Geometry and Classification of Submarine Canyon Confluences

Abstract

Points of confluence are important morphological features of deep-water submarine canyons, but they are still poorly characterized and incompletely understood. Here, high-quality 3-D seismic data are used to investigate the confluence regions of the Miocene (C1) and Modern (C2) submarine canyons of the Rio Doce Turbidite System in the Espírito Santo Basin, on the southeastern Brazilian margin. Growing salt diapirs play a dominant role in the confinement of the studied canyons, because confluence points are located near slope regions with the highest degree of confinement. Continuous salt growth and interaction with evolving sedimentary systems has forced an up-slope migration of the submarine confluence areas during the late Cenozoic. Detailed mapping and measurements of canyon thalwegs reveal two distinct morphologies for both confluences. Tributaries of Canyon C1 merge at an equal junction, whereas in Canyon C2 tributaries merge at an unequal junction, with a vertical offset of 100 m between them. This study also documents the presence of submarine confluence scours. In C1, a confluence scour is located in the central confluence region, whereas in C2 the scour is adjacent to the hanging tributary. Seismic attribute-based analysis of the channel-fill deposits (namely maximum and RMS amplitude) highlight the predominance of sand-prone, high-amplitude deposits along the eastern tributary and postconfluence conduits in both canyons. We propose a classification scheme for submarine confluences based on a combined analysis of channel geometry and the use of seismic attributes to assess sedimentary paths. Confluences may be symmetric or asymmetric, based on the similarity of the angles that the tributaries bear to the postconfluence channel. Left- or right-symmetric confluences define whether the dominant flow path is along the left- or right-hand tributary. Pure or secondary asymmetric confluences depend on whether the dominant preconfluence flow takes place along the main or secondary tributary, respectively. Following these criteria, the studied canyons are classified as pure asymmetric with a predominance of high-amplitude strata in the eastern tributaries and postconfluence path. This classification can be used to characterize the full sedimentary fill of buried canyons and channels and also of specific channel-fill events. The latter allows a detailed assessment of the lateral continuity (or lack of it) of relevant sand bodies from the main canyon conduit into the tributary systems. This has important implications for the estimation of fluid flow paths at confluence regions, and for the assessment of which tributaries constitute the most favourable fluid accumulation compartments.