Sedimentology of Quaternary calcareous tufas from Gafsa,southwestern Tunisia

Well-preserved tufa deposits exhibit a diversity of field fabrics along the major structural feature in Gafsa area and extent between Jebel El Mida in the southeast and Sidi Ahmed Zarrouk and Jebel Ben Younes in northwest. The deposition history of the tufa is defined by three major stages; the first occurred in a perched springline or slope system with dominance of the autochthonous facies (stromatolitic tufa facies, i.e. boundstone sheets of micrite and peloids, and phytoherm framestone facies) in the proximal zone and microdetrital tufa in distal sites. The second stage occurred in a mixture of perched springline and poorly drained paludal environments that characterized by the abundance of allochems and chalk and marl. The third deposition stage occurred in a mixture of paludal and braided fluviatile environments with allochthonous tufa facies (oncoidal cyanolith tufa facies, phytoclast tufa facies and lithoclast and intraclast tufa facies). Tufas are of particular interest since their occurrences are linked to tectonic activity and extensional fault systems. Gafsa strike-slip fault, in addition to its tectonic role in creating fluid paths to the surface through flowing springs, it acts as a major regional sill that controlled paleoflow directions, discharge locations, volume, rate and fluctuations of the water supply. Tufa cessation could be explained by increasing aridity during late Holocene and subsequent reduced rainfall, increased evaporation and water table falling, which reduce the amount of recharge and its subsequent dissolution in the recharge area.