Formation of Cohesionless Debris Flows and Turbidity Currents from Sediment Failures: A Case Study from the Continental Margin of Southwest Orphan Basin, Labrador Sea

Efthymios K. Tripsanas and David J.W. Piper
Bedford Institute of Oceanography, Dartmouth, NS

A series of submarine canyons on the southwest slope of Orphan Basin, termed the Sheridan Canyon system, have been studied with 34 sediment cores and 1165 line-km of ultra high-resolution seismic-reflection profiles. Three distinct near-surface sediment failure events occurred in this area, dated at 12, 9, and 6 ka BP. The last failure is the most distinct and formed a thick (< 20 m), cohesive mud-flow deposit, of limited distribution on the continental rise with a total run-out of ~60 km. On top of this mud-flow, an erosional muddy-gravel deposit is replaced basinward by sorted, inversely graded sand-gravel deposits and well-sorted sand deposits. These deposits are interpreted as the depositional result of a cohesionless debris flow that transformed basinward into a high-density turbidity current. This flow resulted in the excavation of a 5-20 m deep channel, and traveled for a distance over 200 km. No hemipelagic deposits separate the sand-gravel unit from the mud flow. Seismic data indicate that the mud-flow deposit was initiated by a large sediment failure, which acquired a retrogressive character that resulted in the later failure of upper slope and shelf deposits, consisting of coarse-grained till. The cohesionless debris flow/high-density turbidity current was therefore not produced by the flow transformation of the large cohesive mud flow, but from the latest, smaller retrogressive failures.