ALBERT

Description: Many hydrocarbon reservoirs occur within confined turbidite systems in which the depositional pattern of turbidity currents has been strongly influenced by basin-floor topography. In certain settings basin-floor topography may cause the development of anomalously thick (tens of metres) sandstones that are potentially excellent reservoir units. Southern exposures of the Peira Cava outlier (Eocene-Oligocene; Annot Sandstones) provide well-exposed outcrops of such decametre-thick sandstone bodies. These units are located close to basin margins and downstream from an inferred topographic break-in-slope. Several base-of-slope sandstone bodies are examined that illustrate a common sedimentary theme of a complex basal unit, comprising laterally pinching or inter-fingering debrite and turbidite, abruptly overlain by a single, thick normally graded turbidite deposit. One of these sandstone bodies pinches out laterally over less than several hundred metres and sits within a deep (〉20 m) spoon shaped' erosional scour. The scour is similar to morphological features observed in modern base-of-slope settings recently imaged using high-resolution submarine bathymetric surveys. Several different process interpretations may explain the occurrence of such sandstone bodies including remobilization of newly deposited sediment off basinmargins and enhanced deposition due to flow across a break-in-slope. A submarine channel interpretation is not consistent with the field observations. However, these units do share a number of similar features to channels that could lead to the misinterpretation of reservoir geometry.

Description: Submarine debris flows show highly variable mixing behaviour. Glacigenic debris flows travel hundreds of kilometres along the sea floor without undergoing significant dilution. However, in other locations, submarine slope failures may transform into turbidity currents before exiting the continental slope. Rates and processes of mixing have not been measured directly in submarine flow events. Our present understanding of these rates and processes is based on experimental and theoretical constraints. Significant experimental and theoretical work has been completed in recent years to constrain rates of shear mixing between static layers of sediment and overlying turbulent flows of water. This work was driven by a need to predict transport of fluid mud and the erosion of cohesive mud beds in shallow water settings such as estuaries, docks and shipping channels. These experimental measurements show that the critical shear stress necessary to initiate shear mixing (around 0.1 to 2 Pa) is typically several orders of magnitude lower than the yield strength of the debris. Shear mixing should initiate at relatively low velocities (about 10-200 cm s-1) on the upper surface of a submarine debris flow, at even lower velocities at its head (about 1-10 cm s-1), and play an important role in mixing over-ridden water into the debris flow. Addition of small amounts of mud (approximately 3% kaolin) to a sand bed dramatically reduces the rate of mixing at its boundary, and changes the processes by which sediment is removed. Estimates are presented for rates of shear mixing at a given flow velocity, and for the critical velocity necessary for hydroplaning or a transition from laminar to turbulent flow. Although these estimates are crude, and highlight the need for further experimental work, they illustrate the potential for highly variable mixing behaviour in submarine flow events.

Description: Seabed-hugging flows called turbidity currents are the volumetrically most important process transporting sediment across our planet and form its largest sediment accumulations. We seek to understand the internal structure and behavior of turbidity currents by reanalyzing the most detailed direct measurements yet of velocities and densities within oceanic turbidity currents, obtained from weeklong flows in the Congo Canyon. We provide a new model for turbidity current structure that can explain why these are far more prolonged than all previously monitored oceanic turbidity currents, which lasted for only hours or minutes at other locations. The observed Congo Canyon flows consist of a short-lived zone of fast and dense fluid at their front, which outruns the slower moving body of the flow. We propose that the sustained duration of these turbidity currents results from flow stretching and that this stretching is characteristic of mud-rich turbidity current systems. The lack of stretching in previously monitored flows is attributed to coarser sediment that settles out from the body more rapidly. These prolonged seafloor flows rival the discharge of the Congo River and carry ~2% of the terrestrial organic carbon buried globally in the oceans each year through a single submarine canyon. Thus, this new structure explains sustained flushing of globally important amounts of sediment, organic carbon, nutrients, and fresh water into the deep ocean.

Description: Submarine turbidity currents create some of the largest sediment accumulations on Earth, yet there are few direct measurements of these flows. Instead, most of our understanding of turbidity currents results from analyzing their deposits in the sedimentary record. However, the lack of direct flow measurements means that there is considerable debate regarding how to interpret flow properties from ancient deposits. This novel study combines detailed flow monitoring with unusually precisely located cores at different heights, and multiple locations, within the Monterey submarine canyon, offshore California, USA. Dating demonstrates that the cores include the time interval that flows were monitored in the canyon, albeit individual layers cannot be tied to specific flows. There is good correlation between grain sizes collected by traps within the flow and grain sizes measured in cores from similar heights on the canyon walls. Synthesis of flow and deposit data suggests that turbidity currents sourced from the upper reaches of Monterey Canyon comprise three flow phases. Initially, a thin (38–50 m) powerful flow in the upper canyon can transport, tilt, and break the most proximal moorings and deposit chaotic sands and gravel on the canyon floor. The initially thin flow front then thickens and deposits interbedded sands and silty muds on the canyon walls as much as 62 m above the canyon floor. Finally, the flow thickens along its length, thus lofting silty mud and depositing it at greater altitudes than the previous deposits and in excess of 70 m altitude.

Description: The Miocene Marnoso-arenacea Formation (Italy) is the only ancient sequence where deposits of individual submarine density flow deposits have been mapped in detail for long (〉100 km) distances, thereby providing unique information on how such flows evolve. These beds were deposited by large and infrequent flows in a low-relief basin plain. An almost complete lack of bed amalgamation aids bed correlation, and resembles some modern abyssal plains, but contrasts with ubiquitous bed amalgamation seen in fan-lobe deposits worldwide. Despite the subdued topography of this basin plain, the beds have a complicated character. Previous work showed that a single flow can commonly comprise both turbidity current and cohesive mud-rich debris flows. The debris flows were highly mobile on low gradients, but their deposits are absent in outcrops nearest to source. Similar hybrid beds have been documented in numerous distal fan deposits worldwide, and they represent an important process for delivering sediment into the deep ocean. It is therefore important to understand their origin and flow dynamics. To account for the absence of debrites in proximal Marnoso-arenacea Formation outcrops, it was proposed that debris flows originated within the study area due to erosion of mud-rich seafloor; we show that this is incorrect. Clast and matrix composition show that sediment within the cohesive debris flows originated outside the study area. Previous work showed that intermediate and low strength debris flows produced different downflow-trending facies tracts. Here, we show that intermediate strength debris flows entered the study area as debris flows, while low strength (clast poor) debris flows most likely formed through local transformation from an initially turbulent mud-rich suspension. New field data document debrite planform shape across the basin plain. Predicting this shape is important for subsurface oil and gas reservoirs. Low strength and intermediate strength debrites have substantially different planform shapes. However, the shape of each type of debrite is consistent. Low strength debrites occur in two tongues at the margins of the outcrop area, while intermediate strength debrite forms a single tongue near the basin center. Intermediate strength debrites are underlain by a thin layer of structureless clean sandstone that may have settled out from the debris flow at a late stage, as seen in laboratory experiments, or been deposited by a forerunning turbidity current that is closely linked to the debris flow. Low strength debrites can infill relief created by underlying dune crests, suggesting gentle emplacement. Dewatering of basal clean sand did not cause a long runout of debris flows in this location. Hybrid beds are common in a much thicker stratigraphic interval than was studied previously, and the same two types of debrite occur there. Hybrid flows transported large volumes (as much as 10 km 3 per flow) of sediment into this basin plain, over a prolonged period of time.

Description: Turbidite paleoseismology aims to use submarine gravity flow deposits (turbidites) as proxies for large earthquakes, a critical assumption being that large earthquakes generate turbidity currents synchronously over a wide area. We test whether all large earthquakes generate synchronous turbidites, and if not, investigate where large earthquakes fail to do this. The Sumatran margin has a well-characterized earthquake record spanning the past 200 yr, including the large-magnitude earthquakes in 2004 ( M w 9.1) and 2005 ( M w 8.7). Sediment cores collected from the central Sumatran margin in 2009 reveal that surprisingly few turbidites were emplaced in the past 100–150 yr, and those that were deposited are not widespread. Importantly, slope basin deposits preserve no evidence of turbidites that correlate with the earthquakes in 2004 and 2005, although recent flow deposits are seen in the trench. Adjacent slope basins and adjacent pairs of slope basin and trench sites commonly have different sedimentary records, and cannot be correlated. These core sites from the central Sumatran margin do not support the assumption that all large earthquakes generate the widespread synchronous turbidites necessary for reconstructing an accurate paleoearthquake record.