Continental shelf incised valleys as a key to shelf evolution during Quaternary in the Western South Atlantic

Abstract

Shelf-incised valleys, carved by ancient rivers and estuaries during periods of continental shelf exposure, were primarily shaped by base-level fluctuations in the Quaternary. These geological features are prevalent in the western South Atlantic shelf but often go unnoticed as key indicators of the region's evolution. This research unveils the outcomes of a comprehensive investigation that combined sub-bottom profiling (SBP) surveys, historical bathymetric data, and the application of Geographic Information System (GIS) tools to extract drainage patterns within the Santos Basin, a significant sedimentary basin in Brazil.The study provides an unprecedented overview of the interconnected network of the ancient drainage system across the continental shelf, totalizing 91 thalwegs associated with 80 incised valleys in the study area, ranging in length from 250 m to 148 km. This connection between the mapped incised valleys and the upper slope evolved through three distinct phases during the Late Pleistocene. The first phase (Phase A) was characterized by the formation and subsequent destruction of incised valleys, driven by high-frequency sea-level oscillations during MIS5. The second phase (Phase B, with subphases B1 and B2) witnessed the construction of incised valleys during a period of forced regression at MIS4 and MIS3. The final phase (Phase C) marked the establishment of fully developed valleys, connecting them to upper slope canyons during the Last Glacial Maximum (LGM). The correlation of these sea-level changes with the presence of underfilled incised valleys in the Santos Basin shelf represents a significant breakthrough in understanding the geological history of the western South Atlantic shelf. These valleys typically eroded until reaching depths near the transient base level, offering a valuable glimpse into the geological chronicles of sea-level fluctuations over time.