The Influence of Sediment Supply & Caliber on Submarine Canyon Morphology and Turbidity-Flow Character

Coastal areas of high coarse-grained sediment supply generally produce submarine canyons that indent the shelf edge and contain highly erosive morphologies due to the erosive power of sand-rich turbidity flows. Typically, this volume of coarse sediment is transferred through the canyons onto large submarine fans. However, canyons formed in areas of low and/or very muddy sediment supply do not indent the shelf edge, have aggradational morphologies, and lack downslope fans. More dilute, mud-rich turbidity flows and hemipelagic deposition cause canyon aggradation rather than erosion. Temporal changes in the supply and caliber of sediment can significantly alter the types of flows available to a continental margin, thereby influencing the morphology of submarine canyons and the presence of submarine fans. This study uses an example from the continental margin of Equatorial Guinea, West Africa to illustrate this point. During the late Cretaceous, the continental margin of Equatorial Guinea was dominated by an erosional, sand-rich, submarine canyon system. This system was abandoned during the Paleogene, but the relict topography was re-activated in the Miocene during submarine erosion associated with tectonic uplift. Subsequently, a decrease in sediment supply resulted in a drastic transformation in canyon morphology. The result is a modern, muddy, aggradational canyon system that does not indent the shelf edge and has no downslope sediment apron. Draping reflections indicate that hemipelagic deposition aggrades the canyons. Intra-canyon lateral accretion deposits indicate that canyon concavity is maintained by thick (> 150 m), dilute, sluggish, mud-rich turbidity currents of a much different character than those in erosive canyons. Spatial and temporal changes in sediment supply and caliber to the Equatorial Guinean margin have been caused by tectonic uplift, climatic forcing, and shelf morphology. In particular, longshore drift provides high sediment supply to shelfal recesses, creating shelf-indenting, erosional canyons and associated submarine fans. Leeward of these shelfal recesses, the margin is starved of coarse sediment and canyons do not indent the shelf edge and are muddy and aggradational.