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NAME: OLA ITUNU GRACE
LEVEL: 300
MATRIC NYMBER: 13/SCI14/012
COLLEGE: SCIENCES
DEPARTMENT: GEOLOGY
COURSE CODE: GEY 309
COURSE: SEDIMENTARY DEPOSITIONAL
ENVIRONMENT AND BASINS OF AFRICA
QUESTION:
1. DISCUSS ECONOMIC IMPORTANCE OF A TYPICAL
SEDIMENTARY ENVIRONMENT YOU KNOW.
2. WRITE SUCCINCTLY ON BASINS OF AFRICA.
ANSWER
1) A sedimentary environment is a geographic location
characterized by a particular combination of geologic processes
and environmental conditions. Geologic processes include the
currents that transport and deposit sediments (water, wind, or
ice) and the plate tectonic settings that affect sedimentation.
For example, the geologic processes of a beach environment
include the dynamics of waves crashing against the shore,
shoreline currents, and the distribution of sediments on the
beach. Environmental conditions include the kind and amount
of water (ocean, lake, river, arid land), the landscape (lowland,
mountain, coastal plain, shallow ocean, deep ocean), and
biological activity.
ECONOMIC IMPORTANCE OF THE MARINE
ENVIRONMENT
The oceans and seas of the world cover almost three-quarters of
the surface of the planet and are very important areas of
sediment accumulation. The oceans are underlain by oceanic
crust, but at their margins are areas of continental crust that may
be flooded by seawater: these are the continental shelves. The
extent of marine flooding of these continental margins has varied
through time due to plate movements and the rise and fall in
global sea level related to climate changes. The sedimentary
successions in these shallow shelf areas provide us with a record
of global and local tectonic and climatic variations. There is
considerable variety in the sedimentation that occurs in the
marine realm, but there are a number of physical, chemical and
biological processes that are common to many of the marine
environments. Physical processes include the formation of
currents driven by winds, water density, temperate and salinity
variations and tidal forces: these have a strong effect on the
transport and deposition of sediment in the seas. Chemical
reactions in seawater lead to the formation of new minerals and
the modification of detrital sediment. The seas also team with life:
long before there was life on land organisms evolved in the
marine realm and continue to occupy many habitats within the
waters and on the sea floor. The remains of these organisms and
the evidence for their existence provide important clues in the
understanding of palaeoenvironments.
The most important chemical and biochemical sediments in
modern seas and ancient shelf deposits are carbonate sediments
and evaporites, and in the oceans plankton generate large
quantities of carbonate and siliceous sediment. In addition there
are other, less abundant but significant chemical and biochemical
deposits.
The term glauconite is commonly used by geologists to refer to
a dark green mineral that is found quite commonly in marine
sediments. In correct usage the use of this term should be
restricted to potassium- rich mica, which has the mineral name
glauconite, because this is in fact only one member of a group of
potassium and iron-rich phyllosilicate minerals that are closely
related. Glaucony/glauconite is important in sedimentology and
stratigraphy for a number of reasons. Firstly, it is a reliable
indicator of deposition in a shallow marine environment, although
it can be reworked into deeper water and occasionally into
shallower environments by currents. Secondly it is most abundant
within shelf sediments under conditions where sedimentation of
Other material, terrigenous clastic or carbonate, is slow. It
therefore commonly occurs in condensed sections, that is, strata
which have been deposited at anomalously low sedimentation
rates. The recognition of periods of low sedimentation rate on the
shelf is important when assessing evidence of changes in sea level
because outer shelf sedimentation tends to be slowest during
periods of sea level rise. Thirdly, because the mineral is authigenic
and also rich in potassium, it can be dated by radiometric
methods and the age obtained corresponds to the time of
deposition. Direct radiometric dating of sedimentary material is
rarely possible, but Glaucony/glauconite is the exception and
consequently is very important in relating strata to the geological
time scale.
Organic material from dead plants, animals and microbial
organisms is abundant in the oceans and becomes part of the
material that falls to the sea floor. They have considerable
economic importance in sedimentology and stratigraphy as they
are hydrocarbon source rocks
2) The official definition of a sedimentary basin is: a low area in
the Earth’s crust, of tectonic origin, in which sediments
accumulate. Sedimentary basins range in size from as small as
hundreds of meters to large parts of ocean basins. The
essential element of the concept is tectonic creation of relief,
to provide both a source of sediment and a relatively low place
for the deposition of that sediment.
Africa includes large-scale tectonic basins, topographic
depressions and drainage basins, or watersheds, which describe
blocks of country drained by common watercourses. These
landscapes are remarkably varied. They encompass sandy deserts,
vast savannas and some of the biggest, steamiest tropical rain
forests on the globe, roamed by elephants and gorillas.
In a general sense, southern and eastern Africa are more
uplifted than the northern and western parts of the continent,
sometimes they are distinguished as “High Africa” and “Low
Africa” respectively. Large-scale basins separated by
intervening highlands are imposed upon this general trend of
the landscape. Lurching of the Earth’s crust has also staggered
the terrain, most famously along the African Rift Valley system.
There are eighty sedimentary basins in five different types in
African continent, I. e. craton sag basin, foreland basin,
intermountain basin, passive margin basin and rift basin, which
underwent the stress environment of stable
Depression-compression-extension. The first three types of basins
had been intensely influenced by Hercynian and Alpine tectonic
movement, while the later two types of basins always exist in a
stable extension environment. Different basin evolution caused
the obviously hydrocarbon distribution difference. In North Africa,
marginal craton sag and rift basins show great expedition
potential of hydrocarbon, marginal craton sag basins had good
formation and preservation of Lower Silurian hot shale, tectonic
strata traps and migration pathway formed by Hercynian
unconformity, and rift basins had excellent Upper Cretaceous
marine source rocks and good hydrocarbon preservation with
little tectonic activity . Meanwhile, in the salt-containing passive
margin basins and delta basins of West Africa, thick strata
containing high quality source rocks and plastic strata were well
developed. Source rocks of high maturity, good hydrocarbon
preservation, growth faults and deformational structure traps
were formed by abundant overlying sediments and sources
supplied from Tertiary large water system. There exist eighty
sedimentary basins with abundant hydrocarbon resources and
areas about 20121 ×1 0 6 km 2 in Africa, among which oil and gas
have been discovered in fifty four ones (Tong, 2004).
Formation and evolution of sedimentary basins in Africa
Congo shield, Kalahary shield and West Africa shield, developing
early from the Archean, formed the original basement of African
Plate. To Proterozoic, all Archean shields became the early
cratons with sedimentary cover consisting of greenstone belt,
metamorphic rocks, quartzite, apogrite and volcanic rocks. African
Plate essentially of four main Precambrian cratons: Congo carton,
Kalahary craton, West Africa craton and East Africa craton (Guan
et al., 2005). African Plate, as a part of southern Gondwana
continent, collided with northern Laurasia to form the first super
ancient land. From Cambrian to Tertiary, all plates in the Earth,
including African Plate, went into the forming and breaking up
process of Pangaea. During the evolution process with the
frequent change between extrusion and extension, African Plate
developed basins of five types (including nine sub types) by the
different tectonic stress backgrounds, different positions in the
plate, and different basin subsiding mechanism. In general, it is an
collision process for African Plate in Paleozoic when all plates
were converging to each other and the Pangaea were formed at
the end of Paleozoic (Guan et al . , 2005). In the early stage,
African Plate stayed still in lightly extension setting, on the
contrary, in the later stage, African Plate was in intensely
extrusion setting. After Pan African movement at the stage of
Cambrian to Silurian, Laurasia land moved little to the opposite
direction of Gondwana land, when African Plate was still in
tectonic state with uniform rise and subsidence. As a result of less
tectonic movement and much crust thinning, craton sag basins
developed in African Plate along north to west or near north to
south direction of strike (Zheng, 2005). Strata of less changed
thickness and little deformation lay in craton sag basins.
According to different basin locations in African Plate, all craton
sag basins can be divided into two types: intra-craton sag basins
and external craton sag basins. Most external craton sag basins
were located in west Sahara region of North Africa, while intracraton sag basins lie in Middle Africa and South Africa. From Early
Devonian, Caledonian movement was active. Then after
separating 100 Ma, initial collision of Laurasia land and Gondwana
land started to collide again in African. In this compress
background, external craton sag basins suffered press from north
to east direction. Because of differential rise and subsidence of
those basins, strata were folded up or erode away. All the
Deformation was aggravated by Hercynian movement in
Carboniferous and Permian. Hercynian movement was the most
intensely movement that influenced entire African Plate in
geological history, which caused intense erosion of early formed
strata and generally distributed unconformity in North Africa .
Hercynian fold belts were formed at northern and southern
Africa, where foreland basins and intermountain basins developed
along those two fold belts. After the end of Hercynian at late
Permian, a united continent, so called the Pangaea, formed again
as a result of the combination of Gondwana land and Laurasia
land. In the process, African Plate splited away from the neighbor
plates, leading to the formation of passive margin and inner plate
rift activities. From the largest united continent to the smaller
Gondwana land and smaller African Plate, the broken events
descended in a sequence consist of four grades. Firstly, Gondwana
land broke away from Laurasia land from late Triassic to early
Jurassic, causing Africa splited away from North America in
northwestern part ; secondly, a sudden break happened inner
Gondwana land from early Jurassic to late Cretaceous, causing
east Gondwana land separated from west Gondwana land and
Africa splited away from India and Australia in eastern part;
thirdly, Africa broke away from South America of inner west
Gondwana land in early Cretaceous; finally, in African Plate,
because of tensional stress between plates, west Sahara splited
away from other part of Africa in late Cretaceous, causing West
Africa rift zone, Middle Africa shear zone and East Africa rift zone .
At the end of Cenozoic, Hercynian fold belt in North African
continent was slightly influenced by Alpine tectonic movement,
and some tectonic deformation developed into intermountain
basins. During whole process, African continent was always in
passive extension background without any extrusion events, so
passive margin basins developed along Africa continental shelf
and rift ba2 sins developed near rift zones inner plate. Mostly
distributed in eastern and western coast, passive margin basins
developed “double layer structure” above divergent crust (Jiang,
2005), which consists of lower
According to different tectonic-sediment sequences, those
passive margin basins can be divided into four types: saltcontaining passive basins, none salt passive basins, strike-slip
basins and delta basins. The type of delta basins means that
typical basins deposited ultra thick delta sediments at the river
mouth region in Niger, Angola and Egypt in Tertiary (Pei, 2004).
Entirely, the stress field of African Plate under-went a changing
process including extension in Early Paleozoic, extrusion in Later
Paleozoic, extension in Mesozoic and Cenozoic. As extension
background was dominated through the whole geological history
and basin formation mechanism of all types was relatively simple,
all basins were distributed in a uniform law and well preserved.
1. Reggane Basin
2. Ghadames Basin
3. Oude Mya Basin
4. Rharb Basin
5. Tellian Atlas Basin
6. Sirte Basin
7. Chad Basin
8. Senegal Basin
9. Southwest Coast Basin
10.
11.
12.
13.
14.
15.
16.
17.
Low Congo Basin
Cote d’Ivoire Basin
Niger Delta Basin
Tanzania Coast Basin
Melut Basin
Kufra Basin
East African Rift System basins
Nile Delta Basin
To a large number of basins has so many types, petroleum
geological conditions show great differences in each kind of
basins, which lead to hydrocarbon resources distribution
differences in African continent.
Craton sag basins; eighteen craton sag basins developed in Africa,
including eight intra-craton sag basins lay in the middle and
southern part of Africa, and ten external craton sag basins in
North Africa. External craton sag basins, with high degree of
exploration, had good conditions for hydrocarbon to be generated
and preserved in Paleozoic petroleum system, where many large
oil and gas fields were found.
Total thirty eight passive margin basins developed around African
Plate, most of which lay in eastern coast of Atlantic Ocean and
western coast of India Ocean. According to exploration results,
the best petroleum geological conditions developed at west
central coast of Africa (Liu et al., 2008), where Niger Delta Basin
and Congo Fan Basin developed. This region stretches from the
southern edge of the Niger Delta to the Walvis Ridge, including a
few salt basins. Two petroleum systems support above mentioned
conclusion. First one developed at lower sequences below the salt
bed. From Late Jurassic to Early Cretaceous, Africa separated with
South America step by step from southern part of West Africa to
northward, when a series of asymmetrical horsts and grabens
extended parallel to present coastline. At syn-rift stage of the
extensive rifting event, thick sequences of fluvial and lacustrine
rocks were deposited in those rift basins. Ascribed to the
presence of Walvis Ridge, oceanic upwelling and anoxic events
(Hou et al . , 2005; Huang et al . , 2008; Huc et al . , 2004) , a set of
organic-rich and lacustrine black shales of the rift staged
developed in the restricted sea environment, which were the
most important hydrocarbon source rocks in these passive margin
basins . Hydrocarbon generated potential of some kerogen could
be up to 46 t/km2. Overburden pressures from super strata
accelerated the mature of underlying source rocks, promoted
shale and salt deformation and the formation of depositional fault
(Liro et al., 1995), which were helpful to form giant oil and gas
filed. Another preferable region is Guinea Gulf in the northwest of
Niger delta, where strike-slip basins developed. Two points, the
presence of transform fault zone and the absence of evaporate
and salt strata, differing from above mentioned salt basins, led to
moderate petroleum potential in this area (Brownfield et al . ,
2006). Lower Cretaceous marine shale source rocks, trubidite of
post-rift stage, and faults pathway related to transform fault zone
still could develop high2 quality hydrocarbon reservoirs. Some
present discoveries in the deep water at Guinea Gulf have
supported this point.
Rift basins; Eighteen rift basins of small scale developed in
Africa, in which Sirte Basin is the largest one with most abundant
hydrocarbon resources. Typical rift basins distributed in Mesozoic
West Africa Rift Zone, Middle Africa Shear Zone and Cenozoic East
Africa Rift System. Except offshore platform sedimentary system
of marine facies developed in Sirte Basin, fluvial-delta-lake
sedimentary system of terrestrial facies developed in other rift
basins. Two petroleum systems of marine facies and terrestrial
facies were formed in these basins.