Factors influencing the deposit geometry of experimental turbidity currents: implications for sand-body architecture in confined basins

Abstract

AbstractTwo sets of scaled laboratory experiments were performed to examine the effect of flow volume, flow density and grain-size distribution on the transport efficiency of turbidity currents. The experiments employed two sediment analogues (ballotini and silica flour) intended to model medium- to coarse-grained sand and mud respectively. In the first set of experiments each parameter was varied to examine its effect upon deposit geometry. Increases in the initial flow density, volume and proportion of fines had the effect of increasing the amount of sediment that was transferred to the floor of the experimental tank by the turbidity currents. Increase of each of these parameters has a characteristic effect on the three-dimensional geometry of the deposit: the deposits of large-volume flows are elongate, and those of fines-rich flows are broad. Increase of flow density increases the initial potential energy of the flow, thus increasing the runout distance; increase of the initial density beyond a sediment concentration of 13% by mass results, however, in a reverse of the geometrical trend of deposit elongation, possibly because of turbulence suppression at high densities. Increase of flow volume also increases the initial potential energy, and reduces the rate of velocity decrease due to gravitational spreading. Increase in the proportion of fines leads to maintenance of negative buoyancy, as the fine fraction remains suspended until the flow has virtually come to rest; it also decreases the settling velocity of the coarser fraction and thus delays its sedimentation. The second set of experiments was performed to investigate the influence of flow efficiency on the interaction of turbidity currents with topography. A single arcuate obstacle was placed in the path of the flows. In successive experiments flow efficiency was increased by progressively increasing the proportion of fines (silica flour). Both the proportion of sediment reaching the obstructing topography and the proportion of it able to surmount the topography increased as flow efficiency increased. Thus flow efficiency may determine whether or not an enclosed basin hosts deposits whose geometry has been affected by the confinement, and may also determine the relative effectiveness of the topography in confining inbound turbidity currents, and thus trapping their sediment load.