A Geophysical Study of the East Nile Rift Basin – Sudan

Abstract

The regional gravity measurements of the East Nile - North Khartoum area show high Bouguer anomalies south of Umm Ali area. As the area is generally covered with thick sediments, the observed high gravity anomalies cannot be explained by shallow B.C.. Such phenomenon is uncommon in the geology of Central Sudan and more work is needed to investigate the source of the high gravity encountered over the basin features in this area. Gravity measurements are made over the area of high anomaly. All reductions common in gravity method are applied to the field data. When comparing these Bouguer anomalies with those produced by Sun Oil Company for the same location, similar values are obtained; thus the Bouguer anomaly map which was produced by this company in 1984 is taken into consideration. The residual anomaly separation is made using the least-squares method. The residual anomalies are interpreted using a 2D gravmodeller program with the aid of geologic information of the area. The most convenient explanation of the high anomalies is the presence of granulites with density of 3.1 g/cm3. They are suggested to be uplifted during the period of the Pan-African movement to a depth of less than 3000 m below gneissic rocks with a density of 2.7 g/cm3 beneath the Nubian Sandstone with a density of 2.3 g/cm3. Thus they are believed to cause the high anomaly. Additional results are obtained from the interpretation of the gravity data: Several sedimentary basins, which were previously discovered, are delineated here again. The most important one is Atbara Basin in which the total thickness of sediments is about 3000 m. A depth map of the Basement surface of the study area is prepared. It is concluded that the dextral strike slip fault at Sabaloka area extends beyond River Atbara. vi More geoelectrical resistivity (vertical electrical sounding VES) measurements are conducted in the area to reveal the sedimentary sequence and further to investigate the groundwater condition in the area. The measurements are concentrated at Musawarat, El Awatib, Es Salama and Wad Musa areas. The present resistivity data are combined together with the old available data for more details. The VES data are interpreted using IPI2win software. A number of 8 geoelectric/geologic cross sections are prepared. Thus, it is concluded that the subsurface of the area consists of six geologic layers: - The first layer is the surface layer consists of undifferentiated sedimentary facies (gravels, sands, clayey sands, sandy clays and clay). - The second layer consists sandy-clay, clayey sands and sands. - The third layer is saturated sandstones. - The fourth layer is silicified sandstones/claystones (aquifuge). - The fifth layer is saturated sandstones. - The six layer is Basement Complex. Thus, two aquifers are revealed: - The upper aquifer is the third layer. It represents free aquifer. - The lower aquifer is the fifth layer. It represents confined aquifer. It is the thickest one. Its water is the most abundant and the most convenient for drinking and irrigation purposes. Five subsurface geologic maps are prepared. Two of them are depth maps of the top surface of the upper and lower aquifers. The other two are thickness map of the upper and lower aquifers. The last one is a depth map of the Basement surface.