Stratigraphic and structural evolution of the Blue Nile Basin ...

GEOLOGICAL JOURNALGeol. J. (2008)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/gj.1127Stratigraphic and structural evolution of the Blue Nile Basin,Northwestern Ethiopian PlateauN. DS. GANI 1 *, M. G. ABDELSALAM 2 , S. GERA 3 and M. R. GANI 11 Earth and Environmental Sciences, University of New Orleans, New Orleans, LA, USA2 Geological Sciences and Engineering, Missouri University of Science and Technology, Rolla, MO, USA3 Regional Mapping and Geochemistry Department, Geological Survey of Ethiopia, Addis Ababa, EthiopiaThe 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 2008KEY WORDSBlue Nile Basin; Mesozoic rift systems; basin evolution; eastern and central Africa1. INTRODUCTIONThe Blue Nile Basin is situated in the Northwestern Ethiopian Plateau and is bounded to the E and SE by thetectonic escarpment of the uplifted western flank of the Main Ethiopian Rift and to the N and S by theAxum–Adigrat and Ambo lineaments, respectively. The basin contains a 1400 m thick section of Mesozoicsedimentary rocks unconformably overlying Neoproterozoic basement rocks and unconformably overlain byEarly–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: ngani@uno.eduCopyright # 2008 John Wiley & Sons, Ltd.

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) 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 timesduring Gondwana break-up (Figure 1). These rift basins terminate sharply in the northwest against the NE-trendingCentral African Shear Zone, which is considered to be a major dextral strike-slip shear zone (Figure 1; McHargueet al. 1992; Binks and Fairhead 1992). However, there might be some lithospheric extension to the north of the shearzone, 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 poorlyunderstood. There, these basins are covered by 500–2000 m thick pile of Early–Late Oligocene volcanic rocks, andlocally followed by 300 m thick sequence of Quaternary volcanic rocks. These volcanic rocks are associated withthe 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 Nilerift 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 riftbasins connect with each other in the southeast and then connect with the Anza Rift in Kenya (Figure 1; McHargueet al. 1992; Binks and Fairhead 1992). However, the continuation of the Blue Nile rift basin to the southeast fromthe lowlands of Sudan towards the highlands of Ethiopia is not certain for the reasons outlined above. The Blue NileBasin in Ethiopia lies between 98N and 13850’N, and 34850’E and 39850’E where the Blue Nile is incised intothe 2500 m high (average) Northwestern Ethiopian Plateau (Figure 2). The linear exposures in the Gorge of theNile 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 theBlue Nile Basin is related to Mesozoic rift basins of eastern and central Africa (Figure 1; Bosellini 1989, 1992). Thepresence 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 southeastwardthrough 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

lue nile basin evolutionThe Blue Nile Basin is thought to have formed during the Late Jurassic on the basis of K/Ar age (143 6 and124 5 Ma) of two basaltic layers encountered in the Khartoum Basin of the Blue Nile Rift in Sudan (Bosworth1992).The exposures of the Blue Nile Basin within the Northwestern Ethiopian Plateau are bordered by the upliftedtectonic escarpments on the western flanks of the Afar Depression and the Main Ethiopian Rift in the east andsoutheast, respectively, and in the west by the erosional Tana escarpment (Figure 1). The Quaternary-agedE-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 ofoutstanding 10.7–22.4 Ma old ( 40 Ar/ 39 Ar ages of Kieffer et al. 2004) shield volcanoes around which the Blue Nilenavigates (Figure 2). Some 1400 m of Mesozoic sedimentary rocks are exposed where the Blue Nile forms the150 km semi-circular Blue Nile Bend within very rugged and largely inaccessible terrain (Gani and Abdelsalam2006).3. DATA AND METHODSField 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 examinationof various structural styles and orientations (areas 1, 2, 3 and 4 on Figure 2). Additionally, we have used geologicaland structural data collected along the SE-flowing segment of the Blue Nile close to Lake Tana to examine theorientation 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 ReflectionRadiometer (ASTER) data. These data have three visible and near infrared (VNIR) bands with 15 m spatialresolution, six shortwave infrared (SWIR) bands with 30 m spatial resolution and five thermal infrared (TIR) bandswith 90 m spatial resolution. (2) Landsat thematic mapper (TM) data which have four VNIR bands and two SWIRbands with 30 m spatial resolution, and one TIR band with 60 m spatial resolution. (3) Standard beam RADARSATdata 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 DEMswith 15 m x–y resolution. The methods used to process and interpret the remote sensing data used in this study aredescribed in detail in Gani and Abdelsalam (2006).Field studies are focused on mapping different stratigraphic units as well as documenting the orientation andstyle of geological structures. Field studies and remote sensing analysis are used to produce detailed geologicalmaps and geological cross-sections for each of the key areas (an exercise helped by reference to publishedgeological 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 comprehensivestratigraphic column for the Blue Nile Basin; (2) examination of the basin’s structural orientations and styles withinthe framework of regional tectonic stress regimes and (3) construction of an evolutionary model for the basin.4. STRATIGRAPHY, DEPOSITIONAL ENVIRONMENTS AND STRUCTURESGeological maps and geological cross-sections for each key area are presented in Figures 3–6. A comprehensivestratigraphic column for the Blue Nile Basin is shown in Figure 7. The dominant orientations of faults and fracturesfor 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 dominantlyNeoproterozoic basement rocks, Triassic–Early Jurassic Lower Sandstone, and Early–Late Oligocene volcanicrocks. Key area 3 is within the SW-flowing segment of the Blue Nile (Figures 2 and 5). Exposures in this areainclude Triassic–Early Jurassic Lower Sandstone, Early Jurassic glauconitic sandy mudstone, Early–MiddleJurassic Lower Limestone and gypsum, Middle–Late Jurassic Upper Limestone and Early–Late OligoceneCopyright # 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

lue nile basin evolutionFigure 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 phase4.1.1. Neoproterozoic basement rocksThese rocks form the base of the Blue Nile Basin (Figure 7a) and crop out within rugged topography at an altitude of900–1500 m along the entire NW-flowing segment of the Blue Nile (Figures 2–4). The age of the basement rocksis considered to be Neoproterozoic, ranging from 850 to 550 Ma as documented from U-Pb and Rb-Srgeochronologic studies further south of the study area by Ayalew et al. (1990). These rocks are made-up of variablymetamorphosed quartzofeldspathic schists and gneisses, migmatites and plutonic rocks. Neoproterozoicpenetrative NNE-trending sub-vertical ductile planar fabrics are associated with NNE- to NE-trending uprighttight 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- andESE-trending normal faults are more common than NE- and NW-trending faults. In contrast, fractures within theNeoproterozoic basement rocks are dominantly NNE- and ESE-trending (Figure 8a). These fractures are clearlydilational 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

n. ds. gani 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 showingan associated normal fault (attitude 1528/558NE, throw 400 m) juxtaposing Early–Late Oligocene basalt and Middle–Late Jurassic UpperLimestone: location of photomosaic is shown in (a).Copyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

lue nile basin evolutionFigure 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 phaseThe 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). Thissuccession contains evidence for different phases of marine transgression and regression.4.2.1. Lower SandstoneThis 300 m thick unit is also known as the Adigrat Sandstone and is considered to be Triassic–Early Jurassic inage 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 andLemoigne 1975; Russo et al. 1994). The unit is found unconformably overlying Neoproterozoic basement rocksand, 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 segmentCopyright # 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-upwardfacies succession interpreted as fluvial channel deposits within the Lower Sandstone and (c) detailed stratigraphic column showingsedimentological 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-upof pink to red, fine- to coarse-grained sandstones that are rarely interbedded with grey mudstone beds. Sedimentarystructures within this unit include dune-scale trough cross-bedding with set thickness ranging between 10 cm and1 m (Figure 10a) and with occasional pebbles and lithoclasts along foresets. Generally, the Lower Sandstone isCopyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

lue nile basin evolutionFigure 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) Uppervolcanic 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 baseoverlain by lags, interpreted as channel features (Figures 7b and 10b). Lateral accretion surfaces within thesandstones indicate lateral migration of the channels. The average azimuth of palaeocurrents measured from dunecross-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. Insome places, silicified tree trunks up to 4 m long, mud-cracks and vertebrate tracks are found within this unit. TheCopyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

lue nile basin evolutionFigure 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 anaggregate throw of 4m.presence of large mud-cracks (Figure 10c) and vertebrate tracks (Figure 10d) within the sandstones (Figures 10cand 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-trendingnormal faults, as well as NW- and ENE-trending fractures, which are mostly dilational (Figure 8b). Throws on thenormal faults ranges between 50 cm and 8 m, and fault zones range in width between 10 cm and 10 m. In someplaces, the normal faults are characterized by the smearing of mud layers and the presence of multiple internal faultsurfaces (Figure 10e).4.2.2. Glauconitic sandy mudstone unitIn 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 thefirst time by Gani and Abdelsalam (2006), is sandwiched between Lower Sandstone and Early–Middle JurassicCopyright # 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-scalecross-stratification.Lower Limestone. An Early Jurassic age was therefore assigned to this unit based on its stratigraphic position. Theupper 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 sandstoneinterval has also been identified within the upper part of this unit (Figure 7c). Presently, the glauconitic unit ispreserved as mound-shaped erosional remnants which appear festoon-shaped in map view (Figure 5a; Gani andAbdelsalam 2006). This unit is interpreted to be deposited in an offshore to shelfal marine environment.4.2.3. Lower Limestone and gypsum unitThis unit, 450 m thick (Figure 7), also known as the Gohatsion Formation, is of Early–Middle Jurassic (Toarcianto Bathonian) age, as determined from micro– and mega–fossil studies by Assefa (1981). It is exposed along theSW-flowing segment of the Blue Nile where it is underlain by the glauconitic sandy mudstone unit or theTriassic–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 alternatingCopyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

n. ds. gani ET AL.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.Copyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

n. ds. gani ET AL.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.Copyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

lue nile basin evolutionThe Upper Sandstone unit is affected by NW- and NE-trending normal faults and dominantly N-trendingdilational fractures with subordinate NE- and NW-trending sets (Figures 8e, 14b and c). The throw on normal faultsranges 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 BasinThe Mesozoic stratigraphic succession of the Blue Nile Basin is broadly similar to that of the adjacent MekeleBasin, 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 marineenvironment (Bosellini et al. 1997); Late Jurassic Antalo supersequence, a largely carbonate unit, and EarlyCretaceous Upper Sandstone or Amba Aradam sandstone overlain by Tertiary flood basalt (Beyth 1972; Boselliniet al. 1997). Compared to the Blue Nile Basin, the Lower Sandstone is much thicker (670–700 m thick) in theMekele Basin, and consists of grey or red, fine-grained, mature sandstone with cross-bedding, frequentbioturbation, abundant laterite beds and petrified woods (Beyth 1972; Bosellini et al. 1997). This unit ischaracterized by three major fining-upward facies successions (Bosellini et al. 1997). Regionally, the LowerSandstone 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 agebased on foraminiferal fauna) transition bed, made up of shale with intercalations of reddish, highly bioturbatedsandstone and calcarenite (Bosellini et al. 1997). Like the glauconite unit of the Blue Nile Basin, this transitionalunit probably indicates the initiation of a deepening of the basin. The 450 m thick Lower Limestone unit of theBlue Nile Basin is missing in the Mekele Basin, indicating an earlier flooding in the rapidly subsiding Blue NileBasin during this time.The 700 m thick Antalo supersequence which overlies the transitional unit, is a carbonate-marly succession ofLate Oxfordian–Early Kimmeridgian age (based on foraminiferal fauna) and is equivalent to the Upper Limestoneunit of the Blue Nile Basin (Bosellini et al. 1997). The Antalo supersequence consists of four depositionalsequences that include thickening and shallowing-up cycles (Bosellini et al. 1997). Like the Lower Sandstone, thethickness of this unit increases towards the Red Sea (>1420 m thick in Danakil; Hutchinson and Engles 1970). Thisthickening trend towards the east indicates that the Danakil-Red Sea region was a subsiding trough during theJurassic (Bosellini et al. 1997).In the Mekele Basin, the fluvial Upper Sandstone unit (100–200 m thick) was deposited unconformably on theAntalo supersequence during the Early Cretaceous (Bosellini et al. 1997). This unit, characterized byfining-upward cycles, consists of coarse-grained, cross-bedded, conglomerate lens-bearing fluvial sandstone, alongwith shale (Bosellini et al. 1997).4.3. Post-sedimentation phase:4.3.1. Lower volcanic rocksThe Lower volcanic rocks rest unconformably on the Upper Sandstone, with the absence of interveningPaleocene–Eocene rocks. These Early–Late Oligocene flood basalts (26.9–29.4 Ma on the basis of 40 Ar/ 39 Ar agedating and magnetostratigraphy of Hofmann et al. 1997), together with subordinate trachytes and rhyolites covermuch 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 ofwell-developed columnar joints (Figure 15a). Locally, 1–3 cm thick sub-horizontal layering is observed within thebasalts which are generally aphanitic, and locally vesicular, with the vesicles sometimes filled with zeolites, calciteand quartz to form amygdaloidal texture. In a few places, the upper part of the basalts contains 1 m thick horizonsCopyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

n. ds. gani ET AL.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.Copyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

lue nile basin evolutionof dark brown clay topped by a fine- to coarse-grained pyroclastic layer. A few specimens of silicified wood andbaked 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 faultzones ranging between a few cm and 50 m wide. The dominant fractures are dilational and are NNE- andE-trending with subordinate NW-trending set (Figure 8f).4.3.2. Upper volcanic rocksQuaternary volcanic events resulted in the eruption of 300 m thick basaltic rocks (Figure 7a). This unit is notexposed 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, andvesicles 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. DISCUSSION5.1. Structural interpretation within regional tectonic frameworkRegional 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 formationof sub-parallel NW-trending Mesozoic rifts in northern and central Africa (McHargue et al. 1992). (2) LateMiocene NW–SE-directed tensile stress associated with orthogonal opening of the Main Ethiopian Rift. Tensilevectors of this stress regime have been established from the consistency of NE-trending border faults of the MainEthiopian Rift (Ebinger et al. 1993; Chorowicz et al. 1994; Korme et al. 1997; Acocella and Korme, 2002) andpalaeomagnetic studies (Kidane et al. 2006). (3) Quaternary E–W-directed tensile stress associated with obliqueopening 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 withinrift 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-trendingQuaternary 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 withthe evolution of the Afar Depression. These are: (a) NE–SW-directed tensile stress in southern Afar resulting fromnortheastward separation of the Arabian Plate and Africa Plate. This stress regime has been documented from faultplane solutions (Ayele et al. 2006); and (b) Quaternary E–W directed tensile stress in the western margin of the AfarDepression resulting from the stepping of the Red Sea spreading axis into Afar and subsequent S-propagation ofembryonic spreading centre towards the Afar triple junction. This stress regime has been documented from faultplane 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 QuaternaryE-trending Ambo Lineament (Abebe et al. 1998) which is thought to have both normal and dextral strike-slipcomponents resulting from Quaternary NNE–SSW tensile stress that accompanied transverse faults developed as aresult of change of extension within the Main Ethiopian Rift from orthogonal NW–SE extension in the LateMiocene 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 fromthe later Neogene structures related to the development of the Main Ethiopian Rift and the Afar Depression. AgeCopyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

n. ds. gani ET AL.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 Riftand the Afar Depression.relationships between different faults and fracture sets are rather complicated and in many cases are difficult toresolve unequivocally. However, we have observed that many NW-trending faults and fractures, especially in theMesozoic sedimentary section, are relatively older compared to other faults and fracture sets as evidenced bycross-cutting relationship:(1) The orientations of normal faults and fractures within the Neoproterozoic basement rocks are NNE- andESE-trending. These trends are oblique to both NW- and NE-trending normal faults that are expected todevelop in association with Jurassic–Cretaceous NE–SW-directed extension and Late MioceneNW–SE-directed extension, respectively. We explain the presence of NNE-trending normal faults as dueto the influence of the Neoproterozoic regional structures which are dominantly NNE-trending. The presenceof such strong pre-existing regional fabric can result in strain localization NNE-trending planes during NE–SWand NW–SE-directed extension into NNE-trending faults. The presence of ESE-trending normal faults withinthe Neoproterozoic basement rocks can be directly related to NNE–SSW-directed extension related to theE-trending Ambo Lineament which extends westward from the Main Ethiopian Rift and runs just south of theexposures 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 tobasement’ towards the Main Ethiopian Rift in the NE as a result of northeastward stepping down ofhanging-walls along these ESE-trending faults.(2) The Mesozoic sedimentary section is dominated by NW- and NE-trending normal faults. NW-trending faults inthe lower part of the section (Lower Sandstone and Lower Limestone) seem to dominate over NE-trendingCopyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

lue nile basin evolutionfaults. Fractures within the lower part of the stratigraphic section are NE- and NW-trending. However, theupper part of the Mesozoic section (Upper Limestone and Upper Sandstone) shows a fracture pattern in whichN-trend dominates. We interpret these structural observations as follows: (a) The Mesozoic section wasdeposited under a strong Jurassic–Cretaceous NE–SW-directed extension related to Mesozoic rifting ofGondwana. (b) At a later stage, this Mesozoic fill was affected by Late Miocene NW-SE extension related to theopening of the Main Ethiopian Rift. Normal faults associated with this extension are better developed in theupper part of the Mesozoic sedimentary section compared to the lower part. This might be due to the lower partof 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 fracturesin the lower part of the section) can be explained as a combination of two factors: (i) The effect of QuaternaryE–W-directed extension in the western flank of the Afar Depression. Most of our fracture data are collectedfrom key area 4 where the Upper Limestone and Upper Sandstone dominate the Mesozoic section. This keyarea is the closest to the western margin of the Afar Depression compared to other areas (Figure 16). (ii) Theeffect of Quaternary E–W extension related to oblique continuing opening of the Main Ethiopian Rift. ThisE–W-directed extension will be less intense (development of dilational fractures compared to normal faultswith significant displacement) compared to Late Miocene NW–SE-directed extension, because much of theQuaternary extension is localized within the floor of the Main Ethiopian Rift, rather than border faults, as wasthe case during the Late Miocene extension.(3) The Early–Late Oligocene volcanic rocks are deformed by dominant NE-trending faults and less-frequentNW-trending normal faults. Fracture orientations within these volcanic rocks as well as Quaternary volcanicrocks are dominantly NE-, NNE-, NW- and ESE-trending. The NE-trending faults, and NE- and NNE-trendingfractures can be directly related to Miocene extension. We explain the presence of a subsidiary set ofNW-trending faults, and NW- and ESE-trending fractures as a combination of: (a) QuaternaryNNE–SSW-directed extension related the to E-trending Ambo Lineament in the south; and (b) QuaternaryNE–SW-directed extension related to the northeastward separation of the Arabian Plate from the African Plate.5.2. Palaeogeography and basin evolutionWe summarize our stratigraphic and structural results and architecture of the Blue Nile Basin in relation to regionaltectonic 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 musttherefore have been uplifted and subjected to a long period of erosion. Subsequently, Triassic–CretaceousNE–SW-directed extension related to Gondwana break-up dominated the regional stress regime, resulting inthe formation of NNE-trending normal faults (Figure 17a) whose orientation appears to be controlled by theearlier NNE-trending Neoproterozic regional fabric.(2) Initial rifting associated with the break-up of Gondwana started during the Triassic–Middle Jurassic in easternand central Africa resulting in the initiation of the Blue Nile Basin as a series of NW-trending fault-bounded riftbasins, caused by strong NE–SW extension. The Blue Nile Basin in the Northwestern Ethiopian Plateau mighthave developed in structural continuation and synchronous with the Blue Nile Rift in the lowlands of Sudan tothe northwest. NW-trending grabens developed within the Blue Nile Basin served as depocentres for thedeposition 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 increasingsubsidence of the Blue Nile Basin, a shallow marine embayment extended northwestwards from the IndianOcean submerging the newly formed NW-trending Blue Nile Basin. This initial marine transgression within thebasin 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 forCopyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

n. ds. gani ET AL.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 gypsumand limestone strata at the top of the Lower Limestone (Figure 17e). This was followed by a second phase ofmarine 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 ofmarine depositional environment with a continental alluvial/fluvial environment resulting in the deposition ofthe Upper Sandstone. The unconformity at the base of the Upper Sandstone does not just represent a facieschange associated with a regression, but probably coincides with a period of uplift and erosion. The UpperSandstone 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 earlyuplift (the Afar dome). Şengor (2001), based on a tectono-chronostratigraphic calculation, concluded that theAfar dome began to rise in the middle Eocene, reaching an elevation of 1 km by the Early Oligocene. Thisevent was followed by the extrusion of 500–2000 m thick volcanic rocks which covered much of the MesozoicBlue Nile Basin. Subsequently, the stress regime in northern and central Africa changed dramatically fromNE–SW to NW–SE-directed tensile stress resulting in the initiation of the Main Ethiopian Rift as a majorNE-trending continental rift. The northern Main Ethiopian Rift that dissected the Ethiopian Plateau intonorthwest and southeast sections (Figure 1) developed ca. 11 Ma (Ar/Ar geochronologic study of Wolfendenet al. 2004). WoldeGabriel et al. (1990) conclude that the initiation of the western boundary fault of the MainEthiopian Rift was at least 8.3 Ma (K/Ar geochronology and stratigraphic relationships). However, Bonini et al.Copyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

lue nile basin evolution(2005), based on recent structural, petrological and K/Ar geochronological studies, proposed the extensionforming the Main Ethiopian Rift started between 6 and 5 Ma. These studies support the initiation of the MainEthiopian Rift during the Late Miocene. NE-trending faults were formed dominantly within the MainEthiopian Rift, but the related extension has also affected a broader region beyond the border faults of theMain Ethiopian Rift. Hence, NE-trending faults are developed within the Early–Late Oligocene volcanic rocksas well as within the upper part of the Mesozoic sedimentary section superimposed on the NW-trending normalfaults (Figure 17h).(6) The Early–Late Oligocene volcanic event was followed by the extrusion of 300 m thick Quaternary volcanicrocks. The unconformity at the base of the Upper volcanic rocks probably represents a period of uplift anderosion during the Late Miocene to Quaternary time. More than one extension direction (Figure 17) emerged inthe Quaternary and continued to operate on the region up to the present time. These include E–W-directedextension related to oblique opening of the Main Ethiopian Rift and consequently the development ofE-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–Wextension related to stepping of the Red Sea spreading ridge onto Afar. These tensile stresses resulted in thesuperimposition of N-, ESE and NW-trending extensional structures on the Blue Nile Basin resulting in thepresent architecture of the basin.6. CONCLUSIONS1. The Blue Nile Basin has evolved through three main phases, including (i) pre-sedimentation phase involving thepeneplanation of Neoproterozoic basement rocks, (ii) sedimentation phase including deposition of thickMesozoic strata represented by repetitive marine transgression and regression, and (iii) post-sedimentationphase 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–SWextension producing NNE-trending normal faults in the Neoproterozoic basement rocks and NW-trending faultswhich provided the depocentres for the deposition of the Mesozoic sedimentary rocks in marine and continentalenvironments.3. The Afar Mantle Plume resulted in extrusion of Early–Late Oligocene volcanic rocks that covered much of theMesozoic sedimentary section. This volcanic event was followed by NW–SE-directed extension resulting inthe opening of the NE-trending Main Ethiopian Rift and superimposition of NE-trending faults on rocks withinthe Blue Nile Basin.4. The Quaternary Era in the region is characterized by the extrusion of 300 m thick volcanic rocks, and varyingdirections of tensile stresses (E–W, NNE–SSW and NE–SW) related to tectonic events within the MainEthiopian Rift and Afar Depression. These resulted in superimposition of Quaternary N-, ESE- andNW-trending extensional structures on the Blue Nile Basin.ACKNOWLEDGEMENTSThis 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 LandsatTM data. The authors would like to thank the Geological Survey of Ethiopia and Linda Smith for co-operationduring fieldwork. The authors would also like to thank Professors Ian Somerville and John Walsh, and Drs KarlaKane and Steve Drury for their critical comments to improve the manuscript. Part of this work was carried out in theDepartment of Geoscience at the University of Texas at Dallas. This is the University on New Orleans Departmentof Earth and Environmental Sciences contribution number – and Missouri University of Science and TechnologyGeology and Geophysics Program contribution number 10.Copyright # 2008 John Wiley & Sons, Ltd. Geol. J. (2008)DOI: 10.1002/gj

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