Jean-Marie Vila

The evolution of the Lansarine–Baouala salt canopy in the North African Cretaceous passive margin in Tunisia

Detailed geological mapping, dating, and gravimetric and seismic data are used to interpret the Lansarine–Baouala salt structure (North Tunisia) as a salt canopy emplaced during the Cretaceous Period. The extensional tectonic regime related to the Cretaceous continental margin offered at least two factors that encouraged buried Triassic salt to extrude onto the sea floor and flow downslope: (i) extension induced normal faults that provided routes to the surface, and led to the formation of sub-marine slopes along which salt could flow; (ii) this structural setting led to differential sedimentation and consequently differential loading as a mechanism for salt movement. The present 40-km-long Lansarine–Baouala salt structure with its unique mass of allochthonous Triassic salt at surface was fed from at least four stems. The salt structure is recognized as one of the few examples worldwide of a subaerial salt canopy due to the coalescence of submarine sheets of Triassic salt extruded in Cretaceous times.

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.

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.

Reconstitution du substratum jurassique-crétacé basal du domaine des Sellaoua (marge sud-téthysienne, Algérie nord-orientale) à partir des galets du bassin mio-pliocène d’Hamman N’Baïls

Abstract The Mio-Pliocene series filling up the intermontane basin of Hammam N’Bails (Guelma region, NE Algeria) corresponds to a fluviatile sequence beginning by polygenic conglomerates that contain several Mesozoic carbonate pebbles. The inventory and the micropalaeontological study of these pebbles attest the occurrence, at the base of the Sellaoua unit (southern Tellian foreland of Maghrebides), of a Jurassic–Berriasian carbonate-dominated series, unknown in outcrops, showing some affinities with the synchronous North-Atlasic Algerian and Tunisian ones respectively to the W-SW and to the E-NE. This series was deposited on the South-Tethyan margin, to the south of the ‘Tethyan trough’ joining the incipient Atlantic Ocean and the Ligurian Ocean. The Upper Miocene and Pliocene extensional tectonics and the halokinetic motions, induced by the presence of large Triassic evaporites masses, have probably uplifted towards the surface this Mesozoic Sellaoua-type material, which, after erosion, has supplied the herein studied fluviatile conglomerates.