stratigraphic and structural evolution of the blue nile basin,...

GEOLOGICAL JOURNAL

Geol. J. 44: 30–56 (2009)

Published online 9 September 2008 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. ABDELSALAM2, S. GERA 3 and M. R. GANI 1

1Earth and Environmental Sciences, University of New Orleans, New Orleans, LA, USA2Geological Sciences and Engineering, Missouri University of Science and Technology, Rolla, MO, USA

3Regional Mapping and Geochemistry Department, Geological Survey of Ethiopia, Addis Ababa, Ethiopia

The Blue Nile Basin, situated in the Northwestern Ethiopian Plateau, contains �1400 m thick Mesozoic sedimentary sectionunderlain by Neoproterozoic basement rocks and overlain by Early–Late Oligocene and Quaternary volcanic rocks. This studyoutlines the stratigraphic and structural evolution of the Blue Nile Basin based on field and remote sensing studies along theGorge of the Nile. The Blue Nile Basin has evolved in three main phases: (1) pre-sedimentation phase, include pre-riftpeneplanation of the Neoproterozoic basement rocks, possibly during Palaeozoic time; (2) sedimentation phase from Triassic toEarly Cretaceous, including: (a) Triassic–Early Jurassic fluvial sedimentation (Lower Sandstone, �300 m thick); (b) EarlyJurassic 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-upliftand marine regression (alluvial/fluvial Upper Sandstone, �280 m thick); (3) the post-sedimentation phase, including Early–LateOligocene eruption of 500–2000 m thick Lower volcanic rocks, related to the Afar Mantle Plume and emplacement of �300 mthick Quaternary Upper volcanic rocks. The Mesozoic to Cenozoic units were deposited during extension attributed toTriassic–Cretaceous NE–SW-directed extension related to the Mesozoic rifting of Gondwana. The Blue Nile Basin was formedas a NW-trending rift, within which much of the Mesozoic clastic and marine sediments were deposited. This was followed byLate Miocene NW–SE-directed extension related to the Main Ethiopian Rift that formed NE-trending faults, affecting Lowervolcanic rocks and the upper part of the Mesozoic section. The region was subsequently affected by Quaternary E–W andNNE–SSW-directed extensions related to oblique opening of the Main Ethiopian Rift and development of E-trending transversefaults, as well as NE–SW-directed extension in southern Afar (related to northeastward separation of the Arabian Plate fromthe African Plate) and E–W-directed extensions in western Afar (related to the stepping of the Red Sea axis into Afar). TheseQuaternary stress regimes resulted in the development of N-, ESE- and NW-trending extensional structures within the Blue NileBasin. 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, NewOrleans, LA 70148, USA. E-mail: [email protected]

Copyright # 2008 John Wiley & Sons, Ltd
.

Figure 1. (a) Inset map showing the location of Figure 1(b). (b) Tectonic elements within and around the Blue Nile Basin (Modified afterFairhead 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.

blue nile basin evolution 31

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. 44: 30–56 (2009)

DOI: 10.1002/gj

Table 1. Previous stratigraphic studies in the Blue Nile Basin and surrounding areas

Authors Studies

Krenkel (1926) Described limestone, gypsum and shale unit of the Blue NileStefanini (1933), Merla et al. (1973) Produced geological map of Ethiopia, Eritrea and Somalia

accompanied by general stratigraphic description of the regionMohr (1962) Described the regional geology of EthiopiaFiccarelli (1968) Studied marine fossils of the Blue Nile sectionKazmin (1973, 1975) Explain geological map of EthiopiaBeauchamp and Lemoigne (1974, 1975) Reconstructed the palaeogeography and subsidence of the

central Ethiopian sedimentary basins in the Jurassic periodStudied palaeofloral dating of volcanics on the Ethiopian Plateau

Kalb and Oswald (1974) Studied the Mesozoic invertebrate fossils of EthiopiaMcDougall et al. (1975) Determined the age and rates of denudation of Ethiopian

Trap Series basaltCanuti and Radrizzani (1975) Described microfacies of the limestone in the Blue Nile BasinBeauchamp (1977) Determined the age of palaeoflora of the Ethiopian Plateau

volcanic rocksAssefa (1979, 1980, 1981, 1991) Established lithostratigraphic units of the Blue Nile BasinRusso et al. (1994) Outlined the sedimentary evolution of the Abay River BasinMangesha et al. (1996) Generated geological map of EthiopiaHofmann et al. (1997) Determined the age of Ethiopian flood basaltWood et al. (1997) Presented the first palynostratigraphic dates for Mesozoic faunasCoulie et al. (2003) Determined age and duration of the Ethiopian trap series basaltKieffer et al. (2004) Determined ages of Ethiopian flood basalt and shield volcanoesGani and Abdelsalam (2006) Produced geological map of Dejen–Gohatsion region of the Gorge

of the Nile

32 n. ds. gani ET AL.

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


DOI: 10.1002/gj

Figure 2. Hill shade Digital Elevation Model (DEM) extracted from the 90 m x–y resolution Shuttle Radar Topography Mission (SRTM) datashowing 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).


DOI: 10.1002/gj


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 (wavelength¼ 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


DOI: 10.1002/gj

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.


DOI: 10.1002/gj



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.


DOI: 10.1002/gj

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 showingan 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).


DOI: 10.1002/gj




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


DOI: 10.1002/gj

Figure 7. (a) Generalized stratigraphic column of the Blue Nile Basin, (b) detailed stratigraphic column showing the repetitive fining-upwardfacies 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


DOI: 10.1002/gj

Figure 8. Orientation data for normal faults and dilational fractures plotted on equal area stereonets and rose diagrams, for (a) Neoproterozoicbasement rocks, (b) Lower Sandstone, (c) Lower Limestone, (d) Upper Limestone, (e) Upper Sandstone, (f) Lower volcanic rocks and (g) Upper

volcanic rocks.


DOI: 10.1002/gj


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


DOI: 10.1002/gj

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 sandstonebed, (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


DOI: 10.1002/gj

Figure 11. Sedimentary structures of the glauconitic sandy mudstone unit of key area 3. (a) Hummocky cross-stratification and (b) dune-scalecross-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


DOI: 10.1002/gj


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).


DOI: 10.1002/gj

Figure 12. Sedimentary and tectonic structures of the Lower Limestone of key area 3. (a) Gypsum unit, (b) listric normal fault in the gypsumunit shallowing to layer parallel and resulting in the rotation of bedding to almost vertical and (c) orthogonal fractures in the gypsum unit.


DOI: 10.1002/gj


Figure 13. Sedimentary and tectonic structures, and palaeontological features of the Upper Limestone of key area 3. (a) Sub-horizontallimestone beds offset by a listric normal fault with �1 m throw, (b) brachiopod shells and (c) stylolites.


DOI: 10.1002/gj


Figure 14. Sedimentological and tectonic structures of the Upper Sandstone of key area 4. (a) Dune-scale trough cross-bedding with pebbleclasts along foresets (scale is 3 cm), (b) orthogonal fracture set and (c) a normal fault with �3 m throw.


DOI: 10.1002/gj



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


DOI: 10.1002/gj

Figure 15. Geological features of the Lower and Upper volcanic rocks. (a) Columnar joints in the Lower volcanic unit, (b) palaeosol horizonssandwiched between two basaltic flows of the Upper volcanic rocks and (c) orthogonal fractures in Quaternary volcanic rocks.


DOI: 10.1002/gj



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


DOI: 10.1002/gj

Figure 16. Regional tectonic stress regimes within and around the Blue Nile Basin. Boxes show the location of the four key areas. Fault planesolutions 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) T

Cop

he 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) T
he 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

yright # 2008 John Wiley & Sons, Ltd. Geol. J. 44: 30–56 (2009)

DOI: 10.1002/gj

Cop


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) T
he 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) P
alaeozoic 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) I
nitial 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) T
he 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


DOI: 10.1002/gj

Figure 17. A nine-step schematic palaeogeographic model (not to scale) of the Blue Nile Basin from Neoproterozoic to Quaternary times.

Cop


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) D
uring the Oligocene, the Afar Mantle Plume reached the base of the African lithosphere resulting in an early
uplift (the Afar dome). Sengor (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.


DOI: 10.1002/gj

Cop


(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) T
he 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. T
he 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. T
he 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. T
he 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. T
he 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.


DOI: 10.1002/gj


REFERENCES

Abebe T,Maetti P, BoniniM, Corti G, Innocenti F,Mazzarini F, Pecksay Z. 2005. Geological map of the northern Main Ethiopian Rift and itsimplications for the volcano-tectonic evolution of the rift. Geological Society of America, Map and Chart Series MCH094 (1:200,000): 20.

Abebe T, Mazzarini F, Innocenti F, Manetti P. 1998. The Yerer-Tullu Wellel volcanotectonic lineament; a transtensional structure in centralEthiopia and the associated magmatic activity. Journal of African Earth Sciences 26: 135–150.

Acocella V, Korme T. 2002. Holocene extension direction along the Main Ethiopian Rift, East Africa. Terra Nova 14: 191–197.Assefa G. 1979. Clay mineralogy of the Mesozoic sequence in the Upper Abay (Blue Nile) River Valley region. Ethiopian Journal of Science

(Sinet) 3: 37–65.Assefa G. 1980. Stratigraphy and sedimentation of the type Gohatsion Formation (Lias-Malm) Abay River basin, Ethiopia. Ethiopian Journal of

Science (Sinet) 3: 87–110.Assefa G. 1981. Gohatsion formation: a new Liasmal lithostragraphic unit from the Abby River basin, Ethiopia. Geoscience Journal 2: 63–88.Assefa G. 1991. Lithostratigraphy and environment of deposition of the Late Jurassic–Early Cretaceous sequence of the central part of

Northwestern Plateau, Ethiopia. Neues Jahrbuch fur Geologie und Paleontologie Abhandunglen 182: 255–284.Ayalew T, Bell K, Moore JM, Parrish RR. 1990. U-Pb and Rb-Sr geochronology of the Western Ethiopian Shield. Geological Society of

America Bulletin 102: 1309–1316.Ayele A, Jacques E, KassimM,Kidane T, OmarA, Tait S, Nercessian A, de Chabalier JB, KingG. 2007. The volcano-seismic crisis in Afar,

Ethiopia, starting September 2005. Earth and Planetary Science Letters 255: 177–187.Ayele A, Nyblade A, Langston CA, Cara M, Leveque JJ. 2006. New evidence for Afro-Arabian plate separation in southern Afar. In The

Structure And Evolution Of The East African Rift System In The Afar Volcanic Province, Yirgu G, Ebinger C, Maquire PKH (eds). GeologicalSociety London, Special Publications 259: 133–141.

Beauchamp J. 1977. Paleofloral dating of volcanics on the Ethiopian plateau (Shoa). Bulletin of Geophysical Observatory, Addis Ababa 15:125–131.

Beauchamp J, Lemoigne Y. 1974. Presence d’un basin de subsidence en Ethiopie centrale et essai de reconstruction paleogeographique del’Ethiopie durant le Jurassique. Bulletin of the Geological Society of France 7: 563–569.

Beauchamp J, Lemoigne Y. 1975. Paleofloral dating of volcanics on the Ethiopian Plateau (Shoa). Bulletin of Geophysical Observatory, AddisAbaba 15: 125–131.

BeythM. 1972. Paleozoic–Mesozoic sedimentary basin of Mekele Outlier, Northern Ethiopia. The American Association Petroleum GeologistsBulletin 56: 2426–2439.

Bilham R, Bendick R, Larson K, Mohr P, Braun J, Tesfaye S, Asfaw L. 1999. Secular and tidal strain across the main Ethiopian Rift.Geophysical Research Letters 26: 2789–2792.

Binks RM, Fairhead JD. 1992. A plate tectonic setting of Mesozoic rifts of West and Central Africa. Tectonophysics 213: 141–151.BoccalettiM, BoniniM,Mazzuoli R, Trua T. 1999. Pliocene–Quaternary volcanism and faulting in the northern Main Ethiopian Rift (with two

geological maps at scale 1:50,000). Acta Vulcanologica 11: 83–97.BoniniM, Corti G, Innocenti F,Manetti P,Mazzarini F, Abebe T, Pecskay Z. 2005. Evolution of the Main Ethiopian Rift in the frame of Afar

and Kenya rifts propagation. Tectonics 24: 1–21.Bosellini A. 1989. The continental margins of Somalia: their structural evolution and sequence stratigraphy. Memorie di Scienze Geologische,

Padova 41: 373–458.Bosellini A. 1992. The continental margins of Somalia: structural evolution and sequence stratigraphy. Geology and Geophysics, Continental

Margins. Proceedings M.T. Halbouty Continental Margin Conference, American Association of Petroleum Geologists Memoirs, Tulsa 53:185–205.

Bosellini A, Russo A, Fantozzi PL, Assefa G, Solomon T. 1997. The succession of the Mekele Outlier (Tigre Province, Ethiopia). Memorie diScienze Geologische, Padova 49: 95–116.

Bosworth W. 1992. Mesozoic and early Tertiary rift tectonics in East Africa. Tectonophysics 209: 115–137.Bosworth W, Morley CK. 1994. Structural and stratigraphic evolution of the Anza rift, Kenya. Tectonophysics 236: 93–115.Canuti P, Radrizzani CP. 1975. Microfacies from the upper portion of the Antalo limestone (Blue Nile sequence, Ethiopia). Revista Espanola

de Micropaleontologia 7: 131–147.Chorowicz J, Collet B, Bonavia FF, Korme T. 1994. Northwest to north-northwest extension direction in the Ethiopian rift deduced from the

orientation of extension structures and fault-slip analysis. Geological Society of America Bulletin 105: 1560–1570.Coulie E, Quidelleur X, Gillot PY, Coutillot V, Lefevre JC, Chiessa S. 2003. Comparative K-Ar and Ar/Ar dating of Ethiopian and Yemenite

Oligocene volcanism: implication for timing and duration of the Ethiopian traps. Earth and Planetary Science Letters 206: 477–492.Dow DB, Beyth M, Hailu T. 1971. Paleozoic rocks recently discovered in Northern Ethiopia. Geological Magazine 108: 53–59.Ebinger CJ, Yemane T, WoldeGabriel G, Aronson JL, Walter RC. 1993. Late Eocene–Recent volcanism and faulting in the southern Main

Ethiopian Rift. Journal of the Geological Society, London 150: 99–108.Fairhead JD. 1988. Mesozoic plate tectonic reconstructions of the central South Atlantic Ocean. The role of West and Central African rift

system. Tectonophysics 155: 181–181.Ficcarelli G. 1968. Fossili giuresi della serie sedimentaria del Nilo Azzurro meridionale. Rivista Italiana di Paleontologia e Stratigrafia 74:

23–50.Gani ND, AbdelsalamMG. 2006. Remote sensing analysis of the Gorge of the Nile, Ethiopia with special emphasis on Dejen–Gohtsion region.

Journal of African Earth Sciences 44: 135–150.Gani ND, AbdelsalamMG, Mazzarini F, Abebe T, Pecskay Z, Gani MR. 2007. Blue Nile incision on the Ethiopian Plateau: Pulsed plateau

growth, Pliocene uplift, and hominin evolution. GSA Today 17: 4–11.Gortani M. 1973. La fauna mesocretacea degli Strati di Graua. In Missione Geologica dell AGIP nella Dancalia Meridionale sugli Altipiani

Hararini(1936–1938). Accademia Nazionale Dei Lincei, Atti Convegni Lincei 4: 3–98.


DOI: 10.1002/gj


Guiraud R, Maurin JC. 1992. Early Cretaceous rifts of west and Central Africa: an overview. In Geodynamics of Rifting, Volume II, CaseHistory Studies on Rifts: North and South America and Africa, Ziegler PA (ed.). Tectonophysics 213: 153–168.

Hautot S, Whaler K, Gebru W, Desissa M. 2006. The structure of a Mesozoic basin beneath the Lake Tana area, Ethiopia, revealed bymagnetotelluric imaging. Journal of African Earth Sciences 44: 331–338.

Hofmann C, Courtillot V, Feraud G, Rochette P, Yirgu G, Ketefo E, Pik R. 1997. Timing of the Ethiopian flood basalt event and implicationsof Plume birth and global change. Nature 389: 838–841.

Hutchinson RW, Engles GG. 1970. Tectonic significance of regional geology and evaporate lithofacies in northeastern Ethiopia. PhilosophicalTransactions Royal Society London 267: 313–329.

Jepsen DH, AthearnMJ. 1961. Geologic plan and section of the left bank of the Blue Nile Canyon near crossing of Addis Ababa-Debre Marcosroad. US Department of Interior/Ethiopia’s Water Resources Department, Addis Ababa.

Jepsen DH, Athearn MJ. 1964. Land and water resources of the Blue Nile River Basin, Appendix II-Geology. US Department of Interior/Ethiopia’s Water Resources Department, Addis Ababa: 221.

Kalb J, Oswald EB. 1974. Report documenting Mesozoic invertebrate fossil localities in the Blue Nile Gorge. Ethiopia Monograph: 23.Kazmin V. 1973. Geological Map of Ethiopia. Geological Survey of Ethiopia: Addis Ababa, Ethiopia.Kazmin V. 1975. Explanation of the geological map of Ethiopia. Geological Survey of Ethiopia Bulletin 1: 1–15.Kidane T, Platzman E, Ebinger C, Abebe C, Rochette P. 2006. Paleomagnetism constraints on continental break-up processes: observations

from the Main Ethiopian Rift. In The Afar Volcanic Province Within The East African Rift System, Yirgu G, Ebinger CJ, Maguire PKH (eds).Geological Society London, Special Publications 259: 165–183.

Kieffer B, Arndt N, Lapierre H, Bastien F, Bosch D, Pecher A, Yirgu G, AyalewD,Weis D, JerramDA, Keller F,Meugniot C. 2004. Floodand shield basalts from Ethiopia: magmas from the African Superswell. Journal of Petrology 45: 793–834.

Korme T, Chorowicz J, Collet B, Bonavia FF. 1997. Volcanic vents rooted on extension fractures and their geodynamic implications in theEthiopian Rift. Journal of Volcanology and Geothermal Research 79: 205–222.

Krenkel E. 1926. Abessomalien (Absessinien und Somalien). Handbook of Regional Geology, Heidelberg 7: 8.Kurz T, Gloaguen R, Ebinger C, Casey M, Abebe B. 2007. Deformation distribution and type in the Main Ethiopian Rift (MER): a remote

sensing study. Journal of African Earth Sciences 48: 100–114.Mangesha T, Chernet T, Haro W. 1996. Geological Map Of Ethiopia (1: 2,000,000). Geological Survey of Ethiopia: Addis Ababa, Ethiopia.McDougall I, Morton WH, William MAJ. 1975. Ages and rates of denudation of trap series basalts at the Blue Nile Gorge, Ethiopia. Nature254: 207–209.

McHargue T, Heidrick T, Livingstone J. 1992. Tectonostratigraphic development of the interior Sudan rifts, Central Africa. In Geodynamics ofRifting, Volume II. Case History Studies on Rifts: North and South America and Africa, Ziegler PA (ed.). Tectonophysics 213: 187–202.

MegeD, KormeT. 2004. Dyke swarm emplacement in the Ethiopian Large Igneous Province: not only a matter of stress. Journal of Volcanologyand Geothermal Research 132: 283–310.

Merla G, Abbate E, Canuti P, Sagri M, Tacconi P. 1973. Geological map of Ethiopia and Somalia, 1:2,000,000. Consiglio Nazionale delleRicerche Italy, Stabilimento Poligrafico Fiorentino, Firenze.

Millegan FS. 1990. Aspects of the interpretation of Mesozoic rift basins in northern Sudan using potential-field data. Society of ExplorationGeophysicists abstracts 605–607.

Mohr PA. 1962. The Geology of Ethiopia. Addis Ababa University Press, Addis Ababa, Ethiopia: 268.Russo A, Assefa G, Atnafu B. 1994. Sedimentary evolution of the Abby River (Blue Nile) Basin, Ethiopia. Neues Jahrbuch fur Geologie und

Palaeontologie Monatshefte 5: 291–308.Saxena GN, Assefa G. 1983. New evidence in the age of the glacial rocks of northern Ethiopia. Geological Magazine 120: 549–554.Sengor AMC. 2001. Elevation as indicator of mantle-plume activity. Geological Society of America 352: 183–225.Silvestri A. 1973. Orbitoline mesocretacea degli Strati di Graua. In Missione Geologica dell’ AGIP nella Dancalia Meridionale e sugli Altipiani

Hararini (1936–1938). Accademia Nazionale Dei Lincei, Atti Convegni Lincei 4: 101–138.Stefanini G. 1933. Saggio di una carta geologica dell’Eritrea, della Somalia e dell’Ethiopia, 1:2,000,000. Note Illustrative, Firenze: 195.Williams L. 2002. Mineral wealth at the Horn of Africa. Ethiopia. Mining journal, London: 12.WoldeGabriel G, Aronson JL, Walter RC. 1990. Geology, geochronology, and rift basin development in the central sector of the Main

Ethiopia Rift. Geological Society of America Bulletin 102: 439–458.Wolfenden E, Yirgu G, Ebinger C, Deino A, Ayalew D. 2004. Evolution of the northern Main Ethiopian Rift: birth of a triple junction. Earth

and Planetary Science Letters 24: 213–228.Wood CB, Zavada MR, Goodwin MB, Clemens WA, Schaff CR. 1997. First palynostratigraphic dates for Mesozoic vertebrate faunas in the

Blue Nile Gorge region, Ethiopia. Journal of Vertebrate Paleontology 17: 86A.Worku T, Astin TR. 1992. The Karoo sediments (Late Paleozoic to Early Jurassic) of the Ogaden Basin, Ethiopia. Sedimentary Geology 76:

7–21.Wright TJ, Ebinger C, Biggs J, Ayele A, Yirgu G, Keir D, Stork A. 2006. Magma-maintained rift segmentation at continental rupture in the

2005 Afar dyking episode. Nature 442: 291–294.


DOI: 10.1002/gj

stratigraphic and structural evolution of the blue nile basin,...

Documents