Lithological controls on the timing of strath terrace staircase formation in a collisional mountain belt

Abstract

In mountain belts, strath terrace staircases serve as markers for deriving river incision rates and erosional patterns. Distinguishing between terrace patterns influenced by external perturbations like changes in climate and tectonics and those driven by internal dynamics including feedbacks between topography, erosion and sediment transport remains challenging. We demonstrate that in a collisional mountain belt, lithology can act as a first-order control on the spatial and temporal scales of strath terrace formation. Here, we investigate the role of lithology in modulating internal dynamics and the formation of strath terraces in the Mgoun River catchment of the High Atlas in Morocco, a region characterised by constant low-rate rock uplift, a cyclical cool-warm/arid-humid Quaternary climate history and contrasting bedrock lithologies. By collecting (1) modern river and terrace clast data, (2) bedrock strath and strath-top sediment elevations of four terrace levels, (3) terrace sedimentology and (4) integration with published terrace chronology, we found a dominance of local sediment input from hillslopes, mostly from recycled bedrock conglomerates. Additionally, we found valley width, controlled by the stratigraphic and structural configuration of lithological erodibility, significantly impacts sediment connectivity. The isolation between valleys with varying widths results in varied timescales of river channel response to hillslope coupling, with hillslope-derived stochastic sediment gravity flows preserved in fluvial terraces in some river reaches and not in others. Furthermore, asynchronous terrace formation and abandonment ages result from the low longitudinal river connectivity between multiple valleys formed in erodible rock separated by gorges in high-strength rock. These gorges limit knickpoint migration rates, inhibiting the ability of terraces formed in one valley to spread through the catchment. These findings can inform future research distinguishing between autogenic and external signals in erosional landscapes and help carefully derive river incision rates and climate insights from terraces.