MODELING EVENT-BED SUCCESSIONS AND LONG-TERM SEDIMENT FLUX ON A STORM-DOMINATED CONTINENTAL SHELF, NORTHERN CALIFORNIA MARGIN

Shelf sediments on the northern California margin, like many storm-dominated margins, consist of thin-bedded sedimentary facies produced by wave reworking of sediment delivered by river floods during winter storms. These successions are seen in surficial box cores, piston cores of Holocene shelf sediments, and uplifted Pleistocene sections in the area. These thin-bedded facies alternate with more massive facies, where bioturbation has apparently removed the primary sedimentary structures, perhaps due to periods of reduced sedimentation or smaller events. Numerical simulations provide a way of testing how variations in storm frequency, sedimentation rate, and storm intensity are reflected in the stratigraphic record. Model runs with large numbers of storm events also provide a way to tie large-scale time-averaged diffusion coefficients used in stratigraphic models to measurable hydrodynamic parameters. The SLICE model, developed by Woodward-Clyde, is a 2-D model of sediment transport that simulates wind-driven circulation, tidal currents, waves, and the resulting bottom boundary shear stress to determine sediment transport during storm events. SLICE has been calibrated against box cores and wave and current measurements collected by the STRATAFORM program. By running large numbers of hypothetical storms based on the historic data of the region, longer successions (representing centuries) are generated that demonstrate the impact of varying climate on stratigraphy, and provide a means of estimating realistic diffusion coefficients for longer term models. A 20% increase in simulated storm wave height shifted the mud line on the shelf from 5.7 km offshore to 6 km offshore, while a 20% decrease in storm wave height moved the mud line to 5.35 km offshore. The zone of maximum sediment deposition also varied with wave height, varying from 10.5 km offshore under current conditions to 11.5 km offshore with a 20% increase in wave height, and 9.5 km offshore with a 20% decrease in wave height. Increased wave height also increased the amount of sediment that bypassed the shelf. Simulations such as these provide a way of testing the possible stratigraphic effects of climatic variations, such as changes in El Nino frequency and millennial-scale fluctuations that have been inferred for the Cascadia margin.