Timing and nature of Quaternary fluvial incision in the Ouarzazate foreland basin, Morocco

The history of alluvial fan and terrace formation within a stretch of the Ouarzazate basin along the southern margin of the Central High Atlas is reconstructed using geomorphological and Be-10 terrestrial cosmogenic nuclide (TCN) methods. Alluvial fan and terrace incision was controlled partially by a drop in base level during the Pliocene or early Pleistocene as the outlet channel, the Draa river, progressively cut through the Anti-Atlas to the south of the Ouarzazate foreland basin, the drainage of which started to become external after a long period of internal drained conditions. The alluvial fans and terrace surfaces have abandonment ages that date to at least the past four glacial cycles. Their formation was strongly modulated by climate on glacial-interglacial time scales as base level dropped. This demonstrates a strong climatic control on sediment transfer in other intracontinental mountain belts. Furthermore, these data show that mean rates of fluvial incision in this region range between 0.3 and 1.0 mm a(-1) for the latter part of the Quaternary. This study provides the first comprehensive TCN chronology for the Atlas Mountains, and it illustrates the applicability and limitations of TCN methods.