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Keywords:

  • Debris flow;
  • Gulf of Mexico;
  • Orphan Basin;
  • Scotian Slope;
  • slide

Abstract

No comprehensive scheme yet exists to describe the depositional products of submarine sediment failures at the scale of piston cores, resulting in misinterpretation of failure deposits and overuse of the genetic term ‘debris flow’. Ninety-nine sediment cores (0·5 to 20 m in length), from offshore eastern Canada and the Gulf of Mexico, are used to propose a descriptive sedimentary facies scheme with genetic implications for mass-transport deposits. Seven facies are distinguished: (i) allochthonous stratified sediment; (ii) distorted stratified sediment; (iii) clast-supported hard-mud-clast conglomerate; (iv) matrix-supported mud-clast conglomerate; (v) thin mud-clast conglomerate (<0·8 m thick); (vi) diamicton; and (vii) sorted sand-gravel deposits (≥0·05 m thick).

Seven genetic types of deposits are recognized. (i) Slumping of coherent sediment blocks (facies I). (ii) Slump and slide deposits (facies I and II). (iii) Debris-avalanche deposits (hard sediment of facies I and II overlain by facies III). (iv) Low-viscosity or large-scale, high-viscosity, cohesive debris flow deposits (facies IV, may have I, II, and III). (v) Very low-viscosity debris flow deposits (facies V). (vi) Cohesionless debris flow deposits (facies VI). (vii) High-density turbidity currents (facies VII). Vertical transitions between the genetic types were analysed by Markov chain analysis.

Although sedimentological transitions are inferred between deposits of slides and cohesive debris flows, their spatial distribution indicates that a cohesive debris flow forms principally in the initial stages of a sediment failure, suggesting that transformation depends mostly on the strength of the sediments. A genetic link is suggested for cohesionless debris flow deposits, which originate from the disintegration of sandy sediment on the upper continental slope, and the closely related turbidity current deposits. Debris avalanches are common in sedimentary marine environments with steep slopes (>10°). In many cases, geometrical and seismic characteristics of debris avalanche, slide and debris flow are similar, requiring core data to verify transport process.