Fig. 6).

5. Discussion

Increases in clastic sediment accumulation rates are related toeither a tectonically driven uplift in the drainage basin, a climaticchange toward more seasonal and erosive conditions, an eustaticsea-level fall exposing the continental shelf, or a combination ofthese parameters. Differentiating between the tectonic, climaticand eustatic causes of recorded high sediment fluxes is oftendifficult (Bonnet and Crave, 2003).

The high sedimentation rate recorded in the Tanzania coastalbasin during the Middle Jurassic is contemporaneous with theonset of rifting between East Africa and the Mada-gascareIndiaeAntarctica landmass. Sediment supply during thisperiod originated from the erosion of the rifting associated uplift.The fall of the flux from 4.9 � 103 km3/Myr during Middle Jurassic(T1) to 1.9 � 103 km3/Myr during Late Jurassic (T2) probably showsthat the major part of the rifting-associated relief was erodedduring the Middle Jurassic.

In the latest Cretaceous, the recorded high sedimentation ratewas synchronous with a basal conglomerate recorded at the San-tonian/Campanian transition which indicates a tectonic upliftaffecting the source areas during that time. This event is describedin many parts of Africa and known as the Santonian tectonic pulse(83 ± 2 Ma) attributed to the onset of the collision between theAfricaneArabian and Eurasian plates (Burke and Dewey, 1974;Guiraud and Bosworth, 1997; Burke and Gunnel, 2008). The sedi-mentary supply issued from this tectonic pulse was reinforcedduring the latest CretaceousePalaeocene by sediments erodedfrom the Anza rift shoulders (Fig. 4) which have been rejuvenatedduring the 70e60 Ma interval (Foster and Gleadow, 1992, 1996;

Fig. 6. Sediment fluxes, corrected for compaction and in situ (e.g. carbonates) production, preserved in the Tanzania coastal basin (in km3/Myr). The purple arrows indicate themajor unconformities bounding the megasequences. The green arrow indicates the presence of basal conglomerates. The curve of Sea level variations is taken from Miller et al.(2005) and the curve of the d18O and the deep Ocean temperature are taken from Zachos et al. (2001). The timings of the rapid cooling of the rift shoulders are taken fromSpiegel et al. (2007). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295294

Spiegel et al., 2007; Bosworth and Morley, 1994). This erosionalevent is not seen in the Anza basin or surrounding area, whichsuggests that the shoulders of the Anza rift were topographicallyhigh (Macgregor, 2015). The rejuvenated rifting was due to anextensional stress regime in Africa resulting probably from thedifference of seafloor-spreading rates between the Central and theSouth Atlantic (Fairhead and Binks, 1991).

The rapid rock cooling recorded by fission-track and helium dataduring the Late Cretaceous (~85e40 �C; Spiegel et al., 2007; Mbede,2001) and the Late Cretaceous-Early Paleogene (Noble et al., 1997;Foster and Gleadow, 1992, 1996) is in good agreement with ourfindings. The high sedimentation rate recorded during the middleEocene time is not coeval with any rapid rock cooling (Spiegel et al.,2007). The fission track ages corresponding to the youngest periodof accelerated cooling proposed by Noble et al. (1997; from 88 ± 6to 48 ± 3 Ma) and Mbede (2001; from 64 ± 6 to 48 ± 4 Ma) and byVan der Beek et al. (1998; from ~40 to 30 Ma) slightly ovelap themiddle Eocene time interval (48e38 Ma), if we consider the stan-dard deviation of the fission track ages. However, it is clear that theoverlap is partial and that most of the middle Eocene correspondsto a period of slow rock cooling. Moreover, the curve of eustaticvariations (Miller et al., 2005) shows a decrease in the sea levelduring the Eocene time synchronous with a decrease of the Oceantemperature as it is evidenced by the d18O measurements (Zachoset al., 2001; Fig. 6). Thus, the middle Eocene sedimentary pulseseems more likely related to changes in the climatic and eustaticconditions. It is worth noting that a change in the lithologyoccurred between early and middle Eocene towards more sandydeposits constituting one of the gas reservoirs in the Zanzibar,Pemba and Mafia channels (Fig. 3). This could be a direct effect of asea level drop and a related forced regression.

The high sedimentation during the Miocene was synchronouswith an accelerated cooling and denudation in the western rift ofthe EARS (Van der Beek et al., 1998). It coincided also with analkaline volcanic activity in central Kenya. This later started at~20 Ma, and most activity occurred between 16 and 8 Ma (Smith,1994). In the Lindi area (Southern part of the Tanzania coastal ba-sin; Fig. 3), erosional truncation between base Miocene and topOligocene are described in the channels (Mpanda, 1997).

During the Miocene, Eastern Africa was subjected to strongannual monsoonal cycles which accentuated the denudation andsedimentation rates. The Indian Ocean monsoonwas established at

~15 Ma (DeMenocal, 1995) and accentuated at ~7e8 Ma (Prell andNiitsuma, 1989; Quade et al., 1989) due to the closing of the Zagrosseaway north of Arabia. During the Plio-Pleistocene, Eastern Africaunderwent an aridification which is attributed either to the closureof the Indonesian seaway (Cane and Molnar, 2001) or to the closingof the Isthmus of Panama (Haug et al., 2001). This aridification wasprobably at the origin of the decrease in the sedimentation rateduring that time. The Ethiopian Highlands situated in the northernpart of the EARS and constituting themajor supplier of sediment forthe Nile delta seems not to be affected by this aridification as it isevidenced by the considerable increase of sediment flux in the Nileduring the Plio-Pleistocene (Macgregor, 2012).

6. Conclusions

Here we used well data to quantify the evolution of the sedi-mentary budgets in the Tanzania coastal basin in order to unravelthe uplift chronology of the sourcing area located in the East AfricanRift System. Our data identify four major periods of high sedi-mentation rate: (1) during the middle Jurassic with a rate of4.9 � 103 km3/Myr, (2) during the CampanianePalaeocene with arate of ~3.2 � 103 km3/Myr, (3) during the middle Eocene with arate of ~3.9 � 103 km3/Myr and (4) during the Miocene with a rateof ~2.5 � 103 km3/Myr. Three of these sedimentation pulses aremore likely related to uplift events in the East African Rift Systemand earlier rift shoulders and plume uplifts. Rapid cooling in the riftsystem and high denudation rates in the sediment source area arecoeval with these recorded sedimentation pulses. However, themiddle Eocene pulse was synchronous with a fall in the sea level, aclimatic change and slow cooling of the rift flanks and thus seemsmore likely related to climatic and eustatic variations. It seems thatthe East African Rift System has inherited its present relief from atleast three epeirogenic uplift pulses of middle Jurassic, Campa-nianePalaeocene, and Miocene ages.

Acknowledgments

This research was funded by Statoil AS through a grant (N�

4502178992) to Kalkulo AS.The authors thank Statoil AS for funding this study and for

providing us with well data. We would especially like to thankJakob Skogseid for fruitful discussions and inspiration. We also

A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295 295

thank Bart Willem Hendrik and an anonymous reviewer for helpfulcomments and Patrick Eriksson for editorial handling.

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