Giant Depressions above Submarine Channels Caused by Flow Activity on The Brazilian Continental Margin

Heinio, Paivi¹, Richard Davies² (1) Cardiff University, Cardiff, United Kingdom (2) University of Durham, Durham, United Kingdom

Neogene-age, roughly circular depressions, which are several hundred meters across and up to 50 m deep, are imaged with 3-D seismic data from the Espirito Santo Basin on the Brazilian continental margin. They are organized into trails following the courses of underlying channels, which are incised into a steep slope just beneath the shelf break. Reflection geometry and the lack of any vertical evidence for fluid escape rule out these being pockmarks, instead we interpret them to be formed by depositional and erosional processes of sediment-gravity flows.

3-D seismic data allow for a detailed analysis of the formation of the depressions at different stages. The erosional channels are initially overlain by reflections with sediment wave-like geometries. These channel-confined waves have steep lee slopes facing downslope and shallow stoss slopes facing upslope forming scour-shaped depressions. These scours evolve into more circular depressions, which eventually become filled with mounded onlap fill.

The waves are interpreted to have initiated from small irregularities on the channel-floor, which caused local changes in flow regime of the turbidity currents within the channels. An undulating surface results in wide ranging Froude numbers leading to bypass and erosion on steeper slopes and deposition at slope breaks and behind obstacles. Several examples on the present day seafloor support this interpretation with depressions above knickpoints on shallowly buried submarine channels. These exceptional data provide the first natural examples of flow-originated depressions imaged by 3-D seismic data and aids in the understanding of flow behavior.