Expressão geomorfológica da queda do nível de base no Baixo São Francisco

Abstract

A large number of studies of flat surfaces on high-elevation passive margins have listed river erosion over bedrock as one of the determining factors in the long-term evolution of the landscape. However, in low-elevation hinterlands over exposed shield, it is often said that rivers play a supporting role in landforms delineation. Rivers, in this context, are routinely described as transporting agents for pre-weathered material. However, physical mechanisms involved in the mechanics of river erosion in rocky beds testifies its importance goes beyond the transport of sediments. This study aims to analyze the fluvial topography of bedrock rivers that drain the Southern Sertaneja Depression, a pervasive, low-elevation flat surface over the Proterozoic shield that borders the Borborema Plateau, in the semi-arid region of Northeast Brazil. We sought to investigate the morphology of the longitudinal profiles, map knickpoints, estimate the magnitude and spatial distribution of the vertical incision of river valleys and evaluate the contribution of structural controls on these morphologies. All these analyzes were based on morphometric techniques based on a Digital Elevation Model (DEM), such as the extraction of the Normalized Steepness Index (Ksn) and the Stream Gradient Index (SL), in addition to the quantification of the slope angle of the hillslopes, analysis of photolineaments of relief and drainage, integral and hypsometric curve, local relief, among others. The target rivers of this study are all tributaries of the left bank of the lower course of the São Francisco River, in the region of its canyon. The hypothesis that guided this study was that a base level fall generated an incision wave that is propagating through the São Francisco River and its tributary network in the flat Proterozoic terrains of the Southern Sertaneja Depression. With the morphometric analyses, the existence of this supposed wave of incision in the tributaries of the São Francisco was demonstrated. This process produces a clear pattern of base level fall in the landscape of the study area, with the increase of the gradient of the channels, hillslopes and the formation of hanging valleys downstream of the main ruptures, which form a fractal at various scales. The valleys and ridges are subordinated to the structural heritage of the basement, as well as the sinuosity of the channels and the confluence angle. It has been suggested, based on independent evidences, that the São Francisco epigeny originated in the Eocene and the incision wave that is observed in the valleys today was formed after this period.