Turbidity currents, and other types of submarine sediment density flow, redistribute more sediment across the surface of the Earth than any other sediment flow process, yet their sediment concentration has never been measured directly in the deep ocean. The deposits of these flows are of societal importance as imperfect records of past earthquakes and tsunamogenic landslides and as the reservoir rocks for many deep-water petroleum accumulations. Key future research directions on these flows and their deposits were identified at an informal workshop in September 2013. This contribution summarizes conclusions from that workshop, and engages the wider community in this debate. International efforts are needed for an initiative to monitor and understand a series of test sites where flows occur frequently, which needs coordination to optimize sharing of equipment and interpretation of data. Direct monitoring observations should be combined with cores and seismic data to link flow and deposit character, whilst experimental and numerical models play a key role in understanding field observations. Such an initiative may be timely and feasible, due to recent technological advances in monitoring sensors, moorings, and autonomous data recovery. This is illustrated here by recently collected data from the Squamish River delta, Monterey Canyon, Congo Canyon, and offshore SE Taiwan. A series of other key topics are then highlighted. Theoretical considerations suggest that supercritical flows may often occur on gradients of greater than , 0.6u. Trains of up-slope-migrating bedforms have recently been mapped in a wide range of marine and freshwater settings. They may result from repeated hydraulic jumps in supercritical flows, and dense (greater than approximately 10% volume) near-bed layers may need to be invoked to explain transport of heavy (25 to 1,000 kg) blocks. Future work needs to understand how sediment is transported in these bedforms, the internal structure and preservation potential of their deposits, and their use in facies prediction. Turbulence damping may be widespread and commonplace in submarine sediment density flows, particularly as flows decelerate, because it can occur at low (, 0.1%) volume concentrations. This could have important implications for flow evolution and deposit geometries. Better quantitative constraints are needed on what controls flow capacity and competence, together with improved constraints on bed erosion and sediment resuspension. Recent advances in understanding dilute or mainly saline flows in submarine channels should be extended to explore how flow behavior changes as sediment concentrations increase. The petroleum industry requires predictive models of longer-term channel system behavior and resulting deposit architecture, and for these purposes it is important to distinguish between geomorphic and stratigraphic surfaces

A self‐accelerating current is a particle‐driven gravity flow moving on a sloping bottom whose velocity increases in the downstream direction as a result of increasing suspended sediment concentration due to sediment entrainment from the bed. This implies that the net balance between deposition from the current onto the bed and erosion into the flow must be favorable to the latter; thus, a larger mass of particles is being picked up into suspension than is settling out. The self‐accelerative stage cannot continue indefinitely. Either the downstream bed slope drops off to the point where self‐acceleration cannot be maintained or an autosuspensive stage may be reached where the net balance between deposition and erosion is zero and the channel bed is partially or completely free of alluvium. Once such a state is reached on a constant bed slope, the current can persist indefinitely without any external supply of energy other than the potential energy offered by the slope itself. This paper documents experimental turbidity currents composed of lightweight plastic particles ranging from 20 to 200 μm with a specific density between 1.3 and 1.5. These particles were either noncohesive or slightly cohesive. The experiments were performed in a 15‐m long flume with a bottom slope of 0.05. Self‐acceleration of the head of the flow was achieved in some of the tests reported here. Measurements of velocity and suspended sediment taken at different stages of head evolution document this self‐acceleration. In addition, these measurements are in agreement with previous empirical studies relating to head thickness, concentration, velocity, and water depth. Stratigraphic analysis of the deposit shows the key role bed material plays in determining whether a given turbidity current will or will not accelerate. This factor ties the dynamics of a self‐accelerating current to the existence of deposits laid down by antecedent currents. The conditions of the present tests appear to fulfill previous autosuspension criteria relating to flow velocity, particle settling velocity, and bed slope. Densimetric Froude number similarity analysis is used to estimate equivalent parameters for field scale turbidity currents.

Seismostratigraphy, coring, and logging while drilling during Integrated Ocean Drilling Program Expeditions 319, 322, and 333 (Sites C0011/C0012) show three Miocene submarine fans in the NE Shikoku Basin, with broadly coeval deposits at Ocean Drilling Program Site 1177 and Deep Sea Drilling Project Site 297 (NW Shikoku Basin). The sediment dispersal patterns have major implications for paleogeographies at that time. The oldest, finer‐grained (Kyushu) fan has sheet‐like geometry; quartz‐rich flows were fed mostly from an ancestral landmass in the East China Sea. During prolonged hemipelagic mud deposition at C0011‐C0012 (~12.2 to 9.1 Ma), sand supply continued at Sites 1177 and 297. Sand delivery to much of the Shikoku Basin halted during a phase of sinistral strike slip to oblique plate motion, after which the Daiichi Zenisu Fan (~9.1 to 8.0 Ma) was fed by submarine channels. The youngest fan (Daini Zenisu; ~8.0 to 7.6 Ma) has sheet‐like geometry with thick‐bedded, coarse‐grained pumiceous sandstones. The pumice fragments were fed from a mixed provenance that included the collision zone of the Izu‐Bonin and Honshu Arcs. The shift from channelized to sheet‐like flows was favored by renewal of relatively rapid northward subduction, which accentuated the trench as a bathymetric depression. Increased sand supply appears to correlate with long‐term eustatic lowstands of sea level. The stratigraphic position and 3‐D geometry of the sandbodies have important implications for subduction‐related processes, including the potential for focused fluid flow and fluid overpressures above and below the plate boundary fault: In sheet‐like sands, pathways for fluid flow have greater horizontal permeability compared with those in channel sands.

Submarine turbidity currents are controlled by gravity acting on suspended sediments that pull water downslope along with them 1 . In addition to suspended sediments, turbidity currents also transport sediments at the base of the flow 2 , which causes the reorganization of basal sediments prior to the settling of suspended grains 3-6 . However, as turbidity currents reach areas with minimal slope, they cross a fall-velocity threshold beyond which the suspended sediments begin to stratify the flow. This process extinguishes the turbulence near the bed 7,8 . Here we use direct numerical simulation of turbidity currents to show that this extinction of turbulence eliminates the ability of the flow to re-entrain sediment and rework the sediment at the base of the flow. Our simulations indicate that deposits from flows without basal reworking should lack internal structures such as laminations. Under appropriate conditions, then, sustained delivery of fine sediments will therefore result in the emplacement of massive turbidites. We suggest that this mechanism can explain field observations of massive deposits 9 that were emplaced gradually by dilute but powerful turbidity currents. We also conclude that turbulence in submarine turbidity currents is more fragile than river systems, and more sensitive to damping by the stratification of suspended sediment in the flow.Turbidites are often characterized in terms of complete or partial manifestations of the Bouma sequence 10 . Units T a to T e in Fig. 1a correspond to a single flow event, with flow waning from bottom to top. Intervals T a and T b , tend to be sand. Interval T b has parallel laminations, indicating bedload reworking as sediment settles 3 . Interval T a is 'massive', that is, lacking sedimentary structures. Massive deep-sea turbidites are prominent features of channelized and unchannelized submarine fans. They are enigmatic in that they show little or none of the internal structure used to interpret emplacement mechanisms.Parallel laminations are seen in fluvial deposits as well as turbidites, and have been reproduced in the laboratory 3-6 . Their formation is due to the organization of sand grains into streaks according to size and orientation by bedload. Mechanisms for the emplacement of massive turbidites are more speculative. These units are common in the ancient rock record of deep-water deposits, as well as in deeper parts of modern continental margins. Individual event beds can be up to metres in thickness (Fig. 1b), and extend for tens to hundreds

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