The origin of massive sands in turbidite successions has commonly been attributed to the rapid dumping of sand due to flow unsteadiness in collapsing, single surge‐type, high‐density turbidity currents. The general applicability of this model is questioned here, and we propose that rapid deposition of massive sands also occurs due to non‐uniformity in prolonged, quasi‐steady high‐density turbidity currents. We attempt to eliminate ambiguity in the use of the terms ‘deceleration’and ‘unsteadiness’with respect to non‐uniform sediment gravity flows, and stress that, as with any particulate current, unsteadiness is not a prerequisite of sediment deposition. We propose a mechanism of gradual aggradation of sand beneath a sustained steady or quasi‐steady current, and upward‐migration of a depositional flow boundary that is dominated by grain hyperconcentration and hindered settling. Formation of tractional structures is prevented by the absence of a sharp rheological interface between the lowest parts of the flow and the just‐formed dewatering deposit. Deposition continues as long as the downward grain flux to the depositional flow boundary is balanced by grain supply from above or from upcurrent. Massive sand deposited in this way is not, strictly, a result of ‘direct suspension sedimentation’in that it is characterized by grain interactions, hindered settling, shear and, possibly, by interlocking of grains. The thickness of the resulting massive sand bears no relation to the thickness of the parental current, and the vertical variation within the deposit may reveal little about the vertical structure of the current, even during deposition. Thin, normally graded tops or mud drapes represent the eventual waning of sustained currents.

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Abstract. Gravity currents are of considerable environmental and industrial importance as hazards and as agents of sediment transport, and the deposits of ancient turbidity currents form some significantly large hydrocarbon reservoirs. Prediction of the behavior of these currents and the nature and distribution of their deposits require an understanding of their turbulent structure. To this end, a series of experiments was conducted with turbulent, subcritical, brine underflows in a rectangular lock-exchange tank. Laser-Doppler anemometry was used to construct a twodimensional picture of the velocity structure. The velocity maximum within the gravity current occurs at y/d • 0.2. The shape of the velocity profile is governed by the differing and interfering effects of the lower (rigid) and upper (diffuse) boundaries and can be approximated with the law of the wall up to the velocity maximum and a cumulative Gaussian distribution from the velocity maximum to the ambient interface. Mean motion within the head consists of a single large vortex and an overall motion of fluid away from the bed, and this largely undiluted fluid becomes rapidly mixed with ambient fluid in the wake region. The distribution of turbulence within the current is heterogeneous and controlled by the location of large eddies that dominate the turbulent energy spectrum and scale with flow thickness. Turbulent kinetic energy reaches a maximum in the shear layer at the upper boundary of the flow where the large eddies are generated and is at a minimum near the velocity maximum where fluid shear is low.

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Submarine channel related thin-bedded turbidites are deposited in environments such as external levees, internal levees, depositional terraces and at times of channel abandonment.Thin-bedded turbidites are defined as beds that are less than 10cm thick, but the described environments can at times contain beds up to 50cm thick which would be classified as medium or thick-bedded (Boggs, 2006). This paper addresses many examples of these environments from the modern seafloor, outcrop and the subsurface to suggest criteria that assist in the differentiation of levees and terraces from an architectural, sedimentological, M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 2 ichnological and hydrocarbon reservoir perspective. External levees confine channel belts and are elongate sedimentary deposits that are a product of over-spill of turbidity currents from the channel belt they confine. External levees often have predictable vertical, lateral and downstream changes but can commonly be modified by collapse of the inner external levee into the channel, collapse on the outer external levee, sediment waves, and interaction of external levees with topographic features such as other channels, other external levees, basin margins or previous slump/slide blocks, which can greatly modify the sand distribution within them.A combination of internal levees, depositional terraces and slide blocks of external levee sediment make up thin-bedded turbidites within channel belts. We differentiate between wedge shaped internal levees and topographically flat or subdued depositional terraces whose differing geometries and sand distribution reflect the fact that the flow processes involved in the formation of these deposits are different. The characteristic wedge shape of an internal levee requires sufficient space within the channel belt for the over-spilling current to decelerate and deposit the majority of its suspended sediment before reaching the bounding topography of the channel belt. In the case of depositional terraces the space available in the channel belt is insufficient for the current to deposit the majority of its sediment before reaching the bounding topography of the channel belt, creating confined sheet-like deposits.External levees, internal levees and depositional terraces have distinct sedimentological characteristics such as sand bed thickness trends and sedimentary structures that can be used to distinguish them. Together with sedimentological characteristics, in some systems these thin-bedded turbidite deposits contain distinctive trace fossil assemblages, where channel proximal deposits such as proximal external levees, internal levees and depositional terraces M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT 3 can have much higher biodiversity than sand-rich channel axes and more mud-dominated outer external levees.The depositional sites for internal levees and depositional terraces within channel belts can be formed by various processes such as entrenchment, point bar accretion, meander bend cutoff, ...

[1] We introduce a computational model for high-resolution simulations of particle-laden gravity currents. The features of the computational model are described in detail, and validation data are discussed. Physical results are presented that focus on the influence of particle entrainment from the underlying bed. As turbulent motions detach particles from the bottom surface, resuspended particles entrained over the entire length of the current are transferred to the current's head, causing it to become denser and potentially accelerating the front of the current. The conditions under which turbidity currents may become self-sustaining through particle entrainment are investigated as a function of slope angle, current and particle size, and particle concentration. The effect of computational domain size and initial aspect ratio of the current on the evolution of the current are also considered. Applications to flows traveling over a surface of varying slope angle, such as turbidity currents spreading down the continental slope, are modeled via a spatially varying gravity vector. Particular attention is given to the resulting particle deposits and erosion patterns.

High-resolution turbulence data from refractive index-matched gravity currents have been used to quantify the mean flow and turbulence structure in steady, experimental gravity currents. Comparison of this data-set with experimental and theoretical gravity current data from the literature and with turbulent wall jets, reveals several new insights into both subcritical and supercritical flows. Existing data collapse approaches can be improved with the use of a new characteristic lengthscale taken from the wall jet literature. Turbulence production from shear has been quantified and is seen to be most significant in subcritical currents. Density stratification is also shown to be an important control on the distribution of turbulent kinetic energy. A slow diffusion zone (SDZ) characterized by low turbulence intensities and reduced vertical mass transport in the lower part of the current, around the level of the velocity maximum, has been identified, and related to both density stratification and reduced turbulence production around the velocity maximum. The results presented in this chapter should lead to improved understanding of gravity current dynamics and provide a good test for the output of numerical models.Particulate Gravity Currents. Edited