Depositional Turbidity Currents in Diapiric Minibasins on the Continental Slope: Experiments--Numerical Simulation and Upscaling

Abstract

The northern continental slope of the Gulf of Mexico is riddled with numerous subsiding diapiric minibasins bounded by ridges, and often connected by channels created by turbidity currents. The region is economically relevant in that these diapiric minibasins constitute excellent focal points for the deposition of sand. These deposits in turn serve as excellent reservoirs for hydrocarbons. A better understanding of the ''fill and spill'' process by which minibasins fill with sediment as the intervening ridges are dissected by canyons may serve to aid in the location of such reservoirs. A theoretical analysis in a companion paper has revealed two key aspects of the ''fill and spill'' process: (1) the formation of an internal hydraulic jump as a turbidity current spills into a confined basin, and (2) the detrainment of water across a settling interface forming at the top of the ponded turbidity current downstream of the hydraulic jump. In that paper it was shown that sufficiently strong detrainment can consume the flow, so that there is no outflow of either water or sediment even with continuous inflow. As the basin fills with sediment, however, overspill is eventually realized. Herein the theory of the companion paper is used as the basis for a numerical model of ponding of turbidity currents. The numerical model is tested and verified against two experiments. In the first of these, detrainment is sufficient to capture an entire sustained turbidity current. In the second of these, detrainment is insufficient to prevent sustained overspill. The principles of similitude using the densimetric Froude number allow upscaling of the experimental results to field scale. A full numerical model is verified against the experiments and applied at field scale. The result is a view of intraslope minibasin sedimentation that has a stronger physical basis than the conceptual models proposed to date.

Depositional Turbidity Currents in Diapiric Minibasins on the Continental Slope: Formulation and Theory

“…If the slow, gradual accumulation of sediment on the bed by deposition from the dilute suspension is neglected for the sake of argument, a perfectly stable settling interface can be created in a minibasin when the rate of detrainment of water across the interface downstream of the hydraulic jump perfectly balances the difference between the rate of inflow of water from upstream and the rate of outflow across the downstream lip of the minibasin. That such a balance can be reached is demonstrated theoretically in the present paper and numerically and experimentally in the companion paper (Toniolo et al 2006). The gradual accumulation of sediment implies, however, that the interface should slowly rise in time as the basin fills.…”

“…As long as the sediment in the turbidity current is uniform, deposit thickness downstream of the hydraulic jump is approximately constant in the streamwise direction, as shown in Figure 4B. This result, which is a direct consequence of ponding, is demonstrated theoretically below and experimentally in the companion paper, Toniolo et al (2006).…”

“…This justification is given later in the present paper and in the companion paper, Toniolo et al (2006). The issues are more easily discussed, however, after they have been introduced.…”

“…It is useful to be more specific as to how long a current might be sustained. Although the issue is addressed in more detail below, Lamb et al (2004) provide evidence that in the case of the minibasins on the north slope of the Gulf of Mexico, a current that requires on the order of at least 30 minutes to pass by a given point should be sufficient to create the conditions described in this paper and its companion, Toniolo et al (2006).…”

Flow Behavior of Ponded Turbidity Currents

“…As the hump reaches the inlet, the basin is filled by a sedimentbearing cloud which is thicker than the original turbidity current and has a sharp subhorizontal upper 269 settling interface ( Fig. 4D; Toniolo et al, 2006aToniolo et al, , 2006b. 270…”