Experiments on the settling of carbonate sand–mud suspensions

This experimental study addresses the settling behavior of natural carbonate sediment suspensions. Natural carbonate suspensions differ hydrodynamically from siliciclastic suspensions due to a broader range of grain sizes, shapes, and solid densities of skeletal grains. These variations also underlie hydrodynamic differences between carbonate and siliciclastic turbidity currents. The experiments used coarse, noncohesive skeletal tropical carbonate sand obtained from the back-reef of Moorea (French Polynesia) and cohesive tropical carbonate mud retrieved from the slopes of Little Bahama Bank. Twenty-one settling experiments were conducted at volumetric sediment concentrations of 9%, 20%, and 30%. The suspensions consisted of carbonate sand mixed with cohesive carbonate mud in sand/mud ratios ranging from 96%/04% to 76%/24%. Textural trends in grain size and composition of the experiment deposits were evaluated by laser diffraction analysis and microscopic observations. Three facies were identified, from base to top: 1) interval A: weakly graded to ungraded rudstone–grainstone to floatstone–packstone, occasionally with a fining-upward base at low bulk concentrations and mud proportions, 2) interval B: normally graded grainstone to packstone, and 3) interval C: normally graded wackestone to mudstone. Interval B, the least muddy (cleanest sand) with the best sorting, has a normalized thickness consistent across experiments and unaffected by sediment concentration or mud proportion. In contrast, interval C thickens at the expense of interval A as mud proportion increased at each sediment concentration, although this trend lessens for deposits of higher-concentration suspensions where interval A is the dominant facies. In deposits of low-concentration suspensions, the fining-upward base of interval A decreased in normalized thickness with increasing sediment concentration and mud proportion. The experiments demonstrate that grain-size segregation becomes less efficient with increasing sediment concentration and/or cohesive-mud proportion. Thus, adding cohesive carbonate mud lowers the critical sediment concentration at which grain-size segregation is suppressed, resulting in thicker ungraded interval A deposits. Compared to previous siliciclastic suspension-settling experiments, grain-size segregation is suppressed at lower sediment concentrations in carbonate suspensions, although the present experiments used much coarser grain sizes. This work contributes to understanding carbonate suspension-flow deposits such as calciturbidites and calcidebrites, by hinting to: i) vertical and longitudinal (proximal to distal) grain-size sorting processes, and ii) grain-shape sorting patterns within individual deposits. Both aspects tie to the hydrodynamic behavior of individual, irregular-shaped grains in sediment suspensions with varying grain composition, as well as rheological changes due to interaction with variable quantities of cohesive carbonate mud.