The Chari-Baguirmi aquifer, located East of Lake Chad, is the main source of water in this region. The groundwater piezometry is marked by a large depression. The work presented here aims to understand the development of this depression. The approach is based on a sedimentological study coupled to a hydrogeochemical study of the groundwater. The sedimentological analysis revealed the grain size heterogeneity of the different layers. Clay mineralogy showed that the deposition of sediments takes place from the periphery to the center of the depression. This sedimentation mode suggests the existence of a morphological cuvette to which sediments are driven by the transport agent. We deduced the existence of a "structural" depression unlike the common assumption of an origin due to accentuated evaporation of the groundwater over the depression area. The combined study of chloride and oxygen-18 has shown that the area between Lake Chad and the piezometric depression is a special evaporation zone. This means that the process of evaporation of groundwater does not occur mainly in the center of the depression. The first results of this work provide new insights into the functioning of this important aquifer system and the establishment of the large Chari Baguirmi piezometric depression.