Sedimentation patterns and fluid flow through sediments: Examples from the Eel River margin, the Juan de Fuca Ridge flank, and San Francisco Bay (California).

摘要

Sediment accumulation patterns result from interactions between sediment input and basin morphology. Sediment thickness and hydraulic properties control the connection between surface water and underlying basement aquifers. I have examined: (1) the sediment distribution on the Eel River margin, (2) the sediment and fluid seepage distributions over a basement ridge on the Juan de Fuca Ridge flank, and (3) groundwater seepage through contaminated sediments into San Francisco Bay and its impact on dissolved metals budgets.; On the Eel River margin thick transgressive deposits have accumulated due to the combination of high sediment-yield and ample accommodation space on a narrow continental shelf. Long-term sediment accumulation patterns on the margin are affected by northwest-southeast trending faults and folds; sediment thickness varies as much along the margin as across it. A set of slope gullies grow and are maintained during regression and sea level lowstand, likely a result of downslope-eroding sediment flows. The evolution of continental slope gullies could provide insight to sea level fluctuations on numerous margins.; On the eastern flank of the Juan de Fuca Ridge, variability in sediment thickness and type causes spatial variations in fluid seepage overlying a buried basement ridge. The distribution of sediments around the basement ridge result from the interaction of turbidity currents with seafloor topography. Sediment physical properties vary consistently with sediment type. Modeled seepage rates, based on estimated basement overpressures and sediment hydraulic impedance, are 0–35 mm/yr. Half of the volume flux of fluid overlying the basement ridge is contributed from 15% of the area with flow rates ≥1.9 mm/yr.; Groundwater seepage into northern San Francisco Bay is quantified, using seepage meters and pore water chemistry, to determine its role in transporting dissolved metals from sediments to surface waters. Seepage velocity changed throughout a year, with best fits to the data ranging from 20 cm/yr upflow to 34 cm/yr downflow (average = 4 cm/yr upflow). Estimated benthic fluxes could account for ≥60% of unknown sources of dissolved cobalt and ≥4% of unknown sources of dissolved silver, cadmium, copper, nickel, and zinc to northern San Francisco Bay.