Lamont-Doherty Earth Observatory, United States

Several mass-transport facies were recovered during drilling on the continental slope and rise off the U. S. East Coast during ODP Leg 150. Distinct facies occur in canyon and intercanyon areas. Intercanyon deposits contain contorted, muddy slumps and slides in which the clast and matrix material are of similar age. In contrast, modern canyons contain muddy debris flows which show an evolution in the degree of internal deformation and clast character down canyon and out onto the continental rise. Sandy turbidites also occur in canyons, but are rare in intercanyon regions. Slumps and debris flows on the rise show even greater deformation (folding, clastic injections, contorted bedding, more than one episode of transport and contain clasts of diverse ages (Eocene to Pleistocene), sizes and degree of induration. The relationship between mass-transport deposits and sequence boundaries is complex. In the Pleistocene and upper Miocene sections, mass-wasting deposits most frequently correlate with sequence boundaries beneath their respective upper paleoslopes, but this correlation becomes less frequent downslope. In contrast, in the lower to middle Miocene section, the occurrence of mass-transport deposits at sequence boundaries is greatest beneath the respective middle paleoslopes, but is less frequent upslope. These observations suggest a correlation between slope erosion leading to generation of mass-transport deposits and sequence boundaries. However, preservation of mass-wasting deposits was also significantly influenced by the morphology and gradient of the existing paleoslope. Steepening of the slope in response to more rapid seaward sediment progradation during the late Miocene to the Pleistocene may have led to increased sediment failure and movement of failed sediments further out into the continental rise (e.g. Site 905). In contrast, the more gentle gradients of the Oligocene to middle Miocene paleoslopes may have resulted in increased deposition and preservation of mass-transport deposits on the more proximal upper- to middle paleoslopes.