The Arabic expressions for salt (melah) and gypsum (zebs), frequently used in local geographic descriptions, draw attention to the widespread occurrence of evaporites in North Africa. The principal occurrences consist of variegated red clay containing gypsum and salt alternating with limestone, dolomite, breccia, and cellular dolomite. The evaporite section is predominantly Triassic, but its base may be Permian, and its top frequently extends into the Liassic.

The evaporite-bearing sediments often occur as homogeneous masses, lacking any stratigraphic sequence, owing to deformation by plastic flow. A great variety of structural conditions has been noted, including: (1) undisturbed thick stratified evaporites (Saharian shelf in southern Tunisia); (2) salt-bearing laminated beds in the hinge area of important faults and flexures (Nara Bou-Dzer fault in Tunisia); (3) anticlinal features, both gentle domes and narrow anticlines, that have been pierced by a diapiric core of evaporites containing coarse breccia and basalts (Tunisian trough and the Saharian Atlas); (4) clay with intercalated salt and gypsum underlying nappes and thrust sheets, for which the material provided a plastic gliding horizon; and (5) genuine diapirs, with the salt-bearing core injected as a result of gravitational instability owing to the presence of overlying denser sediments (Miocene Cheliff basin in Algeria; Mesozoic Haha trough in Morocco).

All but the first (and perhaps last) include phenomena which obviously are related to orogenic deformation by compressive forces; but at least part of the deformation in most cases resulted from gravitational instability and is hence truly diapiric. Thus there appears to be an interaction of diapiric (gravitational) and orogenic structural forces, which probably is characteristic of orogenic areas.

Although the Tertiary orogeny had a direct effect on the structure of the evaporites in most places, diapiric movement probably had started by late Cretaceous time. The Tertiary orogeny pressed out some of the diapiric cores and injected the material along overthrusts and faults.