GEOLOGIC MAP OF THE CONTERMINOUS UNITED STATES: PRE-QUATERNARY BEDROCK 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 Pre-Quaternary bedrock geology layer depicts geologic units older than Quaternary in the Midwest and Northeast, mostly, but not everywhere, buried beneath Quaternary glacial deposits and mapped from outcrop and subsurface data such as boreholes. In most places it depicts geology exposed at the “top of rock” surface at the base of the Quaternary. The map is a composite of 22 state geologic maps depicting pre-Quaternary geologic units, usually referred to as “bedrock” in that region (some maps show Quaternary deposits locally, mostly young alluvium).

About 2400 source map units from 22 maps are assigned to 110 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 selected to show sedimentary, igneous, and metamorphic rocks of different ages (mostly Paleozoic), and locally to distinguish clastic from carbonate sedimentary rocks. 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, such as dikes, key beds, and fold axes.