No abstract

On the Amazon Fan, the meandering Amazon Channel is flanked by levees tens of meters to >100 m high. Grain-size characteristics of the thicker and coarser grained spillover turbidites recovered by coring of the levees can be used to infer the nature of the suspended load and velocity of Pleistocene turbidity currents that transited the channel. Magnetic-mineral alignments in these turbidites provide estimates of flow directions of the overspilling currents. Together, these data augment our understanding of the development and maintenance of large submarine channels. The grain size of spillover turbidites from 10 to 50 m below the seafloor at Ocean Drilling Program Sites 939, 940, 934, 936, 944, and 946 was determined using a Sedigraph 5100 particle-size analyzer. The magnetic alignments were determined by measuring anisotropy of magnetic susceptibility using a Kappabridge KLY-2 susceptibility meter.From the upper to the lower fan, the median size increases and the levee height decreases. Paleoflow during overspill was at a high angle to levee crests, but with considerable dispersion. Paleoflow data can only be properly interpreted in conjunction with information on the local channel shape and the position of low points, or saddles, in the adjacent levee crest. Comparison of (1) the texture of spillover turbidites with (2) grain sizes of sand in the talweg of the Amazon Channel, (3) independent velocity estimates based on differential levee heights, and (4) suspension theory, leads to the conclusion that a single type of mixed-load turbidity current could have transported very coarse sand as bedload along the talweg and contributed to levee growth by spilling a suspension of mainly silt and mud from the flow top. While transiting the Amazon Channel, such turbidity currents were constantly entraining seawater, but were losing a greater volume of dilute suspension from the flow top through overspill. As a result, there is a thirtyfold decrease in channel cross-sectional area from the upper fan to the lower fan. On the lower fan, with levee heights less than 25 m, even some of the sand load from the lower part of turbidity currents was lost to overspill.

No abstract

No abstract

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

No abstract

No abstract

The parautochthonous Cloridorme Formation is a syn‐orogenic flysch succession that was deposited in an elongate foredeep basin as mainly lower middle‐fan, outer‐fan, and basin‐floor deposits. The basin‐floor deposits (about 1.5 km thick) are confined to members β1, β2 and γ1, and are characterized by graded, thick (1–10 m) mud‐rich calcareous greywacke beds previously interpreted as deposits of concentrated, muddy, unidirectional turbidity currents that locally generated backset (antidune) lamination in internally stratified flows. The dominant flow directions were from east to west, but west to east transport also occurred, as seen in the orientation of ripples, climbing ripples, flutes, consistently overturned flames, and grain imbrication. We believe that the flows that deposited these thick calcareous greywacke beds reversed by roughly 180° one or more times during deposition of the lower sandy part of the beds. Flow reversals are consistent with the sharp grain‐size breaks and mud partings within sandy divisions. Measurement of grain fabric relative to stratification in the most celebrated ‘antidune’ bedforms indicates flow from west to east; thus, the bedforms were produced by west‐to‐east migration of megaripples, not by the upcurrent migration of antidunes. The thick muddy beds were deposited by large‐volume, muddy flows that were deflected and reflected from the side slopes and internal topographic highs of a confined basin floor, much like the ‘Contessa’ and similar beds of the Italian Apennines. Large quantities of suspended mud were ponded above the irregular basin floor and settled to produce the thick silty mudstone caps seen on each bed. Because of their mode of emplacement, we propose that these beds be called contained turbidites.