Sedimentological Lithofacies Analysis and Assessment of the River Atbara Basin, Sudan

Abstract

This study investigates the sedimentology, facies and depositional style of River Atbara in east central Sudan. The study included detailed Iithofacies analysis, grain size analysis, heavy mineral analysis and hydrochemical analysis. River Atbara has a meandering parameter of 1.16 in the upper reach (Safawa- Khashim EL Girba reach), 1.54 for the middle reach (Khashim El Girba - Goz Ragab reach) and 1.2 in the lower Reach (Goz Ragab-Atbara town). The river ranges in depth from 20 m in the proximal part to 8 m in the distal part.The width of River Atbara ranges from 150 to 300 m and characterized by mid- channel bars, side bars, and point bars. Lithofacies analysis indicates that the facies assemblage are, matrix supported gravel (Gms), clast- supported gravel (Gms), trough cross stratified gravel (Gt), trough cross - stratified sand (St), planner cross stratified sand (Sp), rippled sand (Sr), horizontally stratified sand (Sh), laminated mud and silt (FI) and massive mud and silt facies (Fm). River Atbara is characterized by stacked finning upward sequences. The Architectrual elements recognized include channel -fill (CH) gravely bars and bed forms (GB) sandy bedforms (SB) and overbank fines (OF). Surface and subsurface facies show downstream decrease in grain size reflecting gradual decrease in river competence caused by discharge loss due to evaporation and infiltration. The stacking pattern of the subsurface facies indicates a vertical change from bedload dominated to mixed-load dominated streams. This may be interpreted as a change from low- sinuosity to high sinuosity streams produced in parts as a result of change in climate from dry to humid conditions. The aerial and stratigraphic distribution of facies, channel pattern and type were influenced by both allocyclic and autocyclic controls. The River Atbara appears to have similar evolutionary pattern and depositional style to those of the Blue Nile. The Heavy minerals assemblage of River Atbara include Zircon, tourmaline, Kaynite, hornblende and staurolite. This assemblage indicates mainly metamorphic source with minor contribution from igneous and sedimentary rocks which originated from the Ethiopian high lands and South Eastern Sudan. Generally groundwater in the study area proved to be fresh, the total dissolved solids range between I 15 ppm and 1000 ppm. The total hardness varies from 72 ppm to 600 ppm, therefore, the groundwater can be classified as moderately soft to very hard water. The groundwater of River Atbara basin is dominated by alkaline earth and weak acid (Ca, Mg) (HCO3). The groundwater quality proved to be suitable and fit for both human and live stock consumption. The River Atbara aquifer geometry and facies architecture reflect the vertical and lateral change of facies as controlled by the evolution of the river system since late Tertiary time.