NEW BEDROCK GEOLOGIC MAP OF THE BLACK MOUNTAIN 1:24,000-SCALE QUADRANGLE, BUNCOMBE AND MCDOWELL COUNTIES, NC

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 Black Mountain 7.5-minute quadrangle in Buncombe County, NC. Bedrock geology of the Black Mountain quadrangle is comprised of the Ashe Metamorphic Suite (AMS), Tallulah Falls Formation (TFF), the Alligator Back Metamorphic Suite (ABMS), Henderson gneiss (Hg), and rocks of the Brevard Zone (BZ).

Foliation and mylonitic foliation generally strike NE and dip SE. Prominent, steeply-dipping fracture sets with azimuths 145° and 300° along with a subordinate fracture set of azimuth 330° were identified from fracture data.

Neoproterozoic to Cambrian metasediments of the AMS in the northwest corner of the quadrangle are the oldest and structurally lowest rocks in the map area. The AMS is a highly deformed migmatitic package of interlayered metamorphosed sedimentary and volcanic rocks. Interlayers include metagraywacke, garnet-mica schist, metaconglomerate, and amphibolite. They have been metamorphosed to kyanite and sillimanite grade and retrograded to chlorite grade near the BZ.

Directly southeast of and structurally above the AMS are Neoproterozoic to Cambrian metasediments and metavolcanics of the ABMS. The ABMS was subdivided into heterogeneous coarse- and fine-grained units, garnet-mica schist, metasandstone, amphibolite, and graphitic schist. The presence of graphitic schists and metasandstones help differentiate the ABMS from the AMS. The ABMS in the map area is rarely migmatitic and has been affected by chlorite grade retrogression associated with shearing along the BZ.

The BZ separates the AMS and the ABMS in the northwest from the structurally higher TFF and Hg in the southeast. The BZ consists of interlayered mylonitic phyllite, metasandstone, metasiltstone, quartzite, meta-arkose, metagraywacke, muscovite schist, porphyroblastic biotite gneiss, and felsic gneiss.

TFF in the map area is an interlayered heterogeneous sequence of metamorphosed sedimentary and volcanic rocks, possibly equivalent to the AMS. It has been strongly affected by BZ mylonitization and chlorite retrograde metamorphism.

To the southeast, the Hg and associated granitic gneisses are structurally above and in fault contact with the TFF and BZ.