Late Cenozoic and present-day hillslope erosion dynamics in a passive margin context: stability or instability? Case studies in Northeast Brazil
Résumé
Steep slopes form the marginal scarp of the semi-arid northern Brazilian “Nordeste”, above depressions or corridors connected to a low erosion plain merging seaward with a coastal erosional-gradational piedmont. Dissected pediments and sparse debris fans visible at the base of some escarpments suggest recent erosive activity and possible slope retreat, although most of these forms are decoupled from major valleys. We describe the morphostructural patterns of chosen escarpments, in the basement as well as in the sedimentary cover, and the morphodynamic conditions of their evolution since the Miocene. Only a few scarps show strong structural control. Many of them are mainly inherited landforms, initiated during the Early Cretaceous rifting or the later margin uplift, a situation which suggests long-term stability. Later on, except in one case (the Araripe scarp), only slight or local backwearing took place, associated with downwearing on low surfaces and pediments, probably in diachronic ways. The Neogene clastic sedimentation on piedmonts and coastal areas mainly reflects the occurrence of dry periods inducing widespread stripping of deep soil horizons and erosion of bare rock slopes and surfaces. Dissection stages occurred in periods of more humid climate and/or low sea level. Marks of strong recent of present activity are mainly registered in the rims of the Chapada do Araripe, owing to favourable structural, hydrogeological and climatic conditions. The moderate volumes of Neogene clastic sediments imply overall low uplift and erosion rates until the Present, favourable to morphological and lithological resistance effects in the landscapes. However, slope instabilities are not uncommon, locally leading to well characterized processes, landforms and deposits of gullying and mass wasting. Therefore, hazards related with slope processes never may be neglected, although only small scale events were recorded in recent times, and times of recurrence of larger events are probably much longer than the historic times.