Submarine sediment density flows are one of the most important processes for moving sediment across our planet, yet they are extremely difficult to monitor directly. The speed of long run-out submarine density flows has been measured directly in just five locations worldwide and their sediment concentration has never been measured directly. The only record of most density flows is their sediment deposit. This article summarizes the processes by which density flows deposit sediment and proposes a new single classification for the resulting types of deposit. Colloidal properties of fine cohesive mud ensure that mud deposition is complex, and large volumes of mud can sometimes pond or drain-back for long distances into basinal lows. Deposition of ungraded mud (TE-3) most probably finally results from en masse consolidation in relatively thin and dense flows, although initial size sorting of mud indicates earlier stages of dilute and expanded flow. Graded mud (TE-2) and finely laminated mud (TE-1) most probably result from floc settling at lower mud concentrations. Grain-size breaks beneath mud intervals are commonplace, and record bypass of intermediate grain sizes due to colloidal mud behaviour. Planar-laminated (TD) and ripple cross-laminated (TC) non-cohesive silt or fine sand is deposited by dilute flow, and the external deposit shape is consistent with previous models of spatial decelerating (dissipative) dilute flow. A grain-size break beneath the ripple cross-laminated (TC) interval is common, and records a period of sediment reworking (sometimes into dunes) or bypass. Finely planar-laminated sand can be deposited by low-amplitude bed waves in dilute flow (TB-1), but it is most likely to be deposited mainly by high-concentration near-bed layers beneath high-density flows (TB-2). More widely spaced planar lamination (TB-3) occurs beneath massive clean sand (TA), and is also formed by high-density turbidity currents. High-density turbidite deposits (TA, TB-2 and TB-3) have a tabular shape consistent with hindered settling, and are typically overlain by a more extensive drape of low-density turbidite (TD and TC,). This core and drape shape suggests that events sometimes comprise two distinct flow components. Massive clean sand is less commonly deposited en masse by liquefied debris flow (DCS), in which case the clean sand is ungraded or has a patchy grain-size texture. Clean-sand debrites can extend for several tens of kilometres before pinching out abruptly. Up-current transitions suggest that clean-sand debris flows sometimes form via transformation from high-density turbidity currents. Cohesive debris flows can deposit three types of ungraded muddy sand that may contain clasts. Thick cohesive debrites tend to occur in more proximal settings and extend from an initial slope failure. Thinner and highly mobile low-strength cohesive debris flows produce extensive deposits restricted to distal areas. These low-strength debris flows may contain clasts and travel long distances (DM-2), or result from more local flow transformation due to turbulence damping by cohesive mud (DM-1). Mapping of individual flow deposits (beds) emphasizes how a single event can contain several flow types, with transformations between flow types. Flow transformation may be from dilute to dense flow, as well as from dense to dilute flow. Flow state, deposit type and flow transformation are strongly dependent on the volume fraction of cohesive fine mud within a flow. Recent field observations show significant deviations from previous widely cited models, and many hypotheses linking flow type to deposit type are poorly tested. There is much still to learn about these remarkable flows.

Subaqueous sediment density flows: Depositional processes and deposit types

Hybrid flows comprising both turbidity current and submarine debris flow are a significant departure from many previous influential models for submarine sediment density flows. 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 flows, and cohesive debris flows more generally, are best classified in terms of a continuum of decreasing cohesive debris flow strength. High-strength cohesive debris flows tend to be clast rich and relatively thick, and their deposit extends back to near the site of original slope failure. They are typically confined 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 flows 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 far-traveled, and meter-sized clasts can be rafted long distances across very low gradients if they are less dense than surrounding flow. Low-strength cohesive debris flows generally lack mud clasts, and as cohesive strength decreases further there is a transition into fluid 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 flows are more likely to be turbulent. Intermediate-strength debris flows can run out for long distances on low gradients without hydroplaning. Very low strength cohesive debris flows 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 seafloor 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, flow viscosity, permeability, and the time taken for excess pore pressures to dissipate that span multiple orders of magnitude. Reduction in flow speed can also cause substantial increases in viscosity and yield strength in shear thinning muddy fluids. Small amounts of sediment can dampen or extinguish turbulence, especially as flow decelerates, affecting how sediment is supported or deposited. This ensures that cohesive debris flows and hybrid flows have a rich variety of behaviors.

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

Recent advances in offshore geotechnics for deep water oil and gas developments

Test data to compare the resulting behavior with that observed under cyclic loading triaxial test conditions are presented. It is concluded that the liquefaction characteristics observed in the two types of tests are qualitatively similar. However, under cyclic simple shear conditions, which provide a close simulation of the stresses induced under field conditions, the cyclic shear stresses required to cause liquefaction were only about 35% of those determined by more approximate triaxial test procedures. It is suggested, however, that limitations of the simple shear test procedure induce liquefaction at stress levels perhaps 40% to 50% lower than those corresponding to actual field conditions, and that a correction factor of this order of magnitude should be applied to laboratory simple shear test data intended for use in the analyses of field problems. The test program also showed that frequency effects, within the range from 1/6 cps to 4 cps, are small.

Closure to “Sand Liquefaction Under Cyclic Loading Simple Shear Conditions”

Deepwater offshore oil and gas developments require an assessment to be made of the risk of infrastructure damage from submarine slides. The likelihood and magnitude of submarine slides, and the co...

/pdf/strength-of-fine-grained-soils-at-the-solid-fluid-transition-87j6mtsew1.pdf

Strength of fine-grained soils at the solid-fluid transition

Rheological properties of fine-grained sediments depending on index properties and salinity were examined. To characterize flow behaviors as a function of soil type, groups were made for convenienc...

Rheological properties of fine-grained sediment: the roles of texture and mineralogy.

The rate-dependent rheological behaviour of soils of different origins and characteristics was studied and the applicability of the power law model was examined. The studied soils were divided into...

Applicability of power law for describing the rheology of soils of different origins and characteristics

Thixotropic and anti-thixotropic behaviors of fine-grained soils in various flocculated systems

Particle–particle interactions in natural clays can be evaluated by their rheological behavior, but the results are often affected by the physicochemical properties of the clays. The behaviors of two fundamentally different types of clays (low-activity and high-activity) differ with respect to salinity and a time factor (duration of shearing at a given shear rate): illite-rich Jonquiere clay (low-activity clay, Canada) and montmorillonite-rich Wyoming bentonite (high-activity clay, USA). The purpose of the present study was to investigate these different behaviors. Most natural clays exhibit shear-thinning and thixotropic behavior with respect to salinity and the volumetric concentration of the solids. Natural clays also exhibit time-dependent non-Newtonian behavior. In terms of index value and shear strength, lowactivity and high-activity clays are known to exhibit contrasting responses to salinity. The geotechnical and rheological characteristics as a function of salinity and the shearing time for the given materials are compared here. The clay minerals were compared to estimate the inherent shear strengths, such as remolded shear strength (which is similar to the yield strength). Low-activity clay exhibits thixotropic behavior in a time-dependent manner. High-activity clay is also thixotropic for a short period of shearing, although rare cases of rheopectic behavior have been measured for long periods of shearing at high shear rates. The change from thixotropic to rheopectic behavior by bentonite clay has little effect at low shearing speeds, but appears to have a significant effect at higher speeds.

The Effects of Salinity and Shear History on The Rheological Characteristics of Illite-Rich and Na-Montmorillonite-Rich Clays

The frequency of ground motions during earthquakes is typically in the order of a few hertz As the earthquake-induced liquefaction of soils is widely assessed by performing laboratory tests, it is necessary to consider various loading frequencies generated by real earthquakes The effect of loading frequency has been studied by cyclic triaxial tests; however, it has rarely been investigated by cyclic direct simple shear tests, which are more similar to the cyclic loading conditions associated with earthquakes In this study, a series of cyclic direct simple shear tests were performed on clean sand with a relative density (Dr) of 40% (loose sand) and 80% (dense sand), obtained from Nakdong River The parameters considered are the initial vertical effective stresses (σv0′ = 50, 100, and 200 kPa) and the loading frequencies (f = 005, 01, 05, and 1 Hz) to evaluate the effect of the loading frequency on the liquefaction prediction of clean sand The results showed that the liquefaction resistance of the sand increases with the increase in the loading frequency, regardless of the initial vertical effective stress and relative density When the loading frequency increased from 01 to 05 or 1 Hz, the maximum increase in the cyclic resistances were 15%, and 19% for loose and dense sand, respectively For a given loading frequency, the liquefaction resistance of the sand decreased when the initial vertical effective stress increased

Sueng Won Jeong

Papers

Rheological properties of fine-grained sediment: the roles of texture and mineralogy.

Applicability of power law for describing the rheology of soils of different origins and characteristics

Thixotropic and anti-thixotropic behaviors of fine-grained soils in various flocculated systems

The Effects of Salinity and Shear History on The Rheological Characteristics of Illite-Rich and Na-Montmorillonite-Rich Clays

Effect of Cyclic Loading Frequency on Liquefaction Prediction of Sand