GEOLOGIC AND GEOPHYSICAL MAPPING OF THE SONOMA VOLCANIC FIELD, CALIFORNIA: A PROGRESS REPORT ON HYDROGEOLOGIC APPLICATIONS IN THE NAPA AND SONOMA BASINS

In the northern San Francisco Bay region, California, units of the Sonoma Volcanic Field (SVF) are increasingly important aquifers. These rocks form many of the hills surrounding Napa and Sonoma Valleys and presumably underlie the alluvium in both valleys. The SVF (~ 7.5 to 2.5 Ma) consists of a diverse assemblage of volcanic rocks erupted from multiple centers. The 3D architecture of these rocks and their hydrologic properties are being investigated through surface and subsurface geologic and geophysical mapping at various scales, combined with hydrologic and geochemical data. Depth to pre SVF basement is greatest in the southern parts of the Napa and Sonoma basins based on gravity. The SVF consists of basaltic-andesite flow fields, felsic flow-dome and tuff cone complexes, and calderas with associated ash flow tuffs. In the hills around southern Napa and Sonoma Valleys, new mapping indicates that distinct packages of volcanic rocks are separated by angular unconformities. An older, southwest-dipping section of silicic units is comprised of a spatially restricted, coalescing flow-dome field with air fall tuff aprons separated by dacite and basaltic-andesite flows. A more gently-dipping upper silicic package consists mostly of flows and, in Napa Valley, locally important ash-flow tuffs. The upper silicic units have primarily fracture permeability whereas the older sections are nonwelded and pumiceous, serving as locally important aquifers and preferential pathways for recharge into the valleys. In contrast, young ash flow tuffs in the northern Napa Valley are regionally extensive and have been traced to their source calderas on the eastern side of the valley, on basis of age and phenocryst assemblages. Widespread hydrothermal alteration in northern Napa Valley generally decreases permeability and aquifer potential and results in elevated levels of boron and arsenic in water. In southern Napa and Sonoma valleys the felsic tuff units are unaltered and are primary aquifers in the Sonoma basin. In the NE part of SVF, NNW trending faults provide structural control for upwelling thermal fluids in northern Napa Valley. The thermal water is a mixture of meteoric water and connate fluid derived from Cretaceous marine sedimentary rocks (B/Cl ratio). This suggests Great Valley Sequence below the SVF in the Calistoga area.