Oceanic flood deposits on the northern California shelf: large-scale distribution and small-scale physical properties

Flooding by northern California's Eel River in January 1995, March 1995 and January 1997 produced sedimentologically and geochemically distinct deposits on the adjacent continental shelf. X-radiographs collected during an extensive suite of box coring cruises revealed that these layers extended along-shelf for up to 50 km, were in water depths of 50-110 m and were up to 8 cm thick. The areal distribution of the three flood layers was remarkably similar, with < 8 km separating their respective centers of mass. Hourly measurements of river discharge made by the US Geological Survey were coupled with an empirically determined rating curve to predict the mass of fine-grained (<63 mum) suspended sediment that entered the ocean during each flood. Comparison of these estimates to independent estimates of flood layer mass suggests that <25% of the fine-grained sediment delivered to the ocean resides in the hood layers. The remaining flood sediment is temporarily sequestered within sand deposits of the inner shelf or transported along- and off-shelf by buoyant plumes and density-driven fluid-mud flows. The Eel River dispersal system therefore has two seemingly contradictory attributes: widespread dispersal of flood sediment (most likely off shelf), but a consistent locus of deposition of flood layers on the shelf. Explanations for this paradox involve the small size of the drainage basin and the extreme suspended-sediment concentrations during floods. The former results in a rapid throughput of sediment in the basin. whereby sediment is delivered to the ocean during storms, when energy levels are high (leading to widespread dispersal), but repeatable (leading to a consistent locus of deposition). In addition, high suspended-sediment concentrations result in fluid muds that may be: a necessary condition for the formation of the flood deposits. Other West Coast river systems (e.g., Columbia, Russian) either have longer sediment throughput times or lower riverine suspended-sediment concentrations, thereby resulting in a greater fraction of sediment deposited on the shelf, but not as thick, recognizable layers (i.e. flood deposits). (C) 2000 Elsevier Science Ltd. All rights reserved.