Deep-Water Processes and Deposits

Abstract

Deep-water environments (>200m in bathymetry) are characterized by gravity-driven downslope processes, which comprise slides, slumps, debris flows, and turbidity currents. Mass-transport processes (i.e., slides, slumps, and debris flows) exhibit elastic and plastic behaviors due to high sediment concentration (25–100% by volume). Turbidity currents are not mass-transport processes and they exhibit viscous fluid behavior due to low sediment concentration (1–23% by volume). Long-runout distances of up to 810km are known for submarine mass-transport deposits (MTDs). Characteristic features of slides, slumps, debrites, and turbidites are basal glide plane, contorted bedding, planar clast fabric, and normal grading, respectively. Deep-water depositional settings include continental slopes, submarine canyons, submarine fans, and abyssal plains. Empirical data for emplacement of coarse sandy and gravelly deposits by mass-transport processes in modern submarine canyons are abundant, whereas observational data for gravelly and coarse sandy turbidity currents (i.e., high-density turbidity currents) are totally absent. Various turbidite facies models are problematic. Among the 21 triggering mechanisms of sediment failures, short-term natural events that represent only a matter of a few minutes to several hours or days (e.g., earthquakes) are more important than long-term sea-level lowstands. The four basic types of deep-water bottom currents are (1) thermohaline-induced geostrophic bottom currents (contour currents), (2) wind-driven bottom currents, (3) tidal bottom currents, and (4) baroclinic currents associated with internal waves and tides. Deposits of all bottom currents exhibit traction structures; however, double mud layers are unique to deep-water tidalites. Offspring deposits of sandy debris flows and bottom currents (i.e., hybrid flows) are common. Deep-water deposits comprise important petroleum reservoirs worldwide. The two recent developments include types and controls of density plumes and soft-sediment deformation structures (SSDS). Although hyperpycnal flows are considered analogous to sediment-gravity flows, hyperpycnites are of no significance in deep-water settings.