Spatial variability in sedimentary processes on the Eel continental slope

The northern California continental margin receives episodically large quantities of sediment from the Eel River, which seasonally discharges to the shelf. Regional circulation broadcasts this sediment widely, not only affecting sedimentation on the shelf, but on the slope as well. A suite of 60 cores has been collected to examine the processes of sediment accumulation and sediment delivery to the Eel continental slope in waters <800 m deep. Spatial surveys of accumulation rate (0.2-1.3 g cm(-2) year(-1)) and surficial sediment grain size (4-8.5 phi) demonstrate that sediment is accumulating throughout the Eel margin, reflecting a rapid and widespread redistribution of the fluvial discharge. Downcore grain-size profiles of heterogeneous sediments suggest that hemipelagic and other episodic sediment-delivery processes may be important for the slope in areas proximal to the river. In more distal areas, downcore profiles reveal relatively homogeneous sediments, delivered dominantly by hemipelagic processes. A budget for fine-grained sediment shows that this upper portion of the slope contains, at a maximum, 20% of the river's annual discharge; a combined budget for the shelf and upper slope suggests that at least 60% of the annual sediment load is not accounted for in these areas. The Eel continental slope, because of the large terrigenous sediment input, the narrow width of the adjacent shelf, and its energetic environment, is a good analogue for slopes during transitional stands of sea level and for modern, tectonically active settings. Processes typically associated with these geological settings (i.e., supply of sediment to the slope, and downslope mass movement) are actively occurring. (C) 1999 Elsevier Science B.V. All rights reserved.