Download GEY_309_1

NAME: DEELE NABARA JOAN
MATRIC NO: 13/SCI14/005
COURSE CODE: GEY 309
COURSE TITLE: SEDIMENTOLOGY
QUESTIONS:
1. Discuss Economic importance of a typical sedimentary Environment you know
2. Write succinctly on Basins of Africa
SEDIMENTARY BASINS OF AFRICA
Sedimentary basins are regions of the earth of long-term subsidence creating
accommodation space for infilling by sediments. The subsidence results from the thinning
of underlying crust, sedimentary, volcanic, and tectonic loading, and changes in the
thickness or density of adjacent lithosphere. Sedimentary basins occur in diverse geological
settings usually associated with plate tectonic activity. The sedimentary basins of Africa are
largely of two types, sag basins and failed rifts. Some of the sag basins, such as the Karoo,
are clearly syn-depositional in origin, with clear evidence that subsidence was coeval with
and exerted a strong control on sedimentation. However, many of the sag basins, notably
those of North Africa, are clearly post-depositional in origin.
THE BASINS IN NORTH AFRICA
The Tindouf Basin: The Tindouf basin is the most westerly of the Saharan basins. It is elongated
east to west. Its western edge is truncated by the cretaceous Atlantic coastal sag basin in
Western Sahara. It is closed off some 700 km to the east in western Algeria. The basin is
asymmetric in cross-section with a steep northerly limb and a gentle southerly limb. It is in filled
by some 8 km of sediments of Cambrian to Carboniferous age that broadly conform to the panSaharan stratigraphy. The Reggane basin lies between the Tindouf and Ahnet basins. It is
elongated northwest to southeast, with a length of some 350 km and a width of some 150 km.
Like the Tindouf basin, it is also asymmetric, with a gentle southwestern limb that dips gently
off the Reguibat massif, and a steeper northeastern limb that is bound by the Ougarta range.
The Sirte Basin of Libya: The Sirte basin is a major sedimentary basin that extends southward
from the Gulf of Sirte in central Libya. It is found that, unlike the basins of southern Libya and
Algeria, there is little surface expression of the Sirte basin. It lies beneath vast sarir gravel plains
with occasional sand seas and escarpments. The existence of the Sirte basin was unknown until
gravity and magnetic surveys were carried out in the late nineteen-fifties, as a part of the quest
for petroleum. The oldest rocks encountered above the Sirte basin unconformity are of
Cenomanian age. This is, thus, taken as the date for the collapse of the Tibesti-Sirte arch. It is
found that these dates come from the wells drilled on the crest of structural highs. The Sirte
basin is extensively dissected by the complex of Late Miocene deep steep-sided sand-filled
channels that drained northward into the Gulf of Sirte.
The Sedimentary Basins of Egypt: The basement complex of Egypt is exposed north and west of
the Red Sea and the Gulf of Suez in the areas of Sinai and the Eastern Desert. It extends south
of latitude 24° north of Jabal Uweinat and Aswan. Basement exposures represent about 10% of
the total area of Egypt, the rest are covered by Phanerozoic sediments that increase
progressively in thickness northward following the regional dip. The Dakhla and Upper Nile
basins are broad intracratonic depocentres that were developed in southern and central Egypt
during the Paleozoic and Mesozoic. They have evolved because of the structural differentiation
and subsidence of the rigid cratonic plate. The Dakhla and Upper Nile basins are separated from
each other by the Kharga and Dakhla basement uplifts that are aligned in N-S and WNW-ESE
directions, respectively. The fluvial-dominated sediments of southern Sinai change northward
and upward into a progressively marine siliciclastic and carbonate sequence of middle Triassic
age.
THE BASINS OF CENTRAL AFRICA
The Iullemmeden Basin: The Iullemmeden are a federation of Touareg people who occupy the
central region of Niger. The gentle, prolonged down warping of the Iullemmeden basin during
the upper Cretaceous-lower Tertiary is characterized by the widespread continuity of deposits.
Doming is also evident to the north of the Iullemmeden, with the northern domes associated
with major fault systems. The Lower Cretaceous Assouas and Tchirezrine formations are similar
to the Abinky formation in that lenses of feldspathic sandstones are found intercalated with
red-brown shales. Both represent the persistence of a meandering river system across the
eastern region of the basin. The lower parts of the Terzikasan formation were deposited in a
sandy braided fiver environment, with a steady discharge, producing a series of linguoid,
sinuous and straight crested bars that migrated downstream to give stacked planar cross-bed
sets.
The Rift Basins of the Sudan: The Sudanese rift structures form intracontinental basins,
bordered on all sides by anorogenic terrain. These basins are seen as the result of a
multistructural system of rifts which appear to have been activated several times since the
Paleozoic. The rapid rate of uplift and subsidence have assisted in the rapid accumulation and
filling of the basins with unconsolidated sediments, ranging from a few hundred metres to
some thousand metres. The rift structures contain sediments of several age groups, origin and
mode of deposition. Some of them are as old as the Paleozoic passing through to Mesozoic,
Tertiary and Quaternary (Bahr El Arab rift and Blue Nile rift). Rift structures in Sudan form the
major ground-water basins, outside these basins ground water is found only in small quantities
and is of poor quality. The rift basins also act as reservoirs for hydrocarbon accumulation; all
the known oil fields of inland Sudan are located in these basins.
The Niger Delta Basin: The Niger Delta basin occupies the Gulf of Guinea continental margin in
equatorial West Africa, between latitudes 3° and 6° N and longitudes 5° and 8° E. It ranks
among the world's most prolific petroleum-producing Tertiary deltas, comparable to the Alaska
North Slope, the Mississippi, the Orionoco, and the Mahakam. The Niger Delta basin occupies
the coastal and ocean ward part of a much larger and older tectonic feature, the Benue trough.
The Benue trough is a NE-SW folded rift basin that runs diagonally across Nigeria. It formed
simultaneously with the opening of the Gulf of Guinea and the equatorial Atlantic in AptianAlbian times, when the equatorial part of Africa and South America began to separate. The
influence of basement tectonics on the structural evolution of the Niger delta was largely
limited to the movements along the equatorial Atlantic oceanic fracture zones that extend
beneath the delta and determined the initial locus into which the proto-Niger built its delta.
The following are other sedimentary basins in central Africa:
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The west African coastal basin
The eastern African rift basins
The coastal basins of Somalia, Kenya and Tanzania.
THE BASINS IN SOUTHERN AFRICA
The Owambo Basin of Northern Namibia: The geology of the Owambo Basin is known from
outcrops along its margins, from interpretation of seismic, aeromagnetic and gravity surveys
and from a few widely spaced wells. The Owambo Basin is floored by mid-Proterozoic crustal
rocks of the Congo Craton and contains possibly as much as 8000 m of sedimentary rocks of the
Nosib, Otavi and Mulden Groups of the late-Proterozoic Damara Sequence, 360 m of Karoo
rocks and a blanket of semi-consolidated to unconsolidated Cretaceous to Recent Kalahari
Sequence sediments up to 600 m thick.
As rifting evolved to spreading, the region of the Owambo Basin became a stable
platform (Northern Platform of the Damara Orogen) marginal to oceans to the south and west.
Carbonates of the Otavi Group were deposited between about 730 and 700 Ma on this
platform.
Continental Cretaceous to Recent aeolian sands and lacustrine clays of the Kalahari
Sequence form a thick blanket over all older rocks in the Owambo Basin. The lacustrine clays
with their associated fluviatile silts and sands may have been transported by Endorheic Rivers
flowing from the northwest before being deposited in shallow lakes in the southern half of the
basin in much the same way that the Okawango Swamps of Botswana are being fed today by
the Okawango River. The present-day Etosha Pan developed as a result of pan-edge retreat.
The Foreland Karoo Basin South Africa: The main Karoo Basin contains strata that range in age
from late Carboniferous to middle Jurassic and attain a maximum cumulative thickness of
approximately 12 km in the southeastern portion of the basin toward the eastern end of the
Karoo trough. The basin covers an area of approximately 700,000 km2, but it was more
extensive during the late Carboniferous to Permian, when it formed one of the major
depocentres in southwest Gondwana with an area of at least 1,500,000 km2. The Karoo basin is
filled with clastic and subordinate igneous rocks belonging to the Karoo super group. The
Dwyka group forms the basal part of the succession and consists of diamictite and other glacialrelated rock types deposited during the late Carboniferous and early Permian. The deposits are
overlain by the mudrock-dominated Permian Ecca group, generally representing suspension
settling.
Late Mesozoic Sedimentary Basin off the South Coast Of South Africa: Late Mesozoic
sedimentation off the south coast of South Africa records a history of initial continental rifting
(Middle-Late Jurassic to latest Valanginian), followed by a transitional episode (latest
Valanginian to Early Aptian) and a drifting episode (Early Aptian to present day), as Africa
separated from South America. Rifting appears to have been initiated by separation of East and
West Gondwana during the Middle to Late Jurassic. Sediments associated with rifting are now
confined to four major basins —Bredasdorp, Pletmos, Gamtoos and Algoa, which are underlain
and bounded by rocks of the Ordovician-Devonian Cape Super group that form prominent
arches between the basins.
ECONOMIC IMPORTANCE OF A TYPICAL SEDIMENTARY ENVIRONMENT
ECONOMIC IMPORTANCE OF FLUVIAL ENVIRONMENT
Fluvial environments are those environments that are dominated by running water and are
characterized as either a meandering stream system or a braided stream system. Braided
stream systems are typically found in areas with high slopes or in areas where the water
contains a high sediment load.
Fluvial deposits are of great economic importance for many reasons. They serve;
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As aquifers or petroleum reservoirs, because of their porosity and permeability
As the hosts of deposits of coal, uranium, and placer minerals
As important hosts for placer ones of detrital heavy minerals notably gold: Placer gold
deposits account for more than two-third of the world’s gold reserves. Placer mining
continues to be an important industry in the province with annual average expenditures
of more than 30million dollars over a survey period 1981 to 1986. Shallow alluvial
placers also account for a large part of world tin and diamond production.
Alluvial valleys can be important paleo environmental indicators. The fluvial
environment is constantly changing over time and space as rivers migrate and sediments
are eroded and deposited. The changes in the balance of the fluvial system are a function of
climate change, tectonic activity, fluctuating sea level, and human land use. These changes
are recorded in the stratigraphic record preserved in alluvium.
Much work have been done on paleoenvironmental reconstruction based on
interpretation of fluvial deposits. Bettis (1992) used stratified deposition sequences in
terraces and flood plains in the Des Moines River Valley in Lowa to develop a model to be
used for differentiating between fluvial deposits and landform of historic, late Holocene and
early middle Holocene age.
The physical environmental context is very important for archaeological sites and
artifacts discovered within alluvial settings. Alluvium contains valuable information about
the physical settings of the archeological site. Alluvium and fluvial landforms indicates a lot
about paleoenvironmental context sediment deposition and paleoclimate information can
be inferred from analysis of stratigraphic profiles. Although sedimentary conditions and
deposition are not always consistent throughout a particular river valley, important
information obtained by alluvial sediment studies can be used to infer settlement patterns
and cultural attributes of a prehistoric group.