The initiation mechanism and source of turbidite sediments to fill both recent and ancient offshore basins can be explained by examining nearshore submarine canyons like those off the Magdelline River, Columbia, and Rio Balsas, Mexico. To generate turbidity currents, large amounts of sediments of mixed grain size must first be stored as a stable deposit in the upper reaches of a canyon, then some mechanisms must create instability and set the entire deposit into motion. Canyons which head at deltas respond to the pulsating sources of sediment and the migration of the river mouth from one area to another. There is a flip-flop in the processes active in the canyon heads from one of erosion when the head is proximal and a large amount of coarse sediment enters directly into he canyon, to one of deposition of fine-grained sediment when the canyon head is distal and processes quiescent. During the distal stage, fine-grained cohesive sediments build up forming V-shaped profiles. The walls literally grow together. The migration of the river mouth back to the vicinity of a formerly quiescent distal canyon head will introduce coarse-grained sediments and reinitiate submarine erosion of the poorly consolidated canyon fill. Erosion forms steep unstable slopes and progressive slumping, creating the mechanism for generating a turbidity current with a large volume of poorly sorted driving sediment.

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