A importância dos rearranjos de drenagem para a organização hidrográfica do nordeste setentrional brasileiro

Abstract

Drainage rearrangements have been reported in Brazilian territory since the 1950s. The great exception is the Semi-arid Equatorial Margin, specifically in the Northeastern Brazil, where the hydrography organization was interpreted as an antecedent and/or superimposed on morphotectonic structures inherited from cretaceous extensive stress that culminated in the Equatorial Atlantic Ocean opening. However, even this region shows drainage rearrangement evidence, that has been little explored in geomorphological evolution models proposed for the region. In this context, the present work investigates drainage rearrangements occurrence in the Northeastern Brazil, analysing morphological and sedimentary evidence obtained from field expeditions and through the extraction of morphometrics such as local relief, paleotopographic modelling and the χ variable. Among the main results, the following stand out: (i) barbed drainage networks discordant with the regional structural fabric; (ii) flattened and low-relief divides drained by coastal rivers with interland weak penetration; (iii) steps of intermontane depressions drained by rivers with incised mouths or limited by low-relief divides (iv); natural damming of river sediments; (v) transversal drainage formed by epigenic interceptions; (vi) areas transfers signatures between adjacent rivers; and (vii) incision differences between rivers flowing over the same dissection surfaces. These results indicate that drainage rearrangements were responsible for transfers of a few thousand km² of area between the watersheds, promoting neogene-quaternary reorganizations for investigated rivers (Acaraú, Aracatiaçu, Jaguaribe, Piranji, Choró, Poti, Apodi-Mossoró and Pajeú ) and their streams. Erosive potential disparities of regional rivers, subordinated to sectors of basement lithostructural weakness zones and conditioned by moisture peaks related to climatic oscillations, promoted river captures by lateral planation, overflow and underground interception. The identification of these evidence suggests that piracy processes may play an active role in relief shaping and in hydrographic organization of the Northeastern Brazilian semi-arid region, even with unfavourable topographic and hydroclimatic conditions for the manifestation of these processes.