Sedimentary Budgets of the Tanzania Coastal Basin and Implications for Uplift History of the East African Rift System

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 implications for 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, Norway b Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Norway article info abstract

Article history: Data from 23 wells were used to quantify the sedimentary budgets in the Tanzania coastal basin in order Received 1 July 2015 to unravel the uplift chronology of the sourcing area located in the East African Rift System. We quan- Received in revised form tified the siliciclastic sedimentary volumes preserved in the Tanzania coastal basin corrected for 12 August 2015 compaction and in situ (e.g., carbonates) production. We found that the drainage areas, which supplied Accepted 14 August 2015 sediments to this basin, were eroded in four episodes: (1) during the middle Jurassic, (2) during the Available online 18 August 2015 CampanianePalaeocene, (3) during the middle Eocene and (4) during the Miocene. Three of these high erosion and sedimentation periods are more likely related to uplift events in the East African Rift System Keywords: Sediment flux and earlier rift shoulders and plume uplifts. Indeed, rapid cooling in the rift system and high denudation Tanzania coastal basin rates in the sediment source area are coeval with these recorded pulses. However, the middle Eocene Uplift history pulse was synchronous with a fall in the sea level, a climatic change and slow cooling of the rift flanks East African rift system and thus seems more likely due to climatic and eustatic variations. We show that the rift shoulders of the East African rift system have inherited their present relief from at least three epeirogenic uplift pulses of middle Jurassic, CampanianePalaeocene, and Miocene ages. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction 2. Geological setting

The volumes of siliciclastic sedimentary rocks preserved in the 2.1. Rifting and uplift history of the East African rift system marginal basins represent good archives of past uplifts and to- pographies over the continent, and are therefore a key to constrain The East African rift system (EARS) is a major topographic the timing and amplitude of vertical movements, and consequently feature in East Africa. It is composed of two branches: the eastern the mechanism(s) responsible of epeirogenic movements. Here we branch and the western branch separated by the East African aim to unravel the vertical motion history of the catchment area Plateau (alternatively known as the Tanzanian plateau; Fig. 1). The located at the East African Rift System which supplies sediment to mechanistic links between topography and rifting as well as the the Tanzania coastal basin. Only few estimates of sedimentation timing and magnitude of the uplift of the East African Plateau are rates in the Tanzania coastal depocentres fed by the East African Rift debated (Wichura et al., 2010). Available geological and geophysical System are published (Macgregor, 2010). They are based on a single data support the presence of limited relief of less than 1000 m prior cross-section and have therefore a low level of confidence. Based on to the onset of volcanism in the EARS (Smith, 1994). The earliest an interpolation between 23 exploratory wells, we carried out a recorded volcanic activity took place ~40e45 Ma in the northern volumetric study of the sediments preserved in the Tanzania Turkana depression (Fig. 1; e.g., Ebinger et al., 1993; George et al., coastal basin from the mid-Jurassic onwards. The presented results 1998; Knight et al., 2003; Furman, 2007). This was followed by provide a better estimate of the sediment fluxes. We then investi- the formation of the Gulf of Aden and Red Sea rifts, which separated gate the implications of our findings for the uplift timings and the the Nubian Plate from Arabia at approximately ~35e27 Ma topographic building of the East African rift system. (Schilling et al., 1992; Pik et al., 1999). Intracontinental rifting in East Africa is believed to have commenced in the region surrounding Lake Turkana at ~25 Ma, with volcanism and fault prop- * Corresponding author. agation moving southward through the Eastern and Western Rift E-mail address: [email protected] (A. Said). branches into Mozambique, while simultaneously propagating http://dx.doi.org/10.1016/j.jafrearsci.2015.08.012 1464-343X/© 2015 Elsevier Ltd. All rights reserved. A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295 289

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: East African Plateau.

northwards into Ethiopia (Watson et al., 2012). The northwards plume initiated in southwestern Ethiopia and sourcing the volca- propagation connected the East African Rift System with the Gulf of nism in the region (e.g., Duncan and Richards, 1991; Ebinger and Aden and Red Sea spreading centers, creating the Afar Triple Sleep, 1998; Ring, 2014). Most of these studies admit that the up- Junction at ~11 Ma (Smith, 1994; Chorowicz, 2005; Furman, 2007; lift of the EARS is mainly related to the plume effect. However, most Watson et al., 2012). The formation of this Triple junction was of the episodes of fast rock cooling are correlated with periods of preceded by the arrival of the Afar plume at the start of the extension in the African plate and development and reactivation of Oligocene (Ebinger and Sleep, 1998). high-angle faults (Noble et al., 1997; Foster and Gleadow, 1992, In the western rift of the EARS, Apatite fission track analysis 1996). They are also synchronous with accelerated deposition of show 3 periods of accelerated cooling and denudation (Van der sediments in the East African basins like in the Selous basin Beek et al., 1998). The oldest denudation event is recorded at (Wopfner and Kaaya, 1991). It seems that the uplift of the EARS around 250e200 Ma and corresponds to late Karoo erosion. A originated from both volcanism and rifting. second denudation phase, corresponding probably to a renewed rifting, was recorded in Late JurassiceEarly Cretaceous (~150 Ma). 2.2. Stratigraphy in the Tanzania coastal basin The youngest denudation event seems to start since ~40 Ma with a major part taking place since 20 Ma. In the Malawi and Rukwa rift The Tanzania marginal basin is composed of the Tertiary sedi- flanks (Fig. 1), the denudation of the Late Karoo and the Late ments associated to the fluvio-deltaic system, the underlying syn- JurassiceEarly Cretaceous events is estimated to 2.0 ± 0.4 km each, rift Mesozoic sediments associated to the East Africa passive however the Cenozoic denudation is estimated to less than margin onset and the older pre break-up system (Fig. 2). This 1.2 ± 0.2 km (Van der Beek et al., 1998). sedimentary column is characterized by several megasequences In the eastern branch of the EARS, thermal history modeling of bounded by major unconformities recognized at Base Pliocene, apatite fission-track and helium data (Spiegel et al., 2007) shows a Base Miocene, Base Middle Eocene, Base Palaeocene, Base Late rapid rock cooling during Late Cretaceous (~85e40 C) and Late Cretaceous, and Base Middle Jurassic (Break-Up Unconformity; Neogene (40 C to surface temperature). During this later period, Fig. 2). The stratigraphy of the coastal basin of Tanzania has been the average denudation rates range between ~0.2 and 0.4 km/Myr. described by a number of authors (Kent et al., 1971; Kajato, 1982; In eastern Tanzania, Noble et al. (1997) show a wide variation in Mpanda, 1997; Kapilima, 2003). The Triassic to Early Jurassic sedi- apatite fission track age from 221 ± 11 to 48 ± 3 Ma, and identify ments in the coastal basin of Tanzania consists of mainly fluviatile three periods of accelerated cooling: during the Early Cretaceous, sandstones (Fig. 2). They unconformably overlie the Precambrian Late CretaceouseEarly Paleogene, and Late EoceneeEarly Oligo- gneissic basement in both Selous-Tanga and Mandawa sub-basins cene, In northern Tanzania, Mbede (2001) identifies three periods (Fig. 3). These two sub-basins contain Karoo rift sequences corre- of rapid rock cooling: during the Early Jurassic (~195e159 Ma), Late sponding to different depositional environments. In the Selous- Cretaceous (~81e75 Ma), and Early Cenozoic (~64e48 Ma). Tanga sub-basin, the Karoo rift sequence is represented by fluvia- It is widely admitted that the EARS developed above a mantle tile sandstones resting on basal conglomerates. In the Mandawa 290 A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295

Fig. 2. Composite stratigraphic column of the Tanzania coastal basin (modified after Orca Exploration Group Inc, 2010). sub-basin, the Karoo rift sequence is formed by evaporitic sequence deposition of fluvial sediments along the shoreline, forming pre- (gypsum, halites and anhydrite) with silty shales. This lithofacies dominantly sandstones. In the Aptian-Albian period, a transgres- variation reflects difference in rates of subsidence and sedimentary sional cycle started (Mpanda, 1997). The Late Cretaceous was depositional environments (Kapilima, 2003). The evaporitic marked by a major transgression (Mpanda, 1997). An argillaceous sequence developed in a more rapidly subsiding part of the basin facies, associated with a deep marine domain, dominates this allowing a restricted marine incursion (Sabkha environment; period (Fig. 2), and probably results from a regional rise in the sea Mpanda, 1997). The coastal basin of Tanzania became a continental level (Haq et al., 1987). The Transgression series which started in shelf during the AalenianeBathonian (174e166 Ma) when the first the Aptian-Albian continued in the Palaeocene (Fig. 2). The Palae- marine transgression overlapped the continental Karoo. Westward, ocene deposited sediments are mostly deep marine, consisting of the transgression was limited by the Tanga and Lindi faults clay and claystone with thin bands of fine sandstone. However, the (Mpanda, 1997; Fig. 3). The depositional environment varied from deltaic type of sedimentation continued along the track of the shallow marine to neritic. In some localities, the Middle Jurassic present Rufiji River (Fig. 5). Between Middle Eocene and Oligocene, sediments rest unconformably on the basement. The Late Jurassic is a regressive phase occurred in the region (Kent et al., 1971; Kent marked by accumulation in a low energy sedimentary environment and Pyre, 1973). The Lower Eocene sediments mainly consist of (mostly marine shales) in the Mandawa basin. The deepening of shales, sandstones and carbonate rocks. The Middle and Late depositional environments is evidenced by the occurrence of more Eocene comprises a sandy deposit transported from the eroded predatory marine organisms such as ammonites and nautilus high relief basement areas in the west. It constitutes the gas (Kapilima, 2003). The thickness of the sediments in general de- reservoir in the Zanzibar, Pemba and Mafia grabens (Fig. 3). A creases southward. During the ValanginianeMiddle Aptian subsidence process allowed marine water to intrude the area and (~140e120 Ma), a regression cycle started, accompanied by the provided convenient conditions to establish the Late Eocene A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295 291

Fig. 3. Major structural features of coastal Tanzania with the main faulting zones and ages (modified after Kajato, 1982). carbonate platform. The Oligocene marine regressional sediments Data from 23 wells drilled in the Tanzania coastal basin were are of relatively restricted occurrence in the coastal belt of Tanzania. used to estimate the volume of sediment accumulated from Middle During the Neogene, the reactivation of the old Tanga and Lindi Jurassic to Present times. To do so, we followed a methodological faults was coeval with the opening of Eastern branch of the EARS approach, which defines 13 time-intervals corresponding to (Mpanda, 1997). Consequently, the maximum subsidence in the different litho-tectonic units, as reported in the stratigraphic col- Pemba, Zanzibar and Mafia channels occurred at that time (Fig. 3). umn in Fig. 2. From the youngest to the oldest, the time-intervals are reported in Table 1. 3. Data and methods To determine the evolution of the sedimentary flux and calculate changes in the sedimentation rate, the volume of sediment The Tanzania coastal basin covers the Selous-Tanga and the preserved in each formation was quantified using interpolation Mandawa sub-basins (Fig. 3). The boundaries of the Tanzania between the well data. The quantification of terrigenous sediment coastal basin have been refined based on geophysical (gravimetry) volumes requires two main corrections: a correction for carbonates and geological (isopachs and structural features) data (Fig. 4). The produced in situ; and a correction for the sediment compaction basin is limited to the West by the Tanga and the Mandawa faults exerted by the overlying sediment layers (Said et al., 2015). which separate it from the basement (Figs. 3 and 4b). To the East, the basin is bordered by a relatively low gravimetric feature parallel - Compaction correction to the Davie Fracture Zone and situated to the east of the isopach 6km(Fig. 4b). The Tanzania coastal basin is laterally adjacent to the The sediment decompaction was calculated based on the Lamu basin in the North and to the Ruvuma basin in the South. porosity-depth law in Allen and Allen (2013). This law assumes that Presently, this basin is connected to the Rufiji catchment basin density changes of a sedimentary unit are caused only by changes which supplies its central part with sediments. The southern part of in the pore space (ignoring, for example, diagenesis) and that the the basin is fed by the Ruvuma catchment (Fig. 5). porosity, f (the ratio between pore space and sediment), is 292 A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295

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. decreasing exponentially with the depth, z: The decompacted thickness of a layer is its net thickness plus this depth-dependent pore volume. The decompacted thickness is fð Þ¼f ð $ Þ; z 0 exp c z determined by an iterative approach: decompacting a layer, removing it and then decompacting the layers beneath it. f where 0 is the porosity at the surface and c is a lithology- ﬁ dependent coef cient that determines the slope of the porosity- - Carbonate correction depth curve. In the well-logs used in this study, the following li- thologies are present: sandstone, shale, limestone and shaly To calculate the terrigenous sediment volume, the non-terrig- f sandstone. For each lithology, we used the 0 and c values as enous contribution is removed from the total sediment in the basin. described by Sclater and Christie (1980). In our case, carbonate deposits are the main non-terrigenous sed- The total pore volume of a sedimentary layer is given by inte- iments present and we assume all carbonates are produced in situ, gration over the depth interval: given the shallow water depths associated with the carbonates in the well-logs. The thickness of carbonate layers for each time in- Zztop terval at each borehole were estimated based on information from f D ¼ f ð $ Þ ¼ 0 ð Þ : the available well-logs. The well data was interpolated to a grid zpore 0 exp c z dz exp cztop exp czbottom c covering the studied region at 0.01 resolution, excluding values zbottom outside the deﬁned limits of the Tanzania coastal basin. The A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295 293

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 to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

interpolation is a two-stage process: first combining the well-data maximum of sedimentation rate in this period was reached during for each 0.2 block as a barycentric approach on a course grid with the Palaeocene (~3.2 103 km3/Myr). The second period (T9) was 0.2 grid spacing, using a weighted combination of the data from the recorded during the Middle Eocene (~3.9 103 km3/Myr), and the nearest three available wells or boundary points. In the case of the third one (T12) was recorded during the Miocene (~2.5 103 km3/ sediment thickness, the thickness was assumed to be zero on the Myr; Fig. 6). boundary and the barycentric interpolation included at most two boundary values. In the second stage, the course grid was refined to 5. Discussion 0.01 resolution using a Delaunay triangulation. Increases in clastic sediment accumulation rates are related to 4. Results either a tectonically driven uplift in the drainage basin, a climatic change toward more seasonal and erosive conditions, an eustatic fl The sedimentary ux is the ratio between a volume of sediment sea-level fall exposing the continental shelf, or a combination of and the time period required for its deposition. Fig. 6 show a Middle these parameters. Differentiating between the tectonic, climatic 3 3 Jurassic high sedimentation rate (4.9 10 km /Myr) which and eustatic causes of recorded high sediment fluxes is often 3 3 dropped during the Late Jurassic to ~1.9 10 km /Myr. The Early difficult (Bonnet and Crave, 2003). e Cretaceous Santonian (T3 to T5) was characterized by low sedi- The high sedimentation rate recorded in the Tanzania coastal 3 3 mentation rates (~1.2 10 km /Myr). After this long sediment basin during the Middle Jurassic is contemporaneous with the starvation, three periods of high sedimentation rate have been onset of rifting between East Africa and the Mada- fi e e recorded. The rst is of Campanian Palaeocene age (T6 T7) and is gascareIndiaeAntarctica landmass. Sediment supply during this marked by the presence of basal conglomerates (Fig. 6). The period originated from the erosion of the rifting associated uplift. The fall of the flux from 4.9 103 km3/Myr during Middle Jurassic 3 3 Table 1 (T1) to 1.9 10 km /Myr during Late Jurassic (T2) probably shows Time periods used in the sedimentary budgets analysis. that the major part of the rifting-associated relief was eroded during the Middle Jurassic. Label Epoch From age (Ma) To age (Ma) In the latest Cretaceous, the recorded high sedimentation rate T13 Plio-Pleistocene 5.3 0 (Present) was synchronous with a basal conglomerate recorded at the San- T12 Miocene 23 5.3 tonian/Campanian transition which indicates a tectonic uplift T11 Oligocene 33.9 23 T10 Late Eocene 38 33.9 affecting the source areas during that time. This event is described T9 Middle Eocene 47.8 38 in many parts of Africa and known as the Santonian tectonic pulse T8 Early Eocene 56 47.8 (83 ± 2 Ma) attributed to the onset of the collision between the T7 Palaeocene 66 56 AfricaneArabian and Eurasian plates (Burke and Dewey, 1974; T6 CampanianeMaastrichtian 83.6 66 T5 ConiacianeSantonian 89.8 83.6 Guiraud and Bosworth, 1997; Burke and Gunnel, 2008). The sedi- T4 CenomanianeTuronian 100.5 89.8 mentary supply issued from this tectonic pulse was reinforced T3 Early Cretaceous 145 100.5 during the latest CretaceousePalaeocene by sediments eroded T2 Late Jurassic 163.5 145 from the Anza rift shoulders (Fig. 4) which have been rejuvenated T1 Middle Jurassic 174.1 163.5 during the 70e60 Ma interval (Foster and Gleadow, 1992, 1996; 294 A. Said et al. / Journal of African Earth Sciences 111 (2015) 288e295

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 the major 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 from Spiegel 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.)

Spiegel et al., 2007; Bosworth and Morley, 1994). This erosional ~15 Ma (DeMenocal, 1995) and accentuated at ~7e8Ma(Prell and event is not seen in the Anza basin or surrounding area, which Niitsuma, 1989; Quade et al., 1989) due to the closing of the Zagros suggests that the shoulders of the Anza rift were topographically seaway north of Arabia. During the Plio-Pleistocene, Eastern Africa high (Macgregor, 2015). The rejuvenated rifting was due to an underwent an aridification which is attributed either to the closure extensional stress regime in Africa resulting probably from the of the Indonesian seaway (Cane and Molnar, 2001) or to the closing difference of seafloor-spreading rates between the Central and the of the Isthmus of Panama (Haug et al., 2001). This aridification was South Atlantic (Fairhead and Binks, 1991). probably at the origin of the decrease in the sedimentation rate The rapid rock cooling recorded by fission-track and helium data during that time. The Ethiopian Highlands situated in the northern during the Late Cretaceous (~85e40 C; Spiegel et al., 2007; Mbede, part of the EARS and constituting the major supplier of sediment for 2001) and the Late Cretaceous-Early Paleogene (Noble et al., 1997; the Nile delta seems not to be affected by this aridification as it is Foster and Gleadow, 1992, 1996) is in good agreement with our evidenced by the considerable increase of sediment flux in the Nile findings. The high sedimentation rate recorded during the middle during the Plio-Pleistocene (Macgregor, 2012). Eocene time is not coeval with any rapid rock cooling (Spiegel et al., 2007). The fission track ages corresponding to the youngest period 6. Conclusions of accelerated cooling proposed by Noble et al. (1997;from88± 6 to 48 ± 3 Ma) and Mbede (2001; from 64 ± 6to48± 4 Ma) and by Here we used well data to quantify the evolution of the sedi- Van der Beek et al. (1998; from ~40 to 30 Ma) slightly ovelap the mentary budgets in the Tanzania coastal basin in order to unravel middle Eocene time interval (48e38 Ma), if we consider the stan- the uplift chronology of the sourcing area located in the East African dard deviation of the fission track ages. However, it is clear that the Rift System. Our data identify four major periods of high sedi- overlap is partial and that most of the middle Eocene corresponds mentation rate: (1) during the middle Jurassic with a rate of to a period of slow rock cooling. Moreover, the curve of eustatic 4.9 103 km3/Myr, (2) during the CampanianePalaeocene with a variations (Miller et al., 2005) shows a decrease in the sea level rate of ~3.2 103 km3/Myr, (3) during the middle Eocene with a during the Eocene time synchronous with a decrease of the Ocean rate of ~3.9 103 km3/Myr and (4) during the Miocene with a rate temperature as it is evidenced by the d18O measurements (Zachos of ~2.5 103 km3/Myr. Three of these sedimentation pulses are et al., 2001; Fig. 6). Thus, the middle Eocene sedimentary pulse more likely related to uplift events in the East African Rift System seems more likely related to changes in the climatic and eustatic and earlier rift shoulders and plume uplifts. Rapid cooling in the rift conditions. It is worth noting that a change in the lithology system and high denudation rates in the sediment source area are occurred between early and middle Eocene towards more sandy coeval with these recorded sedimentation pulses. However, the deposits constituting one of the gas reservoirs in the Zanzibar, middle Eocene pulse was synchronous with a fall in the sea level, a Pemba and Mafia channels (Fig. 3). This could be a direct effect of a climatic change and slow cooling of the rift flanks and thus seems sea level drop and a related forced regression. more likely related to climatic and eustatic variations. It seems that The high sedimentation during the Miocene was synchronous the East African Rift System has inherited its present relief from at with an accelerated cooling and denudation in the western rift of least three epeirogenic uplift pulses of middle Jurassic, Campa- the EARS (Van der Beek et al., 1998). It coincided also with an nianePalaeocene, and Miocene ages. alkaline volcanic activity in central Kenya. This later started at ~20 Ma, and most activity occurred between 16 and 8 Ma (Smith, Acknowledgments 1994). In the Lindi area (Southern part of the Tanzania coastal basin; Fig. 3), erosional truncation between base Miocene and top This research was funded by Statoil AS through a grant (N Oligocene are described in the channels (Mpanda, 1997). 4502178992) to Kalkulo AS. During the Miocene, Eastern Africa was subjected to strong The authors thank Statoil AS for funding this study and for annual monsoonal cycles which accentuated the denudation and providing us with well data. 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