Scarp morphology and identification of large-scale mass movements in tropical tablelands: the eastern Araripe basin (Ceará, Brazil)

The article aims to recognize and analyse large-scale mass movement topography in the rim and piedmont of the eastern part of the Chapada (plateau) do Araripe, Ceara. So far ignored in the interior of northeast Brazil, this morphology was identified by the means of field work combined with the analysis of high resolution satellite images and GIS-based geomorphological mapping. The context is that of topographic inversion of a Mesozoic intracratonic basin. Exposed to the trade-winds, the escarpment bears an evergreen or semi-deciduous forest typical of a sub-humid tropical climate. A high sandstone ledge overlooks dissected pediments shaped into softer rocks within wide erosion amphitheatres open to NE. Abundant deposits rich in metre-scale blocks of sandstone are identified down to 6-8 km from the escarpment, 500 m below the summit surface. Between short segments scalloped by sapping processes, longer segments of the escarpment present smooth and concave slopes interpreted as landslide scars. Various types of mass movements are recognized and measured, ranging from slumps to composite types of landslides and debris flows of pluri-kilometric scale. Degraded landslide scars and pediment covers with weathered block deposits also indicate the involvement of large-scale mass movements in older stages of scarp evolution and retreat. The lithological and hydrogeological conditions encountered in the tabular structure of the inverted basin appear as determining factors in the occurrence of these landslides. Although neither historical testimony nor absolute ages are presently available, a relative chronology and possible correlations with the regional paleoclimatic evolution are proposed. Since these phenomena appear as usual processes in scarp evolution and retreat and may occur in a presently populated area, and since some of them involve catastrophic events, a preliminary approach of landslide-related hazards is proposed.