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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.