Scarp morphology and large-scale mass movements in tropical tablelands: the eastern Araripe basin (Ceará, Brazil)
Résumé
Large-scale mass movement topography is recognized in the rim and piedmont of the eastern part of the Chapada do Araripe, a high tabular plateau elongated EW at the border between the Ceará and Pernambuco states, northeast Brazil. So far ignored in the study area, 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 geomorphic context is that of topographic inversion of a Mesozoic intracratonic basin. Exposed to the trade-winds, the prow of the tableland 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. Various deposits rich in metre-scale blocks are identified down to 6-8 km from the escarpment, 500 m below the summit surface. Between short segments shaped in spurs and gullies 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 simple slumps to composite 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 hydrogeological conditions encountered in the tabular structure of the inverted basin, with aquicludes below the upper layer of deeply weathered sandstones, appear as a determining factor in the occurrence of these landslides. Although neither historical testimony nor absolute age are presently available, a relative chronology and possible correlations with the regional paleoclimatic evolution are proposed. Since some of these processes involve catastrophic mass movements, a preliminary approach of landslide-related hazards is proposed.