Where Did All the Suspended Sediment Go?

Studies of mud transportation and deposition in modern shelf settings indicate that fine-grained siliciclastic sediment is deposited from suspension in various ways. However, contemporary interpretations of ancient analogs (e.g., epeiric seaways such as North America’s Cretaceous Western Interior Seaway, or Devonian Appalachian Basin) focus almost entirely on mud deposition from energetic bottom currents. If suspension deposits are not present in the rock record, but modern studies indicate those deposits should be present, it is appropriate to ask where all the suspended sediment went in the geologic past. In this paper I argue that mud dispersal from a river mouth to the shelf is likely to be a multi-stage process that includes transportation in suspension and in bottom currents. Most or much silt and clay discharged at a river mouth is deposited on the shelf within a few tens of kilometers from hypopycnal plumes. Data from modern rivers suggests that hypopycnal flows are relatively rare events. Plume deposits on the inner shelf can be reworked downslope by wave-enhanced sediment gravity flows. Other types of currents ensure that most mud deposited on a shelf is reworked and transported further offshore and along depositional strike by bottom currents. The sedimentary record preserves evidence only of the last phase of sediment transport, i.e. via bottom currents. Undisturbed muddy suspension deposits might be preserved in very proximal settings (associated with high sedimentation rates), interbedded with sandy storm deposits (as “fair-weather shales”) or in very distal settings as claystones or in marine snow.