The endorheic – Exorheic transition and later stage of fluvial incision in a wet tropical margin setting: The Atlantic draining Paraíba do Sul River basin (Brazil)

Abstract

Present-day endorheic drainage systems are rare in tropical humid regions and/or close to the coast. During the late Cenozoic, under a humid tropical climate, the Paraíba do Sul River basin (SE Brazil) has developed along the South America passive margin. This basin currently drains into the South Atlantic Ocean, but it preserves landforms that are indicative of previous endorheic paleodrainage. This study examines the possibility that this region was endorheic for most of the Neogene, prior to the establishment of the present-day drainage to the Atlantic and discusses the transition from an endorheic to an exorheic system. Data was obtained through analysis of geomorphological features identified by remote-sensing techniques and verified by fieldwork, as well as the interpretation of landscape evolution models elaborated by the Seppômen method. Five drainage convergence areas and possible endorheic paleobasins, previous to the Quaternary (or to the Pliocene) have been identified within the present-day Paraíba do Sul River basin. Each area is associated with a Cenozoic graben and is separated by structural highs which would have formed paleodrainage divides. The mechanism for the transition endorheic-exorheic is the overspill, the inland regressive erosion or, more probable, a combination between these two processes. In fact, these two processes often occur concomitantly and both contribute to the same result: the expansion of an exorheic basin by the incision of a permanent channel into the endorheic basin infill. The geological evolution of the ancestral Paraíba do Sul River, draining to the Atlantic Ocean, was later strongly controlled by the very low sea levels during the Quaternary which determined the stage of fluvial incision. No numerical dating has been yet obtained for the proposed endorheic-exorheic transition; nonetheless, published regional denudation rates suggest that this transition occurred sometime in the interval between 21 and 5 Ma (Miocene to Pliocene). This transition was controlled by a previous decrease in subsidence within the aforementioned grabens and by a much wetter climate that promoted overspill and connection to the Atlantic.