Modern accumulation rates and a sediment budget for the Eel shelf: a flood-dominated depositional environment

The northern California continental margin is periodically impacted by geol ogically significant storms, which have a marked influence on terrigenous s ediment supply, flood deposition, and long-term accumulation of fine-graine d sediment on the Eel shelf. Accumulation of Eel River muds on the adjacent shelf was investigated using Pb-210 and Cs-137 geochronologies, in order t o understand the fate of sediment discharged by the Eel River and to relate patterns of net sediment accumulation (100-yr time scale) to sediment dyna mics. Pb-210 data demonstrate that modem accumulation of river mud occurs f rom the 50-m isobath seaward. Across-shelf accumulation rates decrease from maximum mid-shelf values of 0.6-1.7 g cm(-2) yr(-1) to values of 0.2-0.4 g cm(-2) yr(-1) at the shelf break, with a spatially weighted mean of 0.5 g cm(-2) yr(-1) (0.4 cm/yr) for the entire shelf. Pb-210(xs) sediment-depth p rofiles from the region of highest accumulation rate are characterized by s ubsurface intervals of low and uniform activity, which are produced by floo d deposition. In some cores, particular Pb-210(xs) activity intervals may b e associated with major Eel River floods of 1955, 1964, and 1974. It is pos tulated that, because of the coincidence of high-river-flow events and sout herly winds during cyclonic winter storms, net northward transport allows f or preferential deposition of fine-grained sediment north of the river mout h. Over the past similar to 100 years, fluvial sediment input combined with marine dispersal processes have produced a mid-shelf depocenter, evident b y both the spatial distribution of Pb-210 accumulation rates and by clay-ri ch flood layers partially preserved in shelf deposits. A fine-grained sedim ent budget for the dispersal system, based on hydrological data and Pb-210 geochronologies, demonstrates that a maximum of similar to 20% (3 x 10(9) k g/yr) of the mean annual supply of fluvial mud (14 x 10(9) kg/yr) is trappe d on the shelf. The results of this study demonstrate that: (I) shea-term s edimentation processes associated with floods can influence sediment accumu lation on longer time scales; and (2) a major fraction of fine-grained sedi ment supplied to tectonically active margins by flood-prone mountainous riv ers bypasses narrow continental shelves. (C) 1999 Elsevier Science B.V. All rights reserved.