A NEW BEDROCK GEOLOGIC MAP OF THE SOUTHERN HALF OF THE MONTREAT 7.5 MINUTE QUADRANGLE, BUNCOMBE, MCDOWELL, AND YANCEY 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 southern half of the Montreat 7.5-minute quadrangle in Buncombe, Mitchell, and Yancey Counties, NC. Bedrock geology of the map area is comprised of the Ashe Metamorphic Suite (AMS) and the Alligator Back Metamorphic Suite (ABMS).

The Neoproterozoic to Cambrian AMS represent the oldest and structurally lowest rocks in the map area. The AMS is composed of complexly deformed and migmatitic meta-sedimentary and meta-volcanic rocks. Metagraywacke, conglomeratic and/or schistose metagraywacke, garnet-mica schist, amphibolite, and altered ultramafic bodies are interlayered with each other at all scales. Two sub-parallel horizons of altered mafic/ultramafic rocks may indicate the presence of a ~2 km-scale fold structure or a repetition of AMS stratigraphy by faulting. The AMS has been metamorphosed to upper amphibolite facies. The kyanite/sillimanite isograd strikes NE-SW across the approximate center of the map with sillimanite to the NW and kyanite to the SE.

The Neoproterozoic to Cambrian ABMS lies structurally above and in fault contact with the AMS and is comprised of meta-sedimentary and meta-volcanic rocks. The ABMS was subdivided into five units: 1) metasandstone; 2) schist; 3) amphibolite; 4) graphitic schist; and 5) a mixed unit consisting of interlayered metasandstone, metawacke, and schist. The ABMS is distinguished from the AMS by the presence of a graphitic schist unit, lower biotite content in sandstones and schists, less migmatitic lithologies, and the presence of a prominent “pin-striped” texture in meta-sandstones.

Compositional layering and schistosity are parallel to one another and complexly folded in both the AMS and ABMS. These foliations typically strike NE-SW and dip NW/SE. Mylonitic foliation is prevalent in outcrops near the AMS-ABMS fault contact and in the SE corner of the map near the Brevard Zone (BZ). Mylonitic foliation generally strikes to the NE and dips to the SE. Fold hinges and mineral lineations are subhorizontal to gently plunging and trend mostly NE-SW. Fracture data indicate a prominent steeply-dipping fracture set with azimuth 110°-290° (ESE-WNW).