During the late Triassic and early Jurassic a shallow sea transgressed westwards along the High Atlas Trough. At the western extremity of this trough moderate thicknesses of carbonate tidal flat and shallow subtidal sediments accumulated. Analysis of these deposits has indicated a complex mosaic of facies analagous to Holocene sediments in the Persian Gulf and Shark Bay, Western Australia, and has allowed the preparation of paleogeographic and paleosalinity reconstructions which show a complex of inlets in which salinities became restricted. Sedimentation in the trough was controlled by a series of ancient faults trending ENE-WSW which moved in response to stresses created by plate movements in the Atlantic and Tethyan Oceans. Generally arid climatic conditions increased salinities in intertidal areas and restricted faunal diversity. Sabkha conditions prevailed onshore, while algal mats and large bivalves flourished in intertidal areas. Limited coral, oolite and skeletal grainstones interfingered with mud in the subtidal areas, and in the Upper Pliensbachian Opisoma sp. formed lensoid reefs.