Above the essentially clay-sandy "Continental Intercalaire" series lies a 500-meter-thick carbonate layer, dating from the Upper Albian to the Campanian. This forms the backbone of the Dahar plateau and represents the last marine sediments deposited on the Saharan platform before its final emergence. These carbonate deposits, largely transgressive over the underlying Mesozoic and Paleozoic series, materialize the widest expansion of the Tethys.
Within the Upper Cretaceous carbonates, two lithological groups are distinguished by their stratigraphic position and geomorphological role in the landscape of Southern Tunisia:
- the first group at the base, from the Upper Albian to Turonian age, consists of two massive dolomite bars, about forty meters each, separated by intermediate alternations of chalky dolomites, marls, and gypsum with an average thickness of around 150 meters;
- the second group, of Senonian age (Coniacian to Campanian), is more homogeneous and consists of a significant layer (about 250 m) of recrystallized fossiliferous limestone, organized in highly indurated jointed beds.
From the surroundings of Toujène to the far South of Tunisia, unlike the Matmata region to the north, it is the carbonate layers of the first Upper Cretaceous group (Upper Albian to Turonian) that border the Jebel Dahar plateau. Forming a large escarpment, this carbonate group is topped by the massive dolomite bar of the basal Turonian and forms a powerful cliff dominating the Jeffara plain.
On the reverse side of the great Plateau, the Senonian limestones of the upper group, more exposed to erosion, occupy a much more recessed position to the west and are only represented on the plateau's edge by small isolated massifs and mesas (buttes-témoins).
The waters of the lower Complexe Terminal (CT) aquifer are housed within these Upper Cretaceous carbonates.
Paleogeography and Paleo-environments
With the Upper Cretaceous carbonate deposits, the Tethys reached its maximum extension and recorded its final sedimentary archives before its definitive retreat from the Saharan platform following a significant drop in relative sea level.
These deposits show several signs of syn-sedimentary tectonic activity, testifying to the role of tectonics in this long marine transgression. The great thickness of the Upper Cretaceous carbonates and the numerous levels of bioconstructed rudist limestones intercalated within indicate tropical waters and optimal conditions for carbonate production.
