QUATERNARY STRATIGRAPHY AND NEOTECTONICS OF THE SACRAMENTO-SAN JOAQUIN DELTA, CALIFORNIA: PRELIMINARY FINDINGS FROM RECENT COLLABORATIVE RESEARCH

The U.S. Geological Survey and California Department of Water Resources (CDWR) are developing a 3D Quaternary chronostratigraphic model for the Sacramento-San Joaquin Delta area. Principal objectives of the study are to: a) identify and map subsurface unconformities and flooding surfaces that serve as tools for assessing tectonic deformation and the potential for seismic activity on faults in the region, b) provide a predictive tool for characterizing subsurface conditions to evaluate potential water conveyance system alignments, c) better characterize and map the 3D distribution of geotechnical parameters important for assessing levee vulnerability from earthquake ground shaking and liquefaction, and d) characterize the shallow seismic-velocity structure. Data for the model primarily consist of thousands of geotechnical boring logs and CPT soundings collected during the past six decades, including high-quality logs and core samples from several hundred boreholes and CPT soundings completed since 2008 by CDWR to depths up to 200 ft.

We have initially identified six mid-Pleistocene to Holocene sequences that can be regionally correlated based on log descriptions that suggest the presence of buried soils and measured variations in shear-wave velocity and soil density. Correlations have been aided by the discovery and identification of at least three distinct mid-lower Pleistocene age tephra horizons that occur within the section. Rates of sediment accumulation in the Delta are low, with perhaps >1 m.y of the sedimentary record preserved within ~300 ft of the ground surface. Initial evaluations of chronostratigraphic relations suggest a) middle and late Quaternary tectonic activity associated with the mapped trace of the West Tracy fault, b) post-lower Pleistocene structural relief likely associated with the South Midland fault, and c) possible Quaternary tectonic deformation within the central Delta not associated with any presently known structures.