U-shaped slope gully systems and sediment waves on the passive margin of Gabon (West Africa)

摘要: Abstract 3-D seismic reflection data has enabled the documentation of a system remarkable modern and buried u-shaped gullies which intimately co-exist with upslope migrating sediment waves along 80 km Gabon continental slope. The occur on silty mud-dominated slope in water depths 150–1500 m an ~ 50 km wide gradient 4.5° decreasing to 1.5°. gully sets persist laterally for distances at least 40 km extend downslope up 60 km. are cross-section, relief 5–30 m, widths 50–400 m. Intriguingly, become narrower shallower distance down slope, as well increasing number majority appear be erosional but some have resulted from simultaneous aggradation inter-gully ridges non-deposition adjacent floor. Hence, these interpreted formed mainly response spatially-variable deposition, rather than erosion. Upslope close proximity gullies. Gullies cross fields observed migrate up-slope locally within both aggradational Evidence is lacking any slumping or headward erosion headwall region gullies, discounts formation by very local gravity flows originating shelf-edge collapse, been other v-shaped systems. Based our new data, supported theoretical studies mechanics turbidity currents, we propose that related were diffuse, sheet-like, mud-rich currents presumably originated shelf. Instabilities generated wave-shaped perturbation cross-flow direction leading regularly spaced regions faster slower flow. For non-aggradational it inferred axes experienced flow velocities exceeded settling velocity suspension, thus no deposition occurred. In contrast, indicate lower flanks. Mixed mode also found successive can experience variations properties interspersed depositional events.