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An Integrated Approach Toward a New Quaternary Stratigraphic Model for the Los Angeles Basin, California: A Framework for Refined Seismic Hazards and Groundwater Studies

D. J. Ponti and FOQUS-LA
U.S. Geol Survey, Menlo Park, CA

A new, multidisciplinary initiative by the USGS, in partnership with local agencies, is producing new data in support of a unified Quaternary stratigraphic model for the Los Angeles coastal plain and adjacent continental shelf. We have begun our work in the Long Beach area and on the San Pedro shelf, in partnership with theWater Replenishment District of Southern California and the L.A. County Department of Public Works. The objectives of this study are to provide a detailed 3-D hydrostratigraphic model to address seawater intrusion problems, and to evaluate the seismic potential of theWilmington anticlinorium. New data collection includes two 1400' - deep core holes/wells, nearly 1000 km of marine and land-based hiresolution and multi-channel seismic reflection/refraction lines, and detailed surface gravity and electrical studies. The depositional and tectonic history deduced from these coordinated studies will provide the basis for extending the Long Beach stratigraphic model basinwide. In constrast to older lithostratigraphic and hydrostratigraphic correlations, this new model is sequence-based, and uses modern techniques to define the age, structure, facies architecture, and physical and geochemical properties of the Quaternary basin fill. This refinement provides the necessary framework for: 1) flow and transport models needed for basin ground water management, 2) improved age and geometric constraints for blind-fault evaluations, and 3) refined 3-D velocity constraints for ground motion models.

Building the 3-D regional model requires the development of detailed reference sections throughout the basin. These sections derive from continuously cored boreholes drilled for multiple scientific purposes. Cores are described and sampled on-site in a mobile laboratory, and preserved for future research. The cores provide unprecedented detail on the depositional and geochemical environments, as well as samples for age control and measurements of physical properties. The new data constrain existing well-log interpretations, as well as inferences drawn from borehole geophysics and hi-resolution geophysical investigations that form additional input.

AAPG Search and Discovery Article #90904©2001 AAPG Pacific Section Meeting, Universal City, California