Upper Triassic rocks in the Argana Valley of southern Morocco consist of 2,500 to 5,000 m of coarse to fine-grained red-brown clastic deposits. A succession of eight lithofacies consists essentially of a lower coarse stream-laid deposit derived from nearby uplands, a middle lacustrine and deltaic complex, and an upper aggradational mud plain that passed westward into an extensive salt flat. Variations in thickness and lateral continuity of the sedimentary units are attributed to a rather complex relation between sedimentation and differential movement of basement horsts and grabens during basin development.

Upper Triassic deposits of the Argana Valley are an early sedimentary response to the initiation of extension in the area of the western High Atlas. The valley sequence is a proximal facies that was dispersed westward along the axis of the West Atlas basin. Through a series of west-trending fault blocks within the basin, basement was faulted down with increasing displacement from the Moroccan Meseta in the north toward the older African craton in the south. The early Mesozoic structural configuration of the West Atlas basin was that of a large half-graben with maximum vertical displacement along the boundary between the Variscan orogene and the African craton. No reliable evidence, onshore or offshore, supports an interpretation of down-to-coast basement faulting along NNE-SSW structu al trends in this part of southwestern Morocco.