REGIONAL-SCALE THREE-DIMENSIONAL GEOLOGIC MAPS OF THE INTERMOUNTAIN WEST—A POTENTIAL STARTING POINT TO A NATIONAL-SCALE GEOLOGIC MODEL

Three-dimensional (3-D) geologic maps define the elevation, thickness, extent, and material properties of geologic units in a 3-D volume. 3-D maps foster regional geologic conceptualization, inform energy, mineral, and water resources assessments, and are digital input to numerical process models. The current strategic vision of the USGS National Cooperative Geologic Mapping Program (NCGMP) calls for creation of national-scale digital 2-D and 3-D geologic maps. In 3-D, two digital geologic frameworks might be envisioned: (1) a generalized framework that comprises a small number of regionally extensive surfaces that portray the stratigraphic and structural geologic system in a consistent, generalized manner, and (2) local 3-D frameworks housed inside the regional framework which vary greatly in their depth, level of detail, and portrayal of geologic parameters depending on their end use.

Regional-scale 2-D and 3-D geologic mapping in the Intermountain West by the NCGMP, in partnership with regional-scale water assessment projects conducted within the USGS Water Mission Area, provides a starting point for a national-scale model. In the Intermountain West, deep regional groundwater flow through consolidated rocks requires understanding of the occurrence and juxtaposition of permeable aquifer units or low-permeability confining units in 3-D at the watershed scale. 3-D maps created to date include (1) a 9-layer model in the eastern Great Basin covering 285,000 km² and extending to depths of about 5 km, and (2) two 7-layer models in the Colorado Plateau that extend to depths of 4 km and cover a combined area of 136,000 km². These regional-scale 3-D geologic maps integrate multiple datasets: geologic map data from several states, geologic cross sections, structure contour and isopach data, stratigraphic tops from well data, geophysically-derived surfaces, and results from existing 3D framework models. Future priorities for regional- to national-scale efforts include: (1) development of standardized 3-D data structure that includes input geologic data, output surfaces and 3-D data arrays; (2) inventory and capture of digital data from existing 3-D geologic maps; and (3) establishing workflows for the production, review, and updating of 3-D datasets, and 4) integration with ongoing 2-D mapping.