MULTI-LAYERED MAPS OF THE GEOLOGY OF NORTHERN SACRAMENTO VALLEY, CALIFORNIA: GIS FOR THE GIS-CHALLENGED

The northern Sacramento Valley straddles three tectonic elements of Northern California (accretionary Franciscan Complex, Great Valley Group forearc deposits and underlying ophiolite basement, and the Sierra Nevada batholith and the accreted terranes it intruded). The valley also hosts significant ground and surface water resources. Tectonic analysis and assessment of resources is advanced by spatial data integration that takes advantage of the layered portable document format (pdf), where 2-D maps of geology, geophysics, and well information can be displayed as layers in a single pdf file. GIS (Geographic Information System) databases are exported into the pdf file as layers that are geographically registered and retain the spatial relationships of different datasets. For the northern Sacramento Valley in the Willows and Chico 1:100,000-scale quadrangles, we have augmented our digital geologic map compilation with new surficial and bedrock geologic mapping, compiled whole-rock chemistry and radiometric ages, collected new gravity and aeromagnetic data, and stratigraphic tops and lithologic variations interpreted from electric logs of gas wells and drillers’ logs of water wells to produce data layers in the pdf file. In addition to a traditional geologic map, data layers include filtered versions of the gravity and aeromagnetic data to enhance sources at different depths, the subsurface extent of the Miocene Lovejoy Basalt as mapped by aeromagnetic and well data, the base of the continental Plio-Pleistocene Tuscan and Tehama Formations (which form the main aquifer of the valley), and the tops of the Eocene sedimentary units and the Cretaceous strata within the Great Valley sequence. Within the layered pdf, the mapped extent of the concealed Corning fault in the geologic map layer may be compared with the distribution of wells, the base of the aquifer as defined by those wells, and the filtered gravity data that are the basis of the mapped fault location. Other faults that are postulated to traverse the valley, such as the Willows and Paskenta faults, appear to have minimal offset, given their subtle expression in the filtered aeromagnetic data. The layered pdf allows the user to explore and plumb the spatial relationships between multiple datasets without the need for expensive software or GIS expertise.