GEOLOGIC MAP COMPILATION AT 1:100,000 SCALE WITHIN COLORADO: PROCESSES AND CHALLENGES FOR CREATING DIGITAL GEOLOGIC DATA TO SUPPORT A NATIONAL GEOLOGIC MAP BY THE END OF THIS DECADE

The Colorado Geological Survey (CGS) has been creating compilation geologic maps at ~1:100,000 scale to both support our own programs and mission, but also the USGS’s goal of completing a geologic map of the U.S. by the end of this decade. The resulting maps are used by CGS geologists to create groundwater resources, mineral resources and geologic hazard derivative maps that are utilized by federal, state and local governments as well as private individuals. Compiling the geologic mapping is a multi-step process that varies across our projects; we will use our county-wide hydrogeologic maps as an example. First, a table of available published maps at various scales in the county is created that includes the associated unit descriptions, maximum depths and economic resources for each unit. These data become the core of a regional stratigraphic column. Next, available vector data are pulled into ArcGIS and organized into GeMS; if GIS data are not available it gets digitized from paper maps. Challenges include changes to various geologic unit naming conventions over time, space and by author (especially for Quaternary deposits); edge-matching units at various scales; and extrapolating bedrock layers beneath Quaternary deposits. For the hydrogeologic maps, this extrapolation extends to county boundaries. Using available water or oil and gas well data, depth to top of aquifers and elevation of the surface are mapped as GIS rasters and contours. Challenges include interpreting available geologic logs while trying to minimize the inherent author bias, and inadequate well spatial density.