COMPILATION MAPPING IN THE DANVILLE 100K GEOLOGIC MAP – THE NORTH CAROLINA PERSPECTIVE

The North Carolina Geological Survey (NCGS) collaborated with the Virginia Geology and Mineral Resources Program to compile the Danville 100K geologic map into a Geologic Mapping Schema (GeMS) geodatabase deliverable. The effort was partially supported by the U.S. Geological Survey, National Cooperative Geologic Mapping Program under STATEMAP.

The North Carolina portion of the Danville 100K includes the lower 1/3 of eight 24K-scale quadrangles. In NC, from west to east, the map area includes the following: Blue Ridge Cover Sequence, Smith River Allochthon, Proterozoic Basement Rocks, Sauratown Mountains Anticlinorium, Dan River Basin, Milton terrane, and the Hyco arc portion of the Carolina terrane. Published geologic maps utilized in the compilation included the 1:125K-scale NCGS Region G map (Carpenter, 1982) and portions of two 24k-scale geologic maps published as NCGS Open-file Maps (Hibbard, 2017; Wilkins and Hibbard, 2017) along the Hyco shear zone (a first-order terrane boundary between the Milton and Carolina terranes). Olsen et al. (2015), which revised the stratigraphy of the Dan River basin, was used to significantly revise previous contacts and formations from past mappers.

The NCGS Piedmont mapping program workflow for compilations includes utilizing legacy data at the best available scale. Legacy paper maps are accurately georeferenced to LiDAR and/or road maps. Often legacy paper maps have distortions or shrinkage that cause inaccuracy. To mitigate these inaccuracies, maps are cropped into smaller areas that can be more accurately georeferenced. For this project, NC georeferenced the most detailed geologic maps in VA along the state line to accurately edge-match linework along the NC-VA state line. From January 2022 to May 2022, NCGS staff conducted targeted foot and vehicle traverses to rectify edge-match issues with past workers’ maps, validate contacts from Carpenter’s Region G Map, and correct edge-match issues with the Winston-Salem East 250K (Espenshade et al., 1975). NC and VA staff spent several days together mapping surficial deposits both in NC and VA. This collaboration ensured that the methods of mapping the surficial deposits along the NC-VA state line would be consistent and edge-matched.