New Insights into Geodynamic Evolution of the South-Eastern Termination of the Tunisian Atlas during Early Cretaceous Period from Surface and Subsurface Data: Hydrogeological Implications

Abstract

The Menzel Habib Plain (MHP), the easternmost part of the Southern Tunisian Atlas, exhibits a thick siliciclastic and carbonate succession developed in the Early Cretaceous period. Integration of seismic and gravimetric data coupled with analysis of the syndepositional faults affecting these units proves that the MHP is governed, during this period, by a N-S to NE-SW extensive tectonics induced by the sinistral drifting of Africa with respect to Europe and the beginning of opening of the Eastern Mediterranean Sea. The geodynamic evolution of the MHP is mainly due to the jerky normal activities of the N-S, NW-SE, NE-SW and E-W oriented faults. During extensive period these network of faults architects the sedimentary floor into succession of horst and grabens inducing formation of E-W to NW-SE basins. Seismic lines show that the actual architecture of the MHP obtained after Tertiary tectonic inversion, is that of succession of large synclines separated by narrow anticlines. Porous and permeable unites forming these deep structures host very developed aquifers characterizing the sub soil of MHP and were totally different to the surrounding aquifers of the southern Tunisian margin.

1. Introduction

The Tunisian Atlas, an eastward extension of the Maghrebide chain (Figure 1A) [1, 2, 3, 4, 5], had long recorded a polyphase tectono-sedimentary evolution of the North African margin [6, 7, 8, 9]. Since the initial breakup of the Pangea to the Tethys and Mesogean rifting periods [10, 11] and through the collision events that continue [12, 13], the basins evolution recorded tectono-sedimentary systems with complex facies and structures. Geological work that has targeted the Tunisian margin has shown that this complexity is mainly due to a direct interaction between the brittle tectonics, materialized by the different network of faults, and the sedimentation [7, 14, 15, 16, 17, 18, 19, 20]. Menzel Habib plain (MHP), constituting the Southeastern termination of the Tunisian Atlas (Figure 1B), was not spared from this tectono-sedimentary interaction. This plain characterizes a range of stratigraphic and structural interference between all the major structures of the Tunisian margin (Figure 1B). It occupies an umbilical position between the Eastern platform, the Central Tunisian Atlas, the Southern Atlas, and the Jeffara (Figure 1B). Geodynamically, the evolution of this zone is closely controlled by the jerky activities of the North-South Axis (NSA) and the South Atlasic Fault Corridor (SAFC) represented by the Hadhifa Fault (Figure 1C). By its complex structural and stratigraphic position, the MHP shows a capital interest, both geodynamically and hydrogeologically, to understanding the larger neighboring structures evolve at the expense of major faults that the shreds. Although the geological position of the Menzel Habib plain is very important, works that have targeted this area are very rare and have only focused the mapping of the various stratigraphic and structural units [21]. The aim of this work is (1) determining the different fault networks that affect the study area; (2) showing the tectono-sedimentary evolution of the study area in relation to the major fault systems based on field and subsurface data, (3) giving the outline of the hydrogeological characteristics of potentially aquifer formations, and (4) correlating the evolution of this zone with other examples in neighboring regions.

Figure 1.

Geological context of the study area. A- general position of the Tunisian margin in the Mediterranean context; B- structural zoning of Tunisia [6] showing the structural position of the study area in relation with the major structures of the central and southern atlas, the eastern platform and the Tunisian Jeffara. C- geological map of the study area showing the most master faults (F1 and F17) which affect the Menzel Habib plain (MHP) and the position of the various seismic lines used in this study.

2. Geological framework

2.1 Stratigraphy

(Figure 1C and 2). Triassic beds crop out in Jebel Hadhifa, composed of clay, marl, limestone, gypsum, salt, anhydrite, and sand. The Triassic Germanic facies, typical of the Tunisian margin, is evident. The Lower Cretaceous is outcropped only by the top part of the Bou Hedma formation, the Sidi Aich, and Orbata ones. The BHS, ZB1, BK1, and MAN1 oil wells characterizing the MHP and surroundings (Figure 1C) served to study the non-outcropped formations. In the Chott Fejej area, Mejri et al. [22] attribute the Early Cretaceous successions to the Sened and Gafsa Group defining the Meknassy Super Group [23]. The Melloussi formation, 200 m, comprises interbedded sandstones and claystone with minor dolomite and anhydrite. This formation has been deposited on a shallow marine shelf with an irregular subsidence. The Boudinar formation is made up of white sand, fine to coarse-grained, with fine interbedded levels of green shale, and occasionally thin levels of brown dolomite. The average thick of this formation is 180 m. These two formations constitute the Sened group largely described in the surrounding areas of Fejej Chott.

Figure 2.

Geodynamic and stratigraphic context of the study area; column 1, 2 and 3 show the different lithostratigraphic unites outcropped or crossed by the various oil wells in the MHP; column 4 and 5 expose the principal stages of Africa drifting with respect to Europe and the corresponding tectonic events recorded in the Tunisian margin since Triassic (Ben Chelbi 2013); column 6 summarizes the corresponding hydrostratigraphic units characterizing the subsoil of MHP.

The Gafsa group, starts with the Bou Hedma formation essentially made up by claystone with interbedded sandstone, dolomite, limestone gypsum, and occasional coal. 100 to 150 m of fine white sands characterize the summit of this formation, which usually ends with 3 m of red dolomite forming a boundary between the Bou Hedma formation and the Sidi Aich one. At the core of Zemlet Elbeidha anticline, the Bouhedma formation is represented by 1250 of gypsum, claystone, sandstone, and dolomite alternations (Figure 2) in which 800 m are crossed by the ZB1 petroleum well. The Sidi Aich formation is sandy unit widely spread in Central and Southern Tunisia. In our study area, this formation is composed of interbedded sand and varicolored claystone with occasional dolomite and rare traces of coal. The average thick of the Sidi Aich formation turn around 80 m. This last is usually overlapped by 40 to 60 m of brown and massive dolomites attributed to the Orbata formation. At the South-Eastern flank of the Zemlet Elbeidha anticline, these dolomites are replaced by a bar of polygenic conglomerates formed by dolomite blocks embedded in a sandy and clayey matrix. These dolomites are capped with another dolomitic cliff constituting the base of the Zebbag formation, with a major unconformity. Consequently, if the base of Orbata is approximately synchronous, the top is completely diachronous depending on the sedimentation of the upper members, the subsequent erosion, and the onlaps of the following series [22].

Upper Cretaceous stratigraphyincludesthe Albian beds, which is represented by the Zebbag formation (Upper Albian to Cenomanian), the Aleg formation (Turonian to Lower Campanian), and the Abiod formation (Lower Campanian to Upper Maastrichtian) (Figure 2). These formations are composed of dolomitic, gypsum, and clay alternations, in the lower part, argillaceous and marly deposits, in the middle part, and dolomites and limestones, in the upper part.

The Tertiary and Quaternary consists of predominantly unconsolidated sand and conglomerates at the base overlain by red clays with crystalline gypsum. These deposits are very rare and generally occupy the piedmonts of the great reliefs.

Palaeogeographically, deposits of the Melloussi and Boudinar formations reflect a deltaic regressive detritic body evolving to a deltaic progradation toward the north [23]. Deposits of Bouhedma, Sidi Aich and Orbata formations characterize transgressive sedimentation on a low slope substratum. Facies and thickness changes of these formations are due to the activities of the great faults acting during extensive epochs contemporaneous with sedimentation and to the jerky halokinetic rises of Triassic salt bodies.

2.2 Hydrostratigraphy

Despite the fact that almost all of the MHP is occupied by Plio-Quaternary deposits (Figure 1C), the subsoil contains all the geological formations known for their qualities of forming good aquifers of regional scope. Based on the petrographic and structural characteristics of these different geological formations, as well as on previous work undertaken in the area and its surroundings [21, 24, 25, 26, 27], we have identified and defined several units that may correspond to potential aquifers (Figure 2). Some of these are well known and well-studied, others are not yet and are targets in other neighboring basins.

The most superficial aquifer is lodged in the sandy, silty, and clay deposits of the Pio-Quaternary. In some places, these series are very thick, reaching up to a hundred meters. For decades and considering these hydrogeological characteristics and their accessibility, the Plio-Quaternary formations are extensively exploited. We count more than 350 surface wells that have been dug in these series [21]. Most of these wells are dry, and the rest have a very high salinity of around 5 to 10 g/l.

The second aquifer is known as the “Complexe Terminal” (CT) (Figure 2), which characterizes the Southern Tunisia and South-Eastern of Algeria. This aquifer is a multilayered system stored the Miocene sandy formation, the Upper Cretaceous carbonate members (Campanian-Maastrichtian), and the Zebbag dolomitic formation. Unusually, the Aleg formation, which outcrops at Jebels Belkheir, Chamsi, Berda, and Zemlet Elbeidha is represented, at its base, by fractured white limestones. For this, the latter can be considered as intermediate terms characterizing the TC. In the studied area, only a deep well, which has reached these carbonates and given water with a low salinity (2 g/l). The massive and fractured dolomites attributed to the Orbata formation could constitute an additional aquifer, which can be added to that of the CT due to the stratigraphic continuity and which exists between the two terms.

The Barremian sands, defining the Sidi Aich formation and the summit part of the Bouhedma one, omnipresent in the basements of the MHP, store the most important deep aquifer characterizing the South of Tunisia. This aquifer is called “Continental Intercalaire” (CI) (Figure 2). These aquifers mentioned above are well-known and have been exploited for a long time either for agriculture or for industry.

Considering their petrographic and structural characteristics, the Boudinar and Melloussi formations, respectively represented by sands and dolomitic, and sand alternations may constitute the deepest aquifers of the MHP. These formations were crossed by the BHS1 petroleum well and have shown that they store appreciable quantities of water. Because of their enormous depth, these two last groundwater levels are note previously studied and defined.

Hydrogeological studies undertaken in this area [24, 25] have shown that these aforementioned aquifers are recharged from large reliefs in the North, West, and South, respectively represented by Jebels Belkheir-Chamsi, Jebel Berda, and the North chain of Chotts (Jebels Bir Oum Ali, Jebel Hachichina, Jebel Batoum, Jebel Oum Laagol, Jebel Haira-Es Smaya, and Jebel Zemlet Elbeidha). Moreover, these aquifers are recharged by vertical infiltrations from the endoreic basins (Sabkhet Sidi Mansour-Mhamla, Garaet Hajri, Sabkhet Fejij, Garaet Wali and Garaet Aouled Khoud) through a very dense network of active faults.

2.3 Structural evolution

Since the Triassic period, the Tunisian margin has recorded all the echoes of the geodynamic evolution of the Tethyan domain and the resulting structures (Figure 2) [4]. Indeed, from the first stage of the breakup of Pangea in the early Triassic, a system of regional NS, NE-SW, and NW-SE trending faults greatly jagged sedimentary floor into a mosaic of a variables blocks and expanses. From this period a rifting system was implemented, applying these faults, in response to an extensive tectonics involving a N-S minimum stress. Evidence of the first stages of the Tethyan rifting is dated to the Jurassic-Barremian period [28, 29, 30, 31]. This mechanism of opening is continuing until Barremian period (Figure 2). A radical change occurs during the Aptian that materializes by changing the direction of elongation to become NE-SW [6, 32]. This change in the minimum stress orientation is indicated by reactivation of the NW–SE-trending faults with normal activity, which created several NE–SW and NW-SE basins [6, 15]. These will evolve during the following periods and give the different troughs of the central Tunisian Atlas [15] and the major escarpment of the Southern Tunisian Atlas and Jeffara Zone [1]. Contemporaneous with this activity, the Tunisian Atlas records intense halokinetic activity (Figure 2) forming a large system of diapirs and/or salt glaciers [14, 33].

Tunisian margin was submitted, during the Albian Cenomanian period, to compressive tectonic applying NW-SE maximum stress (Figure 2) [7]. This dynamic activates the N-S, NE-SW, and NW-SE existing faults into sinistral reverse, reverse, and normal dextral activities, respectively.

During the Late Cretaceous the Tunisian margin was controlled by transtensional to transpressional tectonics [6, 9], characterized by reactivation of the north–south-, east– west-, and NE–SW-trending normal faults with dextral component. The general inversion of the structures was recorded during the middle Eocene [14, 15, 19, 32] (Figure 2). The first period of folding is assigned to the Tortonian [34], whereas the ultimate compression leading to the building of the Tunisian Atlas Mountains began during the Villafranchian and continues to the present day (Figure 2) [35]. This long period of compression controlled by NNW–SSE to N-S stress has contributed to the final configuration of the Tunisian Atlas in which NE–SW-, NW–SE-, E-W, and N-S oriented faults were respectively reactivated with sinistral reverse, dextral reverse, reverse, and sinistral slip. The Southern Tunisian Atlas has not escaped this complex evolution. The actual architecture of this part of the Tunisian margin is characterizes by coexisting of E-W trending folds, which are truncated by NW-SE and N-S oriented major faults. These last represent the signature of the SAFC and NSA, respectively. These faults have largely controlled sedimentation, during extensive period, and final structural design, during compressive epochs, of the Southern Tunisian Atlas and the Jeffara escarpment.

3. Geomorphological and hydrologic setting

Cartographically, the study area corresponds to a E-W oriented plain moderately culminating to 150 m (Figure 3). It is bounded on the North and South by two large orographic lines corresponding to the Jebel Chamsi-Jebel Belkhir and the Northern Chain of Chotts, respectively (Figure 3). Jebel Berda constitutes the western limit of this plain. The MHP is drained by several wadis, which flow in the Sabkeht Sidi Mansour, to the West, and in the Sabkhet Zograta and Sabkhet Ouled Khoud, to the East. The medium part of the MHP is shaped by two large topographic convexities, lengthened according to a N-S and NE-SW direction. From West to East, the first erected band is traced between Sabkhet Sidi Mansour and Garaet Hajri, while the second one lengthened the Bled et Teurbia (Figure 3). These convexities constitute a dividing line of discharge of the various wadis. The first band corresponds to the layout of a N-S major fault, which passing by Jebel Es Smaya, to the South, and continues until Jebel Belkhir, to the North (Figure 1C). The second convexity line represent the surface response of a deep NE-SW fault. This late is attested by the rectilinearity of the source line of the wadis, which diversify in Sabkhet Ouali. Oueds characterizing the western province flow either north in Sabkhet Noual or converge toward the Sebkhet Mhamla and Oued Rimth.

Figure 3.

Morphologic and hydrographic context of the study area.

4. Data and methodology

The study of the deep structural architecture of the MHP and neighboring geological structures is inferred from interpretation of gravimetric data and seismic sections, while the evolutionary geodynamic model of this zone is concluded from lithostratigraphic correlation carried out from petroleum wells and field investigations. Seismic sections are obtained from the Tunisian petroleum company (ETAP). The W1 petroleum well is used to show the succession of different stratigraphic sequences characterizing the trough using time-depth conversion of lithological data. Several seismic lines are interpreted to establish the main oriented faults, which participated, firstly, to the facies repartition during sedimentation periods, and secondly to the final architecting of the study area during compressive times. Gravimetric data were the subject of a study carried out by the Ministry of Agriculture during 2016. The gravimetric campaign comprises 461 measurements distributed over a regular mesh covering the study area.

The geodynamic evolution of the MHP is interpreted, first, through the analysis of vertical facies and thicknesses variations of the Lower Cretaceous series on both sides of the NW–SE, E-W, NE-SW, and N-S oriented faults, and secondly through analysis of the main syndepositional faults affecting these formations (Figure 1C). Since the earliest Cretaceous, sedimentation has been largely controlled by this network of faults in which we note a large variation of facies and thicknesses. Four petroleum wells are used to show the succession of stratigraphic sequences characterizing the plain using time–depth conversion of lithological data and to correlate the lower Cretaceous formations on both sides of the major faults characterizing the study area. To understand the hydrogeological characteristics of this area, we have developed piezometric maps of deep aquifers made from well data and from depths and thicknesses maps of Lower Cretaceous formations.

5. Results

5.1 Gravity

5.1.1 Bouguer anomaly map

The Bouguer anomaly grid was established by adopting a process of interpolation of the minimum curvature. The Bouguer anomaly map expresses an amplitude of around 36 mGal, with values distributed between −42 mGal and − 6 mGal (Figure 4). Gravimetric responses are characterized by long wavelength anomalies arranged in a decreasing gravimetric gradient from east to west. Excess masses, reflecting the signature of positive density contrasts in relation to the density used in the Bouguer model, are organized in the eastern part of the study area (Figure 4). A second zone characterized by excess mass is also highlighted in the SW limit of the study zone. This subcircular geometry anomaly has a lower amplitude than those recorded in the eastern part of the study area. The western part has a mass discrepancy with increasingly negative density contrasts toward the western (Figure 4A). The recorded gravity gradient coincides with previous regional scale work carried out in Tunisia (Figure 4B). Thus, the gravimetric work of Midassi [36] reveals decreasing values of Bouguer anomaly toward the West (Figure 4B and C). We also note that this trend can be explained by the geometry of the Moho discontinuity determined by the deep seismic carried out as part of the Geotraverse program (EGT project, 1990), which expresses a lithospheric deepening toward the Gafsa region.

Figure 4.

A- complete Bouguer gravity anomaly map of the Menzel Habib plain; B- regional gravity anomaly map; C- E-W Bouguer anomaly profile of the study area.

5.1.2 Residual anomaly and contact modeling

The residual component was calculated by subtracting the low-frequency anomalies from the total Bouguer anomaly. The map obtained offers a better characterization of the heterogeneities of local densities in the subsoil compared to the density of the Bouguer model. The residual component has more emphasis on gravimetric responses basins following short wavelength characteristics directions. The residual anomaly map (Figure 5A) exposes that a clear N-S dividing line differentiates the study area into two different large gravity provinces. At the western one, three basins accumulating thick series of weak density, characterizing the subsoil of Hajri, Mhamla, and Segui, are identified. The negative anomaly typifies these E-W to ENE-WSW trending zones reaching a value of −3,8 mGal. Moreover, the eastern province is made by a dense substratum reflecting a positive anomaly. The Jebel Oum Laagol, Zemlet Elbeidha, Es Smayaa, Oued Zitoun, and Rabiaat Ouali-Menzel Habib areas reaching a value 3,8 of mGal surround the Sabkhet Fejij area, which shows a negative anomaly reaching a value of −4 mGal. The passage from the eastern province with dense materials toward the western one is front and appears as a straight line defining the layout of the N-S major fault (Figure 5A).

Figure 5.

A- residual anomaly map of MHP; B and C- vertical gradient anomaly maps; D- tilt angle drift map.

5.1.3 Structures and lineaments detection

To better understand the basins distribution, as well as the series which compose them, we proceeded to the elaboration of the vertical derivative maps by accentuations of the gravimetric responses (Figure 5B and C). The application of the first and second-order vertical gradient is responsible for the accentuation of geological structures near the surface (Figure 5B and C). These treatments also make it possible to better separate the anomalies and to better specify the limits of the sources. A reduction in the complexity of gravity anomalies is clearly revealed through better localization of the extensions and geometries of the basins in the study area. The basins already revealed by the residual anomaly map are well expressed by the vertical gradient anomaly maps (Figure 5B and C). The gravimetric signature of these basins shows a strong negative gradient, which proves that these accumulation zones are occupied by low-density sedimentary series. This response reflects the effects of a thick sand and clay series, attributed to the Mio-Plio-Quaternary time and the Lower Cretaceous period, which is characterized by a negative density contrast compared to the average density adopted by the model of Bouguer.

In order to determine the major directions of the different accumulation zones, already defined, we proceeded to the tilt angle derivative technics. This last is widely used to define the limits of gravity sources (geological structures of interest located on the subsurface). Thus, the limits of the sources are expressed by a zero angle. The tilt angle drift map reveals strong gravity gradients occupying the limits of the identified basins (Figure 5D). The geometries released regain that revealed by the first and second-order vertical gradients maps.

The horizontal gradient technics is generally used to highlight lateral variations in density, which may correspond to faults. This technic also makes it possible to characterize the major structural directions of the geological contacts. The latter are represented by gravimetric alignments, which represent the maximum amplitude on the vertical gradient map. The interpretation of the maxima of the horizontal derivative, thus made it possible to identify the discontinuities of density in the subsurface. These discontinuities are interpreted as the possible signatures of major fault systems across the region. The horizontal gradient map (Figure 6A) clearly shows that the study area is differentiated into several subareas, each characterized by various structures and faults. A major N-S fault divides the study area into two provinces. That of the East is characterized by NE-SW, E-W, and NW-SE the western province is affected by faults essentially oriented E-W.

Figure 6.

A- horizontal gradient map of the residual gravity anomaly data; B- structural map deduced from compilation of data presented by the previously gravity mas.

The structural map obtained after application of these different technics shows several lineaments (Figure 6B), which highly contributed in the geodynamic evolution and the configuration of the MHP. In fact, the identified basins are ordered in two major directions. E-W basins characterize the western province of the study area, and NE-SW basins are identified in the neighborhood of Fejij and Zemlet Elbeidha.

The highlighted faults are distributed in the basement of Menzel Habib according to their amplitude and direction (Figure 6B). The N-S fault, dividing the study area into two clear provinces and tracing west of Jebel Es Smayaa and passing through Oued Zitoun, is the most important one. This fault, called F1 (Figure 6B) constitutes the prolongation, toward the South, of the mega N-S accident known as the North-South Axis. The Hadhifa fault, NW-SE direction, named F17 on the structural map (Figure 6B), passing north of Jebel Oum Laagol, shows a major importance in the deep structuring and the geodynamic evolution of MHP; it constitutes a segment of the SAFC.

The E-W faults are concentrated and essentially characterize the western province of the study area (Figure 6B). The most important fault is that which divides this province into two subbasins called Mhamla and the Segui basins. The NE-SW and NW-SE faults characterize only the eastern province of the study area. This province is also controlled by N-S and E-W faults, the most important of which are those which shred the Fejij sub-basin. Figure 6, which shows the main faults that have been identified and affect the basement of Menzel Habib, demonstrates that the eastern province is intensely fractured compared to the western one.

5.2 Seismic interpretation

To understand the deep architecture of the study area and highlight the role of the various faults in the geodynamic evolution of this area, several seismic sections have been interpreted. These sections, obtained from the Tunisian Petroleum Company (ETAP), are calibrated by the W1 well. The different seismic profiles used billow are transversal to the average direction of gravity discontinuities already identified previously. We present bellow five interpreted seismic lines showing the main results obtained after the dots of the different seismic horizons.

Section S 1 is oriented NW-SE (Figure 1C); it is traced between the two blocks mentioned above in the gravimetric study, which are separated by the N-S master fault noted F1 (Figure 7). The mapping of the different horizons demonstrates that this fault has largely controlled sedimentation during the Lower Cretaceous period. Indeed, the SE block exhibits a reduced series of the sandy formations of Boudinar, Bouhedma, and Sidi Aich, while they are thicker in the NW block. The NW end of the seismic section shows that the thickness of the Sidi Aich formation increases considerably. Moreover, this extremity highlights a Triassic saliferous intumescence very well pronounced at depth. The NW block, characterizing the subsoil of Mhamla, Belkhir, Segui, and Aoussaj, located between the fault F1 and the Triassic intumescence, shows that the deep architecture is dominated by a flat-bottomed synclinal in which governs the clay, sandy, gypsum deposits of the Bouhedma formation. The SE block corresponds to the Fejij plain in which the Bouhedma formation outcrops with a very reduced thickness. Three main faults have been mapped on this seismic line and correspond to the so-called F1, F18, and F2 oriented respectively N-S, NE-SW, and E-W defined below after gravimetric study.

Figure 7.

NW-SE interpreted seismic line showing the deep geometry of the MHP in response to the most major fault detected by gravimetric analysis. Note the role played by the F1 fault in the differentiation of two different basins, in the SE side, and the existence of Triassic intumescence, in the NW side.

Section S2 characterizes the NW block of the study area and is mainly oriented N-S (Figure 1C). This section extends between the Northern flank of the Jebel Oum Laagol Anticline and the Southern flank of the Jebel Belkhir Anticline. From North to South, this section has well highlighted the F3 and F4 faults, which are E-W oriented and the F17 (Figure 8), called Hadhifa Fault, which constitutes a segment of the SAFC. Several thickness and facies variations are underlined on this N-S-oriented seismic section. Indeed, from South to North the Bouhedma formation gradually decreases in thickness, while the Sidi Aich formation records a net increase in thickness. Melloussi, Boudinar, and Orbata formations keep a constant thickness throughout the seismic line (Figure 8). Furthmore, two Triassic saliferous bulges characterize this seismic section denoting an intense halokinetic activity having started to individualize at least since the Barremian. This halokinetic activity coupled with the dynamics of faults F17 and F3 established a deep geometry materialized by two synclines separated by a narrow anticline.

Figure 8.

N-S interpreted seismic line showing the important role played by the F17 fault in the structuration of the subsoil of MHP and the thickening of the early cretaceous series in the northern side.

Section S3 initiates from the NW flank of the Zemlet Elbeidha anticline, crosses Fejij plain, Araguib Itama, Hajri, Menzel Habib plain and continues through Sabkhet Sidi Mansour (Figure 1C). It shows that the subsoil of these localities is compartmentalized, from South to North, into three distinct blocks bounded by four NE–SW, E-W, and N-S oriented faults, corresponding respectively to F6, F8/F7 and F1 faults deduced from the gravimetric study (Figure 9). These bordering faults of hidden basins have normal slip and show small reverse bends, which could be induced by progressive structural inversions. In addition to these faults, this section shows the existence of two Triassic saliferous bulges delimiting a clear synclinal basin. The structural-stratigraphic architecture presented by this section proves that these faults, as well as the Triassic intumescences largely controlled sedimentation during the Lower Cretaceous. Indeed, South of the F7 fault, the Melloussi, Boudinar, and Bouhedma formations show a strong reduction in thickness, while the Sidi Aich formation is at its maximum thickness. Gradually toward the North, the Bouhedma formation is gaining in thickness with a small reduction above the salt structure evolving directly above the F1 fault, while a depocentre is observed between the two salt structures for this formation. This architecture denotes that, during Early Cretaceous, sedimentation is made contemporaneous to an active halokinetic rise.

Figure 9.

NNW–SSE interpreted seismic line presenting the tectonostratigraphic role accomplished by the F1 fault and the progressive thickening of the early cretaceous formations toward the NW.

Section 4 (Figure 1C) with a NE–SW orientation, is orthogonal of Section 1 and crosses the subsoil of Zograta, Ouali, and Aouled Khoud characterizing the NE province of the study area (Figure 2b). Several faults have been crossed by this section. From NE to SW, these faults correspond to the F12, F11, F13, F15, F16, and F9 previously defined by the gravimetric study (Figure 10). Mapping of the different horizons shows that these faults have controlled sedimentation during Early Cretaceous. The central part located between faults F15 and F16 shows that the Melloussi, Boudinar, and Bouhedma formations are very reduced in thickness, while the Sidi Aich and Orbata formations are very thick; gradually, toward the NE of fault F15 and to the SW of fault F9, the Sid Aich and Orbata formations lose thickness and the Melloussi, Boudinar, and Bouhedma formations become thicker (Figure 10). The deep architecture offered by this section is that of a large graben formed by the aforementioned faults testifying to a tectonic inversion of the ancient horst occurring from the Upper Barremian (Figure 10).

Figure 10.

NE–SW interpreted seismic line demonstrating the general deep architecture of the eastern province.

Section 5 (Figure 1C), with a E-W orientation, initiated from Hajri and crosses Sidi Mansour Sebkha, Mhamla, and Aoussaj plains. The different seismic horizons pointed at this section make it possible to clarify the role played by F1 fault, oriented N-S, the F17 NW-SE oriented fault representing a segment of the SAFC and ends near a buried salt structure characterizing the subsoil of the plain of Bled Segui (Figure 11). This seismic section shows that the deep structure crossed is compartmentalized in three blocks separated by the predicted faults. The compartment located east of the F1 fault shows that the Early Cretaceous formations are very reduced in thicknesses, but the central block exposes the thickest series. In addition, by approaching toward the salt structure, these formations of the Lower Cretaceous gradually lose thickness and show progressive bevellings. This seismic section clearly proves that the sedimentation of the Lower Cretaceous is largely controlled by the normal dynamics of the N-S and NW-SE faults and by the early halokinetic activity having, probably, started since Late Jurassic time [37, 38], on the one hand, and that the sedimentary architecture induced is that of a mega tilted block toward the West, generated by the activity of the F1 fault, on the other hand (Figure 11).

Figure 11.

E-W interpreted seismic line crossing the MHP and showing the roles of the F1 and F17 faults in the structuration of the study area.

5.3 Facies and thickness variations

The Lower Cretaceous series are exposed only at Jebel Zemlet Elbeidha and Jebel Oum Laagol (Figure 1C). The Melloussi and Boudinar formations, as well as the lower part of the Bouhedma formation, are lacking and do not outcrop. They are crossed only by oil wells drilled in the region. To understand the role of the various faults, thus defined, in the distribution of these series, two transects of correlations were carried out. The first one is trended N-S (Figure 12), using data from oil wells, while the second is oriented NE-SW (Figure 13) highlighting the variations of facies and thicknesses of the outcropped series, at Jebel Zemlet Elbeidha, on both sides of the existing faults. Figure 12 illustrates the first correlation line drawn within a N-S direction for the eastern part of the study area. From South to North, this correlation transect comprises four lithostratigraphic columns representative of the fourth petroleum drilling wells, respectively named ZB1, BHS1, BK1, and MAN1. The two first wells are drilled at the core of Zemlet Elbeidha anticline and at the center of the MHP. Structurally, these two wells are separated by the F6 fault characterizing the NW flank of Zemlet Elbeidha. The BK1 well is drilled at the NE termination of Jebel Belkheir. These two last wells are separated by the layout of the N-S F1 fault. MAN1 petroleum well characterizes the Southern flank of Jebel Bouhedma.

Figure 12.

NNW–SSE correlation line of the early cretaceous formations crossed by the different oil wells characterizing the study area. Noting the clear change in fault dip that occurred from late Barremian inducing a change in subsidence which becomes toward the NW.

Figure 13.

Role of the different network of faults affecting the southern flank of the Zemlet Elbeidha anticline in the distribution of the Barremian and Aptian sedimentation; A- simplified geological map of the southern flank of the Zemlet Elbeidha anticline showing the location of the different lithological columns correlated; B- correlation line of the Barremian-Aptian formation characterizing this site; C and D- fault geometries during sedimentation demonstrating a clear fault dipping change from Barremian to Aptian periods.

Lithostratigraphic columns studied show significant facies and thicknesses variations of the different formations attributed to the Lower Cretaceous. Indeed, from south to north, we note a gradual thickness decrease of the Melloussi, Boudinar, and lower part of the Bouhedma formation. The lithostratigraphic column of the ZB1 petroleum well intersected a Bouhedma formation three times thicker (1300 m) than that crossed by the MAN1 one (420 m). The sandy part, which characterizes the top of the Bouhedma formation, was encountered only at the ZB1 well with a thickness exceeding 150 m, while it is absent or very reduced in the other wells (30 m = m at BHS1). The thickness of the Melloussi and Boudinar formations make about 450 m in Zemlet Elbeidha structure, while they do not exceed to 250 m in the Belkheir and Mansour Structures.

In addition, a clear inversion of subsidence occurred during the Late Barremian-Aptian period. This inversion is materialized by a strong thickness of the Sidi Aich and Orbata formations, in the North (Jebel Belkheir and Mansour 500 m), and a progressive reduction of this thickness toward the South (Zemlet Elbidha structure), where it registers at most 100 m thick. We also note that the Orbata formation, crossed by the MAN1 petroleum well, is formed by clays and dolomites, while it is exclusively dolomitic in the other localities.

Figure 13 illustrates the second correlation line, which is mainly oriented NE-SW. It comprises six lithostratigraphic columns characterizing the Southern flank of the Zemlet Elbeidha anticline (Figure 13A). Each column is separated from another by a directional fault (Figure 13A). Significant thickness and facies variations are highlighted by this correlation line. It shows that the sandy deposits, characterizing the summit part of the Bouhedma formation, as well as the Sidi Aich sandy formation shows, at the L4 locality, the most reduced thickness. On both sides of this locality, these sandy formations recorded a progressive increase in their thickness (Figure 13B). This sedimentary design, during this period, is governed by the normal activities of the different defined faults inducing a horsts and grabens architecture (Figure 13C).

In addition, dolomites attribute to the Orbata formation show continuous variations from one locality to another (Figure 1B). The lithostratigraphic column L3 exposes the most reduced series, and the column L5 demonstrates that the Orbata formation is limited to two meters of conglomerates. The thickest series has been identified in column L4. This sedimentary design is largely controlled by the normal activity of the various faults characterizing the Southern flank of Zemlet Elbeidha structure, which induce a horst and graben architecture (Figure 13D) but conversely with the structuration of the underlying sandy series.

5.4 Synsedimentary faulting analysis

To determine the structural evolution of the NE termination of the Southern Atlas including the MHP, during Early Cretaceous, we measured the orientations of striations on fault planes that affect different formations. Syndepositional fault populations, providing direct dating of tectonic events, have been specially analyzed. Deposits attributed to the Valanginian and the Hauterivian, defining the Melloussi and Boudinar formations, do not outcrop in our sector of study. For this reason, we will be interested to the synsedimentary faults affecting the Bouhedma, Sidi Aich, and Orbata formations, which are exposed at Jebel Zemlet Elbeidha and Jebel Oum Laagol. We present below the main sites fossilizing a tectonic evolution during the Early Cretaceous, which is materialized by synsedimentary faulting.

Red marly, and beige to yellow sand alternations, attributed to the Late Hauterivian period, outcropping at the core of Zemlet Elbeidha anticline, fossilized many syndepositional normal faults. These last are organized into two directional families (Figure 14A). The first family contains N40-60 trending faults, while the second one is represented by NW-SE-oriented faults. These syndepositional faults impose a horst and graben design. The numerical treatment of these fractures shows that the medium Barremian sedimentation is controlled by an extensive tectonic applying a N-S trending minimal horizontal stress (Figure 14A’ and B).

Figure 14.

Syndepositional faulting affecting the Barremian-Aptian deposits recorded at the different outcropped series in the study area (a, C, D, E, F, G and H) and characteristic stereographic projections corresponding to data measurements, Schmidt projection, lower hemisphere (B, a’, C′, D′, E’, F′, G’, H′ and I) (the central figure shows the location of the studied sites); a, A’B) fault planes are great circles; slickenside lineations are small centrifugal arrows (normal faults), black squares with index 1: σ3, black squares with index 2: σ2 black squares with index 3: σ1.

Several normal synsedimentary faults (Figure 14C and D) affecting clay, dolomite, and gypsum alternations, attributed to the Hauterivian and Barremian periods and defining the Bouhedma and Sidi Aich formations, characterizing Jebel Oum Laagol show that this sedimentation was controlled by an extensive tectonics involving a N-S trending minimum stress (Figure 14C′ and D′).

The red dolomitic bar, which marks the summit of the Bouhedma formation, which outcrops between Khanguit Amor and Khanguit Aicha at the Southern flank of Zemlet Elbeidha anticline, is affected by multitudes normal synsedimentary faults (Figure 14E). These are oriented E-W, NE-SW, and NW-SE and draw an architecture in horst and grabens. Numerical analyses of these fractures show that the Late Barremian sedimentation is controlled by an extensive tectonic applying a N-S to NNE-SSW trending minimal horizontal stress (Figure 14E’).

Dolomites attributed to the Aptian period and which characterize the Orbata formation are affected by normal faults trended NE-SW, NW-SE, and E-W (Figure 14F, G, and H). These faults draw a tilted blocks and a horst and graben geometries (Figure 14H). Numerical analyses of these faults prove that the Aptian sedimentation was subjected to an extensive tectonics applying a NE-SW minimal stress (Figure 14F′, G’, H′, and I).

6. Discussion: plaleostructuration and hydrogeological implication

6.1 Basin shapes

The compilation of morphological, gravimetric, and seismic data shows that the MPH subsoil is structured in diamond-shaped to rectangular blocks shaped by the combination of N-S and NW-SE oriented major faults with E-W and NE-SW second-order faults. The NW-SE and E-W faults, which dominate the subsoil of the western province architect this zone into two subsiding basins of lozenge-shaped forms separated by a raised band (Figure 15A). This arrangement is that of a horst and graben system, during periods of sedimentation, evolving after tectonic inversion into two large synclines separated by a narrow anticline (Figure 15B). To the east and near the fault F1, these two synclines are wide and deep. They gradually narrow toward the West near Jebel Berda and are very disturbed by the halokinetic activity molded on the F17 master fault. Geometrically, we point out the progressive deepening of the Lower Cretaceous series from West to East and from South to North. This architecture could affect the underground flow of water contained in the porous and permeable series of the Lower Cretaceous. The overall collapse of the western province is due to the F1 Fault in comparison with the eastern province (Figure 15). This last is constantly controlled by N-S and NE-SW trending faults. Thin formations characterizing this province compared to that identified at the western province drawed a large synclinal basin (Figure 15A) of sub-rectangular shape delimited by two major N-S oriented faults, in the East and West parts, and by two NE-SW trending fault, in the North and South borders. This buried structure is continuously affected by second-order faults. Geometrically, this highly fractured syncline structure shows a regular slope toward the SE.

Figure 15.

General basin shapes of the subsoil of MHP; A- E-W cross section; E-W geological section showing the deep architecture of the study area and the roles of the N-S and NE–SW faults in the individualization of this architecture; B- N-S section showing the deep structure of the study area in relation to the NW-SE and E-W fault systems.

In addition to these different fault networks, which determine the deep geometry of the Menzel Habib plain, there is an intense halokinetic activity, which creates bulges localized essentially in the layout of large faults (Figure 15). The most important salt structures are located below the fault F1, F17, and F8, which characterize the subsoil of Menzel Habib Village-Jebel Belkheir, the North flank of Jebel Oum Laagol and the subsoil of Sabkhet Sidi Mansour-Mhamla, respectively.

6.2 Paleo structuring

The multiscale approach adopted to understand the geodynamic evolution of the MPH, based on the morphological analysis, the interpretation of gravity and seismic data coupled with the inventory of facies and thickness variations of the Lower Cretaceous formations and the examination of the synsedimentary faults prove that this plain is dominated, during the Early Cretaceous time, by a N-S to NE-SW extensive dynamics. This tectonic event results in the individualization of several basins of variable size and extent, evolving at the expense of a complex network of faults. This architecture has greatly influenced the hydrogeological evolution of this plain.

Morphological analysis allowed to note that the MHP is differentiated from East to West into two different hydrological provinces separated by a N-S oriented bulge, which is traced in the center of this plain and serves as a water sharing (Figure 3). It corresponds to a surface response of the F1 fault (Figures 6 and 9) elongated in the same direction. Regionally, this morphologic protuberance constitutes the extension toward the South of the morphostructural line, elongated from Hammem Lif, in the North, to Mezzouna, in the South, which is molded on a N-S paleogeographic fault, commonly called N-S Axis. Although, morphologically not very prominent, this central sub-meridian uplift reflects the effects of a jerky geodynamic evolution and plays a very important role in the hydrogeological differentiation of the Menzel Habib basin.

The most impressive gravimetric differentiation is that managed by an N-S trending fault separating an eastern basin bearing the signature of dense series and a western basin characterized by a significant mass deficit (Figure 5A). This fault coincides with that was identified by morphological analysis and corresponds to the southernmost branch of the N-S axis. The residual map, the vertical and horizontal gradient maps, as well as the tilt angle map show that the subsoil of MHP is shaped into several E-W and NE-SW trending basins, which are largely disturbed by N-S, NE-SW, E-W, and NW-SE oriented faults (Figure 6B).

The interpretation of the available seismic lines confirmed the existence of all the faults detected by the morphological and gravimetric analyzes. These faults had a major impact on the distribution of the various geological formations that characterize the study area during early extensive periods and guided its structural differentiation during later orogenic phases. The F1 and F17 faults, which present a N-S and NW-SE direction, respectively, are the most important accidents that have contributed to the shaping of this study area. In addition, the geodynamic evolution of the study region is also due to the E-W and NE-SW faults, currently exhibiting reverse and sinistral reverse activities respectively (Figures 6–8).

During the Early Cretaceous period, the extensive dynamic governing the Tunisian margin and applying a N-S to NE-SW minimal stress [39] activates the F1 fault with a normal dynamic allowing the uplift of the eastern block and the subsidence of the western one (Figures 16 and 17A). This architecture promotes the accumulation of a thick and continuous series to the West and a reduced one in the eastern province (Figure 4A).

Figure 16.

Superposition of the isochronous maps of the different formations characterizing the subsoil of MHP showing the role played by the F1 fault in the paleostructuration of the study area. Noting essentially the big variation in thickness of Melloussi, Boudinar and Bouhedma formations on both sides of this fault.

Figure 17.

Evolution of the Menzel Habib plain in response to the N-S and NE–SW extensive tectonics during early cretaceous. A- two evolutionary models of Menzel Habib plain in relation to the most important fault which affect the study area; B- relative motion of Africa with respect to Europe. Six steps (a, b, c, d, e and f with different colors) of displacement of Tunis summarize the relative trajectory of Africa during early cretaceous [11]; C and D plate tectonic maps of alpine Tethys and the western embayment of Neotethys during Hauterivian and Aptian periods respectively.

From North to South, Burollet [40], Abbès et al. [41], Boccaletti et al. [42], Dolglioni et al. [43], Martinez et al., [44], and [45] demonstrate that this architecture has been proven on all the different branches of the N-S Axis. Furthermore, this master lineament constitutes a sedimentary limit between two domains, which have different palaeogeographic settings, the Atlas zone, with very thick Lower Cretaceous formations, and the Eastern platform, characterizing a slow subsidence, during this period. Arfaoui et al., [45] denote that this alignment formed a palaeogeographic feature since the Jurassic as the reductions or condensations of the sedimentary sequences often show gaps and discordances.

In addition, and in response to this extensive tectonic, the F17 fault, representing the t branch of the SAFC, is activated as normal dynamic with a NE dipping (Figure 17A). It results accumulation of continuous and thick series in the NE part (3000 m thick of deposits) and reduced ones in the SW province in which the total thickness of the Jurassic and Early Cretaceous series does not exceed 1600 m [46]. This architecture has been highlighted and proven by Said et al., [47, 48] and Arfaoui et al., [45]. These two authors have confirmed that these major faults signed, during the Late Triassic to Early Cretaceous, the initiation and the accommodation of the Tethyan rifting inducing a complex system of horst and graben structures.

The structural associations characterizing these two major palaeogeographic faults (N-S axis and Gafsa-Hadhifa accident) lead to consider them as first-order structures to which the MHP owes its existence and its evolution since the first stages of Tethyan rifting.

Second, the stratigraphic evolution of the MHP depends on the NE-SW fault, which runs along the NW flank of Zemlet Elbeidha, and the E-W fault, which borders Jebel Belkheir and Jebel Chamsi on the south side. During the Valanginian-Hauterivian, these two faults dip toward the South, while they dip toward the North during the Barremian to Aptian period (Figure 17A).

In response to the Early Cretaceous NE-SW to N-S extensive tectonics, these faults compartmentalize the subsoil of Sidi Mansour-Mhamla-Aoussaj into several rhombic basins of different geometry and size with differential migration of the subsidence. This architecture is that of a large graben made by the Hadhifa Fault (F17), which dips toward the North and the major E-W fault bordering the Jebel Chamsi and Jebel Belkheir anticline’s, and which dips to the South (Figure 17A). On the other hand, the F1 fault and the major fault separating the MHP from the plain of Skhira, which plunge toward the West and toward the East, respectively delimit a great horst characterizing the subsoil of Zograta-Rbiat Ouali (Figure 17A).

Detailed analysis of the different syndepositional faults fossilized in the Early Cretaceous series proves confirm that this extensive tectonic applies two successive minimum stress oscillating between N-S and NE-SW poles. The first direction of stress is attributed to the Valanginian to Barremian period, while the second one characterizes the Upper Barremian to Aptian time.

In addition to their synsedimentary activity these faults guided, during the Lower Cretaceous, significant halokinetic activity materialized by Triassic bulges. Some of these Triassic intumescences reach the surface, case of Jebel Hadhifa (Figure 1), others remain buried and mark out the layout of the major faults, case of the Triassic bulge on the F1 fault (Figure 9). These salt bodies implanted on the master faults give an additional argument on the role played by these faults in the geodynamic evolution of that MHP. Chronologically, the N-S extensional tectonics recorded at this region of the Tunisian margin has started since the Triassic period. It has been highlighted at the Southeastern part of Tunisia [5, 35, 49] at the Central Tunisian Atlas [31, 44, 50] and at the Northern Tunisian Atlas [6, 28, 29]. This extensional event evolved in relation with the sub-meridian Tethyan extensional framework, which affected the whole of the North-African margin [51].

This extensive dynamic, which ruled the Tunisian Margin during the Lower Cretaceous, has been widely studied and proven in Libya [52, 53, 54], in Italy [55], in Algeria [56, 57, 58], in Morocco [8, 59] and in Spain [60]. It is responsible for an intense rifting applying N-S and N-–SW faults favoring the development of several NE-SW trending grabens filled with continental deposits. Moreover, this distension is accompanied by a very important basaltic magmatism [61].

The extensive tectonics, which govern the Tunisian margin as well as the neighboring margins and which applies two minimal stress, trended NS and NE-SW, representative of the Valanginian-Hauterivian-Lower Barremian and the Upper Barremian-Aptian times respectively, translate the echoes of a jerky evolutionary motion of Africa and Iberia in respect to Eurasia plates during Tethyan and Atlantic spreading (Figure 17B) [11, 62, 63]. In fact, the first geodynamic stage of extension is contemporaneous to the northward propagation of the Atlantic Ocean spreading along the western margin of Iberia and the sinistral displacement of Africa relative to Europe (Figure 17B and C) [64, 65], which produce the abandonment of the Ligurian–Tethys corridor and the opening of the Bay of Biscay along the already rifted Pyrenean basin with a concurrent counterclockwise rotation of Iberia [11, 66, 67]. The second geodynamic stage characterized by NE-SW dynamics results from the Northward propagation of the Central to North Atlantic, which led to the individuation and eastward motion of the Iberian microplate and Africa with respect to Europe ([11]; Berra and Angiolini [68] (Figure 17B and D) Menant et al. [69], and Ye et al. [70] Guerrera et al. [71]. After this significant plate reorganization and increase of the Iberia-Africa-Eurasia convergence velocity at ~118 Ma [54, 72], results the onset of subduction of the eastern Ligurian part of Alpine Tethys along the Iberian–African plate boundary, which continuous with intracontinental subduction and Eo-alpine orogenesis (Figure 17D) at the northern tip of Adria, where Alcapia and Adria converged obliquely during the same time period [11, 54, 70].

For the MHP case, the change of stress field from a N-S pole to a NE-SW one represents the transition between the opening of the South Atlantic, the sinistral drift toward the SE of Africa in relation to Eurasia and the beginning of subduction of the Neo-Tethys [63, 73, 74, 75], and the beginning of the opening of the Eastern Mediterranean accompanied by the NW migration of Apulia and the development of Back-arc basins above the northern subduction zone, forming oceanic crust now flooring the Black Sea and the Caspian Sea [54, 76, 77, 78, 79, 80].

6.3 Hydrogeological implication

The geometry of the various buried structures characterizing MHP, coupled with the high porosity and permeability of the different sandy and dolomitic series, attributed to the Early Cretaceous time, prove that the subsoil of MHP exhibits a very developed deep aquifer system characterizing the eastern end of the Southern Tunisian Atlas. The inventory of the various water wells, carried out in 2017, confirms that the MHP subsoil contains a very important multilayer aquifer system. These layers correspond to the sandy and dolomitic deposits of the Melloussi formation (300 m), the sandy series of the Boudinar formation (250 m), the top sandy layers of the Bouhedma formation (150 m), the sandy deposits of the Sidi Aich formation (100 m), and the dolomitic dale of Orbata formation (50 m) (Figure 2). The recharge of this aquifer is done either by direct infiltration from the outcropped formations at Jebel Zemlet Elbeidha and Jebel Oum Laagol, which dips toward the NW or by vertical infiltration through the various faults affecting the surface from the numerous sabkhats characterizing this zones.

Therefore, the available piezometric maps ([25, 81]; CRDA Gabès), the structural model (Figures 15 and 16), the hydrographic network (Figure 3), and the underground water flow direction enabled us to draw a detailed piezometric map of the deep aquifer characterizing the MHP. This map shows that this area is compartmentalized into two hydrogeological compartments separated by a water-sharing line corresponding to the layout of the F1 master fault parallel to the dividing line of the surface water. The piezometric map of the eastern compartment clearly shows that the underflow is directed from west to east taking the same direction as the flow of Oued Rimth. In the western compartment, the underground flow is multidirectional converging toward the basement of Sabkhet Sidi Mansour imitating the flow of the different wadis flowing into this Sebkha (Figure 18).

Figure 18.

Piezometric map of deep groundwater table ([25, 81]; CRDA Gabès, 2019).

Consequently, the deep aquifer system of MHP appears by its structural arrangement between the different structures of the central and Southern Tunisian Atlas, the Eastern platform, and the Jeffara by its geometry, as well as by its complex geodynamic evolution in relation to the major faults (NSA and SAFC), as an independent hydrogeological entity and different from other aquifers characteristic of neighboring areas. This multilayered aquifer of MHP is separated from the North Gabès Aquifer (NGA) by the N-S (F1) and the Al Mida NE-SW faults, from the Continental Intercalaire Aquifer (CI) of El Hamma-Kebili by the Hadhifa NE-SW Fault (F17), from the Guettar-Gafsa Nord Aquifer by the Chamsi-Belkheir anticlines and by the Gafsa Fault (Branch of the SAFC), from Sfax Aquifer by a NW-SE fault, which borders Sabkhet Noual and from the coastal Aquifer of Gabès by the N-S fault parallel to the F1 fault (Figure 18).

7. Conclusions

The main results that can be enhanced from our research on structural-stratigraphic field investigations and the analysis of gravimetric and seismic data, targeting the Menzel Habib plain, which represents a hinge zone between the major structural features of the Southern Tunisian margin, can be summarized as follows:

• The hydrographic and morphologic approach proves that the MHP is subdivided into two distinguished hydrologic basins separated by a N-S topographic bulge, which expresses the surface response of the deep activity of the F1 fault.

• The synsedimentary faults affecting the various formations of the Early Cretaceous, as well as the variations in thickness and facies of these formations, on both sides of the major faults, prove that this period was controlled by an extensive tectonics applying a minimal stress oscillate between a N-S pole and a NE-SW pole. The Valanginian-Hauterivian-Lower Barremian period, with a N-S extensive event, constitutes the continuation of the long rifting epoch, which began since the Late Triassic time and which is responsible for the installation of several basins evolving in respect to N-S and NW-SE normal faults. From the Late Barremian period the minimum stress becomes NE-SW. It is responsible for the amplification of the subsidence of the basins following a horst and graben or tilted block architecture. The N-S, E-W and NW-SE faults are the most important and the most imposing syndepositional faults to ensure this evolution.

• The deep architecture of MHP, deduced from the available gravity maps, is that of two basins with different densities, separated by a major N-S oriented fault (F1). The eastern compartment is formed by a thin and dense series, while the western basin is characterized by a significant mass deficit. The residual map, the vertical and horizontal gradient maps as well as the tilt angle map show that the subsoil of MHP is shaped into several E-W and NE-SW trending basins, which are largely disturbed by N-S, NE-SW, E-W, and NW-SE oriented faults.

• The available seismic lines clearly confirmed the existence of all the faults detected by the gravimetric and morphologic analysis. The most important faults highlighted by the seismic interpreted lines are the F17 fault, representing the most eastern branch of the SACF, and the F1 fault, which constitutes the southern segment of the NSA.

• The deep architecture of the MHP appears as a series of basins evolving to the detriment of a dense network of faults and showing varying thicknesses of the Lower Cretaceous formations. These basins constitute, by their geometries and by the composition of the various formations which form them, very developed deep aquifers which will be able to store appreciable quantities of water. These aquifers are totally independent from the usual ones known in the surrounding areas of the Southern Tunisian margin. The N-S and the NW-SE trending faults, characterizing the F1 and F17 faults, and the major various anticlines (Jebel Zemlet Elbeidha, Jebel Aidoudi, Jebel Oum Laagol, Jebel Chamsi and Jebel Belkheir) contribute to the specificities of these deep aquifers.