Reports: DNI850295-DNI8: Quantifying the Morphology and Composition of Thin-bed Levee Deposits in Deep-water Settings

Kyle Straub, PhD, Tulane University

Levees are the primary elements of self-formed submarine channels, yet in comparison to channel thalwegs little is known about their morphodynamics. The primary objective of this project is to improve our ability to invert levee morphology for deposit composition (net-to-gross) in a range of deep-water environments. Further, we aim to improve our ability to predict how these thin-bed deposits are linked to the channel deposits they bound. Thin bed deposits constructed from the overspill of channelized turbidity currents constitute a significant fraction of continental margin stratigraphy and in some settings these deposits form economically valuable hydrocarbon reservoirs. However, at present the processes associated with levee growth and our ability to predict levee composition lags behind our understanding of channelized deep-water deposits.

The proposed research for this project had two components. The first component involves augmenting a previously developed levee growth model by Straub and Mohrig 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 focuses 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 has focused on collecting experimental data which characterizes the growth and composition of inner and outer levees bounding a compound channel. This data is being used to identify processes important to levee construction in compound channels. During the second year of this project we have made significant advances in the first and second component of the project, outlined below.

Motivated by observations of levee stratigraphy in seismic data and levee morphodynamics observed in physical experiments we have developed a new levee growth model. This model, developed in the last year, couples a simple advection settling scheme for suspended sediment with a vertical suspended sediment profile for partially-channelized turbidity currents. Use of an advection-settling scheme is supported by small advection length scales for settling sediment in overbanking flows compared to most levee widths. The channelized (parent) suspended sediment concentration profile in our model is defined for multiple grain sizes using a two-layer method. Suspended sediment concentration below the height of the velocity maximum is defined by the Rouse equation, while the concentration above this height is defined by a near-Gaussian relationship. In our model only the current fraction situated above the elevation of the levee crest is used in the advection settling calculation. As a result early levee growth is associated with coarse and relatively stratified overbanking flow compared to later periods of growth when channel relief is greater. We use this model to link channelized flow properties to both levee morphology and texture. The evolution of levee morphology, specifically levee taper, is shown to depend on the Rouse number of the parent flow with high Rouse number flows corresponding to steep levees. Initial exploration of levee texture has focused on the distribution of particle diameters in the parent flows. We find that a flow composed of a narrow distribution of particle diameters results in a coarser levee than a flow with the same median grain size but broader particle size distribution. Expanding on our work from the first year, we now also track the evolution of the channel center, allowing us to define how the growth of relief occurs in submarine channels.

In conjunction with the development of our new levee growth model, time was spent in the last year performing experiments in Tulane’s Deepwater Basin and analyzing results from the experiments. The first experiment in this basin lasted four months. In this experiment we have ran a sequence of turbidity currents down a straight channel and characterized the evolving flow field, sediment transport field, and evolving topography of this system which is associated with actively aggrading levees. Data quality resulting from this experiment was very high and is currently being compared to predictions from our numerical model. Our experimental results have confirmed the presence of a two-layer concentration profile, which was used in our numerical model. As part of the experiment we collected samples of the deposit at over 800 locations which we are currently analyzing with our grain size analysis line at Tulane to characterize the texture of the deposit and compare this to our numerical model.

This grant has also supported travel to the 2011 AGU conference held in San Francisco, CA.

Performing a second set of experiments that will document the growth of inner and outer levees in a compound channel. This set of experiments will be run by a new post-doc joining Tulane and Straub’s lab, Anjali Fernades, who will be supported by this grant.