Mohamed Ben Youssef

Tectonique en radeaux au toit d'un ≪ glacier de sel ≫ sous-marin albien de Tunisie du Nord-Ouest: exemple du secteur minier de Gueurn Halfaya

Abstract In northwestern Tunisia, the new study of the Gueurn Halfaya mining area allows us to define the relationships of the Triassic masses with the surrounding Cretaceous and Miocene formations, and to specify the genesis of several mineralizations (Fe, Sr, Pb-Zn), from an Aptian-Albian tilted block extensional setting, later tectonically inverted during the Tertiary. During the Lower-Middle Cretaceous, a sedimentary slope with boulders, determined by normal faulting, receives a submarine ‘salt glacier’, like in the Gulf of Mexico. This is then overlain by two successive sedimentary covers (Middle-Upper Albian and Vraconian-Turonian); the depocentres of the second cover directly overlie the saliferous Triassic rocks after the rafting of the first cover. This organisation is similar to the Kuanza basin setting in Angola. The Lower-Middle Cretaceous tectono-sedimentary evolution proposed here, very different to the ‘classical’ forceful diapiric interpretation, is continuously drived from the extensional tectonics (sedimentation and mineralizations, before, during and after the halokinesis), and afterwards tectonically inverted by the two Tertiary contractional events.

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

First microbialites associated to organic-rich facies of the Oceanic Anoxic Event 2 (Northern Tunisia, Cenomanian–Turonian transition)

From northern Tunisia, small-scale well-preserved microbialites, contemporaneous to the global oceanic anoxic event 2 (OAE 2) are first reported on the southern Tethyan Margin. These microbialites are encased within the pelagic organic-rich black shales of the Bahloul Formation (Cenomanian–Turonian transition). Biostratigraphic, petrographic, and geochemical investigations carried out to constrain their biogenicity and genesis character led to consider them as thrombolites and stromatolites occurring in lenticular bioherms/biostromes and columnar bodies co-relatable to the global ‘filament event’ of the authors, close to the base of the Watinoceras ammonite zone. Abundant clotted micrite, cyanobacterial filaments, and algal tissues point to the key process of microbial carbonate precipitation and to a major role played by microbes in the stabilisation and subsequent lithification, which in turn favoured the preservation of the original structure of the microbialites. These microbially induced carbonate formations are considered as favoured by chemosynthetic fauna of bivalve molluscs and lithistid sponges which were able to host symbiotic microbial communities. The latter contributed to the precipitation of authigenic calcite and non-carbonate minerals (e.g. pyrite) fuelled by microbial activity under sulphate-reducing conditions. The carbonate body onset is considered to be initiated by seafloor instability due to syndepositional fault acting that induced the appraisal of uplifted tilted blocks within oxygenated waters but near the anoxic water masses. Generated depressions allowed the development of chemosynthetic-based communities. Deep faults related to Triassic salt domes acted as conducts for hydrocarbon and salt brine expulsion to the seafloor and the microbialite growth was enhanced by an abrupt uprising sea level under warmer conditions.

Comment on 'Geodynamic evolution of Jbel Cheid (Northern Tunisian Atlas) from geophysical and geological data' by R. Benassi (2011)

Benassi (2011) provides a valuable contribution and much needed data (gravity measurements, edge enhancement of Bouger anomalies and 2.5D forward modelling of geologically realistic profiles) regarding the problem of the geodynamic evolution of Jebel Ech Cheid salt body. However, when we viewed the previously published and the new gathered data, we believe the evidence better supports folded sheets of allochthonous salt rather than diapiric walls. For elucidating the geodynamic evolution model of Jebel Ech Cheid salt body (Fig. 1), R. Benassi (2011) supposed an unfractured substratum (Fig.7 of R. Benassi (2011)) which is completely different of the previous interpretations (Ben Slama et al. 2008; Ben Slama et al. 2010; Ben Slama 2011). This structural problem has been a subject of discussion and heated debate between different research groups in the neighbouring salt structures of Jebel Ech Cheid (Jallouli et al. 2005; Talbot 2005). This discussion focuses on the structural and the gravimetric responses of the Jebel Ech Cheid salt body published by R. Benassi (2011). Taking into account many facts (rock densities, geological data, etc.), we believe that in his interpretation R. Benassi (2011) has completely ignored the average rocks densities published by Japan and Metal mining (2001) which can greatly influence the gravimetric responses and consequently the interpretation (Table 1). The laboratory test published by Japan and Metal mining (2001, p.43) and resulting from several samples within the Jebel Ech Cheid salt structure for the Quaternary, Tertiary, Cretaceous and Triassic deposits. The problem is that R. Benassi (2011) has used an average density of the Cretaceous series (2.31 g/cm), which is very close to the average density of the Triassic material (2.32 g/cm) deduced from the values presented in Japan and Metal mining (2001) report. While the gravity map shows (Fig. 2) the variation of the gradient density between the Triassic complex and enclosing series is net (Fig. 2). In addition, the values of average densities of Jurassic, Cretaceous and Tertiary have almost the same values of mean density (2.57 g/cm) which implies a net contrast density between Jurassic, Cretaceous and Tertiary series and Triassic density which is illustrated in the gravimetric map (Fig. 2). Nevertheless, in our analysis we discovered an important dissolved Halite mass coming from the principal salt structure (Ben Slama 2011) (Fig. 3). The salt material of Jebel Ech Cheid has a low-density, probably originated from a shallow level (≃100 m) such as in other salt bodies of Northern Tunisia, also we pointed out the existence of many salt springs around the salt structure which indicate the low density of Triassic complex (Vila et al. 1999, 2002; Ghanmi et al. 2001, M.-M. Ben Slama (*) :M. Ghanmi : F. Zargouni U.R. 11 E.S. 13, Geomatique, Geologie structurale et Appliquee, Faculte des Sciences de Tunis, 2092 Tunis, Tunisia e-mail: mohamedmontassar.benslama@fst.rnu.tn

The structural style of the Southern Atlassic foreland in Northern Chotts Range in Tunisia: field data from Bir Oum Ali Structure

We used field data collected from the Bir Oum Ali structure (BOAS) and take into consideration recent published geophysical works to evaluate the deformation style and the structural evolution of the Southern Atlassic foreland in Northern Chotts Range in Tunisia. Various structural interpretations have been proposed for the genesis of the Northern Chotts Range. The BOAS was interpreted previously as (1) curved, right-stepping “en echelon” geometry folds as the result of NW-trending lateral strike-slip faulting (2), an anticline structure developed and the result of Ramp-related folding deformed the sedimentary cover (thin-skinned model) over the rigid basement during a single tectonic event (3), response of salt tectonic that began in Early Jurassic and allows the migration to nucleated folding during tertiary compressional events. New field data of the BOAS show a deformation style, in which shortening is differently accommodated in the eastern, southern, and northern areas. Data highlight a tight folding and steeply to overturned strata related to the N100–110° E-trending Bir Oum Ali–Hachichina fault systems (BOAHFS). This later exhibits fault kinematics, with striation showing a multiphase history. The second-order fault systems associated to the main trend of the BOAHFS are usually apparent strike-slip faults. Unfolding of the structure shows an inherited normal faulting. In addition, the normal faulting generates syntectonic conglomerates associated to the first normal faulting movement. The E-trending fold-related fault reactivation of the E-trending Northern Chotts Range might result from the reactivation of inherited Mesozoic faulting. A new structural data interpretation give information of tectonic inversion starting in Late Cretaceous (Campanian–Maastrichtian transition?). The Late Cretaceous to present-day history is dominated by two major events, i.e., Eocene Atlassic contractional event and Middle–Late Miocene to present-day Alpine event. The folding style, the partial reverse–reactivation of the pre-existing E-trending BOAHFS major faults, and the abundant thick tertiary siliciclastic growth strata sequences together with the recent published geophysical data provide a coherent model, in which the thick-skinned tectonic style (development with basement inversion at the depth) is synchronous of thin-skinned tectonic (shallow decollement in the sedimentary cover).

Le paléomagnétisme est-il un meilleur outil que la biostratigraphie et la sédimentologie pour fixer la polarité de l'Albien du Nord-Ouest tunisien ?

Abstract The results of a recent palaeomagnetic study, which are considered as proof of the overturning of the Aptian-Albian beds under the supposed saliferous intrusive bodies from northwest Tunisia, are compared with the several incompatible cartographic, stratigraphic and sedimentological data. The geological interpretation of the palaeomagnetic measures supporting the hypothesis of the non-interbedding of the evaporites is discussed and arguments are given in order to demonstrate the lateness of certain magnetizations, borne by magnetites, which are regarded by us as post-diagenetic. The palaeomagnetic tool does not seem, in the studied areas (S of Debadib, Kt ed Dalaa, J. Slata), to support the proposed setting up processes (upturning, lateral injections under cover). The non-interbedding of the evaporites would not explain either the flat-topped initial geometry of the saliferous structures, or the wells, or the gravimetric Algerian and Tunisian data. On the contrary our interpretation of the saliferous bodies emplacement, within wide ‘salt glaciers’ is more convenient to integrate the various data previously obtained on this topic.

Biostratigraphy of upper Cretaceous through Paleocene successions in Grombalia, Tunisia (southern Tethyan domain)—reworking processes and interpretations

Revision of Late Cretaceous through Paleocene planktic foraminiferal zonal schemes for northeastern Tunisia (Khanguet El Hajjej and Sidi Rhilaine section) has provided new insights into the biostratigraphic framework of the Abiod Formation. A late Paleocene age is endowed with this unit based on the occurrence of late Cretaceous reworked microfauna associated with Morozovella velascoensis planktic foraminifera. The biostratigraphic equivalent, between the mayaroensis and velascoensis Zones near J. Barka, includes an occasional submarine reworking of sediments induced by mass flow transport, indicated by the presence of microfaunal content and heterogeneous extraclasts. The internal organization of sediment (olistoliths, mass flow deposits) reflects tectonic activity and clear evidence of redeposition during the late Paleocene.