Rezente Abtragung und geomorphologische Bedingungen im Becken von Quarzazate (Süd-Marokko)
Arbeit im Forschungsprojekt Mobilität aktiver Kontinentalränder
Inst. für Physische Geographie, FU, Berlin
Other online version: http://resolver.sub.uni-goettingen.de/purl?PPN1020553693
Wagner, Peter, 1984: Rezente Abtragung und geomorphologische Bedingungen im Becken von Quarzazate (Süd-Marokko) - Arbeit im Forschungsprojekt Mobilität aktiver Kontinentalränder. Berliner geographische Abhandlungen, 38, 126 S., DOI: 10.23689/fidgeo-2897.
[contains map: Ouarzazate – Hydrographie und Stufen] ; [contains map: Ouarzazate - Oberflächenerodierbarkeit] [contains geomorphological maps] ; Summary: Investigations into current erosional processes and their geomorphological boundary factors in the Ouarzazate basin show the significant inﬂuence here of local relief on the course and intensity of relief-modifying processes. The bedrock’s resistance to weathering and erosion is also important. A limiting factor is the scarcity of water due to the rare (geomorphodynamically effective) heavy rainstorms. The influence of the extremely sparse vegetation as a controlling factor in the erosional process is negligible. There is relatively little erosional activity on the gently inclined plains. The existing rills and rill wash do not destroy the plains. On the basis of simulation runs the surfaces of the individual geological units were classified in the order given below (starting with the surface most prone to erosion): (10) loamy bottom lower terrace (Early Holocene) ; (9) lowest glacis level q1 (Late Pleistocene) ; (8) Mio-Pliocene marl surfaces and top glacis levels q6 (Oldest Pleistocene) ; (7) youngest terrace sediments (Late Holocene) ; (6) areas of Mio-Pliocene sandstones ; (5) Weathering surfaces on Pre-Cambrian Anti-Atlas-Rocks ; (4) middle glacis level q2 (Mid-Pleistocene) ; (3) middle glacis level q3 (Mid-Pleistocene) ; (2) upper (Oldest-Pleistocene) glacis level q5; structural and erosion surfaces in Mio-Pliocene conglomerates ; (1) higher glacis levels q4 (Early Pleistocene) ; Aeolian processes on the plains are prevented by the presence of boulder pavements and the effect of the surface crust. The cuestas in the Ouarzazate basin consist of a caprock of Mio—Pliocene conglomerates (mpc) overlying a basement of Tertiary sandstone and marl and are presently subject to increased geodynamic activity. Even under prevailing arid conditions weathering and transportation of the conglomerate’s debris down the steep slopes may result in undercutting of the resistant caprock. The calcareous blocks on conglomerate break off along joints, are undercut on the slopes and moved downwards by the force of gravity. They are weathered by both mechanical and chemical processes. In the wadis intensiﬁed geomorphodynamic processes caused by an increase in water supply currently lead to erosion of the plains following downcutting of the river bed and the resulting increased undercutting of the banks. Lateral erosion widens the wadi beds. During arid periods aeolian activity is facilitated by the mobile sandy sediments of the wadis. Owing to their erosion-prone surfaces and subsoils and their proximity to the wadis the loamy bottom lower terraces are subject to increased linear erosion which also affects nearby agricultral land during the rare ﬂood phases. In the Ouarzazate basin linear erosion has led to the formation of badlands only in the area west of Skoura, owing to the inﬂuence of the pre-Miocene relief of Anti—Atlas rock which channels runoff into the river Dades. These coexisting relief forms and elements are evidence that processes of preservation (on glacis), erosion (by wadis) and extension of plains (due to cuesta erosion) occur simultaneously and are governed by the climatically controlled availability of water. Knowledge of the regional variations in the wadis and the Tertiary conglomerate scarps of the Ouarzazate basin makes it possible to assess current relief development. East of Assif Toundoute the relief is relatively well preserved. To the west the fluvial network is denser and the Tertiary conglomerate scarps are more frequent; therefore relief modiﬁcation tends to be more effective. First conclusions about the current mass balance of the Ouarzazate basin were drawn from data recorded at the Mansour Eddahbi reservoir. The amount of sedimentation since the construction of the reservoir indicates a rate of erosion of max. 310 m3/km2/yr or 0.3 mm/yr. This corresponds to a longterm rate of erosion of 300 Bubnoff and is distinctly higher than the assumed rate of max. 200 Bubnoff for continental-temperate climatic zones.