Bedrock erosion by sedimentary flows in submarine canyons

How indurated sediment and rock outcrops in the walls of some submarine canyons were exhumed is unclear; the flows traversing them were muddy sedimentary flows, so abrasion is unlikely to have been important in these cases. The answer may lie in the importance of quarrying or plucking. Observations of rivers in extreme floods suggest that such erosion processes begin to operate effectively on jointed bedrock when reach-scale bed shear stress exceeds 100 Pa and become increasingly rapid beyond that stress level, some floods producing >1000 Pa. Here, the sedimentary flow weights that would be needed to produce similar shear stresses are estimated for two canyons where observations from submersibles have revealed exposed bedrock: Monterey Canyon (California, offshore western USA) and Hendrickson Canyon (New Jersey, offshore eastern USA). Assuming that the dense portions of turbidity currents occupied Monterey Canyon to a height corresponding to a 100-m-high steep inner wall, the minimum flow-averaged excess density derived from the estimated flow weight is found to be only 5 kg m -3 , whereas a 1000 Pa condition would suggest values 10 times larger. In contrast, muddy debrites and other mass transport deposits dominate the New Jersey slope and upper rise. The weight of debris flows occupying Hendrickson Canyon was computed using the stress constraints and converted to equivalent sediment thickness using upper slope in situ sediment deposit densities. These thicknesses were found to be 1–10 m, within the range of possible mobile sediment thicknesses suggested by relief of upper slope landslide embayments. The flow density (Monterey) and thicknesses (Hendrickson) are both modest, so bed shear stresses generated by sedimentary flows in continental slope canyons seem adequate to explain exhumation of bedrock by quarrying or plucking erosion mechanisms.