GEOLOGIC INTERPRETATIONS OF GEOTECHNICAL DATA IN THE QUATERNARY SACRAMENTO-SAN JOAQUIN DELTA, CALIFORNIA, U.S.A

Using extensive geotechnical borehole data acquired by the California Department of Water Resources, we calibrate Unified Soil Classification System units to geologic interpretations in the Sacramento-San Joaquin Delta, California. Such geologic translation of geotechnical data can be extrapolated to boreholes without retained cores, extending correlation of subsurface units. In the Delta dataset, we treat Unified Soil Classifications as distinctive geotechnical units and use detailed observation and sampling of retained cores to interpret deposition and facies associations. Retained cores provide an opportunity to observe fine-scale sedimentary and diagenetic structures, and interpret geologic context, mechanisms of deposition, and depositional environments within geotechnical units. Sub-samples from cores, including volcanic ash, diatoms, and paleomagnetic samples, also aid paleoenvironmental interpretations and correlations. Quaternary subsurface deposits in the Delta are primarily sand and clay, with lesser silt and gravel. Lean clay units with abundant carbonate can contain paleosols formed on fluvial floodplains during sea level lowstands, which may represent laterally extensive chronostratigraphic surfaces. Well- to poorly-graded sand and gravel units commonly exhibit subdivisions in fining upward packages that represent river channel deposits with potentially limited lateral extent. Units rich in silt or clay can contain diatom-rich zones indicative of standing water. Organic-rich deposits buried ~100-200 ft below the modern surface are concentrated in the central Delta, occur in lithologies that range from gravelly sands to laminated clays, and potentially represent deposits from previous sea-level highstands. Other less extensive deposits, such as tephra, augment geotechnical units and provide useful correlation tools. Geologic classification of geotechnical units identified in the subsurface of Sacramento-San Joaquin Delta may be applicable to shallower Delta deposits where borehole cores were not retained but potential liquefaction hazards exist. Insights from this study may be relevant to other fluvial-deltaic settings with abundant recorded geotechnical data, where similar changes in depositional environments have occurred.