DERIVATIVE GEOLOGIC MAP AND DATABASE OF KARST GEOLOGY ALONG THE I-81 CORRIDOR, VIRGINIA: STATUS UPDATE

A 1:24,000-scale geologic map and geodatabase are being compiled in karst prone areas along the I-81 corridor in western Virginia. The 2nd phase of the 3-phase project was delivered to the USGS National Cooperative Geologic Mapping Program in Fall 2024. The map and database now cover 38 7.5-minute quadrangles and approximately 200 miles from the Virginia-West Virginia border near Winchester southwest to Blacksburg. The final phase will extend the map and database to the Virginia-Tennessee border at Bristol. The products include (1) the geographic distribution of carbonate bedrock, (2) overlying surficial deposits, (3) faults, folds, dikes, and structural measurements within carbonate bedrock, (4) sinkholes, (5) springs, (6) Virginia Department of Transportation geotechnical borings and sinkhole mitigation locations, (7) carbonate rock sample locations, and (8) a karst density model derived from sinkholes and additional data from the Virginia Cave Board.

A supplemental report accompanies the map and database to discuss relative solubility, residual soil cover, compositional variability, and spatial statistics of sinkholes within carbonate bedrock units and associated surficial deposits. Purer limestone-bearing units like the Lincolnshire Formation and New Market Limestone consistently correlate with higher sinkhole density. Units like the Rome Formation that are more dolomitic and interbedded with clastic rock have median values and interquartile size ranges of sinkholes greater than in other map units. Less-pure carbonate bedrock tend to have a thicker soil cover that may correlate to the larger average area of depressions. The majority of sinkholes across all units were identified along 0 to 16 slope %, highlighting increased groundwater recharge in gently dipping upland areas. Fluvial terrace deposits covering carbonate bedrock may enhance groundwater recharge and chemical weathering and be a catalyst for sinkhole development.

This product is intended to assist landowners, businesses, industries, consultants, and government agencies make more informed decisions in karst terrain and can be downloaded on the Virginia Energy, Geology and Mineral Resources Program’s website. A web viewer is under development to provide additional access to the geodatabase and supplemental information.