Mass-transport complexes (MTCs), mass-transport deposits (MTDs), and associated facies and features are widely recognized in continental slopes around the world. In most current stratigraphic models of MTCs and MTDs, these submarine sediment failures are related to aquifer outflow (sapping, seepage) along continental slope fronts that originated during relative sea-level fall. We test a hypothetical scenario that is favored during early forced regression using reduced-scale physical simulation. A major underground subaerial hydraulic gradient is assumed to flow towards the basin depocenter as a function of relative sea-level fall. We developed an experimental apparatus with slope angles varying between 15 and 30° to test this concept. Hydraulic gradients, aquifer outflow velocities, and triggered collapses induced by the seepage effect were recorded at various positions of the slope. Analysis shows that steeper slope gradients require lower seepage velocities (and shear stresses) to trigger collapse, but gentler slopes remain unchanged. Experimental data are compatible with a seepage effect that could potentially trigger mass failure and the formation of MTCs during relative sea-level fall. The features produced in the experiment have geometries comparable to natural environments, and the experimental seepage velocities are of an order of magnitude similar to those monitored in submarine aquifers. The experimental results advance understanding of mass transport in continental slopes by introducing and testing new methods, and also provide new insights into potential submarine geohazard risks where tectonic uplift operates along some coastal regions.
A laboratory tank experiment tested whether a subsurface flow from a confined aquifer causes slope instability and leads to the formation of pathways for sediment transfer from shallow to deep water when the subsurface flow discharges through the face of a subaqueous slope. A sandy slope with multilayer stratigraphy was built inside the tank, and a confined aquifer was simulated within the stratigraphy. To induce groundwater flow out of the face of the slope, water was injected in the proximal zone of the confined aquifer at progressive increased discharge. Sediment movement on the slope occurred by rolling of particles, fluidized flow, grain flow and slides. The fluctuation of phreatic pressure in the confined aquifer was measured by a set of piezometers, from which the hydraulic gradient generated by the water flow moving towards the slope was determined. This study determined that the mass movements started when the imposed injected flow rate was greater than the hydraulic conductivity capacity of the simulated aquifer, using the flow capacity calculated from the Darcy equation for porous media. The various physical parameters used in the experiment were found to scale well to natural prototypes. Moreover, the patterns of erosion and deposition in the physical simulation resembled natural features observed in seismicgeomorphology maps and modern deep-sea physiography. Therefore, water sapping by a confined aquifer flow is a potential mechanism for slope erosion and instability and for the formation of pathways connecting shallowwater and deep-water environments.
One of the great challenges in the physical modeling of turbidity currents is to acquire data with equipment that do not cause disturbance. Imaging equipment, such as video cameras, are an alternative for recording experiments without disturbances. However, for flows with high concentration, or where the material has unfavorable color (such as coal), visualization and analysis end up being impaired. An alternative for recording such flows is the application of an ultrasonic imaging equipment, which allows the acquisition of data with good quality and resolution. Experiments performed in NECOD laboratory (UFRGS -Brazil) showed that the images obtained by a medical ultrasound equipment could provide good qualitative and quantitative data. In a first application, the ultrasound images suggest that there is a relationship between the different current layers and the velocity and concentration profiles. For this study, the gray scale colors produced by the medical ultrasound equipment were used to calculate numerical values, by the Matlab tool with a new mathematical code. Such values allow obtaining spectra of density correlating them with the measured velocity (UVP) and concentration (UHCM) profiles. In a second experiment, the medical ultrasound enables to measure and map the evolution of the slope profile eroded by turbidity currents. An image sequence obtained during the experiment, registered a series of slope profiles allowed the mapping of the evolution and formation of a subaqueous canyon. The ultrasound is a good tool to acquire images from flow simulations, including for experiments produced in small scale.
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