Pliocene–Quaternary fluvial and aeolian records in the Souss Basin, southwest Morocco: A geomorphological model

The Souss Basin in SW Morocco is filled by Pliocene–Quaternary fluvial, fluvio-lacustrine and aeolian sediments, representing an excellent archive of palaeohydrology, palaeoclimate and the effects of crustal deformation. In general these sediments indicate stream-dominated alluvial systems, influenced by fluctuations in climate (humidity/aridity). Lakes developed within the basin around the Pliocene–Pleistocene transition and persisted into the Early Pleistocene. During this early period, relatively humid conditions are indicated by the dominance of coarse-grained sedimentation in the upper reaches of fluvial systems, the existence of large lakes and the considerable sediment thicknesses in the centre of the basin. Uplift of the surrounding mountain ranges contributed to piedmont formation by providing large amounts of coarse-grained material that accumulated at the lowland margin. Climatic deterioration in the Middle Pleistocene was accompanied by progressively more irregular and disrupted fluvial regimes. These trends were evident in the Late Pleistocene and became clearer after the mid-Holocene, with aeolian activity becoming the dominant sedimentary agent. Differences between upstream and downstream depositional regimes became marked: while coarse-grained sedimentation has characterized the upper reaches of wadi catchments, fine-grained sedimentation has prevailed downstream. Hiatuses in sedimentation throughout the Pliocene and Quaternary are marked by palaeosol horizons interbedded within the sedimentary sequences, indicating alternate vegetated (stable) and unvegetated (unstable/active) phases (biostasy and ‘rhexistasy’).