Evolution of complex giant seafloor depressions at the northern Argentine continental margin (SW Atlantic Ocean) under the influence of a dynamic bottom current regime

Seafloor depressions (SD) are features commonly observed on the ocean floor. They often occur as circular, small-sized (up to 10 s of m) incisions caused by fluid expulsion. Larger depressions (100s m to km) are considerably less abundant, and their origin and development have been scarcely studied. This study investigated two giant morphological depressions (> 5 km) using recently acquired multibeam bathymetry and backscatter, sediment echosounder, and high-resolution seismic data. An arc-shaped (SD-N) and a sub-circular depression (SD-S) are located on the Ewing Terrace at the Argentine Continental Margin north and south of the Mar del Plata Canyon, respectively. The study area is influenced by the Brazil-Malvinas Confluence, where major counterflowing ocean currents affect sedimentation, and northward flowing currents form a large contourite depositional system. Using an existing seismo-stratigraphy, the onset of SD-N was dated to the middle Miocene (similar to 15-17 Ma), whereas SD-S started developing at the Miocene/Pliocene boundary (similar to 6 Ma). Acoustic anomalies indicate the presence of gas and diffuse upward fluid migration, and therefore seafloor seepage is proposed as the initial mechanism for SD-S, whereas we consider a structural control for SD-N to be most likely. Initial depressions were reworked and maintained by strong and variable bottom currents, resulting in prograding clinoform reflection patterns (SD-N) or leading to the build-up of extensive cut-and -fill structures (SD-S). Altogether, this study highlights the evolution of two unique and complex seafloor depressions throughout the geologic past under intense and variable bottom current activity in a highly dynamic oceanographic setting.

Automated summary

This study investigates the origin and development of two giant seafloor depressions using multibeam bathymetry and seismic data. The depressions are likely formed by seafloor seepage and structural control, and their evolution is influenced by strong and variable bottom currents.