24 Outstanding Field Trip Stops in the Carolinas and Beyond for Professionals and Teachers (Posters)

Monday, 2 April 2012
8:00 AM - 12:00 PM
Grand Ballroom

RECENT ROAD CUT ON U.S. 221 IN MCDOWELL COUNTY, NC REVEALS A NEW EXPOSURE OF THE LINVILLE FALLS FAULT

Paper number: 24-2
Program: Theme Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

The North Carolina Geological Survey (NCGS) has examined in detail a bedrock outcrop on U.S. 221 in North Cove, McDowell County, NC. The outcrop is an excellent new exposure of the Linville Falls fault (LFf). Bedrock data were taken along the base of the outcrop and along a cross-section line that extended from the base to the top of the cut slope. At this location the LFf thrusts the Cranberry Gneiss, approximately one-billion-year-old continental basement, over dolomitic marble of the Shady Dolomite within the Grandfather Mountain Window. At its type locality the Shady Dolomite is a fine- to coarse-grained, thin- to thick-bedded dark- to light-gray, blue-gray, and white dolomitic limestone. At this location the original dolomite has recrystallized into a dolomitic marble. Near the LFf fault contact the unit is mylonitic, pyrite bearing and locally sericitic, away from the fault it is semi-massive and highly fractured. This is one of the few easily accessible locations along the Grandfather Mountain Window-basement contact where the Shady Dolomite has not been omitted by faulting. The outcrop is substantial, stretching approximately a quarter mile along U.S. 221 and reaching a height of 325 feet.

The dolomitic marble is exposed at the base of the outcrop and grades successively upward into calcareous phyllonite, ferruginous phyllonite, mylonitic granitic gneiss and semi-massive migmatitic folded orthogneiss. Rocks near the LFf are highly strained and completely overprinted with a mylonitic foliation. Slickenlines and localized breccia zones indicate late stage brittle faulting near the marble-calcareous phyllonite contact. Rocks near the top of the outcrop are slightly less mylonitic and preserve older metamorphic fabrics. Mylonitic foliation generally strikes southwest and dips northwest. Dips range from 18 to 40 degrees and average about 30 degrees throughout the exposure. Immediately to the east, on the opposite bank of the North Fork Catawba River are quartzite exposures of the upper member of the Chilhowee Group that underlies the Shady Dolomite. With this site being readily accessible and well exposed it makes a great location to get a peek inside the Grandfather Mountain Window.