Characterization of the structure, geometry and artesianism of Upper Cretaceous and Paleocene-Eocene aquifers in the coastal sedimentary basin of Benin

Abstract

Groundwater represents a part in the drinking water supply of the populations of the coastal sedimentary basin of Benin (CSB) of which age is Meso-Cenozoic. This is done using groundwater from the Continental Terminal (a detrital sandy-clays formation of siderolithic nature, of post-Middle Eocene to pre-Quaternary age) and Quaternary for populations in the basin southern part. The water supply can be compromised in the medium and long term by demographic pressure and saline intrusion. To overcome this potential depletion water resources, we can consider the deep aquifers exploitation. Our study aims to improve knowledge about the deep aquifers and specifically their structure, geometry and artesianism characterization. The methodology used, is essentially based on lithostratigraphic correlation between boreholes. For this, we made North-South, West-East and North East - South West cross-sections of area and geology containing Eocene-Paleocene and Turonian-Coniacian aquifers. The results showed that two aquifers are deep at the south in the CSB: sands (10 to 40 m thick) and limestones (20 to 50 m thick) of Eocene and Paleocene; Turonian-Coniacian sands and sandstones from around 30 to 60 m of thickness at the North. Eocene-Paleocene aquifer roof altitude varies between + 30 m in the North and - 420 m in the South-East. Upper Cretaceous altitude varies between + 170 m in the North and - 630 m (borehole AT6) in the South. These aquifers plunge towards the south where they have their outlets in Atlantic Ocean. Their outlets are also found in the streams and valleys which cut the basin into plateaus. These valleys and rivers that border the plateaus are characterized by artesianism phenomenon. This is explained by the piezometric surface above the ground surface at these low altitude locations.