A Gravity and Magnetic Survey of the Sabaloka Area (Sudan) with Details on the Ring Complex

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Abd El Atti Ahmed, Sadig
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The Sabaloka area (north of Khartoum, Sudan) has outcrops of he Basement Complex rocks, the Igneous Complex rocks and the Nubian Sandstone rocks. The principal rocks comprising the Igneous Complex are acidic and include rhyolites, ignimbrites and porphyritic micro granites, the latter form the conspicuous marginal Ring Dyke. A gravity and magnetic survey of the area has been made to elucidate i) the density distribution in the Pre – Nubian rocks and ii) the structure of the Sabaloka Igneous Complex. The survey was divided into i) a regional survey in which 300 stations were occupied at intervals of one mile along traverses covering an area of about 2,800 sq. miles and ii) a local survey in which 1,300 stations were established at intervals of 100 yards along 23 selected profiles over the Sabaloka Igneous Ring Complex. The gravity observations have been reduced to Bouguer anomalies. Laboratory measurements show a small density contrast (of – 0,05 g/m3) of rocks of the Igneous Complex with respect to those of the Basement Complex. Within the Basement Complex, however, Meta – basic rocks have considerably higher density than the principal constituents (biotite gneisses and porphyritic granite). The Nubian Sandstone rocks show a density contrast of – 0.35 g/cm3 with respect to the Basement Complex. A broad belt of steep gradients occurs in the region east of the Ring Complex and culminated in high peaks at El Ban Gadeed and Naga. These highs are interpreted as being associated with scattered meta–basic patches which probably have a much greater volume at depth than can be surmised from the surface outcrops. The magnetic anomalies show rapid variations in the zone of the steep gravity gradient, suggesting that these variations may be associated with the meta–basic rocks which have a distinctly higher susceptibility than the principal constituents of the Basement Complex. The local survey provides some details of the broad gravity low, shown in the regional survey, over the Igneous Complex. The maximum thickness of light density rocks underlying the central plateau is about 19,000 ft. There is a gravity evidence for GEOLOGY the presence of a granitic mass underlying the Basement Complex in the northern half of the Igneous Complex. There is a similar evidence that rocks identical to those of the Ring Dyke occur in the southwestern section of the Complex. It is suggested that granitic rocks may also underlie the volcanic rocks of the Plateau. It is necessary that the extrusive and the intrusive groups composing the Igneous Ring Complex be treated as one unit both on the basis of density contrast as well as on the basis of gravity anomalies. The overall shape of the low density rocks of the Sabaloka Igneous Complex is that of a disc, having a diameter of about 20 km and a thickness varying from 3 to 5 km. the side walls of the disc formed by the Ring Dyke are steep with variable inclinations (vertical, inwards or outwards). The measured susceptibility contrast between the Ring Dyks and ignimbrites is not apparent in the magnetic anomalies, partly owing to noise. The prominent observed magnetic anomalies are interpreted as being associated with trachy basalt (with a distinctly high susceptibility) or due variable magnetisation along the contacts of the Ring Dyke.
A Gravity and Magnetic Survey of the Sabaloka Area (Sudan) with Details on the Ring Complex