Hybrid submarine flows comprising turbidity current and cohesive debris flow: Deposits, theoretical and experimental analyses, and generalized models

Hybrid fl ows comprising both turbidity current and submarine debris fl ow are a signifi cant departure from many previous infl uential models for submarine sediment density fl ows.Hybrid beds containing cohesive debrite and turbidite are common in distal depositional environments, as shown by detailed observations from more than 20 modern and ancient systems worldwide.Hybrid fl ows, and cohesive debris fl ows more generally, are best classifi ed in terms of a continuum of decreasing cohesive debris fl ow strength.High-strength cohesive debris fl ows tend to be clast rich and relatively thick, and their deposit extends back to near the site of original slope failure.They are typically confi ned to higher gradient continental slopes, but may occasionally form megabeds on basin plains, in both cases overlain by a thin turbidite.Intermediate-strength cohesive debris fl ows typically contain clasts, but their deposits may be <1 or 2 m thick on low-gradient fan fringes, and are encased in turbidite sand and mud.Clasts may be fartraveled, and meter-sized clasts can be rafted long distances across very low gradients if they are less dense than surrounding fl ow.Low-strength cohesive debris fl ows generally lack mud clasts, and as cohesive strength decreases further there is a transition into fl uid mud layers that do not support sand.Intermediate-and low-strength cohesive debrites are consistently absent in more proximal parts of submarine systems, where faster moving sediment-charged fl ows are more likely to be turbulent.Intermediatestrength debris fl ows can run out for long distances on low gradients without hydroplaning.Very low strength cohesive debris fl ows most likely form through late-stage transformations near the site of debrite deposition, and emplaced gently to avoid mixing with surrounding seawater.The location and geometry of cohesive debrites in hybrid beds are controlled strongly by seafl oor morphology and small changes in gradient.Debrites occur as fringes around raised channel-levee ridges, or in the central and lowest parts of basin plains lacking such ridges.Small variations in mud fraction produce profound changes in cohesive strength, fl ow viscosity, permeability, and the time taken for excess pore pressures to dissipate that span multiple orders of magnitude.Reduction in fl ow speed can also cause substantial increases in viscosity and yield strength in shear thinning muddy fl uids.Small amounts of sediment can dampen or extinguish turbulence, especially as fl ow decelerates, affecting how sediment is supported or deposited.This ensures that cohesive debris fl ows and hybrid fl ows have a rich variety of behaviors.