Analyses of 3-D seismic data in predominantly basin-floor settings offshore Indonesia, Nigeria, and the Gulf of Mexico, reveal the extensive presence of gravity-flow depositional elements. Five key elements were observed: (1) turbidity-flow leveed channels, (2) channel-overbank sediment waves and levees, (3) frontal splays or distributary-channel complexes, (4) crevasse-splay complexes, and (5) debris-flow channels, lobes, and sheets. Each depositional element displays a unique morphology and seismic expression. The reservoir architecture of each of these depositional elements is a function of the interaction between sedimentary process, sea-floor morphology, and sediment grain-size distribution.

(1) Turbidity-flow leveed-channel widths range from greater than 3 km to less than 200 m. Sinuosity ranges from moderate to high, and channel meanders in most instances migrate down-system. The high-amplitude reflection character that commonly characterizes these features suggests the presence of sand within the channels. In some instances, high-sinuosity channels are associated with (2) channel-overbank sediment-wave development in proximal overbank levee settings, especially in association with outer channel bends. These sediment waves reach heights of 20 m and spacings of 2-3 km. The crests of these sediment waves are oriented normal to the inferred transport direction of turbidity flows, and the waves have migrated in an up-flow direction. Channel-margin levee thickness decreases systematically down-system. Where levee thickness can no longer be resolved seismically, high-sinuosity channels feed (3) frontal splays or low-sinuosity, distributary-channel complexes. Low-sinuosity distributary-channel complexes are expressed as lobate sheets up to 5-10 km wide and tens of kilometers long that extend to the distal edges of these systems. They likely comprise sheet-like sandstone units consisting of shallow channelized and associated sand-rich overbank deposits. Also observed are (4) crevasse-splay deposits, which form as a result of the breaching of levees, commonly at channel bends. Similar to frontal splays, but smaller in size, these deposits commonly are characterized by sheet-like turbidites. (5) Debris-flow deposits comprise low-sinuosity channel fills, narrow elongate lobes, and sheets and are characterized seismically by contorted, chaotic, low-amplitude reflection patterns. These deposits commonly overlie striated or grooved pavements that can be up to tens of kilometers long, 15 m deep, and 25 m wide. Where flows are unconfined, striation patterns suggest that divergent flow is common. Debris-flow deposits extend as far basinward as turbidites, and individual debris-flow units can reach 80 m in thickness and commonly are marked by steep edges. Transparent to chaotic seismic reflection character suggest that these deposits are mud-rich.

Stratigraphically, deep-water basin-floor successions commonly are characterized by mass-transport deposits at the base, overlain by turbidite frontal-splay deposits and subsequently by leveed-channel deposits. Capping this succession is another mass-transport unit ultimately overlain and draped by condensed-section deposits. This succession can be related to a cycle of relative sea-level change and associated events at the corresponding shelf edge. Commonly, deposition of a deep-water sequence is initiated with the onset of relative sea-level fall and ends with subsequent rapid relative sea-level rise.

http://www.zenzebra.net/posamentier/posamentier-deep-water-depositional-elements.pdf

Seismic Geomorphology and Stratigraphy of Depositional Elements in Deep-Water Settings

Summary
The literature on the structure and behaviour of gravity currents is reviewed, with emphasis on some recent studies, and with particular attention to turbidity currents, though reference is also made to comparable behaviour in pyroclastic flows. Questions of definition are discussed, in particular the distinction between dense currents, which may deposit en masse, and more dilute currents. High-density dispersions may exist as a discrete, independently moving layer beneath a more dilute flow, as the basal part of a continuous density distribution or possibly as a transient depositional layer. Existing theory appears inadequate to explain the behaviour of some high-density dispersions. Surge-type currents are contrasted with quasi-steady currents, which may be generated by a variety of mechanisms including direct feed by rivers in flood. Such fluvially generated currents provide one means of generating currents with reversing buoyancy. Geologically significant turbidity currents are impractical for direct study owing to their large scale and (often) destructive nature. Small-scale laboratory currents offer a wealth of insights into turbidity current behaviour. This paper summarizes recent experimental studies that focus on the physical structure of gravity currents, with emphasis on the velocity and turbulence structure, the vertical density distribution and the stability of stratification. Preliminary quantification of the turbulence structure (including controls on turbulent entrainment, turbulent kinetic energy, Reynolds stresses and turbulence production) has been facilitated by recent technological developments that have allowed the measurement of instantaneous fluctuations in both velocity and concentration. Laboratory models, however, generally involve substantial simplification, and require compromises in some parameters to achieve adequate scaling of the parameters of most interest. Mathematical modelling also provides important insights into turbidity current behaviour. We discuss various approaches to modelling, ranging from simple hydraulic equations to systems of partial differential equations that explicitly treat conservation of momentum, fluid and sediment mass, and turbulent kinetic energy. The application for which the model is designed (i.e. to calculate mean head velocity or to create an instantaneous two-dimensional contour plot of downstream velocity in a current) determines the complexity of the mathematical model required. The behaviour of suspension currents around topography is complex and depends upon the relative height of the topography, and upon the density and velocity structure of the current. Many interactions with topography are well described by the internal Froude number, Fri. Both reflection and deflection of currents may occur on the upstream side of topography, depending upon Fri. On the downstream side of topography, flow separation, lee waves or hydraulic jumps may occur.

The structure and fluid mechanics of turbidity currents: a review of some recent studies and their geological implications

50 years of the turbidite paradigm (1950s—1990s): deep-water processes and facies models—a critical perspective

Observations of cross-shelf sediment transport conducted in the winter of 1997–1998 as part of the STRATAFORM program reveal that gravitationally forced density flows of fluid mud trapped within the thin wave bottom boundary layer provide a mechanism for forming flood deposits on the Eel river continental shelf. The data from two moored tripods located on the 20 and 60 m isobaths combined with “rapid response” hydrographic surveys, indicate a process whereby the Eel River delivers sediment on to the inner shelf faster than dispersal and transport processes are able to move it offshore. The river does not deliver sediment beyond the inner shelf because the plume is trapped along the coast due to onshore surface flow associated with downwelling favorable winds. However, the final flood deposition region is located seaward of the 50-m isobath. Acoustic backscattering data taken on the 60-m isobath (in the historic flood deposit region) show two depositional events of 6 and 13 cm during a period of high river discharge and high waves in January of 1998. These depositional events are associated with fluid mud layers that scale in thickness with the wave boundary layer. Velocity profiles from a vertical array of current meters spanning the bottom 2 m of the water column show that the current meter closest to the seafloor has the largest offshore velocity during the depositional events, indicating an offshore flow of the fluid mud from the inner shelf to the flood deposit region. During periods of low concentration suspended sediment transport without fluid mud layers present, either no deposition or erosion was found indicating that the offshore flow of the fluid mud is the dominant depositional mechanism.

The role of wave-induced density-driven fluid mud flows for cross-shelf transport on the Eel River continental shelf

Sediment Deposition from Turbidity Currents

If uniform flow conditions are approached, turbidity currents should be supercritical (Froude number greater than unity) while passing through submarine canyons, and subcritical (Froude number less than unity) in the upper submarine fan channel. The change in flow regime requires that the current pass through a hydraulic jump in the vicinity of the canyon mouth. During the jump, the velocity of the flow would be reduced and its thickness markedly increased. The aspects of such hydraulic jumps are examined. The quantitative analysis of hydraulic jumps in turbidity currents is based upon solutions of the momentum flux equation and the equation resulting from the assumption of a continuity of sediment flux. It is found that for the average submarine canyon-channel system, the flow would more than double in thickness and its velocity would be halved during a hydraulic jump. The entrainment of water through the interface of the flow during the jump is significant in reducing the density of the current. The higher the initial Froude number, the greater the expected density reduction. However, the density reduction due to the jump is probably not sufficient to convert a high-density slide into a low-density turbidity current. There must be an initial density reduction by entrainment while the slide is passing through the canyon at high Froude number. Turbulence generated within the hydraulic jump would stir the flow and help to insure suspension of its sediment.

Paul D Komar

Papers

Hydraulic Jumps in Turbidity Currents