How is a turbidite actually deposited?

The deposition of a classic turbidite by a surge-type turbidity current, as envisaged by conceptual models, is widely considered a discrete event of continuous sediment accumulation at a falling rate by the gradually waning density flow. Here, we demonstrate, on the basis of a high-resolution advanced numerical CFD (computational fluid dynamics) simulation and rock-record examples, that the depositional event in reality involves many brief episodes of nondeposition. The reason is inherent hydraulic fluctuations of turbidity current energy driven by interfacial Kelvin-Helmholtz waves. The experimental turbidity current, with realistic grain-size composition of a natural turbidite, used only 26 to 33% of its in-place flow time for deposition, while the remaining time went to the numerous episodes of sediment bypass and transient erosion. The general stratigraphic notion of a gross incompleteness of sedimentary record may then extend down to the deposition time scale of a single turbidite.publishedVersio

Morphodynamics and depositional signature of low-aggradation cyclic steps: New insights from a depth-resolved numerical model

Bedforms related to Froude-supercritical flow, such as cyclic steps, are increasingly frequently observed in contemporary fluvial and marine sedimentary systems. However, the number of observations of sedimentary structures formed by supercritical flow bedforms remains limited. The low number of observations might be caused by poor constrains on criteria to recognise these associated deposits. This study provides a detailed quantification on the mechanics of a fluvial cyclic step system, and their depositional signature. A computational fluid-dynamics model is employed to acquire a depth-resolved image of a cyclic step system. New insights into the mechanics of cyclic steps shows that: (i) the hydraulic jump is, in itself, erosional; (ii) there are periods over which the flow is supercritical throughout and there is no hydraulic jump, which plays a significant role in the morphodynamic behaviour of cyclic steps; and (iii) that the depositional signature of cyclic steps varies with rate of aggradation. Previous work has shown that strongly aggradational cyclic steps, where most of the deposited sediment is not reworked, create packages of backsets, bound upstream and downstream by erosive surfaces. Here the modelling work is focussed on less aggradational conditions and more transportational systems. The depositional signature in such systems is dominated by an amalgamation of concave-up erosional surfaces and low-angle foresets and backsets creating lenticular bodies. The difference between highly aggradational cyclic steps and low aggradation steps can be visible in outcrop both by the amount of erosional surfaces, as well as the ratio of foreset to backset, with backsets being indicative of more aggradation

Controls on upstream-migrating bed forms in sandy submarine channels

Submarine channels parallel river channels in their ability to transport sediment. However, in contrast to rivers, sediment transport and bed-form development in submarine channels are less well understood. Many steep (>1°), sandy submarine channels are dominated by upstream-migrating bed forms. The flow conditions required to form these upstream-migrating bed forms remain debated because the interactions between turbidity currents and active bed forms are difficult to measure directly. Consequently, we used a depth-resolved numerical model to test the role of flow parameters that are hypothesized to control the formation of upstream-migrating bed forms in submarine channels. While our modeling results confirmed the importance of previously identified flow parameters (e.g., densiometric Froude number), we found that basal sediment concentration in turbidity currents is the strongest predictor of upstream-migrating bed-form formation. Our model shows how locally steep gradients enable high sediment concentrations (average >5 vol%) in the basal parts of flows, which allow the development of cyclic step instabilities and their associated bed forms. This new insight explains the previously puzzling observation that upstream-migrating bed forms are abundant in proximal, steep, sandy reaches of submarine channels, while their occurrence becomes more intermittent downslope

Froude supercritical geophysical flows: Their related bedforms and frontal structure

Sediment transport around the globe is dominated by rivers and turbidity currents. While rivers shape the land, turbidity currents shape the ocean floor. These flows can pose hazards to infrastructure placed in their paths. The sedimentary deposits left behind by these flows are key to understanding how our planet evolved over geological timescales. This thesis aims to enhance the general understanding of these geophysical flows by tackling specific areas of flow dynamics and their deposits that have so far remained poorly understood.Rivers transport most of their sediment in rare flooding events. A specific type of bedform can occur during these floods, called cyclic steps. Little is known about these cyclic steps as their occurrence is rare and observations during such powerful floods are difficult to make. Here, cyclic steps are reproduced in a numerical model to constrain bedform morphyodynamics. Additionally, the model is used to predict how the occurrence of cyclic steps can be deduced from studying the deposits that are left behind by floods. The numerical modelling shows that the relation between flow properties and the occurrence of erosion deviate from existing models. The deposits arising from cyclic steps depend on how fast sediment is deposited. Cyclic steps that deposit rapidly form a series of upstream-dipping laminations bound by erosion surfaces. Cyclic steps that deposit slowly, however, form amalgamated concave-up erosion surfaces that are infilled with laminations that dip upstream and downstream. These results now allow geologist to reconstruct river floods throughout geological time with more confidence.In contrast to rivers, where cyclic steps are rare, similar bedforms are abundant in submarine channels, which are major conduits of sediment transport to the deep ocean. These bedforms are an important building block for submarine channels; just as dunes are for rivers. Unlike rivers, the formative controls on these bedforms in submarine channels are presently not well understood. Here I tested three hypotheses on the formation of the bedforms, again using a numerical model. (1) These bedforms only form under fast and thin flow, (2) the bedforms only form under stratified flow and (3) the bedforms only form under flows strong enough to erode sediment from the seafloor. The results of the study showed that (1) not all fast and thin flows create the bedforms, (2) only stratified flows, but not all, create the bedforms and (3) only flows that exceed a threshold of erosion created the bedforms. These results show that the bedforms form in submarine channels that fall within a “sweet spot” of particular grain-size and slope, and thus explains their abundance, but also their local absence.Finally, this thesis presents some direct observations of turbidity currents on the seafloor. These observations confirm the importance of stratification already noticed in the previous numerical modelling work. The observations show that the fronts of stratified turbidity currents are wedge-shaped, and host the fastest and densest part of the flow. These observations contrast the bulbous and dilute front found in unstratified turbidity currents seen in most modelling studies. The frontal structure of stratified turbidity currents shows remarkable similarities to that of stratified pyroclastic density currents and powder-snow avalanches. A fast and dense front on turbidity currents poses substantially larger hazards for any submarine infrastructure such as pipelines and telecommunication cables.<br/

How is a turbidite actually deposited?

The deposition of a classic turbidite by a surge-type turbidity current, as envisaged by conceptual models, is widely considered a discrete event of continuous sediment accumulation at a falling rate by the gradually waning density flow. Here, we demonstrate, on the basis of a high-resolution advanced numerical CFD (computational fluid dynamics) simulation and rock-record examples, that the depositional event in reality involves many brief episodes of nondeposition. The reason is inherent hydraulic fluctuations of turbidity current energy driven by interfacial Kelvin-Helmholtz waves. The experimental turbidity current, with realistic grain-size composition of a natural turbidite, used only 26 to 33% of its in-place flow time for deposition, while the remaining time went to the numerous episodes of sediment bypass and transient erosion. The general stratigraphic notion of a gross incompleteness of sedimentary record may then extend down to the deposition time scale of a single turbidite

Complex and cascading triggering of submarine landslides and turbidity currents at volcanic islands revealed from integration of high-resolution onshore and offshore surveys

Submerged flanks of volcanic islands are prone to hazards including submarine landslides that may trigger damaging tsunamis and fast-moving sediment-laden seafloor flows (turbidity currents) that break critical seafloor infrastructure. Small Island Developing States are particularly vulnerable to these hazards due to their remote and isolated nature, small size, high population densities and weak economies. Despite their vulnerability, few detailed offshore surveys exist for such islands, resulting in a geohazard ‘blindspot’, particularly in the South Pacific. Understanding how these hazards are triggered is important; however, pin-pointing specific triggers is challenging as most studies have been unable to link continuously between onshore and offshore environments, and focus primarily on large-scale eruptions with sudden production of massive volumes of sediment. Here we focus on a situation where volcanic sediment supply produces a long-term elevation over a “normal” regime, which is more similar to the long-term elevated sediment production cases at many sites (volcanic or not) where human-induced vegetation change over-supplies sediments to coastal margins. We address these issues by integrating the first detailed (2 m x 2 m) bathymetry data acquired from Tanna Island, Vanuatu with a combination of terrestrial remote sensing data, onshore and offshore sediment sampling, and documented historical events. Mount Yasur on Tanna has experienced low-magnitude Strombolian activity for at least the last 600 years. We find clear evidence for submarine landslides and turbidity currents, yet none of the identified triggers are related to major volcanic eruptions, in contrast to conclusions from several previous studies. Instead we find that cascades of non-volcanic events (including outburst floods with discharges of &gt;1000 m3/s, and tropical cyclones), that may be separated by decades, are more important for preconditioning and triggering in chronic sediment oversupply regimes such as at Tanna. We conclude with a general model for how submarine landslides and turbidity currents are triggered at volcanic and other heavily eroding mountainous islands. Our model highlights the often-ignored importance of outburst floods, non-linear responses to lands-use and climatic changes, and the complex interactions between a range of coastal and tectonic processes that may overshadow volcanic regimes

Complex and Cascading Triggering of Submarine Landslides and Turbidity Currents at Volcanic Islands Revealed From Integration of High-Resolution Onshore and Offshore Surveys

Submerged flanks of volcanic islands are prone to hazards including submarine landslides that may trigger damaging tsunamis and sediment-laden seafloor flows (called “turbidity currents”). These hazards can break seafloor infrastructure which is critical for global communications and energy transmission. Small Island Developing States are particularly vulnerable to these hazards due to their remote and isolated nature, small size, high population densities, and weak economies. Despite their vulnerability, few detailed offshore surveys exist for such islands, resulting in a geohazard “blindspot,” particularly in the South Pacific. Understanding how these hazards are triggered is important; however, pin-pointing specific triggers is challenging as most studies have been unable to link continuously between onshore and offshore environments, and focus primarily on large-scale eruptions with sudden production of massive volumes of sediment. We address these issues by integrating the first detailed (2 × 2 m) bathymetry data acquired from Tanna Island, Vanuatu with a combination of terrestrial remote sensing data, onshore and offshore sediment sampling, and documented historical events. Mount Yasur on Tanna has experienced low-magnitude Strombolian activity for at least the last 600 years. We find clear evidence for submarine landslides and turbidity currents, yet none of the identified triggers are related to major volcanic eruptions, in contrast to conclusions from several previous studies. Instead we find that cascades of non-volcanic events (including outburst floods with discharges of >1,000 m3/s, and tropical cyclones), that may be separated by decades, are more important for preconditioning and triggering of landslides and turbidity currents in oversupplied sedimentary regimes such as at Tanna. We conclude with a general model for how submarine landslides and turbidity currents are triggered at volcanic and other heavily eroding mountainous islands. Our model highlights the often-ignored importance of outburst floods, non-linear responses to land-use and climatic changes, and the complex interactions between a range of coastal and tectonic processes that may overshadow volcanic regimes

Rykalová, Gabriela (2009): Entwicklung in der Tagespresse : dargestellt an journalistischen Textsorten der deutschsprachigen Zeitungen

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