Southern Appalachian Geology: Inner Piedmont

Monday 3-8-2004
Dr. John M. (Jack) Garihan, guest geologist

(click on pictures for larger versions)

Sassafras Mountain Inner Piedmont: Eastatoe Gap and Table Rock Quadrants Henderson Gneiss at Sassafras Mountain, the highest point in South Carolina, was metamorphosed from a granite intrusion 450-560 myo, intruded during the Ordovician and caught up in deformation. Henderson Gneiss, consisting of quartz, plagioclase and microcline with some muscovite, has microcline crystals modified by shearing. These phenocrysts have white myrmekite around the outside and are pink inside. Feldspar makes “warts” on the surface of the rock because it is more resistant to weathering. This part of the Appalachians is a series of thrust sheets, northeast-trending, with the southeast layers stacked on top of the northwest layers. Some terrane in the area could be part of an island arc. A fault is just a change from one kind of rock to another kind. The Eastatoe fault at Beasley Gap is a ductile fault that pushed southeastern Chauga River schist over northwestern Henderson Gneiss. The Chauga River schist was a megasiltstone (muddy rock) metamorphosed into garnet-muscovite-biotite schist with some feldspar. It is Cambrian age, based on zircon dating. The next layer above, thrust over the Eastatoe fault, is the older Poor Mountain gneiss, with more than 50% amphibolite along with plagioclase and hornblende. It is finely grained and banded, indicating that it was basaltic in origin, perhaps an airfall. The rock weathers to clay; iron adsorbs onto the clay and makes it red. The contact between the Poor Mountain amphibolite and the Table Rock gneiss is an intrusive contact. The Table Rock gneiss intruded into the amphibolite schist. The contact was faulted, but is not visible from this location. As the Table Rock gneiss was folded and compressed, it formed sheath folds. The hinge of a sheath fold is always perpendicular to the direction of push. Chemical weathering made the Table Rock gneiss a soil, but left the original rock features.
	Dr. Jack Garahan points out the unique features of Henderson Gneiss.
	A sheath fold completely wraps a rock layer around rock from the layer below.
	Deeply folded rocks show the intensity of the forces at work in the building of the Appalachians.
	Table Rock, viewed from the east, shows exfoliation features.
Camp Greenville Inner Piedmont: Standingstone Quadrant At Camp Greenville, a thrust sheet of the Chauga River formation created the Six-mile Thrust Sheet over the Table Rock Gneiss. The fault line consists of quartz and feldspar, separating the red amphibolite from the much lighter gneiss below.
	The Six-Mile Thrust Sheet as viewed at Camp Greenville.
	A close-up view of the fault.
Bald Rock Inner Piedmont Bald Rock is an exfoliation dome 450 myo, the same age as the Table Rock formation. It is speculated that circles in the rock around quartz “knobs” are man-made. The granite of the dome was itself intruded by other granite.
	Exfoliation features at Bald Rock.
	Quartz knobs at Bald Rock.
	Granitic intrusion folded in the Bald Rock granite.
Mill Creek Dike Inner Piedmont
	During the Jurassic (180 mya), basalt intruded the Poor Mountain amphibolite (Mill Creek Dike) near Greenville, SC. The fissure into which the basalt intruded resulted from thinning of the continental crust as the Atlantic Ocean reopened.

Notes:

The Henderson gneiss is a medium-gray, biotite granite augen gneiss, which is composed of microline, oligoclase, quartz, biotite, and minor amounts of muscovite, epidote, and titanite. There is a gradual textural progression across strike from southeast to northwest; from strongly deformed Henderson gneiss to protomylonite, to porphyroclastic mylonite, to mylonite, and ultramylonite. The mylonitic rocks are yellowish-gray, fine-grained and thinly laminated, with grayish-pink porphyroclast of microline enclosed by a matrix of fine-grained quartz, plagioclase, and micas. http://www.geo.ua.edu/fieldtrips/trip2001/area4.html

Myrmekite: An intergrowth of plagioclase feldspar (commonly oligoclase) and vermicular quartz, generally replacing potassium feldspar; formed during the later stages of consolidation in an igneous rock or during a subsequent period of plutonic activity. The quartz occurs as blobs.

More intense metamorphosis makes crystals smaller. The minerals are trying to stabilize quickly, so they form smaller crystals.

This is my Field Journal from The University of South Dakota 2004 ESCI 396 Spring Field Trip:
Southern Appalachian Geology. © 2004 Charlotte S. Marek