Matthieu J.B. Cartigny

Supercritical and subcritical turbidity currents and their deposits - a synthesis

Common facies models of turbidite deposits are based on idealized sequences of turbidite units, which are assumed to reflect the depositional processes of a decelerating turbidity current. We show how suites of turbidite units, i.e., distinct turbidite facies associations that are easily described from core and outcrop, may characterize the entire range of large-scale dynamics of turbidity currents, enabling estimates of their densimetric Froude number (Fr; subcritical versus supercritical) and suspension fall-out rate (stratified versus nonstratified flows). The linking of facies associations with large-scale flow dynamics resolves process-facies links that were hitherto unresolved by the common turbidite facies models.

Cyclic steps along the South Taiwan Shoal and West Penghu submarine canyons on the northeastern continental slope of the South China Sea

Large-scale step-like features within the South Taiwan Shoal and West Penghu submarine canyons on the northeastern continental slope of the South China Sea are investigated by integrating high-resolution multibeam bathymetric data and multichannel seismic profiles. These step-like features, ranging from 1.2 to 10.0 km in wavelength and 5.4–80.9 m in wave height, are mostly interpreted as cyclic steps formed by turbidity currents flowing through the canyons, based on their characteristic step-like morphology, in-train alignment, large wavelengths and aspect ratios (ratio of wavelength to wave height), and typical upstream-sloping backset bedding, among others. A train of 19 continuous steps delineated along the thalweg of the South Taiwan Shoal canyon measures up to 100 km and may be the longest ever reported. Nine short trains of scours identified on a terrace of the South Taiwan Shoal canyon are oriented parallel to the distributaries draining over the terrace and roughly perpendicular to the main canyon thalweg, indicating a complicated flow pattern within the canyon valley. Two trains of scours separated by an intracanyon high in the steeper middle reach of the West Penghu canyon are interpreted as transitional bed forms between antidunes and cyclic steps, which develop downstream into a train of five net-depositional cyclic steps with typical backset bedding in the gentler-sloping lower reach of the canyon. Average slope gradients for the canyon reaches with cyclic steps range from 0.26° to 1.24°. Along each thalweg step train, a slope break is identified to separate the net-erosional cyclic steps in the steeper upstream segment from the net-depositional ones in the gentler downstream segment. Rough estimations indicate that the paleoflows are 100 to 300 m thick with maximum velocities of up to 10 m s–1. The estimated flow depths match well with those inferred from geomorphologic analysis. Estimated paleodischarges of ?7–23 × 105 m3 s–1 are equivalent to ten times the discharge of the modern Amazon River.

Morphodynamics of submarine channel inception revealed by new experimental approach

Submarine channels are ubiquitous on the seafloor and their inception and evolution is a result of dynamic interaction between turbidity currents and the evolving seafloor. However, the morphodynamic links between channel inception and flow dynamics have not yet been monitored in experiments and only in one instance on the modern seafloor. Previous experimental flows did not show channel inception, because flow conditions were not appropriately scaled to sustain suspended sediment transport. Here we introduce and apply new scaling constraints for similarity between natural and experimental turbidity currents. The scaled currents initiate a leveed channel from an initially featureless slope. Channelization commences with deposition of levees in some slope segments and erosion of a conduit in other segments. Channel relief and flow confinement increase progressively during subsequent flows. This morphodynamic evolution determines the architecture of submarine channel deposits in the stratigraphic record and efficiency of sediment bypass to the basin floor.

Supercritical-flow structures on a Late Carboniferous delta front: Sedimentologic and paleoclimatic significance

Deposits of fluvial systems in highly seasonal tropical climates possess unique architectural and facies characters owing to a flood-prone regime alternating with lengthy periods of ineffective discharge. Distally linked deltaic successions should also feature distinctive attributes, with great potential to preserve the stratigraphic evidence of exceptional discharge events. We describe Late Carboniferous delta-front, valley-confined sandstones from the Pennine Basin (UK), originally deposited at paleoequatorial latitudes during final assembly of the Pangean megacontinent and characterized by giant sedimentary structures with repetitively sigmoidal geometry. Facies traits indicate geologically instantaneous deposition of a large sediment volume from a density current at sustained supercritical-flow conditions, leading to aggradation of cyclic steps, recently identified bedforms developing in high-energy flows and of which this is the first complete outcrop example. The lack of unconformable erosional surfaces and absence of different associated facies point to a single aggradational event during which the structures attained dimensions comparable to those indicated by seismic data sets from which they are remotely detected on modern seafloors. Cyclic-step formation in a deltaic setting suggests that Pangean megamonsoons could have triggered hydrologic events capable of imprinting sedimentologic signatures on shallow-marine deposits.

Newly recognized turbidity current structure can explain prolonged flushing of submarine canyons

Runaway turbidity currents stretch into the deep ocean to form the largest sediment accumulations on Earth. 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.

A new model for turbidity current behavior based on integration of flow monitoring and precision coring in a submarine canyon

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.

Morphodynamics of supercritical turbidity currents in the channel-lobe transition zone

This study aims to resolve process-facies links at both bed and environmental scales for the channel lobe transition zone (CLTZ). Data comes from existing experimental and modern CLTZ studies and from new outcrop studies. The experiments show that the CLTZ architecture of supercritical turbidity currents is complex and different from their counterparts where flows are subcritical throughout. Supercritical CLTZ’s are characterised by erosive channels formed by supercritical turbidity currents, by offset stacked lobes deposited from subcritical turbidity currents and by hydraulic jump related mouth bar deposits and upslope onlapping backfill deposits at the down slope end of the transition zone. Erosive channels and backfill features can be resolved by high resolution seismic data, yet evidence for supercritical flow must come from facies analysis of core data. Outcrop examples of the CLTZ from the Tabernas submarine fan (SE Spain) and the Llorenc del Munt deep-water delta slope (N. Spain) are used to establish such links between seismic scale architecture and facies recognised in cores. The outcrops described here were mapped as transition zone, and show 100 m sized, spoon-shaped scours filled with sediment containing sandy to gravelly backsets up to 4 m in height. Their facies and architecture is indicative of deposition by hydraulic jumps, can be recognized from cores, and is a good proxy for further predicting CLTZ architecture constructed by supercritical turbidity currents.

Direct monitoring of active geohazards: emerging geophysical tools for deep-water assessments

Seafloor networks of cables, pipelines, and other infrastructure underpin our daily lives, providing communication links, information, and energy supplies. Despite their global importance, these networks are vulnerable to damage by a number of natural seafloor hazards, including landslides, turbidity currents, fluid flow, and scour. Conventional geophysical techniques, such as high-resolution reflection seismic and side-scan sonar, are commonly employed in geohazard assessments. These conventional tools provide essential information for route planning and design; however, such surveys provide only indirect evidence of past processes and do not observe or measure the geohazard itself. As such, many numerical-based impact models lack field-scale calibration, and much uncertainty exists about the triggers, nature, and frequency of deep-water geohazards. Recent advances in technology now enable a step change in their understanding through direct monitoring. We outline some emerging monitoring tools and how they can quantify key parameters for deepwater geohazard assessment. Repeat seafloor surveys in dynamic areas show that solely relying on evidence from past deposits can lead to an under-representation of the geohazard events. Acoustic Doppler current profiling provides new insights into the structure of turbidity currents, whereas instrumented mobile sensors record the nature of movement at the base of those flows for the first time. Existing and bespoke cabled networks enable high bandwidth, low power, and distributed measurements of parameters such as strain across large areas of seafloor. These techniques provide valuable new measurements that will improve geohazard assessments and should be deployed in a complementary manner alongside conventional geophysical tools.

How to recognize crescentic bedforms formed by supercritical turbidity currents in the geologic record: Insights from active submarine channels

Submarine channels have been important throughout geologic time for feeding globally significant volumes of sediment from land to the deep sea. Modern observations show that submarine channels can be sculpted by supercritical turbidity currents (seafloor sediment flows) that can generate upstream-migrating bedforms with a crescentic planform. In order to accurately interpret supercritical flows and depositional environments in the geologic record, it is important to be able to recognize the depositional signature of crescentic bedforms. Field geologists commonly link scour fills containing massive sands to crescentic bedforms, whereas models of turbidity currents produce deposits dominated by back-stepping beds. Here we reconcile this apparent contradiction by presenting the most detailed study yet that combines direct flow observations, time-lapse seabed mapping, and sediment cores, thus providing the link from flow process to depositional product. These data were collected within the proximal part of a submarine channel on the Squamish Delta, Canada. We demonstrate that bedform migration initially produces back-stepping beds of sand. However, these back-stepping beds are partially eroded by further bedform migration during subsequent flows, resulting in scour fills containing massive sand. As a result, our observations better match the depositional architecture of upstream-migrating bedforms produced by fluvial models, despite the fact that they formed beneath turbidity currents.

Which triggers produce the most erosive, frequent and longest runout turbidity currents on deltas ?

Subaerial rivers and turbidity currents are the two most voluminous sediment transport processes on our planet, and it is important to understand how they are linked offshore from river mouths. Previously it was thought that slope failures or direct plunging of river flood water (hyperpycnal flow) dominated the triggering of turbidity currents on delta-fronts. Here we re-analyse the most detailed time-lapse monitoring yet of a submerged delta; comprising 93 surveys of the Squamish Delta in British Columbia, Canada. We show that most turbidity currents are triggered by settling of sediment from dilute surface river plumes, rather than landslides or hyperpycnal flows. Turbidity currents triggered by settling plumes occur frequently, run out as far as landslide-triggered events, and cause the greatest changes to delta and lobe morphology. For the first time, we show that settling from surface plumes can dominate the triggering of hazardous submarine flows and offshore sediment fluxes.