A GEMS-COMPLIANT FRAMEWORK FOR LAYERING GEOLOGIC MAP DATA IN THE FALL ZONE OF VIRGINIA

The Geologic Map Schema (GeMS) is the U.S. Geological Survey standard for the digital publication of geologic map databases. As part of the National Cooperative Geologic Mapping Program’s U.S. GeoFramework Initiative, the Virginia Department of Energy - Geology and Mineral Resources Program (VAGMRP) routinely converts paper and digital geologic maps into GeMS-compliant products. The Geologic Map of the Chesterfield quadrangle (Carter and others, 2010) straddles the Fall Zone and depicts crystalline Piedmont bedrock, Coastal Plain sediments, and modern surficial deposits. The conversion of the map to GeMS required generating a geodatabase that could accommodate the three-dimensional relationship between these map units.

A typical VAGMRP GeMS database follows a traditional model for geologic maps by portraying one continuous surface of geologic map units intersected by contact and fault lines. Although three-dimensional relationships are portrayed through map pattern, correlation charts, structural data, and cross section diagrams, the data is not explicitly structured for digital modelling or analysis. Some VAGMRP maps in the Fall Zone depart from this traditional approach by identifying concealed crystalline bedrock contacts beneath unconsolidated Coastal Plain map units. For the Chesterfield GeMS map conversion, VAGMRP integrated this spatial relationship into the map body as layers. The database includes two sets of “MapUnitPolygons” and “ContactsAndFaults” feature classes, each representing a discrete topological surface in the geologic map. This approach also required a minor addition to the “DescriptionOfMapUnits” table.

The Chesterfield database offers a model for structuring geologic data in the Fall Zone, where the shallow depth of Coastal Plain sediments allows for definition of underlying bedrock contacts. This database structure passes level-3 GeMS validation while preserving the layered structure of the map. It provides a GeMS-compliant method for integrating stratigraphic relationships into geologic map databases.