Determination de la structure geologique de la partie Sud de la plaine du Saiess (bassin de Meknes-Fes, Maroc) par la methode geoelectrique

摘要

The paper reports a geo-electrical study carried out across the southern part of the Saiss plain, Morocco, which forms part of the Meknes-Fes Basin. This continental basin is situated between the Rif Range (south of Tangiers) to the north and the Middle Atlas range to the south. The geology in the Meknes-Fes Basin extends from the Quaternary to the Carboniferous (Visean). Two aquifers are present in the Saiss plain: a deeper aquifer in the Lias and a younger one in the calcareous rocks of Pliocene/Quaternary age. The electrode configuration used a symmetric Sclumberger array and included 100 soundings with an electrode spacing not exceeding 4,000 m. As drilling had also been undertaken, it was possible to compare the geo-electrical data with the established lithological sequence. The qualitative analysis of the electrical soundings identified three horizons: set Ⅰ- a superficial resistance level corresponding to the Pliocene/Quaternary deposits; set Ⅱ - an intermediate conductive level corresponding to deposits of Miocene age; set Ⅲ - a resistant substratum corresponding to the Liassic deposits beneath the unconformity. The quantitative results obtained from the interpretation of all the electrical soundings are presented in the form of transverse and longitudinal geo-electrical profiles and an isohypse map of the top of the resistant Lower Jurassic/Liassic substratum. The geo-electrical soundings indicated disturbance of the Liassic platform, which has in part been interpreted as a horst and graben structure. The main fault structures trend NE-SW and NW-SE. From an interpretation of the geo-electrical data it is clear that the area has been affected by structural deformation on a number of occasions. The substratum dips towards the northwest at an average gradient of 7-8%. The work has provided details of the structural complexity of the area, particularly of the older strata which has been obscured by the superficial morphology and deposits of the Saiss plain. This structural interpretation has allowed the production of a new hydrogeological model. As a consequence, it is now possible to optimise the exploitation of the aquifers by better positioning of boreholes/well holes.