The Chapada do Araripe (Northeast Brazil): shallow basin inversion, implications for regional uplift and denudation history
Résumé
At the border between the Ceará and Pernambuco states (northeast Brazil), the Chapada do Araripe is a high tabular plateau (900-1000 m a.s.l.) shaped into Cenomanian fluvial conglomerates and sandstones underlain by fossiliferous lacustrine or marine Albian layers. The Araripe basin lay at the time at a palaeoelevation close to palaeosea level. We analyse the mechanisms of relief inversion in the Araripe sedimentary basin in relation with local or regional crustal upwarp, in the light of original field and cartographic data. The contrast between the plateau – a near-structural surface – and the surrounding depressions is explained through a study of the erosion scarps that fringe the chapada above sedimentary depressions (east, southwest: segments with cuesta-like landforms and wide inland “peripheric” depressions; cuesta-like landforms and deep box-valleys without wide depressions) or basement surfaces (north, west, south: glints and outliers). No evidence of tectonic inversion is found. Sapping, landslides and other mass movements appear as common processes of relief inversion and scarp retreat, the rates and results of which appear to be controlled by sediment litholoy and paleolandforms of the rifted basement. Geometric relationships with regional stepped surfaces, in particular the low Sertaneja Surface, and exhumed paleosurfaces are analysed. Our estimation of the amplitude of relief inversion (0.6-0.7 km) differs significantly from AFT-derived estimates reported from Palaeozoic and Mesozoic sandstones, which would imply burial by a considerable thickness of younger section and, later, 1.5 km of post-rift denudation. The exhumation of surrounding basement surfaces probably began very early, as shown to the northwest of the chapada by the presence of widespread Palaeogene laterites. A later stage of basin topographic inversion, in the Oligocene and later, would correspond to the major stages of river incision, partial planation and basin inversion.