Geology and hydrogeology
According the Schneider and Wolff (1992), the Lake Chad Basin is composed of a sequence of layers of different ages and thicknesses (Fig. 1).
Figure 1: Geology of the Lake Chad Basin (BGR, 2012).
The deepest known layer being the Continental Terminal (Oligocene/Miocene) composed of sandstones of around 200 m in thickness and located at depths between 400 and 600 m below the surface (Fig. 2). Overlying, the Lower Pliocene composed of fluvio-lacustrine sands is encountered. The Upper Pliocene follows as a massive layer of clays of 200-300 m of thickness. The Quaternary composed the uppermost layer and is made of sands with different sub-formations as follows in the Central Chad Basin (BGR, 2014):
- The Moji Series (from early Pleistocene) which is a fluvio-lacustrine clayey series with evaporates (gypsum) to the North of Kanem,
- The eolian sand dunes of the "Ogolien" age (lying over the Moji Series) corresponding to dunes formed from 20 000 to 13 000 y BP (Swezey, 2001). These dunes appear mainly in the North of Lake Chad and are essentially composed of quartz sands. The interdunal valleys up to a distance of 30 km from the ancient lake shore are often occupied by sodium-carbonate containing evaporate minerals such as natron (Na2CO3, 10 H2O) still exploited as sodium carbonate salt.
Figure 2: Cross-section (A-A', see Fig. 1) through the Chad lake Basin from Maiduguri (SW) to Faya Largeau (NE) after Schneider & Wolff, 1992 (in BRG, 2012).
At the south of the Lake, Quaternary deposits are overlaid by the Yaéré et Naga plains, where is found an alternation of sandy layers, indicator of past arid conditions, and clayey layers either lacustrine or fluvial, resulting from more humid periods when the size of the lake increased and the Logone and Chari riverbeds were much wider.
In CAF, at the very southern border of the basin, Quaternary deposits can also be found lying on the Continental Terminal deposits that crop out directly on the Proterozoic substratum.
From the hydrogeological point of view, the Continental Terminal is an artesian aquifer in the surroundings of the Lake Chad. It is outcropping in the southern part of the Basin especially in CAF where it can be also hidden by Quaternary deposits. The lower Pliocene contains freshwater and can also be artesian around the Lake Chad.
The Quaternary forms the major transboundary regional aquifer characterised by different depositional environments. The aquifers vary between semi-confined and confined depending on their relative position and the occurrence of clay layers. The Pleistocene and contemporaneous sands are characterised by relatively good hydraulic conductivity and groundwater of drinking quality.
Most of the Lake Chad Basin is covered by Quaternary sands (Fig. 1) of different depositional origins. In the northern part of the basin prevails aeolian deposition with the presence of dunes (Kanem region). Fluviatile, lacustrine and deltaic depositions that result in alternating sequences of thin layers of sand and clay and mainly clayey soils on the surface are typical in the south. Regionally, these Quaternary sands act as an unconfined transboundary aquifer with flow direction towards the Lake Chad and the NE along the Bahr el Ghazal. South of the 14°N parallel this aquifer shows a low hydraulic conductivity, especially vertical, due to the sequences of sand and clay. Furthermore, due to its flatness and low gradient (in average 0.5‰), the horizontal flow is very slow (BGR, 2009).
At a depth of some 75 to 100 m appears a thick layer of some 280 m of clay from the Upper Pliocene age (Fig. 2). This layer separates the Quaternary sands above from the Lower Pliocene below. The Lower Pliocene is composed of sand and sandstone and has a thickness of 30 m, underlain by the sandstones of the Continental Terminal (Tertiary) with a thickness of some 150 m.
The Upper Pliocene is almost impermeable and builds thus an aquitard that confines the sandstones of the Lower Pliocene and Continental Terminal (CT) from the upper Quaternary aquifer causing widespread artesian conditions in the central part of the basin. According to Eberschweiler (1993), both transboundary aquifers the Pliocene and the CT have similar good hydrogeological properties and comparable water chemistry, therefore they can be considered as a single aquifer.
The Quaternary aquifer groundwater flow map (Fig. 3) shows the presence of three important piezometric depressions: Chari-Baguirmi, Komadougou-Yobé and Pays-Bas. To the South of the basin, groundwater flow is oriented from the South to the North in direction of Lake Chad and Chari-Baguirmi depression. To the North, the Pays-Bas depression acts as a final collector of groundwater flowing from the east (Chad) and west (Niger). Groundwater flow for the Lower Pliocene and the Continental Terminal is less documented and sparse information is only available for the region around the Lake Chad (BGR, 2009).
Figure 3: Potentiometric map of the Quaternary aquifer (BGR (2009).
For the Lower Pliocene aquifer, groundwater flow is also oriented globally from West to East and from South to North in the direction of Lake Chad. No piezometric information is available for the Eastern Chad region for this aquifer (Fig.4).
Figure 4: Groundwater contour line for the Lower Pliocene (BGR, 2009).
Groundwater flow in the Continental Terminal aquifer is organized from South to North in the direction of Lake Chad and then to the North-East of the Lake (Fig.5). Very few information is available up to now on this aquifer and this potentiometric map must be considered as schematic. Recent investigations based on 36Cl by Bouchez (2015) have evaluated the residence time of the CT groundwater east of the Lake Chad to more than 300,000 years.
Figure 5: Groundwater contour lines for the Continental Terminal aquifer (BGR, 2009).
Data source: Integrated and Sustainable Management of Shared Aquifer Systems and Basins of the Sahel Region. Annadif Mahamat Ali ABDELKARIM, Mahamat Salah HACHIM, Beatrice KETCHEMEN TANDIA, Wilson Yetoh FANTONG, 2017
Regional geological setting
The Chad Basin lies within a vast area of Central and West Africa at an elevation of between 200 m and 500 m above sea level. The basin is centered around lake Chad (Fig. 1) and occupies an area of approximately 2,500, 000 km 2 extending over parts of the Republic of Niger, Chad, Sudan and the northern portions of Cameroon and Nigeria. The origin of the Chad Basin has been generally attributed to the rift system that developed in the early Cretaceous when the African and South American lithospheric plates separated and the Atlantic opened. Pre-Santonian Cretaceous sediments were deposited within the rift system.
