Continental Slope Morphodynamics



Sediment deposition and erosion on the continental slope is dominated by the mechanics of turbidity current systems. Turbidity currents are defined as sediment gravity flows in which the gravitational driving force is supplied by an excess density associated with the suspension of particles. Turbidity currents can construct submarine canyons and channels that are similar in form to their terrestrial cousins. Sediment deposition from these flows construct expanded sections of sedimentary strata that preserve important records of past environmental conditions. Our group works on advancing our understanding of the interactions between turbidity currents and topography and therefore improve our ability to decipher the geological record preserved in their deposits. To accomplish this we utilized laboratory experiments, remote sensing of subsurface sedimentary deposits, and numerical analyses of targeted transport processes and sedimentary deposits. Quantitative measurements collected with these tools are used to study a range of conditions, spanning a flow spectrum from channelized currents to fully unconfined sheet-flows.

Projects:
Quantifying the morphology and composition of thin-bed levee deposits in deep-water settings
Research Assistantship available for this project


The objective of this project is to improve our ability to invert levee morphology for deposit composition in deep-water environments. At present the processes associated with levee growth and our ability to predict levee composition lags behind our understanding of channelized deep-water deposits. We are building upon an earlier investigation of levee morphology and growth that proposed a levee growth model that utilized an advection settling model for sediment transport coupled to a vertical sediment concentration profile. In this project we are extending the capabilities of this levee growth model to allow it to predict the composition of levees bounding an array of channel configurations.

The first component of the project involves augmenting the levee growth model of Straub and Mohrig, 2008 to track the evolving composition of levees bounding straight channels in aggradational settings and to compare model results against reduced scale laboratory experiments. The second component of the project will focus on improving our ability to predict the composition of inner and outer levees bounding compound submarine channels. Levees of compound channels form important repositories for thin-bedded fine-grained sands. Our investigation will focus on collecting experimental data which characterizes the growth and composition of inner and outer levees bounding a compound channel. This data will then be used to identify processes important to levee construction in compound channels.

Experiments for this project will take place in our new Deepwater Basin. This project is supported by the American Chemical Society’s Petroleum Research Fund. The grant includes funds for graduate and undergraduate support (please contact me if you are interested in pursuing research on this topic).

Reconstructing ancient passive margin dynamics by relating geomorphic and stratigraphic surfaces: with Ben Sheets (U. of Washington)
Research Assistantship available for this project


The primary objective of this project is to improve our ability to invert the stratigraphic record for paleo-surface-dynamics and topography. The architecture of continental margin stratigraphy contains features which resemble bars, channels, and channel networks, yet scientists cannot precisely reconstruct the relationship between these preserved deposits and the geomorphic processes that constructed them. This is unfortunate, as the stratigraphic records of modern systems contain invaluable information that could improve our ability to interpret paleo climatic and tectonic records in active and ancient rift settings.

Due to the large spatial and temporal scales associated with natural systems, we are investigating this through reduced scale experiments, in which surface topography is monitored at high spatial and temporal scales. Coupled with digital images of stratigraphy produced by these experimental systems, we are quantifying the relationship between geomorphic and stratigraphic surfaces. Our range of planned experiments will allow for the targeted investigation of controls on the construction of channelized passive margin stratigraphy, including parameters such as channel mobility, avulsion rate, and flow type.

Data collected from each experiment will be used to construct statistical relationships between morphodynamics and stratigraphic surfaces in one, two, and three dimensions. These data, in turn, will be used to benchmark the development of deterministic cellular models of channelized flow. Finally, we are planning to apply these insights to the inversion of physical stratigraphy from two field sites: 1) the Quaternary stratigraphy of the Mississippi Delta below Breton Sound and the Cretaceous Ferron Sandstone.

Scale dependant compensational stacking of channelized sedimentary deposits measured in outcrops: with David Pyles (Colorado School of Mines)


Compensational stacking is the tendency of flow event deposits to fill topographic lows and as a result smooth topographic relief. We are utilizing a recently developed technique that quantifies the strength of compensation in sedimentary basins to quantify the stratigraphic architecture of turbidite deposits of the Ross sandstone. We are applying the compensation index to mapped surfaces of the Ross sandstone outcropping along the southwest coast of Ireland in an attempt to characterize how compensation varies with the scale of deposits.

Bengal fan Channel: with Tilmann Schwenk (U. Bremen)

Using recently acquired bathymetry data on one of the Bengal Fan channels we are studying the morphology of sinuous submarine channels. The survey was collected by colleagues at the University of Bremen. The bathymetry survey tracks the present location of the channel over ~900 km of its length. The Bengal submarine channel system is the largest submarine channel system in the world. We will be comparing several aspects of the channels morphology to terrestrial river systems. Comparison of terrestrial and submarine channel morphologies should improve our understanding of sediment transport processes in the deep marine, especially in sinuous channels which are the dominant submarine channel type.

Laboratory experiments on interactions of turbidity currents with sinuous submarine channels: with David Mohrig (U Texas Austin) & Carlos Pirmez (Shell Petroleum)


Channels are the most significant morphologic feature of the submarine landscape on the continental slope. Many of these channels are highly sinuous in planform (s = 1.3) and persist from 10 km to 1000 km downslope, yet the processes by which these channels evolve and organize themselves are incompletely known. There are still very few direct observations of turbidity currents moving through sinuous channels because infrequent occurrence, great water depths, and high current velocities make measurements difficult to obtain. We are examining the interactions between turbidity currents and topography that construct the continental slope. Part of this research has focused on development of aggrading sinuous submarine channels through well-controlled laboratory experiments. These reduced-scale experiments provide us with the opportunity to directly monitor sediment-laden density currents. Results from my laboratory experiments document several previously unknown relationships and processes and test several hypothesized but unverified conceptual models for submarine-channel evolution. One key finding suggests that channel sinuosity itself influences the runout length of turbidity currents. The mechanism(s) allowing for the transport of sediment through submarine channels many hundreds of km in length has puzzled scientist for decades. In attempting to explain this transport a contribution from the planform channel shape had until now never been considered. My measurements of currents traversing straight and sinuous channels show enhanced turbulence and vertical mixing of suspended sediment at channel bends, reducing deposition rates and increasing current runout lengths. This study illustrates that increased form drag on currents does not automatically decrease their transport efficiency and provides an intriguing explanation for why almost all submarine channels greater than 100 km in length are moderately to highly sinuous in their plan form.

Other laboratory results characterize the interaction of currents with channel bends and the resulting patterns of channelized and overbank sedimentation that arise from these interactions. Properties of the depositional turbidity currents are compared to those described in better studied river systems. A key observation made in my experiments was that the large cross-channel superelevation observed in channel bends cannot solely be described by the balance of a centrifugal and pressure-gradient force as is commonly done for rivers. The vertical velocities associated with the runup of currents against the channel sidewalls cannot be discounted. Runup forces result in relatively large superelevations for turburdity currents compared to rivers due to differences in the ratios of current density to ambient-fluid density in the two settings. Large current superelevations at bends of experimental channels allowed for the deposition of thick, relatively coarse-grained levees along the outer bank of channel bends. The resulting high rates of overbank sedimentation acted to preserve channel form even under conditions of rapid bed aggradation, similar to observations from natural submarine channels. Our measurements of the velocity field, depositional/erosional patterns and grain size and sorting are being used to benchmark numerical models describing fully three-dimensional turbidity currents that are being developed by my colleagues Dr. Jasim Imran, University of South Carolina (Department of Civil and Environmental Engineering), and Dr. Carlos Pirmez, Shell, Shell International Exploration and Production Inc. Results from our laboratory experiments also have implications beyond the study of marine geology. Comparison of the evolution of submarine channels to better studied river systems allows us to constrain what processes are general to channelized landscapes and which processes are environment specific. This knowledge will aid in interpretation of channelized landforms observed on the surfaces of other planets and moons. Specifically, the study of channels and drainage basins on planetary bodies that have atmospheres denser than Earth’s, such as Titan and Venus, will be facilitated by studies of channels in the deep marine.
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and even more.

Levee Construction: with David Mohrig (U. Texas Austin)


We are using an industry-grade 3D seismic survey to connect submarine sediment-transport processes with the construction of deep-marine stratigraphy. Research questions addressed with this data set are motivated by a combination of observations made during regional mapping of the seafloor and shallow subsurface horizons and observations taken from the laboratory. This seismic survey, made available for study by Brunei Shell Petroleum Inc., covers an area of 2400 km2 off of the northern coast of Borneo that encompasses a network of submarine channels. I am using this data to address the morphodynamics of submarine levee construction. Levees, a primary element of self-formed channels, are faithful recorders of channel history and connect channels to their overbank surfaces. Thickness data derived through the differencing of mapped seismic horizons is used to unravel the process of levee and channel growth. With this thickness and topographic data I have determined sedimentation trends; particularly the relationship between channel relief and levee taper. Levee taper increases rapidly as channel relief grows from 0 to 40 m, but increases at ever diminishing rates for progressively deeper channels. A similar relationship between channel relief and levee taper was observed in the lab and lead to the development of a levee growth model based on an advection-settling scheme for sedimentation combined with a vertical profile for suspended-sediment concentration defined by the Rouse equation. The model reproduces field and laboratory observations of levee growth and suggests that the most important parameters controlling levee development are the degree to which currents are confined to a channel and the vertical structure of suspended-sediment concentration profile. The model is now being used to estimate the thicknesses of currents that constructed leveed submarine channels observed on the sea floor.
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Evolution of the NW Borneo Margin: with David Mohrig (U. Texas Austin) & Carlos Pirmez (Shell Petroleum)


The present-day continental slope offshore Brunei Darussalam displays several networks of submarine channels possessing planform attributes similar to those observed in better studied river systems. We use shallow 3D seismic data to study one tributary network in detail. This network is located directly downslope from the shelf-edge Champion and the channels in this network initiate just down dip of the shelf-edge and are not directly linked to a terrestrial river system. Mapping of shallow seismic horizons reveals that the tributary channel network is an aggradational feature constructed on top of a relatively smooth slide plane associated with a large mass failure event. This observation highlights differences between network construction in submarine settings compared to terrestrial settings where tributary networks are net ersosional features. Observations suggest that this channel network was constructed from turbidity currents that initiated at the shelf-edge as sheet-flows prior to transitioning down slope into weakly confined flows through the construction of aggradational channels. We are able to use the channel morphology in this region to estimate the thicknesses of channel forming turbidity currents.
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Submarine Canyon Growth: with David Mohrig (U. Texas Austin)


We are also using the seismic survey from offshore Borneo to examine the growth and preservation of constructional canyons via sheet-flow turbidity currents. The formation and deepening of submarine canyons are typically attributed to purely erosional processes. Using maps of past seafloor topographies preserved in the subsurface we have found that is not always the case. Some canyons can deepen as a result of net depositional processes. A comparison of canyon axis versus ridge deposition shows that slightly higher rates of sedimentation on the ridges between the canyons have lead to an increase in their relief through time. These observations will require the community interpreting submarine landscape evolution using only surface topography to adjust their metrics.
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