NEW BEDROCK GEOLOGIC MAP OF THE CRAGGY PINNACLE 1:24,000-SCALE QUADRANGLE, BUNCOMBE COUNTY, NC

The North Carolina Geological Survey produced a new 1:24,000-scale bedrock geologic map of the Craggy Pinnacle 7.5-minute quadrangle in Buncombe County, cooperatively funded through the STATEMAP component of the National Cooperative Geologic Mapping Program. Bedrock in the quadrangle is almost entirely comprised of the Ashe Metamorphic Suite-Tallulah Falls Formation (AMS-TFF), although the Earlies Gap biotite gneiss (EG), a sizeable migmatite body (Pzmg), and outcrop scale trondhjemite dikes are also present.

Foliation and mylonitic foliation are extensively folded exhibiting a variety of fold interference patterns, but generally strike NE-SW and dip SE and NW. A prominent, steeply-dipping joint set of azimuth 125°-305° along with a subordinate joint set of azimuth 45°-225° were identified.

Mesoproterozoic basement gneisses of the EG underlie a small portion of the northwest corner of the quadrangle and are the oldest rocks in the map area. EG rocks on the quadrangle are mylonitic to ultramylonitic varieties of biotite gneiss and layered biotite granitic gneiss. Mylonitization along the EG/AMS-TFF contact and ultramylonites present within the AMS-TFF to the SE within the map area are attributed to deformation along the Holland Mountain Fault/Burnsville Shear Zone.

The Neoproterozoic to Cambrian metasediments of the AMS-TFF are structurally above and in fault contact with the EG, and span the remainder of the quadrangle. The AMS-TFF here consists of metagraywacke, schistose metagraywacke, garnet mica schist, amphibolite, altered ultramafics, and biotite hornblende gneiss. AMS-TFF has been metamorphosed to upper amphibolite facies and is locally migmatitic. Kyanite and sillimanite coexist in several localities.

A 0.25 to 0.75 km wide SW-NE striking, segmented migmatite body with a total strike length of over 7 km occurs in the map area. The migmatite is weakly to well foliated and also commonly occurs as thin layers and lenses within the AMS-TFF. Too small to be mapped at a 1:24,000 scale, trondhjemites occur as thin dikes and sills ranging in thickness from centimeters to meters. A similar trondhjemite dike on the adjoining Weaverville quadrangle has a reported age of 410-420 Ma (Miller and others, 2000).