TOWARDS A 3D GEOLOGIC MAP OF NORTH AMERICA: A PROGRESS REPORT

U.S. Geological Survey (USGS) science planning documents and decadal strategic plans for the USGS National Cooperative Geologic Mapping Program (NCGMP) call for geologic mapping across the Nation to become increasingly three-dimensional (3D) in nature. Recent Congressional mandates call for the NCGMP to compile and serve seamless 2D and 3D geologic information to the public at detailed national- and continental-scale resolution in cooperation with State geological surveys. Adding a 3D component to the regional- and national- scale mapping program poses significant scientific and data management challenges in parsing the abundant, local-scale stratigraphic detail present on geologic maps and in boreholes to arrive at a tractable set of subsurface units. One potential solution is to create a 3D rendering of the Geologic Map of North America (GMNA) for the contiguous United States. This map offers seamless geologic coverage of the entire lower 48 states with unambiguous map-unit descriptions. However, the compiled map units are chronostratigraphic in nature, thus reconciliation of the GMNA units with lithostratigraphic units that appear on State geologic maps or in borehole data requires a study of regional subsurface stratigraphic correlation diagrams and the USGS lexicon of geologic names.

In this presentation we discuss our working methodology for creating a national-scale 3D model from the GMNA through two case studies being conducted on (1) the Colorado Plateau and (2) the Midcontinent. In the incised Colorado Plateau, we find that mapped contacts from State geological maps along with structure contour and isopach maps are sufficient to determine which formations fall within the GMNA units and define their 3D configuration, except within Neogene basins, where greater reliance on borehole data is needed. In the Midcontinent, comparing borehole data to regional subsurface stratigraphic correlation diagrams is necessary to link GMNA units to formational units and available data sets. When complete, this coarse-resolution 3D representation can be used to inform national-scale process models (i.e. water and seismic risk) and provide a framework in which to fit more detailed subsurface models that will be disseminated in a non-proprietary, consistent, and versionable format to the public.