Contribution of seismic velocity studies to structural and lithostratigraphic reconstructions : Salt-intruded corridor ceiling and Lower Turonian Beida anhydrite deposits' outline in the central-southern Atlas of Tunisia

The Triassic salt dome/ripple ceiling and the Lower Turonian anhydrite Beida distribution are the main geological research objectives in central Tunisia. 2D seismic interpretation and outcrop studies show the thickness and the facies variations of the Turonian deposits identified along the structures recognised in the Gafsa area. Sedimentary distribution in the sub-basin and on the platform zones of the Gafsa plain is controlled by the WNW-ESE salt-intruded Gafsa corridor, the north-south Sidi Ali Ben Aoun and NE-SW Sidi Aich master strike-slip faults. The conventional seismic velocity enables the extraction of an iso-velocity section that allows us to follow the Sehib-Berda salt-intruded ceiling identified in the Turonian deposits. The interval velocity map was helpful to predict the facies nature and, therefore, to delineate the Lower Turonian anhydrite Beida Formation that filled a local part of the northern Gafsa plain and does not exceed the Sidi Aich fault to the north and the Sidi Ali Ben Aoun fault to the southeast. The palaeogeographic distribution is the result of a broadly NE-SW extensional tectonic event. The Early Turonian distension is followed by the Middle and Upper Turonian sinistral transtension, causing an inversion of the structuring and a new distribution of the deposits.Original Abstract: Les etudes en affleurement et l'interpretation de la sismique 2D montrent des variations d'epaisseur et de facies des depots turoniens le long des structures de la region de Gafsa, dans l'Atlas centro-meridional. La distribution sedimentaire est controlee par les couloirs de failles NW-SE, nord-sud et NE-SW, associes aux intrusions triasiques. Les vitesses de sommation montrent que le toit des intrusions triasiques le long du couloir tectonique de Sehib-Berda se situe au sein des depots turoniens. La carte des vitesses d'intervalle permet de delimiter l'extension des anhydrites de la formation Beida au niveau de la plaine nord de Gafsa. La distribution paleogeographique est le resultat d'un evenement tectonique globalement extensif NE-SW. La distension du Turonien inferieur est suivie par une transtension senestre au Turonien moyen-superieur, qui entraine une inversion de la structuration et une nouvelle repartition des depots.