A FURTHER GEOLOGIC STUDY ON THE TRIASSIC FORMATIONS OF NORTH‐CENTRAL ALGERIA WITH SPECIAL EMPHASIS ON HALOKINESIS

Up‐to‐date lithostratigraphic classifications of the Triassic detritals of the Upper Triassic and Lower Jurassic evaporitic series have been made in North‐Central Algeria. It is believed that the formation of the Lower Triassic fluvio‐deltaic deposits and the principal Upper Trias‐Lias evaporites may be explained in terms of plate tectonics. Correlation of the Triassic formations were found possible between the two different tectonic units known in Algeria. The Epihercynian and Pre‐cambrian platforms attained their main tectonic features after the Early Alpine “Post‐Liassic” orogenic movements; whereas the successive southerly migrations of the Equator during Later Triassic time led to the formation of several lagoons. To the E of the High Plateaux, a NE‐SW lagoonal basin probably existed in the central part of which salt accumulated, overlain by shales and anhydrites, whereasat the edges only anhydrites were deposited. In the NW of the Algerian Sahara‐studied area, open Tethys sea sedimentation partly prevailed and carbonate rocks were deposited during Late Triassic. Triassic‐Liassic diapirs of different ages of emplacement are known in N Algeria and are represented on the surface by saliferous shales and other associated rocks mainly produced by halotectonic processes. Precise photogeologic and seismic studies may reveal the presence of salt‐dome structures in areas of halokinetic piercing diapirs, as well as in other areas away from the direct effects of regional tectonics. No Triassic‐Liassic outcrops or diapirs are known on the Saharan platform, the Triassic basins having developed on the Paleozoic craton.