CLASSIFICATION OF SUBMARINE CONFLUENCES AND IMPLICATIONS FOR INTRA-CANYON SEDIMENT DISTRIBUTION

Confluences points are important features of deepwater submarine canyons, but still poorly characterised. High-quality 3D seismic data is used to investigate a Miocene buried canyon and a modern, active one on the Rio Doce Turbidite System in the Espírito Santo Basin (SE Brazil). Detailed mapping of canyon thalwegs reveal two distinct confluence morphologies. Tributaries of the buried canyon merge at an equal junction of identical thalweg depth. Tributaries of the modern canyon merge at an unequal junction with a vertical offset of 100m between them. Submarine confluence scours are also observed, which occur either at central confluence region or preferentially adjacent to a tributary. The analysis of seismic amplitude attributes is used to highlight the preferential flow/depositional paths of sand-prone strata. In our examples both systems show a predominance of high amplitudes along the eastern tributaries and extending into the post-confluence path.

We propose a classification for submarine confluences based on a combined analysis of canyon geometry and the use of seismic attributes. Confluences may be symmetric or asymmetric based on the similarity of the angles the tributaries bear to the post-confluence channel. Asymmetric confluences are primary or secondary depending on whether the dominant pre-confluence sediment flow (or deposition) takes place along the main or secondary tributary, respectively. This classification can be used to characterise the full sedimentary fill of buried canyons and channels and also of specific channel-fill events. The detailed assessment of the lateral continuity (or lack of it) of sand-prone strata shows that these can extend into the tributaries or be limited to the post-confluence segments. Furthermore, the strata geometry suggests episodic flow shifts and/or diachronous sediment deposition between tributaries. This has implications for the estimation of fluid flow paths at confluence regions, and to assess which tributaries constitute favourable fluid accumulation compartments.