EXPERIMENTS ON THE ENTRAINMENT THRESHOLD OF WELL-SORTED AND POORLY SORTED CARBONATE SANDS

Few studies have examined the hydrodynamic behaviour of carbonate sedi ments. The data presented here are the result of preliminary research on entrainment in well- and poorly sorted carbonate sands. Experiments were performed using naturally occurring sediments in a tilting, reci rculating freshwater flume. Results indicate that when of similar size , shape and density, the transport threshold of carbonate sands is sim ilar to that of quartz. However, owing to their lower density and ofte n platy or irregular shape, skeletal sands require a lower shear stres s to initiate transport. Because the density of carbonate particles ma y increasingly vary with grain size, the threshold of motion in coarse carbonate grains may differ more markedly from that of quartz. In poo rly sorted samples, results show that the coarse-grained constituents move before the finer-grained components. Grain properties and boundar y-layer dynamics are believed to explain this phenomenon. Rollability, of the larger grains combined with physical trapping and immersion wi thin a low velocity sublayer are believed to prevent finer particles f rom moving. Given the appropriate sediments and flow conditions, it ma y therefore be possible to deposit and preserve fine-grained sediments in a flow regime typically thought to transport such materials.