Colluvial sedimentation in a hyperarid setting (Atacama Desert, northern Chile): Geomorphic controls and stratigraphic facies variability DARIO VENTRA*, GUILLERMO CHONG D  IAZ † and POPPE L. DE BOER* *Faculty of Geosciences, Utrecht University, Budapestlaan 4, 3584CD, Utrecht, The Netherlands (E-mail: d.ventra@uu.nl) † Departamento de Ciencias Geol  ogicas, Universidad Cat olica del Norte, Avenida Angamos 0610, Antofagasta, Chile ABSTRACT Research on colluvial depositional systems has recently emphasized peri- glacial and high-altitude settings, and the relations between Quaternary slope stratigraphy and climate change. This article examines the role of vari- able slope morphology, surface hydrology and microclimate in controlling colluvial sedimentation along a coastal tract of the hyperarid Atacama Desert in northern Chile. Direct accessibility of active surfaces is accompanied by uninterrupted stratigraphic exposures along the base of slopes, allowing direct comparisons between surface processes and the resulting sedimentary record. Four slope sectors are identified, based on differences in morphology and processes over active surfaces. Colluvial sedimentation is controlled by complex interactions of slope gradients and profiles, exposure to dominant winds, and potential runoff pathways, which vary considerably between different sectors. Major differences are evident between these hyperarid deposits and slope sedimentation in periglacial and temperate settings, including the complete absence of pedogenic activity and clay minerals; the volume of aeolian deposits and their role in controlling processes which redistribute sediment downslope, extending colluvial aprons; and the occurrence of runoff processes only where favoured by particular topo- graphic configurations. Depositional surfaces range from steep talus cones, to debris-flow-dominated and aeolian-dominated colluvial aprons, to an aeolian ramp subject to reworking by mass flows and flash floods. Consequently, facies associations and architectures at outcrop are highly variable and high- light the importance of spatial variations in slope morphology and processes in producing distinct, coeval colluvial stratigraphies within a single environ- mental context. Discrepancies between active processes and the correspond- ing stratigraphic signatures are also evident in some sectors; for example, preservation of alluvial and aeolian facies in stratigraphic sections does not always reflect the dominant processes over active slopes. Together with the spatial variability in processes and deposits along these slopes, this suggests that caution is required when extracting palaeoenvironmental information from analyses of colluvial successions. Keywords Aeolian ramp, alluvial fan, Atacama Desert, colluvium, slope morphology, slope sediments, talus. © 2013 The Authors. Journal compilation © 2013 International Association of Sedimentologists 1257 Sedimentology (2013) 60, 1257–1290 doi: 10.1111/sed.12029