Blue Nile Basin
Encyclopedia
The Blue Nile Basin is a major geological formation in the northwestern Ethiopian Plateau formed in the Mesozoic
Era during a period of crustal extension
associated with the break-up of Gondwana
, and filled with sedimentary deposits. The modern Blue Nile
river cuts across part of the sedimentary basin
.
rocks aged about 750 Ma that had become a peneplain
, possibly during the Paleozoic
era (540 MaMa = Million years ago - 250 Ma).
The basin was formed due to rifting during the Mesozoic era (250 Ma - 65 Ma).
Between the Triassic and early Jurassic
, about 300 m of fluvial
sediments were deposited by rivers and streams. During the Jurassic (200 Ma - 145 Ma) the basin was twice covered by an arm of the Indian Ocean
for extended periods, creating a lower limestone sediment 450 m thick and an upper limestone sediment 400 m. In the late Jurassic and early Cretaceous
periods the basin rose, and the 280 m upper sandstone sediments, both alluvial or fluvial, were deposited. In total, about 1.4 km of sediment was deposited over the basement rocks in this period.
Later, the Afar mantle plume caused volcanic eruptions in the early and late Oligocene
(34 - 23 Ma), depositing volcanic rock
s between 500 m and 2000 m thick, with further eruptions in the Quaternary
depositing another 300 m of rock. These layers have been exposed where the Blue Nile river has cut through the strata
, creating a 1,600 m gorge where the rocks of different periods can be studied.
However, the architecture of the basin is not well known in other areas due to the thick upper layer of comparatively recent volcanic rock.
and early Cretacious periods through forces related to the rifting of Gondwana.
As the Blue Nile basin formed, it was filled with clastic and marine sediments.
In the late Miocene
further rifting occurred in a NW-SE extension associated with the Main Ethiopian rift, which formed NE-trending faults. In the Quaternary
(2.5 Ma - present) the region was subject to further stresses as the Main Ethiopian rift opened obliquely, creating N, ESE and NW trending extensions within the basin.
Other rift basins with the same orientation formed in the region at the same time. The Blue Nile rift
, Melut Basin
and Muglad Basin
all terminate on the line of the Central African Shear Zone
, a major strike-slip shear zone
. It is now known that the Muglad and Melut connect in the southeast, and then connect to the Anza trough
in Kenya
. It is possible that the Blue Nile basin may be a southeastern extension of the Blue Nile rift in the Sudan
, and may also extend southeast of the Main Ethiopian rift, connecting to the Ogaden Basin
in southeast Ethiopia
.
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
Era during a period of crustal extension
Extensional tectonics
Extensional tectonics is concerned with the structures formed, and the tectonic processes associated with, the stretching of the crust or lithosphere.-Deformation styles:...
associated with the break-up of Gondwana
Gondwana
In paleogeography, Gondwana , originally Gondwanaland, was the southernmost of two supercontinents that later became parts of the Pangaea supercontinent. It existed from approximately 510 to 180 million years ago . Gondwana is believed to have sutured between ca. 570 and 510 Mya,...
, and filled with sedimentary deposits. The modern Blue Nile
Blue Nile
The Blue Nile is a river originating at Lake Tana in Ethiopia. With the White Nile, the river is one of the two major tributaries of the Nile...
river cuts across part of the sedimentary basin
Sedimentary basin
The term sedimentary basin is used to refer to any geographical feature exhibiting subsidence and consequent infilling by sedimentation. As the sediments are buried, they are subjected to increasing pressure and begin the process of lithification...
.
Strata
The Blue Nile basin originated in an area of NeoproterozoicNeoproterozoic
The Neoproterozoic Era is the unit of geologic time from 1,000 to 542.0 ± 1.0 million years ago. The terminal Era of the formal Proterozoic Eon , it is further subdivided into the Tonian, Cryogenian, and Ediacaran Periods...
rocks aged about 750 Ma that had become a peneplain
Peneplain
A peneplain is a low-relief plain representing the final stage of fluvial erosion during times of extended tectonic stability. The existence of peneplains, and peneplanation as a geomorphological process, is not without controversy, due to a lack of contemporary examples and uncertainty in...
, possibly during the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
era (540 MaMa = Million years ago - 250 Ma).
The basin was formed due to rifting during the Mesozoic era (250 Ma - 65 Ma).
Between the Triassic and early Jurassic
Jurassic
The Jurassic is a geologic period and system that extends from about Mya to Mya, that is, from the end of the Triassic to the beginning of the Cretaceous. The Jurassic constitutes the middle period of the Mesozoic era, also known as the age of reptiles. The start of the period is marked by...
, about 300 m of fluvial
Fluvial
Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
sediments were deposited by rivers and streams. During the Jurassic (200 Ma - 145 Ma) the basin was twice covered by an arm of the Indian Ocean
Indian Ocean
The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
for extended periods, creating a lower limestone sediment 450 m thick and an upper limestone sediment 400 m. In the late Jurassic and early Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...
periods the basin rose, and the 280 m upper sandstone sediments, both alluvial or fluvial, were deposited. In total, about 1.4 km of sediment was deposited over the basement rocks in this period.
Later, the Afar mantle plume caused volcanic eruptions in the early and late Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
(34 - 23 Ma), depositing volcanic rock
Volcanic rock
Volcanic rock is a rock formed from magma erupted from a volcano. In other words, it is an igneous rock of volcanic origin...
s between 500 m and 2000 m thick, with further eruptions in the Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
depositing another 300 m of rock. These layers have been exposed where the Blue Nile river has cut through the strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
, creating a 1,600 m gorge where the rocks of different periods can be studied.
However, the architecture of the basin is not well known in other areas due to the thick upper layer of comparatively recent volcanic rock.
Rifting
The NW trending rift basin originated when the basement rocks were extended between the TriassicTriassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
and early Cretacious periods through forces related to the rifting of Gondwana.
As the Blue Nile basin formed, it was filled with clastic and marine sediments.
In the late Miocene
Miocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
further rifting occurred in a NW-SE extension associated with the Main Ethiopian rift, which formed NE-trending faults. In the Quaternary
Quaternary
The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...
(2.5 Ma - present) the region was subject to further stresses as the Main Ethiopian rift opened obliquely, creating N, ESE and NW trending extensions within the basin.
Other rift basins with the same orientation formed in the region at the same time. The Blue Nile rift
Blue Nile rift
The Blue Nile rift is a major geological formation in the Sudan, a rift with a NW trend that terminates on the Central African Shear Zone.It was formed through crustal extension during the break-up of Gondwana....
, Melut Basin
Melut Basin
The Melut Basin is a rift basin in South Sudan. It is situated in the states of Upper Nile and Jonglei, south of the capital Khartoum and east of the river Nile. The Basin contains several hydrocarbon accumulations, although oil exploration, as elsewhere in Sudan, has been hindered by conflict...
and Muglad Basin
Muglad Basin
The Muglad Basin is a large rift basin in Northern Africa. The basin is situated within southern Sudan and covers an area of approximately 120,000 km2. It contains a number of hydrocarbon accumulations of various sizes, the largest of which are the Heglig and Unity oil fields...
all terminate on the line of the Central African Shear Zone
Central African Shear Zone
The Central African Shear Zone is a wrench fault system extending in an ENE direction from the Gulf of Guinea through Cameroon into Sudan.The structure is not well understood....
, a major strike-slip shear zone
Shear zone
A shear zone is a very important structural discontinuity surface in the Earth's crust and upper mantle. It forms as a response to inhomogeneous deformation partitioning strain into planar or curviplanar high-strain zones. Intervening blocks stay relatively unaffected by the deformation...
. It is now known that the Muglad and Melut connect in the southeast, and then connect to the Anza trough
Anza trough
The Anza trough is a rift in Kenya that was formed in the Jurassic Period of the Mesozoic Era.The trough runs inland from the coast in the northwest direction....
in Kenya
Kenya
Kenya , officially known as the Republic of Kenya, is a country in East Africa that lies on the equator, with the Indian Ocean to its south-east...
. It is possible that the Blue Nile basin may be a southeastern extension of the Blue Nile rift in the Sudan
Sudan
Sudan , officially the Republic of the Sudan , is a country in North Africa, sometimes considered part of the Middle East politically. It is bordered by Egypt to the north, the Red Sea to the northeast, Eritrea and Ethiopia to the east, South Sudan to the south, the Central African Republic to the...
, and may also extend southeast of the Main Ethiopian rift, connecting to the Ogaden Basin
Ogaden Basin
The Ogaden Basin is an area of Ogadkenya that may hold significant reserves of crude oil and natural gas. The basin covers an area of some 350,000 squarekilometres and is formed from sedimentary rocks up to 10,000 meters thick...
in southeast Ethiopia
Ethiopia
Ethiopia , officially known as the Federal Democratic Republic of Ethiopia, is a country located in the Horn of Africa. It is the second-most populous nation in Africa, with over 82 million inhabitants, and the tenth-largest by area, occupying 1,100,000 km2...
.