Contourite features in the Northern Scotia Sea: tectonic, sedimentary and palaeoceanographic implications

The flow of near bottom-water masses controls the along-slope sedimentary processes in oceanic basins and continental margins. The association of largest depositional accumulations (contourite drifts) and related erosional features (as contourite-terraces, channels, etc) has been termed as Contourite Depositional System (CDS). The present work aims to understand the interaction between bottom-water masses and the northern Scotia Sea slope due to the water exchange between the Scotia Sea and the South Atlantic Ocean. This work emphasis is on regional morpho-sedimentary and seismo-stratigraphic analysis based on single- and multichannel seismic surveys conducted along the western end of the North Scotia Ridge (NSR), close to the shallow Burdwood Bank. The regional tectonic complexity of the Scotia Sea is strongly conditioned by the last steps of the Gondwana break-up and the onset and evolution of the Drake Passage. The NSR is a complex of shallow banks and submarine ridges that formed the quasi-continuous topographic link existing from southern South America to South Georgia and Antarctica. This feature forms the so-called northern edge of the Scotia Sea; located close to the sinistral strike-slip boundary of South-America and Scotia plates. Due to its relatively shallow depth, the NSR is an important morphologic obstacle to the northward flow of Antarctic water masses from the Scotia Sea into the South Atlantic Ocean. Five seismic units are identified within the sedimentary record of the study area. From bottom to top these units are referred to as V through I. These units are limited by stratigraphic horizons defined by reflections of high amplitude and lateral continuity. Around Unit III, a change in the seismic facies is observed. The lower units (V & IV) present high amplitude reflections, however, in the upper units (III, II & I) reflections present reduced amplitude and show wavy or drift morphologies superimposed to their wedge shape. We infer that the growth patterns of the lower units are highly controlled by the structure of the acoustic basement, whereas the upper units are mostly determined by the CDS associated to the local water masses circulation. Interfaces between these water masses are related to main morphologic changes along the slope.