James L. Buttles

Interactions between turbidity currents and topography in aggrading sinuous submarine channels: A laboratory study

We present results from a laboratory experiment documenting the evolution of a sinuous channel form via sedimentation from 24 turbidity currents having constant initial conditions. The initial channel had a sinuosity of 1.32, a wavelength of 1.95, an amplitude of 0.39 m, and three bends. All currents had a densimetric Froude number of 0.53 and an initial height equal to the channel relief at the start of the experiment. Large superelevation of currents was observed at bend apexes. This superelevation was 85%–142% greater than the value predicted by a balance of centrifugal and pressure-gradient forces. An additional contribution to the superelevation was the runup of the current onto the outer banks of bends. This runup height is described by a balance between kinetic and potential energy. Runup resulted in deposition of coarse particles on levee crests that were indistinguishable from those deposited on the channel bottom. Deposit thickness and composition showed a strong cross-channel asymmetry. Thicker, coarser, steeper levees grew on the outer banks relative to the inner banks of bends. Zones of flow separation were observed downstream from bend apexes along inner banks and affected sedimentation patterns. Sedimentation from currents caused the channel to aggrade with almost no change in planform. However, channel relief decreased throughout the experiment because deposition on the channel bottom always exceeded deposition at levee crests. The first bend served as a filter for the properties of the channelized current, bringing discharge at the channel entrance into agreement with the channel cross-sectional area. Excess discharge exited the channel at this filtering bend and was lost to the overbank surface.

Deep turbidity currents in shallow channels

Experimental results presented here quantify how the thickness of turbidity currents affects the evolution of an aggrading subaqueous channel. Ten sediment-laden currents with identical initial conditions were released into a tank to determine how depositional flows interact with and modify a pre-existing channel form. Deposition was always greatest on the channel floor, systematically reducing channel depth and causing the ratio of initial current thickness to channel depth to increase with each flow. The ratio of longitudinal to lateral current velocity decreased exponentially with increasing relative current thickness. This change in lateral spreading demarcated regimes of channelized, quasi-channelized, and unconfined flow. Channelized currents have thicknesses that are less than 1.3 times local channel depth; unconfined currents are greater than five times channel depth; and quasi-channelized currents occur in between. The unconfined cases were consistent with prior experiments and theory defining the flow field for nonchannelized turbidity currents. Sedimentation patterns for even the thickest currents were influenced by subdued channel topography.

The role of buoyancy reversal in turbidite deposition and submarine fan geometry

Although recent work has shown that changing interstitial fluid density within turbidity currents is a frequently overlooked factor affecting the texture and internal architecture of turbidites, little is known about its influence on submarine fan morphology. Here we present the results of three-dimensional flume experiments of turbidity currents that clearly demonstrate the role of low-density interstitial fluid, in combination with sediment concentration and basin gradient, on submarine fan geometry. The experiments show that turbidity currents with reversing buoyancy, and their resulting deposits, are narrower than those that remain ground hugging. Furthermore, wider deposits result from increases in sediment concentration and/or basin-floor gradient. We also propose that Taylor-Gortler vortices associated with currents traveling over a break in slope may lead to the deposition of wider lobes compared with those traveling over a constant gradient.

Turbidity Current Flow Out of Channels And Its Contribution to Constructing the Continental Slope

We combine analysis of shallow seismic data from industry grade 3D volumes with results from physical models that resolve channel-to-overbank sedimentation by turbidity currents to explore how regional surfaces are constructed by unconfined flows. Depositional patterns measured from seismic and laboratory data are used to define properties of proximal versus distal overbank sedimentation. Both data sets reveal a significant drop in variability of depositional thickness associated with the transition to distal sedimentation. This drop in standard deviation is a useful metric for defining the transition from levee to distal overbank deposits.