Experimental modeling of the spatial distribution of grain-size developed in a fill and spill mini-basin setting

In many deep-water slope settings, turbidity currents are inferred to fill discrete basins that are linked streamwise (e.g., Gulf of Mexico ponded mini-basin settings). As an upstream basin is filled with sediment, progressively more overspill is directed into the next basin downstream. Turbidity currents are, however, vertically stratified in terms of grain concentration and grain size, especially during deposition. Thus the degree of confinement should potentially affect the degree of grain size fractionation between the two basins. Accordingly, two separate experimental programs were conducted to assess spatial trends in flow and deposit character developed within a pair of linked basins. Two sills, sinusoidal in section and arranged transverse to flow, were positioned between confining lateral walls. In the Series I experiments, individual flows were obstructed by an upstream sill, whose height was varied between flows, and a downstream sill of fixed height. Measurements of flow velocity, concentration, grain size, and the resultant deposit thickness were taken. In the Series 2 experiments, both sills were fixed in height whilst 18 repeat flows were run one after the other. Each flow overran the deposit of its predecessor/s. Sampling both along and through the resultant composite deposit allowed the mapping of systematic changes in grain size both horizontally and vertically. Both sets of experimental results show a strong relationship between the depth of the experimental flow, the height of the confining topography, and the degree of grain size partitioning between the two basins. Progressively greater proportions of coarser-grained material are bypassed downstream as the degree of confinement is reduced, whilst the mean grain size of that retained in the upstream basin also increases. At the natural scale, this may result in the production of systematic vertical trends in mean grain size, sorting (skewness), and sand-to-shale ratio in both the upstream and downstream basins.