Reconstruction of the evolution of the Niger River and implications for sediment supply to the Equatorial Atlantic margin of Africa during the Cretaceous and the Cenozoic

Abstract

This paper presents a reconstruction of the palaeodrainage evolution of the Niger River in West Africa in order to contribute to the understanding of sediment supply to the Niger Delta. It has been covered extensively in literature that the Niger River has undergone changes along its course in the Holocene, as implied by the large bend it makes in Mali. However, other enigmatic bends further downstream are indicative of an older and more complicated history that has yet to be understood, and is the focus of this paper. Until now, sediment supply from the Niger River has been considered as being negligible compared to that of the Benue River. The results of this study imply that the contribution from the Niger River was more important than previously thought. The Niger River obtained its present-day geometry in three phases: a Bida Basin phase (Maastrichtian-Miocene); a Iullemmeden Basin phase (Miocene-Pleistocene); and a presentday Niger River phase (Holocene). In the Miocene, an important capture event occurred, increasing the incipient drainage basin by 10(6) km(2), thereby changing the provenance of the sediment supplied to the Niger Delta from mainly crystalline basement to mixed lithologies including sandstone, shale, limestone and volcanic outcrops.

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Geological Society, London, Special Publications Online First
April 9, 2014; doi 10.1144/SP386.20
, first publishedGeological Society, London, Special Publications
Kathelijne P. M. Bonne
Cenozoic
Atlantic margin of Africa during the Cretaceous and the
implications for sediment supply to the Equatorial
Reconstruction of the evolution of the Niger River and
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Reconstruction of the evolution of the Niger River and implications
for sediment supply to the Equatorial Atlantic margin of Africa
during the Cretaceous and the Cenozoic
KATHELIJNE P. M. BONNE
Getech, Kitson House, Elmete Hall, Elmete Lane, Leeds LS8 2LJ, UK
(e-mail: kathelijne_bonne@hotmail.com)
Abstract: This paper presents a reconstruction of the palaeodrainage evolution of the Niger River
in West Africa in order to contribute to the understanding of sediment supply to the Niger Delta. It
has been covered extensively in literature that the Niger River has undergone changes along its
course in the Holocene, as implied by the large bend it makes in Mali. However, other enigmatic
bends further downstream are indicative of an older and more complicated history that has yet to be
understood, and is the focus of this paper. Until now, sediment supply from the Niger River has
been considered as being negligible compared to that of the Benue River. The results of this
study imply that the contribution from the Niger River was more important than previously
thought. The Niger River obtained its present-day geometry in three phases: a Bida Basin phase
(Maastrichtian–Miocene); a Iullemmeden Basin phase (Miocene– Pleistocene); and a present-
day Niger River phase (Holocene). In the Miocene, an important capture event occurred, increas-
ing the incipient drainage basin by 10
6
km
2
, thereby changing the provenance of the sediment
supplied to the Niger Delta from mainly crystalline basement to mixed lithologies including sand-
stone, shale, limestone and volcanic outcrops.
The geological history of large rivers in the world
has been studied extensively because of both the
scientific and commercial value of the large deltas
and submarine fans fed by such systems. There is
an increasing interest towards understanding the
link between the evolution of large river systems
supplying sediment from the hinterland, and the
marine record; that is, the source to sink relation-
ships. Reconstruction of palaeodrainage and the
sediment routing systems contributes to the under-
standing of these source to sink relationships,
thereby allowing the quantification of sediment
supply parameters such as type, flux, volume and
timing, which in petroleum exploration ultimately
leads towards the assessment of reservoir qual-
ity. The overall Cenozoic histories of many of the
world’s largest rivers have been reconstructed,
albeit with varying degrees of uncertainties within
the proposed hypotheses, which are often the sub-
ject of debate. Many uncertainties are due to over-
printing and the erosion of patterns in the drainage
network, and the landscape that could have pro-
vided evidence, and to the complex interwoven
nature of the processes that drive the dynamics
leading to drainage changes.
The geological evolution of large rivers is
closely linked to large-scale landscape evolution,
primarily governed by tectonics and denudation
that interact as an intricate coupled system. A con-
siderable volume of literature is available on
this topic, including Summerfield (1991), Burbank
& Pinter (1999), Montgomery (2003), Pazzaglia
(2003) and Allen (2008). The complex interplay
of interdependent factors driving erosion, including
climate, base level, vegetation, slope, relief, out-
crop lithology and soil formation, all act upon the
tectonically driven landscape by means of the
river system, which ultimately forms the interface
between the counteracting endogenic and exogenic
forces. The consequent denudation of the landscape
and the loading of the basins generate geodynamic
feedback (i.e. isostatic and flexural response),
which in turn rejuvenate the landscape. Climate is
a fundamental component in the feedback system
that affects the rate of erosion, and dictates sediment
fluxes and volumes (Tucker & Slingerland 1997).
Drainage changes, such as stream captures, caused
by the evolving landscape, affect the size of the
drainage basin or catchment of a river system,
which eventually has an impact on sediment flux
and type as the source areas have changed. Through-
out the history of a river system, the dynamic
events undergone are preserved at varying resol-
utions within the sedimentary successions beyond
its mouth. Understanding the dynamics of landscape
evolution affecting a given river system is therefore
essential for the prediction of reservoir quality.
Many of the large-scale drainage rearrange-
ments that have been detected in large rivers were
triggered by regional tectonic events and directly
affected distribution of petroleum reservoirs. The
Amazon, for example, formed in the late Miocene
From:Scott, R. A., Smyth, H. R., Morton,A.C.&Richardson, N. (eds) Sediment Provenance Studies in
Hydrocarbon Exploration and Production. Geological Society, London, Special Publications, 386,
http://dx.doi.org/10.1144/SP386.20
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or later due to uplift of the Andes, causing an east-
ward tilt of the Amazon Basin and an eastward
shift in drainage (Figueiredo et al. 2009). The
Yangtze River is at least 22 Ma old and formed
during a regional drainage reorganization coeval
with the formation of very high relief in Asia
(Zheng et al. 2013). Several evolutionary hypoth-
eses exist for the Nile River. Each of the hypothe-
ses has in common that the sediment flux towards
the Nile Delta is greatly influenced by the Mio-
cene rift shoulder uplift of the Red Sea and by
uplift of the Afar Dome (Macgregor 2011). It is
accepted that the Congo River started flowing to
its current outlet due to late Cenozoic capture near
the Malebo Pool (Stankiewicz & de Wit 2006),
but the location of the palaeo-outlet of the large
Central African drainage is still debated. Rift-
controlled drainage such as the Rufiji (Tanzania)
(Stankiewicz & de Wit 2006) and Benue rivers
(Nigeria) (Markwick & Valdes 2004) are possible
candidates for the palaeo-Congo outlet.
The Niger Delta is one of the world’s most impor-
tant petroleum provinces, yet, surprisingly, only
the Holocene history of the Niger River has been
described in literature (Goudie 2005 and references
therein). This recent history includes the formation
of the large bend in Mali (Fig. 1), by the merg-
ing of NE- and SE-flowing sections of the Niger
River, related to the formation of the Sahara
Desert (Goudie 2005). The older history of this river
remains enigmatic and referral to it in papers on
African drainage is minimal (Summerfield 1991;
Burke 1996; MacGregor et al. 2003; Goudie 2005;
Gupta 2007). The Benue River is regarded as the
only significant sediment supplier to the Niger
Delta, and sediment contribution from the Niger
River is assumed as negligible (e.g. Burke 1996).
This perception is attributed to the very late cap-
ture of the Upper Niger and to the arid Sahelian
climate presently dominating large parts of the
drainage basin of the Niger River. However, even
without the drainage of the Upper Niger, the catch-
ment of the Middle and Lower Niger still covers
an area of more than 10
6
km
2
that should not
be neglected as a possibly important provenance
area for the Niger Delta. Furthermore, the present-
day aridity is mainly a late Neogene phenomenon
and climate conditions were more humid before
(Micheels et al. 2009), suggesting that sediment
fluxes from this region may have been higher than
previously assumed. It is the aim of this paper to
find out how the Niger River evolved through time
as an integral part of the larger Niger– Benue
system feeding the Niger Delta, and to assess how
it could have contributed to deposition.
The Niger Delta is fed by two geologically dis-
tinct drainage systems: the Niger River draining
a large part of the West African Craton and the
Pan-African Mobile Belt (Dirks et al. 2009); and
the Benue River, the downstream left-bank tributary
of the Niger River that occupies the Benue Trough,
a Cretaceous aulacogen related to the opening of
the South Atlantic (Burke & Dewey 1973; Obaje
2009; Ukaegbu & Akpabio 2009; Opeloye 2012).
This paper focuses mainly on the course of the
actual Niger River. The Niger River is divided
into three sections called the Upper, Middle and
Lower Niger, as indicated in Figure 1. The main
components of the Niger River and its drainage
basin are shown in Figure 1, and the outcrop geo-
logy is shown in Figure 2.
The large bend of the Niger River, located in
Mali (Fig. 1), is a well-known feature extensively
covered in English and French literature. It is
further referred to as the Large Bend to differenti-
ate it from other bends discussed in this paper. As
opposed to the Large Bend, the origin of two other
prominent bends located in Nigeria, here called
the Nigerian Bends (Fig. 1), remains largely uncov-
ered. Anomalies in the stream networks, such as
prominent bends, especially at large scales are often
indicative of past changes along a river’s course
and can provide clues towards reconstructing
the palaeodrainage evolution (Summerfield 1991;
Twidale 2004). Following this line of thought, we
assume that understanding the origin of the Nige-
rian Bends is pivotal to reconstruct the palaeodrai-
nage of the pre-Holocene Niger.
Physiography of the Holocene Niger River
The present-day Niger River is 4100 km long and
has an annual suspended sediment load of 32 Mt
(Gupta 2007). It is the largest river of West Africa
and the third largest of Africa, after the Nile and
the Congo. The present-day active catchment sur-
face is 1.1 × 10
6
km
2
(Goudie 2005) but this surface
can be expanded to 6.4 × 10
6
km
2
if it includes
the entire area hydrologically contributing to the
Niger Delta (Fig. 3). This includes the depressions
of the Taoudenni and Chad basins. This full drai-
nage capacity can potentially be reached if pre-
cipitation is sufficient to fill the latter endorheic
basins to spill-point and to cause overflow. For the
Chad Basin, overflow happened at a sill at Bongor
(Bridges 1990; Goudie 2005), and, for the Taou-
denni Basin, this would be north of the Large
Bend (Fig. 3). The now nearly dry Chad Basin
was filled to its maximum capacity, up to 320 m
asl (metres above sea-level), forming Lake ‘Mega-
Chad’ at 6500 BP, following several Pleistocene –
Holocene dry–wet cycles (Thiemeyer 2000). These
regions are now part of the Sahel region straddling
the Sahara Desert, and are major dust pans (Varga
2012).
K. P. M. BONNE
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Fig. 1. Large rivers draining to the Equatorial Atlantic and details of the Niger River. The Large Bend and the Nigerian
Bends are discussed in this paper.
Fig. 2. Regional geology and large rivers of the Equatorial Atlantic. Geology from American Geoscience Institute
(2014). The Niger River flows from its sources in the Guinea Highlands through the Taoudenni Basin, the Iullemmeden
Basin and the Bida Basin. The Benue River joins the Niger River before it enters the Atlantic Ocean via the Niger Delta.
The Benue Trough is shown in more detail in Figure 4.
PALAEODRAINAGE OF THE NIGER RIVER
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The source of the Niger River is located in the
Fouta Djallon Plateau in the Guinea Highlands,
at an elevation of 800 m and at only 300 km from
the Atlantic coast (Figs 1 & 2). The headwaters
flow into the interior of West Africa, along a NE
course into Mali, where the river’s Large Bend
is located. There the gradient decreases and the
sediment load is deposited into the Inland Niger
Delta (Fig. 2), a large area of marshes, swamps
and pools, which is ecologically endangered by
the encroaching Sahara Desert (Jacobberger 1981;
Olivry & Boule
`
gue 1995; Goudie 2005; Makaske
et al. 2007). Owing to evaporation and infiltration,
the flow rate decreases by two-thirds at the Inland
Niger Delta (FAO 1997; Andersen et al. 2005).
The Large Bend represents the location where two
Fig. 3. (a) Drainage networks of the Niger River drainage basin generated from the SRTM30 DEM at a resolution of
100 km
2
. The dashed white line represents the divide between the actively contributing area and the internally draining
basins. The black dots show the locations of the spill-points between both. (b) Drainage networks of the Equatorial
Atlantic margin of Africa generated from the SRTM3 DEM at a resolution of 10 km
2
. Straight lines in the networks are
GIS artefacts generated where topographical depressions are filled.
K. P. M. BONNE
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separate river systems merged in the Holocene, due
to encroaching of the Sahara Desert (e.g. Goudie
2005), as discussed below.
East from the Inland Niger Delta, the Niger
passes through the Large Bend and flows NE
through the intracratonic Iullemmeden Basin in
the Republic of Niger (Kogbe 1991; Obaje 2009)
(Fig. 2). Several large tributaries, all of them loca-
ted to the north, are sourced in the Hoggar Massif
in the Sahara and are mostly dry, therefore the
water flux remains low (Goudie 2005). An impor-
tant tributary, now also dry, was the Dallol Bosso
River (Fig. 1). Tributaries for the southern bank of
the Niger River are short. After entering Nigeria,
the Niger River makes two subsequent abrupt
bends, the Nigerian Bends. The Lower Niger River
is located within a subtropical environment, increas-
ing the water flux (FAO 1997). The river flows
through the Bida (Nupe) Basin (Fig. 2), a small
Late Cretaceous extensional basin (Obaje 2009;
Obaje et al. 2011). After the confluence with the
Benue River, the Niger River reaches the Niger
Delta. This delta is one of the world’s largest
deltas, spanning 19 135 km
2
, starting progradation
in the Oligocene (Gupta 2007; Reijers 2011).
The Large Bend of the Niger River has been
known to people for millennia, as for many desert
populations it was an important source of water
and a meeting point for traders. Scientists recog-
nized early that the Large Bend formed by the
joining of two separate rivers, later recognized to
be caused by processes related to climate oscil-
lations, and the effect of those on the formation
of the Sahara Desert and associated hydrological
factors (e.g. Urvoy 1942; Palausi 1955; Monod
1964; Tricart 1965; Iloeje 1981; Jacobberger 1981;
McIntosh 1983; Doust & Omatsola 1989; Bridges
1990; Burke 1996). According to Urvoy (1942),
the Upper Niger used to flow NE into the Sahara,
forming a large lake in the Azawagh (also
Azawad), a region located north of the Large Bend
covering a large part of NE Mali (Fig. 2). The
Azawagh is geologically located in the Taoudenni
Basin (Craig et al. 2009). Palausi (1955) discovered
abandoned river channels within the same region,
the presence of which was later confirmed by
Jacobberger (1988) by the use of satellite images.
Climate fluctuations in the Quaternary, responsi-
ble for alternations between lacustrine and aeo-
lian phases in the Sahara (e.g. Gasse et al. 1990;
Gasse 2000; Giresse 2008; Lespez et al. 2011),
were most probably the cause of obstructing the
previous NE flow by dune formation. In following
wetter phases, a lake formed at the location of
the Inland Niger Delta, with dunes effectively
blocking throughflow. This lake, referred to as
Lake Araouane (Iloeje 1981; Akaa et al. 2008), epi-
sodically spilled over a sill at Tosaye (Fig. 1).
Lake Araouane is suggested by Bridges (1990) to
have risen to spill-point during the pluvial period
between approximately 15 000 and 10 000 BP,
and was possibly drained at around 5000 BP to
form the Inland Niger Delta as a remnant of the
ancient large lake. The actual connection between
the Upper and Middle Niger was established by a
combination of two interacting processes: over-
flow of Lake Araouane (e.g. Bridges 1990); and
breaching of the sill due to headward erosion of
the Middle Niger (e.g. Akaa et al. 2008). A pos-
sible chronology could be that excess overflow
from Lake Araouane flowed into the Iullemme-
den Basin (Kogbe 1991; Burke 1996), which was
drained by the palaeo-Middle Niger, this extra dis-
charge increasing the stream power of the latter.
Consequently, headward erosion along the Middle
Niger possibly occurred or increased, resulting in
the upstream propagation of incision, eventually
breaching the sill of Lake Araouane. As such, the
water was drained from that lake, and the present-
day Niger River and Inland Niger Delta may have
formed.
The idea of the Upper Niger previously flowing
NE into a lake in the Azawagh region in the inte-
rior of the African continent (Urvoy 1942) is con-
sistent with the Cenozoic history of Africa during
which a ‘basin-and-swell’ dynamic topography for-
med (Burke 1996). This caused many large rivers
to flow into endorheic basins (e.g. into the Congo,
Chad, Sudd, Taoudenni and Iullemmeden basins:
Burke 1996). At several locations in Africa, large
pluvial lakes existed but they have since dried up
(e.g. Varga 2012).
Geological setting
Most drainage changes are caused by the response
of the landscape to tectonic events. Therefore, any
palaeodrainage reconstruction should be based on
the geological background. Whereas the drainage
patterns at a given moment in time give clues of
past events and changes, the timing and process of
the change can only be understood through the
knowledge of the geological history. An overview
of this history is, hence, given in order to understand
the setting of the landscape upon which the stream
network of the Niger River has evolved.
In the catchment of the Niger River, the out-
crop geology is dominated by Precambrian base-
ment comprising gneisses, migmatites and granites
(Obaje 2009). The Archaean Reguibat and Leo-
Man shields are the cores of the Palaeoproterozoic
West African Craton (Bertrand-Sarfati et al. 1991;
Lompo 2009), surrounded by Pan-African defor-
mation zones (e.g. the 3000 km-long Trans-Saharan
Belt to the east: Kro
¨
ner & Stern 2004) (Fig. 2).
PALAEODRAINAGE OF THE NIGER RIVER
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Table 1. Summary of Cretaceous events
Period Epoch Age Regional
events
Equatorial
Atlantic
Benue
Trough
Iullemmeden
Basin
Anambra
Basin
Bida
Basin
Drainage
events
Cretaceous Late Maastrichtian Uplift affecting the
Benue Trough and
Anambra Basin
Benue Trough
became
entirely
emergent
Marine transgression
from the Tethys
Sea
End-Cretaceous
uplift temporarily
terminated
marine
sedimentation
Subsidence and
deposition in
the Bida Basin
Drainage from
palaeo-Benue,
which was a
small river
sourced in the
Middle Benue
Trough, and
palaeo-Niger in
the Bida Basin.
Both deposited in
the Anambra
Basin
Campanian Marine
sedimentation
Palaeo-Niger formed
in the Bida Basin,
draining a small
catchment centred
on the Nigerian
Shield
Santonian ‘Santonian’ event due
to plate
reorganization
caused regional
deformation and
compression.
Trans-Saharan
Seaway migrated to
the west of the
Hoggar Massif
Transform motion
ceased,
establishment of
passive margins
Compression and uplift,
regression of seas.
The Middle Benue
Trough formed a
drainage divide, with
seas from the north
(Tethys) still
submerging the
Upper Benue
Trough. Westward
shift in the
depositional axis
Subsidence of the
Anambra Basin,
which received
sediment from
the uplifted
Benue Trough
Disruption of
integrated
drainage through
Upper Benue
Trough
Coniacian Marine transgression
from the Tethys
Sea
Turonian Trans-Saharan Seaway
connected Tethyan
and Atlantic waters
via the Chad Basin
and the Benue
Trough
Sea levels rose to
establish a marine
connection between
the Benue Trough
and rifts in the Chad
Basin
K. P. M. BONNE
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Cenomanian The Bima Formation
formed a delta in the
Upper Benue
Trough. The rest of
the Benue Trough
remained submerged
Drainage from a
possibly
substantial river
forming a delta in
the Upper Benue
Trough, draining
rift shoulders of
basins along the
Central African
Shear Zone. The
palaeo-Congo
may have been
part of this
system
Early Albian Establishment
of open
marine conditions
Marine transgression
from the
Equatorial
Atlantic
Aptian Break-up in the South
and Equatorial
Atlantic
Seafloor
spreading and
establishment of
a narrow seaway
across very
narrow, deep,
rapidly subsiding
basins undergoing
transform motion
Main rifting
phase
Barremian Development of
the Central
African Rift
System
Initial shearing,
forming a series
of dextral en
echelon strike-slip
basins
Initial
shearing
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The Dahomeyides (Fig. 2) are a part of the Trans-
Saharan Belt located in Ghana, Togo and Benin,
comprising suture zone nappes thrust westwards
over the West African Craton (Castaing et al.
1993; Kro
¨
ner & Stern 2004; Guiraud et al. 2005;
Attoh & Brown 2008; Attoh & Nude 2008; Dos
Santos et al. 2008; Obaje 2009). The Nigerian
Shield and the Jos Plateau, both in Nigeria, are
also part of the Trans-Saharan Belt, and consist of
strongly varying lithologies that represent reworked
Palaeoproterozoic basement and Neoproterozoic
oceanic assemblages (Kro
¨
ner & Stern 2004; Obaje
2009). At 200 Ma, magmatic rocks were intruded
as, for example, ring complexes in many parts of
West Africa, Cameroon and Nigeria during the
Central Atlantic Magmatic Event (Wilson 1992;
Torsvik et al. 2012).
The actual history of the Niger Delta begins
with the creation of the accommodation space
for its sedimentary successions; that is, the forma-
tion of the triple junction between the South and
Equatorial Atlantic and the failed rift of the Benue
Trough in Cretaceous times (Burke & Dewey
1973; Benkhelil 1989; Basile et al. 2005; Antobreh
et al. 2009; Nemc
ˇ
ok et al. 2012). A summary of
the most noteworthy events of the Cretaceous
Period is given in Table 1. The geological history
can be summarized into three main events that sig-
nificantly affected the palaeogeography and palaeo-
topography of the study area: (1) the Cretaceous
rifting in Africa leading to the oblique opening of
the Equatorial Atlantic (Wilson & Williams 1979;
Guiraud et al. 1992; Wilson & Guiraud 1992; Genik
1992; Jones et al. 1995; Gasperini et al. 2001;
Mohriak & Rosendahl 2003; Basile et al. 2005;
Bigot-Cormier et al. 2005; Antobreh et al. 2009;
Turner & Wilson 2009; Moulin et al. 2010); (2)
the marine transgressions from the Tethys and the
Trans-Saharan Seaway (Guiraud et al. 2005); and
(3) the creation of the basin-and-swell topography
since the Oligocene (e.g. Burke 1996).
Cretaceous rifting
The Cretaceous rifting episode comprised the
oblique dextral rifting in the Equatorial Atlantic
(Basile et al. 2005) and sinistral rifting in the
Benue Trough (Benkhelil 1989). Initial shearing
started between the late Barremian and the Aptian,
forming a series of dextral en echelon strike-slip
basins (Basile et al. 2005). The continental break-
up started in the Aptian, and a narrow seaway
was established since that time (Jones et al. 1995).
This seaway also transgressed the Benue Trough
since the Albian. While oceanic accretion took
place along the spreading axes, deformation oc-
curred along the transform faults, forming very
deep, narrow, rapidly subsiding basins, comparable
to that of the present Dead Sea (Basile & Allemand
2002). Open marine conditions appeared in the late
Albian (Basile et al. 2005). In the Late Cretaceous,
the newly formed basins were active transform
basins until the Santonian, when they finally
became passive margins (Basile et al. 2005).
The Benue Trough was connected to the north
and east to other Cretaceous rift basins of, respect-
ively, the West and Central African Rift System
(Grant 1971; Fitton 1980; Adighije 1981; Ojo &
Pinna 1982; Ofoegbu 1985; Benkhelil 1989; Doust
1990; Fairhead & Okereke 1990; Ofoegbu &
Onuoha 1991; Genik 1992; Shemang et al. 2001;
Basile et al. 2005; Obaje 2009; Ukaegbu &
Akpabio 2009; Chukwuebuka et al. 2010; Akande
et al. 2012). The Benue Trough contains over
6 km of sediments, is 150 km wide and 800 km
long, and is arbitrarily subdivided from west to east
in the Lower, Middle and Upper Benue (Obaje
2009) (Fig. 4).
The Benue Trough underwent rifting and subsi-
dence in the Aptian coeval with the Equatorial
Atlantic, and during Albian– Cenomanian times a
delta (the Bima Formation) developed in the Upper
Benue Trough (Benkhelil 1989). The Bima Delta
was probably fed by sediments eroded from the
rift shoulders of basins located along the Central
African Shear Zone (Binks & Fairhead 1992;
Genik 1992) and arguably also by the palaeo-Congo
River (Markwick & Valdes 2004). Throughout
the Cenomanian and Turonian, a marine connection
with the Tethys formed across the Benue Trough
(Petters 1978; Benkhelil 1989; Guiraud et al.
2005; Mathey et al. 2006; Obaje 2009). In the San-
tonian, the Middle Benue Trough became emergent
as a result of regional uplift and deformation, creat-
ing a regional drainage divide between Tethyan
and Atlantic waters (Ofoegbu 1985; Benkhelil
1989; Genik 1992). The depositional axis of the
Benue Trough had since then shifted westwards to
the Anambra Basin (Fig. 4) (Obaje 2009; Akande
et al. 2011). Uplift at the end of the Cretaceous
left the Benue Trough entirely emergent (Benkhelil
1989). Palaeogeographies of the Benue Trough
(Benkhelil 1989, p. 258, fig. 4) imply that the
palaeo-Benue River was a short river at the start
of the Cenozoic, because large parts of the Middle
Benue Trough were uplifted and were a sediment
source. Deposition continued in the SW Anam-
bra Basin in the Cenozoic, and from the Eocene
onwards progradation of the Niger Delta started
(Obaje 2009).
The Bida Basin (Fig. 2) is a NW– SE-trending
fault-bounded basin that forms an embayment of
the Anambra Basin and is structurally linked to
the Benue Trough, possibly in a pull-apart sett-
ing (Benkhelil 1989; Kogbe et al. 1983). It under-
went continental–marginal marine sedimentation
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throughout the Campanian and the Maastrichtian.
The Lower Niger River flows along its axis of the
Bida Basin.
Trans-Saharan Seaway
While tectonics affected the Equatorial Atlantic
margin of Africa, the interior of Saharan Africa
was submerged by major transgressions (Late Cre-
taceous– Eocene), due both to the high eustatic sea
level and the overall low elevation of the continent
(Petters 1978; Benkhelil 1989; Genik 1992; Jones
et al. 1995; Guiraud et al. 2005; Burke & Gunnell
2008; Obaje 2009; Markwick et al. 2010). The
Trans-Saharan Seaway connected the Tethys in the
north with the incipient Atlantic Ocean (Fig. 5).
The shape and timing of this seaway gives indi-
cations towards the palaeogeography throughout
Cretaceous and Cenozoic times (Fig. 5), which
enable recognition of the regional drainage divides
through time. The Trans-Saharan Seaway was
across the Benue Trough via the Chad Basin from
Albian to Turonian times (Guiraud et al. 2005).
Since the uplift of the Benue Trough in the San-
tonian, the marine connection was terminated
(Guiraud et al. 2005; Obaje 2009). The Iullemme-
den Basin was transgressed via the Taoudenni
Basin through the narrow Gao Trough (Fig. 2)
(Kogbe 1991; Obaje 2009). A short-lived Paleocene
connection between the Atlantic and the Tethys
via the Iullemmeden Basin and the Bida Basin
has not been proven (e.g. Guiraud et al. 2005) but
is strongly suggested by Kogbe (1989) based on
similarities between Tethyan and Atlantic fauna
during that time, a view supported by Obaje (2009).
The marine conditions over North and West Africa
continued into the Cenozoic, with Tethyan seas in
the Iullemmeden Basin finally retreating in Barto-
nian times, as indicated by the depositional envi-
ronment of sediments (Kogbe 1991). Then fluvial
and, to a minor extent, lacustrine deposition took
place in that basin (Kogbe 1991). Figure 5 shows
the palaeogeographical evolution of the Trans-
Saharan Seaway during each stage of the Cretaceous
Period (Markwick et al. 2010). These palaeogeo-
graphies are based on plate tectonic models and
interpretations from literature (Petters 1978; Genik
1992; Guiraud & Maurin 1992; Guiraud et al.
1992; Jones et al. 1995; Burke 1996; Benkhelil
et al. 1998; Gonc¸alves & Ewert 1998; Basile et al.
2005; Guiraud et al. 2005; Antobreh et al. 2009;
Obaje 2009 amongst others).
Oligocene uplift
A final return to continental deposition took place
in the Oligocene, at 30 Ma, when epeirogenic pro-
cesses started to cause uplift throughout Africa cre-
ating a ‘basin-and-swell’ topography (Burke 1996;
Burke & Gunnell 2008). The basin-and-swell topo-
graphy was responsible for the creation of endorheic
basins all over Africa, including the Taoudenni
Basin into which the palaeo-Upper Niger drained.
The uplifts have been sustained by upper mantle
convection throughout the Neogene and represent
Africa’s main topographical features (Al-Hajri et al.
2009). The overall starting age of the magmatic
events and volcanism is Oligocene but varies locally.
Fig. 4. Outcrop geology of the Benue Trough (from American Geoscience Institute 2014).
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Fig. 5. Palaeogeography of the stages of the Cretaceous Period (Markwick et al. 2010). Top left of the figure shows the relative locations of Africa and South America. T, Taoudenni
Basin; IB, Iullemmeden Basin; H, Hoggar Massif. Shallow seas are in white and lowlands are dark grey. The Aptian stage shows the onset of marine conditions between
Africa and South America. The epicontinental seas submerge large parts of northern and equatorial Africa throughout the Cretaceous. The Iullemmeden Basin is flooded from
Cenomanian to Coniacian times, and again from Campanian to Maastrichtian times. At the location of the Niger Delta and in the Benue Trough, a seaway has been present since
Albian times. In the Campanian and Maastrichtian, rifting in the Bida Basin caused subsidence, forming a small marine embayment.
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In the Hoggar Massif, the first volcanic rocks are
upper Eocene (Lie
´
geois 2006). In the Adamawa and
Hoggar massifs, and on the Jos Plateau, volcanism
and/or doming continued through the Neogene and
the Quaternary (Stuart et al. 1985; Lie
´
geois 2006;
Obaje 2009). Important uplift of the Jos Plateau is
recorded to have started in the Neogene (Obaje
2009). Erosional products from the swells accumu-
lated in the basins (e.g. in the Iullemmeden Basin).
The Niger Delta
The Niger Delta is located at the SW end of the
Benue Trough and progradation has been taking
place since the Eocene (Reijers 2011). Syntheses
of the structure, stratigraphy and petroleum geology
of the Niger Delta are given by Doust & Omatsola
(1989), Doust (1990), Tuttle et al. (1999), Reijers
(2011) and references therein. In the Eocene, the
delta started to spread out over the continental –
lithospheric transition zone, and since the Oligocene
it has been prograding over oceanic crust of the
Gulf of Guinea (Obaje 2009). The total distance of
progradation exceeds 250 km and sediments reach
a thickness of 12 km. It is one of the world’s largest
deltas and its three megasequences show an over-
all upward transition from marine shales (Akata
Formation) through a sand –shale paralic interval
(Agbada Formation) to continental sands (Benin
Formation) (Obaje 2009).
The Niger Delta developed in pulses dictated
by hinterland movements and sea-level oscillations.
Reijers (2011) summarized the megasequences
of the Niger Delta, and during the following time
spans increased progradation rates occurred. At
21.8 Ma, sediment supply increased. In the Early
Miocene, between 19.4 and 15.9 Ma, prograda-
tion occurred in pulses and reached in places 8 –
15 km Ma
21
. In the early Middle Miocene,
between 14.4 and 14.0 Ma, there was an increase
in progradation from 2 to 16– 22 km Ma
21
. In the
Middle Miocene, between 12.8 and 11.5 Ma, an
increase in progradation to 16– 22 km Ma
21
was
observed. Reijers (2011) attributes this specifically
to the increased uplift of the hinterland. Between
9.5 and 5.0 Ma, progradation reached 13–17 km
Ma
21
. Especially after 8.5 Ma, Reijers (2011)
postulates that the hinterland shedded extensive
amounts of clastic sediment. Generally during the
Miocene, the average progradation rate was 1 km
Ma
21
, with a rising hinterland increasing prograda-
tion through the Middle– Late Miocene.
Methodology
The palaeodrainage of the Niger River is
reconstructed using a multidisciplinary approach
comprising geographical information system (GIS)-
based geomorphological techniques. This approach
aims at tying the drainage anomalies, and the chron-
ology of events extracted from the analysis of the
drainage network and the landscape, to the known
geological history.
Data and processing
Shuttle Radar Topography Mission (SRTM) digital
elevation models (DEMs) were used for landscape
and geomorphological interpretation. SRTM30
(30 arcsec) and SRTM3 (3 arcsec) DEMs were pro-
cessed to generate stream networks and drainage
basins at 100 and 10 km
2
, respectively (Fig. 3).
The resolution refers to the minimum area that
each generated stream segment drains. Topographi-
cally generated stream networks were preferred
over mapped river networks because the latter can
be prone to different perceptions of scale, map-
ping resolution and mapping exploration at differ-
ent locations. Since a large part of the study area
is arid, dry riverbeds would be overlooked using
mapped rivers. The Hoggar Massif, for example,
was once drained by large rivers, leaving behind
well-developed river valleys with an integrated
network of tributaries. Almost none of those are
fully displayed on regional maps. It is necessary to
check whether the generated rivers match the map-
ped ones, which they most often do in non-arid
or non-glacial regions. Where dunes occur in arid
landscapes, such as the Sahara, the generated net-
work appears artificial. The generated stream
lines occupy the interdune valleys, creating paral-
lel patterns. In such cases, the drainage network
cannot be used as a starting point to interpret
palaeodrainage.
The stream network is generated through an
iterative process, including the filling of internal
topographical depressions to force flow from each
given point on the land to drain into the oceans
(O’Callaghan & Mark 1984; Tarboton et al. 1991;
Verdin 1997; Verdin & Verdin 1999; Tarboton &
Ames 2001). Primarily, the filling of depressions
is essential to reduce the effect of DEM artefacts
such as pits (one pixel surrounded by higher
pixels; hence not allowing outward flow) and lakes
or dams along the river courses. On a larger scale,
this also integrates the large naturally occurring
depressions, such as the Chad or Taoudenni Basin,
in the drainage basin of the Niger River via the
lowest point along their drainage divides (i.e. via
the spill-points: Fig. 3).
Orders of magnitude (stream numbers) were
assigned to each stream segment of the network,
allowing better visual interpretation and highlight-
ing the higher order streams (Fig. 3). The ordering
hierarchy used here is that of Strahler (1957), where
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first-order streams start at the sources and a higher
order forms when two same-order streams join.
Landscape and drainage patterns
The landscape and drainage analysis starts by iden-
tifying anomalous patterns that are indicative of
changes in the stream network, as described exten-
sively by Summerfield (1991) and Twidale (2004).
Drainage patterns give clues towards past events
and the chronology of events (Twidale 2004). If a
stream network develops upon a flat and homo-
genous surface, a tree-like dendritic pattern devel-
ops. Such patterns are often observed on recently
deposited alluvium in the downstream reaches
of many rivers. However, at most scales, hetero-
geneous lithologies, underlying structure, uplift
and subsidence, and the impact of climate, vol-
canism, glaciations and mass movements, and so
on will cause the dendritic patterns to alter or not
to develop at all. Patterns diverging from dendritic
patterns can therefore be interpreted as being indica-
tive of an underlying mechanism and/or structure.
Amongst textbook examples of typical patterns
occurring at varying scales are centripetal (in inter-
nal depressions), radial (around volcanoes and
doming regions), parallel (due to tilting or uplift),
trellis (in fold and thrust belts) and rectangular pat-
terns (in limestone) (Summerfield 1991). In essence,
all such patterns are determined by slope and struc-
ture (e.g. Twidale 2004).
Equally indicative of drainage change are anom-
alous bends (Summerfield 1991; Twidale 2004).
Although somewhat arbitrary, we here define
an anomalous bend as a bend that is abrupt, large
and/or singular in map view. The Large Bend and
the Nigerian Bends of the Niger River are examples.
With abrupt, a sudden/sharp change in flow direc-
tion is referred to, very often due to underlying
structure. Large bends are those along the trunk
stream or along an important tributary, representing
an obvious regional feature, and having an impact
on the overall geometry and shape of the river
network. Finally, with singular, we indicate that
the bend in question is not part of a network of
many smaller anomalous bends; for example, the
many abrupt bends observed in trellis and rectan-
gular drainage patterns (both caused by very local
drainage readjustments influenced by the bedrock
and structure) or along meandering streams. A
main exception is when smaller grouped abrupt
bends occur as barbed confluences. In that case the
abrupt bends occur along several tributaries of a
trunk stream and they indicate that the flow direc-
tion in this stream was opposite in the past, as
illustrated in Figure 6. Anomalous bends can be
elbows of captures that form when a more erosive
stream beheads the head waters of another stream,
as described, for example, in the Yangtze River
(Zheng et al. 2013). In many rivers, anomalous
bends at varying scales are prominent features
that are very often associated with important drai-
nage reorganizations.
Whereas for recent and active geological events,
the effects on the drainage network and the topo-
graphy are evident, older patterns systematically
are scarcer and more complex to interpret owing
to overprinting, erosion or deposition. Over time,
geomorphic evidence of more ancient events even-
tually becomes obliterated at rates dependent on,
for example, lithology, uplift rate and climate.
Therefore, understanding geological evolution and
palaeogeography becomes increasingly important
when taking palaeodrainage reconstructions back
in time. Finally, for the more ancient recon-
structions, only geological evidence and palaeo-
geographies and palaeotopographies can give clues
of past drainage. For this reason, Cretaceous palaeo-
geographies of the study (Fig. 5) have been used to
interpret the early palaeodrainage history of the
Niger River.
Several geomorphological features in the land-
scape are major sources of information on uplift,
and subsequent erosion, denudation and incision
(Burbank & Pinter 1999). Such features include, but
are not limited to, erosional scarps such as at palaeo-
surfaces and amphitheatre-shaped valleys. Palaeo-
surfaces stand as highs in the landscape, and are
the relicts of ancient erosion surfaces that formed
due to denudation and peneplanation during an
Fig. 6. Development of barbed confluences: (1) a river
flows to the left with tributaries joining at sharp angles;
(2) the river changes flow direction and the inner banks
of the tributaries start being eroded; (3) in time, the
barbed confluences migrate away from the main stream
and become less sharp, as erosion on the inner banks
continues. The patterns eventually disappear.
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episode of tectonic stability (Twidale 1994; Burke
& Gunnell 2008). Amphitheatre-shaped valleys are
caused by headward erosion, and their presence
indicates that an erosive river system is actively
expanding its catchment at the expense of the area
of the adjacent drainage basin. Understanding the
above processes is useful because, if the age of the
incised strata is known, a lower age for incision
and, hence, a lowering in base level can be fixed
and linked to a geological or eustatic event.
Longitudinal (long) profiles of rivers are used to
identify knickpoints that give information on the
propagation of erosion after an episode of uplift,
or are caused by a drainage capture. The long pro-
file of the Niger River is shown in Figure 7. The
long profile consists of a nearly graded upstream
section (between 450 and 270 m in elevation),
which corresponds to the Upper Niger. The profile
of the Upper Niger flattens out at 270 m elevation
where it merges into the Inland Niger Delta. The
maturity suggested by the concave-up shape of the
profile supports the hypothesis of a long-lived drai-
nage setting in which the palaeo-Upper Niger
flowed into an endorheic basin in the Azawagh
region in the Taoudenni Basin. The knickpoint (i.e.
the convex-up segment between 250 and 200 m in
elevation: Fig. 7) marks the location where Holo-
cene capture took place.
Results
In the Niger River the most obvious anomaly is the
Large Bend, where the river turns 908 from a
NE to SE course since the recent past. The Nigerian
Bends, a northern and a southern one, show a more
abrupt change in flow, even though the change in
flow direction is only 458, towards the south and
then the SE, respectively. The northern Nigerian
Bend is located at the southern end of the Iullem-
meden Basin, whereas the southern Nigerian Bend
is located at the SW side of the Bida Basin,
200 km further downstream. The Nigerian Bends
suggest that a drainage reorganization may have
occurred between the Iullemmeden and the Bida
basins. This is in agreement with the palaeogeo-
graphical setting indicating that, in the Cretaceous,
these basins were not connected hydrographi-
cally (Fig. 5).
The Lower Niger flows along the axis of the
Bida Basin, which underwent fault-controlled
subsidence in the Campanian and Maastrichtian
(Obaje 2009). The palaeo-Lower Niger started to
drain the newly exposed land since the sea had
regressed from the Bida Basin in the latest Cretac-
eous. No major tectonic activity has later affected
the Bida Basin, therefore it is assumed that the pos-
ition of the Lower Niger has not significantly
changed since then. Coevally, the Iullemmeden
Basin was submerged by Tethyan seas that trans-
gressed from the NW via the Taoudenni Basin
(e.g. Guiraud et al. 2005). There was, hence, a sill
or drainage divide between the Tethyan and Atlantic
domains in the Iullemmeden and Bida basins,
respectively, and this divide ran across the 200 km
present-day stream segment between the Nigerian
Bends (Fig. 8). This sill was later breached to
form the integrated Niger River.
The drainage patterns near the Nigerian Bends
show evidence of the above setting: (1) tributaries
of the Niger diverge from a SW-trending ridge,
interpreted to be the palaeodrainage divide (Fig. 9);
and (2) barbed confluences in the SE Iullemmeden
Basin, which are indicative of a change in flow
direction (Fig. 9). Flow in that part of this basin
was, hence, to the NW; that is, towards the centre
of the Iullemmeden Basin. The palaeo-Middle
Niger in the Iullemmeden Basin was either endor-
heic or drained to the Taoudenni Basin. The endor-
heic option is favoured because, after final retreat
of the sea in Bartonian times (41 – 38 Ma), lacustrine
deposition occurred, which is suggestive of the lack
of an efficient outlet (Kogbe 1991). Furthermore,
centripetal drainage is common in intra-cratonic
basins (e.g. Summerfield 1991).
Other drainage patterns are also observed.
(1) Along the Middle Niger in the Iullemme-
den Basin, drainage is asymmetrical, with long
tributaries to the north and only short ones to the
south (Fig. 3). The asymmetry can be linked to
southward tilting, which causes a shift to the south
of the drainage in the Iullemmeden Basin. This
tilting was caused by the epeirogenic uplift of the
Hoggar Massif that started in the late Eocene and
continued in pulses through the Neogene (Lie
´
geois
2006). (2) Radial drainage at the Jos Plateau
(Fig. 9), which is typical for doming and volcanism
that has been continuous since the Pliocene up to
very recent time (Obaje 2009).
Fig. 7. Longitudinal profile of the Niger River.
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The palaeodrainage divide between the Nigerian
Bends could not have been breached earlier than the
late Eocene (post-Bartonian) because until then
there was a marine embayment in the Iullemmeden
Basin, connected to the Tethys Sea. To better con-
strain the age of breaching, we analysed the geo-
morphology of the Iullemmeden Basin. This basin
is recognized by a regional incised palaeosurface
(Fig. 10), in the landscape appearing as flat-topped,
steep-sided hills capped by ironstone (Obaje 2009).
The strata on which the palaeosurface developed are
intercalated massive white clays and sandstones of
the fluvial/lacustrine Gwandu Formation (Kogbe
1991; Obaje 2009). The Gwandu Formation is
tentatively of Eocene –Miocene age and outcrops
extensively in the Iullemmeden Basin (Kogbe
1991). The observed incision can only have started
after a base-level lowering that terminated the lacus-
trine/fluvial deposition. This could have occurred
when a hydrological connection to the sea was
made; that is, by breaching of the aforementioned
drainage divide. Detritus from the Iullemmeden
Basin and its catchment has since then been trans-
ported to the Niger Delta. The upper age of the
Gwandu Formation is the Miocene; hence the
capture occurred in the Miocene or later.
A possible trigger for the capture event to happen
was the combination of the onset of headward
erosion along the NW edge of the Bida Basin, and
the southward tilting and uplift in the Iullemmeden
Basin, which facilitated the effective draining of the
formerly endorheic basin. The epeirogenic uplift of
the Hoggar Massif and the Jos Plateau played a
pivotal role in initiating both processes.
The catchment of the palaeo-Lower Niger that
has drained the Bida Basin since the Maastrichtian
included the flanks of the Nigerian Shield in the
south and the Jos Plateau in the north. The Jos
Plateau started to undergo epeirogenic uplift in the
Oligocene (Burke 1996), with the oldest volcanic
deposits being pre-Neogene (Obaje 2009). This
regional uplift may also have affected, to a lesser
degree, the Nigerian Shield. The uplift caused a
relative lowering in base level, triggering a pulse
Fig. 8. Maastrichtian palaeogeography (Markwick et al. 2010). The distribution of land and sea shows that the
Tethyan transgression submerged the Iullemmeden Basin. The last time this took place was in Bartonian times (Eocene).
Until at least then, there was a continental drainage divide between the depositional Iullemmeden Basin as part of the
Tethyan domain, and the Bida Basin, part of the Atlantic domain. The palaeo-Upper Niger flowed from the Guinea
Highlands into the Taoudenni Basin, which was depositional throughout the Cenozoic.
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of erosion into the Jos Plateau. Thereby significant
volumes of sediment may have been removed
since the Oligocene. Headward erosion was facili-
tated near the NW edge of the Bida Basin due to
the presence of a NNE-trending Pan-African schist
belt (Obaje 2009), detectable in the topography as
lineaments (Fig. 11). Propagation of erosion along
this structurally weaker zone eventually resulted
in breaching of the palaeodrainage divide and
capture of the drainage in the Iullemmeden Basin.
Several features in the geomorphology and the
drainage network are the result of the latter events:
(1) the amphitheatre-shaped valleys (Fig. 11) obser-
ved in the landscape are indicative of headward
erosion; (2) the radial drainage developed around
the Jos Plateau testifies of the domal uplift origin
of this plateau (Fig. 9); (3) the Maastrichtian
marine sediments in the Bida Basin are incised,
which is the result of lowering of the base level
due to the uplift of the Jos Plateau; and (4) the
Lower Niger River is shifted to the south with
respect to the depositional axis of the Bida Basin,
also interpreted to be due to the uplift.
The process of the capture may have been
gradual rather than sudden. Erosion first took
place in the SE of the Iullemmeden Basin and then
propagated into the rest of the basin as a result
of the increased potential energy of the stream
network. An effective stream draining SE out of
the basin became ultimately established. This was
the newly formed Middle Niger, which until now
has been recognized by barbed confluences along
its course.
As a last phase of the evolution of the Niger
River, the Upper Niger was captured in the Holo-
cene, as explained at the start of this paper. Owing
to deposition in the Inland Niger Delta, the sediment
contribution of the Upper Niger River is interpreted
as negligible for the Niger Delta.
At present, the water flux within the Iullem-
meden Basin is greatly reduced, especially in the
north where many streams are defunct owing to
the formation of the Sahara Desert. As a result of
the aridity, the sediment flux of the Niger River is
lower than that of the Benue River, which drains
an area with much a higher run-off due to the
Fig. 9. Stream patterns of the Middle and Lower Niger. Red circles show the Nigerian Bends. Blue lines accentuate
several streams showing evidence of drainage change. Dashed grey lines surround Cretaceous–Cenozoic basins.
Barbed confluences (in small black circles) are shown along the Middle Niger in the Iullemmeden Basin. The white
dashed line west of the Niger River shows the palaeodrainage divide between the Iullemmeden and Bida basins.
Lower-order tributaries diverge from this ridge. East of the Niger River, overprinting by the radial drainage patterns of
the Jos Plateau may have erased geomorphological evidence of the drainage divide.
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tropical climate. The climate in the Niger River drai-
nage basin has not always been as arid as today. The
climate was tropical until the end of the Miocene
when open grasslands, replacing forests, started to
appear (Micheels et al. 2009). Overall, the Sahara
Desert started developing in the Pliocene (Micheels
et al. 2009). In the Nigerian part of the Iullemmeden
Basin, the region had a hot and humid climate until
the Quaternary, as evidenced by the presence of
laterites of Pliocene –Early Quaternary age (Obaje
2009). Therefore, past run-off contributed to higher
sediment loads and considerably affected sedimen-
tation in the Niger Delta.
Discussion
We have discovered that the Niger River for-
med through three phases: the Bida Basin phase
(Maastrichtian– Miocene); the Iullemmeden phase
(Miocene– Holocene); and the present-day Niger
River phase (Holocene). The capture events to
form the Nigerian Bends and the Large Bend
represent the transitions between the three phases,
and hence their prominence in today’s drainage
patterns has been proved to be indicative of major
events within the palaeodrainage evolution of the
Niger River. The chronological summary of the
evolution of the Niger River is given together with
the implications on sediment supply to the Niger
Delta. Figure 12 illustrates the suggested evolution
of the Niger River.
The Bida Basin phase was preceded by the
development of the Cretaceous triple junction
between the Benue Trough, and the South and Equa-
torial Atlantic. The Bida Basin formed in the Cam-
panian as an embayment of the Benue Trough, and
marine deposition continued into the Maastrichtian.
After retreat of the sea in the embayment of the Bida
Basin, the palaeo-Lower Niger started to drain the
emerged basin. At the same time, the palaeo-Benue
River drained the Lower Benue Trough, and has had
sources along the flanks of the Benue Trough and
the uplifted Middle Benue Trough since the Santo-
nian. The palaeo-Lower Niger and palaeo-Benue
Rivers were similar in length, and gradually filled
in the Anambra Basin until the Niger Delta started
to prograde. The catchment of the palaeo-Lower
Niger remained limited to the Bida Basin and
its immediate surroundings until the Miocene.
Despite the small size of that basin, it might have
had a considerable impact on the progradation of
Fig. 10. Shaded relief of the Iullemmeden Basin showing palaeosurfaces incised by the Middle Niger and tributaries.
The palaeosurfaces are the flat areas between the steep river valleys.
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the Niger Delta since the Oligocene, when uplift and
volcanism started in the Jos Plateau. The onset of
progradation of the Niger Delta coincides with the
onset of regional magmatic uplifts of Burke
(1996). Erosion both north and south of the Bida
Basin was of Precambrian units of the Nigerian
Basement Complex of Pan-African age (Obaje
2009). The Basement Complex consists mainly of
the ‘Migmatite– Gneiss–Quartzite Complex’, and
with secondary ‘Schist Belts’ made up of phylites,
schists, pelites, quartzites, marbles and amphibolites
(Obaje 2009). Even though the Schist Belts were
less abundant in surface area, they were eroded
more deeply due to their less resistant lithologies
and foliated texture. Overall, the quartz content
in the eroded rock is high, which has a positive
impact on reservoir quality. Uplift of the Jos Pla-
teau became more important through the Miocene
(Obaje 2009), and hence this area remained an
important sediment source for deposition in the
Niger Delta. The several Miocene pulses in progra-
dation in the Niger Delta are possibly related to
pulses in uplift of the Jos Plateau, the latter supply-
ing sediment via both the palaeo-Lower Niger and
the palaeo-Benue.
The Iullemmeden phase started in the Miocene.
A more precise timing cannot be determined.
The palaeo-Lower Niger had breached through the
palaeodrainage divide, eroding through the structu-
rally controlled weaker lithologies of the Schist
Belt (Obaje 2009) in the NW of the Bida Basin.
The deposition of the Gwandu Formation stopped
and, due to upstream progradation of erosion, the
entire Iullemmeden Basin and its catchment, an
area of roughly 10
6
km
2
, was incorporated to the
provenance area of the Niger Delta. An increase in
sediment supply to the Niger Delta is therefore
expected to have occurred. At the end of the
Miocene, the climate started to become more arid
but savannah vegetation still covered large parts
of North Africa, and Saharan conditions were
not established until the Pliocene and Pleistocene
(Micheels et al. 2009). This suggests that run-off
and erosion in the catchment of the palaeo-Middle
Niger was more important than at present. Mag-
matic uplift affecting the Hoggar Massif further
increased erosion and the sediment load of the
palaeo-Middle Niger. Not knowing exactly when
the capture happened, the postulated increase in
sediment load can be linked to either of the
Fig. 11. Topography of the Bida Basin, with the Lower Niger and confluence of the Benue and Niger rivers. White
arrows show the direction of headward erosion, most of them within amphitheatre-shaped valleys. The red lines show
the structural fabric of the Pan-African belt. Eroding streams probably exploited those structures while eroding
backwards into the Iullemmeden Basin and eventually capturing the drainage of that basin. The dashed white line
is the interpreted palaeodrainage divide between the drainage systems of the Bida and Iullemmeden basins. The inset
figure shows the location and the Meso- and Cenozoic geology, highlighting the distribution of the Cretaceous –
Cenozoic basins.
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Fig. 12. Overview of the evolution of the Niger River drainage basin. The coloured areas are the previously separate drainage basins that were significant in the evolution of
the Niger River. The green drainage basin is the one that evolves into the present-day Niger River drainage basin. Red circles highlight the locations of the magmatic uplifts that
started in the Oligocene.
K. P. M. BONNE
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Miocene increases in progradation at 14.4–14.0,
12.8– 11.5 or post-8.5 Ma. A change in sediment
composition must have accompanied the increase
in sediment supply. Cretaceous–Cenozoic sedi-
ments were eroded from the Iullemmeden Basin.
The most widely exposed sediments are those of
the Cenozoic, including the Gwandu Formation.
They consist of clays and sands (Kogbe 1991).
The Cretaceous, Paleocene and Eocene rocks are
mainly marine limestones with some sandstones
and mudstones (Kogbe 1991). Palaeozoic rocks,
exposed on the SE side of the Hoggar Massifs
contain conclomerates, sandstones (some with fer-
ruginous oolites), arkoses, shales, calcareous and
gypsiferous shales, and magmatic detritus (Kogbe
1991). In the Hoggar Massif and the Adrar des
Iforas, Pan-African basement is exposed, compris-
ing oceanic island arcs, cratonic cores, ophiolites
and eclogites (Lie
´
geois 2006). Volcanism occurred
between 35 and 30 Ma, the voluminous phase
between 20 and 12 Ma, and the last phase between
3 Ma and the Late Quaternary (Lie
´
geois 2006).
Depending on the timing of drainage capture, the
volcanic phases could have negatively influenced
sediment type and reservoir quality within the
Niger Delta, even though the volcanic impact of
the Hoggar would probably have been swamped
by volcanic rocks transported by the Benue River.
From the southern side of the Iullemmeden Basin,
the West African Craton also contributed sediment
to the Niger Delta, eroded mainly from gneisses,
migmatites and granites.
The third phase of the present-day Niger River
did not significantly affect sediment supply to the
Niger Delta. Rather, the opposite happened. The
climate started to become more arid throughout
the Pliocene and into the Quaternary, decreasing
the importance of the Iullemmeden Basin and espe-
cially its northern regions as a provenance area for
the Niger Delta, and dune formation and deposi-
tion in the Inland Niger Delta have further hin-
dered any significant transport of sediment from
the Upper Niger.
Conclusions
In this study, the pre-Holocene palaeodrainage evol-
ution of the Niger River was reconstructed and it
was shown that this river evolved through three
phases, of which only the last one was previously
described. Sediment supply of the Niger River
to the incipient Niger Delta was initially from
a small catchment near the Bida Basin, draining
mainly crystalline basement including migmatites,
gneisses and granites, and positively influencing
reservoir quality. The second phase started in the
Miocene when the drainage basin increased by
10
6
km
2
due to capture of the drainage in the Iul-
lemmeden Basin. This increased the sediment flux
to the Niger Delta, and changed the sediment type
to mixed lithologies eroded from large exposures
of clay, sand and limestone, and smaller exposures
of conglomerates, arkoze, shales, volcanics and
ultramafics. This change in lithology could have
negatively affected reservoir quality. The onset of
aridification in the Pliocene reduced the run-off
and, hence, sediment supply from the Iullemmeden
area. Owing to the aridity and further development
of the Sahara in the Holocene, the capture of
the Upper Niger (third phase) did not affect sedi-
ment supply.
The drainage patterns and anomalies observed
within the network of the Niger River match the
geological history, and were used as a tool to
reconstruct palaeodrainage. For future develop-
ment, better dating of strata, especially that of the
Gwandu Formation, would enable the proposed
timing of stream capture between the Bida and
Iullemmeden basins to be refined. This would also
allow a better correlation with the episodes of
increased progradation in the Niger Delta and,
hence, to make interpretations about the sediment
type at specific locations within the delta stratigra-
phy. A detailed palaeodrainage study of the Benue
River would increase knowledge on the combined
effect of both the Niger and Benue river systems
on sediment supply to the Niger Delta.
This study was initiated at Getech (Leeds, UK), which is
gratefully acknowledged for giving permission to pub-
lish. Thanks are extended to L. M. Wilson for processing
morphometric and GIS data, and for discussions and
support, and to P. J. Markwick for methodology support.
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