A ‘braided’ distributary system on the Orinoco Deep-Sea Fan

Abstract

A long-range side-scan sonar (Gloria) survey has been made of part of the Orinoco Deep-Sea Fan that abuts against the deformation front of the Barbados Outer Ridge, i.e. in a pre-orogenic setting. Immediately east of the deformation front sinuous channels up to 65 m deep and 3 km wide extend downslope and become very shallow (a few metres or less deep) and branch into a pattern that resembles stream braiding near the foot of the slope. Here there is a possible suprafan lobe and an almost flat perched sediment pond dammed by a buried basement ridge. In this locality many of the acoustic contrasts on the sonographs are primarily due to differences in sediment type and small-scale roughness rather than to variations in slope angle. Possible debris flows aligned along some of the ‘braided’ channels have been identified from high-resolution profiles. This ‘braided’ system of shallow channels then converges into a single channel that overtops the dam. Because of their braided nature, the internal structure of the resulting deposits is likely to be complex. These deposits should eventually be incorporated into the Barbados Ridge Complex. In contrast to this pre-orogenic setting, Gloria sonographs also suggest that some Orinoco sediment moves down along synclinical axes of folds on the Barbados Ridge Complex, i.e. in a syn-orogenic setting.