Geophysical data revealing the control of geological structures in the El Gouna springs in Souss river valley in Morocco

Abstract

Groundwater, in the Souss-Massa Basin (South western Morocco), occurred under unconfined conditions in the Pliocene and Quaternary formations of the plain, often overlying one or several confined sub-aquifers, such as Neogene and Turonian deep aquifer. The ensemble constituted the multi-layer aquifer system of Souss-Massa Basin. The El Gouna springs emergence, flowing from the Turonian deep aquifer, was one of the natural outlets of the system, with a low flow rate in the downstream part. This study focused on the El Gouna springs area and aimed to understand its hydrogeological functioning based on geological structures and their role in the appearance/disappearance of springs in this area, then studied the relationship between deep and shallow aquifers. For this reason, vertical electrical soundings (VES) using the Schlumberger array and seismic sections using were carried out in the study area. All geophysical data were calibrated by mechanical well logging and geological outcrops, and used to build geoelectrical and seismic cross-sections and structural map.The geoelectrical results showed the alternance of conductive and resistive zones related to the Plio-Quaternary deposits. This alternance followed by a conductor layer (<30 Ωm) related to the Senonian marl and marly limestones. The deep resistant layer corresponded to the Cenomano-Turonian limestone aquifer (170 Ωm) with a thickness of about 95 m.Seismic sections showed that the fault pattern in the studied area was mainly concentrated along corridors with a major direction of ENE-WSW and secondary direction of NNE–SSW. The first one constituted an extension of regional faults such as the El Klea and Agadir faults.The highlighted structures bringing the two aquifers into contact, allowing the emergence of some springs in the area. These resurgences came from the Turonian limestone and dolomite deep aquifer. Despite deepening of the impermeable covering deposits (Senonian marls), the water was forced to flow along higher permeability paths of the fault zone.This upward flow was controlled by structural discontinuities in the impermeable Senonian marls. This hypothesis could explain the spatial variation of salinity in the El Gouna area.