Triassic-Liassic Deposits of Morocco and Eastern North America: Comparison

Abstract

Late Triassic to middle Liassic (Early Jurassic) basin deposits in Morocco are mainly red-bed-evaporite sequences; a few in the Atlas trends in the southwest are thick detrital fill. Essentially coeval deposits in eastern North America comprise detrital sequences along the inner Alleghenian-Acadian belt, and evaporites on the outer continental margin. In Late Triassic time the two continental terrains lay north of the paleoequator, on opposite flanks of a broad, fracturing cratonic arch. Faulting and detrital basin filling began earlier, in Carnian time, along the Alleghenian and Acadian orogenes and the east-trending Cornwall-South Atlas fracture zone. In southern United States these older deposits record a warm, rather moist climate that induced deposits of coal. Later fracturing and graben development along the central Atlantic trend and on the Moroccan Meseta and adjacent African platform were accompanied by westward spread of Tethys in the Glosscap-Gibraltar fracture zone. The seaway leaked into northern marginal salt basins in Morocco and Atlantic Canada as well as down the central Atlantic belt from which it seeped laterally into marginal Atlantic grabens. During this latest Triassic-early Liassic episode the climate became hotter and drier throughout northwestern Morocco and much of eastern North America. Long after basin filling had begun, basaltic lava was extruded along old basin faults, but without disturbing the balance of basin subsidence and sedimentation. As preserved, the lava is much more widespread in Morocco, thicker in basins, and very thick along landward segments of fracture zones. By middle Liassic time a vast expanse of brine basins and mudflats covered the Tethyan and Atlantic margins of easternmost North America and northwestern Africa, as well as much of interior Morocco. But salt never accumulated along the belt of earlier fracturing on the Alleghenian and Acadian orogenes. In late Liassic time an open-marine flooding from Tethys, with a marginal-sabkha facies, spread westward into most of the Moroccan basins, into the central Atlantic rift, and into all the marginal salt basins in eastern North America, but it never reached the western basins on the Moroccan Meseta or the North American detrital basins. Unlike all of the other basins, the latter were broken up and intruded by cross-cutting dikes during an episode of renewed extensional deformation. This reconstruction of the North Atlantic basin implies that Late Triassic-Liassic salt accumulated largely on continental crust, before appreciable opening of the Mid-Atlantic rift, and that no salt accumulated in the North Atlantic realm after Liassic time.