PRELIMINARY COMPILED BEDROCK GEOLOGIC MAP OF FRANKLIN COUNTY, NORTH CAROLINA
The western portion of the county is underlain by Neoproterozoic to Cambrian gneisses and schists metamorphosed to amphibolite facies and include the Falls Lake, Crabtree, and Raleigh terranes. The center is dominated by the Pennsylvanian to Permian Rolesville granitoid batholith and the north-northeast portion is underlain by schists and gneisses of the Raleigh and/or newly proposed Warren terrane(s). In the east, the Gupton and Castalia plutons roughly separate the greenschist to amphibolite facies metavolcanic and metasedimentary rocks of the Spring Hope terrane into northern and southern portions. U-Pb zircon age-dates from the Spring Hope terrane in Franklin and neighboring Nash County range from 525 to 620 Ma. Terranes are separated by a number of mainly NNE-trending late Paleozoic faults showing evidence of dextral and thrust motion.
Dikes of Jurassic diabase intrude crystalline rocks of the county. Multiple brittle fault zones with silicified quartz breccia trend mostly ENE. Outliers of Coastal Plain and/or other unconsolidated sediments are present within the county, though their extent is not well documented. Quaternary alluvium is present in most major drainages.
Naturally occurring groundwater contaminants that are of some concern include radon in the granitoid plutons and arsenic in the Spring Hope terrane. The abandoned Portis Mine, located in the northeast corner of the county in the Spring Hope terrane, produced over $9 million in gold between 1835 and 1936.