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<p>Abstract:</p><p>Karst aquifers belong to the fractured aquifer family. The Zaghouan region located in NE of Tunisia (North Africa) is characterized by a high degree of karstification due to the climate impact and the development of fracture network. Survey using electrical resistivity tomography (ERT) is deployed to provide a cost-effective characterization of the subsurface karst environments. A total of three ERT profiles with a length of 300 meters were evaluated in Zaghouan region.</p><p>The area represents an anticline of Jurassic limestone rocks, which is overlain by a thin clay layer. In this study, an ERT survey was conducted to examine the spatial distribution and shape of underground cavities in the karst area of Jebel Bent Saidan. In this study, geological, hydrogeological and electrical resistivity tomography (ERT) methods were applied to determine the geometry of the karst aquifer in the Zaghouan area (NE Tunisia). The area is characterized by fractured and karstic limestone aquifer of Jurassic. Three resistivity profiles were carried out along the study area (Jebel Bent Saidan). The correct resistivity data was interpreted using ZONDRes 2D software.  The results of the interpreted geo-electrical sections showed that the resistivity of the carbonate aquifer ranges from 350 to over 4000 Ωm. The thickness of the aquifer varies between 15 and 30 meters, while its depth from the surface is between 10 and 40 meters. The ERT not only provided accurate near-surface information, but was also very useful in establishing the geometry of the aquifer. It was also very useful in establishing the 3D geometry and position of several potential karst cavities and conduits. The results show the presence of two large isolated cavities at different depths. The low resistivity of karst cavities in the Jurassic carbonate of Jebel Bent Saidane was explained by the saturation of groundwater. The ERT imaging technique using to identify and characterize the discontinuities, faults and water investigation of the fractured and karstified limestone aquifers in the Bent Saidan Mounts. The conducted research demonstrated that the ERT method was an effective tool for imaging the subsurface in the karst terrain.</p><p><strong>Keywords:</strong> Bent Saidan (NE Tunisia), karst aquifers, electrical resistivity tomography (ERT), cavities.</p><p> </p>

RESUMEEn Tunisie Centro-Septentrionale, la bordure septentrionale du fossé de Kalâat Khasba a été considéré comme une bordure d'effondrement à jeu normal, ayant fonctionnée du Crétacé inférieur jusqu'au Quaternaire. Une étude de terrain et l'interprétation de profils de sismique réflexion permettent de proposer un nouveau modèle tectonique pour ce fossé et du massif adjacent de Bou El Hanèche.La bordure septentrionale du bassin de Kalâat Khasba a été réactivée à l'Eocène en faille inverse. Cette bordure a une valeur de front de chevauchement sur lequel se met en place le pli de propagation de Bou El Hanèche par rapport à l'avant pays de la chaîne alpine.La compression Eocène entraîne la mise en place du pli de Bou El Hanèche à la faveur d'un niveau de décollement situé dans les séries triasiques. Cette déformation est contrôlée par un réseau de failles normales héritées de la phase distensive présumée aptienne. La dynamique compressive continue depuis l'Eocène engendrant la propagation de ce pli. Lorsque la rampe atteint les calcaires de la formation Abiod, le pli se bloque. Ce blocage entraîne la rupture du dressant et la création, vers le Nord du pli, d'un deuxième chevauchement, ce qui implique une évolution du système de chevauchement dans une séquence inverse ou «out of sequence».Mots clés: Tunisie Centro-Septentrionale, Kalâat Khasba, chevauchement, pli de propagation de rampe, séquence inverse, sismique réflexion. ABSTRACTIn Central-northern Tunisia, the septentrional border of Kalâat Khasba trough was considered as a normal fault which has functioned since the Cretaceous to Quaternary periods. A stratigraphic and geophysical recent study of the Kalâat Khasba trough and the Jebel Bou El Hanèche fold area, allows us to propose a new tectonic model. The Northern border was reactivated as reverse fault during the Eocene and was considered as a thrust front. A compressive Eocene event resulted in formation of Bou El Hanèche fold (fault propagation fold) and in determination of a decollement level situated in the Triassic series. This deformation was controlled by the reactivation of normal faults inherited from a distensive aptien phase.The compressive deformation was amplified by the propagation of this fold and, when a ramp reached the competent carbonates series of late Campanian-lower Maâstrichtien times, the fold was stacked, involving a breakthrough steep limb and creating a second thrust on the North. The order of evolution of these events shows an out of sequence thrust.

Apart from the Triassic salt bodies, there are several structural elements that characterize the Oued Zarga area. The new field data allowed us to deduce that the structural evolution of this region is essentially based on two generations of dextral strike-slip faults: an E-W direction to WNW-ESE and a later NW-SE second trend. In the study area, the Triassic material of J. El Matria is framed by two sedimentary contacts. Lower contact in clansayesian clays characterized by a glauconic to Triassic insoluble conglomerates and an upper contact where the Triassic material is covered by a thick pelagic series attributed to the lower Albian. This configuration results from a rapid lateral outpouring during the Lower Cretaceous in an extensive tectonic context and re-folded during the tertiary compressive phases, interpreted like most Triassic structures in northern Tunisia, in terms of the "salt glacier" with Lower Cretaceous age.

The industrial development of the outcrops in the Tunis-Zaghouane area could represent an important economic driver for the country. The main structures are introduced and illustrated with onsite field observations. Moreover, samples which were extracted from the selected regions and are characterized based on measurements of porosity, water absorption and abrasion loss coefficients. The experimental results showed a negligible variation of densities, demonstrated a linear correlation between connected porosity and overall water absorption. In addition, experimental result confirms that outcrops from Jebel Zaghouane, Jebel Hammam Zriba and Jebel Hammam Jedidi could be developed as construction materials.