A NEW 1:24,000-SCALE BEDROCK GEOLOGIC MAP OF THE NORTHERN HALF OF THE MOFFITT HILL 7.5-MINUTE QUADRANGLE, MCDOWELL, RUTHERFORD, AND BUNCOMBE COUNTIES, NORTH CAROLINA

The North Carolina Geological Survey has produced a new 1:24,000-scale bedrock geologic map of the northern half of the Moffitt Hill 7.5-minute quadrangle in McDowell, Rutherford, and Buncombe counties, cooperatively funded through the STATEMAP component of the National Cooperative Geologic Mapping Program. From northwest to southeast, bedrock geology of the map area is comprised of elements of the Ashe Metamorphic Suite-Tallulah Falls Formation (AMS-TFF), the Brevard zone, and a mixed gneiss unit.

AMS-TFF units on the quadrangle include metasandstone and a porphyroclastic biotite gneiss. The porphyroclastic biotite gneiss unit is heterogeneous, consisting predominantly of coarse-grained biotite granitic gneiss with lesser amounts of metagraywacke. It is locally protomylonitic to ultramylonitic. Within the granitic gneiss, porphyroclasts are generally granule-sized but vary to gravel-sized locally, and are heterogeneously distributed throughout the matrix. A U-Pb zircon crystallization age of ~360 Ma was obtained for a granitic gneiss sample from the unit (A. Merschat, personal communication). This age differentiates it from nearby Henderson Gneiss or Precambrian basement. The granitic portion of the unit may be related to intrusions of the Spruce Pine Plutonic Suite or similar plutons to the SW.

Rocks of the Brevard Zone on the Moffitt Hill quadrangle are strongly layered mylonites and phyllonites that are locally brecciated. Pervasive ductile and brittle overprinting makes protolith identification difficult within this unit.

The mixed gneiss unit consists of Henderson Gneiss with biotite gneiss, granitic orthogneiss, and mylonite of unknown affinity. The biotite gneiss 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 correlative with the Tallulah Falls Formation or Henderson Gneiss.

Foliations and mylonitic foliations generally strike NE-SW and dip predominantly to the SE. As observed in other areas along strike, foliations dip more gently SE of the Brevard zone. Most measured lineations and fold axes have shallow NE or SW plunges. Joint data indicate a prominent, steeply-dipping, NW-SE trending fracture set with minor E-W and NE-SW sets.