Turbidity current sediment waves on irregular slopes: observations from the Orinoco sediment–wave field

Abstract

The Orinoco sediment–wave field covers an area of at least 29 000 km2 on the southern margin of the Orinoco Valley, at a water depth of 4400–4825 m. Wave dimensions are highly variable across the wave field, with wavelengths of 110–2600 m, and wave heights of 1–15 m. Slope gradients are also very variable, with values of 0.14–0.48°. Overall, the sedimentary sequence on the upslope wave flanks is about 40% thicker than that on the downslope flanks, leading to upslope wave migration in the manner of antidunes. In addition, reflectors on the upslope flanks generally display higher reflectivity than those on the downslope flanks, suggesting that a higher proportion of coarser sediment occurs on the upslope flank. An unconfined turbidity current origin is proposed for the Orinoco sediment waves, based upon detailed analysis of regional stratigraphic/seismic facies, and sediment wave distribution, morphology and dimensions. Sediment waves are not related to flows passing along (or spilling out of) the Orinoco Valley, or to bottom currents flowing parallel to the slope. Turbidity currents responsible for wave generation are interpreted as originating from slope failures on the adjacent Venezuela, Guyana and Suriname continental margins. Simple numerical modelling has enabled turbidity current flow characteristics across the Orinoco sediment waves to be estimated: internal Froude number=0.7–1.1, flow thickness=24–645 m, and flow velocity=31–82 cm s−1. A key finding of this study is that there appears to be a close relationship between the changes in slope gradient and those in wave dimensions across the wave field. The irregular gradient of the present-day wave field is partly a reflection of the irregular bounding surface of the sediment waves, which is represented by mass-flow deposits and associated mud diapirs. The changes in slope gradient along this lower boundary lead to variations in the flow thickness and flow velocity of passing turbidity currents, which in turn control the wave dimensions. Generally, on lower gradients beyond minor breaks of slope, flow thickness increases and flow velocity decreases, leading to an increase in wavelength and a decrease in height.