A NEW BEDROCK GEOLOGIC MAP OF THE PISGAH FOREST QUADRANGLE, TRANSYLVANIA AND HENDERSON COUNTIES, NORTH CAROLINA

The North Carolina Geological Survey, in conjunction with the United States Geological Survey STATEMAP program, has produced a new 1:24,000-scale bedrock geologic map of the Pisgah Forest 7.5-minute quadrangle in Transylvania and Henderson Counties, NC. Bedrock geology of the Pisgah Forest quadrangle comprises components of the Ashe Metamorphic Suite-Tallulah Falls Formation (AMS-TFF), Pink Beds pluton (PB), Brevard Zone (BZ), and Henderson Gneiss (HG).

Foliation and mylonitic foliation generally strike NE and dip SE. NW dip directions were observed in folded AMS-TFF rocks. Dip values decrease markedly in rocks SE of the BZ. A prominent, steeply-dipping fracture set of azimuth 305°-125° along with a subordinate fracture set of azimuth 225°-45° were identified from fracture data. Both linear and planar fabric orientations are similar to those mapped in the Horse Shoe quadrangle to the east and the Brevard quadrangle to the south.

Neoproterozoic to Cambrian metasediments of the AMS-TFF underlie most of the northwestern half of the quadrangle, and are the oldest rocks in the map area. AMS-TFF rocks are metagraywackes, mica schists, amphibolites, and altered ultramafics and show increasing mylonitization nearer the Brevard Zone. They have been metamorphosed to upper amphibolite facies and are locally migmatitic. The main body of the Pink Beds pluton outcrops in the extreme northwestern corner of the quadrangle. PB is typically trondhjemitic in composition, unfoliated to weakly foliated, and has reported Devonian crystallization ages (Miller et al., 2000; Jubb, 2010). Small igneous bodies, possibly of PB affinity, were mapped within the AMS-TFF. Markedly cross-cutting, weakly foliated to mylonitic granitoid intrusives were observed within the AMS-TFF along the contact with the BZ.

SE and structurally above the AMS-TFF in fault contact are BZ rocks consisting mainly of greenschist-facies chlorite-sericite/graphitic phyllite, marble, and mylonitic granitic gneiss/phyllonite. The protolith of much of the mylonitic granitic gneiss/phyllonite is interpreted to be HG.

SE and structurally above the BZ in fault contact is a large volume of lineated HG. We identified subdivisions within the HG based on composition, grain size and degree of deformation.