The imprint of sedimentary processes in the acoustic structure of deposits on a current-dominated continental shelf

Abstract

This study aims to investigate the spatial variability of surficial sedimentation processes on a large sector of the Brazilian shelf by using echo-character mapping. To reach this goal, almost 3200 km of high-resolution seismic lines collected with a 3.5 kHz Chirp source are compared with surficial sediment-related distribution maps. Based on seismic characteristics, such as sharpness, amplitude, and continuity of sub-bottom reflections, six echo types were identified in the study area. These echo types were grouped into two main classes according to the nature of seafloor morphology. Class 1 is characterized by flat-bottom echoes with a variable amplitude of reflections and acoustic penetration. This class is predominantly found on the inner and middle shelves. Class 2 consists of irregular seafloor morphologies and is classified based on the occurrence of internal reflections and relief variations. This class is mainly found on the outer shelf. The distribution of echo types and associated surficial sediment maps exhibit three main shelf domains: inner, middle, and outer. The echo types are mostly controlled by the influence of along-shelf currents, as well as by mid-shelf fronts associated with the entrance of cold-water tongues under changing wind conditions and upwelling phenomena. Whereas the inner shelf is mainly affected by variable wind-driven currents, the outer shelf is more persistently influenced by the southwestward movement of the Brazil Current. Between these areas, the middle shelf is affected by seasonal intrusions of the South Atlantic Central Water and northward extensions of the Brazilian Coastal Current. The studied area can be considered a case model for a shelf driven by hydrodynamic processes.