THE TECTONIC HISTORY OF THE PRE-SAHARAN DEPRESSION (MOROCCO) - A GEOMORPHOLOGICAL INTERPRETATION

During the Cretaceous and Tertiary periods the Pre-Saharan depression between Ouarzazate and Errachidia was an area of deposition in a spati ally and chronologically highly differentiated pattern. The variable s tructural and tectonic history of this region in also reflected in its denudational development. Until the late Pliocene/early Pleistocene t he western part (Ouarzazate basin) was a depositional basin for lacust rine and alluvial sediments. Pediment formation did not start before t he Pleistocene period. In the central part of the depression (between Boumalne and Tinerhir) denudational activity on cuesta scarps started already in the late Miocene to early Pliocene as can be deduced from a new dating of the Foum el Kous volcano (2.9 my) and from Djebl Sarhro gravels on the crest of the Paleogene scarp. In the easternmost part of the depression (between Goulmima and Errachidia) there are no Neoge ne sediments, and the calculations of rates of scarp retreat demonstra te that scarp backwearing must have begun in the late Eocene. Thus the shift from depositional to erosional activity in the Pre-Saharan depr ession ranges from the late Eocene to the Pleistocene. This is at the same time an expression of its complex tectonic history.