Generation and migration of coarse-grained sediment waves in turbidity current channels and channel–lobe transition zones

Abstract

Large-scale sediment waves, composed of gravels and sands, have been studied using deep-water sidescan systems. New data are presented from submarine channels off the Canary Islands and from canyon mouths off Portugal. Data from other areas are briefly reviewed, including a re-interpretation of data from Laurentian Fan, in order to summarise the varied morphology and setting of these bedforms. Coarse-grained sediment waves are found in the proximal, dominantly bypassing areas of deep-water turbidite systems, within canyons, channels and channel–lobe transition zones. Wave heights are in the region of 1–10 m, and wavelengths are up to several hundred metres. The distribution of waves, and sparse sedimentological evidence from modern and ancient sediment wave fields, suggests that initial transport and deposition of coarse sediment occurs within a high-density turbidity current, and not as a non-Newtonian debris flow. In some cases the development of pronounced wave asymmetry, and evidence of wave disruption and reworking, suggests that the wave morphology is at least partially controlled by a later phase of low-density turbidity flow. Grain size also appears to exert some control on wave morphology, for example, gravel-rich waves have a greater height for the same wavelength than sand-rich waves. Coarse-grained sediment waves are often difficult to recognise on the seafloor because of reworking or burial by younger turbidity currents, and are equally difficult to recognise in outcrop because of their large size.