Experiments evaluating subsidence generated within a subaqueous deformable substrate due to varying differential sediment loading patterns

The intraslope areas of many passive margins display a complex bathymetry of topographic depressions and crests that form series of minibasins. These minibasins are linked to the deformation of subsurface salt layers and act as localized sediment traps. Many mechanisms have been proposed for the initiation of minibasins, including tectonic forces (both extensional and contractional), regional gravitational sliding, density inversion between salt layers and overburden, and differential sediment loading. Regardless of initiation mechanism, it is widely recognized that synkinematic deposition plays a active role in determining subsidence patterns and sediment routing within and among the minibasins. We undertook a series of simplified 1-D and 2-D experiments 1) to evaluate the feasibility of developing a series of well-defined minibasins created exclusively by differential sediment loading and 2) to quantitatively determine the effects of substrate thickness, density contrast, and sedimentation rate on the resultant subsidence pattern. We also present an initial non-dimensionalized formulation of the problem that relates density contrasts, clinoform thickness, substrate thickness, progradation rate, and viscosity of the deformable substrate. Two sets of experiments were performed. The first set (1-D) vertically loaded a subaqueous corn syrup substrate (capturing the rheology of subsurface salt as a Newtonian fluid) with walnut sand. The second set (2-D) of experiments prograded a walnut sediment clinoform across a corn syrup substrate. We systematically varied sedimentation rate, substrate thickness, and, in the case of the prograding clinoform, base level. In no cases did we successfully reproduce a series of minibasins similar to those observed in natural settings. Instead the substrate was simply displaced laterally as sediment was deposited, forming a single depression. High sedimentation rates tended to produce wider zones of subsidence, however, if given sufficient time subsidence tended towards a similar overall geometry for both slow and fast sedimentation rates. This occurred as long as a sufficiently thick substrate was maintained. In our experiments, this was a ratio greater than 1:4 of substrate-to-clinoform thickness. When the substrate is insufficiently thick, weld structures formed as the overlying sediments were deposited onto the base of the flume. These structures appear similar to salt welds in natural systems. We also noted small zones of enhanced subsidence below the toeset and foreset of the prograding clinoform as sediment was deposited. These minor features are approximately one tenth the size of the overall basin formed in the substrate, and locally trap additional sediment during progradation. These are potentially related to infrequent grain flow events. Overall our experiments suggest that sediment differential loading is an insufficient mechanism in isolation to produce series of minibasins, but that the loading does significantly affect the initial geometry and rate of subsidence within these basins.