MARINE DISPERSAL OF FINE-GRAINED RIVER SEDIMENT: ENVIRONMENTAL CONTROLS ON SUBAQUEOUS-DELTA CLINOFORMS

Rivers discharge sediments and associated carbon and nutrients to continental shelves around the world, and the fate of these materials is important to understanding the history of the Earth and its future. Research has shown that waves and tides have a dominant influence on coastal morphology and stratigraphy of modern river deltas, and classification schemes have been developed to categorize different deltaic systems (e.g., tide-dominated). These studies, however, largely ignored the deposits created by fine-grained sediments, and these materials typically represent the greatest fraction of a river’s sediment load. This research illustrates that tidal range, mean significant wave height, sediment load, and continental-shelf width at river mouths are the dominant factors controlling fine-grained river-sediment dispersal on the continental shelf, and specifically, the development of subaqueous-delta clinoforms.

Quantitative studies of fine-grained sediment dispersal in the ocean have been made for several river systems using radionuclides and seismic-reflection methods. On the basis of this work, we propose four basic types of dispersal: proximal-deposition dominated (PDD), subaqueous-delta clinoform (SDC), marine-dispersal dominated (MDD), and canyon-captured (CC). A GIS database with information (e.g., continental shelf width) at >100 major river mouths was developed to evaluate characteristics critical to the formation of the four types of dispersal systems. These data indicate that PDD systems have a mean significant wave height of <2 m, while the opposite is true for MDD systems. SDC development requires not only a large sediment load (~100 million tonnes) but also a large tidal range (>2 m) and moderate wave climate (>1 m mean significant wave height). Continental-shelf width is also important because shelves less than 50 km wide are more likely to experience shelf sediment bypass and canyon capture. Additional measurements from quantitative dispersal systems studies reveal strong correlations between the depth and distance of nearest maximum shelf deposition and mean significant wave height and tidal range, respectively. These relationships further emphasize the importance of waves and tides in regulating fine-grained sediment accumulation on continental shelves.