Magnetotelluric soundings and the geological structure and tectonics of the Senegalo‐Mauritanian Basin in northern Senegal, West Africa
The results of the magnetotelluric soundings provide new insights on the geological structure and tectonics of the onshore Senegalo-Mauritanian basin in northern Senegal (West Africa). Electrical resistivities can be correlated with the major geological units. In the upper 1000 m, resistivity variations indicate a transition from freshwater saturated sediments near the surface to saline water saturated sediments above the basement. The resistivity of the sediments is dominated by their porosity and the resistivity of the contained fluids. The resistivity of the Cretaceous sediments is unexpectedly low, especially in the western part of the basin. The top of the metamorphic/granitic basement cannot be located on the west portion of the profile because of the feeble resistivity contrast between the impervious, compacted Aptian-Jurassic limestones and the metamorphic/crystalline rocks. The magnetotelluric estimates of the depth of the basement range from 600 m in the east to 4000 m in the west. Based on the magnetotelluric interpretation and other independent geophysical and geological information, a cross section through the Senegalo-Mauritanian basin along the magnetotelluric traverse is presented. A zone of high electrical conductivity is observed in the crust, at depths of 4–6 km, on the eastern margin of the deep basin. This conductive structure may be the extension of one further south that was revealed by an earlier investigation. The entire conductive zone, which runs north–south between longitudes 15° and 16°W, may be a major fracture zone related to the opening of the Atlantic Ocean; however, its origin remains to be established.