GEOLOGIC MAP OF THE CONTERMINOUS UNITED STATES: PRECAMBRIAN GEOLOGY

The USGS National Cooperative Geologic Mapping Program (NCGMP) was directed by Congress to create a new geologic map of the United States by “bring[ing] together detailed national and continental-resolution 2D and 3D information produced throughout the Survey and by federal and state partners” (House Report 116-100). To fulfill this goal, we have created a database engine for synthesizing geologic maps (described in a companion poster) based on the Geologic Map Schema (GeMS) and used it to bring together geologic maps published by the USGS and State Geological Surveys and create derivative thematic geologic maps.

This Precambrian geology layer depicts Precambrian rocks in the central United States, where they are mostly buried beneath Phanerozoic sedimentary rocks and mapped from boreholes and geophysical data. The map is a composite of 3 state geologic maps and 7 USGS state and regional geologic maps depicting Precambrian rocks in the surface and subsurface. Although outcrop areas of Precambrian rocks in the upper Midwest and Rocky Mountains are included, the map does not show all Precambrian rock outcrops in the conterminous U.S.

About 560 source map units from 10 maps are assigned to 60 synthesis map units, broadly grouped by age and lithology, from the International Commission on Stratigraphy (ICS) timescale and GeMS Geomaterials fields, respectively. Synthesis units were chosen to highlight major Precambrian terranes and structures, including Archean cratons, Proterozoic orogenic belts, and the Midcontinent rift, and to show different ages of igneous intrusions. Synthesis map units are colored according to USGS guidelines such that they are readable as a traditional geologic map. The locations of contacts and faults are unchanged from the original sources, but only those that bound synthesis units are solid; those within them are symbolized as “internal.” Additional line features are included, primarily showing Proterozoic dikes.