A NEW 1:24,000-SCALE BEDROCK GEOLOGIC MAP OF THE OLD FORT 7.5-MINUTE QUADRANGLE, MCDOWELL AND YANCEY COUNTIES, NORTH CAROLINA

The North Carolina Geological Survey has produced a new 1:24,000-scale bedrock geologic map of the Old Fort 7.5-minute quadrangle in McDowell and Yancey counties, cooperatively funded through the STATEMAP component of the National Cooperative Geologic Mapping Program. Bedrock geology of the map area comprises mylonite/phyllonite, porphyroclastic biotite gneiss, mixed gneiss, and Neoproterozoic metasedimentary rocks.

The Brevard Fault Zone is a prominent NE-SW striking feature on the Old Fort quadrangle. Multiple ductile and brittle activations in the Paleozoic have caused rocks in this zone to become mylonitic to ultramylonitic making protolith identification difficult.

The porphyroclastic biotite gneiss unit is locally protomylonitic to ultramylonitic. Porphyroclasts are generally granule-sized but vary to gravel-sized locally, and are heterogeneously distributed throughout the matrix. U-Pb age dating has produced an age of ~360 Ma (A. Merschat, personal communication). This age precludes its origin as nearby Henderson Gneiss or Precambrian basement. The unit may be related to the Spruce Pine Plutonic Suite or several similar granitic intrusions to the SW with ages from 320-400 Ma.

The mixed gneiss unit consists mostly of biotite gneiss with minor granitic orthogneiss, and mylonite. Correlation of the biotite gneiss is difficult as it is heterogeneous, contains granule- to gravel-sized porphyroclasts, pegmatitic layers, ribboned felsic layers, and is protomylonitic to mylonitic. Different portions of this unit may be correlated to the Tallulah Falls Formation or the Henderson Gneiss.

Neoproterozoic to early Cambrian metasediments consist of metasandstone, graphitic schist, amphibolite and an undivided unit of metawacke, meta-arenite, and schist. Notably, the metasandstone unit lacks schist, amphibolite, and garnet. This may represent a shift in depositional environment.

Foliations and mylonitic foliations generally strike NE-SW and dip moderately to steeply to the SE. Joint data indicate a prominent, steeply-dipping, WNW-ESE trending fracture set with a minor E-W set. Most measured fold axes have a shallow NE or SW plunge.