Controls on dryland mountain landscape development along the NW Saharan desert margin: Insights from Quaternary river terrace sequences (Dadès River, south-central High Atlas, Morocco)

Abstract

This study documents river terraces from upstream reaches of the Dad es River, a major fluvial system draining the south-central High Atlas Mountains. Terraces occur as straths with bedrock bases positioned at 10 m altitudinal intervals up to 40 m (T1-T5) above the valley floor, becoming less common between 50 and 140 m. The rock strength, stratigraphy and structure of the mountain belt influences terrace distribution. Terraces are absent in river gorges of structurally thickened limestone; whilst welldeveloped, laterally continuous terraces (T1-T4) form along wide valleys occupying syncline structures dominated by weaker interbedded limestone-mudstone. Terrace staircases develop in confined canyons associated with weaker lithologies and influence from structural dip and stratigraphic configuration. Terraces comprise a bedrock erosion surface overlain by fluvial conglomerates, rare overbank sands and colluvium. This sequence with some OSL/IRSL age control, suggests terrace formation over a 100 ka climate cycle with valley floor aggradation during full glacials and incision during glacial-interglacial transitions. This integrates with other archives (e.g. lakes, glaciers, dunes), appearing typical of landscape development along the NW Saharan margin south of the High Atlas, and similar to patterns in the western-southern Mediterranean. The 100 ka climate cycle relationship suggests that the terrace sequence documents Late-Middle Pleistocene landscape development. Consistent altitudinal spacing of terraces and their distribution throughout the orogen suggests sustained base-level lowering linked to uplift-exhumation of the High Atlas. Low incision rates (<0.2 mm a À1) and general absence of terrace deformation suggests dominance of isostatically driven base-level lowering with relief generation being Early Pleistocene or older.

Supercritical river terraces generated by hydraulic and geomorphic interactions

“…The influence of tectonics in generating strath terraces is usually limited to the consideration to the initiation of headward migrating incision waves (e.g., Finnegan 2013), while the impact of regional landscape tilting is rarely considered. A clear identification of the mechanism of strath terrace formation is important for the interpretation of the presence and pattern of strath terraces within a landscape to date past climate changes (e.g., Fuller et al, 2009;Stokes et al, 2017), estimate fluvial incision rates (e.g., Lavé and Avouac, 2001), or to help calibrate empirical models of fluvial erosion (Finnegan, 2013). However, a universal model of strath terrace formation is currently limited by an incomplete understanding of the mechanics of vertical and lateral erosion in bedrock and difficulties in identifying the erosional pattern during the response of channel processes to perturbations in Qs/Qsc from preserved strath terrace age distributions.…”

“…The first two experiments explore the response of a channel following a sudden perturbation to Qs/Qsc from the same initial conditions while the third explores the impact of a gradual decrease in Qs/Qsc, through progressive tilting of the experiment. The experiments identify patterns of erosional response including the resulting preserved terrace morphology, aiding the interpretation of terrace age distributions in natural landscapes for purposes such as the reconstruction of climate or tectonic histories (e.g., Fuller et al, 2009;Stokes et al, 2017)…”

Tectonic Evolution of the Western High Atlas of Morocco: Oblique Convergence, Reactivation, and Transpression

“…The northern front of the WHA (F‐VIII) crosscuts Quaternary river terraces, indicating recent deformation (Figure ). Evidence of recent deformation along the South Atlas fault (F‐V) is constrained by Holocene deposits and deformation (Leprêtre et al, ; Pastor et al, ; Teixell et al, ) and by the presence of a river knickpoint (Boulton et al, ; Stokes et al, ). This finding is also confirmed by tilted Quaternary terraces along the northern rim of the Ouarzazate basin (Figure S8 in the supporting information).…”

Rock strength and structural controls on fluvial erodibility: implications for drainage divide mobility in a collisional mountain belt

“…The High Atlas is in a post-orogenic state, with long-term isostatic rock uplift rates of 0.17-0.22 mm yr -1 since 15 Ma related to lithospheric thinning (Babault et al, 2008). The lack of recent tectonic deformation is apparent in the undeformed continuous Quaternary river terraces forming parallel river long profiles throughout the fold-thrust belt and thrust front (Stokes et al, 2017).…”

“…Thus, for our purposes the main control on the Plio-Quaternary evolution of the High Atlas river network is the incision through the inherited tectonic architecture of lithological units and their contrasting strengths. Drainage development in the High Atlas is considered to be primarily the product of the exhumation of structurally distributed lithologies with different hardness, controlling where river terraces develop (Stokes et al, 2017) and affecting the occurrence of diffusive and advective slopes (Mather and Stokes, 2018). The High Atlas is set in a semi-arid climate, which means that the effect of weathering on rock erodibility is expected to be low.…”