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In northwest Africa, Triassic and Early Jurassic deposits accumulated in two different tectonic provinces, the African platform and the Variscan (Hercynian) orogene.
In southern Algeria and Tunisia, Lower, Middle, and Upper Triassic sediments, essentially devoid of volcanic materials, were deposited on the stable African platform south of the Saharan flexure (South Atlas fault zone). Here the basal Triassic beds lie on a post-Variscan unconformity which marks a progressively longer hiatus northward toward the orogene. The sequence consists mainly of nonmarine arenaceous and argillaceous evaporite facies, with intercalations of marine limestone and dolomite increasing toward the northeast. These deposits resemble correlative ones in western Europe. On the unstable eastern cratonic margin and adjacent southwestern corner of the Pelagian block (Djeffara Plains) Lower Triassic detrital paralic sedimentary rocks conformably overlie a thick succession o Permian marine strata.
Most of the Variscan province apparently was stable and emergent during Early and Middle Triassic times. Nevertheless, the mid-Triassic Tethyan transgression that spread a carbonate-evaporite mantle across central and northern Tunisia did encroach westward onto the eastern part of the orogene. Early Mesozoic extensional deformation of the Variscan domain produced differential vertical displacement along the Saharan flexure between the orogene and the African platform, as well as on the orogene along the central Altantic, Middle Atlas, and Gibraltar fracture zones. In the west, this deformation outlined the Moroccan and Oranian mesetas which remained relatively stable and were extensively eroded during early Mesozoic time. Red beds, evaporites, and basaltic flows filled the basins. Alt ough poorly dated by fossils, their linear pattern and discordant boundaries, and radiometric ages of the volcanic rocks, indicate that the basins were formed during the Late Triassic extensional phase that disrupted eastern North America.
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