Mohamed Hédi Inoubli

The Chotts Fold Belt of Southern Tunisia, North African Margin: Structural Pattern, Evolution, and Regional Geodynamic Implications

At the North of the old African continent, craton and shields having more than two billion years, Tunisia, Algeria and northern Morocco underwent a complex geodynamic and structural evolution during the Mesozoic and Cenozoic times (Dercourt et al., 1985; Bouillin, 1986; Frizon de Lamotte et al., 2009). This evolution resulted in the development of varied paleogeographic fields, in relation with the Tethyan and Atlantic movements. Its end led to the genesis of the North-African alpine orogen (Dercourt et al., 1985; Martinez et al., 1990) formed by the Maghrebid and Atlassic domains (Fig. 1). Tunisia occupies the eastern part of this orogen, located at the north of a large Saharan platform, developed on the stable African craton, not deformed during the alpine cycle and bounded by a major structural lineament « South Atlassic fault » composed of complex overlapping folds trending NE-SW, E-W and NW-SE (Caire,1971; Zargouni, 1985; Turki, 1988; Zouari et al., 1990; Ben Ayed, 1993; Boukadi, 1994; Bedir, 1995; Bouaziz, 1995; Zouari, 1995; Bouaziz et al., 1999, 2002; Abbes, 2004; Zouaghi et al., 2005a, b, 2011; Ouali, 2007; Melki et al, 2010). Structures of the North African margin were usually subject of discussion. This domain could be considered as a passive margin, close to the oceanic opening, characterized by a strong subsidence marked by accumulations of prograding deposits (Dercourt et al., 1985; Biju-Duval et al., 1976). For others, it is a transform margin related to displacements of the African plate compared to the Eurasian plate. These movements generated opening of the Paleo-Tethys (Arthaud and Thomas, 1977). The Africa-Europe relative motions would be at the origin of the recent ocean floor spreading of the Mediterranean (Taponnier, 1977; Reading, 1980; Olivet et al., 1982; Alvarez et al., 1984; Ricou, 1994). The study area belongs to the North African margin and the northern edge of the Saharan platform. Studies undertaken on Paleo-Tethys show the development of deformed and subsiding zones between the cratonic blocks and the basins (Caire, 1974; Arthaud and Thomas, 1977; Aubouin and Debelmas, 1980; Bernoulli and Lemoine, 1980; Durand-Delga and Fonrbote, 1980; Bousquet and Philip, 1981; Dercourt et al., 1992). The geodynamic

Advantages of using the kriging interpolator to estimate the gravity surface, comparison and spatial variability of gravity data in the El Kef-Ouargha region (northern Tunisia)

A comparative study on two interpolator methods for Bouguer anomaly mapping was performed in the El Kef-Ouargha region, Tunisia. In particular, we compare the results of minimum curvature gridding and kriging. We undertake a geostatistical study using both anisotropic and isotropic analysis for studding the spatial variability and kriging the random variable: “Bouguer anomaly”. The model parameters of each method are calculated to compare the results. The anisotropic analysis most closely approximates the measured data. Consequentially, the Bouguer anomaly and vertical gradient maps using anisotropic kriging have the best correlation with the geological map, best represent density distributions at depth and most closely approximate the measured anomaly amplitudes.

Evidence for a new regional NW–SE fault and crustal structure in Tunisia derived from gravity data

A new Tunisian gravity map interpretation based on the Gaussian filtered residual anomaly, total horizontal gradient, and Moho discontinuity morphology established from gravity data exhibit a new regional northwest–southeast fault extending from Eastern Kairouan to Ghardimaou (Algeria–Tunisia Boundary). It presents a horizontal gradient maximum lineament that terminates the north–south Jurassic structures in the Kairouan plain. Further, this interpretation reveals other known fault systems and crustal structures in Tunisia. The new regional northwest–southeast fault constitutes with the north–south axis and Gafsa–Jefara faults the deepest faults coinciding with the Moho flexures, which had an important role in their initiation. They constitute the border intra-continental crust faults of the Mesozoic rift. The newly recognized deep fault has critical implications for mineral and petroleum perspectives.

Geodynamic Evolution of Northeastern Tunisia During the Maastrichtian-Paleocene Time: Insights from Integrated Seismic Stratigraphic Analysis

Abstract The Maastrichtian–Paleocene El Haria formation was studied and defined in Tunisia on the basis of outcrops and borehole data; few studies were interested in its three-dimensional extent. In this paper, the El Haria formation is reviewed in the context of a tectono-stratigraphic interval using an integrated seismic stratigraphic analysis based on borehole lithology logs, electrical well logging, well shots, vertical seismic profiles and post-stack surface data. Seismic analysis benefits from appropriate calibration with borehole data, conventional interpretation, velocity mapping, seismic attributes and post-stack model-based inversion. The applied methodology proved to be powerful for charactering the marly Maastrichtian–Paleocene interval of the El Haria formation. Migrated seismic sections together with borehole measurements are used to detail the three-dimensional changes in thickness, facies and depositional environment in the Cap Bon and Gulf of Hammamet regions during the Maastrichtian–Paleocene time. Furthermore, dating based on their microfossil content divulges local and multiple internal hiatuses within the El Haria formation which are related to the geodynamic evolution of the depositional floor since the Campanian stage. Interpreted seismic sections display concordance, unconformities, pinchouts, sedimentary gaps, incised valleys and syn-sedimentary normal faulting. Based on the seismic reflection geometry and terminations, seven sequences are delineated. These sequences are related to base-level changes as the combination of depositional floor paleo-topography, tectonic forces, subsidence and the developed accommodation space. These factors controlled the occurrence of the various parts of the Maastrichtian–Paleocene interval. Detailed examinations of these deposits together with the analysis of the structural deformation at different time periods allowed us to obtain a better understanding of the sediment architecture in depth and the delineation of the geodynamic evolution of the region.

Prospection géophysique pour la recherche d’eau souterraine dans le Nord-Ouest de la Tunisie

RÉSUMÉ Des sondages électriques verticaux, des enregistrements diagraphiques et des données de forages sont utilisés pour identifier et caractériser les aquifères de la région de Bou Salem (Nord-Ouest de la Tunisie). Les données de résistivité apparente mesurées à chaque sondage électrique vertical pour les différentes distances AB sont interprétées unidimensionnellement. Les enregistrements diagraphiques ont permis la restitution précise des colonnes lithologiques des forages préexistants et par conséquent l’étalonnage des sondages électriques. L’analyse et l’interprétation des données révèlent que les alluvions quaternaires perméables se limitent aux zones proximales des oueds Kasseb et Bouhertma. Cette distribution est contrôlée par les mécanismes de sédimentation dans les plaines d’inondation et par la réactivation des accidents N–S en failles normales durant le Quaternaire récent. Il ressort aussi que les calcaires éocènes et crétacés correspondant au substratum des alluvions quaternaires et caractérisés par des fortes résistivités peuvent constituer d’intéressants aquifères.

Seismic Reflection Contribution to the Study of the Jerid Complexe Terminal Aquifer (Tunisia)

According to Kilan (1931) the term « Continental Terminal » concerns the sandy and clayey continental Formations dated Miocene-Pliocene. In 1966, Bel & Demargne highlighted a vertical communication between the aquifer contained in these Formations and the Eocene, Senonian and Turonian aquifers. Consequently, the Continetal Terminal is redefined as a multi-layered aquifer which extends from the Late Cretaceous to the Miocene –Pliocene. Frequently, this hydrogeological system is designated by the term «Complexe Terminal» proposed by Bel & Cuche in 1969.

Insights into Mejerda basin hydrogeology, Tunisia

The present study concentrates on the interpretation of Vertical Electrical Soundings (VES) and well logs to understand the geometry and the functioning of the Ghardimaou multilayered aquifer, a potential target for water supply in the Mejerda basin (Tunisia). The analysis of isobath and isopach maps established in this study, shows a tectonic influence on the reservoirs structure; the Villafranchian folding and the NE–SW, and E–W normal faulting in the recent Quaternary created an aquifer system compartmentalized by raised and tilted blocks. Geoelectrical cross sections reveal that this structure influences the thickness of permeable formations and the groundwater circulation. These results will be useful for rationalizing the future hydrogeological research that will be undertaken in the Mejerda basin.

Contribution of Geophysics to the Management of Water Resources: Case of the Ariana Agricultural Sector (Eastern Mejerda Basin, Tunisia)

In Tunisia, the Mejerda basin is the most important agricultural sector. Identification of additional water sources and good management of current and new resources is required to maintain and increase its productivity. The present study concerns the Ariana region (north-east of Tunisia) covering the eastern part of Mejerda basin. Lithological columns, well logs, vertical electrical soundings, and gravity data were analyzed and interpreted to give more precise information for exploitation of the water resources of the Ariana region. Lithological columns, well logs, and geoelectrical models reveal that the permeable Quaternary alluvial deposits, the Upper Campanian–Lower Maestrichtian, Albian and Barremian limestones, and Valanginian–Hauterivian sandstones are the main aquifers in Ariana. Comparison of geoelectrical cross sections to the Bouguer anomaly map of the Ariana region reveals tectonic influence on the geometry of the aquifers. The folded structures attributed to the Tortonian compressive stress and the tectonic movements in N–S, E–W, NW–SE, and NE–SW directions have resulted in the compartmentalization of Ariana’s aquifers into subsided and raised blocks. This reconstituted geometry of the region influenced the depth of burial of permeable lithologies and inhibited groundwater circulation in some localities.

Geophysical Contributions to Hydrogeological Study in the Chemtou–Sidi Miskine Agricultural Sector (Northwestern Tunisia)

AbstractThis study presents an example of the use of geophysics for agricultural water management. Borehole data, gravity measurements and vertical electrical soundings were used to reconstruct a precise hydrogeological scheme of the Chemtou–Sidi Meskine area (Northwest of Tunisia) in order to assist exploitation and judicious management of the underground water resources for irrigation. The Chemtou–Sidi Meskine area belongs to Mejerda basin, the most important agricultural sector in Tunisia. It is characterized by the coexistence of alluvial and basement aquifers, which are very scare in this country. The computed gravity maps emphasize an E–W lineament separating Hairech metamorphic formations and Mejerda alluvial deposits. The apparent resistivity maps confirm the presence of this lineament. The interpreted vertical electrical soundings are correlated with lithological columns to give more accurate structural patterns. The resulting geoelectrical cross sections highlight a normal fault network that collapsed metamorphic aquifers, controlled the Quaternary alluvial aquifers thickness and created closed areas with high water salinity that is unfavorable for exploitation.

Overview of the Structural Pattern in Northern Tunisia Inferred Gravity Interpretation

me zone in the Atlassic region. The gravity analysis consists in mapping the contrasting gravity responses: complete Bouguer anomaly, residual anomaly and derivative maps and Euler Deconvolution. The main results display a positive amplitude gravity anomaly as the response of Triassic evaporite bodies and important NE–SW-trending features at the boundaries between the Triassic outcrops and their enveloping strata. In contrast with gravity calculations of a salt dome structure usually resulting in negative gravity anomaly models, they express a positive gravity anomaly generated by a higher density of the Triassic mass contrasting with a lighter Meso-Cenozoic rock envelope. Gravity lineaments allow distinction of alignments extending NE-SW, which retrace master fault systems bordering the Triassic masses; the contact between Triassic and Cretaceous systems (or more recent), correspond to a part of steep gravity gradient. These faults are thought to have given issue to ascending evaporite-bearing masses.