Drowned barriers and valleys: A morphological archive of base level changes in the western South Atlantic

Drowned paleo-coastal and fluvial landforms preserved on continental shelves are important indicators of sealevel changes during the Quaternary. Morphological analyses of paleo-coastal and fluvial submerged features using multibeam bathymetry as basic datasets, were carried out on the Southeast Brazilian Shelf. The aim was to investigate the interaction between incised valleys and submerged barriers and their temporal relationship. Six partially infilled incised valleys with sinuous and meandering features, and up to similar to 40 m of fluvial incision were observed, including several morphologies associated with meanders (e.g., point bars, neck, cut-bank). Paleocoastal features, mainly related to barriers, were found at four depth ranges (DR) in a dip orientation: DR1 (>60 m); DR2 (55 m-60 m); DR3 (50 m-55 m); DR4 (45 m-50 m). The geomorphic results show that valleys and coastal barriers coexisted and that lower and narrower barriers tend to occur near the shelf break. Preliminary geomorphic evolution between incised valleys and coastal barriers encompasses two possible origins: I) paleocoastal features of DR 2-4 developed during the interstadial fluctuations of sea-level in MIS 3-5 and/or multiple genetic phases with different base level cycles. The valleys had two phases of incisions related to the base level drop below the shelf break: MIS 4 in the shallow incisions (terraces) and MIS 2 in the main incisions. Moreover, the development of DR 1 was related to the post-LGM base level rise; II) All submerged coastal (DR 1-4) and fluvial features developed during post-LGM base level rise (i.e., fluvial and coastal dynamics in the same cycle of base level change). The multiple phases of base-level changes inducing phases of valley incisions and barrier formation is more likely the be a better explanation. From this perspective, geochronological studies using sedimetary cores in the paleo-coastal and fluvial features are crucial to validate these scenarios, and thus contributing to fill knowledge gaps regarding morphosedimentary responses to base-level changes on the western Atlantic margin.