Erosion around a large-scale topographic high in a semi-arid sedimentary basin: Interactions between fluvial erosion, aeolian erosion and aeolian transport

Large-scale aeolian erosion features such as pans usually develop upon a relatively homogeneous erodible sedimentary rock substratum and show simple concave morphologies. However, some erosional depressions in semi-arid to arid settings develop in more complex geological contexts, associating sedimentary basins and outcrops of non-erodible basement. In semi-arid basins, the geomorphic evolution is controlled by interplay between the processes of fluvial erosion and deposition (especially of the finer sediment fraction in ephemeral lakes) and the processes of aeolian erosion and deflation. The occurrence of non-erodible basement outcrops within a region of overall deflation complicates drainage systems and wind-flow patterns. Both are liable to evolve rapidly through time and their interactions remain poorly understood. In this study, we use a detailed geomorphological analysis, including mapping of the aeolian sand-motion direction within the 16,300 km(2) Tsagaan Els depression (Gobi Desert, Mongolia) as well as in situ-produced cosmogenic Be-10 and Al-26 based quantitative constraints, to propose a conceptual model for the morphologic evolution of geologically complex semi-arid basins. Based on theoretical studies of wind-flow patterns around obstacles, we demonstrate that the occurrence of erosion-resistant basement highs inside the basin induces the localization of preferential deflation zones and aeolian and fluvial carving of large-scale relief. The enhanced deflation zones form sub-depocenters controlling local drainage systems that mobilize clastic material and transport it to the deflation zone. These river systems are frequently captured by neighboring streams, which creates unstable conditions of erosion and deposition. Such rapid modifications of the topography through time affect the hydrogeologic structure of the basin and may impact potential sediment-hosted mineral deposits. (C) 2021 Elsevier B.V. All rights reserved.