Sedimentological Investigation around the State of Khartoum and on the Northern- Central Part of the Gezira ,Central Sudan

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Bireir, FatheLrahman Ali
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University of Khartoum
This study has been carried out on the Upper Cretaceous strata (Omdurman Formation and the Mid Tertiary to Quaternary strata (Gezira Formation) which represent the uppermost par of the Khartoum basin in central Sudan. Lithofacies analysis, followed by the grain size, heavy minerals, clay minerals and geochemical analyses, represent the methodology in the present work. The predominantly sandy Omdurman Formation is subdivided here into: Lower Omdurman Formation and Upper Omdunnan Formation. A paleosol horizon is proposed to represent the boundary between them. The Lower Omdurman Formation (Albian - Cenomonian), composed of fine and much coarse grained, poorly to moderately sorted, silisiclastic sediments. The heavy mineral associations consist of 70% stable minerals (zircon, tourmaline & rutile) and 30% of semi stable and unstable ones (staurolite, garnet, hornblende, epidote & kynite). Zircon is the remarker and the assemblage indicate a derivation from igneous - metamorphic terrain and partly from arenaceous sediments ,. The abundance of kaolinite and the minor traces of smectite and illite recorded in this Formation suggested a tropical humid paleocilmate with short dry seasons. The relatively high concentration of Vadnium, copper and molybdenum in some horizons showed that the sediments of the Lower Omdurman Formation are partly bituminous rather than deposited in oxidizing environment. The sediments of this Formation were almost transported as suspended particles to which rolling adds a considerable amount and deposited under fluvio - lacustrine conditions. The Upper Omdurman Formation (Cenomenian - Turonian) comprises the coarse and much less fine grained, poorly sorted silisiclastic sediments cropping out in the state of Khartoum and reveal almost finning upward sequences. Trough cross bedding is the dominant sedimentary structure and display mainly NW paleocurent direction. Compared to the Lower Omdurman Formation, the stable heavy minerals decreases to 57% while the semistable and unstable minerals increases to 43%. Staurolite is the remarker mineral and the assemb1age as a whole reflected a metamorphic, igneous and/or arenaceous source rocks. Detrital kaolinite is the dominant clay mineral which suggested tropical humid paleoclimate. The continental indicator elements Cr and Ra are highly concentrated in this formation. The depositional environment of the' Upper Omdurnan Formation is braided river of Donjeke type and the mode of transportation was almost a combination of bedload and suspended loads. The term Gezira Formation is modified in the present work by informally dividing the formation into Lower Gezira Formation and Upper Gezira Formation which are composed of poorly to moderately consolidated fluviatile sediments. The Lower Gezira Formation (Oligocene - Miocene) composed of interbeded sand and clays, rarely consolidated to form poorly sorted sandstone and claystone. The stable heavy minerals reach 70% and the unstable ones reach 30%. Zircon is the remarker mineral and the identified assemblage stem from metamorphic, igneous and less probably from polycyclic sediments. This formation contain about 60% detrital smectite and up to 40% authigenic kaolinite. Such clay mineral association suggested a tropical climate with long dry seasons. In this formation their trace elements Pb and Mn show anomalous values and few pyrite crystals are recorded. The sediments of this formation were almost transported in the form of rolled grains to which suspension adds small amount and deposited in an acidic environment. The Lower Gezira Formation represents sandy meandering deposits. The Upper Gezira Formation (Miocene - Quaternary) is composed of unconsolidated sands, clays and gravels which are poorly sorted and display strong lateral facies changes. They contain considerable amount of carbonate materials and volcanic rock fragments. The unstable heavy minerals are dominant (60%) among which hornblende is the remarker while the stable ones reach (40%). Aegerine augite and traces of allanite are recorded only in this formation. The heavy mineral assemblages suggest a metamorphic source as a permanent area of sediment supply. Compared to the Lower Gezira Formation, smectite increase to reach sometimes more than 90% while kaolinite decrease to reach in some horizons less than 6% which suggest a dominance of; arid conditions during the deposition of this Formation. The trace elements Nb, Sc, Zn, and Co are concentrated in higher amount in the Upper Gezira Formation. The sediments of this formation were almost transported in the form of rolled grains and suspended particles and deposited by high sinuosity (braided) streams in an environment characterized by increasing saline conditions. This formation represents the last episode in the history of the Khartoum basin and is overlain by river alluvium and other superficial deposits.
State of Khartoum; Gezira; Sudan; Omdurman