Implications of drainage rearrangement for passive margin escarpment evolution in southern Brazil
Résumé
Although several authors have pointed out the importance of earth surface process to passive margin escarpments relief evolution and even drainage rearrangements, the dynamics of a consolidated capture area (after a drainage network erodes the escarpment, as the one from the Itajai-Acu River) remain poorly understood. Here, results are presented from radar elevation and aerial imagery data coupled with in-situ-produced 1 Be concentrations measured in sand-sized river-born sediments from the Serra Geral escarpment, southern Brazil. The Studied area's relief evolution is captained by the drainage network: while the Itajai-Acu watershed relief is the most dissected and lowest in elevation, it is significantly less dissected in the intermediate elevation Iguacu catchment, an important Parana River tributary. These less dissected and topographically higher areas belong to the Uruguai River catchment. These differences are conditioned by (i) different lithology compositions, structures and genesis; (ii) different morphological configurations, notably slope, range, relief; and (iii) different regional base levels. Along the Serra Geral escarpment, drainage features such as elbows, underfitted valleys, river profile anomalies, and contrasts in mapped chi-values are evidence of the rearrangement process, mainly beheading, where ocean-facing tributaries of the Itajai-Acu River capture the inland catchments (Iguacu and Uruguai). The Be-10 derived denudation rates reinforced such processes: while samples from the Cacador and Araucarias Plateaus yield weighted means of 3.1 +/- 0.2 and 6.5 +/- 0.4 m/Ma respectively, samples from along the escarpment yield a weighted mean of 46.8 +/- 3.6 m/Ma, almost 8 times higher. Such significant denudation rate differences are explained by base-level control, relief characteristics, and the geology framework. The main regional morphological evolutionary mechanism is headward denudation and piracy by the Itajal-Acu River tributaries, As the escarpment moves from east to west, Itajal-Acu River tributaries develop, leading to regional relief lowering and area losses within the Iguacu and Uruguai catchments. Such processes were accelerated since Itajai-Acu tributaries reached into sedimentary and volcanic rocks. From this moment on, Serra Geral became the main hydrographic divide between the ocean- and inland facing-catchments in the area.