Jeffrey G. Marr

Fluvio-deltaic sedimentation: A generalized Stefan problem

We present a model of sedimentation in a subsiding fluvio-deltaic basin with steady sediment supply and unsteady base level. We demonstrate that mass transfer in a fluvio-deltaic basin is analogous to heat transfer in a generalized Stefan problem, where the basin’s shoreline represents the phase front. We obtain a numerical solution to the governing equations for sediment transport and deposition in this system via an extension of a deforminggrid technique from the phase-change literature. Through modication of the heat-balance integral method, we also develop a semi-analytical solution, which agrees well with the numerical solution. We construct a space of dimensionless groups for the basin and perform a systematic exploration of this space to illustrate the influence of each group on the shoreline trajectory. Our model results suggest that all subsiding fluvio-deltaic basins exhibit a standard autoretreat shoreline trajectory in which a brief period of shoreline advance is followed by an extended period of shoreline retreat. Base-level cycling produces a shoreline response that varies relative to the autoretreat signal. Contrary to previous studies, we fail to observe either a strong phase shift between shoreline and base level or a pronounced attenuation of the amplitude of shoreline response as the frequency of base-level cycling decreases. However, the amplitude of shoreline response to base-level cycling is a function of the basin’s age.

Experiments on subaqueous sandy gravity flows: The role of clay and water content in flow dynamics and depositional structures

Deep-water deposits consisting mainly of massive sand are commonly identified as deposits of turbidity currents (i.e., turbidites). Speculation has risen in recent years as to whether some of these massive sandy deposits could have instead been deposited by debris flows. This possibility is explored here by examining the flow mechanics of sand-rich subaqueous gravity flows by means of laboratory experiments. In these experiments, sandy gravity flows were generated when well-mixed slurries of sand, clay, and water were released into a tank filled with tap water and allowed to flow under gravity over a slope that declined from 4.6° to 0.0°. The observed flow mechanics and resulting depositional features were strongly tied to the “coherence” of the debris flows (i.e., the ability of the slurry to resist being eroded and broken apart by the shear and pressure undergone by the flow). Low water content and high clay content resulted in strongly coherent debris flows, whereas high water content and low clay content resulted in weakly coherent flows. As little as 0.7 to 5 wt% of bentonite clay or 7 to 25 wt% of kaolinite clay at water contents ranging from 25 to 40 wt% was required to generate coherent gravity flows. Weakly coherent and moderately coherent flows produced significant, low-concentration subsidiary turbidity currents, and their deposits developed coarse- tail grading, water-escape structures, and minor increases in thickness at the base of the slope. Strongly coherent debris flows commonly hydroplaned and generated only minor subsidiary turbidity currents. Their deposits were structureless and ungraded, commonly showing tension cracks, compression ridges, water-escape structures, detached slide blocks, and a significant increase in thickness at the base of the slope. Application of distorted geometric scaling suggests that many aspects of these experiments appropriately scale up to the field scale of natural submarine debris flows.

Role of Turbidity Currents in Setting the Foreset Slope of Clinoforms Prograding into Standing Fresh Water

Clinoforms produced where sand-bed rivers flow into lakes and reservoirs often do not form Gilbert deltas prograding at or near the angle of repose. The maximum slope of the sandy foreset in Lake Mead, for example, is slightly below 1°. Most sand-bed rivers also carry copious amounts of mud as wash load. The muddy water often plunges over the sandy foreset and then overrides it as a muddy turbidity current. It is hypothesized here that a muddy turbidity current overriding a sandy foreset can substantially reduce the foreset angle. An experiment reveals a reduction of foreset angle of 20 percent due to an overriding turbidity current. Scale-up to field dimensions using densimetric Froude similarity indicates that the angle can be reduced to as low as 1° by this mechanism. The process of angle reduction is self-limiting in that a successively lower foreset angle pushes the plunge point successively farther out, so mitigating further reduction in foreset angle.

Submarine mass-wasting on glacially-influenced continental slopes: processes and dynamics

Abstract Submarine slides and debris flows are common and effective mechanisms of sediment transfer from continental shelves to deeper parts of ocean basins. They are particularly common along glaciated margins that have experienced high sediment flux to the shelf break during and after glacial maxima. During one single event, typically lasting for a few hours or less, enormous sediment volumes can be transported over distances of hundreds of kilometres, even on very gentle slopes. In order to understand the physics of these mass flows, the process is divided into a release phase, followed by break-up, flow and final deposition. Little is presently known regarding release and break-up, although some plausible explanations can be inferred from basic mechanics of granular materials. Once initiated, the flow of clay-rich or muddy sediments may be assumed to behave as a (non-Newtonian) Herschel-Bulkley fluid. Fluid dynamic concepts can then be applied to describe the flow provided the rheological properties of the material are known. Numerical modelling supports our assertion that the long runout distances observed for large volumes of sediments moving down gentle slopes can be explained by partial hydroplaning of the flowing mass. Hydroplaning might also explain the sharp decrease of the friction coefficient for submarine mass flows as a function of the released volume. The paper emphasizes the need for a better understanding of the physics of mass wasting in the submarine environment.

Numerical simulation of mud-rich subaqueous debris flows on the glacially active margins of the Svalbard-Barents Sea

Abstract Seismic images and sediment core data from the Bear Island and Isfjorden fans localized along the Svalbard–Barents Sea continental margin, reveal an interesting depositional system consisting of stacked debris flow lobes. The frequent release of debris flows was associated with large volumes of sediment rapidly delivered to the shelf break during periods of maximum glaciation. The compositions of the lobes for both fans are similar, consisting of mainly clay and silt. The data show, however, a dramatic difference in runout distances for the two areas. Isfjorden debris lobes are 10–30 km in length whereas Bear Island lobes are 100–200 km in length. Even more intriguing is the fact that the large runout distances on the Bear Island fan occurred on slopes less than 1° whereas the Isfjorden fan flows occurred on slopes greater than 4°. Depth-averaged non-linear one-dimensional equations for balance of mass and linear momentum are applied to simulate the subaqueous debris flow. The equations are solved by the numerical model BING, describing the flow as a visco-plastic Bingham fluid. The model is employed to study the effect yield strength, viscosity and bathymetry have on debris flow runout. The study shows that the large runout distances can be achieved on the Bear Island fan by visco-plastic flows with sufficiently low yield strength. High yield strength sediments require an additional mechanism, such as hydroplaning, to reach measured runout distances. Most importantly, this study shows the necessity of good rheological measurements for accurate numerical modeling of subaqueous debris flows.