Event Stratigraphy on the Northern California Continental Shelf: Role of High and Low Concentration Suspension

Seaward of its nearshore zone, the Eel River sector of the Northern California shelf is mantled by a seaward-fining pro-delta wedge of Holocene mud, the upper portion of which is accessible by box coring. Xradiographs reveal a sequence of thin to very thin muddy beds and laminae deposited by storm-related currents during or shortly after flood episodes ("flood beds"), intercalated with sandier beds and laminae reworked from the flood beds. While most beds are of more or less recent flood provenance, most of this material has undergone multiple resuspensions, and is storm packaged. The beds are thus "tempestites" but their sediment is in various stages of textural maturity, depending on the number of resuspensions to which it has been subjected. Observations of fluid motion and sediment transport from the Eel River sector of the Northern California Shelf during two storms in the winter of 1996 have been assembled, and observations of box cores that penetrate the 1996 have been collected by participants in the STRATAFORM Project of the Office of Naval Research. A two-dimensional, across-shelf sediment transport model has been developed to simulate sediment re-suspension, deposition and bed evolution. The observations and simulations show that rather than dividing beds into "flood" and "storm" beds, it is more meaningful to divide the event beds into the deposits of high concentration regimes and low concentration regimes. Coast-hugging surface flood plumes occur on the inner shelf during the winter season. The plumes generate dense, near-bottom suspensions, which may attain fluid mud concentrations (> 5 g/l) as particles settle. The period of storm-heightened waves may continue into the flood period, leading to gravity-driven seaward displacment of the bottom suspension, or the wave regime may ameliorate, leaving the suspension to consolidate as a short-lived inner-shelf flood bed. Such beds tend to be resuspended within days or weeks by subsequent storm events that may recreate the original high concentrations. The sediment is dispersed seaward, by either gravity or wind-driven flows to be deposited as a muddy "flood bed" on the central shelf. In contrast, low concentration regimes occur during storm periods when there has been no recent flood deposition on the inner shelf. The shelf floor is better consolidated than in the previous case, and the resulting suspended sediment concentrations are lower. As a consequence, beds deposited are thinner and sandier. In multi-year event bed successions, flood beds stand out, not only because more and finer material has been supplied to them, but because the change in the rate and character of supply has itself altered the dynamics and shifted the regime toward accumulation.