Bahr El Arab rift comprises two major structures, the Baggara graben and the Sudd graben (Salama 1985 a & b). Baggara graben covers the northern part of the rift and extends in an east west direction. While the Sudd graben covers the southern part and extend in a NW - SE direction. The grabens and horsts indicate a step-like subsidence of separate blocks. The intensity of the faulting and the subsidence increase southward, where it attains a depth of more than five kilometers in the Baggara graben and an estimated eleven kilometers in the Sudd graben.
As outlined in this figure, the Bahr El Arab rift basin is a longitudinal basin extending in a NW-SE direction, it is nearly 1150 km in length, 600 km in width in its northern part and about 330 km in width in its southern part. It is surrounded by the high uplifted blocks; the Darfur dome in the north, the Nuba mountains in the east, the Congo-Nile divide in the south and west, and the J.Marra massif in the NW. The rate of uplift of the East African highs to be about 0.1 mm year-1 in the mid Tertiary. Using this figure to calculate rates of uplifts in the Sudanese blocks, gives the following heights; 2500 m for the Congo-Nile uplift, 3800 m for the Nuba mountains, 1500 m for the Darfur dome and 2000 m for J.Marra massive. The highs were eroded depositing sediments in Bahr El Arab troughs.
The Tertiary sediments are divided into three groups:
a) Recent-Upper Miocene: these range in thickness from 6004-1000 m of poorly sorted, iron stained sands and siltstones with interbedded clays, changing southward to light colored (volcanic ash) claystone. This active period of deposition was followed by a hiatus, and a period of slow deposition.
b) Oligocene-Upper Eocene: varying in thickness from 600-43000 m of interbedded, clear quartz sands and multicolored clays. Local development of lacustrine shale in axial regions of rift basin.
c) Eocene-Pliocene: varying in thickness from 6004-3000 m, of massive sand deposits composed mainly of coarse clear and white quartz grains with locally abundant pyrite.
In the Gileizan area, thick carbonate deposits were recorded (Salama 1985a) from shallow wells and drilled boreholes.
Sudd Lake
Salama 1987 showed that a series of fresh water lakes exists at the edges of Bahr El Arab; including Lake Keilak, Lake Abyad and Lake Kundi, together with the main Sudd lake which covers most of the central part of this river system. Based on hydrological data he concluded that if the Victoria Nile was not connected to the White Nile the Sudd would be a closed lake system.
Independent evidence showing that the White Nile was not connected to the main Nile prior to 12500 years BP based on hydrological and geological evidence. This evidence indicates that the Sudd depression of Southern Sudan was a closed lake system. It is estimated that the ancient lake was 250 - 655 miles in length from north to south and that the Blue Nile flowed southwards to join this lake. A lake of these dimensions, and average evaporation rate of 3 mm l-1 over the lake would be sufficient to dispose of all rain and river water entering the lake, since the average annual rate of evaporation from open water surfaces at Mongalla, Malakal and Khartoum are 3.0, 4.5 and 7.5 mm respectively. Salama 1987 yet there is enough evidence to show that there was always a water body in the Sudd region. However, the lake was not formed by a dammed up Nile. It was formed in a closed basin caused by subsidence in Bahr El Arab rift during the Sage period (Salama, 1985 a & b). The size fluctuated according to the paleoclimatological events prevailing at the time.
Salama 1987 using saturation indices of minerals and salinity parameters, showed that a saline lake occupied the central area of the Sudd. He estimated that the lake fluctuated in size from a maximum of 32,000 km2 to a minimum of 336 km2 (the area enclosed between isosalinity lines of 1000 mg l-1 and 30000 mg l-1 ). The depth of the lake ranged from few meters at the borders of the lakes to about 200 meters in the central part (calculated from groundwater gradients, versus thickness of saline zone). The presence of thick carbonate deposits at Bahr El Arab, suggests that the size of the lake was larger than the figures calculated or perhaps indicates the presence of another separate saline lake in that part.
Salama (1987) reported the presence of carbonate deposits 0.5 to 1.5m thick, along the banks of Bahr El Arab. Thick lacustrine carbonate deposits at the western edge of Bahr El Arab rift, where a known fault zone were also recorded from shallow wells and borehole records (Salama 1987). The presence of a closed saline lake might be responsible for the deposition of the carbonate by contact of calcium - rich fresh water with saline water bodies. It is possible that as lakes always form on the down faulted part of the fault zone which is continuously subsiding, the surface drainage entering the lake in the form of sheet flow will deposit the carbonate at the contact zone between the fresh and saline water bodies. This might also explain the concentration of sulphate in the central part of the lakes. As the water will be depleted in its carbonate content, the central part will continuously exhibit increase in sulphate and chlorides. The widespread occurrence at various depths of carbonate indicate that this area was always a contact zone between the saline areas and the fresh waters entering the lakes.
