Download Stratigraphic and structural evolution of the Blue Nile Basin

GEOLOGICAL JOURNAL
Geol. J. (2008)
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/gj.1127
Stratigraphic and structural evolution of the Blue Nile Basin,
Northwestern Ethiopian Plateau
N. DS. GANI 1*, M. G. ABDELSALAM 2, S. GERA 3 and M. R. GANI 1
1
Earth and Environmental Sciences, University of New Orleans, New Orleans, LA, USA
Geological Sciences and Engineering, Missouri University of Science and Technology, Rolla, MO, USA
3
Regional Mapping and Geochemistry Department, Geological Survey of Ethiopia, Addis Ababa, Ethiopia
2
The Blue Nile Basin, situated in the Northwestern Ethiopian Plateau, contains 1400 m thick Mesozoic sedimentary section
underlain by Neoproterozoic basement rocks and overlain by Early–Late Oligocene and Quaternary volcanic rocks. This study
outlines the stratigraphic and structural evolution of the Blue Nile Basin based on field and remote sensing studies along the
Gorge of the Nile. The Blue Nile Basin has evolved in three main phases: (1) pre-sedimentation phase, include pre-rift
peneplanation of the Neoproterozoic basement rocks, possibly during Palaeozoic time; (2) sedimentation phase from Triassic to
Early Cretaceous, including: (a) Triassic–Early Jurassic fluvial sedimentation (Lower Sandstone, 300 m thick); (b) Early
Jurassic marine transgression (glauconitic sandy mudstone, 30 m thick); (c) Early–Middle Jurassic deepening of the basin
(Lower Limestone, 450 m thick); (d) desiccation of the basin and deposition of Early–Middle Jurassic gypsum; (e)
Middle–Late Jurassic marine transgression (Upper Limestone, 400 m thick); (f) Late Jurassic–Early Cretaceous basin-uplift
and marine regression (alluvial/fluvial Upper Sandstone, 280 m thick); (3) the post-sedimentation phase, including Early–Late
Oligocene eruption of 500–2000 m thick Lower volcanic rocks, related to the Afar Mantle Plume and emplacement of 300 m
thick Quaternary Upper volcanic rocks. The Mesozoic to Cenozoic units were deposited during extension attributed to
Triassic–Cretaceous NE–SW-directed extension related to the Mesozoic rifting of Gondwana. The Blue Nile Basin was formed
as a NW-trending rift, within which much of the Mesozoic clastic and marine sediments were deposited. This was followed by
Late Miocene NW–SE-directed extension related to the Main Ethiopian Rift that formed NE-trending faults, affecting Lower
volcanic rocks and the upper part of the Mesozoic section. The region was subsequently affected by Quaternary E–W and
NNE–SSW-directed extensions related to oblique opening of the Main Ethiopian Rift and development of E-trending transverse
faults, as well as NE–SW-directed extension in southern Afar (related to northeastward separation of the Arabian Plate from
the African Plate) and E–W-directed extensions in western Afar (related to the stepping of the Red Sea axis into Afar). These
Quaternary stress regimes resulted in the development of N-, ESE- and NW-trending extensional structures within the Blue Nile
Basin. Copyright # 2008 John Wiley & Sons, Ltd.
Received 21 June 2007; accepted 7 May 2008
KEY WORDS Blue Nile Basin; Mesozoic rift systems; basin evolution; eastern and central Africa
1. INTRODUCTION
The Blue Nile Basin is situated in the Northwestern Ethiopian Plateau and is bounded to the E and SE by the
tectonic escarpment of the uplifted western flank of the Main Ethiopian Rift and to the N and S by the
Axum–Adigrat and Ambo lineaments, respectively. The basin contains a 1400 m thick section of Mesozoic
sedimentary rocks unconformably overlying Neoproterozoic basement rocks and unconformably overlain by
Early–Late Oligocene and Quaternary volcanic rocks. The architecture of this basin is poorly known, but it is
* Correspondence to: N. DS. Gani, Department of Earth and Environmental Sciences, University of New Orleans, 2000 Lakeshore Drive, New
Orleans, LA 70148, USA. E-mail: [email protected]
Copyright # 2008 John Wiley & Sons, Ltd.
n. ds. gani
ET AL.
Figure 1. (a) Inset map showing the location of Figure 1(b). (b) Tectonic elements within and around the Blue Nile Basin (Modified after
Fairhead 1988; Bosellini 1989; Guiraud and Maurin 1992; Binks and Fairhead 1992; Worku and Astin 1992; Russo et al. 1994; Mege and Korme
2004; Hautot et al. 2006); Ax–Ad, Axum–Adigrat Lineament.
considered to have formed during the Mesozoic break-up of Gondwana, similar to NW-trending Mesozoic rifts that
exist throughout northern and central Africa (Figure 1; Bosellini 1989; Russo et al. 1994).
Previous stratigraphic studies on the Blue Nile Basin and surrounding areas are summarized in Table 1.
Regardless of these important studies, the stratigraphic and structural evolution of the Blue Nile Basin is not fully
understood since much of the basin’s geological record (Mesozoic and Precambrian rocks) is buried beneath the
extensive 500–2000 m thick Cenozoic volcanic rocks (Hofmann et al. 1997; Coulie et al. 2003; Kieffer et al. 2004)
and no subsurface data are available. However, the 1600 m deep Gorge of the Nile (Gani and Abdelsalam 2006;
Gani et al. 2007) formed by the Blue Nile River on the Northwestern Ethiopian Plateau (Figure 2) provides good
surface exposures suitable for focused stratigraphic and structural studies that can be used for regional
reconstruction of the geological history of the Blue Nile Basin. The Blue Nile flows SE from Lake Tana, then S and
SW, before it assumes a NW-flowing direction as it approaches the lowlands of Sudan (Figure 2; Gani and
Abdelsalam 2006). Exposures in this gorge start with Neoproterozoic basement rocks (of 750 Ma age). Mesozoic
sedimentary rocks are sandwiched unconformably between the Neoproterozoic basement rocks and Early–Late
Oligocene volcanic rocks (Gani and Abdelsalam 2006). No Palaeozoic sedimentary rocks are exposed within the
key study areas of the Gorge of the Nile. Only Late Palaeozoic–Triassic rocks are exposed at the lower reaches of
the gorge (Mangesha et al. 1996). This is in contrast to the presence of an extensive Palaeozoic sedimentary section
at the base of the Ogaden Basin, to the southeast of the Blue Nile Basin (Figure 1; Williams 2002). The scarcity of
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
blue nile basin evolution
Table 1. Previous stratigraphic studies in the Blue Nile Basin and surrounding areas
Authors
Studies
Krenkel (1926)
Stefanini (1933), Merla et al. (1973)
Described limestone, gypsum and shale unit of the Blue Nile
Produced geological map of Ethiopia, Eritrea and Somalia
accompanied by general stratigraphic description of the region
Described the regional geology of Ethiopia
Studied marine fossils of the Blue Nile section
Explain geological map of Ethiopia
Reconstructed the palaeogeography and subsidence of the
central Ethiopian sedimentary basins in the Jurassic period
Studied palaeofloral dating of volcanics on the Ethiopian Plateau
Studied the Mesozoic invertebrate fossils of Ethiopia
Determined the age and rates of denudation of Ethiopian
Trap Series basalt
Described microfacies of the limestone in the Blue Nile Basin
Determined the age of palaeoflora of the Ethiopian Plateau
volcanic rocks
Established lithostratigraphic units of the Blue Nile Basin
Outlined the sedimentary evolution of the Abay River Basin
Generated geological map of Ethiopia
Determined the age of Ethiopian flood basalt
Presented the first palynostratigraphic dates for Mesozoic faunas
Determined age and duration of the Ethiopian trap series basalt
Determined ages of Ethiopian flood basalt and shield volcanoes
Produced geological map of Dejen–Gohatsion region of the Gorge
of the Nile
Mohr (1962)
Ficcarelli (1968)
Kazmin (1973, 1975)
Beauchamp and Lemoigne (1974, 1975)
Kalb and Oswald (1974)
McDougall et al. (1975)
Canuti and Radrizzani (1975)
Beauchamp (1977)
Assefa (1979, 1980, 1981, 1991)
Russo et al. (1994)
Mangesha et al. (1996)
Hofmann et al. (1997)
Wood et al. (1997)
Coulie et al. (2003)
Kieffer et al. (2004)
Gani and Abdelsalam (2006)
Palaeozoic sedimentary rocks within the Blue Nile Basin might be due to uplift during the Palaeozoic Era resulting
in extensive erosion throughout this area.
This study represents the first comprehensive examination of the evolution of the Blue Nile Basin through the
integration of stratigraphic and structural data and evaluation of the basin’s architecture within the regional tectonic
elements. The objectives of this work include documentation of the stratigraphic history and structural architecture
of the Blue Nile Basin, examination of the evolution of the basin in relation to regional tectonics and definition of
the palaeogeographic history of the basin. These objectives were addressed in field studies along exposures within
the Gorge of the Nile supplemented by the analysis of orbital optical and radar remote sensing data and digital
elevation models (DEMs). We have focused on four key areas, covering different parts of the Blue Nile Basin along
the Gorge of the Nile (Figure 2), where different stratigraphic units are exposed and where various orientations of
extensional structures can be documented. The Quaternary volcanic rocks are not exposed in any of the four key
areas. Hence, to examine this unit we have relied on exposures close to Lake Tana where the Blue Nile flows SE
(Figure 2).
2. REGIONAL STRUCTURAL FRAMEWORK
During the Triassic–Cretaceous time, northern and central Africa was affected by lithospheric extension associated
with NE–SW extension (Fairhead 1988). This formed NW-trending Mesozoic rift basins including the Muglad, the
Melut, the Blue Nile and the Anza rift basins (Figure 1; McHargue et al. 1992; Binks and Fairhead 1992). Bosellini
(1989, 1992) and Russo et al. (1994) interpreted these structures as NW-trending aulacogen-like rift basins
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
n. ds. gani
ET AL.
Figure 2. Hill shade Digital Elevation Model (DEM) extracted from the 90 m x–y resolution Shuttle Radar Topography Mission (SRTM) data
showing the Gorge of the Nile and the location of the four key areas used in this study.
extending northwestward from the NE-trending Karoo rift which was formed in Late Palaeozoic–Jurassic times
during Gondwana break-up (Figure 1). These rift basins terminate sharply in the northwest against the NE-trending
Central African Shear Zone, which is considered to be a major dextral strike-slip shear zone (Figure 1; McHargue
et al. 1992; Binks and Fairhead 1992). However, there might be some lithospheric extension to the north of the shear
zone, especially in the vicinity of the Blue Nile Basin (Millegan 1990; McHargue et al. 1992).
The southeastern continuation of the Mesozoic rift basins, especially in the highlands of Ethiopia, is poorly
understood. There, these basins are covered by 500–2000 m thick pile of Early–Late Oligocene volcanic rocks, and
locally followed by 300 m thick sequence of Quaternary volcanic rocks. These volcanic rocks are associated with
the Afar Mantle Plume and subsequent opening of the Afar Depression and the Main Ethiopian Rift (Hofmann et al.
1997; Abebe et al. 2005). Most of the published work has concentrated on the Melut, the Muglad and the Blue Nile
rift basin in Sudan, and the Anza rift basin in Kenya (Figure 1; Binks and Fairhead 1992; Guiraud and Maurin 1992;
McHargue et al. 1992; Bosworth and Morley 1994). These studies have shown that the Melut and the Muglad rift
basins connect with each other in the southeast and then connect with the Anza Rift in Kenya (Figure 1; McHargue
et al. 1992; Binks and Fairhead 1992). However, the continuation of the Blue Nile rift basin to the southeast from
the lowlands of Sudan towards the highlands of Ethiopia is not certain for the reasons outlined above. The Blue Nile
Basin in Ethiopia lies between 98N and 13850’N, and 34850’E and 39850’E where the Blue Nile is incised into
the 2500 m high (average) Northwestern Ethiopian Plateau (Figure 2). The linear exposures in the Gorge of the
Nile make it difficult to trace the trend of extensional structures related to the Blue Nile Basin. Nevertheless,
Mesozoic sedimentary sections and a few observed NW-trending faults have led some authors to suggest that the
Blue Nile Basin is related to Mesozoic rift basins of eastern and central Africa (Figure 1; Bosellini 1989, 1992). The
presence of NW-trending sub-basins underneath Lake Tana has been taken as evidence to support this notion
(Hautot et al. 2006). Furthermore, it has been suggested that the Blue Nile Basin in Sudan continues southeastward
through Ethiopia, across the NE-trending Main Ethiopian Rift to join the Ogaden Basin in southeastern Ethiopia
(Figure 1; Bosellini 1989; Russo et al. 1994).
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
blue nile basin evolution
The Blue Nile Basin is thought to have formed during the Late Jurassic on the basis of K/Ar age (143 6 and
124 5 Ma) of two basaltic layers encountered in the Khartoum Basin of the Blue Nile Rift in Sudan (Bosworth
1992).
The exposures of the Blue Nile Basin within the Northwestern Ethiopian Plateau are bordered by the uplifted
tectonic escarpments on the western flanks of the Afar Depression and the Main Ethiopian Rift in the east and
southeast, respectively, and in the west by the erosional Tana escarpment (Figure 1). The Quaternary-aged
E-trending Axum–Adigrat and Ambo lineaments (Abebe et al. 1998) bordered this region in the north and south,
respectively (Figure 1). The topography of the Northwestern Ethiopian Plateau is shaped by the presence of
outstanding 10.7–22.4 Ma old (40Ar/39Ar ages of Kieffer et al. 2004) shield volcanoes around which the Blue Nile
navigates (Figure 2). Some 1400 m of Mesozoic sedimentary rocks are exposed where the Blue Nile forms the
150 km semi-circular Blue Nile Bend within very rugged and largely inaccessible terrain (Gani and Abdelsalam
2006).
3. DATA AND METHODS
Field and remote sensing studies have been focused on four accessible key areas along the Gorge of the Nile
(Figure 2) that expose representative Mesozoic and Cenozoic stratigraphic successions and allow for examination
of various structural styles and orientations (areas 1, 2, 3 and 4 on Figure 2). Additionally, we have used geological
and structural data collected along the SE-flowing segment of the Blue Nile close to Lake Tana to examine the
orientation and style of geological structures within the Quaternary volcanic rocks.
Remote sensing data used in this study include: (1) the Advanced Spaceborne Thermal Emission and Reflection
Radiometer (ASTER) data. These data have three visible and near infrared (VNIR) bands with 15 m spatial
resolution, six shortwave infrared (SWIR) bands with 30 m spatial resolution and five thermal infrared (TIR) bands
with 90 m spatial resolution. (2) Landsat thematic mapper (TM) data which have four VNIR bands and two SWIR
bands with 30 m spatial resolution, and one TIR band with 60 m spatial resolution. (3) Standard beam RADARSAT
data which have a C-band (wavelength ¼ 6 cm), and 25 m spatial resolution. (4) Digital Elevation Models (DEMs)
extracted from the Shuttle Radar Topography Mission (SRTM) data with 90 m x–y resolution. (5) ASTER DEMs
with 15 m x–y resolution. The methods used to process and interpret the remote sensing data used in this study are
described in detail in Gani and Abdelsalam (2006).
Field studies are focused on mapping different stratigraphic units as well as documenting the orientation and
style of geological structures. Field studies and remote sensing analysis are used to produce detailed geological
maps and geological cross-sections for each of the key areas (an exercise helped by reference to published
geological maps (Mangesha et al. 1996)) and to document the general trends of faults (dominantly normal faults)
and fractures (mostly dilational). These studies provide the basis for: (1) production of a comprehensive
stratigraphic column for the Blue Nile Basin; (2) examination of the basin’s structural orientations and styles within
the framework of regional tectonic stress regimes and (3) construction of an evolutionary model for the basin.
4. STRATIGRAPHY, DEPOSITIONAL ENVIRONMENTS AND STRUCTURES
Geological maps and geological cross-sections for each key area are presented in Figures 3–6. A comprehensive
stratigraphic column for the Blue Nile Basin is shown in Figure 7. The dominant orientations of faults and fractures
for each stratigraphic unit are shown in Figure 8.
Key areas 1 and 2 occur where the Blue Nile flows NW (Figures 2–4). Here, the exposures are dominantly
Neoproterozoic basement rocks, Triassic–Early Jurassic Lower Sandstone, and Early–Late Oligocene volcanic
rocks. Key area 3 is within the SW-flowing segment of the Blue Nile (Figures 2 and 5). Exposures in this area
include Triassic–Early Jurassic Lower Sandstone, Early Jurassic glauconitic sandy mudstone, Early–Middle
Jurassic Lower Limestone and gypsum, Middle–Late Jurassic Upper Limestone and Early–Late Oligocene
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
n. ds. gani
ET AL.
Figure 3. Geological map (a) and cross-section (b) for key area 1; the cross-section is along line A–B shown in (a).
volcanic rocks. Key area 4 occurs where the Blue Nile flows S and exposes Middle–Late Jurassic Upper Limestone,
Late Jurassic–Early Cretaceous Upper Sandstone and Early–Late Oligocene volcanic rocks (Figures 2 and 6).
Results from the four key areas are discussed below, organized into eight stages grouped into pre-sedimentation,
sedimentation and post-sedimentation phases.
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
blue nile basin evolution
Figure 4. Geological map (a) and cross-section (b) for key area 2; the cross-section is along line A–B shown in (a).
4.1. Pre-sedimentation phase
4.1.1. Neoproterozoic basement rocks
These rocks form the base of the Blue Nile Basin (Figure 7a) and crop out within rugged topography at an altitude of
900–1500 m along the entire NW-flowing segment of the Blue Nile (Figures 2–4). The age of the basement rocks
is considered to be Neoproterozoic, ranging from 850 to 550 Ma as documented from U-Pb and Rb-Sr
geochronologic studies further south of the study area by Ayalew et al. (1990). These rocks are made-up of variably
metamorphosed quartzofeldspathic schists and gneisses, migmatites and plutonic rocks. Neoproterozoic
penetrative NNE-trending sub-vertical ductile planar fabrics are associated with NNE- to NE-trending upright
tight folds.
The Neoproterozoic basement rocks are affected by normal faults with throws ranging between 5 cm and 5 m
(Figures 3, 4 and 9). The orientation of these faults varies considerably (Figure 8a). However, NNE- and
ESE-trending normal faults are more common than NE- and NW-trending faults. In contrast, fractures within the
Neoproterozoic basement rocks are dominantly NNE- and ESE-trending (Figure 8a). These fractures are clearly
dilational with openings ranging between 10 and 50 cm sometimes filled with tectonic breccias.
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
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ET AL.
Figure 5. Geological map (a) and cross-section (b) for key area 3; the cross-section is along line A–B shown in (a). (c) A photomosaic showing
an associated normal fault (attitude 1528/558NE, throw 400 m) juxtaposing Early–Late Oligocene basalt and Middle–Late Jurassic Upper
Limestone: location of photomosaic is shown in (a).
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
blue nile basin evolution
Figure 6. Geological map (a) and cross-section (b) for key area 4; the cross-section is along line A–B shown in (a).
4.2. Sedimentation phase
The Blue Nile Basin is characterized by 1400 m thick horizontal to sub-horizontal successions of both fluvial/
alluvial siliciclastic and marine carbonate rocks, ranging in age from Triassic to Cretaceous (Figure 7a). This
succession contains evidence for different phases of marine transgression and regression.
4.2.1. Lower Sandstone
This 300 m thick unit is also known as the Adigrat Sandstone and is considered to be Triassic–Early Jurassic in
age based on some biostratigraphic data and comparison with adjacent areas providing fossil ages (e.g.
Permian–Triassic age from palynological evidence; Jepsen and Athearn 1961, 1964; Mohr 1962; Beauchamp and
Lemoigne 1975; Russo et al. 1994). The unit is found unconformably overlying Neoproterozoic basement rocks
and, in turn, is overlain by Early–Late Oligocene volcanic rocks in the NW-flowing segment of the Blue Nile
(Figures 3 and 4). However, the unit occupies the basal part of the stratigraphic section in the SW-flowing segment
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
n. ds. gani
ET AL.
Figure 7. (a) Generalized stratigraphic column of the Blue Nile Basin, (b) detailed stratigraphic column showing the repetitive fining-upward
facies succession interpreted as fluvial channel deposits within the Lower Sandstone and (c) detailed stratigraphic column showing
sedimentological characteristics of the glauconitic sandy mudstone unit.
of the river where it is overlain by a Early–Middle Jurassic Lower Limestone unit (Figure 5a). This unit is made-up
of pink to red, fine- to coarse-grained sandstones that are rarely interbedded with grey mudstone beds. Sedimentary
structures within this unit include dune-scale trough cross-bedding with set thickness ranging between 10 cm and
1 m (Figure 10a) and with occasional pebbles and lithoclasts along foresets. Generally, the Lower Sandstone is
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
blue nile basin evolution
Figure 8. Orientation data for normal faults and dilational fractures plotted on equal area stereonets and rose diagrams, for (a) Neoproterozoic
basement rocks, (b) Lower Sandstone, (c) Lower Limestone, (d) Upper Limestone, (e) Upper Sandstone, (f) Lower volcanic rocks and (g) Upper
volcanic rocks.
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
n. ds. gani
ET AL.
Figure 9. Extensional structures within the Neoproterozoic basement rocks of key area 1. (a) Normal fault displacing sub-horizontal quartz vein.
(b) Complex fracture network. Scale bar is 5 cm in both figures.
characterized by repetitive fining-upward facies successions. An individual cycle starts with an erosional base
overlain by lags, interpreted as channel features (Figures 7b and 10b). Lateral accretion surfaces within the
sandstones indicate lateral migration of the channels. The average azimuth of palaeocurrents measured from dune
cross-strata is 1108 (Figure 7b). Locally, channels are vertically stacked and produce amalgamated sandstones
(Figures 7b and 10b), indicating high-energy and/or depositional setting with low accommodation volume.
Subordinate structures include lateral accretion surfaces, horizontal stratification and ripple cross-lamination. In
some places, silicified tree trunks up to 4 m long, mud-cracks and vertebrate tracks are found within this unit. The
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
blue nile basin evolution
Figure 10. Sedimentary and tectonic structures of the Lower Sandstone of key area 3. (a) Erosional channel base and lateral accretion, (b)
dune-scale trough cross-bedding with set thickness ranging between 10 cm and 1 m, (c) large mud-cracks preserved at the base of a sandstone
bed, (d) vertebrate tracks on the surface of a sandstone bed and (e) NW-trending normal fault with multiple internal fault surfaces and an
aggregate throw of 4 m.
presence of large mud-cracks (Figure 10c) and vertebrate tracks (Figure 10d) within the sandstones (Figures 10c
and d) suggest sub-aerial exposure of the flood plains in a continental fluvial environment.
The Lower Sandstone unit is affected by dominant NW-trending normal faults and less dominant N-trending
normal faults, as well as NW- and ENE-trending fractures, which are mostly dilational (Figure 8b). Throws on the
normal faults ranges between 50 cm and 8 m, and fault zones range in width between 10 cm and 10 m. In some
places, the normal faults are characterized by the smearing of mud layers and the presence of multiple internal fault
surfaces (Figure 10e).
4.2.2. Glauconitic sandy mudstone unit
In key area 3, the Lower Sandstone is overlain by a 30 m thick unit of greyish-green glauconitic sandy mudstones
(Figures 5, 7a and 11a), demarcating the first marine transgression in the Blue Nile Basin. This unit, reported for the
first time by Gani and Abdelsalam (2006), is sandwiched between Lower Sandstone and Early–Middle Jurassic
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
n. ds. gani
ET AL.
Figure 11. Sedimentary structures of the glauconitic sandy mudstone unit of key area 3. (a) Hummocky cross-stratification and (b) dune-scale
cross-stratification.
Lower Limestone. An Early Jurassic age was therefore assigned to this unit based on its stratigraphic position. The
upper part of this unit is characterized by hummocky cross-stratification (Figure 11a) and wave ripples
(Figure 11b), indicating storms and waves in a marine environment. A trough cross-stratified shoreface sandstone
interval has also been identified within the upper part of this unit (Figure 7c). Presently, the glauconitic unit is
preserved as mound-shaped erosional remnants which appear festoon-shaped in map view (Figure 5a; Gani and
Abdelsalam 2006). This unit is interpreted to be deposited in an offshore to shelfal marine environment.
4.2.3. Lower Limestone and gypsum unit
This unit, 450 m thick (Figure 7), also known as the Gohatsion Formation, is of Early–Middle Jurassic (Toarcian
to Bathonian) age, as determined from micro– and mega–fossil studies by Assefa (1981). It is exposed along the
SW-flowing segment of the Blue Nile where it is underlain by the glauconitic sandy mudstone unit or the
Triassic–Early Jurassic Sandstone and overlain by a Middle–Late Jurassic Upper Limestone unit (Figures 5 and 7a).
The unit consists of a lower thinly bedded (average 20 cm) limestone interval and an upper interval of alternating
Copyright # 2008 John Wiley & Sons, Ltd.
Geol. J. (2008)
DOI: 10.1002/gj
blue nile basin evolution
limestone and gypsum beds (Figures 7a and 12a). The bedded limestone, grey in colour, is sparsely fossiliferous
with burrows including Thalassinoides, Planolites and Ophiomorpha. The gypsum beds are characterized by
mottled texture, and are inter-bedded with glauconitic mudstone beds and rare thin sandstone beds. Deposition of
the Lower Limestone indicates deepening of the basin. However, the alternation of gypsum and limestone in the
upper part of the unit indicates repetitive drying and flooding of an evaporitic basin.
The Lower Limestone is cross-cut by NW-trending normal faults, NE- and NW-trending dilational fractures and
less-frequent NE-trending normal faults (Figures 8c, 12b and c). The fault planes exhibit both planar and listric
geometry (Figure 12b) with throws ranging between 10 cm and 2 m and fault zones ranging in width from 15 cm to
1 m. Tilting roll-over anticlines, and complex splay structures are common. In places, listric faults, which
occasionally flatten out to become layer-parallel structures (Figure 12b), result in the tilting of bedding planes to
almost vertical.
4.2.4. Upper Limestone
This 400 m thick unit (Figure 7a) comprises thinly bedded (average 10 cm) to massive limestone (Figure 13a). It
is also known as the Antalo Limestone, which is of Middle–Late Jurassic age on the basis of Callovian to
Kimmeridgian benthic foraminifers and macrofaunas (Canuti and Radrizzani 1975; Russo et al. 1994). It is found
in the SW-flowing segment of the Blue Nile sandwiched between the Early–Middle Jurassic Lower Limestone unit,
and either the Late Jurassic–Early Cretaceous Upper Sandstone unit or Early–Late Oligocene volcanic rocks
(Figure 5). Although the base of this unit is not exposed in the S-flowing segment of the Blue Nile, it is overlain by
the Late Jurassic–Early Cretaceous Upper Sandstone unit (Figure 6). The middle part of the Upper Limestone is
fossiliferous with alternating yellowish limestone and grey calcareous mudstones. The fossils found within this unit
are dominantly brachiopod shells (Figure 13b), bivalves and gastropods. Locally, the bedded limestone is followed
by nodular limestone containing a few tepee structures (diagenetic sedimentary structures formed as
pseudoanticlines due to the expansion of surface sediment layers). The bedded limestone is also characterized
by the occasional presence of 2–3 m thick stylolitic (Figure 13c) and intensely bioturbated horizons. The deposition
of the Upper Limestone indicates a second major marine transgression in the Blue Nile Basin.
The Upper Limestone is affected by NW- and NE-trending normal faults (Figure 8d), the throws of which
generally range between a few cm and 60 m (Figure 13a), but with one fault having a 400 m throw (Figure 5c). Fault
zones range from a few cm to 50 m wide. Fractures within this unit are dilational and dominantly N-trending with
subordinate ENE- and NW-trending sets (Figure 8d).
4.2.5. Upper Sandstone
This unit, also known as Debre Libanos Sandstone, unconformably overlies the Upper Limestone unit. Since no
biostratigraphic or radiometric age data are available, this unit is determined to be of Late Jurassic–Early
Cretaceous age based on its stratigraphic relationship with overlying and underlying units (Assefa 1991; Russo
et al. 1994). The only palaeontological age dating for the Upper Sandstone unit was documented as Early
Cretaceous (Aptian–Albian) in southeastern Ethiopia (Gortani 1973; Silvestri 1973). The sequence thickness varies
from 200 to 500 m with an average thickness of 280 m (Figure 7a). The Upper Sandstone is encountered in the
S-flowing segment of the Blue Nile below the Early–Late Oligocene volcanic rocks (Figure 6). It comprises thickly
to thinly bedded sandstones, with bed thickness ranging from 1 to 40 cm. The sandstones are white to pink in colour,
and are medium to coarse grained. The Upper Sandstone shows dune-scale trough cross-bedding and horizontal
stratifications (Figure 14a). Distinct pebbles horizons are locally present and small channels with lateral accretion
surfaces are rarely observed. The overall depositional environment of this unit is interpreted to be continental
alluvial to fluvial. Therefore, the unconformity (disconformity) at the base of this unit marks a regional regression
when rocks of Early Cretaceous are absent (Figure 7a). The boundary between the Upper Sandstone and the
overlying Early–Late Oligocene volcanic rocks is indicated by a whitish–pinkish baked sandstone horizon which
consists of sandstone with distorted sedimentary structures and also represents a major hiatus (Figure 7a). A
detailed stratigraphic description of the Upper Sandstone has been given by Assefa (1991).
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Figure 12. Sedimentary and tectonic structures of the Lower Limestone of key area 3. (a) Gypsum unit, (b) listric normal fault in the gypsum
unit shallowing to layer parallel and resulting in the rotation of bedding to almost vertical and (c) orthogonal fractures in the gypsum unit.
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Figure 13. Sedimentary and tectonic structures, and palaeontological features of the Upper Limestone of key area 3. (a) Sub-horizontal
limestone beds offset by a listric normal fault with 1 m throw, (b) brachiopod shells and (c) stylolites.
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Figure 14. Sedimentological and tectonic structures of the Upper Sandstone of key area 4. (a) Dune-scale trough cross-bedding with pebble
clasts along foresets (scale is 3 cm), (b) orthogonal fracture set and (c) a normal fault with 3 m throw.
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The Upper Sandstone unit is affected by NW- and NE-trending normal faults and dominantly N-trending
dilational fractures with subordinate NE- and NW-trending sets (Figures 8e, 14b and c). The throw on normal faults
ranges between 2 and 80 m, and fault zones range between 2 and 10 m wide.
4.2.6. Comparison with the Mesozoic succession of the nearest Mekele Basin
The Mesozoic stratigraphic succession of the Blue Nile Basin is broadly similar to that of the adjacent Mekele
Basin, situated north of the study area (Figure 1). The Mekele Basin stratigraphy consists of from older to younger,
Triassic–Middle Jurassic fluvial Adigrat or Lower Sandstone unit underlain by Palaeozoic glacial rocks (Dow et al.
1971; Saxena and Assefa 1983); a shale unit (named ‘transition beds’ intercalated with calcarenite and sandstone)
of Late Callovian to Early Oxfordian age (based on foraminiferal fauna) and deposited in shallow marine
environment (Bosellini et al. 1997); Late Jurassic Antalo supersequence, a largely carbonate unit, and Early
Cretaceous Upper Sandstone or Amba Aradam sandstone overlain by Tertiary flood basalt (Beyth 1972; Bosellini
et al. 1997). Compared to the Blue Nile Basin, the Lower Sandstone is much thicker (670–700 m thick) in the
Mekele Basin, and consists of grey or red, fine-grained, mature sandstone with cross-bedding, frequent
bioturbation, abundant laterite beds and petrified woods (Beyth 1972; Bosellini et al. 1997). This unit is
characterized by three major fining-upward facies successions (Bosellini et al. 1997). Regionally, the Lower
Sandstone unit thins westward to about 80 m (Beyth 1972) and thickens towards the Red Sea coast (1775 m thick;
Hutchinson and Engles 1970).
The Lower Sandstone is overlain by 20–30 m thick Middle–Late Jurassic (Late Callovian to Early Oxfordian age
based on foraminiferal fauna) transition bed, made up of shale with intercalations of reddish, highly bioturbated
sandstone and calcarenite (Bosellini et al. 1997). Like the glauconite unit of the Blue Nile Basin, this transitional
unit probably indicates the initiation of a deepening of the basin. The 450 m thick Lower Limestone unit of the
Blue Nile Basin is missing in the Mekele Basin, indicating an earlier flooding in the rapidly subsiding Blue Nile
Basin during this time.
The 700 m thick Antalo supersequence which overlies the transitional unit, is a carbonate-marly succession of
Late Oxfordian–Early Kimmeridgian age (based on foraminiferal fauna) and is equivalent to the Upper Limestone
unit of the Blue Nile Basin (Bosellini et al. 1997). The Antalo supersequence consists of four depositional
sequences that include thickening and shallowing-up cycles (Bosellini et al. 1997). Like the Lower Sandstone, the
thickness of this unit increases towards the Red Sea (>1420 m thick in Danakil; Hutchinson and Engles 1970). This
thickening trend towards the east indicates that the Danakil-Red Sea region was a subsiding trough during the
Jurassic (Bosellini et al. 1997).
In the Mekele Basin, the fluvial Upper Sandstone unit (100–200 m thick) was deposited unconformably on the
Antalo supersequence during the Early Cretaceous (Bosellini et al. 1997). This unit, characterized by
fining-upward cycles, consists of coarse-grained, cross-bedded, conglomerate lens-bearing fluvial sandstone, along
with shale (Bosellini et al. 1997).
4.3. Post-sedimentation phase:
4.3.1. Lower volcanic rocks
The Lower volcanic rocks rest unconformably on the Upper Sandstone, with the absence of intervening
Paleocene–Eocene rocks. These Early–Late Oligocene flood basalts (26.9–29.4 Ma on the basis of 40Ar/39Ar age
dating and magnetostratigraphy of Hofmann et al. 1997), together with subordinate trachytes and rhyolites cover
much of the Northwestern Ethiopian Plateau (Figure 7a) and range in thickness from 500 to 2000 m (Hofmann et al.
1997). Isolated shield volcano building events emplaced volcanic rocks of 10.7–22.4 Ma age (Kieffer et al. 2004)
which are not exposed within the study areas. The basaltic rocks of this unit are characterized by the presence of
well-developed columnar joints (Figure 15a). Locally, 1–3 cm thick sub-horizontal layering is observed within the
basalts which are generally aphanitic, and locally vesicular, with the vesicles sometimes filled with zeolites, calcite
and quartz to form amygdaloidal texture. In a few places, the upper part of the basalts contains 1 m thick horizons
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Figure 15. Geological features of the Lower and Upper volcanic rocks. (a) Columnar joints in the Lower volcanic unit, (b) palaeosol horizons
sandwiched between two basaltic flows of the Upper volcanic rocks and (c) orthogonal fractures in Quaternary volcanic rocks.
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of dark brown clay topped by a fine- to coarse-grained pyroclastic layer. A few specimens of silicified wood and
baked clay horizons are also found within this unit.
Normal faults in the Early–Late Oligocene basalts are dominantly N- to NE-trending and less often NW-trending
(Figure 8f). These faults have throws ranging from a few cm to 50 m, and rarely 400 m (Figure 5c), with fault
zones ranging between a few cm and 50 m wide. The dominant fractures are dilational and are NNE- and
E-trending with subordinate NW-trending set (Figure 8f).
4.3.2. Upper volcanic rocks
Quaternary volcanic events resulted in the eruption of 300 m thick basaltic rocks (Figure 7a). This unit is not
exposed in any of the four key areas, but is encountered close to Lake Tana where the Blue Nile flows SE (Figure 2).
Here, these rocks are relatively fresh, lack columnar joints and are characterized by the presence of sheet joints, and
vesicles ranging in diameter between 2 mm and 1.5 cm. These are filled with green zeolite, calcite and quartz.
Locally, this basaltic unit contains a few cm-thick reddish baked clay beds, and 50 cm-thick pyroclastic layers.
Patchy trachytic volcanic mounds are locally present. Red to brown palaeosol horizons of 30 cm thickness
(Figure 15b) indicate several eruption pulses. No normal faults are observed in the Quaternary volcanic unit.
However, this unit is characterized by the presence of NW- and NE-trending fractures (Figures 8g and 15c).
5. DISCUSSION
5.1. Structural interpretation within regional tectonic framework
Regional stress regimes that might have affected the structural architecture of the study area include (Figure 16): (1)
Triassic–Cretaceous NE–SW-directed tensile stress associated with Gondwana break-up leading to the formation
of sub-parallel NW-trending Mesozoic rifts in northern and central Africa (McHargue et al. 1992). (2) Late
Miocene NW–SE-directed tensile stress associated with orthogonal opening of the Main Ethiopian Rift. Tensile
vectors of this stress regime have been established from the consistency of NE-trending border faults of the Main
Ethiopian Rift (Ebinger et al. 1993; Chorowicz et al. 1994; Korme et al. 1997; Acocella and Korme, 2002) and
palaeomagnetic studies (Kidane et al. 2006). (3) Quaternary E–W-directed tensile stress associated with oblique
opening of the Main Ethiopian Rift. The shift of rift opening from orthogonal to oblique (Abebe et al. 1998;
Boccaletti et al. 1999) has been attributed to the change in stress accommodation from within border faults to within
rift floor as a result of magma-maintained extension through segmented diking during the Quaternary (Kurz et al.
2007). This Quaternary E–W-directed stress regime is deduced from the presence of abundant N-trending
Quaternary faults within the floor of the Main Ethiopian Rift which are oblique to the NE-trending border faults
(Kurz et al. 2007) as well as geodetic surveying (Bilham et al. 1999). (4) Quaternary stress regimes associated with
the evolution of the Afar Depression. These are: (a) NE–SW-directed tensile stress in southern Afar resulting from
northeastward separation of the Arabian Plate and Africa Plate. This stress regime has been documented from fault
plane solutions (Ayele et al. 2006); and (b) Quaternary E–W directed tensile stress in the western margin of the Afar
Depression resulting from the stepping of the Red Sea spreading axis into Afar and subsequent S-propagation of
embryonic spreading centre towards the Afar triple junction. This stress regime has been documented from fault
plane solutions and Interferometric Synthetic Aperture Radar (InSAR) studies (Wright et al. 2006; Ayele et al.
2007). (5) In addition, the structural architecture of the region might have been affected by the Quaternary
E-trending Ambo Lineament (Abebe et al. 1998) which is thought to have both normal and dextral strike-slip
components resulting from Quaternary NNE–SSW tensile stress that accompanied transverse faults developed as a
result of change of extension within the Main Ethiopian Rift from orthogonal NW–SE extension in the Late
Miocene to oblique E–W extension in the Quaternary (Abebe et al. 1998).
In the following sections, we will examine our structural observations within these regional tectonic regimes.
However, careful attention will be given to differentiating the basin-forming Mesozoic extensional structures from
the later Neogene structures related to the development of the Main Ethiopian Rift and the Afar Depression. Age
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Figure 16. Regional tectonic stress regimes within and around the Blue Nile Basin. Boxes show the location of the four key areas. Fault plane
solutions are generated from http://discoverourearth.org/webmap/ and show the orientation and nature of faults within the Main Ethiopian Rift
and the Afar Depression.
relationships between different faults and fracture sets are rather complicated and in many cases are difficult to
resolve unequivocally. However, we have observed that many NW-trending faults and fractures, especially in the
Mesozoic sedimentary section, are relatively older compared to other faults and fracture sets as evidenced by
cross-cutting relationship:
(1) The orientations of normal faults and fractures within the Neoproterozoic basement rocks are NNE- and
ESE-trending. These trends are oblique to both NW- and NE-trending normal faults that are expected to
develop in association with Jurassic–Cretaceous NE–SW-directed extension and Late Miocene
NW–SE-directed extension, respectively. We explain the presence of NNE-trending normal faults as due
to the influence of the Neoproterozoic regional structures which are dominantly NNE-trending. The presence
of such strong pre-existing regional fabric can result in strain localization NNE-trending planes during NE–SW
and NW–SE-directed extension into NNE-trending faults. The presence of ESE-trending normal faults within
the Neoproterozoic basement rocks can be directly related to NNE–SSW-directed extension related to the
E-trending Ambo Lineament which extends westward from the Main Ethiopian Rift and runs just south of the
exposures of the Neoproterozoic basement rocks within the Gorge of the Nile (Figure 16). Abebe et al. (1998)
attributed the exposures of the Neoproterozoic basement rocks in the southwest and the deepening of ‘the top to
basement’ towards the Main Ethiopian Rift in the NE as a result of northeastward stepping down of
hanging-walls along these ESE-trending faults.
(2) The Mesozoic sedimentary section is dominated by NW- and NE-trending normal faults. NW-trending faults in
the lower part of the section (Lower Sandstone and Lower Limestone) seem to dominate over NE-trending
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faults. Fractures within the lower part of the stratigraphic section are NE- and NW-trending. However, the
upper part of the Mesozoic section (Upper Limestone and Upper Sandstone) shows a fracture pattern in which
N-trend dominates. We interpret these structural observations as follows: (a) The Mesozoic section was
deposited under a strong Jurassic–Cretaceous NE–SW-directed extension related to Mesozoic rifting of
Gondwana. (b) At a later stage, this Mesozoic fill was affected by Late Miocene NW-SE extension related to the
opening of the Main Ethiopian Rift. Normal faults associated with this extension are better developed in the
upper part of the Mesozoic sedimentary section compared to the lower part. This might be due to the lower part
of the Mesozoic section being concealed under 3000 m of sedimentary and volcanic rocks during extension.
(c) The presence of dominantly N-trending dilational fractures (as opposed to NE- and NW-trending fractures
in the lower part of the section) can be explained as a combination of two factors: (i) The effect of Quaternary
E–W-directed extension in the western flank of the Afar Depression. Most of our fracture data are collected
from key area 4 where the Upper Limestone and Upper Sandstone dominate the Mesozoic section. This key
area is the closest to the western margin of the Afar Depression compared to other areas (Figure 16). (ii) The
effect of Quaternary E–W extension related to oblique continuing opening of the Main Ethiopian Rift. This
E–W-directed extension will be less intense (development of dilational fractures compared to normal faults
with significant displacement) compared to Late Miocene NW–SE-directed extension, because much of the
Quaternary extension is localized within the floor of the Main Ethiopian Rift, rather than border faults, as was
the case during the Late Miocene extension.
(3) The Early–Late Oligocene volcanic rocks are deformed by dominant NE-trending faults and less-frequent
NW-trending normal faults. Fracture orientations within these volcanic rocks as well as Quaternary volcanic
rocks are dominantly NE-, NNE-, NW- and ESE-trending. The NE-trending faults, and NE- and NNE-trending
fractures can be directly related to Miocene extension. We explain the presence of a subsidiary set of
NW-trending faults, and NW- and ESE-trending fractures as a combination of: (a) Quaternary
NNE–SSW-directed extension related the to E-trending Ambo Lineament in the south; and (b) Quaternary
NE–SW-directed extension related to the northeastward separation of the Arabian Plate from the African Plate.
5.2. Palaeogeography and basin evolution
We summarize our stratigraphic and structural results and architecture of the Blue Nile Basin in relation to regional
tectonic elements in a nine-step palaeogeographic model (Figure 17):
(1) Palaeozoic stratigraphic records appear to have been largely eroded in the vicinity of the Blue Nile Basin.
During late Palaeozoic time, the Neoproterozoic basement rocks and the overlying Palaeozoic section must
therefore have been uplifted and subjected to a long period of erosion. Subsequently, Triassic–Cretaceous
NE–SW-directed extension related to Gondwana break-up dominated the regional stress regime, resulting in
the formation of NNE-trending normal faults (Figure 17a) whose orientation appears to be controlled by the
earlier NNE-trending Neoproterozic regional fabric.
(2) Initial rifting associated with the break-up of Gondwana started during the Triassic–Middle Jurassic in eastern
and central Africa resulting in the initiation of the Blue Nile Basin as a series of NW-trending fault-bounded rift
basins, caused by strong NE–SW extension. The Blue Nile Basin in the Northwestern Ethiopian Plateau might
have developed in structural continuation and synchronous with the Blue Nile Rift in the lowlands of Sudan to
the northwest. NW-trending grabens developed within the Blue Nile Basin served as depocentres for the
deposition of the Lower Sandstone during the Triassic–Early Jurassic in a continental fluvial environment.
Palaeocurrent studies indicate that the Lower Sandstone was deposited as a result of SE-flowing rivers
(Figure 17b).
(3) The Indian Ocean emerged in the Early Jurassic as a result of separation of India from Africa. With increasing
subsidence of the Blue Nile Basin, a shallow marine embayment extended northwestwards from the Indian
Ocean submerging the newly formed NW-trending Blue Nile Basin. This initial marine transgression within the
basin is manifested by the deposition of the Early Jurassic glauconitic sandy mudstone interval (Figure 17c).
The Blue Nile Basin continued to deepen as a result of continuation of NE–SW-directed extension allowing for
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Figure 17. A nine-step schematic palaeogeographic model (not to scale) of the Blue Nile Basin from Neoproterozoic to Quaternary times.
the deposition of Early–Middle Jurassic extensive marine strata represented by the Lower Limestone
(Figure 17d). Towards the end of the Middle Jurassic, the Blue Nile Basin turned into an evaporite basin,
and underwent several cycles of flooding and drying as evidenced by the deposition of alternating gypsum
and limestone strata at the top of the Lower Limestone (Figure 17e). This was followed by a second phase of
marine transgression during the Middle–Late Jurassic resulting in the deposition of the Upper Limestone
(Figure 17f).
(4) A final marine regression occurred during the Late Jurassic–Early Cretaceous allowing for the replacement of
marine depositional environment with a continental alluvial/fluvial environment resulting in the deposition of
the Upper Sandstone. The unconformity at the base of the Upper Sandstone does not just represent a facies
change associated with a regression, but probably coincides with a period of uplift and erosion. The Upper
Sandstone was deposited during the continued NE–SW-directed extension (Figure 17g).
(5) During the Oligocene, the Afar Mantle Plume reached the base of the African lithosphere resulting in an early
uplift (the Afar dome). Şengor (2001), based on a tectono-chronostratigraphic calculation, concluded that the
Afar dome began to rise in the middle Eocene, reaching an elevation of 1 km by the Early Oligocene. This
event was followed by the extrusion of 500–2000 m thick volcanic rocks which covered much of the Mesozoic
Blue Nile Basin. Subsequently, the stress regime in northern and central Africa changed dramatically from
NE–SW to NW–SE-directed tensile stress resulting in the initiation of the Main Ethiopian Rift as a major
NE-trending continental rift. The northern Main Ethiopian Rift that dissected the Ethiopian Plateau into
northwest and southeast sections (Figure 1) developed ca. 11 Ma (Ar/Ar geochronologic study of Wolfenden
et al. 2004). WoldeGabriel et al. (1990) conclude that the initiation of the western boundary fault of the Main
Ethiopian Rift was at least 8.3 Ma (K/Ar geochronology and stratigraphic relationships). However, Bonini et al.
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(2005), based on recent structural, petrological and K/Ar geochronological studies, proposed the extension
forming the Main Ethiopian Rift started between 6 and 5 Ma. These studies support the initiation of the Main
Ethiopian Rift during the Late Miocene. NE-trending faults were formed dominantly within the Main
Ethiopian Rift, but the related extension has also affected a broader region beyond the border faults of the
Main Ethiopian Rift. Hence, NE-trending faults are developed within the Early–Late Oligocene volcanic rocks
as well as within the upper part of the Mesozoic sedimentary section superimposed on the NW-trending normal
faults (Figure 17h).
(6) The Early–Late Oligocene volcanic event was followed by the extrusion of 300 m thick Quaternary volcanic
rocks. The unconformity at the base of the Upper volcanic rocks probably represents a period of uplift and
erosion during the Late Miocene to Quaternary time. More than one extension direction (Figure 17) emerged in
the Quaternary and continued to operate on the region up to the present time. These include E–W-directed
extension related to oblique opening of the Main Ethiopian Rift and consequently the development of
E-trending transverse faults, such as the Ambo Lineament, that are accompanied by NNE–SSW extension,
NE–SW extension in southern Afar related to the northeastward separation of Arabia from Africa and E–W
extension related to stepping of the Red Sea spreading ridge onto Afar. These tensile stresses resulted in the
superimposition of N-, ESE and NW-trending extensional structures on the Blue Nile Basin resulting in the
present architecture of the basin.
6. CONCLUSIONS
1. The Blue Nile Basin has evolved through three main phases, including (i) pre-sedimentation phase involving the
peneplanation of Neoproterozoic basement rocks, (ii) sedimentation phase including deposition of thick
Mesozoic strata represented by repetitive marine transgression and regression, and (iii) post-sedimentation
phase involving emplacement of extensive Early–Late Oligocene and Quaternary volcanic rocks.
2. The early stage in the evolution of the Blue Nile Basin was dominated by Jurassic–Cretaceous NE–SW
extension producing NNE-trending normal faults in the Neoproterozoic basement rocks and NW-trending faults
which provided the depocentres for the deposition of the Mesozoic sedimentary rocks in marine and continental
environments.
3. The Afar Mantle Plume resulted in extrusion of Early–Late Oligocene volcanic rocks that covered much of the
Mesozoic sedimentary section. This volcanic event was followed by NW–SE-directed extension resulting in
the opening of the NE-trending Main Ethiopian Rift and superimposition of NE-trending faults on rocks within
the Blue Nile Basin.
4. The Quaternary Era in the region is characterized by the extrusion of 300 m thick volcanic rocks, and varying
directions of tensile stresses (E–W, NNE–SSW and NE–SW) related to tectonic events within the Main
Ethiopian Rift and Afar Depression. These resulted in superimposition of Quaternary N-, ESE- and
NW-trending extensional structures on the Blue Nile Basin.
ACKNOWLEDGEMENTS
This project is funded by National Science Foundation (NSF). The National Aeronautics and Space Administration
(NASA) Jet Propulsion Laboratory (JPL) provided SRTM data, the Earth Resources Observatory System (EROS)
provided ASTER data, the Alaska SAR Facility (ASF) provided RADARSAT data and EarthSAT provided Landsat
TM data. The authors would like to thank the Geological Survey of Ethiopia and Linda Smith for co-operation
during fieldwork. The authors would also like to thank Professors Ian Somerville and John Walsh, and Drs Karla
Kane and Steve Drury for their critical comments to improve the manuscript. Part of this work was carried out in the
Department of Geoscience at the University of Texas at Dallas. This is the University on New Orleans Department
of Earth and Environmental Sciences contribution number – and Missouri University of Science and Technology
Geology and Geophysics Program contribution number 10.
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ET AL.
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